ASNT Level III Study Guide
ElectroDlag etic Testing second edition
The American Society for Nond estructive Testing
ASNT Level III Study Guide
ElectroDla Testing second edition
The American Society for Nondestructive Testing, Inc.
etic
Publi!>hed by The Ameriean Society for Nondestructive Testing. Inc. 17 11 Arlingale Lune PO Box 285 18 Columbu!>. OH 43228-05 18 No purt of this book may be reproduced or transmitted in any form, by means electronic or mechanical including photocopying , recording . or otherwise . without the expressed prior written pennission of the publisher. Copyright 10 2007 by The American Society for Nondestructive Testing. Inc. ASNT is not responsible for the authenticity or accuracy of infonnation herein. Products or services that are advertised or mentioned do not carry the endorsement or recommendation o f ASNT. IRRSP, NDT Handbook. The NDT Technician and www.asnt.org are trademarks of The American Society for Nondestructive Testing. Inc. ACep, ASNT, Level III Study Guide, Materials Evaluation . Nondestructive Testing Handbook, Research in Nondestructive Evalllation and RNDE and are registered trademarks of The American Society for Nondestructi ve Testing, Inc. ASNT Mission Statement: ASI\/'[ exists to create a safer world by promOTing the profession alld rechnologies afnondestructive resting.
ISBN-13: 978-1·57117-164-1 Printed in the United States of Amerie:l
09/07 first printing
II
Foreword ASNT methods committees, at the direction of the Technical and Education Council, have prepared Level III Study Guides that aTe intended to present the major a(('as in each nondestructive testing method. This Study Guide was updated and revised v..r:ith the assistance of the Electromagnetics Committee. The Lcvcllll candidate should use ASNT Level III Study GJ/ide: Electromagnetic Testillg Method only as a review, as it may not contain all of the informa tion necessary to pass a typical ASNT Level III cxaminCltion. The electromagnetic testing method has several subdisciplines. The general consensus at the time of this revision is that there are four specific field techniques: eddy current testing. flux leakage testing. remote field testing and alternating current field measurement. Each of these techniques may provide some information in specific material testing applications that the others may not be able to provide in the same test situation. The primary focus of this document wilt be eddy current testing. Some information is provided to define how the other electromagnetic testing techniques might be applied.. In using this Study Guide, the reader ,vill be given specific references, including page numbers, where additional detailed information can be obtained. Typical Level III question s are available at the end of each chapter to aid in detennining comprehension of the material. A typical use of this Study Guide might include the foUo·wing sequence: An individual should review the qucstions at the end of each chapter in the Study Guide to detcnnine if his or her comprehension of electromagnetic testing is adequate. The questions will serve as an indicator of the individual 's ability to pass a Level III examination. If the individual finds questions in a certain chapter of the Study Guide to be difficult, it is suggested that the individual carefully study the information presented in that chaptcr. This review of the information in the Study Guide will serve to refresh one's mt.'mory of theory and forgotten facts. If the individual encounters information that is new or not clearly understood, then it is important to note the specific references given tluoughout the Study Guide and carefully read this information. Referenccs are indicated by parentheses and the reference number: (N).
iii
Preface Early experimenters in the field of magnetism and electromagnetism established the basis fo r the principles of electromagnetic nondestructive testing used today. In 1820, Hans Christian Oersted discovered the magnetic field surrounding a conductor when current "w as passed through the conductor. In 1820, Andre-Marie Ampere discovered that equal currents flowing in opposite directions in adjacent conductors cancelled the magnetic effect. This discovery has led to development of modern coil arrangements and shielding techniques. In 1824, Dominique F. Arago discovered that the vibration of a m agnetic needle was rapidly damped when it was placed neM a n onmagnetic conducting disk. Michael Faraday discovered the principles of electromagnetic induction in 1831. James Clerk Maxwell integrated the results o f these and other works in a two-volume work published in 1873 and Max.vell's equations are still the basis for investigations of the magnetic and electromagnetic phenomena. The application of these laws and principles has led to the development of an industry whose purpose is to qualitatively and quantitatively investigate the properties and characteristics of electrically conductive materials using nondestructive electromagnetic techniques. As in any industry, controls and guidelines must be established to ensure consistent and reproducible products or services. This Study Guide is intended to provide ASNT Level III candidates w ith a concise reference with which to prepare for the ASNT Level III Examination.
iv
Acknowledgments A special thank you to the technical editor who coordinated this revision and updated major portions himself: Jim Cox, JECNDT, LLC A special thank you goes to the follow ing reviewers who helped with this publication: Claude Davis, Unified Testing & Engineering Services, Inc. Darrell Harris, Anchorage, Alaska Gary Heath, All T~ch Inspection, Inc. Michael J. Ruddy, Tuboscope NOV The Publications Review Committee includes: Chair, Joseph L. Mackin, Intemational Pipe Inspectors Association Stephen P. Black. Clermont, Florida Mark A. Randig, Team Industrial Services, Tnc.
Cynthia Meter Leeman Educational Materials Supervisor
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Table of Contents Foreword ................ • ....•....•.... • .... •. .... Preface .................. . .. , . . .. .......... . . • . . . .•. Acknowledgments .......... . , .............. , .... . . .
. . . Ill . . . . 1V
. . . . . .. V
Chapter 1 - Principles of Eddy Current Testing · ....... ... . .... 1 · . . . . . .. . . ... . .. 1 Historical Background Generation of Eddy Currents ....... 2 Field Intensity ........ . ...... . . ..... .3 Current Density ....... . . .. .4 Phase ! Amplitude and Current Time Relationships . .5 Chapter 1 - Review Questions. , . . ..... . . . . .. 7 Chapter 2 - Test Coil Arrangements .... ... . . . •. . . . •. .. . •... ...... 9 Probe Coils ....... . ........ .•. . . .• . .. . • . . . . •. ...... .. 9 Encircling Coils ...........................•.. .. • ....... .. 9 Bobbin Coils .............. • .... • . . .. • .... • .... • .... . ... 10 Absolute Coils .............. ... . . . . . . . • .. . .. . .......10 Differential Coils . . .. . ...... . · •.• . .• ...... . .10 Hybrid Coils . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . . . . .11 Additional Coil Characteristics . . . . . . . . . . . . . . . . . . .12 Chapter 2 - Review Questions. . . . . ....... . • . ...... . .. ... . . .13 Chapter 3 - Test Coil Design .... . .15 .. . .15 Resistance Inductance ..... . . .. 15 Inductive Reactance ...... . .... . .... . .... • . ... •. . .... .... 16 Impedance ............................. . .... ... .. ...... 17 Q or Figure of Merit . .. . . ... . ... . . ... . . .18 Permeability and Shielding Effects .... 18 Coil Fixtures ......... . .1 9 Chapter 3 - Review Questions. . . . . . . . . . . . .20 Chapter 4 - Effects of Test Object on Test Coil . . . . . .21 Electrical Conductivity .................... . . ... 21 Permeability .... ... ....... ... ..... ....... . .... . • .. ..... 22 Skin Effect ......... . • . . . . .• . . . . • . . . • •. .. . •. ..... .23 Edge Effect ... .. •.. .. •.. .. .. . . . .... . ....•...... .23 End Effect .......•. . ..•.... • .. ... .. 23 Lift Off ....... . . . • . .. .•. ..... .23 Fill Factor .......... . . . . . . . . .24 Discontinuities .. ... .•. . . . .25 Signal-to-Noise Ratio .25 Chapter 4 - Review Questions . . . . . • ..... . • . . •.•.... • . .•. ... ... 26
vii
Ch apter 5 - Selection of Test Frequency .. Frequency Selection .... . . . .... . Single Frequency Systems Multifreql1ency Systems Chapter 5 - Review Questions ...
