Electromagnetic Testing-ASNT Level III Study Guide ET

Electromagnetic Testing

Study Guide Electromagnetic Testing Part 1 My ASNT Level III Pre-Exam Preparatory Self Study Notes 17th April 2015

Charlie Chong/ Fion Zhang

E&P Applications


E&P Applications


http://independent.academia.edu/CharlieChong1 http://www.yumpu.com/zh/browse/user/charliechong http://issuu.com/charlieccchong



Fion Zhang at Shanghai 17th April 2015

http://meilishouxihu.blog.163.com/


乱七八糟 – 随看随记


乱七八糟 – 随看随记




http://greekhouseoffonts.com/


http://www.naturalreaders.com


http://www.naturalreaders.cn/

IVONA TTS Capable.


http://www.naturalreaders.com/

Chapter 1 Principles of Eddy Current Testing


EDDY CURRENT an Overview Description of Eddy Current Detectors Coil configurations Appropriate coil selection is the most important part of solving an eddy current application, no instrument can achieve much if it doesn’t get the right signals from the probe. Coil designs can be split into three main groups: 1. 2. 3.

Surface probes used mostly with the probe axis normal to the surface, in addition to the basic ‘pancake’ coil this includes pencil probes and special-purpose surface probes such as those used inside a fastener hole. Encircling coils are normally used for in-line inspection of round products, The product to be tested is inserted though a circular coil. ID probes are normally used for in-service inspection of heat exchangers. The probe is inserted into the tube. Normally ID probes are wound with the coil axis along the centre of the tube.

Absolute probes These categories are not exhaustive and there are obviously overlaps, for example between non-circumferential wound ID probes and internal surface probes. To this point we have only discussed eddy current probes consisting of a single coil. These are commonly used in many applications and are commonly known as absolute probes because they give an ‘absolute’ value of the condition at the test point. Absolute probes are very good for metal sorting and detection of cracks in many situations, however they are sensitive also to material variations, temperature changes etc.

Differential’ probe Another commonly used probe type is the ‘differential’ probe this has two sensing elements looking at different areas of the material being tested. The instrument responds to the difference between the eddy current conditions at the two points. Differential probes are particularly good for detection of small defects, and are relatively unaffected by lift-off (although the sensitivity is reduced in just the same way), temperature changes and external interference. (assuming the instrument circuitry operates in a "balanced“ configuration)


http://www.eng.morgan.edu/~hubert/IEGR470/eddycurrent.html

Note the characteristic "figure of eight for differential probe" response as first one probe element, then the other, move over the defect. In general the closer the element spacing the wider the "loop" in the signal. Lift-off should be cancelled out assuming that the probe is perfectly balanced, but there will still be a "wobble" response as the probe is moved and tilted slightly.

Reflection or driver pick-up probes have a primary winding driven from the oscillator and one or more sensor windings connected to the measurement circuit. Depending on the configuration of the sensor windings reflection probes may give response equivalent to either an absolute or differential probe. The two coils (differential or absolute plus balancing coil) form the ‘legs’ of a bridge. When the bridge is balanced the measured voltage will be zero. Any change in the condition of either coil will result in an unbalanced bridge, the degree of imbalance corresponds to the change in coil impedance.

The diagram shows a typical response from a differential probe.


Driver pick-up: As can be seen the essential elements are the same for a driver pick-up configuration as for a bridge, the necessary changes can be achieved by simple switching or probe connection changes


Tangential Probe



Orthogonal Probe



Electromagnetic Testing Advantages The following characteristics of the method can be used to advantage :  it can be used without making physical contact with the product ;  it does not need a coupling medium such as water ;  it is capable of being used at high throughput speeds.


EN 12084 : 2001

Factors Affecting Eddy Current Responses The basic parameters which influence the measured quantity are all of the following properties of the product to be tested, alone or in combination :    

the conductivity of the material ; the magnetic permeability of the material ; (magnetic factor) the size and geometry of the product to be tested ; (magnetic factor) the geometry between the eddy current probe and the product to be tested. (magnetic factor)


EN 12084 : 2001

Factors Affecting Eddy Current Response Material conductivity The conductivity of a material has a very direct effect on the eddy current flow: the greater the conductivity of a material the greater the flow of eddy currents on the surface. Conductivity is often measured by an eddy current technique, and inferences can then be drawn about the different factors affecting conductivity, such as material composition, heat treatment, work hardening etc.

Permeability This may be described as the ease with which a material can be magnetised. For non-ferrous metals such as copper, brass, aluminum etc., and for austenitic stainless steels the permeability is the same as that of ‘free space’, i.e. the relative permeability (μr) is one. For ferrous metals however the value of μr may be several hundred, and this has a very significant influence on the eddy current response, in addition it is not uncommon for the permeability to vary greatly within a metal part due to localised stresses, heating effects etc.

Frequency As we will discuss, eddy current response is greatly affected by the test frequency chosen, fortunately this is one property we can control.

Geometry In a real part, for example one which is not flat or of infinite size, geometrical features such as curvature, edges, grooves etc. will exist and will effect the eddy current response. Test techniques must recognise this, for example in testing an edge for cracks the probe will normally be moved along parallel to the edge so that small changes may be easily seen. Where the material thickness is less than the effective depth of penetration (see below) this will also effect the eddy current response



Proximity / Lift-off The closer a probe coil is to the surface the greater will be the effect on that coil. This has two main effects: The "lift-off" signal as the probe is moved on and off the surface. A reduction in sensitivity as the coil to product spacing increases.

Depth of penetration The eddy current density, and thus the strength of the response from a flaw, is greatest on the surface of the metal being tested and declines with depth. It is mathematically convenient to define the "standard depth of penetration" where the eddy current is 1/e (37%) of its surface value. The standard depth of penetration in mm is given by the formula:

Where: δ is standard depth in mm ρ is resistivity in μΩ.cm f is frequency in Hz μr is relative permeability



from this it can be seen that depth of penetration: 1. Decreases with an increase in frequency 2. Decreases with an increase in conductivity 3. Decreases with an increase in permeability: this can be very significant penetration into ferrous materials at practical frequencies is very small.

δ

δ

The graph above shows the effect of frequency on standard depth of penetration. It is also common to talk about the "effective depth of penetration" usually defined as three times the standard depth, where eddy current density has fallen to around 3% (5%?) of its surface value. This is the depth at which there is considered to be no influence on the eddy current field.



The Impedance Plane Eddy current responses of a single coil may be conveniently described by reference to the "impedance plane". This is a graphical representation of the complex probe impedance where the abscissa (X value) represents the resistance and the ordinate (Y value) represents the inductive reactance.




The Impedance Plane


Note that, while the general form of the impedance plane remains the same, the details are unique for a particular probe and frequency. The display of a typical CRT eddy current instrument represents a ‘window’ into the impedance plane, which can be rotated and "zoomed" to suit the needs of the application. For example in the above impedance plane diagram a rotated detail of the "probe on aluminum" area would appear as below:

This shows the display when moving over a series of simulated cracks of varying depths. Note that in the example shown both the amplitude and the phase of response from the different sized cracks varies.

Reliability Eddy currents are often generated in transformers and lead to power losses. To combat this, thin, laminated strips of metal are used in the construction of power transformers, rather than making the transformer out of one solid piece of metal. Insulating glue, which confines the eddy currents to the strips, separates the thin strips. This reduces the eddy currents, thus reducing the power loss. Beside that, Eddy-Current Detectors are very reliable as far as their industrial usage. They are so reliable that nuclear plants are using robots to the tests, instead of risking real human beings.



Robotic


Robotic


Measurement Techniques (EN!) a) Absolute measurement. The measurement of the deviation from a fixed reference point. The reference point is defined by a calibration procedure and can be generated by a reference voltage or coi l. This technique can be used for sorting the product into classes based on physical properties such as hardness, dimensions or chemical composition. It can also be used for the identification of continuous or gradually changing discontinuities.

b) Comparative measurement. The subtraction of two measurements, one of which is taken as a reference. This technique is normally used to sort the product into classes.

c) Differential measurement. The subtraction of two measurements made at a constant distance between the measurement locations and on the same scanning path. This measurement technique reduces the background noise due to slow variations in the product to be tested. (?)

d) Double differential measurement. The subtraction of two differential measurements. This measurement technique provides high-pass filtering of a differential measurement independent of the relative speed between the probe and the product to be tested.

e) Pseudo differential measurements The subtraction of two measurements made at a constant distance between the measurement locations.


EN 12084 : 2001

Historical Background Before discussing the principles of eddy current testing, it seems appropriate to briefly discuss the concept of magnetism and electromagnetism that serve as the foundation for this study. In the period from 1775 to 1900, scientific experimenters Andre Marie Ampere, Françios Arago, Charles Augustin coulomb, Michael Faraday, Lord William Thomson Kelvin, James Clerk Maxwell and Hans Christian Oersted had investigated and cataloged most of what is known about magnetism and electromagnetism. Arago discovered that the oscillation of a magnet was rapidly damped when a nonmagnetic conductor disk was placed near the magnet. He also observed that by rotating the disk, the magnet was attracted to the disk. In effect, Arago had introduced a varying magnetic field into the metallic disk causing eddy currents to flow in the disk. This produced a secondary magnetic field in the disk that affected the magnet. Arago's simple model is a basis for many automobile speedometers used today. This experiment can be modeled as shown in Figure 1.1.


http://pegna.vialattea.net/2Arago_Disk.htm

Figure 1.1 Arago’s Experiment


Arago’s Disk Experiment Arago discovered that the oscillation of a magnet was rapidly damped when a nonmagnetic conducting disk was placed near the magnet. He also observed that by rotating the disk, the magnet was attracted to the disk. In effect, Arago had introduced a varying magnetic field into the metallic disk causing eddy currents to flow in the disk. This produced a secondary magnetic field in the disk that affected the magnet. Arago's simple model is a basis for many automobile speedometers used today.

■

https://www.youtube.com/embed/sChcqdkcLGE


https://www.youtube.com/watch?v=sChcqdkcLGE

Oersted discovered the presence of a magnetic field around a current carrying conductor and observed magnetic field developed in a perpendicular plane to the direction of current flow in a wire. Ampere observed that equal and opposite currents flowing in adjacent conductors cancelled this magnetic effect. Ampere's observation is used in differential coil applications and to manufacture non inductive precision resistor. Faraday's first experiments investigated induced currents by the relative motion of magnet and a coil (Figure 1.2). Faraday's major contribution was the discovery of electromagnetic induction. His work can be summarized by the example shown in Figure 1.3. A coil "A" is connected to a battery through a switch, "S", A second coil, B, connected to a voltmeter is near by. When switch S is closed it produces a current in coil A in the direction shown (a). A momentary current is also induced in coil in direction (b) opposite to the current flow in coil A. If S is now opened, a momentary current will appear in coil B having the direction of (c). In each case current flows in coil B only while the current in coil A is changing.


Figure 1.2: Induced current with coil and magnet


Figure 1.3: Induced current electromagnetic technique A coil "A" is connected to a battery through a switch, "S", A second coil, B, connected to a voltmeter is near by. When switch S is closed it produces a current in coil A in the direction shown (a). A momentary current is also induced in coil in direction (b) opposite to the current flow in coil A. If S is now opened, a momentary current will appear in coil B having the direction of (c). In each case current flows in coil B only while the current in coil A is changing.


Electromagnetic induction is the production of an electromotive force across a conductor when it is exposed to a varying magnetic field. It is described mathematically by Faraday's law of induction, named after Michael Faraday who is generally credited with the discovery of induction in 1831. Electromagnetic induction was first discovered by Michael Faraday, who made his discovery public in 1831. It was discovered independently by Joseph Henry in 1832. In Faraday's first experimental demonstration (August 29, 1831), he wrapped two wires around opposite sides of an iron ring or "torus" (an arrangement similar to a modern toroidal transformer). Based on his assessment of recently discovered properties of electromagnets, he expected that when current started to flow in one wire, a sort of wave would travel through the ring and cause some electrical effect on the opposite side. He plugged one wire into a galvanometer, and watched it as he connected the other wire to a battery. Indeed, he saw a transient current (which he called a "wave of electricity") when he connected the wire to the battery, and another when he disconnected it. This induction was due to the change in magnetic flux that occurred when the battery was connected and disconnected. Within two months, Faraday found several other manifestations of electromagnetic induction. For example, he saw transient currents when he quickly slid a bar magnet in and out of a coil of wires, and he generated a steady (DC) current by rotating a copper disk near the bar magnet with a sliding electrical lead ("Faraday's disk"). Faraday explained electromagnetic induction using a concept he called lines of force. However, scientists at the time widely rejected his theoretical ideas, mainly because they were not formulated mathematically. An exception was Maxwell, who used Faraday's ideas as the basis of his quantitative electromagnetic theory. In Maxwell's model, the time varying aspect of electromagnetic induction is expressed as a differential equation which Oliver Heaviside referred to as Faraday's law even though it is slightly different from Faraday's original formulation and does not describe motional EMF. Heaviside's version (see Maxwell– Faraday equation below) is the form recognized today in the group of equations known as Maxwell's equations. Heinrich Lenz formulated the law named after him in 1834, to describe the "flux through the circuit". Lenz's law gives the direction of the induced EMF and current resulting from electromagnetic induction (elaborated upon in the examples below). Following the understanding brought by these laws, many kinds of device employing magnetic induction have been invented.


http://en.wikipedia.org/wiki/Electromagnetic_induction




http://en.wikipedia.org/wiki/Homopolar_generator


http://en.wikipedia.org/wiki/Homopolar_generator

Faraday's Law - Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil. No matter how the change is produced, the voltage will be generated. The change could be produced by changing the magnetic field strength, moving a magnet toward or away from the coil, moving the coil into or out of the magnetic field, rotating the coil relative to the magnet, etc. Faraday's law is a fundamental relationship which comes from Maxwell's equations. It serves as a summary of the ways a voltage (or emf) may be generated by a changing magnetic environment. The induced emf in a coil is equal to the negative of the rate of change of magnetic flux times the number of turns in the coil. It involves the interaction of charge with magnetic field.


http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html

The law of physics describing the process of electromagnetic induction is known as Faraday's law of induction and the most widespread version of this law states that the induced electromotive force in any closed circuit is equal to the rate of change of the magnetic flux enclosed by the circuit. Or mathematically,

ε = dфB/ dt where ε (epsilon) is the electromotive force (EMF) and ΦB (Φ= BA) is the magnetic flux. The direction of the electromotive force is given by Lenz's law. This version of Faraday's law strictly holds only when the closed circuit is a loop of infinitely thin wire, and is invalid in some other circumstances. A different version, the Maxwell–Faraday equation (discussed below), is valid in all circumstances. For a tightly wound coil of wire, composed of N identical turns, each with the same magnetic flux going through them, the resulting EMF is given by

ε = -N dфB/ dt Faraday's law of induction makes use of the magnetic flux ΦB through a hypothetical surface Σ whose boundary is a wire loop. Since the wire loop may be moving, we write Σ(t) for the surface. The magnetic flux is defined by a surface integral:

фB = ∫Σ(t) B(r,t)∙dA where dA is an element of surface area of the moving surface Σ(t), B is the magnetic field, and B·dA is a vector dot product (the infinitesimal amount of magnetic flux). In more visual terms, the magnetic flux through the wire loop is proportional to the number of magnetic flux lines that pass through the loop.







Lenz's Law When an emf is generated by a change in magnetic flux according to Faraday's Law, the polarity of the induced emf is such that it produces a current whose magnetic field opposes the change which produces it. The induced magnetic field inside any loop of wire always acts to keep the magnetic flux in the loop constant. In the examples below, if the B field is increasing, the induced field acts in opposition to it. If it is decreasing, the induced field acts in the direction of the applied field to try to keep it constant.



Magnetic Force The magnetic field B is defined from the Lorentz Force Law, and specifically from the magnetic force on a moving charge:

The implications of this expression include: 1. The force is perpendicular to both the velocity v of the charge q and the magnetic field B. 2. The magnitude of the force is F = q∙v∙B sin θ where θ is the angle < 180 degrees between the velocity and the magnetic field. This implies that the magnetic force on a stationary charge or a charge moving parallel to the magnetic field is zero. 3. The direction of the force is given by the right hand rule. The force relationship above is in the form of a vector product.

When the magnetic force relationship is applied to a current-carrying wire, the right-hand rule may be used to determine the direction of force on the wire. From the force relationship above it can be deduced that the units of magnetic field are Newton seconds /(Coulomb meter) or Newtons per Ampere meter. This unit is named the Tesla. It is a large unit, and the smaller unit Gauss is used for small fields like the Earth's magnetic field. A Tesla is 10,000 Gauss. The Earth's magnetic field at the surface is on the order of half a Gauss Charlie Chong/ Fion Zhang

Lorentz force In physics, particularly electromagnetism, the Lorentz force is the combination of electric and magnetic force on a point charge due to electromagnetic fields. If a particle of charge q moves with velocity v in the presence of an electric field E and a magnetic field B, then it will experience a force

F = - q∙ [ E + (v x B) ] (in SI units). Variations on this basic formula describe the magnetic force on a current-carrying wire (sometimes called Laplace force), the electromotive force in a wire loop moving through a magnetic field (an aspect of Faraday's law of induction), and the force on a charged particle which might be traveling near the speed of light (relativistic form of the Lorentz force). The first derivation of the Lorentz force is commonly attributed to Oliver Heaviside in 1889, although other historians suggest an earlier origin in an 1865 paper by James Clerk Maxwell. Hendrik Lorentz derived it a few years after Heaviside.


http://en.wikipedia.org/wiki/Lorentz_force

Generation of Eddy Currents When a conductor is place in the area influence by the primary field, eddy current is induced in the conductor, see Fig. 1.4. Following Lenz’s law, the induced eddy current IE will produce a secondary field фE that oppose the фP. The magnitude of фE is proportional to IE. The test objet, conductor B’s characteristic like, material conductivity, permeability and geometry will affect the IE, this in turn cause variation in фE. The variation in фE is reflected in conductor CA by фE influences on фp. The variations are recorded in media like meter, CRT, digital read out or chart. The

Ip = Primary Current Фp =Primary magnetic flux ФE = Secondary Eddy current magnetic flux IE = Secondary Eddy current Figure 1.4: Induced current relationships


Generation of Eddy Currents


http://www.suragus.com/en/company/eddy-current-testing-technology

Factors Affecting Inductance There are four basic factors of inductor construction determining the amount of inductance created. These factors all dictate inductance by affecting how much magnetic field flux will develop for a given amount of magnetic field force (current through the inductor's wire coil): NUMBER OF WIRE WRAPS, OR "TURNS" IN THE COIL: All other factors being equal, a greater number of turns of wire in the coil results in greater inductance; fewer turns of wire in the coil results in less inductance. Explanation: More turns of wire means that the coil will generate a greater amount of magnetic field force (measured in amp-turns!), for a given amount of coil current. L ∝ N2


http://www.allaboutcircuits.com/vol_1/chpt_15/3.html

COIL AREA: All other factors being equal, greater coil area (as measured looking lengthwise through the coil, at the cross-section of the core) results in greater inductance; less coil area results in less inductance. Explanation: Greater coil area presents less opposition to the formation of magnetic field flux, for a given amount of field force (amp-turns). L ∝ A



COIL LENGTH: All other factors being equal, the longer the coil's length, the less inductance; the shorter the coil's length, the greater the inductance. Explanation: A longer path for the magnetic field flux to take results in more opposition to the formation of that flux for any given amount of field force (amp-turns). L ∝ (l)-1

COIL LENGTH

L ∝ (l)-1


COIL LENGTH

L ∝ (l)-1


CORE MATERIAL: All other factors being equal, the greater the magnetic permeability of the core which the coil is wrapped around, the greater the inductance; the less the permeability of the core, the less the inductance. Explanation: A core material with greater magnetic permeability results in greater magnetic field flux for any given amount of field force (amp-turns). L∝μ

μ0 = 4π x 10-7 H.m-1


μr = 600, μiron = 600 x μ0 μ0 = 4π x 10-7 H.m-1


Coil Inductance L An approximation of inductance L, for any coil of wire can be found with this formula: The electromagnetic field produced about an unloaded test coil can be described as decreasing in intensity with distance from the coil and also varying across the coil's cross section. The field is most intense near the coil's surface. The field produced about this coil is directly proportiona1 to the magnitude of applied current, rate of change of current or frequency and the coil parameters. Coil parameters inc1ude inductance, diameter, length， thickness, number of turns of wire and core material.

L = μr• (N2 x A /l) • 1.26 x 10-6 Henry μ0 = 4π x 10-7 H.m-1 or 1.26 x 10-6 H.m-1 EMF = L di/dt Volt Where: L= inductance in Henry H N = Numbers of turn in coil wire (straight wire N=1) μr = relative permeability l = average length of coil in m A = area of coil (not wire area?) in m2 μo = relative permeability in air 4π x 10-7 H.m-1 or 1.26 x 10-6 H.m-1



Coil Inductance L



Note the direction of the primary current (Ip) and the resultant eddy current (IE). IE extends some distance into the test object. Another important observation is that IE is generated in the same plane in which the coil is wound. Figure 1.6 emphasizes this point with a loop coil surrounding a cylindrical test object (4).

Important observation is that IE is generated in the same plane in which the coil is wound.

Figure 1.6 Induction current flow in a cylindrical part. Charlie Chong/ Fion Zhang

Note the direction of the primary current (Ip) and the resultant eddy current (IE). IE extends some distance into the test object. Another important observation is that IE is generated in the same plane in which the coil is wound. Figure 1.6 emphasizes this point with a loop coil surrounding a cylindrical test object (4).

Important observation is that IE is generated in the same plane in which the coil is wound & in opposite direction of Ip

Figure 1.6 Induction current flow in a cylindrical part. Charlie Chong/ Fion Zhang

Generation of Eddy Current With a primary current 1p flowing through the coil, a primarr electromagnetic field фp is produced about the coil. When this excited test coil is placed on an electrically conductive test object, eddy currents IE will be generated in that test object Figure 1.5 illustrates this concept.

Figure 1.5 Generation of eddy current IE in a test object


It must be understood that this formula yields approximate figures only. One reason for this is the fact that permeability changes as the field intensity varies (remember the nonlinear "B/H" curves for different materials). Obviously, if permeability (µ) in the equation is unstable, then the inductance (L) will also be unstable to some degree as the current through the coil changes in magnitude. If the hysteresis of the core material is significant, this will also have strange effects on the inductance of the coil. Inductor designers try to minimize these effects by designing the core in such a way that its flux density never approaches saturation levels, and so the inductor operates in a more linear portion of the B/H curve. If an inductor is designed so that any one of these factors may be varied at will, its inductance will correspondingly vary. Variable inductors are usually made by providing a way to vary the number of wire turns in use at any given time, or by varying the core material (a sliding core that can be moved in and out of the coil). An example of the former design is shown in this photograph:




Permeability changes as the field intensity varies (remember the nonlinear "B/H" curves for different materials).

Figure 1: This unit uses sliding copper contacts to tap into the coil at different points along its length. The unit shown happens to be an air-core inductor used in early radio work. Figure 2: A fixed-value inductor is shown in the next photograph, another antique air-core unit built for radios. The connection terminals can be seen at the bottom, as well as the few turns of relatively thick wire: Figure 3: Here is another inductor (of greater inductance value), also intended for radio applications. Its wire coil is wound around a white ceramic tube for greater rigidity: Figure 4: The two inductors on this circuit board are labeled L1 and L2, and they are located to the right-center of the board. Two nearby components are R3 (a resistor) and C16 (a capacitor). These inductors are called "toroidal" because their wire coils are wound around donut-shaped ("torus") cores. Figure 5: Like resistors and capacitors, inductors can be packaged as "surface mount devices" as well. The following photograph shows just how small an inductor can be when packaged as such: A pair of inductors can be seen on this circuit board, to the right and center, appearing as small black chips with the number "100" printed on both. The upper inductor's label can be seen printed on the green circuit board as L5. Of course these inductors are very small in inductance value, but it demonstrates just how tiny they can be manufactured to meet certain circuit design needs.



A Dual1: Variable inductors Figure This unit uses sliding copper contacts to tap into the coil at different points along its length. The unit shown happens to be an air-core inductor used in early radio work.


Figure 2: A fixed-value inductor is shown in the next photograph, another antique air-core unit built for radios. The connection terminals can be seen at the bottom, as well as the few turns of relatively thick wire:


Figure 3: Here is another inductor (of greater inductance value), also intended for radio applications. Its wire coil is wound around a white ceramic tube for greater rigidity:


Figure 4: The two inductors on this circuit board are labeled L1 and L2, and they are located to the right-center of the board. Two nearby components are R3 (a resistor) and C16 (a capacitor). These inductors are called "toroidal" because their wire coils are wound around donut-shaped ("torus") cores.



Figure 4: The two inductors on this circuit board are labeled L1 and L2, and they are located to the right-center of the board. Two nearby components are R3 (a resistor) and C16 (a capacitor). These inductors are called "toroidal" because their wire coils are wound around donut-shaped ("torus") cores.



Figure 5: Like resistors and capacitors, inductors can be packaged as "surface mount devices" as well. The following photograph shows just how small an inductor can be when packaged as such: A pair of inductors can be seen on this circuit board, to the right and center, appearing as small black chips with the number "100" printed on both. The upper inductor's label can be seen printed on the green circuit board as L5. Of course these inductors are very small in inductance value, but it demonstrates just how tiny they can be manufactured to meet certain circuit design needs.



Grundig radio satellit 750

■ https://www.youtube.com/embed/yD7WAcSwz8o


http://www.universal-radio.com/catalog/portable/0750.html

Phasor Vector Diagram of Coil Voltage A more precise method of describing the relationships of magnetic flux, voltage and current is the phase vector diagram or phasor diagrams (4). Figure 1.7 compares the electromagnetic events associated with an unloaded test coil and what happens when that same coil is placed on a nonferromagnetic test object. The components of phasor diagrams are as follows:

Fig.17(b) Ep = Primary coil voltage I = Exciting current (Primary coil current) Фp = Primary flux Фs = Secondary flux

Fig.17(b) Ep = Primary coil voltage I = Exciting current (Primary coil current) Фp = Primary flux Фs = Secondary flux Es = Secondary voltage ET= Total voltage ФT = Total flux


Figure 1.7: Phasor Diagram of Coil Voltage (?)

In Figure 1.7(a) the current (I) and primary magnetic flux (фp) are plotted in phase. The primary voltage (Ep) is shown separated by 90 electrical degrees. The secondary magnetic flux (фs) is plotted at zero because without a test object no secondary flux exists.


Figure 1.7(b) represents the action of placing the coil on a nonferromagnetic test object Observing the figure, one can see by vectorial addition of Ep and Es that a new coil voltage (ET) is arrived at for the loaded condition. The primary magnetic flux фp and secondary magnetic flux фs are also combined by vectorial addition to arrive at a new magnetic flux (фT) for the loaded coil.

In Figure 1.7(a) the current (I) and primary magnetic flux (фp) are plotted in phase. The primary voltage (Ep) is shown separated by 90 electrical degrees. The secondary magnetic flux (фs) is plotted at zero because without a test object no secondary flux exists. Figure 1.7(b) represents the action of placing the coil on a nonferromagnetic test object Observing the figure, one can see by vectorial addition of Ep and Es that a new coil voltage (ET) is arrived at for the loaded condition. The primary magnetic flux фp and secondary magnetic flux фs are also combined by vectorial addition to arrive at a new magnetic flux (фT) for the loaded coil. Notice that for the condition of the test object in the test coil, фT is no longer in phase with the excitation current I. Also observe that the included angle between the excitation current and the new coil voltage ET is no longer at 90 electrical degrees. These interactions will be discussed in detaillater in this study guide.


Current Density The distribution of eddy currents in a test object varies exponentially. The current density in the test object is most dense near the test coil. This exponential current density follows the mathematical rules for a natural exponential decay curve (1/ e) where ε (epsilon) is 2.718. Usually a natural exponential curve is illustrated by a graph with the ordinate (Y axis) representing magnitude and the abscissa (X axis) representing time or distance. A common point described on such a graph is the knee of the curve. The knee occurs at the 37% value on the ordinate axis. This 37% point is chosen because changes in X axis values produce significant changes in Yaxis values from 100% to 37% and below 37% changes in X axis values þroduce less signlficant changes in Y axis values (?).


Applying this logic to eddy current testing, a term is developed to describe the relationship of current distribution in the test object. The eddy current generated at the surface of the test object nearest the test coil is 100%. The point in the test object thickness where this current is diminished to 37% of its previous strength is known as the standard depth of penetration. The term δ (delta) is used to represent this point in the material. Figure 1.8 is a relative eddy current density curve for a plane wave of infinite extent with magnetic field parallel to the conducting test object surface.