. ..... 27 . .. 27 .... 27 . ..30 . .. • ... .. . . . ... .32
Chapter 6 - Instrument Systems . . . . . . . . . . . . .. 33 Impedance Testing . . . .. . .... . ......... . . . . . ... 34 Phase Analysis Testing . .. . ... ... . ... ...... .... ..34 Vector Point . . . ....... . .. .... . ... ... .. .. .. . . •. . . . . ..34 Ellipse .. . . . . ... .. . .. . . . . ... . . . ........ . . . ..34 Linear Tune Base ...... .. . . . . ...... . . . . . . . . . . .. 34 Impedance Plane Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35 Mode of Operation ...... . . . . . . . .. . . . .. . . . . .•....... 36 Signal Compensation . . . . . . . . . . . . . . . . . . . .. 36 Test Coil Ex("; tation ..... .. .. . .. .. . . . . .. .36 Read Out Mechanisms . . . .. . . . . . . . . . . . . . . . . . . ..38 Indicator Lights .. . .. ........ .38 Audio Alarms .. .38 Meters .. . • .. . ... .39 Digital Displays .39 Cathode Ray Tubes ... 4() Recorders ... 4() Computers . . . . ..... .. . . . . .. .. ... . .. . . . .. Al Test Object Handling Equipment . . .... . . . . . . . . .. Al Probe Delivery Systems . . ... . . .. ...... . .42 Chapter 6 - Review Questions ..... .. ... . .. . . . . . .. 44 Chapter 7 - Eddy Current Applications ... ..A5 Discontinuity Detection . • ........ .45 Dimensiona l MeaslUements .. . . • .• ... .. . .47 Conductivity Measurements . . . . . .• . .•. • . ... • . . ..... 48 Hardness MeaslUements . . ..... . . . . ... 48 Alloy Sorting .. . . . . .... . ... .. ...... .. ..... . .. .. .. . .. . . .48 Chapter 7 - Review Question s .. . ............ .. .......... 50 Chapter 8 - Other Electromagnetic Techniq ues ...... . ............ .51 Alternating Current Field Measu rement .................. .51 Advantages Compared to Magnetic Particle and Dye Penetrant Inspection . ... ..... . . .. . . ....... . . . . .. . . . .. 51 Flux Leakage Testing .... .............. . .. . . . . . .52 Remote Field Testing .... . . . . . . ...53 Chapter 8 - Review Questions . . . . . . . . . . . . ....57 Chapter 9 - Eddy Current Procedures, Standards and Specifications .59 Ametican Society for Testing and Materials. . . . . . . , .59 Military Standard ....... ........... . . .. .60 American Society of Mechanical Engineers .......... . . .. . .60 S
viii
..68 ..69 . . .70
Chapter 1 Principles of Eddy Current Testing Historical Background
Figure 1.1: Arago's Experiment, 182 1
Before d iscus..<;ing the principles o f eddy current
testing. it seems appropriate to briefly discuss the facets of magnetism and electromagnetism that
serve as the fo undation for this study. in the period from 1
n s to 1900, scientific
experimenters Andre-M a ri e Ampere,
F ran~ios
Arago, Charles Augu stin Coulo m b, Micha el Faraday, Lord William Thomso n Kelvin, James
Clerk Maxwell and Hans Christian Oersted investigated and cataloged most of what is known about magnetism and electromagnetism. Arago d iscovered that the oscillation of a
magnet was rapidly damped when a nonmagnetic Magnet
conducting disk was p laced near the m agnet. 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 c.a u ~ing eddy current" 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. Oersted discovered the presence of a magnetic field around a current carrying conductor and observed a magnetic field developed in a perpendicular plane to the direction of current flow in a wire. Ampere observed that equal and opposite cu rrents flowing in adjacent conductors cancelled this magnetic effect. Ampere's observiltion is used in differential coit app lications and to manufilctu re noninductive, precision resistors. Faraday's first experiments investigated ind uced currents by the relative motion of magnet and a coit (Figure 1.2). Faraday's major contribution was the discovery of electromagnetic ind uction. Hi s 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, V, is nearby. When switch 5 is closed it produces a current in coil A in the direction shown (a). A momentary current is also induced in coil B in a direction (b) opposite to the current flow in coil A. If 5 is now o pened, 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 chflnging.
Conductive material
Conductive material Changing flu x density in material
1
The electromotive force (voltage) induced in coil B of Figure 1.3 can be expressed as follows:
in 1873. Maxwell not only chronicled most of the work done in electricity and magnetism at that time, but he also developed and published a group of relations known as Maxwell's E.quations for the electromagnetic field. These equations form the base that mathematically describes most of what is known about electromagnetism today (1). In 1849 Lord Kelvin appUed Bessel's Equation to solve the elements of an electromagnetic field. The principles of eddy current testing depend on the process of electromagnetic induction. This process includes a test coil through which a v
E = N/;'~ K /;.1
Equation 1 where: E l:J./l:J.I
= = =
K
=
N
average induced voltage, number of hlrns of wire in coil B ra te of change of magnetic lines of fo rce affecting coil B, I ll'
Maxwell produced a two-volume work, A
Treatise on Electricity Qnd Magnetism, first published
Generation of Eddy Currents
Fig ure 1.2: Induced current with coil and magnet
N
When an electrically conductive test objecl is placed in the primary field, an electrical current will be induced in the test object. This current is known as the eddy current. Figure 1.4 is a simple model that illustrates the relationships of primary and secondary electromagnetic events. Conductor A represents a portion of a test coil. Conductor B represents a portion of a test object. Following Lenz's L..w/ and indicati.ng the instantaneous direction of the primary current (Ip) a primary field (411') is developed about Conductor A. When Conductor B is brought into the influence of $r an eddy current (IE) is induced in Conductor D. This electrical current (IE) produces a secondary electromagnetic field ($E) that opposes tht: primary
t
Figure 1.4: Induced current relations hips Conductor B (Test object)
Figure 1.3: Induced current, electromagnetic technique CoilA
Coil B
~
b
.t ljl
~b Core
I/v I\.
Conductor A (Test coil)
'-----
", 'E "
= Primary current = Primary magnetic field = Secondary (eddy)
2
current Secondary magnetic field
electromagnetic field (4)p). The magnitude of $E is directly proportional to the magnitude of IE' Characteristic changes in Conductor B such as conductivity, permeability or geometry will cause IE to change. When IE varies $E also varies. Variations of ~E are renectcd to Conductor A by changes in ~p. These changes are detected and displayed on some type of readout mechanism that relates these variations to the characteristic thai is of interest.
Figure 1.6: Induced current flow in part
Field Intensity
--
The electromagnetic field produced about an unloaded test coil can be described as decreasing in i.ntensity 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 proportional to the magnitude of applied current, rate of change of current or frequency and the coil parameters. Coil parameters include inductance, diameter, length, thickness, number of tums of wire and core material. To better understand the principles under discussion, it is important to again look at the instantaneous relationships of current and magnetic nux. The exciting current is supplied to the coil by an alternating current generator or oscillator. With a primary current Ir nowing through the coil, a primary electromagnetic field $p is produced abolll the coil. When thiS excited test coil is placed on an electrically conductive test object, eddy cu rrents IE will be generated in that test object. Figure 1.5 illustrates this concept.
A more precise method of describing the relationships of magnetic flux, voltage and current is the plUJse 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:
Figure 1.7(a) Ep primary coil voltage I excitation current Qp primary magnetic flux 'lis secondary magnetic flux Figure 1.7(b)
Ep = I = 4!p =
FIgure 1.5: Generation of eddy current in a test object Ip
Es 4'5 Er th
• Coil
.. t - - - - ..
a cylindrical
=
primary coil voltage excitation current primary magnetic flux secondary coil voltage secondary magnetic flux total coil voltage total magnetic flux
In Figure 1.7(a) the current (I) and primary magnetic flux (4!p) are plotted in phase. The primary voltage (1;,) is shown separa ted by 90 electrical degrees. The secondary magnetic flux (4'5) 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 Figure 1.7(b) one can see by vectorial addition of ~ and Es that a new coil voltage (Er) is arrived at for the loaded condition. The primary magnetic flux Qp and secondary magnetic flux 4'5 are also combined by vectorial addition to arrive at a new magnetic flux ('h) for the loaded coil.
'E @16=37%
-·--··--i· le@26=13.5% .........:.,. 'e@3S = 5%
Note the d irection of the primary current (Ir) and the resultant eddy current (IE)' IEextends some distance into the test object. Another important observation is that IE is generated ill tlJe same plane in which the coil is wound. Figure 1.6 emphasizes this point with a loop coil surrounding a cylindrical lest object (4). 3
d istance. 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. TItis 37% point is chosen because change!'; in X axis values produce significant changes in Y axis values from ]00% to 37% and below 37% changes in X axis values produ ce less significant changes in Y axis values. App lying this logic to ed dy current testing, a ternl is developed to describe the relationship of current d istribution in the test object. The eddy current generated at the surface of the test object nearest the test coil is ]00%. The point in the test objt.>ct thickness where thi!'; current is diminished to 37% of its previous strength is known as the standard depth of penetration. The term 0 (delta) is used to represent this point in the material. Figure ] .8 is a relative eddy current deruity cu rve for a plan e \'lave of infinite ex tent with magnetic field parallel to the conducting test object surface.
Figure 1.7: Phasor diagram of coil voltage
Glp
¢l 5 " 0
(a) Phasor diagram of coil voltage withoullesl object
E,
Figure 1.8: Rela tive eddy current density 1.2
1.0 0.8 0.6
0,
0.4
.p
0.2
/
~Iandard depth of penetr ation where density of eddy cu rrent 37% of density at the s urface
\ II = \ I'"
I'---
(b) Phasor diagram of coil voltage with test object
o
2
3
4
5
6
Units 01 depth (in multiples 01 the standard depth of penetration)
Notice that for the condition of the test object in the test coil, ~j' is no longer in phase with the excitation current L Also observe tha t the included angle between the exci tation current and the new coil voltage ET is no [anger at 90 electrical degrees. These interactions will be discussed in detail later in this study guid e.
The current density at any depth can be calcula ted as:
J X -- J oe -.f,JIf!/KI Equation 2
Current Density
where:
1., /,
The d istribution 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 d ensity follow!'; the mathematical rules for a natura l exponential decay curve (1 / e) where e (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
n f ~
x
cr e 4
= = =
current density at depth x current density at surface, amperes per square meter 3.1416 frequency in hertz magnetic permeability, henries per meter (H i m) depth from !';urface, meters electric conductivity, siemens per meter 2.7] 8
Using 1.35 m m 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 d ensity of 0.37 or 37%. What is the relative eddy current density at 3 mm? Depth x equals 3 mm and depth of penetration is 1.35 mm, therefore:
Magnetic permeability, ~, is a combination of terms. For nonmagnetic materials:
I' = 41t • 10" Hi m Equation 3 For magnetic materials: tl;: IJ., ).10 where:
3+ 1.35 = 2.222
).l, ;: relati ve penneab ility
1'0 = 4,,·10-' Him
This ratio ind icates a relative eddy current d ensi ty of about 0.'1 or 10%. With only 10% of the available current flowing at a depth of 3 mm, delectability of variables such as condu ctivity, permeability and discontinuities would be very difficult to detect. The obvious solution for grea ter detectability at the 3 mm depth is to lower the test frequency. Frequency selection w ill be covered in detail later in this text.
The stand ard deplh of penetration can be c
Equation 4 where: 0 = rr
f ~
a
= = = =
standard depth o f penetration. meters
Phase/Amplitud e and Current Time Relationships
3.141 6
frequency in hertz magnetic permeability. HI m electric conducti vity, siemens per meter
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 penet rate the test part. Phase and time are analogous meaning - p/mst is an electrical term used to describe timing relationships of electrical waveforms.
It should be observed at this point that as frequency, conductivity o r permeabi li ty 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 exa mple of a very lhick block of sta inless steel being interroga ted with a surface o r p robe coil opcr
0=
Figure 1.9: Radians lagging
3 Radians lagging
,
I J lt f .t"J
0=
J3.
2
I 1416.100,000·4". 10-' ·0 .14· 10'
/ 1/ ,
/ •
2
•
3
depth X de pth of penetralion
Ii = c:-:--:-:c 743.438
Phase is usually expressed in either degrees o r radia ns. There arc 2n: radians per 360 degrees. Each radian therefore is about 57 degrees. Using the surface eddy current ncar the test coil as a reference, eddy current occurring deeper in the test object lags
o= 0.00 135 meters 8;: 1.35 nun
5
the su rface current. The amount o f phase lag is determined by
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 O.017mm. That would be considered excellent resolution. There is an obvious limitation. Refer to Figure 1.8 and observe the resultant relative current density with an x I S ratio of 5. The relative current density is near O. 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. In summary, eddy currents have been explai ned, how they are created and how they move through electrically conductive materials. Once the application of these rules in the real world is understood, eddy current testing can be used for a wide range of inspection applications in electrically conductive materials: • It is possible to measure the size or shape of parte;. • It is possible to measure variations in the grade or chemiStry (alloy) of those parts. • It is possible to determine if and how those parts have been heat treated. • Eddy current testing can be used to help determine if there are manufacturing discontinuities that need to be addressed. • It is possible to determine if there are service induced discontinuities that may limit the use of the part.