Figure 1.8: Relative eddy current density


The current density at any depth can be calculated as:

Jx =J0 e - x√(πfμσ) Where: Jx = Electrical density at depth x in A∙m-2 J0 = Electrical density at the surface x=0 x = distance fro surface in meter m f = Frequency of the AC primary current Hz μ = Permeability of the test object in H∙m-1 σ = Conductivity of the test object in Siemen∙m-1 e = Natural logarithm


Relative Magnetic Permeability

Permeability of free space μ0 = 4π x 10-7 HM-1 Permeability of material can be expressed as relative to μ0 μmaterial = μr∙μ0


The Standard Depth of Penetration δ The Standard Depth of Penetration can be expressed as:

δ = (πfμσ) -½ Where:

δ = One standard depth of penetration; 1/e of the surface current density (37%) in meter, m f = Frequency of the AC primary current in Hz μ = Permeability of the test object in Henry per meter, H∙m-1 σ = Conductivity of the test object in Siemens per meter, S∙m-1


It should be observed at this point that as frequency, conductivity or permeability is increased, the penetration of current into the test object will be decreased. The graph in Figure 1.8 is used to demonstrate many eddy current characteristics. Using an example of a very thick block of stainless steel being interrogated with a surface or probe coil operating at a test frequency of 100 kHz, the standard depth of penetration can be determined and current densities observed at other depths. Stainless steel (300 Series) is nonferromagnetic. Magnetic permeability (μ) is 4πX 10-7 H∙m-1, the conductivity σ is 0.14 X 107 siemens (mhos) per meter for 300 Series stainless steel. δ = (πfμσ) -½ δ = (π x 100 x 103 x 4 x π x 0.14) -½ δ = 0.00135m or 1.35mm#


δ = (π x 100 x 103 x 4 x π x 0.14) -½ as 1000*(pi*x*4*pi*0.14*10^3)^(-.5)


http://graph-plotter.cours-de-math.eu/



http://fooplot.com/



http://rechneronline.de/function-graphs/

Using 1.35 mm as depth x from surface, a ratio of depth/depth of penetration would be 1 Referring to Figure 1.8, a depth/ depth of penetration of 1 indicates a relative eddy current density of 0.37 or 37%. What is the relative eddy current density at 3 mm? The relative standard depth Drelative of x = 3mm is: Drelative = 3/δ = 3/1.53 mm = 2.22δ This ratio indicates a relative eddy current densityof about 0.1 or 10% [ (1/e)2.22 = 10.9% ]. With only 10% of the available current flowing at a depth of 3 mm, detectability of variables such as conductivity, permeability and discontinuities would be very difficult to detect. The obvious solution for greater delectability at a depth of 3 mm depth is to lower the test frequency. Frequency selection will be covered in detaillater in this text.


Relative current density

f(x) = (1/e)x where x = depth/δ

Relative Standard depth x = depth/δ


http://rechneronline.de/function-graphs/

Standard Depth for Different Conductive Materials


https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/PopUps/applet7/applet7.htm

Phase/Amplitude and Current Time Relationships Figure 1.9 reveals another facet of eddy current. Eddy currents are not generated at the same instant in time throughout the part. Eddy currents require time to penetrate the test part. Phase and time are analogous meaning - phase is an electrical term used to describe timing relationships of electrical waveforms. Phase Lag = x/δ radian Where: x =depth below surface δ = Standard depth


Current (?) Lagging Voltage lagging or current lagging?


Current (?) Lagging Voltage lagging or current lagging?


σº∙πμ■δ∝∞ωΩθ√ρβααδπ


Phase is u,sually expressed in either degrees or radians. There'are 2πradians per 360 degrees. Each radian therefore is about 57 degrees (360/2π). Using the surface eddy current near the test coil as a reference, the deeper the eddy current the greater the phase lag. The amount of phase lag is determined by:

β = x/δ = x∙√(πfμσ) β or Φ = Phase lag angle in radian. Others as defined earlier


Figure 1.9 should be used as a relative indicator of phase lag. The exact phase relationship for a particular system may be different due to other variables, such as coil parameters and excitation methods. The amount of phase lag for a given part thickness is an important factor when considering resolution. Resolution is the ability to separate variables occurring in the test object; for example, distinguishing two discontinuities occurring at different depths in the same test object. As an example, using a standard depth of penetration at 1 mm in a 5 mm thick test object. Refer to Figure 1.9 and observe the phase lag of the current at one standard depth of penetration. Where depth of interest (x) is 1 mm and depth of penetration (δ) is 1 mm, the x/ δ ratio is 1 and the current at depth x lags the surface current by 1 radian or 57 degrees.


Projecting this examination, observe the phase lag for the entire part thickness. The standard depth of penetration is 1 mm, the part thickness is 5 mm; therefore, the ratio x/δ equals to 5. This produces phase lag of 5 radians or about 287 degrees for the part thickness. Having a measurement capability of 1 degree increments, the part thickness could be divided into 287 parts each part representing 0.017mm. That would be considered excellent resolution. There is an obvious Iimitation. Refer to Figure 1.8 and observe the resultant relative current density with an x/δ ratio of 5. The relative current density is near 0. It should become apparent that the frequency can be adjusted to achieve optimum results for a particular variable. These and other variables will be discussed in Chapter 5 of this Study Guide.




Chapter 1 Review Questions


Q.1.1 Generation of eddy currents depends on the principle of: A. wave guide theory. B. electromagnetic induction. C. Magnetostriction force D. All of the above Q.1.2 A secondary field is generated by the test object and is; A. Equal and opposite to the primary field B. Opposite to the primary field but much smaller C. In the same plane as the coil is wound. D. In phase with the primary field. Q.1.3 When a non ferromagnetic part is placed in the test coil, The coil' s voltage: A. increases B. remains constant because this is essential. C. decreases. D. shifts 90 degrees in phase.


Q.1.4 Refer to Figure 1.7(b). If ET was produced by the test object being stainless steel, what would the effect be if the test object were copper? A. ET would decrease and be at a different angle. B. ET would increase and be at a different angle. C. Because both materials are non-ferromagnetic, no change occurs D. None of the above.


Q.1.5 Eddy current generated a test object flow; A. in the same plane as magnetic flux B. in the same plane as the coil is wound C. 90 degrees to the coil winding plane. D. eddy currents have no predictable direction. Q.1.6 The discovery of electromagnetic induction is credited to A. Arago B. Oersted. C. Maxwell. D. Faraday. Q.1.7 A standard depth of penetration is defined as the point in a test object where the relative current density is reduced to: A. 25%. B. 37% C. 50%. D. 100%


Q.1.8 Refer to Figure 1.8. If one standard depth of penetration was established at 1 mm in an object 3 mm thick, what is the relative current density on the far surface? A. 3 B. <0.1 C. 1/3 D. indeterminate


Q.1.9 Refer to Figure1.9 using example in question 1.8, what is the phase difference between the near and far surfaces? A. the far surface current leads the near surface current by 57 degrees. B. the far surface current leads the near surface current by 171 degrees. C. the far surface current lags the near surface current by 171 degrees. D. the far surface current lags the near surface current by 570 degrees. Q.1.10 Calculate the standard depth of penetration at 10KHz in Copper with σ = 5.7 x 107 Siemens per meter. A. 0.1 mm (3.9 x10-3 in.) B. 0.02 mm (7.9 x10-4 in.) C. 0.66 mm (0.026 in.) D. 66 mm (2.6 in.) β = x/ δ x 57.3º δ = (πfμσ) -½ = √(10 x 103 x π x 4 π x 10-7 x 5.7 x 107) x 1000 mm


The Answers


Chapter 2 The Coil Arrangements


Test Coil Arrangement Test coils can be categorized into three main mechanical groups: probe coils, bobbin coils and encircling coils. (Surface coil, internal bobbing coil, encircling coil)

Probe Coils Surface coil, probe coil, flat coil or pancake coil are all common terms used to describe the same test coil type. Probe coils provide a convenient method of examining the surface of a test object. Figure 2.1 below illustrates a typical set of probe coils used for several surface scanning applications.


Probe coils and probe coil forms can be shaped to fit particular geometries to solve complex inspection problems. As an example, probe coils fabricated in a pencil shape (pencil probe) are used to inspect threaded areas of mounting studs and nuts or serrated areas of turbine wheels and turbine blade assemblies. Probe coils may be used where high resolution is required by adding coil shielding (2). When using a high resolution probe coil, the test object surface must be carefully scanned to ensure complete inspection coverage. This careful scanning is very time consuming. For this reason, probe coil inspections of large test objects are usually limited to critical areas. Probe coils are used extensively in aircraft inspection for crack detection near fasteners and fastener holes. In the case of fastener holes (bolt holes, rivet holes), the probe coil may be rotated either manually or mechanically to provide a helical scan of the hole using a spinning probe technique (Figure 2.2).


Figure 2.2: Bolt hole inspection probes


Encircling Coils Encircling coil, outside diameter coil and feed through coil are terms commonly used to describe a coil that surrounds the test object. Figure 2.3 illustrates a typical encircling coil. Encircling coils are primarily used to inspect tubular and bar-shaped products. The tube or bar is fed through the coil (feed through) at relatively high speed. The cross section of the test object within the test coil is simultaneously interrogated. For this reason, the circumferential location of discontinuities cannot be determined with an encircling coil. The volume of material examined at one time is greater using an encircling coil than a probe coil; therefore, the relative sensitivity is lower for an encircling coil. The additional advantage that a probe coil would have over the encircling coil is that the probe coil could define where within the circumferential plane the discontinuity exists. The encircling coil cannot make that distinction. If there are multiple signal sources within the coil's field of view the encircling coil response will indicate the average of all of those events.


Figure 2.3: Encircling coil


Discussion Subject; Encircling Coil If there are multiple signal sources within the coil's field of view the encircling coil response will indicate the average of all of those events. Question: Why average? why not sum of all signals?


When using an encircling coil, it is important to keep the test object centered in the coil. If the test object is not centered, a uniform discontinuity response is difficult to obtain. To ensure proper centering it is corrunon practice to run the calibration standard several times, each time indexing the artificial discontinuities to a new circumferential location in the coil. As in all discontinuity detection schemes, it is essential to select a reasonable operating frequency, properly adjust the system display parameters and ensure that the tube is centered in the coil at all times to achieve optimum test sensitivity.


Bobbin Coils Bobbin coil, inside diameter coil and inside probe are terms that describe coils used to inspect from the inside diameter or bore of a tubular test object. Bobbin coils are inserted and withdrawn from the tube inside diameter by long, semi flexible shafts or simply blown in with air and retrieved with an attached pull cable. These mechanisms will be described later in the text. Bobbin coil information follows the same basic rules stated for encircling coils. Figure 2.4 illustrates a typical bobbin coil.


Coil Arrangements Probe coils, encircling coils and bobbin coils can be additionally classified. These additional classifications are determined by how the coils are electrically connected. The three coil categories are absolute, differential and hybrid. Figure 2.5 shows various types of absolute and differential coil arrangements.


Figure 2.5: Test coil configurations for eddy current testing of small-diameter tubing




Absolute



Differential- Self comparisons, external reference



Thru transmission



Reflection (Double) Sending & Receiving

Absolute Coils An absolute coil makes its measurement without direct reference or comparison to a standard as the measurement is being made (6). Some applications for absolute coil systems would be measurements of conductivity, permeability, dimensions and hardness.


Differential coils Differential coils consist of two or more coils electrically connected to oppose each other. Differential coils can be categorized into two types: (1) self comparison differential and (2) external reference differential. a) The self-comparison differential coil compares one area of a test object to another area on the same test object. A common use is two coils, connected opposing, so that if both coils are affected by identical test object conditions, the net output is 0 volts or no signal change. The self-comparison arrangement is insensitive to test object variables that occur gradually. Variables such as slowly changing wall thickness, diameter or conductivity are effectively discriminated against with the selfcomparison differential coil. Only when a different condition affects one or the other test coils will an output signal be generated. The coils usually being mechanically and electrically similar allows the arrangement to be very stable during temperature changes. Short discontinuities such as cracks, pits or other localized discontinuities with abrupt boundaries can be readily detected using the self-comparison differential coli.


b) The external reference differential coil, uses an external reference to affect one coil while the other coil is affected by the test object. Figure 2.6 illustrates this concept. This system is used to detect differences between a standard object and test objects. It is particularly useful for comparative conductivity, permeability and dimensional measurements. Obviously in Figure 2.6 it is imperative to normalize (or balance) the system with one coil affected by the standard object and the other coil affected by an acceptable test object. The external reference differential coil system is sensitive to all measurable differences between the standard object and test object. For this reason it is often necessary to provide additional discrimination to separate and define variables present in the test object.


Figure 2.6: External reference differential system


Hybrid Coils Hybrid coils may be defined as driver/pickup, through transmission, reflection or primary/secdndary coil assemblies. Hybrid coils may or may not be the same size and are not necessarily adjacent to each other. Figure 2.7 shows one possible hybrid coil arrangement. In the through transmission coil, the excitation coil is on one side of the test object and the sensing coil is on the other. The driver coil induces eddy currents and a secondary magnetic field in the test specimen. Any variation of these secondary events should be detected by the smaller probe coil on the opposite side of the thin plate.


Figure 2.7: Hybrid coil (through transmission)


A hybrid coil arrangement consists of an excitation coil and a sensing coil (reflection coils). In most cases a single probe housing assembly contains both the driver and the pickup coil(s). The primary magnetic flux interacts with both coils. The voltage developed in the sensing coil is a function of the current magnitude and frequency applied to the excitation coil, coil parameters of the exciting and sensing coils and the test object characteristics. Most hybrid coils are designed to improve test sensitivity for a specific application. One example of this is for better detection of subsurface discontinuities in multilayer structures. The concept of using a smaller pickup coil enhances the ability to detect lower level impedance variations from small volume discontinuities deeper in the test sample. It should be pointed out that if larger volume discontinuities are encountered that a measurable impedance change might be generated by both the exciter and the pick up coil(s).


Additional Coil Characteristics Coil configuration is but one of many factors to consider when setting up test conditions. Other coil characteristics of importance are mechanical, thermal and electrical stability; sensitivity, resolution and dimensions. The geometry of the coil is usually dictated by the geometty of the test object. Selection of smaller probe sizes may affect test sensitivity and/or resolution. The relative importance of the coil characteristics depends on the nature of the test. A blend of theory and experience usually succeeds in selection of proper coil parameters. Coil design and interactions with test objects will be discussed later in this Study Guide.




Q.2.1 Differential coils are usually used in: A. bobbin coils. B. probe coils. C. outside diameter coils. D. any of the above. Q.2.2 When using a probe coil to scan a test object: A. the object must be dry and polished. B. the object must be scanned carefully to ensure inspection coverage. C. the object must be scanned in circular motions at constant speeds. D. the probe must be moving at all times to get a reading. Q.2.3 A spinning probe would most likely be: A. a bobbin coil B. an inside diameter coil. C. an outside diameter coil. D. a probe coil.


Q.2.4 A feed through coil is: A. a coil with primary/ secondary windings connected so that the signal is fed through the primaq to the secondary. B. an encircling coil. C. an outside diameter coil. D. both Band C. Q.2.5 When inspecting a tubular product with an encircling coil, which statement is not true? A. Outside diameter discontinuities can be found. B. Axial discontinuity locations can be noted. C. Circumferential discontinuity locations can be noted. D. Inside diameter discontinuities can be found.


Q.2.6 An absolute coil measurement is made: A. by comparing one spot on the test object to another. B. without reference to or direct comparison with a standard. C. only with probe coils. D. by comparative measurement to a known standard. Q.2.7 When coils in a self-comparison differential arrangement are affected simultaneously with the same test object variables, the output signal: A is directly proportional to the number of variables. B. is 0 or near 0. C. is indirectly proportional to the number of variables. D. is primarily a function of the exciting current.


Q.2.8 Which coil type inherently has better thermal stability? A bobbin B. absolute C. outside diameter D. self-comparison differential Q.2.9 A hybrid coil is composed of two or more coils. The coils: A. must be aligned coplanar to the driver axis. B. may be of widely different dimensions. C. must be impedance matched as closely as possible. D. are very temperature sensitive. Q.2.10 Proper selection of test coil arrangement is determined by: A. shape of test object. B. redolution required. C. sensitivity required. D. stability. E. all of the above.


The Answers


Chapter 3 Test Coil Design


As discussed earlier test coil design and selection is a blend of theory and experience. Many factors must be considered. These important factors are determined by the inspection requirement for resolution, sensitivity, impedance, size, stability and environmental considerations. To better understand coil properties and electrical relationships, a short refresher in alternating current theory is necessary. First, the electrical units must be examined. For example, current and its representative symbol I. Current not only suggests electron flow but also the amount. The amount of electrons flowing past a point in a circuit in 1 second is expressed in amperes: 2π x 1018 electrons passing a point in 1 second is called 1 ampere.


Ampere The ampere (SI unit symbol: A), often shortened to "amp", is the SI unit of electric current (dimension symbol: I) and is one of the seven SI base units. It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics. The ampere is equivalent to one coulomb (roughly 6.241×1018 times the elementary charge) per second. Amperes are used to express flow rate of electric charge. For any point experiencing a current, if the number of charged particles passing or the charge on the particles is increased, the amperes of current at that point will proportionately increase. The ampere should not be confused with the coulomb (also called "ampere-second") or the ampere-hour (A·h). The ampere is a unit of current, the amount of charge transiting per unit time, and the coulomb is a unit of charge. When SI units are used, constant, instantaneous and average current are expressed in amperes (as in "the charging current is 1.2 A") and the charge accumulated, or passed through a circuit over a period of time is expressed in coulombs (as in "the battery charge is 30000 C"). The relation of the ampere to the coulomb is the same as that of the watt to the joule, and that of metre per second to metre.


http://en.wikipedia.org/wiki/Amperec

Demonstration model of a moving iron ammeter. As the current through the coil increases, the plunger is drawn further into the coil and the pointer deflects to the right.


http://en.wikipedia.org/wiki/Amperec

Resistance Resistance is an opposition to the flow of electrons and is measured in ohms. Ohm's Law is stated by the equation: E = IR Where: I = Current in Ampere A R = Resistance in Ohm Ω E = Electrical potential difference in volt V

The resistance of a coil is determined primarily by the length of wire used to wind the coil; its specific resistance is determined by the type of wire (e.g., copper, silver) and the cross-sectional area of the wire. Resistance = (Specific resistance X Length) / Area Resistance =


Thus, the resistance of a 10 ft length of 40 gage copper wire with a specific resistance of 10.4 circular-mil-foot at 20ºC would be found as follows: R = (10.4 X 10) / 9.888 = 10.518 ohm. In an alternating current circuit containing only resistance, the current and voltage are in phase. In phase means the current and voltage reach their minimum and maximum values, respectively, at the same time. The power dissipated in a resistive circuit appears :in the form of heat. For example, electric toasters are equipped with resistance wires that become hot when current flows through them, providing a heat source for toasting bread.


Circular-Mill-Foot A circular-mil-foot (figure 1-3) is a unit of volume. It is a unit conductor 1 foot in length and has a cross-sectional area of 1 circular mil. Because it is a unit conductor, the circular-mil-foot is useful in making comparisons between wires consisting of different metals. For example, a basis of comparison of the RESISTIVITY (to be discussed shortly) of various substances may be made by determining the resistance of a circular-mil-foot of each of the substances. In working with square or rectangular conductors, such as ammeter shunts and bus bars, you may sometimes find it more convenient to use a different unit volume. A bus bar is a heavy copper strap or bar used to connect several circuits together. Bus bars are used when a large current capacity is required. Unit volume may be measured as the centimeter cube. Specific resistance, therefore, becomes the resistance offered by a cube-shaped conductor 1 centimeter in length and 1 square centimeter in cross-sectional area. The unit of volume to be used is given in tables of specific resistances.


http://www.tpub.com/neets/book4/11b.htm

SPECIFIC RESISTANCE OR RESISTIVITY Specific resistance, or resistivity, is the resistance in ohms offered by a unit volume (the circular-mil-foot or the centimeter cube) of a substance to the flow of electric current. Resistivity is the reciprocal of conductivity. A substance that has a high resistivity will have a low conductivity, and vice versa. Thus, the specific resistance of a substance is the resistance of a unit volume of that substance. Many tables of specific resistance are based on the resistance in ohms of a volume of a substance 1 foot in length and 1 circular mil in cross-sectional area. The temperature at which the resistance measurement is made is also specified. If you know the kind of metal a conductor is made of, you can obtain the specific resistance of the metal from a table. The specific resistances of some common substances are given in table 1-1.



The resistance of a conductor of a uniform cross section varies directly as the product of the length and the specific resistance of the conductor, and inversely as the cross-sectional area of the conductor. Therefore, you can calculate the resistance of a conductor if you know the length, cross-sectional area, and specific resistance of the substance. Expressed as an equation, the "R" (resistance in ohms) of a conductor is



Problem: What is the resistance of 1,000 feet of copper wire having a cross-sectional area of 10,400 circular mils (No. 10 wire) at a temperature of 20°C? Solution: The specific resistance of copper (table 1-1) is 10.37 ohms. Substituting the known values in the preceding equation, the resistance, R, is determined as R = ρ∙ l / A = 10.37 x 1000 / 10400 = 1Ω approximately This equipment operates on the principle that the resistance of a line varies directly with its length. Thus, the distance between the test point and a fault can be computed accurately.



Inductance Heat generation is an undesirable trait for an eddy current coil. If the 10 ft length of wire used in the previous example was wound into the shape of a coil, it would exhibit characteristics of alternating current other than resistance. By forming the wire into the shape of a coil, the coil also would have the property of inductance. The role of inductance is analogous to inertia in mechanics, because inertia is the property of matter that causes a body to oppose any change in its velocity. The unit of inductance is the Henry (H). A coil is said to have the property of inductance when a change in current through the coil produces a voltage in the coil. More precisely, a circuit in which an electromotive force of 1 V is induced when the current is changing at a rate of 1 Ampere per second will have an inductance of 1 H.


The inductance of a multilayer air core coil can be expressed by its physical properties or coil parameters. Coil parameters such as length, diameter, thickness and number of turns of wire affect the coil's inductance. Figure 3.1 illustrates typical coil dimensions required to calculate coil :inductance. An approximation of small, multilayer, air core coil inductapce is as follows: L = 0.8(rN)2 ∙ (6∙r+9∙l+10∙b) -1 μHenry L = self inductance in μH N = number of turns r = mean radius in inches l = length of coil in inches b = coil depth


Multi Layer Induction Coil


http://info.ee.surrey.ac.uk/Workshop/advice/coils/air/area.xhtml

Example: A coil whose dimensions are as follows: r = 0.1 in I = 0.1 in b = 0.1 in N = 100 turns

for L = 0.8(rN)2 ∙ (6∙r+9∙l+10∙b)-1 μH L = 0.8(0.1x100)2 (6x0.1+9x0.1+10.0.1)-1 L = 32 μH As stated earlier, this inductance is analogous to inertia in mechanical systems in that inductance opposes a change in current as inertia opposes a change in velocity of a body. In alternating current circuits the current is always changing; therefore inductance is always opposing this change. As the current tries to change, the inductance reacts to oppose that change. This reaction is called innductive reactance.


Inductive Reactance The unit of inductive reactance (XL) is in ohms. For a given coil the inductive reactance is a function of the rate of change of current or frequency. A formula relating frequency, inductance and inductive reactance is: XL = ωL = 2πf L Where: XL = Inductive reactance Ohm f = Frequency Hz L = Inductance Henry


Example: Using the 32 μH coil calculated earlier operating at 100 KHz, its inductive reactance would be found as follows:

L = 32 μH or 0.000032 H f = 100 KHz or 100000 Hz

XL = ωL = 2πfL = 2π x 100 x 103 x 32 x 10-6 = 20.106Ω Therefore, this coil would present an opposition of 20.096 ohms to currents with a rate of change of 100 kHz due to its reactive component. Unlike a resistive circuit, the current and voltage of an inductive circuit do not reach their minimum and maximum values at the same time. In a pure inductive circuit the voltage leads the current by 90 electrical degrees. This means that when the voltage reaches a maximum value, the current is at 0.


Power is related to current and voltage as follows: Power P = E x I P = Power E = Potential volt I = Current in Ampere

Notice that in a pure inductive circuit, when the voltage is maximum, the current is 0. Therefore, the product P = E x I = 0, Inductive reactances consume no alternating power where resistive elements consume power and dissipate power in the form of heat. The opposition to current flow because of the resistive element of the coil and the reactive element of the coil do not occur at the same time; therefore, they cannot be added as scalar quantities. . A scalar quantity is one having only magmtude, that is a quantity fully described by a number, but which does not involve any concept of direction. Gallons in a tank, temperature in a room, miles per hour, for example, are all scalars.


Impedance To explain the addition of reactance and resistance witha minimum of mathematical calculations, it is ·possible to use vector or phasor diagrams. A vector diagram constructed with imaginary units on the ordinate or Y axis and real units on the abscissa or X axis is shown in Figure 3.2. Z=√(XL2+R2) Observation of Figure 3.2 reveals XL, R and Z appear to form the sides of a right triangle. The mathematical solution of right triangles states the square of the hypotenuse is equal to the sum of the squares of the other two sides, or c2 = a2 + b2

Figure 3.2 Vector Diagram


Substituting Z, XL and R, the statement becomes: Z2 = XL2 + R2, further simplified Z = √(XL2+R2)

Example: Example: What is the impedance of a coil having an inductance. of 100μH and a resistance of 5 ohms and being operated at 200 kHz? XL = 2π x 200 x 103 x 100 x 10-6 = 125.7Ω Z =√(XL2 + R2) = (125.72 + 52) .5 = 125.8 Ω First, convert inductance to inductive reactance and then, by vector addition, combine inductive reactance and resistance to obtain the impedance.


Maximum transfer of power is accomplished when the driving impedance and load impedance are matched. If, for instance, an eddy current instrument had a driving impedance of 50 ohms, the most efficient test coils would also have impedances of 50 ohms. Other, more common examples of impedance matching are home stereo systems rated at 100 W per channel into 8 ohms. Impedance can be discussed in a more detailed manner by mathematically noting variables using imaginary numbers). The square root of a negative number is known as an imaginary number (√-1). The imaginary number √(-16) could be written as √(-1x16) or √-1∙ √(16) or √(-1)∙4. The notation √(-1) is used extensively and is mathematically noted by a lower case letter "i". Because i is also used in electrical terms for current, the i notation for electrical calculations is changed to the letter "j". The term j, often called operator j, is equal to the √(-I). Instead of noting √(-16) as √(-1)∙4. note it as j4. Because reactance is known as an imaginary component, then impedance In Cartesian form:

Z = R + jXm where the real part of impedance is the resistance R and the imaginary part is the reactance X.


http://en.wikipedia.org/wiki/Electrical_impedance

Rectangular Notation Because reactance is known as an imaginary component, then impedance In Cartesian form: Z = R + jX = |Z|∠ θ = The term R + jX is known as a rectangular notation. As an example, a resistance of 4 ohms in series with an inductive reactance of 3 ohms could be noted as Z = 4 + j3 ohms. The impedance

j3 ohms

Z

|∠ 5 | =

4

. Ω º 8.59

Z = 4 + j3 ohms.

4 ohms Charlie Chong/ Fion Zhang


Rectangular Notation Because reactance is known as an imaginary component, then impedance In Cartesian form: Z = R + jX = |Z|∠ θ = The term R + jX is known as a rectangular notation. As an example, a resistance of 4 ohms in series with an inductive reactance of 3 ohms could be noted as Z = 4 + j3 ohms. The impedance

Rectangular Notation j3 ohms

Z

. Polar Notation Ω º 9 5 . 8 4 ∠ | 5 =|

Z = 4 + j3 ohms.



Polar and Rectangular Notation In order to work with these complex numbers without drawing vectors, we first need some kind of standard mathematical notation. There are two basic forms of complex number notation: polar and rectangular. Polar form is where a complex number is denoted by the length (otherwise known as the magnitude, absolute value, or modulus) and the angle of its vector (usually denoted by an angle symbol that looks like this: ∠). To use the map analogy, polar notation for the vector from New York City to San Diego would be something like “2400 miles, southwest.” Here are two examples of vectors and their polar notations: (Figure below)



Vectors With Polar Notations. Standard orientation for vector angles in AC circuit calculations defines 0º as being to the right (horizontal), making 90º straight up, 180º to the left, and 270º straight down. Please note that vectors angled “down” can have angles represented in polar form as positive numbers in excess of 180º, or negative numbers less than 180. For example, a vector angled ∠ 270º (straight down) can also be said to have an angle of -90º. (Figure below) The above vector on the right (7.81 ∠ 230.19º) can also be denoted as 7.81 ∠ -129.81º.