8 = X,J7r/,u(J
= depth/depth of penetration Equation 5
where 9 equal,> the phase ang le lag in radians. Figure 1.9 should be used a .. a relative indicator of phase lag. The exact phase relationship fo r 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 abilHy to separate variables occurring in the test object; for example, distinguishing two d iscontinuities occurring at different depths in the same test object. A.. an example, using a standard depth of penetration at 1 nun in a 5 nun 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 ] mm and depth o f penetration (15) is 1 mm, the xl 15 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 j therefore, the ratio x IS equals 5. This produces a
6
Chapter 1 Review Questions Q.1.1
Q.1.2
Generation of eddy currents depends on the principle of: A. wave guide theory. B. electromagnetic induction. C. magnetorestrictive forces. D. all of the above.
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%.
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.
c.
50%. D. 100%.
Q.1.8 Q.1.3
When a nonferromagnetic 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.
A. 3
B. <0.1 C. 1/ 3 D. indeterminate Q.l.9
Q.l.4
Q.1.5
Refer to Figure 1.8. If one standard depth of penetration was established at 1 mm in an object 3 nun thick, what is the relative current density on the far surface?
Refer to Figure 1.7(b). If Er 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. E-r would increase and be at a different angle. C. Because both materials are nonferromagnetic, no change occurs. D. None of the above.
Refer to Figure 1.9. Using the example in question 1.8, what is the phase difference behveen the near and far su rfaces? 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.l.10 Calculate the standard depth of penetration at 10 kHz in copper. eu has a cr = 5.7 x107 siemens per meter. A. 0.1 mm (3.9 x 10-3 in.) B. 0.02 mm (7.9 x 10--1 in. ) C. 0.66 mm (0.026 in.) D. 66 mm (2.6 in.)
Eddy currents generated in 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.
7
Chapter 2 Test Coil Arrangements Test coils can be c
either manually or mech i11lically to provide a hel ical scan of the hole using a spimllllg probe technique (Figure 2.2 l.
Probe Coils
Figure 2.2 : Bolt hole inspection probes
Surface coil, probe coil, flat coil or pl/Hcake coil are all common terms used to describe the same test coil type. Probe coils provide a convenien t method of examining the surface of a test object. Figure 2.1 illustrates a typical set of probe coils used for 5e\'eral surface scanning applications. Figure 2.1: Probe coil
Probe coils and probe coil forms can be shaped to fit particular geometries to solve complex inspection probkms. As an exampk, probe coils fabricated in a JJf'llrii s hape (pencil probe) are used lo inspect threaded areas of mounting studs and nuts or serrat ed areas of lurbine wheels and turbine blade assemblies. Probe coiL<; 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 scalUled 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
Encircling Coils Encirclil1g coil. olltside diame ter wil and feed thro1lgh coil are terms commonly used to describe a coil that surrounds the test object. Figure 2.3 illustra tes a typical encircling coil. Encircling coils aTe primarily u sed to inspect hlbular and bar-shaped products. The tube or b ar is fed through the coil (feed through) at relatively high speed. The cross section of the test object '''ithin the test coil is simultaneously interrogated. For th is 9
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 frequen cy, properly adjust the system display parameters and enSlUe that the tube is centered in the coil at all time.c; to achieve optimum test sensitivity.
Figure 2.3: Encircling coil Crack
Direction of tube travel
Bobbin Coils Bobbin coil, inside dinmeter coil and illside 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 hlbe inside diameter by long, semiflexible shafts or simply blown in with air and retrieved with an attached pun cable. These mechanisms will be described later in the tex t. Bobbin coil information follows the same basic rules stated for encircling coils. Figure 2.4 illustrates a typical bobbin coil
\ Tube
Figure 2.4: Bobbin coil
Probe coils, encircling coils and bobbin coils can be additionally classified (16). These additional c1ac;sifications 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 d ifferential coil arrangements.
reason, the circumferential location of discontinuities calUlot be determined with an encircling coil (4). 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 oVt:!r the encircling coil is that the probe coil could define where within the circumferential plane the discontinuity t:!xists. The encircling coil cannot make that distinction. If there arc multiple signal sources within the coil's field of vicw the encircling coil response will indicate thc average of all of those events. 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 discontinu ity response is difficult to obtain. To ensure proper centering it is common practice to run the
Absolute Coils
An absolute coil makes its measurement without direct reference or compa rison to a standard as the measurement is being made (6). Some applications for absolute coil systems would be measurements of conductivity, penneability, dimensions and hardness. Differential Coils
Differential coils consist of two or more coils electrically connected to oppo~ each other. Differential coils can be categorized into two types, 10
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 \\lith one coil affected by the s tandard object and the other coil affected by an acceptable test object. The external rderence differential coil system is sensitive to all measurable differences between the stan dard 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 2.5: Test coil configurations for eddy current testing of small-diameter tubing Differential
Absolute
k b .-c
F'\ d{HV4 II tr:J--
b
2222YA Fr~1
I.
f
rUuWiJlF',
J
.
:50 0 =
mO
IIc
00
,3t==
J
:!!
·6
°0
D
e
"-
•
:0
"0
0
OF -
Hybrid Coils Hybrid coils may be defined as driver/pickup, through transmission, reflection or primary/secondary coil assembhes. Hybrid coils mayor may not be the same size and are not necessarily adjacent to each other (4). Figure 2.7 shows one possible hybrid coil arrangement. In the through transmission coil, the excitati on coil is on one side of the test object and the sensing coil is on the other. The driver coil illduces eddy currents and a secondary magnetic fi eld in the test specimen. Any variation of these secondary events should be d etected by the smaller probe coil on the opposite side of the thin plate.
self-comparisml differential and external reference differelltial. The self-comparison differe1ltial coif compares one area of a test object to another area on the same test object. A common u se is tv.'o coils, emillected opposing, so that if both coils are affected by identical test object conditions, the net output is ovolts or 110 signal change. The self-comparison arrangement is insensitive to test object variables that occu r gradually. Variables such as slowly changing wall thickness, diameter or conducti vity are effectively discriminated against with the selfcomparison differential coil. Only when a d ifferent condition affects one or the other test coils will an output signal b e 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 coil. The e:rternal reference differential coil, uses an external reference to affect one coil while the other coil is affected by the test object (4). Figure 2.6 illu strates this concept. This system is used to detect differences between a standard obiect and test
Figure 2.6: External reference differential system
R
R
vol tmeter
AC
n...,
'\---'> R
R ____ Reference
Inspection ~
,,"
Te,'
~
" ,I
V ~"e" o"
sample
11
~
•
sample
A hybrid coil arrangement consists of an cxcit
this is for bettcr 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 encountcred that a measurable impedance change might be generated by both the exciter and the pick up coil(s}. Additional Coil Characteristics
Figure 2.7: Hybrid coil (through transmission)
$
Transmittin g circuit
f"\..)