The Vector Compass Rectangular form, on the other hand, is where a complex number is denoted by its respective horizontal and vertical components. In essence, the angled vector is taken to be the hypotenuse of a right triangle, described by the lengths of the adjacent and opposite sides. Rather than describing a vector's length and direction by denoting magnitude and angle, it is described in terms of “how far left/right” and “how far up/down.” These two dimensional figures (horizontal and vertical) are symbolized by two numerical figures. In order to distinguish the horizontal and vertical dimensions from each other, the vertical is prefixed with a lower-case “i” (in pure mathematics) or “j” (in electronics). These lower-case letters do not represent a physical variable (such as instantaneous current, also symbolized by a lower-case letter “i”), but rather are mathematical operators used to distinguish the vector's vertical component from its horizontal component. As a complete complex number, the horizontal and vertical quantities are written as a sum: (Figure below)



Rectangular Notation – Prefix “j” These two dimensional figures (horizontal and vertical) are symbolized by two numerical figures. In order to distinguish the horizontal and vertical dimensions from each other, the vertical is prefixed with a lower-case “i” (in pure mathematics) or “j” (in electronics). These lower-case letters do not represent a physical variable (such as instantaneous current, also symbolized by a lower-case letter “i”), but rather are mathematical operators used to distinguish the vector's vertical component from its horizontal component. As a complete complex number, the horizontal and vertical quantities are written as a sum: (Figure below)



Rectangle & Polar Notation  In rectangular notation, the vertical and horizontal components’ ordinate and abscissa are shown. Z = R + jX, where j denoting vertical component.  In polar notation, vector length hypotenuse and angle are shown |Z|∠ θ, where |Z| denoting vector length and ∠ θ denoting the angle. Z = R + jXm = |Z|∠ θ =

j3 ohms

Z

|∠ 5 | =

4

. Ω º 8.59

Z = 4 + j3 ohms.



Imaginary Component In “rectangular” form the vector's length and direction are denoted in terms of its horizontal and vertical span, the first number representing the horizontal (“real”) and the second number (with the “j” prefix) representing the vertical (“imaginary”) dimensions. The horizontal component is referred to as the real component, since that dimension is compatible with normal, scalar (“real”) numbers. The vertical component is referred to as the imaginary component, since that dimension lies in a different direction, totally alien to the scale of the real numbers. (Figure below)



Vector compass showing real and imaginary axes. The “real” axis of the graph corresponds to the familiar number line we saw earlier: the one with both positive and negative values on it. The “imaginary” axis of the graph corresponds to another number line situated at 90º to the “real” one. Vectors being twodimensional things, we must have a two-dimensional “map” upon which to express them, thus the two number lines perpendicular to each other: (Figure below)



Imaginary


Vector compass with real and imaginary “j” number lines. Either method of notation is valid for complex numbers. The primary reason for having two methods of notation is for ease of longhand calculation, rectangular form lending itself to addition and subtraction, and polar form lending itself to multiplication and division. Conversion between the two notational forms involves simple trigonometry. To convert from polar to rectangular, find the real component by multiplying the polar magnitude by the cosine of the angle, and the imaginary component by multiplying the polar magnitude by the sine of the angle. This may be understood more readily by drawing the quantities as sides of a right triangle, the hypotenuse of the triangle representing the vector itself (its length and angle with respect to the horizontal constituting the polar form), the horizontal and vertical sides representing the “real” and “imaginary” rectangular components, respectively: (Figure below)

To convert from polar to rectangular notation



To convert from rectangular to polar notation, find the polar magnitude through the use of the Pythagorean Theorem (the polar magnitude is the hypotenuse of a right triangle, and the real and imaginary components are the adjacent and opposite sides, respectively), and the angle by taking the arctangent of the imaginary component divided by the real component: Z = 4+j3 Magnitude vector in terms of real (4) and imaginary (j3) components.



REVIEW on Rectangular & Polar Notations  Polar notation denotes a complex number in terms of its vector's length and angular direction from the starting point. Example: fly 45 miles ∠ 203º (West by Southwest).  Rectangular notation denotes a complex number in terms of its horizontal and vertical dimensions. Example: drive 41 miles West, then turn and drive 18 miles South.  In rectangular notation, the first quantity is the “real” component (horizontal dimension of vector) and the second quantity is the “imaginary” component (vertical dimension of vector). The imaginary component is preceded by a lowercase “j,” sometimes called the j operator.  Both polar and rectangular forms of notation for a complex number can be related graphically in the form of a right triangle, with the hypotenuse representing the vector itself (polar form: hypotenuse length = magnitude; angle with respect to horizontal side = angle), the horizontal side representing the rectangular “real” component, and the vertical side representing the rectangular “imaginary” component.



More Reading on Polar & Rectangular Vector Notations http://en.m.wikipedia.org/wiki/Vector_notation http://www.daycounter.com/Calculators/Polar-To-RectangularCalculator.phtml https://filebox.ece.vt.edu/~LiaB/Lectures/Ch_9/Slides/Mathematics.pdf http://www.allaboutcircuits.com/vol_2/chpt_2/5.html


The term Z = R + jX is known as a rectangular notation. As an example, a resistance of R= 4 ohms in series with an inductive reactance of jX = 3 ohms could be noted as Z = 4 + j3 ohms. The impedance calculation is then : Z = √(42 + 32) = √(25) = 5Ω In coil design it is often helpful to know also the included angle between the resistive component and impedance. A convenient method of notation is the polar form where tan θ =XL+ R and θ is the included angle between resistance and impedance. In the previous example the impedance magnitude is 5 ohms, but at what angle? A proper form of notation is Z∠θ where Z is impedance and ∠θ is the included angle. Therefore, the complete notation for R=3, XL=4 is: Z = √(42 + 32) = √(25) = 5Ω tan θ = ¾ = 0.750 = 36.9º Z = |5|∠ 36.9º or Z = 3+j4


Eddy current coils with included impedance angles of 60 degrees to 90 degrees usually make efficient test coils. As the angle between resistance and impedance approaches 0 degrees, the test coil becomes very inefficient with most of its energy being dissipated as heat.


Q or Figure of Merit The term used to describe coil efficiency is Q or merit of the coil. The higher the Q or merit of a coil, the more efficiently the coil performs as an inductor. The merit of a coil is mathematically stated as: Q = XL / R For example, a coil having an inductive reactance of 100 ohms and a resistance of 5 ohms would have a Q of 20.


Permeability and Shielding Effects The addition of permeable core in certain designs dramatically improves “Q” factor. For example, are wound on a form that allow the powdered iron rod or slug to be placed in the center of coil. It is common to increase the coil impedance by a factor of 10 by the addition of coil materials. This increase in impedance without addition winding greatly enhances the Q of the coil. Some core materials are cylindrical or cup shaped. A common term is cup core (Fig3.3). the coil is first wound and then placed inn the cup core. In the case of a probe coil in the cup core,not only is the impedance increased, but the benefit of shielding is also gained. Shielding with a cup core, prevent the electromagnetic field from spreading at the sides of the coil. This greatly reduces signal produced by edge effect of adjacent member of the test area, such as fasteners on air wings. Shielding, while improving resolution, usually sacrifices some amount of penetration into fue part.


Figure 3.3: Effects of cup cores

(a) Unshielded coil -field spread might be up to twice the coil diameter. (b) Shielded coil - magnetic field extension restricted to the core geometry.


Another technique of shielding uses high conductivity material, such, as copper or aluminum, to suppress high frequency interference from other sources and also to shape the electromagnetic field of the test coil. A copper cup would restrict the electromagnetic field in much the same manner as the powdered iron cup core. A disadvantage of high conductivity, low or no permeability shielding is that the coil's impedance is reduced when the shielding material is placed around the test coil. The net effect is that the coil's “Q” factor is less than it was when the coil was surrounded by air.


Discussion Subject 1: Shielding, while improving resolution, usually sacrifices some amount of penetration into fue part. Subject 2: A disadvantage of high conductivity, low or no permeability shielding is that the coil's impedance is reduced when the shielding material is placed around the test coil. The net effect is that the coil's “Q” factor is less than it was when the coil was surrounded by air.


Saturation Approach Another coil design used for inspection of ferromagnetic materials is the saturation approach. A predominant variable that prevents eddy current penetrating in ferromagnetic material is called permeability. Permeability effects exhibited by the test object can be reduced by means of magnetic saturation(Figure 3.4). Saturation coils for steels are usually very large and surround the test object and test coil. A steady state (DC) current is applied to the saturation coil. When the steel test object is magnetically saturated it may be inspected in the same manner as a nonferromagnetic material. In the case of mild steel many thousands of tesla are required to produce saturation. In some inherently nonferromagnetic tubing materials like high temperature nickel chromium alloy there may be low level permeability variations because of manufacturing discontinuities. In this case the use of small permanent magnets adjacent to the bobbin probe coils may improve the inspection quality by reducing the permeability effects. Figure 3.5 shows the use of disk type magnets placed close to the coil. It is also possible to use an array of bar magnets arranged around the probe housing if higher magnetic potential is required to offset the material permeability characteristics.


Figure 3.4: Magnetic saturation inspection process


Figure 3.5: Magnetic bias probe


Coil Fixtures Coil fixtures or holders may be as varied as the imagination of the designers and users. After the size, shape and style have been decided on, the next consideration should be the test environment. Characteristics of wear, temperature, atmosphere, mechanical stress and stability must be considered (4). Normally wear can be reduced by selection of wear resistant compounds to protect the coil windings. If severe wear is expected, artificial or genuine jewels may be used. Less expensive and very effective wear materials, such as aluminum oxide or ceramics, are more commonly used. Temperature stability may be accomplished by using coil holder material with poor heat transfer characteristics. Metals have high heat transfer characteristics and often coils made with metal holders are sensitive to temperature variations caused by human touch. For high temperature applications, materials must be chosen carefully. Most common commercial copper coil wire may be used up to 150°C to 200°C. For temperatures above 200 ac, silver or aluminum wire with ceramic or high temperature silicone insulation must be used.


Materials must be chemically compatible with the test object. As extreme examples, a polystyrene coil form would not be used to inspect an acetone cooler or a lead or graphite housing allowed to come in contact with a high temperature nickel chromium alloy jet engine tail cone. The chemical interactions between these material combinations could cause cracking and lead to component failure. Mechanical and electrical stability of the test coil can be enhanced by an application of epoxy resin between each layer of coil winding. This accomplishes many objectives: 1) it seals the coil to exclude moisture; 2) it provides additional electrical insulation; and 3) it provides mechanical stability. Characteristics listed are not in order of importance. The importance of each characteristic is determined by specific test requirements.




Q.3.1 A coil's resistance is determined by: A. wire material. B. wire length. C. wire cross-sectional area. D. all of the above. Q.3.2 Inductance might be referred to as being analogous to: A. force. B. volume. C. inertia. D. velocity. Q.3.3 The unit of inductance is the: A. henry. B. maxwell. C. ohm. D. farad.


Q.3.4 The inductance of a multilayer air core coil with the dimensions I = 0.2, r = 0.5, b = 0.1 and N = 20, is: L = 0.8(rN)2 ∙ (6∙r+9∙l+10∙b) -1 μHenry A. 1.38 H. L = self inductance in μH B. 13.8 μH. N = number of turns r = mean radius in inches C. 13.8 ohms. l = length of coil in inches D. 1.38 ohms. b = coil depth

Q.3.5 The inductive reactance of the coil in Q.3.4, operating at 400 kHz, would be: A. 1380 ohms. XL= 2πfL = (2π∙400∙103∙13.8∙10-6) B. 5520 ohms. C. 34.66 ohms. D. 3466 oluns. Q.3.6 The impedance of a 100μH coil with a resistance of 20 ohms operating at 100kHz would be: A. 62.8 ohms. XL = 2πfL, Z =√(XL2 + R2) B. 4343.8 ohms. C. 628 ohms. D. 65.9 ohms. Charlie Chong/ Fion Zhang

Q.3.7 The Q or merit of a coil is denoted by the ratio: A. Z/XL B. XL/Z c. XL/R D. R/XL Q.3.8 The incorporation of ferromagnetic shielding materials around a coil: A. improves resolution. B. decreases field extension. C. increases impedance. D. Does all of the above. Q.3.9 The purpose of a steady state winding w near a test coil is to: (? – scanned copy missing wording) A. reduce material permeability effects. B. produce possible magnetic saturation in the test material. C. provide a balance source for the sensitive coil. D. both A and B.


Q.3.10 The most important consideration when selecting a test coil is: A. sensitivity. B. resolution. C. stability. D. meeting established inspection criteria


The Answers


Chapter 4 Effects of Test Object on Test Coil



Jackfruit Tree

Operating or Test Variables As previously seen, the eddy current technique depends on the generation of induced currents within the test object. Disturbances in these small induced currents affect the test coil. The result is a variation of the test coil impedance due to test object variables. These variances are called operating or test variables. The range of test variables encountered might include electrical conductivity, magnetic permeability, skin effect, lift off, fill factor; end effect, edge effect and signal-to-noise ratio. Coil impedance was discussed at length in Chapter.3. In this chapter coil impedance changes will pe represented graphically to more effectively explain the interaction of the operating variables.


Electrical Conductivity σ In electron theory the atom consists of a positive nucleus surrounded by orbiting negative electrons. Materials that allow these electrons to be easily moved out of orbit around the nucleus are classified as conductors. In conductors electrons are moved by applying an outside electrical force. The ease with which the electrons are made to move through the conductor is called conductance. A unit of conductance is the siemens (mho). The siemens is the reciprocal of the ohm or conductance G = l / R where G is conductance in siemens and R is resistance in ohms. In eddy current testing, instead of describing conductance in absolute terms, an arbitrary unit has been assigned. Since the relative conductivity of metals and alloys varies over a wide range, the need for a conductivity bench-mark is of prime importance.


The International Electrochemical Commission established in 1913 a convenient technique of comparing one material to another. The commission established that a specific grade of high purity copper, 1 m in length, with a uniform cross section of 1 mm2, measuring 0.017241 ohms at 20°C would be arbitrarily considered to be 100% 'conductive. The symbol for conductivity is σ (sigma) and the unit is percent IACS or percent of the International Annealed Copper Standard. Table 4.1 lists materials by their electrical properties: conductivity and resistivity. A statement can be made about a conductor in terms of conductance or resistance. Note that a good conductor is a poor resistor. Conductance and resistance are direct reciprocals as stated earlier. Conductivity and resistivity, however, have different origins and units; therefore, the conversion is not so direct.


As previously discussed, conductivity is expressed on an arbitrary scale in percent IACS. Resistivity is expressed in absolute terms of micro-ohmcentimeters. To convert values on one scale to the other system of units a conversion factor of 172.41 is required. Once you know either the conductivity or the resistivity value for a material the other electrical property can be calculated.

Shanghai- 上海

172.41 Charlie Chong/ Fion Zhang

%IACS & Resistivity ρ (rho) in micro-ohm-centimeter ■

%IACS = 172.41 / ρ micro-ohm-centimeter (μΩ∙cm)

■

ρ micro-ohm-centimeter = 172.41 / %IACS

These numerical values will be necessary when additional calculations are needed to determine issues of frequency choice, depth of penetration and I or phase spread to meet specific inspection criteria. As the test coil is influenced by different conductivities, its impedance varies inversely to conductivity. A higher conductivity causes the test coil to have a lower impedance value. Figure 4.1 illustrates this concept.


Figure 4.1; Conductivity curve


Discussion Subject: A higher conductivity causes the test coil to have a lower impedance value. Figure 4.1 illustrates this concept.- Reason out the statement.


The coil's inductive reactance is represented by theY axis and coil resistance appears on the X axis. The 0% conductivity point, or air point, is when the coil's empty reactance (XL) is maximum. Figure 4.1 represents a measured conductivity locus . Conductivity is influenced by many factors. Table 4.1 is a comparative listing of materials with various chemical compositions. There are various manufacturing or in situ factors that must be considered when hying to measure the conductivity of various alloys. In metals, .as. the temperature is increased, the conductivity wlll decrease. This is a major factor to consider when accurate measurement of conductivities is required.


Figure 4.1; Conductivity curve Key Word: The 0% conductivity point Air point


Effects of Heat Treatment on Conductivity Heat treatment affects electrical conductivity by redistributing elements in the material. Dependent on materials and degree of heat treatment, conductivity can either increase or decrease as a result of heat treatment. Stresses in a material due to cold working produces lattice distortion or dislocation. This mechanical process changes the grain structure and harness of the material, changing its electrical conductivity. Hardness in age hardened aluminum alloy changes the electrical conductivity of the alloy. The electrical conductivity decreas as hardness increase. As an example Brinell hardness 60 is represented by conductivity 23 while Brinell hardness 100 of the same alloy would have a conductivity of 19.


Table 4.1: Electrical resistivity and conductivity of several metals and alloys

ρ = 172.41 / %IACS


Permeability Permeability of any material is a measure of the ease with which its magnetic domains can be aligned or the ease with which it can be establish lines of force. Materials are rated on a comparative basis. Air is assigned a permeability of 1. Ferromagnetic metals and alloys including nickel, iron and cobalt tend to concentrate magnetic flux lines. As discussed in Chapter 3, some ferromagnetic materials or sintered ionic compounds are also useful in concentrating magnetic flux. Magnetic permeability is not constant for a given material. The permeability in a test sample depends on the magnetic field acting on it. As an example, consider a magnetic steel bar placed in an encircling coil. As the coil current is increase, the magnetic field of the coil will increase. The magnetic flux within the steel will increase rapidly at first and then will tend to level off as the steel approaches magnetic saturation. This phenomenon is called the saturation effect.


BH Curve


When increases in the magnetizing force produce littile or no change on the flux within the steel bar, the bar is magnetically saturated. When ferromagnetic materials are saturated, permeability becomes constant. With magnetic permeability constant, ferromagnetic materials may be inspected using tpe eddy current method. Without magnetic saturation ferromagnetic materials exhibit such a wide range of permeability variation that signals produce by discontinuities or conductivity variations are masked by the permeability signal.


Skin Effect Electromagnetic tests in many applications are most sensitive to test object variables nearest the test coil because of skin effect. Skin effect is a result of mutual interaction of eddy currents, operating frequency, test object conductivity and permeability. The skin effect, the concentration of eddy currents in the test object nearest the test coil, becomes more evident as test frequency, test object conductivity and permeability are increased. For current density or eddy current distribution in the test object, refer to Figure 1.8 in Chapter 1. δ = (πfμσ) -½


http://www.geocities.ws/raobpc/EC-Def.html

Skin Effect


http://www.jmag-international.com/catalog/50_SteelWire_InductionHeating.html

Edge Effect The electromagnetic field produced by an excited test coil extends in all directions from the coil. The coil's field precedes the coil by some distance determined by coil parameters, operating frequency and test object characteristics. As the coil approaches the edge of a test object, eddy current flow in the test sample becomes distorted by the edge. This is known as edge effect. Edge effect can create a change in the coil's impedance that is similar to a discontinuity (Figure4.2). The response would move back along the conductivity curve toward the air point. The coil is responding to a slightly less conductive situation (air) at the leading edge of the coil's field of view. It is therefore essential that edge effect be eliminated as a variable during a surface scanning test. Response to the edges of test objects can be reduced by: incorporating magnetic shields around the test coil, increasing the test frequency, reducing the test coil diameter or by changing the scanning pattern used. Edge effect is a term most applicable to the inspection of sheets or plates with a probe coil.


Discussion Subject: Response to the edges of test objects can be reduced by: incorporating magnetic shields around the test coil, increasing the test frequency, reducing the test coil diameter or by changing the scanning pattern used.


Figure 4.2: Edge effect


Lift Off Electromagnetic coupling between test coil and test object is of prime importance when conducting an eddy current examination. The coupling between test coil and test object varies with spacing between the test coil and test object. This spacing is called lift off (4). The effect on the coil impedance is called lift off effect. Figure 4.3 shows the relationship between air, conductive materials and lift off. The electromagnetic field, as previously discussed, is strongest near the coil and dissipates with distance from the coil. This fact causes a pronounced lift off effect for small variations in coil to object spacing. As an example, a spacing change from contact to 0.0254 mm (0.001 in.) will produce a lift off effect many times greater than a spacing change of 0.254mm (0.010 in.) to 0.2794 mm (0.011 in.) (15), Lift off effect is generally an undesired effect causing incre,ased noise and reduced coupling resulting in po6r measming ability (12). In some instances, equipment having phase discrimination capability can readily separate lift off from conductivity or other variables. Lift off can be used to advantage when measuring nonconductive coatings on conductive bases. A nonconductive coating such as paint or plastic causes a space between the coil and conducting base, allowing lift off to represent the coating thickness. Lift off is also useful in profilometry and proximity applications. Lift off is a term most applicable to testing objects with a surface or probe coil.


Fill Factor Fill factor is a term used to describe how well a test object will be ectromagnetically coupled to a test coil that surrounds or is inserted into the test object. Fill factor then pertains to inspections using bobbin or encircling coils. Like lift off, electromagnetic coupling between test coil and test object is most efficient when the coil is nearest the surface of the part. The area of a circle (A) is determined using the equation: A Area = πd2/4 Fill factor can be described as the ratio of test object diameter to coil diameter squared (Figure 4.4). The diameters squared is a simplified equation resulting in the.division of effective coil and part areas. Because the term π /4 both the numerator and the denominator of this fractional equation the term π/4 cancelled out, leaving the ratio of the diameters squared; η (eta) = d2/D2 , fill factor


Fill factor will always be a number less than 1 and efficient fill factors approaching 1, fill factor of 0.99 is more desirable than a fill factorof 0.75. The effect of fill factor on the test system is that poor fill factors do not allow the coil to be sufficiently coupled to the test object. This is analogous to the effect of drawing a bow only slightly and releasing an arrow. The result is, with the bow slightly drawn and released, little effect is produced to propel the arrow. In electrical terms it is said that the coil is loaded by the test object. How much the coil is loaded by the test object due to fill factor can be calculated in relative terms. A test system with constant current capabilities being affected by a conductive nonmagnetic bar placed into an encircling coil can be used to demonstrate this effect.


For this example, the system parameters are as follows: (a) Unloaded coil voltage equals 10 V. (b) Test object effective permeability equals 0.3 (c) Test coil inside diameter equals 25.4 mm (1 in.) (d) Test object outside diameter equals 22.9 mm (0.9 in.) η (eta) = d2/D2 , fill factor = (0.9/1)2 = 0.81 An equation demonstrating coil loading is given by: E = Eo (1- η + η∙μeff ) Where: Eo = Coil voltage with coil affected by air E = Coil voltage with coil affected by the test material η = Fill factor μeff = Effective Permeability


Figure 4.4: Fill factor ratios

Whena nonfermmagnetic test object is inserted into the test coil, the coil's voltage will decrease. E = Eo (1- η + η∙μeff ) E = 10 (1-0.81 + 0.81 x 0.3) E = 4.33 Volts This allows 10- 4.3 or 5.7 V available to respond to test object changes caused by discontinuities or decreases in effective conductivity of the test object. It is suggested that the reader calculate the resultant loaded voltage developed by a 12.7 mm (0.5 in.) bar of the same material and observe the relative sensitivity difference. Charlie Chong/ Fion Zhang

Figure 4.4: Fill factor ratios

Whena nonfermmagnetic test object is inserted into the test coil, the coil's voltage will decrease. ∆E = Eo (1- η + η∙μeff ) ∆E = 10 (1-0.81 + 0.81 x 0.3) ∆E = 4.33 Volts This allows 10- 4.3 or 5.7 V available to respond to test object changes caused by discontinuities or decreases in effective conductivity of the test object. It is suggested that the reader calculate the resultant loaded voltage developed by a 12.7 mm (0.5 in.) bar of the same material and observe the relative sensitivity difference. Charlie Chong/ Fion Zhang

Example: Calculate the resultant loaded voltage developed by a 12.7 mm (0.5 in.) bar of the same material and observe the relative sensitivity difference. η (eta) = d2/D2 , fill factor = (0.5/1)2 = 0.25 An equation demonstrating coil loading is given by: ∆E = Eo (1- η + η∙μeff) ∆E = 10 (1-0.251 + 0.25 x 0.3) ∆E = 8.25 Volts This allows 10- 8.25 or 1.75 V available to respond to test object changes caused by discontinuities or decreases in effective conductivity of the test object.


Discontinuities Any discontinuity that appreciably changes the normal eddy current flow can be detected. Discontinuities, such as cracks, pits, gouges, vibrational damage and corrosion, generally cause the effective conductivity of the test object to be reduced. Discontinuities open to the surface are more easily detected than subsurface discontinuities. Discontinuities open to the surface can be detected with a wide range of frequenciesi subsurface investigations require a more careful frequency selection. Discontinuity detection at depths greater than 12.7 mm (0.5 in.) in stainless steel is very difficult. This is in part due to the sparse distribution of magnetic flux lines at the low frequency required for such deep penetrations.


Figure 1.8 is again useful to illustrate discontinuity response because of current distribution. As an example, consider testing a nonferromagnetic tube at a frequency that establishes a standard depth of penetration at the midpoint of the tube wall. This condition would allow a relative current density of about 20% on the far surface of the tube. With this condition, identical near and far surface discontinuities would have greatly different responses. Due to current magnitude alone, the near surface discontinuity response would be nearly five times that of the far surface discontinuity. Discontinuity orientation has a dramatic effect on response. As seen earlier, discontinuity response is maximum when eddy currents and discontinuities are at 90 degrees or perpendicular. Discontinuities parallel to the eddy current flow produce little or no response. The easiest technique to ensure detectability of discontinuities is to use a reference standard or model that provides a consistent means of adjusting instrumentation.




Signal-to-Noise Ratio Signal-to-noise ratio is the ratio of signals of interest to unwanted signals. Common noise sources are test object variations of surface roughness, geometry and homogeneity. Other electrical noises can be due to such external sources as welding machines, electric motors and generators. Mechanical vibrations can increase test system noise by physical movement of test coil or test object. In other words, anything that interferes with a test system's ability to define a measurement is considered noise. Signal-to-noise ratios can be improved by several techniques. If a part is dirty or scaly, signalto-noise ratio can be improved by cleaning the part. Electrical interference can be shielded or isolated. Phase discrimination and filtering can improve signal- to-noise ratio. It is common practice in non destructive testing to require a minimum signal-to-noise ratio of 3 to 1. This means a signal of interest must have a response at least three times that of the noise at that point.




The Answers


Q.4.1 Materials that hold their electrons loosely are classified as: A. resistors. B. conductors. C. semiconductors. D. insulators. Q.4.2 100% IACS is based on a specified copper bar having a resistance of: A. 0.01 ohms. B. 100 ohms. C. 0.017241 ohms. D. 172.41 ohms. Q.4.3 A resistivity of 13 μohm cm is equivalent to a conductivity in percent IACS of: A. 11.032. B. 0.0625. c. 1652. D. 13.26.


Q.4.4 A (?) prime factor affecting conductivity is: A. temperature. B. hardness, C. heat treatment. D. all of the above. Q.4.5 Materials that tend to concentrate magnetic flux lines are: A. conductive. B. permeable. c. resistive. D. inductive. Q.4.6 Diamagnetic materials have: A. a permeability greater than air. B. a permeability less than air. C. a permeability greater than ferromagnetic materials. D. no permeability.


Q.4.7 Edge effect can be reduced by: A. shielding. B. selecting a lower frequency. C. using a smaller coil. D. both A and C. Q.4.8 Calculate the effect of fill factor when a conducting bar 12.7 mm (0.5 in.) in diameter with an effective permeability of 0.4 is placed into a 25.4 mm (1 in.) diameter coil with an unloaded voltage of 10V. The loaded voltage is: A. 2V. η = (0.5)2 = 0.25 B. 4.6V: ∆E = Eo ( 1-η+ ηxμeff) C. 8.5V: ∆E = Eo ( 1-0.25 + .25x.4) = 8.5 Volts D. 3.2V. Q.4.9 Laminations are easily detected with the eddy current method. A. True B. False


Q.4.10 Temperature changes, vibration and environmental effects are test coil inputs that generate: A. unwanted signals. B. magnetic fields. c. eddy currents. D. drift.


Chapter 5 Selection of Test Frequency


soursop-prickly-custard-apple-durian-belanda-annona-muricata

Test Frequencies It is the responsibility of nondestructive testing engineersand technicians to provide and perform non destructive testing that in some way ensures the quality, usefulness of industly products. To apply a nondestructive test, it is essential that the parameters affecting the test be understood. Usually industry establishes a product or component and then seeks a method to inspect it. This practice establishes test object geometry, conductivity and permeability before the application of the eddy current examination. Instrument, test coil and test frequency selection become the tools used to solve the problem of inspection. Test coils were discussed previously and instrumentation will be discussed later in this text. Test frequencies and their selection will be examined in detail in this Chapter.