AIt,m";'9 ,,,,,,,
Transmitting coil
source
Materiat
Receiving coil
I
I
tndica11ng instrumen1
I
Receiving circuit
12
I
Coil configuration is but one of many factors to consider when setting up test cond itions. Other coil characteristics of importance are mechanical, thl'rmal and electrical stability; sensitivity. resolution and dimensions. The geometry of the coil is usually d ictated by the geometry of the test object. Selection of smaller probe sizes ll'L.'ly affect test sensitivity and / or resolution. The relative importanct' uf tLost coil characteristics depends on the nalure of the test. A blend of theory and experience usually succeeds in selection of proper coil parameters. Coil design and interactions w ith test objects will be d iscussed later in this Study Guide.
Chapter 2 Review Questions Q. 2.1
Differential coils arc usually used in: A. bobbin coils.
Q.2.6
An absolute coil measurement is made: A. by comparing one spot on the test object to another. 8. without reference to o r 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-compa ri son differential arrangement are affected simultaneously with the same test object variables, the output signal: A is directly proportional to the number of variables. 8. is 0 or near O. C. is indirectly proportional to the number of variables. D. is primarily a function of the exciting current.
Q.2.B
Which coil type inherently has better thermal stability? A. bobbin 8. absolute C. outside diameter O. 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.
B. probe coils.
e.
outside diameter coils. O. 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 scalmed in circular
motions at constant speeds. O. the probe must be moving at all times to get a reading.
Q.2.3
Q.2.4
Q.2.5
A spilllling probe would most likely be: A. a bobbin coil. B. an inside diameter coil. C. an outside diameter coil. D. a probe coil.
A feed tllmugll coil is: A a coil with primary / secondary windings connected so that the signal is fed through the primary to the secondary. S_ an encircling coil. C. an outside diameter coil. D. both Band C.
When inspecting a tubular product with an enci rcling 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.2.10 Proper selection of test coil arrangement is determined by: A. shape of test object. 8. resolution required . C. sens itivity required. D. stability. E. all of the above.
13
Chapter 3 Test Coil Design Thus, the resistance of a 10 fllength of 40 gage copper w ire with a specific resistance of lOA circula r milfoot at 20 °C would be found as follows:
As discussed eartier, 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, s tabi lity and environmen tal considerations. To better understand coil properties and electrical relationshi ps, a shorl refresher in alternating current theory is necessary. First, the electrical units must be examined . For example, current and its representative symbol I. Cu rrent not only suggests electron flow but also the amount. The amount of elect rons flow ing past a point in a circuit in 1 second is expressed in amperes : 2nx lOIS electrons passing a point in 1 second is called 1 ampere.
R~
10.4 · 10 9.888
~IO.518ohm s
In an alternating current circuit containing only resistance, the current and voltage are in phase. III phase means the current and voltage reach their minimum and maximum values, respectively, at th{ same time. The power d issipated 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 .
Resistance
Inductance
Resistance is an opposition to the flow of electrons and is measured in ohms. O hm' s Law is stated by the equati on:
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 th( property of i"ductallce. The role of inductance is analogous to inertia in mechanics, because inertia i1 the property of matter that causes a body to opPOS{ 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 whid an electromotive force of 1 V is induced when the current is ch anging at a rate of 1 A /s w ill have an inductance of 1 H . The inductance o f 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 d imensions required to calculate coil inductance. An approxima tion of small, multilayer, air core coil inductance is as follows:
E
I ~-
R Equation 6
where:
" R "
[
E "
current in amperes, resistance in ohms, electrical potential d ifference in volts,
The resistance of a coil is determined primarily by the length of wire used to wind the coil; its s pecific resistance is determined by the type of wire (e.g., copper, silver) and the cross-sectional area of the wire.
. s_~pe=c~i~fi=c=re=s~is~t=an~c~e~x_Le ==n~g~t~ h = ReSlstance Area Equation 7
where: resistance speci fic resistance area length
= =
ohms ohms / circular mil-foot circu lar mils fec t
15
Inductive Reactance
Figu re 3.1: Multilayer coil
1-<-
,X
<
k~
ryy
,
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:
-t _t f
>-tx-
~
r..j
,)r r: I
,
,
x, = 2JCi L
-b-1
Equation 9 where: XL -
f
08 (rN)'
L
f or example, using the 32 J.1H euil calculated earlier, operating at 100 kH?" its inducti ve reactance would be found as follows:
Equation 8
whefe: I.
• • • • •
N
,
f b
•
self·inductance in microhenries (J.1H), total number of tum~, mean radius in inches, length of coil in inches, coil depth or thickness in inches.
1.
=
f
•
2.
•
f b N
•
Therefore, thiS coil would present an oppo~ition of 20.096 ohms to cu rrents \'lith a rate of change of 100 kHz due to its reactivc component. Unlike a resistive circuit, the current and voltagl..· of an inductive circuit do not reach their minimum and maximum values at thc same time. In a pure inductive circuit the voltage leads the current b}' 90 electrical degrees. This means that when the voltage reaches a maximum value, the current is at O. Power is related to current and voltage as follows:
0.1 in. 0.1 in. 0.1 in. 100 turns
would have an inductance of:
L=-
0. 8 (0. 1 . 100)'
6 . 0. 1 + 9 . 0.1 + 10 . 0.1
L=
32 !JH or 0.000032 I I 100 kHz or 100 000 Hz 6.28 6.28 x 100 000 x 0.000032 20.0% ohms
For example, a coil whose dimensions Me as follows: r
inductive reactance (in ohms) frequency (in hertz ) inductance (in henries)
0.8 (100)
P =E · /
= 80 = 32 0.6 + 0.9 + I 2.5
Equation 10 where:
L = 321lH
p
,. E
A~
stated (;OIrlier, this inductance is analogous to inertia in mechanical systems in that inductance opposes a chOinge in current as inertia opposes a change in velocity of a bod y. In alternating current circuits the current is always changing; therefore inductance is always oppo!oing this change. As the current tries to change, the inductance reacts to oppose that change. This reaction is called i.nductive renctnllcc.
power in watts volts current in amperes
Kotice that in a pure ind uctive circuit, when the voltage is mOlximum, the current is O. Therefore, the product E • I = O. Inductive reactances consume no alternating power where resistive elements consume power and diSSipate power in the form of heat. The opposition to curren t flow because of the resistive dement of the coil and the reactive element 16
of the coi l do not occur at the same time; therefore, they cannot be added as scala r quantities. A scalar qualltify is one having only magnitude, 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.
vector Z is known as impedallce. Impedance is the total opposition to current flow. Example: What is the impedance of a coil haVing an inductance of 100 JlH and a resistance of 5 ohms and being operated at 200 kH z?
x, = 27rfL X, =6.28 0 200 000 Hz 0 0.0001 H
Impedance
X, =6.28 0 20= 12S.6 ohms To explain the addition of reactance and resistance with a minimum of mathematical calculations, it is possible to use vector or phasor d iagrams (15). A vector d iagrnm constructed with tmaginary units on the ordinate or Y ax is and real units on the abscissa or X axis is shown in Figure 3.2.
z = J(I25.6)' +(S)' = ,!is 800.36 z = 12S.7 ohms First, convert inductance to inductive reactance and then, by vector addition, combine inductive reactance and resistance to obtain the impedance. The importance of knowing the impedance of the test coil is more one of instrument consideration than coil design . Maximum transfer of power is accomplished when the driving impedance and load impedance are matched. If, for instance, an eddy current instrument had a driv ing 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. Impeda nce can be discussed in a more detailed manner by mathematically noting variables using imaginary numbers (4). The square root of a negative number is known as an imaginary number. The imaginary number r-t6 can be written J(-1)16 or -p>J!6 or F>. 4 . The notation ,rr:::!) 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 .pj . Instead of noting r-t6 as ~ _ 4 note it as ;4. Because reactance is known as an illlagillary componellf, then impedance:
Figure 3.2 : Vector diagram
o
R
Observation of Figure 3.2 reveals Xv Rand Z appear to form the sides of a right triangle. The malhematical solution of right triangles states the square of the hypotenuse is equal to the sum of the squares of the other two sides, or
Equation 11
Substituting Z, XL and R, the statement becomes:
Equation 12
further simplified:
Z
=R+ jX," =lzlLe Equation 14
where: Equation 13
Tan Substituting inductive reactance (Xl) and resistance (R) it is possible to find the resultant of the vector addition of X L and R. This resu ltant
e= XL +- R Equation 15
The term R + jXm is known as a rectangltiar notatiol/ . As an example, a resistance of 4 ohms in 17
Permeability and Shielding Effects
series with an inductive reactance of 3 ohms cou ld be noted a~ Z - 4 + j3 ohms. The impedance calculation is then:
The addition of permeable core materials in certain coil designs dramatically improves the Q factor. Permeable con..>s are usually constructed of high permeability powdered iron. Probe coils, for example, are wound on a form thai allows a powdered iron rod or slug to be placed in the center of the coil (4). It is common to increase the coil impedance by a factor of 10 by the addition of core materials. This increase in impedance withou t additional wind ing greatly enhances the Q of the coil. Some core materials are cylinder or cup shaped. A common term is Clip core (Figure 3.3). The coil i .. first "Wou nd and then p laced into the cup core. In the case of a probe coil in a cup core, not only is the impedance i"creased, but the benefit of sJri!!lrling is also gained. Shielding with a cup core prevents the electromagnetic field from spreading at the sides of the coil. This greatly reduces the signals producl,.'d by edge effect of adjacent members to the te~ t area, such as fasteners on an aircra ft wing. Shielding, while improving resolution, !lSI/ally sncrifices some amount of penetration into the part. Another technique of sh ielding uses high cond uctivity material, such as copper or aluminum, to suppress high frequency interference &om other sources and a1so to shape the electromagnetic field o f the test coil. A copper Clip 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 impedllllce is reduced when the
Equation 16
In coil design \t 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 9 = Xl. -:- R and e is the induded angle between resistance and impedance. In the previous example the im}>l>
R = 3+4=0.750 = 36.9 degrees
Tan 8= X, An; Tan
+
e
Z = 5L36.9° Equation 17
Eddy current coils with included impedance anglt's of 60 degrees to 90 degrees usually make efficient test coils. As the angle beh....een resistance and impedance approaches 0 degrees, the test coil becomes very inefficient w ith most of its energy being dissipated as heat.
Q or Figure of Meri! The term u sed 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:
Figure 3.3: Effects of cup cores
lal
Equation 18
I
where:
~D ~L
XI. = inducti ve reactance
R = resistance
Ibl
For example, a coil haVing an inductive reactance of 100 ohms and a resistance of 5 ohms would have a Q of 20.
(a) Unshielded coil - fietd spread might be up to twice the coil diameter. (b) Shielded coil - magnetic field extension restricted to the core geometry.
18
shielding material is placed around the test coil. The net effect is that the coil's Q is less than it was when the coil was surrounded by air. Another coil design used for inspection of ferro magnetic materials is the saturation approach. A predominant va riable that prevents eddy current penetration in ferromagneti c material is called J'ermeability. Permeability effects exhibited by the test object can be reduced by means of magnetic sahlranon (Figure 3.4). Sahlration 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 red ucing 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.
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, temperahlre, atmosphere, mechanical stress and stability must be considered (4). Normally wear can be reduced by selection of wear resistant compound s 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 cerami cs, 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 temperahlre variations caused by human touch. For high temperature Figure 3.4 : Magnetic saturation inspection applications, materials must be chosen carefully. process Most common commercial copper coil wire may be used up to 150°C to 200 dc. For temperatures above Large saturation w ts II dd col (DC) ma er e y 200 °C, silver or aluminum wire with ceramic or 1 current coil (AC) high temperahlre 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 Fen;ti, (steel) tube ~ 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 Figure 3.5: Magnetic bias probe lead to component failure . Mechanical and electrical stability Poly shaft of the test coil can be enhanced by an Permanent magnets application of epoxy resin between each layer of coil w inding. This accomplishes many objectives: 1) it seals the coil to exclude moisture; 2) it provides additional electrical insulation; and 3) it provides mechanical stability. Differential coils Characteristics listed are not in order of importance. The importance of each characteristic is determined by Non-metallic probe body specific test requirements.
I
(
i": : i
I
19
Chapter 3 Review Questions Q.3.1
Q.3.2
A coil's resistance is determined by: A. wire material. 13. wire length. C. wire cross-sectional area. D. all of the above.
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.R
The incorporation of ferromagnetic shield ing materials around a coil: A. improves resolution. B. decreases fie ld extension. C. increases impedance. D. Does all of the above.
Q.3.9
The purpose of a steady state winding u'>Cd near a test coil is to: A. reduce material permeability effects. B. produce possible magnetic sahlfation in the test materia 1. C. provide a balance source fOT the sensing coil D. both A and B.
Inductance might be referred to as being analogous to: A. force .
B. volume. C. inertia. O. velocity.
Q.3.3
Q.3.4
The unit of inductance is the: A. henry. B. m axv,'cl1. C. o hm. D. farad .
The inductance of a multilayer air core coil wi th the dimensions I = 0.2, r = 0.5, b = 0.1 and N = 20, is: A. 1.38 H . B. 13.8 ~ I-l . C. 13.8 ohms. D. 1.38 ohms.
Q.3.5
The inductive reactance of the coil in Q.3.4, opera ting at 400 kHz, would be: A. 1380 ohms. B. 5520 ohms. c. 34.66 ohms. D. 3466 ohms.