Frequency Selection In Chapter 1, it was observed that eddy currents are exponentially reduced as they penetrate the test object. In addition, a time or phase difference in these currents was observed (current lagging with penetration w.r.t surface current). The currents near the test coil happen first or lead the current that is deeper in the object. A high current density allows good detectability and a wide phase difference between near and far surfaces allows good resolution. Keypoint: Current deeper into the test object lag the surface current by β = x/δ radian.


Single Frequency Systems Unfortunately, if a low frequency is selected to provide good penetration and detectability, the phase difference between near and far surface is reduced. Selection of frequency often becomes a compromise. It is common practice in inservice inspection of thin-wall, nonferromagnetic tubing to establish a standard depth of penetration just past the midpoint of the tube wall. This permits about 25% of the available eddy current to flow at the outside surface of the htbe wall. In addition, this establishes a phase difference of about 150 degrees to 170 degrees between the inside and outside surface of the tube wall. The combination of 25% outside, or surface current and 170 degrees included phase angle provides good detectability and resolution for thin-wall tube inspection. (This is accomplished by properly selecting the driving frequency of the coil to limit the penetration)

Calculation: δ = (πfσμ) -½ β = x/ δ radian = x / δ∙ 57.3º Where: x = depth below surface


Standard penetration depth δ The depth that eddy currents penetrate into a material is affected by the frequency of the alternating current, the electrical conductivity and magnetic permeability of the sample. The depth of penetration decreases with increasing frequency and increasing conductivity and magnetic permeability. The depth at which eddy current density has decreased to 1/e, or about 37% of the surface density, is called the standard depth of penetration (δ or 1 δ) and used as criteria of ideal measurement. At three standard depth of penetration (3δ), the Eddy Current density is down to only 5% of the surface density. So, defects or variation deeper than the three standard depth of penetration cannot be recognized because the EC density in this depth is too low to detect. Thus, achieving the standard penetration depth is the most important factor at Eddy Current testing and this is realized by selecting appropriate frequency suitable for a material property.


http://www.suragus.com/en/company/eddy-current-testing-technologyc

Eddy Current Density Since the sensitivity of Eddy Current inspection depends on the Eddy Current density at the defect location, it is important to know the strength of the Eddy Currents at this location. When detect flaws, a frequency is often selected which places the expected flaw depth within one standard depth of penetration. This assures that the strength of the Eddy Currents would be sufficient to produce a flaw indication.


http://www.suragus.com/en/company/eddy-current-testing-technology

Eddy Current Density of a Solid bar D=2δ

D=2δ

δ 1/e = 37% of surface current density 2δ (1/e)2 = 13.5% of surface current density


Applicable Scenario Eddy Current Density of a Solid bar D≤2c

The field in air on the far surface ?


http://www.suragus.com/en/company/eddy-current-testing-technologyc

Eddy Current Density of a Solid bar D>3δ 100% surface current density δ

D>3δ 2δ

32δ

1/e = 37% of surface current density (1/e)2 = 13.5% of surface current density (1/e)3 = 5% of surface current density at 3δ


Standard Depth δ Formula The depth of penetration formula discussed in Chapter 1, although correct, has rather cumbersome units. Conductivity is usually expressed in percent of the International Annealed Copper Standard (% IACS). Resistivity is usually expressed in terms of micro-ohm-centimeter (μΩcm) (16). Depths of penetration are normally much less than 12.7 mm (0.5 in.). A formula using these units may be more appropriate and easier to use. In Chapter 1 a formula for calculating depth of penetration in the metric units was presented. Another derivative of this formula using resistivity, frequency and permeability with δ expressed in mm or inches can be expressed as follows: (standards form δ = (πf∙σμrμo)-½ , σ = 1/ρ)

δ = K√ [ρ/(fμr)] ， δ = K [ρ/(fμr)] ½ Where: δ = Standard penetration in mm or inches K = 50 for δ in mm and 1.98 for δ in inches ρ = Resistivity in micro-ohm-centimeter (μΩcm) f = Frequency μr = Relative permeability (for non-magnetic conductor μr=1)

δ = K√ [ρ/(f)] for non-magnetic conductor where μr=1


Frequency Selection – Non Ferromagnetic Conductor The prime variable is frequency. By adjusting frequency technicians can be selectively responsive to test object variables. Solving the nonferromagnetic depth of penetration formula for frequency requires a simple algebraic manipulation as follows: δ = K√ [ρ / (f)] for non-magnetic conductor where μr=1 ρ /f = (δ/K)2, f = ρ∙(K/δ)2


Example 1: As an example of how this may be used, consider inspecting a 7.6 mm (0.3 in.) thick aluminum plate, fastened to a steel plate at the far surface. Effects of the steel part are undesirable and require discrimination or elimination. The aluminum plate has a resistivity of 5 μΩ∙cm. By establishing a depth of penetration at 2.54 mm (0.1 in., the far surface current will be less than 10% (5%) of the available current, thus reducing response to the steel part. The frequency required for this can be calculated by applying: δ = 0.1” f = ρ∙(K/ δ)2 , f = 5(1.98/0.1)2 = 1960Hz (use inches)

0.3 in. Thick Al.

Steel plate


Example 2: If detection of the presence of the steel part was required, the depth of penetration could be reestablished at 7.6 mm (0.3 in.) in the aluminum plate and a new frequency could be calculated. δ = 0.3 in. f = ρ∙(K/ δ)2 , f = 5(1.98/0.3)2 = 218Hz (use inches)


Frequency Selection – Ferromagnetic Conductor

For ferromagnetic conductor δ = K√ [ρ/(fμr)], the parameter μr ≠ 1, need to be addressed in the above example:

f = ρ /μr ∙(K/ δ)2 instead of f = ρ ∙ (K/ δ)2


Bessel function for fg (?)

Another approach to frequency selection uses argument A of the Bessel function where argument A is equal to unity or 1.

A = f μrσd2 / 5066 Where: f = frequency Hz μr = Relative permeability σ = Conductivity in meter / Ω.mm2 d = Diameter of the coil in cm


A frequency can always selected to established factor A = 1, the frequency is known as limiting frequency and is denoted by fg. By substituting 1 for A and fg for f, the equation becomes;

1 = fg∙ μr∙ σ∙ d2 / 5066 fg = 5066/ (μr∙ σ∙ d2) Limiting frequency fg is then established in term of conductivity, permeability, some dimensional properties and a constant 5066. Because limiting frequency fg is based on these parameters, a techniques of frequency determination using a test frequency to limit frequency ration f/fg can be accomplished. High f/fg ratios are used for near surface tests and lower f/fg ration is used for subsurface tests.


Often results of such tests are represented graphically by diagrams,These diagrams are called impedance diagrams. Impedance illustrated by vector diagrams in Chapter 3 shows inductive reactance represented onthe Y axis, ordinate and resistance on the X axis, abscissa. The vector sum of the reactive and resistive components is impedance. This impedance is a quantity with magnitude and direction that is directly proportional to frequency. To construct a universal impedance diagram valid for all frequencies, the impedance must be normalized (4). Figure 5.1 illustrated a normalization process.


Figure 5.1(a) shows the effect on primary impedance Zp with changes in frequency (ω= 2πf). Figure 5.1(a) represents primary impedance without a secondary circuit or test object. Figure 5.1(b) illustrates the effect of frequency on primary impedance with a secondary circuit or test object present. The primary resistance R, in Figure5.1(a) has been subtracted in Figure 5.1(b) because resistance is not affected by frequency. The term ωLsG in Figure 5.1(b) represents a reference quantity for the secondary impedance. The units are secondary conductance (G) and ωLs (secondary reactance).


Figure 5.1: Effects of frequency change (a) Primary impedance without secondary circuit (b) Primary impedance with secondary circuit. XL =ωL = 2πf∙L

f


Figure 5.1: Effects of frequency change (a) Primary impedance without secondary circuit (b) Primary impedance with secondary circuit.

B, C, D, E, F, loci for selected values of Zp G = secondary conductance Zp =primary impedance ω = angular frequency = 2πf ωLs = secondary reactance

G The primary resistance R, in Figure5.1(a) has been subtracted in Figure 5.1(b) because resistance is not affected by frequency.


Further normalization is accomplished by dividing the reactive and resistive components by the term ωLo or the primary inductive reactance without a secondary circuit present. Figure 5.2 shows a typical normalized impedance diagram. The terms ωL/ωL0 and R/ωL0 represent the relative impedance of the test coil as affected by the test object. Signals generated by changes in ωL or R caused by test object conditions such as surface and subsurface discontinuities may be noted by ∆ωL or ∆R. The ∆ωL and ∆R notation indicates a change in the impedance.


Figure 5.2: Normalized impedance diagram for long coil encircling solid cylindrical nonferromagnetic bar and for thin-wall tube. Coil fill factor = 1.0 k = √(ωμσ) = Electromagnetic wave propagation constant for conducting material r = radius of the conductor in m μ = magnetic permeability of bar = 4π∙10-7 H.m-1 if bar is non-magnetic (μ = μo) ω = angular velocity = 2πf √(ωLoG) = equivalent of √(ωμσ) for simplified electrical circuit, where G=conductance (Siemens) and Lo = inductance in air (Henry)


Figure 5.3 shows the impedance variation in a nonferromagnetic cylinder caused by surface and subsurface discontinuities. Figure 5.3 also illustrates a sensitivity ratio for surface and subsurface discontinuities. Notice with an flfg ratio of 50, a relatively high frequency, the respouse to subsurface discontinuities is not very prononounced. Figure 5.4 shows responses to the same discontinuities with an f/fg ratio of 15. This lower frequency allows better detection of subsurface discontinuities as shown in Figure 5.4.


Figure 5.3: Impedance variations caused by surface and subsurface cracks Impedance variations caused by surface and subsurface cracks in nonferromagnetic cylinders, at a frequency ratio f/fg = 50.



Figure 5.3: Impedance variations caused by surface and subsurface cracks


Impedance variations caused by surface and subsurface cracks in nonferromagnetic cylinders, at a frequency ratio f/fg = 15.




Multiparameter Techniques It becomes obvious that the technician must have a good working knowledge of current density and phase relationships to make intelligent frequency choices. The frequency choice discussed to date deals with coil systems driven by only one frequency. Test systems driven by more than one frequency are called multifrequency or multi parameters systems. It is common for a test coil to be driven with three or more frequencies. Although several frequencies may be applied simultaneously or sequentially to a test coil, each of the individual frequency techniques follows rule established by a single frequency techniques. Signals generated at the various frequencies are often combined or mixed in electronic circuits that algebraically add or subtract signals to obtain the desired result.


Multiparameter Techniques – Broadband Signals One multifrequency approach is to apply a broadband signal, with many frequency components to the test coil. The information transmitted by the signal is proportional to its bandwidth and the logarithm of 1 plus the signalto-noise power ration. The relation is stated by the equation: C = B∙Log2(1+ S/N) C = rate of information transmitted in bits per second B = bandwidth of the signal S/N = signal-to-noise ratio This is known as the Shannon-Hartley theorem.


Multiparameter Techniques - Multiplexing Another approach to multiparameter techniques is to use multiplexing process. The multiplexing process places one frequency at a time on the test coil. This results in zero crosstalk between each frequency and eliminates the need for channel specific band pass filters. The major advantages of a multiplex system, in addition to the crosstalk reduction issues, are lower cost and greater flexibility in frequency selection. If the multiplexing switch rate is sufficiently high, both broadband and multiplex systems have essentially the same results. The characterization of eddy current signals by their phase angle and amplitude is a common practice and provides a basis for signal mixing to suppress unwanted signals from test data. Two frequencies are required to remove each unwanted variable.


Example – Multiparameter Technique Practical multi parameter frequency selection can be demonstrated by the following example: Problem: Eddy current inspection of installed thin wall nonferromagnetic heat exchanger tubing. Tubing is structurally supported by ferromagnetic tube supports at several locations. It is desired to remove the tube support response signal from tube wall data. Solution: Apply a multiparameter technique to suppress the tube support signal response.


First, a frequency is selected to give optimum phase and amplitude information about the tube wall. This is called the prime frequency. At the prime frequency, the response to the tube support and to a calibration through wall hole are about equal in amplitude. They may also have about the same phase angle. A second frequency called the subtractor frequency is selected on the basis of the phase angle of the tube support response. Because the tube support surrounds the outside diameter of the tube, a lower frequency is selected. At the subtractor frequency the tube support signal response is about 10 times greater than the calibration through wall hole. The phase difference between the support signal and the through wall hole in this lower frequency will be about 90 degrees. Parameter separation limitations are greatest for those parameters producing nearly similar signals, such as dents .


If the prime and subtractor channels have been selected properly then signal subtraction algorithms should be able to suppress the tube support signal leaving only slightly attenuated prime data (discontinuity) information. For suppression of inside or near surface signals, a higher subtractor frequency would be chosen. A combination of prime, low and high subtractor frequencies is often used to suppress both near and far surface signals, leaving only data pertaining to the part thickness and its condition. Bandwidth of the coil is of prime importance when operation over a wide frequency range is required in multifrequency/multi parameter testing. Optimization of a test frequency (or frequencies) will therefore depend on the desired measurement or parameter(s) of interest.


More Reading on Characteristic Parameter Pc / Frequency fg


http://www.geocities.ws/raobpc/Probes.html

EDDY CURRENT PROBES (SENSORS) INTRODUCTION Eddy current (EC) testing is used to ensure pre-service quality and to assess in-service health of industrial components made of electrically conducting materials, by way of detection and characterization of defects or discontinuities. This technique works on the principles of electromagnetic induction and test phenomenon can be explained using the Maxwell’s equations. In this technique, as shown in Fig. 1, a coil (also called probe) is excited with sinusoidal alternating current (frequency, f) to induce eddy currents in the component under test and the change in coil impedance is measured. Defects such as cracks, inclusions, notches, microstructure variations etc. cause a discontinuity in electrical conductivity, and/or magnetic permeability, hence, distort the eddy current flow and in turn, change the coil impedance. The measured impedance change is correlated with defect parameters e.g., length, depth, location, and orientation etc. The locus of impedance change during the movement of an EC probe is usually called an EC signal or the impedance-plane trajectory. Eddy current test phenomenon is controlled by the skin effect, according to which the depth of penetration (also standard depth of penetration [SDP]), depends on frequency and material properties (see Fig.1). Due to skin-effect, the detection and characterization of surface defects is more reliable as compared to buried or sub-surface defects. Popular industrial applications of eddy current testing include defect detection, material property measurement, alloy sorting, and material as well as coating thickness measurements. It is also used for proximity sensing, level measurements, metal particles/debris in non-conducting media (cardboards, bakery products, currency notes, underground mines, insulators etc. ) Eddy current probe is the main link between the eddy current instrument and the component under test. Success of eddy current testing for a specific inspection application depends on sensor, instrument and optimization of test parameters. The probe plays two important roles: it induces the eddy currents, and it senses the distortion of their flow caused by defects. Design of probe / sensor is an important task and a variety of aspects such as component geometry, impedance matching, magnetic field focusing, and environment etc. need to be considered for its design and development. In this contribution, some important aspects concerning probe design and development are covered.



Figure 1: Eddy Current Testing



TYPES OF EDDY CURRENT PROBES / SENSORS Design and development of eddy current probes is very important as it is the probe that dictates the probability of detection and the reliability of characterization. In general, defects that cause maximum perturbation of eddy currents are detected with high sensitivity. The shape, cross-section, size and configuration of coils are varied to design an eddy current probe for a specific application. Depending on the geometry of the component three types of eddy current probes viz. surface pancake, encircling and bobbin probes shown in Fig.2 are employed. The three types of probes can be operated in absolute, differential or send-receive modes. In absolute mode only one coil is used for exciting and sensing eddy currents. The differential probes with two coils usually wound in opposite direction, and the sendreceive probes with separate receiver coils, employ different bridge circuits. The absolute and differential modes exhibit different characteristics (Table.1) and selection depends, primarily on inspection requirement.



Figure 2: Types of Probes





Surface (Pancake) Probes Surface probe or Pancake probe, usually a spring mounted flat probe or a pointed pencil type probe, allows determining the exact location of a defect. The probe may be hand held, may be mounted on automated scanners or may even be rotated around to get e.g. a helical scan in tube/rod inspections. Surface probes possess directional properties i.e. regions of high and low sensitivity (Table.2). Usually ferrite cores (absolute cylindrical as well as splitD differential types) and shields are used for enhanced sensitivity and resolution. Besides ferrites, copper coils are used for shielding purpose. Surface probes are extensively used in aircraft inspection for crack detection in fastener holes and for detection of corrosion/exfoliation in hidden layers. When the component geometry is complex, it is not uncommon to use probe guides, shoes, centering-mechanisms to maintain uniform lift-off and detection sensitivity. Surface probes were developed for EC imaging, for measurement of liquid sodium level in steel tanks and also for measurement of thickness of coatings.



σº∙πμ■δ∝∞ωΩθ√ρβααδπ∠δ



Encircling Probes Encircling probes are used to inspect rods, tubes and wires. In an encircling probe the coil is in the form of a solenoid into which the component is placed. In this arrangement, the entire outside circumferential surface of the component covered by the coil is scanned at a time, giving high-inspection speeds. These probes may not detect circumferential defects (Table.2) as the edy currents flow parallel to them without getting distorted. Popular industrial application of encircling probes is high-speed inspection of tubes from outside during the manufacturing stages. Encircling probes were developed NDE of thin-walled cladding tubes and thick-walled steam generator magnetic tubes.



Bobbin Probes These probes are the most widely used ones in eddy current NDE. Bobbin probes consist of a coil arrangement in the form of a winding over a bobbin, which passes through components such as tubes and scans the entire inside surface in one-go. Popular application of bobbin probes is high-speed multifrequency inspection of heat exchanger tubes in-situ for detection of cracks, wall thinning and corrosion in tubes as well as under support plate regions. The directional properties of these probes are identical to encircling probes. In some instances, bobbin type probes are employed for inspection of bolt holes. For inspection of critical components, phased-array probes are slowly replacing the traditional bobbin/encircling probes as regards to detection and location of circumferential and short defects.



Design of eddy current probes In most EC instruments excitation current is kept constant (in a few tens of mA range) and the inductance may vary by a factor of one thousand. The usual input impedance could range between 20 and 200 ohms. The number of turns and wire gauge (between SWG 30 and SWG 45) are fixed such that the coils fill the available cross sectional space in uniform layers and turns per layer so that inter-winding effects are minimal. In some situations, it may be necessary to use a number of bridge circuits as well as probes operating simultaneously, essentially to cover larger area. For good sensitivity to small defects, small diameter probes are used. Similarly, in order to detect subsurface and buried defects, large diameter high throughput probes are necessary. As a general rule, the probe diameter should be less than or equal to the expected defect length and also comparable to the thickness of the component.



The sensing area of a probe is the physical diameter of the coil plus an extended area governed by magnetic field spread. Hence, it is common to use ferrite cores/shields (high permeability and low conductivity) to contain the lateral extent of magnetic fields without affecting the depth of penetration. It is essential to operate EC probes below the probe/cable resonance frequency, especially while using long probe cables and at very high frequencies. The probe bodies are usually made of non-conducting plastics. Wear of probes is normally be reduced by giving wear resistant coating to the probe heads or tips. It must be noted here that such coatings add to the builtin lift-off of probes and tend to reduce signal amplitudes. Temperature stability of probes is usually accomplished by using coil holder material with poor heat transfer characteristics. Most common commercial copper wires are used up to about 150º C. For temperatures above this, silver or aluminum wires with ceramic or high temperature silicon insulation or MIC are used. The probe material must be chemically compatible with the component.



In brief, probe design is usually done considering the following:           

Geometry of the component e.g. rod, tube, plate etc. Type of discontinuity expected e.g. fatigue cracks, conductivity variation etc. Likely location of defect e.g. surface, sub-surface. Coil impedance and its matching with the bridge circuit of the EC instrument. Frequency range of the probe i.e. for simultaneous multi-frequency excitation Inspection requirement e.g. detection, evaluation of length, depth etc. Material characteristics e.g. ferromagnetic or non-ferromagnetic. Coil response to a notch, drilled hole or other reference discontinuity. Field distribution in space and eddy current flow distribution in the material. Shape and dimensions of core, coil /coils and lift-off characteristics. Environmental characteristics such as wear, temperature and chemical attack.

As many factors need to be considered, three different approaches viz. experimental, analytical and numerical are often resorted to for designing eddy current probes.



Experimental Approach This approach usually involves trail and error fabrication of probes suiting the geometry. In this approach, the coil dimensions and the test frequency are usually optimized by comparing the detection sensitivity of artificial reference notches as well as natural cracks if available. This approach was used to design encircling EC probes for inspection of stainless steel cladding tubes of Fast Breeder Test Reactor (FBTR) and also to design probes for Cr-Mo steam generator tubes of Prototype Fast Breeder Reactors (PFBR). In another instance, in order to minimize low sensitivity zones of phased-array eddy current probes for inspection of heat exchanger tubes, tandem probe was developed.



Analytical Approach (Pc & fg) Analytical approaches for probe design involve analyzing the eddy current testing phenomenon and calculating the coil impedance and examining the operating point on the impedance plane as well as the effect of variations in coil radius r, shape, material conductivity, thickness t and test frequency f. Two popular impedance plane diagram based methods are 1) calculation of characteristic parameter, Pc introduced by Deeds and Doods for planar geometries and 2) calculation of characteristic frequency ratio f/fg, where fg is the characteristic frequency introduced by Förster for tubular geometries. (fg = 5066/ (μr∙ σ∙ d2) Using these two methods, coils are designed such that the operating point is in the “knee” region on the normalized impedance plane diagrams.



Numerical Approach Eddy current testing phenomenon can also be analyzed numerically using finite difference, finite element (FE), boundary element (BEM) and other methods. In this approach, coil and core dimensions are varied systematically and signals are predicted for a reference defect and the dimensions that result in maximum detection sensitivity are chosen. Not only signal amplitude, but phase angle from lift-off is also considered for decision making. A few typical applications of axis-symmetric FE model, are discussed elsewhere. In this model, the region consisting of EC probe and component, is discritised into triangular elements and variational principles are applied to compute the vector potential at the vertices of the elements. From the vector potential, the probe impedance is calculated and in turn, the impedance plane trajectories. This model has been used to optimize eddy current probes for location of garter springs in the coolant channels of Pressurized Heavy Water Reactors (PHWRs)



RECENT TRENDS IN EDDY CURRENT PROBE DESIGN Detection of cracks emanating from edges and corners of components is very important. Often, strong signal from the edges mask the small/weak signal from a potentially harmful crack. Focused surface probes are being explored and likewise appropriate signal processing methods are being incorporated to suppress edge contributions. In the case of heat exchanger tubes, rotating surface probes or array probes with multiplexing are preferred for detection and characterization of defects along the tube circumference (location). For detection of defects at roll joints special array probes are being tried. In order to inspect components with complex geometries, flexible probes are being tried. These probes can be mounted/scanned over a region for inspection purpose and be easily removed. Similarly, for detection of sub-surface and deep-seated defects in multi-layer and other structures eddy current probes are mounted and integrated with Hall probes, SQUID, GMR and AMR sensors. The main objective in these strategies is to detect the weak magnetic fields from defects, rather than the traditional impedance changes. When more than one sensors is used and data fusion methods are adopted to combine the sensors data to form a comprehensive global picture of investigated regions. At times, it may be beneficial to combine information of a single sensor, but operating at different frequencies to get enhanced information of defects. Such an approach has been used in an intelligent imaging scheme to obtain accurate and quick 3-dimensional pictures of defects. Inspection of ferromagnetic tubes is difficult due to high and varying magnetic permeability. For testing such tubes from outside, encircling D.C. saturation coils are used, where as remote field eddy current probes and permanent magnet based probes are used for testing from tube inside. Optimization of frequency and location of receiver coil (usually about 3 to 4 tube diameters away from exciter) in the remote field eddy current testing method is very important. FE model and experimental based approaches have been successfully used this purpose. When surface EC probes are scanned in a raster and the impedance data is displayed, Eddy current C-scan images of defects can be formed. EC images provide valuable information of defects. However, these images are blurred due to distributed point spread function of the probe. FE model based approach was used to optimize ferrite-core probes for eddy current imaging. In case of heat exchangers and steam generators, probes have to negotiate U-bend regions and detect defects, if any, in those regions. Design of flexible probes that are insensitive to bend regions is very challenging. For inspection of bend regions in ferromagnetic steam generator tubes, flexible remote field eddy current probe, with WC rings on either sides, was developed and wavelet transform based signal processing method was incorporated to suppress disturbing signals from bend regions.



Pressurized Heavy Water Reactors (PHWRs)


Pressurized Heavy Water Reactors (PHWRs)




The Answers


Q.5.1 What frequency is required to establish a standard depth of penetration of 7.6mm (0.1 in.) in Zirconium? A. 19.6kHz δ = K√ [ρ/(f)] B. 196Hz f = ρ(K/ δ)2 , ρZr= 40μΩ∙cm C. 3.4 kHz f = 40(50/7.6)2 = 1731 Hz D. 340Hz f = 40(1.98/.3)2 = 1742 Hz Q.5.2 To reduce effects of far surface indications, the test frequency: A must be mixed. B. must be raised. C. mnst be lowered. D. has no effect. Q.5.3 The frequency required to establish the Bessel function argument A equal to 1 is called: A an optimum frequency. B. a resonant frequency. C. a limit frequency. n. a penetration frequency.


Q.5.4 Calculate the limit frequency for a copper bar (σ = 50.6 meter I ohm mm2) 1 cm in diameter. The correct limit frequency is: A. 50kHz. fg= 5066/ (μr x σ x d2) B. 50.6Hz. fg = 5066/ (1x50.6x12) = 100Hz C. 100Hz. D. 100kHz. Q5.5 Using the example in Question 5.4, what is the f/fg ratio if the test frequency is 60kHz? A. 1.2 B. 120 c. 60 D. 600


Q.5.6 In Fig.5.1(b) the value of ωLsG equaling 1.4 would be indicative of A. a high resistivity metal. B. a high conductivity metals. C. a low conductivity metals. D. a nonconductor.


Q.5.7 Primary resistance is subtracted from Fig.5.1(b) because A. resistance is always constant. B. resistance is not frequency dependent. C. resistance is not added to impedance. D. none of the above. Q5.8 The reference quantity is different for solid cylinder and thin wall tube in Fig 5.2 because: A. the frequency is different. B. the conductivity is different. C. the skin effect is no longer negligible. D. the thin wall tube has not been normalized. Note: Both materials having the same conductivity. The thin wall tube will form a weaker eddy current thus weaker secondary flux that oppose the primary flux


Discussion: Subject: Q5.8 The reference quantity is different for solid cylinder and thin wall tube in Fig 5.2 because: A. the frequency is different. B. the conductivity is different. C. the skin effect is no longer negligible. D. the thin wall tube has not been normalized. Further normalization is accomplished by dividing the reactive and resistive components by the term ωLo or the primary inductive reactance without a secondary circuit present. Figure 5.2 shows a typical normalized impedance diagram. The terms ωL/ωLo and R/ωLo represent the relative impedance of the test coil as affected by the test object. Signals generated by changes in ωL or R caused by test object conditions such as surface and subsurface discontinuities may be noted by ∆ωL or ∆R. The ∆ωL and ∆R notation indicates a change in the impedance. k = √(ωμσ) = Electromagnetic wave propagation constant for conducting material r = radius of the conductor in m μ = magnetic permeability of bar = 4π∙10-7 H.m-1 if bar is non-magnetic (μ = μo) ω = angular velocity = 2πf √(ωLoG) = equivalent of √(ωμσ) for simplified electrical circuit, where G=conductance (Siemens) and Lo = inductance in air (Henry)


Q5.9 A 25% deep crack open to the near surface give a response ____ times greater than the same crack 3.3% of diameter under the surface (refer to Fig 5.4) A. 10 B. 3 C. 2 D. 5


Q5.10 When using multifrequency system, low subtractor frequencies are used to suppress: A. conductivity changes B. far surface signals C. near surface signal D. permeability changes Note: For suppression of inside or near surface signals, a higher subtractor frequency would be chosen. A combination of prime, low and high subtractor frequencies is often used to suppress both near and far surface signals, leaving only data pertaining to the part thickness and its condition. The subtractor frequency is actually the “detecting secondary” frequency which will be subtracted from the prime frequency signal.


Chapter 6 Instrument Systems



Eddy Current Instrumentation Most of the eddy current instrumentation is categorized by its final output or display mode. There are basic requirements common to all type of eddy current instruments. Five different elements are usually required to produce a viable eddy current instrumentThese function are excitation, modulation, signal preparation, signal analysis and signal display. An optional sixth component would be test object handling equipment. Figure 1 illustrated how these components interrelate. The generator provide excitation signals to the test coil. The signal modulation occurs in the electromagnetic field of the test coil assembly. Next, the signal preparation section, usually a balancing network, prepares the signal for demodulation and analysis. In the signal preparation stage, balance network are used to null out steady value alternating current signals. Amplifiers and filters are also part of this section to improve signal-to-noise ratio and raise signal levels for the subsequent demodulation and analysis stage.


Figure 6.1: Internal functions of the electromagnetic nondestructive test




Eddy current testing – Signal Balancing The demodulation and analysis section is made up of detectors, analyzers, discriminators, filters and sampling circuits. Detectors can be a simple amplitude type or a more sophisticated phase/amplitude or coherent type. The signal display section is the key link between the test equipment and its intended purpose. The signals generated can be displayed many different ways. The type of display or readout depends on the test requirements. In some tests, a simple GO/NO-GO indicator circuit may be all that is required. However some applications may require recording of 100% of all raw data generated during a test. This data may be imported into other digital devices that allow sophisticated data analysis or engineering statistics to be generated. One example of this is the inspection of large inservice nuclear components so that discontinuity growth can be monitored for determining potential failure rates or replacement cycles. Signal display processes will be discussed more in Chapter 7.


Chernobyl Nuclear Power Plant


Series of simple eddy current instruments is shown in Figure 6.2. In Figure 6.2(a), the voltage across the inspection coil is monitored by an alternating current voltmeter. This type of instrument could be used to measure large lift off variations where accuracy was not critical. Figure 6.2(b) shows an impedance bridge circuit. This instrument consists of an alternating current exciting source, dropping resistors and a balancing impedance. Figure 6.2(c) is similar to Figure 6.2(b). In Figure 6.2(c) a balance coil similar to the inspection coil is used to provide a balanced ridge. Figure 6.2(d) illustrates a balancing coil affected by a reference sample. This is commonly used in external reference differential coil tests. In all cases, because only the voltage change or magnitude is monitored, these systems can all be grouped as impedance magnitude types.


Figure 6.2: Four types of simple eddy current instruments


In Figure 6.2(a), the voltage across the inspection coil is monitored by an alternating current voltmeter. This type of instrument could be used to measure large lift off variations where accuracy was not critical.


Figure 6.2(b) shows an impedance bridge circuit. This instrument consists of an alternating current exciting source, dropping resistors and a balancing impedance.


Figure 6.2(c) is similar to Figure 6.2(b). In Figure 6.2(c) a balance coil similar to the inspection coil is used to provide a balanced ridge.


Figure 6.2(d) illustrates a balancing coil affected by a reference sample. This is commonly used in external reference differential coil tests.




Eddy current testing – Signal Analysis can be divided into three broad groups. The groups are impedance testing, phase analysis testing and modulation analysis testing. 1. Impedance testing is based on gross changes in coil impedance when the coil is placed near the test object. 2. Phase analysis testing is based on phase changes occurring in the test coil and the test object's effect on those phase changes. 3. Modulation analysis testing depends on the test object passing through the test coil's magnetic field at a constant feed rate or speed. These systems act like a tuned circuit. The operating frequency of the tester is changed (modulated) as a discontinuity passes through the test coil's field. The amount of modulation is a function of the transit time of the discontinuity through the coil's field. The faster the transit time the greater the modulation. If a system is set up for one speed and then the parts are scanned at a much slower speed the discontinuities may not be detected.


Modulation analysis testing depends on the test object passing through the test coil's magnetic field at a constant feed rate or speed. These systems act like a tuned circuit. The operating frequency of the tester is changed (modulated) as a discontinuity passes through the test coil's field. The amount of modulation is a function of the transit time of the discontinuity through the coil's field. The faster the transit time the greater the modulation. If a system is set up for one speed and then the parts are scanned at a much slower speed the discontinuities may not be detected.


Impedance Testing With impedance magnitude instrumentation it is often difficult to separate desired responses, such as changes in conductivity or permeability, from dimensional changes. A variation of the impedance magnitude technique is the reactance magnitude instrument. In reactance magnitude tests, the test coil is part of the fundamental frequency oscillator circuit. This operates like a tuned circuit where the oscillator frequency is determined by the test coil's inductive reactance. As the test coil is affected by the test object, its inductive reactance changes, which in turn changes the oscillator frequency. The relative frequency variation ∆f/f is, therefore, an indication of test object condition. Reactance magnitude systems have many of the same limitations as impedance magnitude systems.


Phase Analysis Testing – Earlier Test System Phase analysis processes can be divided into many subgroup depending on the type of display. Some of the earlier test system output were called vector point, ellipse and linear time base.


Phase Analysis Testing - Vector Point The vector point display would simply be a point of light on an analog cathode ray tube (Figure 6.3), The point is the vector sum of Y and X axis voltages present in test coil. By proper selection of frequency and phase adjustment a response in the vertical plan might represent dimensional changes (magnetic permeability’s factor) while voltage shift inthe horizontal plane could represent change in conductivity.


Phase Analysis Testing - Ellipse As with the vector point techniques, the test object and reference standard are used provide a balanced output. A normal balanced output is a straight horizontal line. Fig 6.4 shown typical ellipse responses. Figure 6.4: Cathode ray tube displays for dimension and conductivity


Phase Analysis Testing - Ellipse As with the vector point techniques, the test object and reference standard are used provide a balanced output. A normal balanced output is a straight horizontal line. Fig 6.4 shown typical ellipse responses. Figure 6.4: Cathode ray tube displays for dimension and conductivity


Figure 6.4: Cathode ray tube displays for dimension and conductivity


Figure 6.4: Cathode ray tube displays for dimension and conductivity


Phase Analysis Testing - Linear Time Base An early test system that was better suited to compensate for harmonic distortions present in the fundamental waveform used the linear time base technique. The linear time base unit applies a sawtooth shaped voltage to the horizontal deflection plates of a CRT. This provides a linear trace of the CRT beam from left to right across the CRT screen. The timing of the linear trace function is set to same value as the alternating current energy applied to the coil. This allows one complete cycle of the sine wave voltage applied to the coil to appear on the CRT. Figure 6.5 illustrates a linear time base display. A slit or narrow vertical scale is provided to measure the amplitude of signals present in the slit. The base voltage is normally adjusted to cross the slit at 0 volts, the 180 degree point on the sinewave.

The slit value M is used to analyze results. The slit value M is described by the equation: M = A sin θ Where: M = slit value A =amplitude of the measurement in the slit θ = angle between base angle and measurement effect.

In Figure 6.5 the angle difference A to B is about 90 degrees.


Figure 6.5: Screen image of a linear time base instrument with sinusoidal signals

M = A sin θ M = slit value A =amplitude of the measurement in the slit θ = angle between base angle and measurement effect.

θ= 90º


Presence Method – The Impedance Plane Testing The three tester types that have been defined so far (vector point ellipse and linear time base) were early attempts to correlate electromagnetic changes detected by a test system with material variables. The circuits that they used were fairly primitive by today's standards. These techniques were limited by the level of technology available at the time they were built. They were not very sensitive to small changes in materials and could not readily display small variations in the signal changesthat they did detect. As the field of electronics advanced, more sophisticated components became available. In today's marketplace many eddy current test systems have the capability to display data in multiple modes. The classic X-Y type display mode is a simple way of showing what is meant by an impedance plane test system. In Chapter 4 impedance plane diagrams were defined. These graphs and curves allow technicians to look at complex sets of information for a number of test variables simultaneously. Test systems that provide the ability to view both the direction (phase) and amplitude (voltage) of the voltage shift across an inspection coil provide much greater detail than the early model test systems that were looked at in this chapter. These modern systems give the ability to sort or measure material parameters with a much higher degree of accuracy. Some impedance measurement systems may only display part of the information derived (meterbased technology) but most use a two-dimensional output device.


Impedance Plane Testing - Mode of Operation Test instruments may also be classified by their mode of operation. The mode of operation is determined by two functional areas within the instrument. 1. The first functional consideration might be the degree of compensation, or nulling, and the type of detector used. 2. The second consideration is the method of test coil excitation. The types of excitation include single frequency or multifrequency sinusoidal, single or repetitive pulse and swept frequency.


Impedance Plane Testing - Signal Compensation Mode 1. Null balance with amplitude detector Mode 2. Null balance with amplitude phase detectors, (Figure 6.6) and Mode 3. Selected off null balance with amplitude detector. Mode 1 responds to any signal irrespective of phase angle. These would typically be meter-based instrumentation capable of showing only the voltage change or amplitude of the signal of interest. Mode 2, using amplitude and phase detectors, can be used to discriminate against signals having a particular phase angle. With this type of system, the total demodulated signal can be displayed in an X-Y screen presentation format to show both amplitude and phase relationships. A classic example of the advantage of this XY screen presentation in surface scanning applications is to put lift off responses on the horizontal with discontinuities responding up on the screen. Mode 3 systems are phase sensitive systems although they have only amplitude detector. They achieve phase sensitivity by operating in a manually selected off balanced condition. Based on this selection, the off null signal change can be set so that it may appear larger than the inherent impedance change due to test object variables. Charlie Chong/ Fion Zhang

Figure 6.6: Null balance instrument with amplitude phase detectors

A classic example of the advantage of this X-Y screen presentation in surface scanning applications is to put lift off responses on the horizontal with discontinuities responding up on the screen.


Test Coil Excitation The second consideration that was previously mentioned for determining the mode of operation of a test unit could be the way the probe is being energized. Figure 6.7 a typical surface riding pancake coil responses to an array of EDM notches on a calibration standard. Fig 6.8 shows a block diagram of a stepped, single frequency, phase amplitude instrument. The circuit in Fig 6.8 is capable of operating at any of the four frequency, if the four frequencies are spread over a wide range, several different test coils may be required to use the instrument over the entire range. Most modern single frequency instruments use this principle; however one variable frequency generator with a wide operating range usually replaced the four individual fixed generators. A typical frequency for such an instrument is in the low hertz range (50Hz to 100Hz) to several megahertz (8MHz to 10MHz). This large dynamic range gives these units a wide variety of possible applications.


Figure 6.7: Typical surface riding pancake coil response to an array of EDM notches on a calibration standard


Figure 6.8: Single frequency selectable instrument


For deep subsurface crack detection (more than 5mm) the lower frequency would be required. This test might be performed with hybrid (driver/pick-up) coil to improved detection of the low amplitude responses from smaller discontinuities deeper in a product. For detection of very small stress or fatigue crack in a near surface inspection process the higher frequency range could improve sensitivity to smaller cracks. The compromise at very high frequencies is the issue of skin effect or surface noise. Special probe or scanning process may be required for this type of test also. Figure 6.9 shows a block diagram for a multifrequency instrument operating at three frequencies simultaneously. In modern systems this is referred to as simultaneous injection. This diagram shows three dedicated frequency modules but more recent adaptations use multiple variable frequency circuits. In Figure 6.9, excitation currents at each selected frequency are impressed across the coil at the same time. You will recall from earlier chapters that the electromagnetic envelope around an alternating current driven coil is very dynamic. It is very difficult to model what the combined electromagnetic flux pattern would look like with more than one frequency affecting the coil at a given moment in time.


Figure 6.9: Multifrequency instrument operating at three frequencies simultaneously


Multiple circuits are used throughout the instrument. The test coil output carrier frequencies are separated by filters. Multiple dual phase amplitude detectors are used and their outputs summed to provide separation of several test object parameters. A system similar to this is described in Inservice Inspection of Steam Generator Tubing Using Multiple Frequency Eddy Current Techniques. Another approach to the multifrequency technique uses a sequential coil drive called multiplexing. The frequencies are changed in a step-by-step sequence with such rapidity that the test parameters remain unchanged. The multiplex technique has the advantages of lower cost, continuously variable frequencies and little or no crosstalk between channels. Figure 6.10 illustrates a multifrequency instrument capable of generating up to 16 channels of data sequentially. Each channel or time slot may be adjusted over a wide range of frequencies. In addition, this digital system provides for the creation of mixed channel combinations for suppression of unwanted test variables. Results of such suppression are described in Multifrequency Eddy Current Method and the Separation of Test Specimen Variables . Charlie Chong/ Fion Zhang

Figure 6.10: Commercial multifrequency instrument


This type of digital instrumentation allows all of the test setup parameters to be stored to either internal or external storage media. This allows preprogrammed test setups to be recalled and used by semi-skilled personnel. Systems can be created with programs having supervisory code interlocks that prevent reprogramming by other than authorized personnel. These instruments can also interface with robotic or computer-based systems for both process control and raw data recording purposes. A test system using pulsed excitation is shown in Figure 6.11. A pulse is applied to the test coil, compensating networks and analyzers simultaneously. Systems having analyzers with one or two sampling points perform similar to a single frequency tester using sinusoidal excitation. Pulsed eddy current systems having multiple sampling points perform more like the multifrequency tester shown in Figure 6.10.


Figure 6.11: Pulsed waveform excitation




Read Out Mechanisms Eddy current test data may be displayed or indicated in a variety of ways. The type of display or readout depends on the test requirements. Some common readout mechanisms are indicator lights, audio alarms, meters, digital displays, CRTs, recorders and computer interfaces.

Read Out Mechanisms - Indicating Light A simple use of the indicating light is to monitor the eddy current signal amplitude with an amplitude gate circuit, When the signal reaches a preset amplitude limit, the amplitude gate switches a relay that applied power to an indicator light or automatic sorting device. With the amplitude gate circuit, high-low limits could be preset to give GO/NO-GO indications.


Read Out Mechanisms - Audio Alarms Audio alarm can be as much same as the indicator light. The alarm gives qualitative indication without giving any quantitative information.


Read Out Mechanisms - Meter Display The test information generated by any analog system can be processed through an analog-to-digital converter if additional signal processing is required. Meter-based technology signal responses fall into one of two categories: either quantitative or qualitative. One example of a quantitative meter response would be a system used for measuring conductivity (Figure 6.12). When the needle deflects and reaches a specific point on the scale the number indicated on the scale should correlate to a specific percent IACS value if the system has been properly set up. Some meter-based devices (Figure 6.13) that might be used for simple discontinuity detection do not give the operator a numerical value other than a percent of full scale. A given crack could generate either a small amplitude voltage at a low gain e setting or a larger amplitude response at a higher gain setting. This would be a qualitative type response. These systems are not used for discontinuity sizing.


An qualitative meter response could be used in a test situation where a minimum discontinuity amplitude response can be accurately defined. This might be an EDM ,, notch of a specified depth in a calibration block. As long as the meter stays below the preset voltage level from the selected discontinuity then the sample is acceptable. If that voltage level is exceeded then the part is deemed unacceptable. In some online inspections, this type of voltage threshold or gate is used to rapidly sort or grade materials. The use of these types of output displays should be limited to applications where a qualitative value or discontinuity threshold can be established and would be acceptable to meet test criteria.


Figure 6.12: A quantitative meter response indicating a specific conductivity (in percent of the IACS)


Figure 6.13: A qualitative analog meter response showing only percent of full scale


Read Out Mechanisms - Digital Displays Numerical digital displays can also be used to provide qualitative information. These might have several applications but the most common would be for measuring conductivity values.


Read Out Mechanisms - Cathode Ray Tubes Cathode ray tubes, or CRT type displays, play an important role in the display of eddy current information. In more recent times many eddy cm-rent systems have become available with digital representations of CRT type screens. In the original analog system there were three main elements: the electron gtm, the deflection plates and a fluorescent screen. The electron gun would generate, focus and direct the electron beam toward the face or screen of the CRT. The deflection plates were sih1ated between the electron gun and he screen, arranged in two pairs, usually called horizontal and vertical or X and Y. The plane of one pair would be perpendicular to the other pair. The screen is the imaging portion of the CRT. The screen consists of a coating or coatings that produce photochemical reactions when struck by the electron beam. The photochemical action appears in two stages. Fluorescence occurs as the electron beam strikes the screen. Phosphorescence is the chemical process that allows the screen to continue to give off light after the electron beam has been removed or has passed over a section of the screen. All analog CRT screen materials possess both fluorescence and phosphorescence.


The duration of the photochemical effect is called persistence. Persistence can be grouped as either low, medium ol' high persistence. To display repetitive signals, a low or medium persistence CRT may have been used. To display noru:ecurxent or single events, a high persistence CRT would have been used. Many modern digital cathode ray tube type systems are available. Because analog CRTs are no longer manufactured, those systems are being replaced with other options digital system provide the additional flexibility for the selection of various color and contrastconditions (Figure 6.14). This allows the operator a thoice otcolor options that can be established on the swnesystem to compensate for use :in different lighting conditions. Because the data are outpuftotlte screen in a digital format varying persistence values can be selected by defining the timing factor of a rolling data buffer or memory. This selection process allows the operators to choose how long the digital images created stay on the screen for viewing.


Figure 6.14: Numerical readouts/digital conductivity tester


Read Out Mechanisms - Recorders Data recorders might be required to meet the inspection criteria. Recording is sometimes accomplished on analog paper strip charts or on magnetic tape formats. With most modern equipment providing recording capability some form of digital media would be used. The data could be stored internally in some test systems, but more often than not the data are exported to an external storage device. Most of these digital recording media can retain the files created for offline analysis and long term historical use. Early digital systems were write once - read many devices. The more recent recording media can be erased and reused. The advantage of digital systems is that all of the raw data created by a multifrequency test system can be viewed in multiple display formats at the same time. Tubing exam data are often reviewed using both the X-Y and strip chart modes to optimize discontinuity detection and sizing. The s trip chart format is often used where the discontinuity's location down the length of a rod or tube is critical. The strip chart length is indexed to time or distance and signal response deviation from the baseline indicates various material conditions. The amplitude of the X-Y lissajous response in Figure 6.15 (6.66 V) is an 1nd1cat01' of the volume of the discontinuity. The phase angle with respect to the/( axis (114 degrees) represents discontinuity depthl(in this case, 41%) and discontinuity origin (tube outside diameter), indicating whether the discontinuity originated on the inside or outside surface of the tube (13). Many comp uter-based systems have multiple display modes available for the same raw data set.


Read Out Mechanisms - Computer Eddy current testing may be display with the used of computer. The electronic components and connectors that are linked to a remote computer via a local area network (LAN) cable. The computer itself handles data display and processing functions as well as adjusting tester operating parameters, such as frequency, gain, probe drive voltage and mode of operation, etc. Figure 6.16 shows a multimode output responses of a rotating pancake coil inspection in a bolt hole application. The same crack response can be seen in all four display formats.


Figure 6.16: Multimode output responses: rotating pancake coil inspection in a bolthple application. The same crack response can be seen in all four display formats.




Test Object Handling Equipment Test object handling equipment is often a necessary component of an online test system. Bars and tubes can be fed through encircling coils by means of roller feed assemblies. Consistent centering of the material is essential. The stock being fed through the coils is usually transported at a constant speed. The transport speed needs to be adjusted to allow adequate time for testing and for the reject, cutting or marking systems to perform their tasks. Should product centering or speed change during the examination system performance could be limited. Automatic sorting devices are very common in online inspection systems used in a manufacturing environment. When a volumetric test is required for heat treatment or hardness verification the probe assembly may interrogate the entire test specimen (or some critical region of the specimen) in one view. For small specimens like ball bearings this could take just fractions of a second per sample. In larger specimens the volumetric test may take a few seconds per sample. When crack detection is required the part is normally rotated with one or more coils positioned near the surface of the specimen. This type of inspection ensures 100% inspection of critical areas in one test The eddy current technique can often demonstrate much higher discontinuity sensitivity and more rapid economical testing for surface discontinuities in parts than any of the other nondestructive testing processes. If unacceptable material conditions were encountered at any inspection station the part would be dropped into rejection bin. A digital counter and or remote sensor can be used to track the number of rejection and alert the plant staff on the manufacturing processes.


Probe Delivery System Instead of moving the part through an inspection station there are situation where motorized probe delivery system is used. These are normally employed outside of the of the manufacturing environment to perform in situ inspection on existing materials. The term spinning probes originally comes from the pipe manufacturing environment. The coil is typically a fairly small specialized coil to improve detection potential for small cracks. A probe was rotated around the circumference of a tube or bar. The tested material was moved past the inspection point at a controlled rate of speed. The probe rotational speeds would have been set to be compatible with instrument response and translation speeds to obtain the desired inspection coverage and test sensitivity.


As technology has been improved it has been possible to create other types of spinning probe possibilities. There are now many situations where spinning probes can be used. High speed probe guns are used to perform bolt hole inspection after removal of fastener in aerospace structures. For heat exchanger inspection, Tubes to be inspected, are identified and their coordinates are loaded into a database. Positive feedback is supplied to computerized positioning system by encoder or digital pattern recognition routines. Although these systems are quite automated, visual. Verification of the inspection is confirmed by an inspector via a remote video system. As the probe is inserted and withdrawn from each tube the test results are monitored in real time for data quality but the data are also recorded for later analysis. Remotely Operated Vehicles (ROV) can also be looked at as part of the array of technology to enhance eddy current systems in hostile environments. These electromechanical devices can be used to perform a wide array of nondestructive testing tasks. This could include applications for underwater eddy current array probe inspection of welds in either piping or support structures for offshore platforms.




Answers


Q.6.1 Signal preparation is usually accomplished by: A. detectors. B. samplers. C. balance networks. D. discriminators. Q.6.2 Most eddy current instruments have ______ coil excitation. A. square wave B. triangular wave C. sine wave D. sawtooth wave Q.6.3 Eddy current systems can be grouped by: A. output characteristics. B. excitation mode. C. phase analysis extent. D. both A and B.


Q.6.4 A multifrequency instrument that excites the test coil with several requencies sequentially uses the ______ concept. A. multiplexing B. time base C. broadband D. cartesian Q.6.5 Reject limits should always be adjusted to: A. one-half the screen height. B. 5 volts. C. ensure unacceptable components are properly identified. D. reduce operator training costs. Q.6.6 Display requirements are based on: A. test applications. B. records requirement. C. need for automatic control. D. all of the above.


Q.6.7 Amplitude gates provide a technique of controlling: A. reject or acceptance limits. Note: this is different from UT equipment B. instrument response. “gate” where area of interest is high lighted to display the necessary information, like C. display amplitude. depth, %FSH etc. D. All of the above. Q.6.8 Alarms and lights offer only: A. qualitative information. B. quantitative information. C. reject information. D. accept information. Q.6.9 The length of a strip chart presentation can indicate: A. discontinuity severity. B. distance or time. C. orthogonality. D. all of the above.


Q.6.10 A top view display of the test results from a specimen can be referred to as: A. an X-Y display. B. a C scan. C. a crosshatch presentations


Chapter 7 Eddy Current Applications


Eddy Current Applications Electromagnetic induction and the eddy current principle can be affected in many different ways. These effects may be grouped by discontinuity detection, measurement of material properties, dimensional measurements and other special applications. With the discontinuity or the detection group, we are concerned with locating cracks, corrosion, erosion and mechanical damage. The material properties group includes measurements of . conductivity, permeability, hardness, alloy sorting or chemical composition and degree of heat treatment. Dimensional measurements commonly made are thickness, profilometry, spacing or location and coating or cladding thickness. Special applications include measurements of temperature, flow metering of liquid metals, sonic vibrations and anisotropic conditions. Regardless of the specific application, once the test system has been properly calibrated there should not be any fundamental changes made to it during the testing process. If it has been determined that the instrument has been set up incorrectly or is not working as specified in the operational procedures being used, all material should be retested since the last time the correct setup and proper system operation was verified.


Discontinuity Detection The theoretical response to discontinuities has been discussed in previous chapters of this guide. In this chapter, some actual examples are given to enhance the understanding of the applied theory. A problem common to the chemical and electric power industries is the corrosion of heat exchanger tubing. This tubing is installed in closed vessels in a high density array. It is not uncommon for a nuclear steam generator or main condenser to contain many thousands of tubes. This high density and limited access to the inspection areas often precludes the use of other nondestructive testing methods. A bobbin coil inspection provides a volumetric inspection of the tube wall in a cost effective process. Heat exchanger inspection systems and results are described by Libby, Dodd, Sagar and Davis.


Heat Exchanger


Heat Exchanger


Heat Exchanger


Heat Exchanger


Heat Exchanger


Heat Exchanger


Heat Exchanger


Heat Exchanger


Figure 7.1: American Society of Mechanical Engineers (ASME) thin-walled tubing standard


Heat Exchanger TSP


Heat Exchanger TSP


Heat Exchanger TSP


Phase angle and amplitude relationships are usually established by using reference standards with artificial discontinuities of known and documented values. These discontinuities should reflect expected damage modes as close as possible. In most thin-walled tubing cases the severity of the discontinuity can be determined by analyzing the eddy current signal phase and/ or amplitude. The phase angle of small volume discontinuities (cracks, pits) is used to establish a phase-to-depth calibration curve (Figure 7.2) and to verify the originating surface (inside diameter or outside diameter) of that discontinuity. The signal amplitude is an indicator of discontinuity volume. For volumetric tube wall loss conditions such as wear and fretting, a volts-todepth calibration curve can be created (Figure 7.3). When used properly, these curves will provide a more accurate sizing process for mechanically driven discontinuity mechanisms.


Figure 7.2: Phase-to-depth calibration curve


Figure 7.3: Volts-to-depth calibration curve


The geometry of real discontinuities may differ from reference standard discontinuities. This difference produces interpretation errors as discussed by Sagar. Placement of real discontinuities near tube support members causing a complex coil impedance change is also a source of error. This, of course, is dependent on the size of the discontinuity and its resultant eddy current signal in relation to the tube support signal. This follows the basic principle of signal-tonoise ratio. The signal-to-noise ratio can be improved at tube-to-tube support intersections by the use of multifrequency techniques. In multifrequency applications, an optimum (or prime) frequency is chosen for response to discontinuities within the tube wall. A lower than optimum or suppression frequency (subtractor frequency?) is chosen for response to the tube support. The two signals are processed through comparator circuits called mixers where the tube support response is subtracted from the tube wall response signal, leaving only the response to the tube wall discontinuity. (See Figures 7.4 and 7.5.) Both channels must be able to detect both the discontinuity and the noise source that is being suppressed. Another market sector that uses eddy current testing extensively is the aerospace industry. Many eddy current examinations are conducted on engine and airframe structures.


Figure 7.4: A multifrequency application without discontinuity A: The nominal response to a tube support plate at the prime frequency. B: The nominal response to a tube support plate at the subtractor frequency. A-B: The mixer channel residual response after support plate suppression.


Figure 7.5: A multifrequency application with discontinuity A: The response to a tube support plate with a discontinuity at the prime frequency. B: The response to a tube support plate with a discontinuity at the subtractor frequency. A-8: The mixer channel response to the discontinuity after support plate suppression.


A common problem with turbines is fatigue cracking of the compressor blades or disks in the root areas. Given the potential safety risks if these components fail, the inspection criteria thresholds are set to detect extremely small artifacts. Special probe designs and inspection techniques are required to deal with the difficult sample geometries and small discontinuity detection limits. Many other aircraft inspections are designed to deal with cracking or corrosion processes that may not lead to immediate catastrophic failures but that do need to be handled in a timely manner. Portable inspection devices are often used to perform these tests. Careful test system calibration using appropriate procedures and reference specimens is required to maintain aircraft fleet serviceability.


Aerospace Applications


Aerospace Applications


The reference specimen and its associated discontinuities are very critical to the success of the test. Often models are constructed with artificial discontinuities that are exact duplicates of the item being inspected. Field degraded specimens are also used to verify test discontinuity sensitivity. D.J. Hagemaier discussed low frequency eddy current inspection of aircraft structures for subsurface discontinuity detection in an article published in Materials Evaluation in 1982. A low frequency (100Hz to 1000Hz) technique can be used to locate cracks in thick or multiple layer, bolted or riveted aircraft structures. Again, models are constructed with artificial cracks and their responses are compared to responses in the actual test object. Most of these examinations are performed using single or multifrequency sinusoidal alternating current processes. Pulsed eddy current systems, if available, might also be used for crack detection in thick structures.


Pulsed Eddy Currents Systems

Learn more on Pulsed Eddy Currents Systems


Dimensional Measurements Dimensional measurements, such as thickness, shape and position, or proximity of one item to another, are important uses of the eddy cunent technique. Materials are often clad with other materials to present a resistance to chemicals or to provide wear resistance. Cladding or plating thickness then becomes an important variable to the serviceability of the unit. For nonconductive coatings on conductive bases, the probe-to-specimen spacing, or lift off technique can be applied. The case of conductive plating or cladding on conductive bases requires more refinement. The thickness loci respond in a complex manner on the impedance plane. The loci fm multilayered objects with each layer consisting of a material with a different conductivity follow a spiral pattern. In certain cases, two frequency or multifrequency systems are used to stabilize results or minimize lift off variations on the thickness measurement.