Q.3.6
The impedance of a lOO ll H coil with a res istance of 20 ohms operating at 100 kH z would be: A. 62.8 ohms. B. 4343.8 ohms. C. 628 ohms.
Q.3.1O The most important consideration when selecting a test coil is: A. sensitivity. B. resolution. C. stability. D. meeting established inspection criteria.
D. 65.9 ohms.
20
Chapter 4 Effects of Test Object on Test Coil conductor is a poor resistor. Conductance and resistance are direct reciprocals as s tated 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 lACS. Resistivity is expressed in absolute terms of micro-ohm-centimeters. 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.
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 variance of the test coil impedance due to test object variables. These variances are called operating or test <.>ariablcs (15). The range of test variables encountered might include electrical conductivity, magnetic permeability, skin effect lift off, fill factor, end effecl, edge effect and signal-ta-noise ratio. Coil impedance was d iscussed at length in Chapter 3. In this chapter coil impedance changes
will be represented graphically to more effectively explain the intcmction of the operating variables.
172.42
% [ACS= ~~~~~~~---
Electrical Conductivity
Resistivity (in rnicro-ohm-cm)
In electron theory the atom consists of a positive nucle us surrounded by orbiting negative electrons. Materials that allow these electrons to be easily moved out of orbit around the nucleus are classified as condllctors. In conductors electrons are moved by applyi ng an outside electrical force. The ease with which the electrons are made to move through the conductor is called cOlldl/Ctallce. A unit of conductance is the siell1e11s (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 benchmark 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 o( 1 mm 2, measuring 0.017241 ohms at 20 °C would be arbitrarily considered to be 100% conductive. The symbol for conductivity is (} (sigma) and the unit is percent lACS or percent of the International Annealed Copper Standard. T
0'
. .. (. . h ) 172.4 1 ReSlstlvlty 10 rnlcro-o rn-cm =
%IACS
Equation 19 These numerical values will be necessary when additional calculations are needed to determine issues of frequency choice, depth of penetration and / or phase spread to meet specific inspection criteria. As the tcst 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 th is concept. The coil's inductive reactance is represented by the Y axis and coil resistance appears on the X tlxis. The 0% conductivity pOint, or air point, is when the coil's empty reactance (XLQ) is maximum. Figure 4.1 represents a measured conductivity locus (4). Conductivity is influenced by many factors. Table 4.1 is a comparative listing of materials with various chemical compositions. There are various ma nufactu ring or in s itu factors that must be considered when try ing to measure the conductivity of various alloys. In metals, as the temperature is increased, the conductivity will decrease. This is a major factor to consider when accurate measurement of conductivities is requ ired. 21
Heat treatment affecl'> 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 materia l due to cold working produces lattice distortion or dislocation (2). This
mechanical process changes the grain structure and hardness of the material, changing its electrical conductivity. Hardness in age hardellnble aluminum alloys changes the electrical conductivity of the alloy. The electrical conductivity decreases as hardness in creases. As an example, a Brinell hardness of 60 is represented by a conductivity of 23 and a Brinell hardness of 100 of the same alloy would have a conductivity of 19.
Fig ure 4.1: Conductivity curve
IN--.! I I tr o(air) % I r--. Co~d";IiV;~ I '< I
Permeability Permeability of any material is a measure of the ease with v·:hich its magnetic domains can be aligned or the ease with which it can establish lines of force (2). Materials are rated on a comparative basis. Air is assigned a permeability of l. Ferromagnetic metals and alloys induding nickel. iron and cobalt tend to concentrate magnetic flux lines (15). A5 discussed in Chapter 3, some ferromagne tic materials or sifltered ionic compollnds are also useful in concentrating magnetic flux (4). Magnetic permeability is not constant for a given material. The permeability in a test samp le 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 increased, the magnetic
"
t
\
2%
15-
,%
'\
10% .....
I I
100% lACS ......
Resistance -
Table 4.1: Electrical resistivity and conductivity of several metals and alloys Material
Resistivity micro-ohm-cm Utncm)
6.90 2.65 4.10 5.30
Admiralty Brass Aluminum (99.9)
6061-T6 7075-T-6 2024-T4
5.70 12.00
Aluminum Bronze Copper Copper Nickel 90-10 Copper Nickel 70-30 Gold Corrosive Resistant Nickel Alloy High Temperature Nickel Chromiu-m Alloy Lead Magnesium (99%) Stainless Steel 304 Stainless Steel 316 TItanium 99% Tung sten Zirconium
1.72 18.95 37.00 2.35 130.00
100.00 20.77 4.45 72.00 74.00
48.60 5.65 40.00
22
Conductivity % lACS
25.00 64.94 42.00 32.00 30.00 14.00 100.00
9.10 4.60 75.00 1.30
1.72 8.30 38.60 2.39
2.33 3.50
30.00 4.30
field of the coil wi ll increase. The magnetic flux within the steel will increase rapidly at first and then will tend to level off as the s teel approaches magnetic saturation. This phenomenon is called the
Figure 4.2: Edge effect
Good coupling
Bllrkha/lsell effect (4). When increases in the magnetizing fo rce produce little or no change on the flu x w ithin the steel bar, the bar is magnetically satu rated. When ferromagnetic materials are satu rated, permeability becomes constant. With magnetic permeability constant, ferromagnetic materials may be inspected using the eddy current method. Without magnetic saturation, ferromagnetic materials exhibit such a wide range of permeability variation that signals produced by discontinuities or condu ctivity variations are masked by the permeability signal (15).
(
--•
-•
.-
)
~
Skin Effect
Decreased coupling
(
r-
OO
Electromagnetic l'esLs in many applications are most sensitive to test object va riables nearest the test coil because of skin effect. Skill effect is a result of mutua l interaction of eddy currents, operating frequency, test object conductivity and permeability. The skin effect, the concentration of eddy currents it' the tes t object nearest the test coil, becomes more evident as test frequency, tes t object conductivity and permeability are increased (4). For current density or eddy current distribution in the test object, refer to Figure l.8 in Chapter 1.
E"d effect follows the same logic as edge effect. End effect is the signal observed when the end of a product approaches the test coil. Response to end effect can be reduced by coil shielding or reducing coil width in outside diameter encircling or inside diameter bobbin coils. End effect is a term most applicable to the inspection of bar or tubular products.
Edge Effect
Lift Off
The electromagnetic field produced by an excited test coil extends in all directions from the coil. The coil's field p recedes the coil by some distance (2) deh'!rmined by coil par,mleters, operating frequency ""d lest 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 (.li r) at the leading edge of the coil's field of view. It is therefore essen tial that edge effect be eliminated as a variable during a surface sca nning test. Response to the edges of test objects can be reduced by: incorporating magnetic s hields 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.