Figure 7.6 shows a single frequency hardness tester output presentation. The depth of case hardening can be determined by measuring the nitride case thickness in stainless steel. The nitride case thickness produces magnetic permeability variations. The thicker the nitride layer the greater the permeability. The coil's inductive reactance increases with a permeability increase. This variable is carefully monitored and correlated to actual metallographic results. Eddy current profilometry is another common way to measure dimensions. One example is the measurement of the inside diameters of tubes using a lift oft technique. For this measurement, several small pancake coils are mounted radially in a coil form. The coil form is inserted into the tube and each coil's proximity to the tube wall is monitored. The resultant output of each coil can provide detailed information about the concentricity of the tube. This is especially useful when the amount of tube wall deformation due to either manufacturing or operational conditions may require corrective action. An obvious problem encmmtered with this technique is centering of the coil holder assembly. The center of the coil holder must be near the center of the tube. When inspecting for localized dimensional changes, a long coil holder is effective in maintaining proper centering. Another function o£ the long coil form is to keep the coils from becoming tilted in the tube. This also requires higher probe fill factors than might normally be used during other types of tube inspections.


Figure 7.6: A single frequency hardness tester output presentation


Eddy Current Profilometry is another common way to measure dimensions. One example is the measurement of the inside diameters of tubes using a lift oft technique. For this measurement, several small pancake coils are mounted radially in a coil form. The coil form is inserted into the tube and each coil's proximity to the tube wall is monitored.


Conductivity Measurements Conductivity is an important measured variable. In the aircraft industry, aluminum is used extensively. Aluminum conductivity varies not only with aHoy but also with hardness and tensile strength. Eddy current instruments scaled in percent IACS are normally used to inspect for conductivity variations. Secondary conductivity standards are commonly used to check instrument calibration. Common secondary conductivity standards range from 8% IACS to about 100% IACS. The secondary standards are usually certified accurate to within ±0.35% or ±1% of value, whichever is less. Temperature is an important variable when making conductivity measurements. Most instrument and standards are certified at 20°C. Primary conductivity standards are maintained at a constant temperature by oil bath systems. Primary standards are measured with precision maxwell bridge type instruments. This circuit design increases measurement accuracy and minimizes frequency dependence of the measurement. The secondary standards used for field tester set up and calibration are often required to have their listed values recertified on an annual basis.


Hardness Measurements (Conductivity variable?) Hardness measurements can be performed on both ferritic and nonferritic materials. Some hardness measurements are performed with a two coil comparative process but this is not a strict requirement. When using a two- oil system the reference and test coils are both balanced with sample parts of known hardness. As parts of unknown hardness affect the test coil, the instrument output (impedance) varies. The amount of output variation depends on the degree of imbalance created by the unknown test object hardness. The detected signal variations can be correlated to test object hardness. If an X-Y type display were to be used to display this hardness information, the specimens exhibiting an acceptable hardness could be adjusted to one region of the screen while those specimens defined as unacceptable, or unhardened, could appear in a different region of the screen. Once this calibration process is completed a high-speed automated system can be allowed to make the measurements using an alarm gate process.


Alloy Sorting Alloy sorting can also be accomplished with a two coil comparator bridge process but again it is not a strict requirement. Other types of coil arrangements may also provide useful information. The key element to keep in mind with alloy sorting is that this is not the same as material identification. Two very different materials may provide the same load to the coil. Alloy sorting using electromagnetic must be verified with the additional verification of the mechanical properties of these materials. In the inspection of nonferromagnetic alloys it is easiest to separate one alloy or heat treat type from another when there is a unique range of conductivities associated with each material. This is not always the case within families of alloys. Different alloys and heat treats of the alurninum family may have the same conductivity value. This could lead to misidentification of the materials being inspected.


All comparative tests will be strongly influenced by the selection of correct and accurate reference specimens. Because most eddy current instruments respond to a wide range of variables, the reference specimen parameters must be controlled carefully. Test object and reference specimens must be the same or very similar in the following characteristics: 1. geometry, 2. heat treatment, 3. surface finish, 4. residual stresses, 5. metallurgical structure.


In addition, it is advisable to have more than one reference specimen for backup in case of loss or damage. In the case of steel parts, they should be completely demagnetized information .With the right equipment, probes, techniques and to remove the effects of residual magnetism on instrument readings. As in most comparative tests, temperature of specimen and test object should be the same or compensated. Many other measurements can be made using eddy current techniques. The electromagnetic technique produces so much information about a material that its application is only limited by the ability to decipher this training, the experienced operator should be capable of making the required distinctions between relevant and nonrelevant indications.




Answers


Q.7.1 Conductivity, hardness and composition are part of the group. A. discontinuity detection B. material properties C. dimensional D. special Q.7.2 Using an inside diameter coil on tubing and applying the phase I amplitude technique of inspection, a signal appearing at 90 degrees on a CRT would be caused by: A. inside diameter discontinuity. B. outside diameter discontinuity. C. a dent. D. a bulge. Q.7.3 Discont:inuities in heat exchangers at tube support locations are easier to detect because the support plate concentrates the electromagnetic field at that point. A. True B. False Charlie Chong/ Fion Zhang

Q.7.4 Using multifrequency techniques on installed heat exchanger tubing, a tube support plate signal can be suppressed by subtracting a ____ frequency signal from the optimum frequency signal. A. low B. high C. A orB D. None of the above. Q.7.5 In the aircraft industry, a common problem in gas turbine engines is: A. corrosion. B. fatigue cracking. C. vibration damage. D. erosion.


Q.7.6 Subsurface discontinuities located in thick or multilayered aircraft structures could be detected by: A. low frequency sinusoidal continuous wave instruments. B. high frequency sinusoidal continuous wave instruments. C. pulsed systems. D. A or C. Answer to a Mistake – a mistake make is a good lesson learned. D.J. Hagemaier discussed low frequency eddy current inspection of aircraft structures for subsurface discontinuity detection in an article published in Materials Evaluation in 1982. A low frequency (100Hz to 1000Hz) technique can be used to locate cracks in thick or multiple layer, bolted or riveted aircraft structures. Again, models are constructed with artificial cracks and their responses are compared to responses in the actual test object. Most of these examinations are performed using single or multifrequency sinusoidal alternating current processes. Pulsed eddy current systems, if available, might also be used for crack detection in thick structures.


Q.7.7 Response to multilayer varying conductivity structures follow _____ loci. A. orthogonal B. spiral C. linear D. stepped Q.7.8 Nitride case thickness variations can be detected in stainless steel (μr≈ 1) cylinders by measuring: Answer to mistake: The depth of case hardening can be determined by measuring the nitride case thickness in stainless steel. The nitride case A. conductivity. thickness produces magnetic permeability variations. The thicker the nitride layer the greater the permeability. B. dimensions. Question: Both conductivity & permeability count and the weighted C. permeability. significant dictated the prime factor? In this case the permeability effect dominates. D. none of the above. Q.7.9 Conductivity is not affected by temperature. A. True B. False


Q.7.10 Residual stresses in the test part produce such a small effect that they are usually ignored when selecting reference specimens. A. True B. False


Chapter 8 Other Electromagnetic Techniques


Electromagnetic Testing Eddy current testing is just one of a group of teclmiques that as a whole are defined as the electromagnetic testing method. The sub disciplines or teclmiques listed within the method continue to expand. Following are the techniques that fall under this method at the time of publication: Method: Electromagnetic Testing Techniques:    

alternating current field measurement, ACFM eddy current testing, ECT flux leakage testing, MFLT remote field testing, RFT

The borders are sometimes a little gray between one process and another. These techniques have been grouped in this fashion more on the basis of their specific market area or specialized applications in the field testing envirorunent rather than on a purely scientific basis. Electromagnetics is a very broad term. It covers a wide range of energy levels, sources and measurement tools.


Some other technologies that have been suggested to be included in electromagnetic testing are:    

microwave systems, superconducting quantum interference devices, magneto-optical inspection devices, flux leakage testing*. (*Now accepted as a stand -alone method for tank floor, wire rope, and down-hole pipe inspection work.)


The ASNT Electromagnetics Committee, at the time of this revision, has selected the first four teclmiques because they are currently available and fairly well established to perform specific nondestructive testing inspections in the field. In this chapter the generic differences between these techniques will be explained. Eddy current testing is most commonly used for detection of surface or near surface discontinuities in nonferromagnetic materials. In materials with little or no permeability eddy current testing is effective to about 5.08 mm (0.2 in.) below the test surface. For material thicknesses of greater than 5.08 mm (0.2 in.) special probes and/ or electronics packages are needed to improve the performance of eddy current testing. Although there are applications for eddy current tests on ferritic materials, eddy current has no ability to provide subsurface discontinuity detection in ferromagnetic alloys. Surface crack detection in ferromagnetic materials, especially for weld inspection, is a very viable eddy current process when the right technology is applied. Eddy current is often more sensitive and more cost effective than either magnetic particle inspection or penetrant inspection in this role.


Alternating current field measurement, flux leakage testing and remote field testing are all special electromagnetics testing teclmiques that, if used properly, can provide useful non destructive testing information about ferromagnetic components. The deciding factor of one over the other is the type of material, part size or geometry and the type and size of discontinuities that need to be detected. There is no reason to believe that any of these three techniques would show any significant advantage over eddy current in the nonferromagnetic world except for material thicknesses over 5.08 mm (0.2in.), where remote field testing may be used to provide enhanced sensitivity to outside diameter discontinuities. Manufacturers and users will debate the various capabilities of one of these techniques over another. The following discussion will be made as generic as possible. Note: ACFM does not need magnetic saturation for ferromagnetic materials, unlike ECT. RFT more sensitive to outer surface discontinuities detection than ECT.


Alternating Current Field Measurement Primary application: Inspection of weldments Power source: Alternating current Advantages Compared to Magnetic Particle and Dye Penetrant Inspection  Works through nonconductive coatings [up to 10 mm (0.4 in.) thick] so there is no need to remove and then reapply paint or to clean off rust.  Provides information on depth as well as length, saving time on removing discontinuities of insignificant depth.  Relatively insensitive to material property changes, so it is ideal for inspecting at welds. (permeability μ, thus no need to saturated the specimen & conductivity σ)  Relatively insensitive to probe lift off, allowing deployment through coatings and on rough surfaces.  Allows depth sizing of discontinuities up to about 25 mm (1 in.), depending on probe type.


Figure 8.1 shows the basic principles of the technique. With no discontinuity present and a uniform current flowing in the Y direction, the magnetic field is uniform in the X direction perpendicular to the current flow, while the other components are 0. The presence of a discontinuity diverts current away from the deepest parts and concentrates it near the ends of a crack The effect of this is to produce strong peaks and troughs in Bz above the ends of the crack, while Bx shows a broad dip along the whole discontinuity with amplitude related to the depth. Alternating current field measurement has been developed from the alternating current potential drop technique. Alternating current potential drop uses current injection and contact potential drop probes. This technique required extensive surface preparation of the weld under examination. It could be used to produce crack depth measurements.


Figure 8.1: Alternating current field measurement qualitative explanation of the magnetic forces above a notch. Z

X Y


Bx =magnetic flux component normal to electric field and parallel to test surface Bz =magnetic flux component normal to test surface T = time or scan distance (relative scale)

http://www.ndt.net/article/wcndt00/papers/idn233/idn233.htm


ACFM Principle

ACFM

X

Z

Y



ACFM

Z

X



http://iic-hq.co.jp/english/03sp/01ii/01ni/KH-04.html

ACFM Butterfly Plot


Alternating current field measurement has been developed from the alternating current potential drop ACPD technique. Alternating current potential drop uses current injection and contact potential drop probes. This technique required extensive surface preparation of the weld under examination. It could be used to produce crack depth measurements. Alternating current field measurement ACFM has its origins in alternating current potential drop but instead of using a contact type probe the current is induced in the test specimen. The contact probes previously used in alternating current potential drop have been changed to (noncontact) magnetic field sensitive coils. The models developed in alternating current potential drop for mapping surface magnetic fields and electric cmrents have been utilized in alternating current field measurement.


Offshore Subsea ACFM Applications


http://www.ndt.net/article/platte/platte.htm

The technique in its simplest form uses a handheld probe containing a uniform field induction system and two magnetic field sensors. The induced alternating current is generated in a limited region of the test specimen where the alternating electric current AC is considered to be lineal: In this region a magnetic field is produced which is also linear. Any disturbances in this region produced by surface discontinuities will affect the components of this linear magnetic field. Two or more air wound coils mounted with orthogonal axes within a probe will detect these disturbances. This is the foundation of altemating current field measurement which is different from eddy current testing. Keywords: The induced alternating current...


The alternating current field measurement technique is being used by inspection companies and owners of fabricated components for weld inspection in petrochemical process plants, pharmaceutical plants, offshore well structures, highway bridges and roller coasters. Originally introduced to the offshore industry for subsea weld inspection, the use of alternating current field measurement has now broadened to include inspection of pressure vessels, process piping and drillpipe threads and risers. Recent developments have included automated and semiautomated systems to reduce the reliance on operators and the use of array technology to increase inspection speeds.


Alternating current field measurement can be used for the inspection of nonferromagnetic materials but is less effective in this role. The effective depth of penetration in nonferritic materials with alternating current field measurement is dramatically reduced. This is in sharp contrast to standard eddy current philosophy. It should also be noted that volumetric discontinuities, such as corrosion pitting or porosity, give much weaker signals than planar discontinuities, so it is not recommended that alternating current field measurement be used in this role.


Keywords: The effective depth of penetration in nonferritic materials with alternating current field measurement is dramatically reduced. This is in sharp contrast to standard eddy current philosophy.


http://v-e.vn/en/alternating-current-field-measurement-acfm-crack-microgauge.html

Magnetic Flux Leakage Testing Primary Application: Ferromagnetic Materials: pipe, plate, wire, oil field tubulars and pipelines Power Source. Permanent magnets or direct current coils Flux leakage testing has been extensively used in the pipe inspection industry. This entails the introduction of a moving direct cmrent magnetic field into a ferromagnetic test piece. Any localized (normally surface breaking) discontinuities that lie within the inspection zone will cause the field to bend or leak and extend above the surface at that point. These flux lines cut across a moving coit or other magnetic sensors and are used to detect this direct current leakage field.


In pipe inspection, flux leakage testing is used to look for corrosion pits and cracks. The locally thinned area puts a higher magnetic flux distribution in the space nearer to the flux detection device. This relative increase in field strength can be measured. Any discontinuity with its major axis parallel to the direction of the flux flowing in the material has little chance of being detected using this method. The pull speed of the flux leakage testing probe must be maintained at a fairly constant rate or the accuracy of the test is decreased even further. Pipeline inspections are performed with what are called smart pigs (Figure 8.2). These devices can simultaneously carry out multiple nondestructive testing tests. The most common is flux leakage testing.


Figure 8.2: Equipment for magnetic flux leakage testing of pipes and tubes: (a) pig tool; and (b) data acquisition from pig sensors. (a)

(b)


Defect Detections Any discontinuity with its major axis parallel to the direction of the flux flowing in the material has little chance of being detected using this method.

Low Detectability


Magnetic Flux Leakage Testing


http://www.ndt.net/apcndt2001/papers/11921/11921.htm

The most commonly used inservice inspection tools utilize flux leakage testing to detect internal or external cmrosion. The flux leakage testing inspection pig uses a circumferential array of detectors positioned between the poles of strong permanent magnets to magnetize the pipe wall to near saturation flux density. Abnormalities in the pipe wall, such as corrosion pits, result in flux leakage testing near the pipe's surface. The leakage flux may be detected by hall effect probes or passive induction coils.

MFLTMagnetize the pipe wall to near saturation flux density


http://www.ndt.net/apcndt2001/papers/11921/11921.htm

The most commonly used inservice inspection tools utilize flux leakage testing to detect internal or external cmrosion. The flux leakage testing inspection pig uses a circumferential array of detectors positioned between the poles of strong permanent magnets to magnetize the pipe wall to near saturation flux density. Abnormalities in the pipe wall, such as corrosion pits, result in flux leakage testing near the pipe's surface. The leakage flux may be detected by hall effect probes or passive induction coils. The demands now being placed on magnetic inspection tools are shifting from the mere detection, location and classification of pipeline discontinuities, to the accurate measurements of discontinuity size and geometry. Modern, high resolution flux leakage testing inspection tools are capable of giving very detailed signals. However, converting these signals to accurate estimates of size requires considerable expertise, as well as a detailed understanding of the effects of inspection conditions and the magnetic behavior of the type of steel used.


Magnetic Saturation


http://www.electronics-tutorials.ws/electromagnetism/magnetic-hysteresis.html

Magnetic Saturation


http://202.141.40.218/wiki/index.php/Hysteresis

Magnetic Saturation Hysteresis, in general, is defined as the lag in a variable property of a system with respect to the effect producing it as this effect varies. In ferromagnetic materials the magnetic flux density B lags behind the changing external Magnetizing field Intensity H. Hysteresis curve is drawn by plotting the graph of B-field vs H (or M-H) by taking the material through a complete cycle of H values as follows First, consider an unmagnetized sample of ferromagnetic material. The magnetic field intensity H is initially zero at O. It is increased monotonically, then magnetic induction B increases nonlinearly along the curve (OACDE) called as the magnetization curve. At point E almost all of the magnetic domains are aligned parallel with the magnetic field. An additional increase in H does not produce any increase in B. E is called as the point of magnetic saturation of the material. Values of permeability μ derived from the formula μ = B / H along the curve are always positive and show a wide range of values. The maximum permeability as large as 105 μo occurs at the ``knee (point D) of the curve. Next H is decreased till it reduces to zero. B reduces from its saturation value at "E" to that at point "F". Some of the magnetic domains lose their alignment but some maintain alignment i.e. some magnetic flux density B is still retained in the material. The curve for decreasing values of H (i.e. Demagnetization curve EF) is offset by an amount FO from that for increasing values of H (i.e. Magnetization curve OE). The amount of offset “FO” is called the retentivity or the remanence or the level of residual magnetism. As H is reversed in direction and increased, the curve moves to point "G", where B is reduced to zero. Most of the domains are flipped and oriented randomly so that net flux density within the material is zero. Portion corresponding to “GO” denotes “coercivity”. As H is increased to large values in the negative direction, B reaches saturation but in the opposite direction at point "I ". Almost all of the magnetic domains are aligned in opposite direction to that at point E of positive saturation. H is varied from its maximum negative value to zero. Then B reaches point "J." This point shows residual magnetism equal to that achieved for positive values of H (OF =OJ) H is increased back from zero to maximum in the positive direction. Then B reaches zero value at “K” i.e. it does not pass through the origin of the graph. OK indicates the amount of field H required to nullify the residual magnetism OJ retained in the opposite direction. H is increased from point “K” further in the positive direction, then again the saturation of B is reached at point “E” and the loop is completed.



The magnetization curve is not retraceable. The domains forced to coalesce into large domains aligned with the external field maintain the alignment and retain magnetism even after the external field is removed. The state of a system depends on the history of its state. The state (value and direction) of B depends upon the previous state of H (value=zero/ +ve/ -ve and direction increasing/ decreasing). Ferromagnetic materials have "memory" of previous exposure to magnetism or magnetic history. This phenomenon is called as Hysteresis. This property has been used to advantage in magnetic memory devices e.g. recording of audio tape/ video tape, and the magnetic storage of data on computer disks. From the hysteresis loop, important magnetic properties of a material can be determined as follows 1. Retentivity : A measure of the residual flux density corresponding to the saturation of a magnetic material. It is a material's ability to retain a certain amount of residual magnetic field when the magnetizing force is removed after achieving saturation (The value of B at point E on the hysteresis curve). 2. Residual Magnetism or Residual Flux : The magnetic flux density B that remains in a material when the magnetizing field intensity H is zero. Residual magnetism and retentivity are same only when the material is magnetized to the saturation point. However, it may be lower than the retentivity value otherwise. 3. Coercive Force : The amount of reverse magnetizing field intensity which must be applied to a magnetic material to make the magnetic flux density return to zero. (The value of H at point G on the hysteresis curve). 4. Permeability, μ : A property of a material that measures the ease with which a magnetic flux is established in it. μ is negative in the II and IV quadrants and positive in the I and III quadrants of the B-H graph (i.e. the Hysteresis curve). 5. Reluctance : Is the opposition that a ferromagnetic material shows to the establishment of a magnetic field. Reluctance is analogous to the resistance in an electrical circuit. The knowledge of these properties of materials is useful for selecting materials appropriate for different applications e.g. materials having both a large remanence and a large coercivity are selected for designing a permanent magnet. Materials possessing small remanences and small coercivities are selected for making transformer circuits.









Magnetic Flux Leakage The basic principle behind MFL involves magnetizing a ferrous metal object to saturation level with a powerful magnetic field.


http://www.mdpi.com/1424-8220/14/6/10361/htm

Magnetic Flux Leakage Rare earth neodymium iron boron magnets power the magnetizer of the inspection unit, providing the ultimate strength to meet most pipeline wall thicknesses for the best feature detection and sizing. Special designs can cover extra heavy wall applications. ILI tool drive section is the sealing unit that pulls the pig through the pipeline. All sizes of the ILI tool can accommodate multiple wall thickness in the same run. Longitudinal distance measurement to assure accurate location of anomalies. Magnetic sensors give 3 digital ticks per foot and analog sinusoid quadrature signals to allow for distance interpolation and forward/backward movement discrimination. Closely spaced individually calibrated Hall-effect sensors measure the magnetic flux and record MFL leakage caused by anomalies in Gauss units. A typical ¼” nominal sensor spacing provides for a true High Resolution inspection result. All tools are articulated for short capsule length to achieve bend radius of 1.5 D. The versatility of adding capsules or removing capsules allows the recording life of the tool to be changed with batteries to meet most pipeline lengths. Up-to-date computer hardware and components, flash memories and signal conditioning electronics record the signals from the sensors in full, without any filtering criteria, to allow for best post-run signal analysis and comparison with future runs. ID /OD Sensors discriminate between internal and external anomalies. Each signal captured by the sensor is compared with the signals captured with the array of Hall-effect sensors monitoring the total body wall response to the magnetic field.


http://www.pipeway.com/skins/pipeway/standard.aspx?elid=82

Magnetic Flux Leakage Magnetic Flux Leakage (MFL) testing is a widely used, Non-Destructive Testing (NDT) method for the detection of corrosion and pitting in steel structures. MFL is often used for integrity assessment of pipelines and storage tanks, but the principle can be applied to assets in any industrial sector. The Principle The basic principle behind MFL involves magnetizing a ferrous metal object to saturation level with a powerful magnetic field. Where the object has no flaws, the magnetic flux will remain undisturbed. High magnetization levels are required to differentiate corrosion from other pipeline features such as hard spots, stress and strain variations and to minimize the effects of remnant magnetization and velocity. Where there is internal or external metal loss, the magnetic flux leaks from the object. In the MFL testing device, a magnetic sensor is placed between the poles of a magnet yoke to record the leakage field by Hall-effect sensors. Eddy current sensors integrated in the magnetic flux sensors are used to improve the differentiation between internal and external defects. Applications MFL is used to detect metal loss defects (such as corrosion) in a wide range of settings.


http://www.rosen-group.com/global/company/explore/we-can/technologies/measurement/mfl.html

Remote Field Testing RFT Remote field testing should not be looked at as a typical eddy current test. There are papers and other reference materials that include remote field eddy current, however, to prevent confusion on the range of applications and material test situations, the attempt is being made to phase out that terminology. Both American Society for Testing and Materials (ASTM) and American Society of Mechanical Engineers (ASME) have remote field testing listed as a specific technique within electromagnetic testing. For the purpose of generic discussion this book will discuss remote field testing as it applies to inspection of ferromagnetic tubing in various heat exchangers. Remote field testing is an electromagnetic test that utilizes an alternating current excitation source. This alternating current electromagnetic energy travels along the htbe wall for some distance in both directions from an exciter coil. The distribution of the primary Held is dependent on the magnetic properties of the tube, the tube wall thickness and the presence of surrounding support structures. The transmitted field may be affected by discontinuities within the tube wall or support structures on the tube outside cliameter. The changes in the strength (amplitude) and phase shift or phase angle of the received signal are measured a few tube diameters away from the exciter coil.


Special hybrid (driver/pick up) coils are necessary to perform remote field testing inspections. Because of the need for a significant spacing between the exciter coil(s) and the receiver or pick up coils the probes tend to be longer that the typical eddy current probe. Remote field testing probe types are shown in Figure 8.3. The high magnetic permeability of ferromagnetic materials dramatically impacts standard eddy current testing inspection techniques. Some electromagnetic testing techniques attempt to compensate for and/ or suppress the permeability effects by the use of strong magnets or direct current chiven saturation coils. The remote field testing RFT process requires no magnetic saturation. Instead it makes use of the natural tendency of ferromagnetic materials to channel magnetic energy. Like the keeper of a horseshoe magnet, the magnetic lines of flux from the exciter coil take the path of least reluctance. They will flow down the tube wall; which acts as a wave guide, for a considerable distance.

Probes tend to be longer that the typical eddy current probe Charlie Chong/ Fion Zhang

Figure 8.3: Remote field testing probe types From top to bottom: Larger diameter tubing with either single or dual exciters, smaller diameter tubing and boiler tubing.


At distances in excess of two tube diameters from the internal exciter coil, the flux field has become homogenous and the passive receiver coils, positioned two to three tube diameters away from the exciter, receive practically all of their energy from the flux in the tube wall. The direct field from the exciter has been almost completely attenuated, or absorbed by the tube wall, and the external field is actually stronger than the field inside the tube. Through transmission is a term that is often used to describe the remote field testing process. This term normally implies that there is a source of energy that transmits through a medium. For example through transmission,. in both eddy current and ultrasonic testing, implies that the power source is on one side of the test product and the receiver element is on the opposite side of the material (through wall). In remote field testing some of the alternating current primary magnetic energy does extend to the outside diameter of the tube. It travels down the tube wall and eventually propagates back through the tube to the tube inside diameter. The concept of calling a remote field testing test a through wall technique may be hard to visualize, but the energy path is actually twice through the wall; once out at the exciter and then in at the detector. It is for this reason that short discontinuities show two distinct signals when the exciter and detector pass the discontinuity at different moments in time. The short discontinuity has interrupted the through transmission path twice. Charlie Chong/ Fion Zhang

In remote field testing inspection of tubing it is probably more accurate to look at the tube wall as a conduit or wave guide. Magnetic fields are modeled as closed loops. The following graphic shows the magnetic flux lines traveling out from the exciter coil (at 0 in Figure 8.4), mixing with incoming exciter energy in a transition zone (one to two diameters) and finally becoming homogenous in the remote field zone (two to three diameters) where the detector should be located. The main concern is to determine where along the length of the tube the primary magnetic flux lines will reverse their direction and start their return path back to the driver coil. It is at that point on the tube inside diameter that the remote field testing pick up coils should be placed. The driver or exciter coil supplie.s a low frequency alternating current magnetic field which couples to the tube wall. Electromagnetic induction occurs twice. In the near field or direct coupled zone, eddy currents are created in the tube wall. These actually decrease the efficiency of the process. Eddy currents are also created through induction as the field flux lines cut across the pickup coils on reentering the tube inside diameter.


Figure 8.4: Remote field testing energy distribution


By making careful measurements it is possible to map the strength and distribution of the driver coil's flux density as it travels down the tube wall. A graph can be generated, such as Figure 8.4, using experimental data that shows there are three distinct areas of interest. In an attempt to define the variations in the alternating current energy distributions that are present in the tube wall the following terminology has been developed:  Near Field (direct coupled) Zone - (0-1.5 tube diameters from the driver coil)  Transition Zone - (1.5-2 tube diameters from the driver coil)  Remote Field Zone - (2-3 tube diameters from the driver coil)


Remote-Field Energy Zones - in remote field testing. Profiles of B field just inside and outside pipe wall are used to indicate direct field region, transition and remote field zones.


Remote-Field Zone


http://www.ndt.net/article/v04n08/krzywosz/krzywosz.htm

Remote-Field Zone

Transition Zone


http://www.ndt.net/article/v04n08/krzywosz/krzywosz.htm

Near Field Zone - Within the near field zone the eddy currents generated in the tube wall by the alternating current driven exciter coil create a shielding effect of the exciter's flux. As eddy currents propagate through the material's inner wall, an opposing secondary magnetic flux is developed in the material that attenuates the primary field strength and limits its extension. Logically, the near zone would be the area where there is the greatest sensitivity to discontinuities because of the high concentration of magnetic flux. However the field tends to be concentrated near the inner surface of the tube, next to the exciter and this strong field tends to mask any signals from the tube outside diameter which are much weaker. In remote field testing the pickup coils are placed at some distance away from the exciter coil in an effort to get outside the high internal field area of the near field zone.


RFT - Flux distribution in pipe (a) 0 rad (0 deg);

Legend IS = inside surface OS = outside surface PA = pipe axis


Transition Zone -The region just outside the near field zone is known as the transition zone. It is an area that is currently not considered to contain reliable data because the location of the transition zone changes with changes in wall thickness, permeability and conductivity. In this zone there is a great deal of interaction between the flux of one field that is diffusing outward from the exciter and the flux of the returning energy that is diffusing inward from the outside surface of the tube. The total or resultant field strength in this area · tends to be weaker because of the negative interaction of fields with differing directional characteristics. When the two opposing fields meet, the result is a cancellation of some of their respective energy.


Remote Field Zone - The third definable region starts to occur at about two tube diameters from the exciter coil. The detector coil's signal amplitude bottoms out at the base of the logarithmic curve and starts a linear decay. Notice that the curves (Figure 8.4) describing signal amplitudes of the inner and outer walls parallel each other and are linear after peaking at maximum values. Considering the rate of attenuation of the inner wall field strength, the result is that in the area where the remote field zone starts, the outer wall field strength can be 10 to 100 times the strength of the inner wall field.


Phase - The phase change of the signals detected at the pick up coil can be used to estimate the loss of wall. A thinner wall allows the flux traversing the wall to arrive at the detector sooner (similar to the time of flight of ultrasonic testing signals). Discontinuities of differing depths can be evaluated accurately based on measured phase shift information. In eddy current testing there is a well defined difference in phase angle responses for inside diameter and outside diameter events; however, in remote field testing data inside diameter and outside diameter discontinuities of the same depth will have about the same phase angle.


Amplitude (voltage) - The remote field testing system senses a decrease in wall thickness as a stronger alternating current magnetic field cutting across the pick up coil. This induces a stronger voltage in the coil. Discontinuities of larger volume increase the amplitude of the signal while smaller volume discontinuities produce small amplitude signals, but the signal phase still represents the wall loss at the discontinuity. Signal location (at or near a support versus in free span tube) goes a long way to assisting in signal interpretation. The use of specialized voltage dependent phase analysis curves can also improve discontinuity resolution. Because some of the primary magnetic field extends out beyond the tube outside diameter tube support plates or baffles interfere with the magnetic field distribution. Any metallic material on the tube outside diameter will tend to block the energy transfer down the length of the tube. Because of the spacing between exciter and pick up coils this could lead to decreased sensitivity at these locations. Remote field testing is capable of detecting both small and large volume discontinuities in most ferromagnetic tubing found in a wide range of tubes and pipes such as heat exchangers, boilers, piping and pipelines. Some limitations do exist, for example in fin fan tubing found in air fin coolers.


The base tubing is carbon steel, however to improve heat transfer rates, large diameter fins of high conductivity metal (normally aluminum) are installed on the tube outside diameter. The induced energies in the fins themselves prevent the primary magnetic field distribution along the outside diameter surface of the tube which dramatically limits the remote field testing inspection process. ASTM E-2096 is a good reference document for anyone considering remote field testing applications. It references remote field testing technology as well as personnel training criteria. It provides a guide to the types of minimum detection capability that should be demonstrated by inspection personnel when they apply the proper tools and techniques while performing remote field testing examinations.


RFT


http://www.mdpi.com/1424-8220/14/12/24098/htm

Remote Field Eddy Current Technique (RFT) This process is well adapted to the inspection of small-bore ferromagnetic tubes such as carbon steel. Using electromagnetic techniques this is now the industry standard inspection for boilers and heat exchangers due to its low frequency (typically 50-1000Hz). The probe consists of two coils in a send-receive configuration which are inserted into the tube. The energized exciter coil transmits a signal to the detector coil located some distance away. This signal passes through to the outside tube wall returning to arrive at the detector coil. With wall thinning there is less shielding hence the return time (greater phase) and attenuation (greater amplitude) is shorter. Phase and amplitude traces are generated as the probe is pulled through the tube as recorded data identifies the metal loss. Flaw sizing is also possible with RFT enabling depth, length and circumference to be accurately calibrated.


http://www.itcl.org.uk/itcl-services/tube-inspection/

The Remote Field Technique is an electromagnetic examination, which utilizes a through-transmission process. The resultant field is affected by either ID or OD tube wall anomalies. RFT signal measurements are made a few tube diameters away from the AC excitation coil without any attempt at tube wall magnetization or saturation. A pair of pick-up coils located in the remote field zone measures the resultant field to give both a differential and an absolute signal. The signal phase and amplitude information is used to determine defect depth and volume


Remote Field Eddy Current Technique (RFT)


Remote Field Eddy Current Technique (RFT)


http://www.olympus-ims.com/en/ms-5800-tube-inspection/

RFT- Tube Cleanliness Tube Cleanliness is as important for the process reasons (i.e. heat transfer) as it is for the Remote Field inspection. Inspections that go the smoothest are ones where the tubes are adequately cleaned prior to the inspection. Not only does this save inspection time and money, but the data acquired from clean tubes VS dirty tubes make the inspection much more accurate. Non-relevant indications can occur from Iron deposits, calcium deposits, etc. These non-relevant indications can mask real defects located underneath. So how can you tell when the tubes are cleaned enough for a Remote Field inspection? We have developed a “Dummy” probe chart that customers can use to build probe heads to check for tube cleanliness. These probes can be made to screw on to hydro-blasters lance’s and used after the cleaning process is complete to make sure there is proper clearance for the Eddy Current probe. Final Reports - After the inspections and final data analysis is completed a formal report is generated showing a tube sheet diagram with the tubes inspected color coded to a percentage wall loss. Additional tube sheet diagrams can be generated showing the worst case scenarios for tube plugging or selective re-tubing. In addition to this information our reporting format can generate corrosion rates, and a projection based on the established corrosion rates.


http://www.techcorr.com/services/Inspection-and-Testing/Remote-Field-Testing.cfm

More Reading: RFT Remote Field Testing or "RFT" is one of several electromagnetic testing methods commonly employed in the field of nondestructive testing. Other electromagnetic inspection methods include magnetic flux leakage, conventional eddy current and alternating current field measurement testing. Remote field testing is associated with eddy current testing and the term "Remote Field Eddy Current Testing" is often used when describing remote field testing. However, there are several major differences between eddy current testing and remote field testing which will be noted in this section. RFT is primarily used to inspect ferromagnetic tubing since conventional eddy current techniques have difficulty inspecting the full thickness of the tube wall due to the strong skin effect in ferromagnetic materials. For example, using conventional eddy current bobbin probes to inspect a steel pipe 10 mm thick (such as what might be found in heat exchangers) would require frequencies around 30 Hz to achieve the adequate I.D. to O.D. penetration through the tube wall. The use of such a low frequency results in a very low sensitivity of flaw detection. The degree of penetration can, in principle, be increased by the use of partial saturation eddy current probes, magnetically biased probes, and pulsed saturation probes. However, because of the large volume of metal present as well as potential permeability variations within the product, these specialized eddy current probes are still limited in their inspection capabilities. The difficulties encountered in the testing of ferromagnetic tubes can be greatly alleviated with the use of the remote field testing method. The RFT method has the advantage of allowing nearly equal sensitivities of detection at both the inner and outer surfaces of a ferromagnetic tube. The method is highly sensitive to variations in wall thickness and tends to be less sensitive to fill-factor changes between the coil and tube. RFT can be used to inspect any conducting tubular product, but it is generally considered to be less sensitive than conventional eddy current techniques when inspecting nonferromagnetic materials. Charlie Chong/ Fion Zhang

https://www.nde-ed.org/EducationResources/CommunityCollege/Other%20Methods/RFT/RFT_Intro.htmc

RFT Theory of Operation A probe consisting of an exciter coil and one or more detectors is pulled through the tube. The exciter coil and the detector coil(s) are rigidly fixed at an axial distance of two tube diameters or more between them. The exciter coil is driven with a relatively low frequency sinusoidal current to produce a magnetic field.

axial magnetic flux induced perpendicularly circumferential eddy currents

This changing magnetic field induces strong circumferential eddy currents which extend axially, as well as radially in the tube wall.



These eddy currents, in turn, produce their own magnetic field, which opposes the magnetic field from the exciter coil. Due to resistance in the tube wall and imperfect inductive coupling, the magnetic field from the eddy currents does not fully counterbalance the magnetic exciting field. However, since the eddy current field is more spread out than the exciter field, the magnetic field from the eddy currents extends farther along the tube axis. The interaction between the two fields is fairly complex but the simple fact is that the exciter field is dominant near the exciter coil and the eddy current field becomes dominant at some distance away from the exciter coil. Eddy current field


Exciter field


The receiving coils are positioned at a distance where the magnetic field from the eddy currents is dominant. In other words, they are placed at a distance where they are unaffected by the magnetic field from the exciter coil but can still adequately measure the field strength from the secondary magnetic field. Electromagnetic induction occurs as the changing magnetic field cuts across the pick-up coil array. By monitoring the consistency of the voltage induced in the pick-up coils one can monitor changes in the test specimen. The strength of the magnetic field at this distance from the excitation coil is fairly weak but it is sensitive to changes in the pipe wall from the I.D. to the O.D.



The Zones Direct Couple Zone The region where the magnetic field from the exciter coil is interacting with the tube wall to produce a concentrated field of eddy currents is called the direct field or direct coupled zone. This zone does not contribute a great deal of useful data to the RFT inspection due to problems with rather high noise levels due to the intense varying magnetic field from the excitation coil.

Transition Zone The region just outside the direct couple zone is known as the transition zone. In this zone there is a great deal of interaction between the magnet flux from the exciter coil and the flux induced by the eddy currents. As can be seen in the graph, the interaction of the two opposing fields is strongest near the ID of the tube and fairly subtle at the OD of the tube. The "resultant" field strength (the magnetic field at the sum of the two fields) in this region tends to change abruptly on the ID due to the interaction of the fields with differing directional characteristics of the two fields. The receiver coil's signal phase, with respect to the exciter coil, as a function of distance between the two coils is also shown in the graph. When the two coils are directly coupled and there is no interference from a secondary field, their currents are in phase as seen at location zero. In the transition zone, it can be seen that the phase swiftly shifts, indicating the location where the magnetic field from the eddy currents becomes dominate and the start of the remote field.



Remote Field Zone The remote field zone is the region in which direct coupling between the exciter coil and the receiver coil(s) is negligible. Coupling takes place indirectly through the generation of eddy currents and their resulting magnetic field. The remote field zone starts to occur at approximately two tube diameters away from the exciter coil. The amplitude of the field strength on the OD actually exceeds that of the ID after an axial distance of approximately 1.65 tube diameters. Therefore, RFT is sensitive to changes in material that occur at the outside diameter as well as the inside diameter of the tube.

1.65 tube diameters



RFT Probes Probes for inspection of pipe and tubing are typically of the bobbin (ID) variety. These probes use either a single or dual excitation coil to develop an electromagnetic field through the pipe or tube. The excitation coils are driven by alternating current. The sensing coil or coils are located a few tube diameters away in the remote field zone. Probes can be used in differential or absolute modes for detection of general discontinuities, pitting, and variations from the I.D. in ferromagnetic tubing. To insure maximum sensitivity, each probe is specifically designed for the inside diameter, composition, and the wall thickness of a particular tube.



RFT Instrumentation Instruments used for RFT inspection are often dual use eddy current / RFT instruments employing multifrequency technology. The excitation current from these instruments is passed on to the probe that contains an exciter coil, sometimes referred to as the driver coil. The receiving coil voltage is typically in the microvolt range, so an amplifier is required to boost the signal strength. Certain systems will incorporate a probe excitation method known as multiplexing. This utilizes an extreme high speed switching method that excites the probe at more than one frequency in sequence. Another method of coil excitation that may be used is simultaneous injection. In this coil stimulation technique, the exciter coil is excited with multiple frequencies at the same time while incorporating filter schemes that subtract aspects of the acquired data. The instrument monitors the pickup coils and passes the data to the display section of the instrument. Some systems are capable of recording the data to some type of storage device for later review.



RFT Signal Interpretation The signals obtained with RFT are very similar to those obtained with conventional eddy current testing. When all the proper conditions are met, changes in the phase of the receiver signal with respect to the phase of the exciter voltage are directly proportional to the sum of the wall thickness within the inspection area. Localized changes in wall thickness result in phase and amplitude changes. These changes can be indicative of defects such as cracks, corrosion pitting or corrosion/erosion thinning.



RFT Signal Interpretation



RFT Reference Standards Reference standards for the RFT inspection of tubular products come in many variations. In order to produce reliable and consistent test results, the material used for manufacturing calibration standards must closely match the physical and chemical properties of the inspection specimen. Some of the important properties that must be considered include conductivity, permeability and alloy content. In addition, tube dimensions including I.D., O.D. and wall thickness must also be controlled. The type of damage mechanisms that are expected to be encountered must also be carefully considered when developing or selecting a reference standard. In order to get accurate quantitative data, artificial discontinuity conditions are typically machined into the standards that will closely match those conditions that may be found in the tubing bundle.





Answers


Q.8.1 Which of the following electromagnetic testing techniques does not use an alternating current coil excitation process? A. alternating current field measurement B. eddy current testing C. flux leakage testing D. remote field testing Q.8.2 Which of the following electromagnetic testing techniques should provide the best discontinuity depth and length sizing capability for cracks in ferromagnetic weldments? A. alternating current field measurement B. eddy current testing C. flux leakage testing D. remote field testing Q.8.3 Which of the following techniques should perform best in nonferromagnetic materials? A. alternating current field measurement B. eddy current testing C. flux leakage testing D. remote field testing


Q.8.4 A generally accepted definition of remote field testing is: A. electromagnetic testing done at remote locations. B. the electromagnetic field which has been transmitted through the test object and is observable beyond the direct coupling of the exciter. C. through transmission eddy currents, detected on the far side of a material or object under test by a remote receiver coil. D. the opposite of direct field. Q.8.5 When a nonferromagnetic tube is inspected with a self-comparison differential encircling coil arrangement a non-detection could occur when a discontinuity is: A. filled with water. B. deep but very narrow. C. long with slowly varying depth. D. short and wide.


Q.8.6 The most common electromagnetic testing technique used to locate corrosion thinning in large diameter cross country piping systems would be: A. alternating current field measurement. B. eddy current testing. C. flux leakage testing. D. remote field testing. Q.8.7 Considering the full range of typical probe designs currently in use, in which of the following electromagnetic testing techniques could the term passive receivers be used? A. alternating current field measurement B. eddy current testing C. flux leakage testing D. remote field testing E. All of the above.


Q.8.8 The region of intense electromagnetic interaction at the interface between an alternating current coil's outside diameter surface and a tube wall's inside diameter surface is called the: A. direct couple zone. B. fresnel zone. C. near field zone. D. Both A and C. E. None of the above. Q.8.9 The operating frequencies that are selected to perform remote field testing inspections are: A. usually higher than those used in conventional eddy current tests. B. usually lower than those used in conventional eddy current tests. C. identical to those used in conventional eddy current tests. D. about one half of those used in conventional eddy current tests.   

Near Field (direct coupled) Zone - (0-1.5 tube diameters from the driver coil) Transition Zone - (1.5-2 tube diameters from the driver coil) Remote Field Zone - (2-3 tube diameters from the driver coil)


Q.8.10 The amplitude or voltage of the detected response from a discontinuity is most often related to: A. the width of the discontinuity. B. the location of the discontinuity. C. the depth of the discontinuity. D. the volume of the discontinuity.


Chapter 9 Eddy Current Procedures, Standards and Specifications


Procedures, specifications and standards are produced to provide a means of controlling product or service quality. Written instructions that guide a company or individual to a desired end result and are acceptable to industry, are the basis of procedures, specifications and standards. Many publications are available to guide or instruct us. Some of the most frequently used references are the American Society for Testing and Materials (ASTM), American Society of Mechanical Engineers (ASME), American National Standards Institute (ANSI) and Military Standards (MIL-STD-XXXX). These publications are laboriously produced by committees made up of scientific and technical people. Usually after a committee produces a draft document, it is submitted to industry and the scientific community for comment and subsequent revision. In certain cases, standards combine to assist each other. As an example, ASME Section V Article 8- Appendix IV uses ASTM E1316 to provide Standard Terminology for Nondestructive Testing. The military standard, MIL-STD-1537C Electrical Conductivity Test for Verification of Heat Treatment of Aluminum Alloys, Eddy Current Method, references ASTM B193 Resistivity of Electrical Conductor Materials and ASTM E18 Rockwell Hardness and Rockwell Superficial Hardness of Metallic Materials.


American Society for Testing and Materials American Society for Testing and Materials (ASTM) standards (practices or guides) usually include in the written instructions headings such as scope, referenced documents, terminology, significance and use, basis of application, apparatus, reference standards, standardization, procedure and keywords. Scope makes a general statement about the document's applicability and intent. Referenced Documents refers to other publications used as references within the standard. The terminology section usually may contain definitions of unique terms specific to the equipment or examination covered by the standard. Significance and Use is a more detailed discussion of test results and probable causes of indications expected during the examination. The Basis of Application section identifies items which are subject to contractual agreement between the parties using or referencing the standard such as personnel qualification, qualification of nondestructive testing agencies, procedures and techniques, surface preparation, timing of examination, extent of examination, reporting criteria/ acceptance criteria, reexamination of repaired/reworked items. Apparatus describes the general requirements for the inspection system including instrumentation, coils, positioning and driving mechanisms. The fabrication requirements for artificial discontinuity standards used for standardization are discussed under reference standards. A discussion of the reference specimen and the geometrical requirements of the artificial discontinuities in it is usually included. Standardization provides instructions for adjustment of the apparatus used for the examination. The response to known discontinuities in the reference standard is usually described in this section. Detailed instructions to process the inspection appears under procedure. These instructions may include acceptance limits and the handling of components that are not acceptable.


ASTM publishes several standards pertaining to the eddy current method. These standards are numbered; for example:  E 571- 98. "E 571" refers to the standard and "98" refers to the year of revision. Some ASTM standards that pertain to the eddy current method are:  E 215 Standard Practice for Standardizing Equipment for Electromagnetic Examination of Seamless Aluminum-Alloy Tube.  E 243 Standard Practice for Electromagnetic (Eddy Current) Examination of Copper and Copper-Alloy Tubes.  E 426 Electromagnetic (Eddy-Current) Testing of Seamless and Welded Tubular Products, Austenitic Stainless Steel and Similar Alloys.  E 571 Standard Practice for Electromagnetic (Eddy Current) Examination of Nickel and Nickel Alloy; Tubular Products.  E 690 Standard Practice for In Situ Electromagnetic (Eddy-Current) Examination of Nonmagnetic Heat Exchanger Tubes.  E 1316 Standard Terminology for Nondestructive Testing.


Military Standard The United States Military uses the Military Standard document to control testing and materials. Standard procedures are provided by a series of MIL-STD-XXXXX documents. Special requirements are specified by the Military Specification system. For example, MIL-STD- 537C refers to Electrical Conductivity Test for Verification of Heat Treatment of Aluminum Alloys, Eddy Current Method. The Calibration System Requirements for MIL-STD-1537C are contained in Military Specification MIL- -45662. The MIL-STD usually contains several parts and is very descriptive. These parts normally include Scope, Applicable Documents, Definitions, General Requirements, Detail Requirements and Notes. The Scope contains a general statement of applicability and intent of the Standard. Applicable Documents pertains to other reference or controlling documents such as other MIL-STD, Military Specification or ASTM publications. Definition contains precise definitions of key words and phrases used in the Standard. Under General Requirements, equipment, reference specimen and personnel requirements are described in sufficient detail to implement the Standard. Included in this part is instrument sensitivity and response, test object variables, reference specimen requirements and personnel qualification requirements. Detail Requirements describes the specific procedure to implement the Standard. Notes contains pertinent statements about the process and guidelines for reporting results.


American Society of Mechanical Engineers In 1911 the American Society of Mechanical Engineers (ASME) set up a committee to establish rules of safety for design, fabrication and inspection of boilers and pressure vessels. These rules have become known throughout industry as the ASME code. The ASME Boiler and Pressure Vessel Committee is a large group from industry and the scientific community. The Committee has many subcommittees, subgroups and working groups. Each subcommittee, subgroup and working group combines as a unit for a specific area of interest. For example, the Subcommittee on Pressure Vessels (SC VIII) has two working groups and five subgroups reporting to it. The purpose of these groups is to interface with industry to keep pace with changing requirements and needs of industry and public safety. The ASME Boiler and Pressure Vessel Code is divided into 11 sections. ASME Section V, Nondestructive Examination/ is divided into two subsections, A and B. Subsection A deals with Nondestructive Methods of Examination. Article 8 is Eddy Current Examination of Tubular Products. Subsection B is Documents Adopted by Section V. Eddy current standards are described in Article 26. In this case, the ASTM E215 document has been adopted by ASME and reassigned the designation SE215. ASME Section V, Article 8, Appendix I gives detailed procedure requirements for Eddy Current Examination Method for Installed Nonferromagnetic Heat Exchanger Tubing. A procedure designed to meet this requirement can be illustrated by the following example, Document QA 3.


EDDY CURRENT INSPECTION OF NONFERROUS TUBING BY SINGLE FREQUENCY TECHNIQUES Procedure No. QA 311-1 A. PURPOSE This procedure describes the equipment and methods as well as the personnel qualifications to be utilized for the performance of the eddy current examination of steam generator tubes. It meets the requirements of the NRC Regulatory Guide 1.83, ASME Section XI, Appendix IV and ASME Section V, Article 8 of the ASME Boiler and Pressure Vessel Code. B. SCOPE The scope of the examination to be performed is contained in the eddy current inspection program document applicable to the specific plant to be inspected. C. PREREQUISITES 1 . Plant Condition The plant must be shut down with the primary system drained. The steam generators shall be open on the primary side for access to the channel head and the shell cool down sequence shall be complete. Air movers shall be attached to circulate air through the generator to dry the tube sheet.


2. Equipment The examinations shall be performed utilizing an XXXX/XX multifrequency eddy current instrument with bobbin coil probes designed for testing from the inside of the tubes. The inspection performance shall be monitored by the use of a phase sensitive vector display and recorded for later evaluation. a. Equipment utilized shall be: i. XXXX/XX eddy current instrument. ii. Bobbin coil probes capable of operation in the differential and absolute modes. iii. Digital recording device(s). iv. Communications system. v. Reference standard The reference standard shall be manufactured from a length of tubing of the same size and type of material that is to be examined in the vessel. The standard shall contain 6 intentional discontinuity areas as follows: aa. 100% through the wall drill hole (0.052 in. for 0.750 in. outside diameter tubing and smaller, and 0.067 in. for larger tubing). bb. Flat bottomed drill hole 5/64 in. diameter X 80% through from the outer tube wall surface. cc. Flat bottomed drill hole 7/64 in. diameter X 60% through from the outer tube wall surface. dd. Flat bottomed drill hole 3/16 in. X 40% diameter through from the outer tube wall surface. ee. Four flat bottom holes, 3/16 in. diameter, spaced 90 degrees apart around the tube circumference, 20% through the tube wall. ff. Circumferential groove 20% deep by 1/16 in. long by 360 degrees on the inside tube wall surface. gg. Circumferential groove 10% deep by 1/8 in. long !jy 13,60 degrees on the outer tube wall surface. hh. Each standard shall be identified by a serial number etched on one end and be traceable to the master standard stored at the facility.


b. Probe positioning and feeding shall be accomplished remotely for in-service inspection. Baseline inspection may be done manually. c. Personnel communications devices shall be provided. 3. Personnel Qualifications Personnel collecting data in accordance with this procedure shall be qualified to Level! or higher in accordance with Document QA 101. Personnel interpreting data collected in accordance with procedure shall be qualified to Level IIA or higher in accordance with Document QA 101. Prior to receiving a certification, the applicants shall have completed the program recommended by SNT-TC-1A (1984 edition}, Supplement E. D. PRECAUTIONS 1. All personnel to be engaged in eddy current inspection programs at operating plants shall have received instructions in and understand the radiation protection rules and guidelines in effect on the plant site. 2. All personnel to be engaged in the test program shall wear protective clothing to the extent of the type defined by the exclusion area work permit. 3. All personnel entering a radiation work area will have proven their ability to work in a face mask by successfully passing the pulmonary function test during their annual physical. 4. No entries shall be made into the steam generator channel head without the presence of a qualified health physics technician. 5. Ensure that nozzle covers (when applicable) are securely in place inside the vessel before commencement of the eddy current inspection program.


E. PERFORMANCE 1 . Preparation a. Establish location of data acquisition control center. b. Arrange power distribution at data acquisition control center. c. Install communications system control box at the data acquisition control center. d. Establish communication with one or more headsets at the steam generator. e. Install XXXX!XX eddy current test instrument, pusher puller and fixture control boxes as the steam generator. f. Install remote digital data acquisition computers and recording devices at the data acquisition control center. 2. Equipment Calibration a. Prior to the commencement of the eddy current examination, of the steam generator tubes and after the replacement of any component, the equipment shall be calibrated in accordance with the following steps: Insert the reference bobbin coil probe into a reference standard. i. ii. iii. iv. v.

Insert the test bobbin coil probe into a section of the reference standard, which is tree of discontinuities. Select the desired frequencies as per the Site Specific Data Acquisition Procedure. Select the probe drive voltage and channel gain as per the Site Specific Data Acquisition Procedure. Perform a hardware null. Remotely pull the test probe through the reference standard at the speed selected for actual testing in the heat exchanger. Data from the heat exchanger will also be acquired on the pull unless noted. vi. Set the display sensitivity setting for each channel per the site specific calibration procedures. vii. Set the rotation (phase) value so that the probe motion signals in the discontinuity sensitive differential channels are horizontal (as per the specific calibration procedure) with the first lobe of the 100% through the wall drill hole going down first as the probe is withdrawn from the standard. viii. Set the rotation (phase) value so that the probe motion signals in the discontinuity sensitive absolute channels are horizontal (as per the specific calibration procedure) with the response of the 100% through the wall drill hole going up as the probe is withdrawn from the reference standard. ix. Complete the digital calibration summary form, update it with all pertinent information and store this information to the selected digital storage device.


3. Tube Inspection General (Refer to Site Specific Calibration Procedure QA 2) a. Eddy current inspection activities shall be performed with equipment sensitivities and speeds set per the Site Specific Data Acquisition Procedure. b. Visual verification of the identity of the specific tube being inspected shall be performed before and after each fixture change and at the beginning and end of each row or column. Verification of the positive identification of tube location shall be noted by a digitally recorded message. c. Should the performance of the tube identity verification reveal an error has occurred in the recording of probe location, all tubes examined because the previous verification of location shall be reexamined. d. The equipment calibration shall be verified and recorded at the beginning and end of each calibration cycle. At a minimum, the calibration will be verified at 4 hours intervals and after any equipment change. e. Should the equipment be found to be out of calibration, the equipment will be recalibrated as per Section E-2 of this procedure. The data interpreter will determine if it is necessary to re-inspect any of the tubes. 4. Tube Inspection Manual a. The data recording shall be made during probe withdrawal. Withdrawal speed is 14 in. per second maximum. No minimum speed specification is required, but a good uniform pull of 12 in. per second is preferred. b. Because no inspection is performed during probe insertion, the speed may be as rapid as possible. c. Due to radiation exposure probe pusher/pullers should be used to facilitate the inspection.


5. Tube Inspection Automatic Remote NOTE: Ensure that all probe positioner, probe feeder and probe and communication connecting cables are clear of access walkways and secured to available supports. a. Install remotely operated probe feeder local to steam generator. b. Check the operation of the remotely operated eddy current positioner and connect the flexible probe conduits to the probe guide tube and the probe pusher. c. Install remotely operated probe positioner on the manway or the tube sheet of the steam generator to provide coverage of the area to be examined. d. Connect power and air supply lines to remote hardware as required. e. Verify the correct operation and control of the remotely operated platform hardware. f. Operate the position er to locate the probe beneath the tube to be examined. g. If probe insertion is to be done manually, utilize the probe pusher controls to feed the probe into and up the tube to the desired height. Monitor the extent of insertion by reference to impedance signals from known tube reference locations (tube end, top of tube sheet, supports) on the display screen. h. If operating in the Auto Acquire mode, verify that the proper landmark tables have been installed, axial encoders are functioning properly and that the correct voltage thresholds have been established for auto locate of supports and tube ends. i. If performing manually or automatically ensure that the tube alphanumeric identifier has been properly updated. Monitor the withdrawal of the probe from the tube until the impedance signal on the screen indicates that the probe is clear of the tube sheet. Concurrent with the probe withdrawal, visually monitor the signals on the display screen while recording all data in real time. j. Reposition the probe beneath the next tube selected for examination. k. Repeat the procedures described in the preceding steps until all the tubes selected for inspection have been examined. Charlie Chong/ Fion Zhang

F. INSPECTION RESULTS AND DOCUMENTATION 1 . Requirements a. The data interpreter shall be certified to Level IIA or IIIA as per Procedure QA 101. b. Data shall be collected with an eddy current test system with a current certification of calibration as per CSP procedure. c. The data collection system shall be calibrated with an approved reference standard that is serialized and traceable to a master reference standard. d. The identify of the plant site, the steam generator, the operators name and certification, the date, the test frequencies, the reference standard serial numbers, equipment serial numbers and certification dates, software revisions and probes design and serial number shall be recorded at the start of each calibration cycle. e. data collection station shall be set up and calibrated as per Procedure OA 3.


2. . Performance a. The data interpreter shall: i. Determine that all tubes selected for inspection have been tested. ii. Report tubes whose data are incomplete or un-interpretable. iii. Require a retest of any tubes exhibiting excessive noise or unusual responses. iv. ln-service inspections a. Report all discontinuities > 19%. b. Report all other indications that appear to be relevant. c. Identify the axial position of all indications with respect to a known structural member. v. Pre-service inspections a. Report all indications observed. Include the axial position of the indication with respect to a known structural member. b. Interpretation i. All data shall be reported on a digital Final Report form. ii. The conversion from signal phase angles (or amplitudes) to discontinuity depths shall be accomplished per calibration curves established on the appropriate channels using the calibration standards and techniques defined in the site specific data analysis specifications. iii. All data shall be reviewed in its entirety. iv. Any abnormal signals observed shall be reported.


G. REFERENCES The following documents or files are required for the performance of eddy current inspection programs utilizing the methods described in this procedure. 1. Required Documentation a. Eddy current inspection specific calibration procedure documents applicable to the plant to be inspected. b. Inspection plans showing tube sheet maps marked to designate the extent of examination to be performed and extent of completion. c. Final Reports including all indications resolved by the Data Resolution Analyst.




Answers


Q.9.1 A precise statement of a set of requirements to be satisfied by a material, product, system or service is a: A. standard. B. specification. C. procedure. D. practice. Q.9.2 A statement that comprises one or more terms with explanation is a: A. practice. B. classification. C. definition. D. proposal. Q.9.3 A general statement of applicability and intent is usually presented in the of a _____ standard? A. summary B. scope C. significance D. procedure Charlie Chong/ Fion Zhang

Q.9.4 Military Standards are designated by MIL-C-(number). A. True MIL-STD-XXXXX B. False Q.9.5 In the structure of American Society of Mechanical Engineers (ASME) the subcommittee reports to the subgroup. A. True B. False Q.9.6 In example QA 3, personnel interpreting results must be: A. Level I or higher. B. Level II or higher. C. Level IIA or higher. D. Level Ill.


Q.9.7 The prime artificial discontinuity used to calibrate the system described in QA 3 is: A. 20% inside diameter. B. 50% outside diameter. C. 100%. D. 50% inside diameter. Q.9.8 In QA 3, equipment calibration must be verified at least: A every hour. B. each day. C. every 4h. D. every 8 h. Q.9.9 QA 3 specifies a maximum probe traverse rate of: A. 305 mm/ s (12 in./ s). B. 355.6mm/s (14in./s). C. 152.4 mm/s (6 in./s). D. not specified.


Q.9.10 The system in QA3 is calibrated with an approved standard that is traceable to: A. NBS. B. American Society of Mechanical Engineers (ASME). C. a master standard. D. American Society for Testing and Materials (ASTM). Q.9.11 In accordance with QA 3, a tube whose data are incomplete must be: A. reinspected. B. reported. C. reevaluated. D. removed from service.


■ωσμ∙Ωπ∆º≠δ≤>η


More Reading http://www.allaboutcircuits.com/vol_1/index.html


Further Reading


Discussion Subject: discuss on the standard requirements on the differences in frequency Hz used on specific applications for thickness checks and weld examination.

BS EN 1711:2000 6.4.2 Surface probes 6.4.2.1 Probes for measuring thickness of coating and material evaluation relative to calibration block To be acceptable for this purpose, the probe shall be capable of providing a full screen deflection lift off signal on the instrument when moved from an uncoated spot on a calibration block to a spot covered with the maximum coating thickness expected on the structure to be tested. The probe shall operate in absolute mode at a selected frequency in the range from 1 kHz to 1 MHz. All the probes shall be clearly marked with their operating frequency range. (See Figure 1). 6.4.2.2 Probes for weld examination For examination of ferritic welds, probes specially designed for this purpose shall be used. The probe assembly shall be differential, orthogonal, tangential or equivalent which is characterized by having a minimal dependency on variations in conductivity, permeability and lift off in the welded and heat-affected zones. The diameter of the probe shall be selected relative to the geometry of the component under test. Such probes shall be able to operate when covered by a thin layer on non-metallic wear-resistant material over the active face. If the probe is used with a cover, then the cover shall always be in place during calibration. The probe shall operate at a selected frequency in the range from 100 kHz to 1 MHz. Key • 1,2,3,4 Deflections representing variations of thickness of simulated coatings on calibration block • 5 Deflection representing material of calibration block • 6,7 Deflection representing range of material to be examined using calibration block 0 Balance


BS EN 1711:2000

Discussion Subject: discuss on the standard requirements on the frequency used on the specific applications for thickness testing and defect detections. BS EN 1711:2000 6.4.2 Surface probes 6.4.2.1 Probes for measuring thickness of coating and material evaluation relative to calibration block To be acceptable for this purpose, the probe shall be capable of providing a full screen deflection lift off signal on the instrument when moved from an uncoated spot on a calibration block to a spot covered with the maximum coating thickness expected on the structure to be tested. The probe shall operate in absolute mode at a selected frequency in the range from 1 kHz to 1 MHz. All the probes shall be clearly marked with their operating frequency range. (See Figure 1). 6.5.2 Procedure for examination of welds in ferritic materials 6.5.2.1 Frequency The frequency shall be optimized with respect to the sensitivity, the lift off and other unwanted signals. Underusual conditions a frequency of about 100 kHz is recommended.

Key • 1,2,3,4 Deflections representing variations of thickness of simulated coatings on calibration block • 5 Deflection representing material of calibration block • 6,7 Deflection representing range of material to be examined using calibration block 0 Balance


BS EN 1711:2000


Comparison of OD and ID Eddy Current Inspection of Tubing Scope The Eddy Current test (ECT) test is the primary nondestructive test (NDT) used in tube mill certification testing for condenser, feedwater heater, and balance of plant (BOP) power generation tubing. When tested at the tube mill, the procedure is performed using encircling differential outside diameter (OD) coils. Such OD testing techniques are well accepted by industry and consumers alike. This technique has, in fact, been the standard tubing NDT practice for several decades and is incorporated into the ASME Boiler & Pressure Vessel Code. Details of this type of test and its advantages and limitations are defined in the HEI Tech Sheet #129. Although ultrasonic testing (UT), remote field testing (RFT) and flux leakage testing may be acceptable alternatives, they are only used when specified by the customer, or by a small number of product specifications such as ASTM B338. As a result, this document will only discuss the EC test. Once the tubing is manufactured, the owner or end user may specify an additional EC test using an ID probe. The purpose of this document is to describe the major differences between the two tests and what each is intended to accomplish. Charlie Chong/ Fion Zhang

http://www.heatexchange.org/pdf/techsheets/TechSheet132.pdf

Inspection from the Tube OD Most ASTM and ASME tubular product specifications require a nondestructive electric test NDE. The NDE tests may include eddy current testing, ultrasonic testing, or flux leakage testing. The product specifications do not necessarily designate which of these three must be used, and unless agreed upon in the purchase order, the test choice is at the option of the tube producer. The test that is the quickest, with the highest reliability and provides good sensitivity for finding sharp, abrupt defects is the OD eddy current test. It is the overwhelming choice of both tube manufacturers and end users.



The ASTM has developed recommended practices on how those tests may be performed. These include as follows: 1. ASTM E309 / SE309 –Standard Practice for Eddy-Current Examination of Steel Tubular Products Using Magnetic Saturation 2. ASTM E426 / SE426 –Standard Practice for Electromagnetic (EddyCurrent) Examination of Seamless and Welded Tubular Products, Austenitic Stainless Steel and Similar Alloys 3. ASTM E571 / SE571 –Standard Practice for Electromagnetic (EddyCurrent) Examination of Nickel and Nickel Alloy Tubular Products 4. ASTM E2096 - Standard Practice for In Situ Examination of Ferromagnetic Heat-Exchanger Tubes Using Remote Field Testing 5. ASTM E-690 / ASTM E690 - Standard Practice for In Situ Electromagnetic (Eddy-Current) Examination of Nonmagnetic Heat Exchanger Tubes



Approximately 25 years ago, the ASTM A01.09/A01.10 NDE task group recognized that the “E” practices identified above did not have sufficient detail to ensure that tube mills were incorporating all of the necessary ASTM requirements. In addition, there was no validation that the procedures used at one manufacturing plant would provide similar test results as those at another mill. As a result, the ASTM developed a number of additional requirements which were then added into the general tubular product specifications. These additional requirements specified items such as calibration size & location (artificial defect size & type, i.e., drilled hole or notch), and calibration procedures to ensure consistent & repeatable results. These requirements also included training and certification of operators, signal to noise ratio recommendations, and required equipment calibration standards. The general specifications that include the additional requirements are as follows: ■ ASTM A450 / A450M – Standard Specification for General Requirements for Carbon and Low Alloy Steel Tubes

■ ASTM A1016/A1016M - Standard Specification for General Requirements for Ferritic Alloy Steel, Austenitic Alloy Steel, and Stainless Steel Tubes



The ASTM/ASME OD EC testing of tubes is usually accomplished using only one frequency, typically within a range of 25 KHz to 100 KHz. As the magnetic field penetrates the metal, the ability to receive a signal lessens or attenuates. This phenomenon is called standard depth of penetration or alternatively, the “skin effect”. The depth of penetration decreases with increasing frequency, conductivity and magnetic permeability. As a result, the signal returning from an imperfection near the OD will be stronger than an identically sized imperfection away from the OD surface. The specifications do not address the imperfection’s location. Rejection is typically decided on a go/no-go signal amplitude criteria from an artificial defect described in the general specification or in the supplementary requirements of the product specifications. Magnetic properties or anomalies can be created in non-magnetic materials through minor parent metal alloy excursions, manufacturing, welding, strain-induced cold work and other processes and may not be detected using conventional OD saturation. These signals are defined as anomalies or discontinuities and are not considered a manufacturing defect. This magnetic coupling is achieved by using encircling coils to create a saturating magnetic field. This magnetic saturation does not necessarily improve the testing sensitivity or repeatability but does allow penetration of eddy currents in magnetic materials.



One additional advantage of OD ECT inspection is that the tubing can be fully magnetically saturated during testing to ensure the maximum level of sensitivity and repeatability, vastly reducing the occurrence of false indications on those materials which have ferromagnetic domains. As noted earlier, carbon and alloy steels, stainless steels and some nickel alloys may contain small magnetic domains that must be magnetically coupled during testing to minimize “noise” providing for “quiet” or higher signal to noise inspection with eddy currents. Even austenitic stainless steels which are considered to be non-magnetic, may have small magnetic regions from residual delta ferrite formed during the welding process or strain induced martensite from cold working. Special tube configurations such as integral ID and/or OD fins will require unique technologies that are not covered in this document but should be reviewed with the manufacturer prior to the onset of testing.



Inspection from the tube ID ID eddy current testing employing internal probe coils was developed as an in-service or baseline inspection tool to identify tube damage, discontinuities or operational wear. This damage may include pitting, cracking, wear from vibration or abrasion and other environmentally induced mechanisms. The indications are identified using ID probes that are passed down the length of the tube on a tethered cable that is connected to specially designed equipment containing an alternating current power source and electronics for recording and analyzing the output. The probes can be designed with differential encircling coils highly sensitive in identifying sharp, abrupt or axial damage (similar to OD testing), or can use pancake coils to identify longitudinally oriented damage. Internal probe coils can be operated in both the differential and absolute modes simultaneously for identifying both abrupt and gradually occurring discontinuities. The ID test is not only sensitive to tube damage and wear but may also identify other discontinuities including scratches and dents caused by transport and handling, installation, and OD and ID debris that can come from a variety of sources. Therefore, if a one-time test is performed on an existing heat exchanger, it may be difficult to determine which indications are the results of service vs. those that are a result of the manufacturing and installation process.



The baseline eddy current test The baseline eddy current test was developed to separate service related damage vs. manufacturing/installation process defects. The baseline test is most effective when performed immediately after the installation of the tubing and is typically done in the fabricator’s shop. It should be noted however that even with specialized electronics, the use of a bobbin coil will have considerable difficulty determining the precise discontinuity shape. Depth can be determined with a single frequency but multiple frequencies will improve the analysis. Signal length and a comparison of the absolute and differential signals from the same discontinuity can also help. The ability & knowledge of the signal analyst to correctly & accurately interpret potential failure mechanisms for the tubes serviced and the signals’ location in the heat exchanger becomes of paramount importance. A baseline test can be performed for the following reasons: ■ To determine if the tube was damaged during installation in the heat exchanger ■ To develop a database of discontinuities and anomalies, including their locations in the heat exchanger for comparison with future examinations



An initial ID baseline map facilitates in-service evaluations by comparing the initial readings to a second test after a given service exposure. Tracking indication changes becomes a useful tool to understand the effect of pits, cracks and other wall loss damage in the tubing over the service timeline. This information can then facilitate predictive maintenance programs. During ID testing, indications are normally identified as a percentage of wall loss which is determined by a combination of phase angle shift and different responses to multiple frequencies. Because natural damage may not provide identical size data to the artificial defects used to calibrate the equipment, accurate sizing of the damage needs to be verified by removing samples with indications and comparing them to the calculation made during the analysis. However, removal of actual samples may not always be practical; in that case, the analysis must rely on past experience.


There is no standard accept/reject criteria for ID electric testing of feedwater heater, condenser or balance of plant heat exchangers. Steam generator tubing has several criteria for rejection and many are associated to the artificial defects machined into the reference standard per ASME, Section V, Article 8, Appendix II. This reference standard does not simulate the feedwater heater, condenser and BOP heat exchanger indications and may result in excessive reject rates from non-injurious indications. ID testing to develop a baseline condition map is a mature technology and can be a very useful tool to help track tube damage and wear and future heat exchanger tube problems. Testing should be performed after the tubing is installed, rolled, seal welded and other surrounding manufacturing processes are completed. Detailed test information such as frequencies, probe speeds, phase angles and other parameters need to be carefully documented as well as probe descriptions and model numbers. When the test is duplicated, it can be compared easily to the baseline map to identify any changes to the tubes.


Conclusion Investigative efforts in producing this HEI Tech Sheet have not identified any known research or studies that have been performed comparing the results of OD EC testing vs. ID EC testing of new commercial grade tubes. As such, this Tech Sheet is compelled to identify concerns relative to this issue as follows: 1. 2. 3.

4. 5.

The impact of attenuation needs to be better understood and addressed. Testing performed from the OD will accentuate OD imperfections while ID testing will accentuate those on the ID. The use of different frequencies will also have a significant impact on the signal vs. depth of the discontinuity. With any eddy current testing, fill factor, or the distance between the coil and the tube, is critical for determining discontinuity sizing. A high fill factor and precise coil centering improves sensitivity while a low fill factor results in a less precise response. When OD testing is performed the tubing is rigidly held and centering within the coil is ensured through the use of stationary rolls in both in-line and offline testing. Depending on the calibration process, OD-tested tubes can either be held stationary or rotated during testing. ID probes rarely have effective centering devices and no requirement or specification currently exists to prove centering. In the case of ID probe coils, a high fill factor results in better centering. Poor centering results in less sensitivity in the hemisphere of the tube that has a larger gap between the probe coil and tube wall. In a baseline test, a good fill factor is usually achievable because the tubes are clean. Testing tubes that have been in service may result in lower fill factors because of ID fouling. Most ID eddy current probes do not have a method for saturation to ensure that small magnetic domains do not produce false indications. Those probes with saturation only have sufficient energy to saturate thin walls and the testing is significantly slower. If the ID testing is performed before installation in the bundle, imperfections developed during the installation process are typically ignored.

The OD ECT is the current industry norm for NDE certification of new tubing. Considering all of the issues above and in the absence of detailed comparative studies, the use of ID testing as an acceptance criterion for new tubing is not only controversial but highly subjective. In light of these concerns, it is therefore recommended that users discuss these issues in detail with the proposed tube manufacturers before specifying an ID test. Charlie Chong/ Fion Zhang

Pulsed Eddy Currents Systems


Reading 1： Pulsed Eddy Currents - PEC Technology Conventional eddy current testing uses a single frequency sinusoidal to excite a coil and, among many applications, measure flaw responses as voltage and phase changes on an impedance plane. Since different frequencies present different sensitivity behaviors, multi-frequency testing is sometimes performed. In that case, multi-frequency eddy current measurements are either performed by simultaneous injection or multiplexing of multiple frequency components. In pulsed eddy current (PEC) testing, multi-frequency inspections are performed by driving a coil with a broadband pulse instead of a monochromatic excitation. This results in broader frequency contents than standard eddy current signals, as well as offering a better penetration into the depth of a material. The measured response of a PEC inspection is a waveform, similar to an ultrasonic A-Scan, from which features can be extracted to characterize flaws or for example perform thickness measurements. A temporal analysis of the transient response of the coil that results from this excitation can provide useful information about the depth of a defect. Pulsed eddy current is an ongoing research field as novel probes as well as new ways of interpreting and quantifying results are still required to fully exploit their potential.


http://www.pecscan.ca/BasicsPECtech.html

Although still an active research field, PEC technology already has its place among the NDT techniques. The first application that benefits from the use of PEC is the detection of corrosion under insulation (CUI), where PEC has been used for many years to measure the remaining wall thickness of material buried below up to 6” of insulation material. PEC has also been used to detect deeply embedded corrosion or cracks in the multi-layered aluminum structures used in the aerospace industry.



PEC Definitions Transient response: A transient event or response is a short-lived oscillation caused by a sudden change of voltage, current, or load. In pulsed eddy current, it expresses the time-dependant behavior of the coil-inspected material response to the input pulse. Balanced signal: Result of the subtraction of a voltage response by a reference signal, generally taken on an unflawed area of a test sample. The balanced signal is null for unflawed regions and displays amplitude variations when a defect or thickness change is encountered. It is similar in nature to performing a null with a standard eddy current apparatus.



LOI (Lift-Off point of Intersection): The LOI is the location of the crossing point between a transient voltage response acquired on a sample and a response taken with a certain probe lift-off: the LOI is a position where the signal does not vary with probe lift-off. Monitoring the voltage response in the vicinity of the LOI point location therefore provides a mean of performing Pulsed Eddy Current inspections that are free of lift-off.



Monitoring the actual displacement of the LOI point also provides useful information as the position of the LOI is dependant of the sample properties (material, thickness, etc.). An important application of shift is the possibility to perform thickness measurements from the variations of the LOI point coordinates.



Gap Point : Similar to the LOI, the gap point defines a coordinates of the voltage response that is independent of gap variations between two layers of a multi-layered sample. This point is located further in time than the LOI. Spectral analysis: Spectral analysis consists of performing a conventional eddy current analysis of the frequencies contained in a PEC signal. The spectral analysis approach is a variation of the multi-frequency eddy current field but benefits a complete spectrum instead of finite frequencies.



Probes Probes play an important part in Pulsed Eddy Current inspections. Their selection basically depends on the dimensions and shape of the flaws that need to be detected in relation with the properties of the part (material, number of layers, etc.). The probe that is most commonly used for Pulsed eddy current inspections is a reflection type, which means that the device used to induce the pulsed eddy currents is different than the device that receives their effects. Different combinations of driving/receiving sensors can be used to perform pulsed eddy current measurements. Driving Coil: Induction of Pulsed eddy currents The generation or induction of the pulsed eddy currents is typically done using a coil. The purpose of the coil is to convert an electrical pulse (driving pulse) into a magnetic field which induces eddy currents into the tested material following Faraday's laws of induction. The physical and electromagnetic characteristics of the driving coil partly define the bandwidth and footprint of the induced pulsed eddy currents.



Driving Coil



Coil receiver: Conventional eddy current probes Conventional eddy current probes use both a coil as the driving and receiving sensor. The reception of the eddy currents is again based on Faraday's induction laws. When a voltage I applied to the driving coil, it creates a magnetic field that induces eddy currents in the tested material. In return, these eddy currents generate an additional magnetic field that interacts with the initial one. A receiving coil picks up the variations of that resulting magnetic field and converts it into a measurable electrical signal.

Driving Coil Receiving Coil



Hall effect sensors In opposition to coils which measure variations of a magnetic field, Hall effect sensors allow for the direct measurement of a magnetic field. This difference allows for a better measurement of magnetic fields that do not vary rapidly. GMR sensors Giant Magneto Resistance sensors (GMR sensors) make use of a phenomenon discovered in 1988 and observed in thin film structures composed of alternating ferromagnetic and nonmagnetic layers, where the electrical resistance of the GMR varies in the presence of a magnetic field. While it does not rely on the same principles, this sensor is equivalent to a Hall sensor in the sense that it also provides a voltage output that is proportional to the magnetic field.

GMR sensors



PEC Results PEC is considered a new technology rather than an improvement of conventional eddy current. By changing the pulse excitation to a square wave, we input and receive signals that are quite different form conventional eddy currents. For this reason, PEC requires particular signal processing techniques which differ from the usual amplitude and phase analysis techniques. There is no denying that considerable information is available in the temporal and spectral analysis of these pulses. Because of the considerable amount of information available and inherent to the technology, the physical phenomenon must be well understood to discriminate between flaws and other artifacts



Crack Detection using Pulsed Eddy Currents - PEC Crack detection on multilayered aircraft structures is achieved with two different PEC analysis methods. The PEC analysis method is selected based on the layer thickness and the rivet head physical properties. The designated PEC system requires proper calibration to obtain the desired detection.

Introduction The detection of cracks is of great importance in aerospace structures as they can rapidly grow to cause catastrophic failures. Eddy currents, ultrasounds and radiography are the most common ways of inspecting this type of defect. While radiography has a limited use in tight spaces and because of security reasons, eddy current and ultrasonic inspections fail to detect cracks in all situations. Ultrasonic inspections require a mechanical bonding in order to propagate through multiple layers, which is not always the case for riveted structures. On the other hand, eddy currents can penetrate through unbounded layers, but at limited depths (typically 2 layers). Like eddy currents, Pulsed Eddy Currents have the particular advantage of being able to monitor multiple layers without the need for mechanical bonding. In the case of multilayered aerospace structures, a magnetic field that is strong enough to penetrate all layers of interest must be generated. When this is achieved, pulsed eddy currents are produced on both surfaces of each layer and, from the principles of mutual-inductance, generate an additional magnetic field that interact with the one coming from the driving coil. The presence of cracks affects the pulsed eddy currents and can be monitored in the resulting field. Multiple features can be used to detect cracks from either the transient waveform or its spectral representation.


http://www.pecscan.ca/PDFs/Application-note-Cracks-Revised.pdf

Experiments C-Scan inspections of multilayered aircraft structures can be done using the ARMANDA scanner (figure 1a), which is a portable scanner that can be fixed on the structure. A PEC inspection was performed using this scanner on a riveted eddy current standard (2 aluminum layers of 0.04” with the bottom layer containing EDM notches of lengths of 0.250”, 0.200”, 0.150” and 0.100” on the rivet holes edge and identified from {1} to {4} on figure 2a). For sample inspection, we selected a conventional reflection eddy current probe (700 Hz - 15 kHz). The PecScan™ driver/receiver unit (figure 1b) is used to drive the probe, generate and receive the PEC signals.



Fig. 1 (a) ARMANDA - Automated scanner Pulsed Eddy Current generation and reception. utomated for PEC testing (b) PecScan™ Driver/Receiver unit for Pulsed Eddy Current generation and reception.



A picture of the sample is presented in figure 2a. Figure 2b shows the result obtained by analyzing the PEC waveforms using a temporal method (feature: total energy in a time gate) in the form of a C-Scan image. On the other hand, figure 2c shows the C obtained through spectral analysis (feature: single frequency component of 10 kHz extracted from the PEC waveforms). This spectral analysis allows displaying the content of the selected frequency on an impedance plane the same way it is performed in conventional eddy current inspections. Based on the impedance response measured on a good rivet, a rotation is applied on the 10 kHz component to minimize the effects of the rivet edge, leading to the result presented in figure 2 (c).



Fig. 2. Images of the Eddy current standard samples {1}, 0.200” {2}, 0.150” {3} and 0.100” waveforms (energy within a time gate); all scales in analysis of the PEC waveforms: imaginary part of the 10 kHz component after rotation of the rivet edge signals. (d) Color palette used to display the C samples. (a) Picture showing the EDM notches of 0.250” {4}. (b) C-Scan obtained from the temporal analysis of the PEC mm. (c) CScan obtained from the spectral C-Scans.



Pulsed Eddy Currents Systems Pulsed Eddy Currents offer great potential for corrosion detection and location in thick structures. The wide band frequency spectrum of Pulsed Eddy Currents allows the determination of a large number of parameters, such as defect size and location. In fact, Pulsed Eddy Current techniques have the potential to become the primary method of corrosion detection in multi-layered structures. Our research concerning Pulsed Eddy Current technologies concentrates on the detection of corrosion and measurement of wall thickness of insulated pipelines. In order to optimize inspection productivity and costs, it is imperative to improve the quality of inspection and corrosion data interpretation. Our research efforts therefore revolve around the interpretation of corrosion data and the integration of Pulsed Eddy Current techniques to commercial inspection systems.


http://www.tecscan.ca/solutions/advanced/pulsed-eddy-current/

Reading 2： 4.9.2.3 Pulsed Eddy Current Testing. Conventional multifrequency systems usually utilize two or three frequencies. Additional frequencies require very complex multiplex mixing systems to analyze the information from the test. A variety of experimental techniques have utilized the multifrequency characteristics of a short electrical pulse to achieve the same type of results as the multifrequency test technique. In principle, this technique is advantageous in that it requires simpler electronics to process the data. It can potentially generate higher frequencies than fixed frequency systems. This would allow testing of thinner materials, and materials with very low electrical conductivity (high resistivity). The eddy current pulse can also be a very short, high voltage pulse that can be used to momentarily produce magnetic saturation in a ferromagnetic part. This will allow detection of subsurface flaws in ferromagnetic materials.

4.9.2.4 Low Frequency Eddy Current Inspection. In the past most eddy current testing utilized test frequencies of 10 kHz to 1 MHz .Improved equipment and data processing techniques now allow the use of test frequencies as low as 55 Hz. Along with impedance plane equipment to measure signal phase, this has provided a means for testing multilayer materials and thick materials. Detection of deep subsurface cracks, cracking in intermediate layers of material, and corrosion on the backside of a material are possible.

4.9.2.5 Barkhausen Noise Testing Of Ferromagnetic Materials. Abnormal stresses induced by shot peening, other cold working processes, and grinding burns affect the structural properties of a material and can lead to flaw growth and part failure. In ferromagnetic materials, these processes affect the ease with which the magnetic domains in the surface of the material can be moved. In un-magnetized ferromagnetic material, the magnetic domains are randomly oriented. If the material is subjected to a magnetic field, the magnetic domains tend to align themselves in the direction of the magnetic field. When the domains move to align themselves, electrical pulses are generated during the domain movement. This is called Barkhausen noise. This electrical noise can be detected and measured by Hall effect sensors. If the material is free of abnormal stresses, the domains are relatively free to move and little Barkhausen noise is generated. Areas of tensile stress parallel to the applied magnetic field cause an increase in Barkhausen noise. Examples of applications of this test method are ferromagnetic engine components and landing gear. Barkhausen noise measurements are also used to detect the quality of drilling and reaming of holes in ferromagnetic material.


http://chemical-biological.tpub.com/TM-1-1500-335-23/css/TM-1-1500-335-23_419.htm

Good Luck


Good Luck


https://www.yumpu.com/en/browse/user/charliechong Charlie Chong/ Fion Zhang

Electromagnetic Testing-ASNT Level III Study Guide ET

Recommend Documents