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
End Effect
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 w id1 distance from the coil. This fact causes a p ronounced 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 increased noise and reduced coupling resulting in poor measuring ability (12). In some instances, equipment haVing phase d iscrimination capability can readily separa te lift off from cond uctivity o r other variables. Lift off can be
23
used to advantage when measuring nonconductive coatings on conductive bases. A nonconductive coating such as p aint or plastic causes a space betv/een 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 m ost applicable to testing objects with a smfilce or probe coil.
equation resulting in the d ivision of effective coil and part area,>. Because the term rr. / 4 appear.:; in both the numerator and the denominator of this fractional equation the term rr. / 4 cancels out, leaving the ratio of the diamctf::!rs squared:
d'
- , = 11 = Fill Factor D-
Fill Factor
Equation 21
Fill factor will always be a number less than 1 and efficif::!nt fill factors approach L A fill factor of 0.99 is more desirable than a fill factor of 0.75. The effect of fill factor on the test system is that poor fUl factors do not allow the coi! 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 thf::! 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 d emonstra te this f::!ffect. For this example, the system parameters are as follows: (a) Unloaded coil voltage equals 10 V. (b) Tes t object effective permeability equals 0.3. (c) Test coil ins ide diameter equals 25.4 mm (1 in.) (d) Test object outside diameter equals 22.9 mOl (0.9 in.)
Fill facfor is a term used to describe how well a test object will be electromagnetically coupled. to a test coil that surround s or is in serted into the test object. fill fac tor then pertains to inspections using bobbin or cllCi rclin g coils. Like lift off, electromagnetic coupling between test coil and test object is most efficient when the coil is neaTCst the surface of the pilIt. The area of a circle (A) is determined using the equation:
Eq uation 20
Fill factor can be described as the ratio of test object diameter to coil diameter sq uared (Figure4.4). The d iameters squared is a simplified Figure 4.3 : Lift off/conductivity relationships
90" 0% lACS
(09)'
Fi II Faclor q~ - ;-
~ 0.81
Equation 22
An equation demonstrating coil loading is given by: Angle A
I
olr---L--~
__-1-_-1-_ --'-_ --'-_--"-_
100% lACS ---"' 0'
where: Eo
Resistance - --
E lift off: The change in coil impedance due to a changing (air) gap
between the coil and the material being tested.
'I .uiff =
24
coil voltage with coil affected by air coil voltage with coil affected by test object fill factor effective permeability
establishes a standard depth of penetration at the midpoint of the rube wall. Th is condition would allow a Encircling Rod 10 relative current coillD OD - - density of about 20% on the far surface of the tube. With this condition, identical near and far surface discontinuities \·..,ould have greatly different responses. Due to current magnirude 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. D iscontinuities 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 (12).
Figure 4.4: Fill factor ratios OR Compare either: I
,,
,,, ,,
Tube Bobbin 10 ID - - - coil 00
,,,
\
,,
When a nonferromagnetic test object is inserted into the test coil, the coil's voltage wil l decrease.
E E E
E
=
10[(1 - 0.81) + (0.81) (0.3)] 10[0.19 + 0.243J 10[0.433] 4.3 V
This allows 10 - 4.3 or 5.7 V available to respond to test object changes caused by discontinuities o r
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.
Signal-Io-Noise Ralio
Discontinuities
Signal-fa-liaise ratio is the ratio of signals of interest to unwanted signals (4). 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 lIoise. Sigllal-to-noise ratios can be improved by several techniques. If a part is dirty or scaly, signal-to-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 nondestructive 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.
Any d iscontinuity 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 (15). Discontinuities open to the surface can be detected with a wide range of frequencies; 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
25
Chapter 4 Review Questions Q.4.1
Materials that hold their electrons loosely are classified as: A. resistors. 8. conductors. C. semiconductors. O. insulators.
QA.6
Diamagnetic materials have: A. a permeability greater than air. B. a permeability less than air. e. a permeability greater than ferromagnetic materials. D. no permeability.
Q.4.2
100'%, lACS is based on a specified copper bar h aVing a resistance of: A. 0.01 ohms. B. 100 ohms. C. 0.017241 ohms. D. 172.41 ohms.
Q.4.7
Edge effect can be reduced by: A. shielding. B. selecting a lower frequency. C. u sing a smaller coil. D. both A and C.
Q.4.8
Q.4.3
A resistivity of 13llohrn em is equivalent to a conductivity in percent lACS of: A. 11.032. 6. 0.0625. C. 1652. D. 13.26.
Ca lculate the effect of fill factor when a conducting bar 12.7 mm (0.5 in.) in diameter with an effective pi!I meability of 0.4 is placed into a 25.4 mm (1 in.) diameter coil with an un l oa d ~d voltage of 10V. The loaded voltage is: A. 2V. B. 4.6V. C. 8.5V
Q.4.4
D. 3.2V.
A prime factor affecting conductivity is: A. temperature.
B. hardness. QA.9
C. heat treatment. D. a ll of the
Q.4.5
Materials that tend to concentrate magnetic flux lines are: A. conductive.
Laminations are easily detected with the eddy current method. A. True B. False
Q.4.lD Temperarure changes, vibration and envuorunental effec~ are test coil inputs that generate: A. unwanted Signals. B. magnetic fields. C. eddy currents. D. d rift.
B.
permeable. C. resistive. D. inductive.
26
Chapter 5 Selection of Test Frequency It is the responsibility of nondestructive testing engineers and technicians to provide and perform nondestructive testing that in some way ensures the quality or usefulness of industry 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 . Instrumentation, test coil and test freque ncy selection become the tools used to solve the problem of inspection. Test coils were discussed previously and instrumentation will be d iscussed later in this text. Test frequencies and their selection will be examined in detail in this Chapter.
The depth of penetration formul a discussed in Chapter 1, although correct, has rather cumbersome units. Conducti vity is u sually expressed in percent of the International Annen/cd Copper Sta/1dard (% lACS). Resistivity is usually expressed in terms of micro-olim-centimeter (llncm) (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 fo r calculating depth of penetration in the metric units was presented. Another derivative of this formula using resistivity, frequency and permeabili ty with S expressed in inches can be expressed as follows:
Frequency Selection Equation 23 In Chapter 1, it was observed that eddy currents are exponentially reduced as they penetrate the test object. In addition, a time o r phase difference in these currents was observed. 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 .
where: Ii K
P
f !irel =
s tandard depth of penetrati on 50 (for millimeter) or 1.98 (for inches) resistivity (in micro-ohm-centimeter) frequency (in hertz) 1 (for nonferromagnetic materials)
For nonferromagnetic materials the term II rei is ignored. The equation then becomes:
Single Frequency Sy stems Unforrunately, 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 stand ard 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 tube 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.
Eq uation 24 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:
27
8~K~
la) Ib)
Ie)
Id)
lei
where:
f a d
!~~ 8'
p
K' K'
f f
8'
p
K' p
8'
,U ....1
frequency in hertz conductivity meter / ohm-rrun 2 diameter of test object, em relative permeability
A frcqllency can always be selected to establish
factor A equal to 1. This frequency is knmvn as the limit frequency and is noted by the term fg . By substituting 1 for factor A and fg for f, the equation becomes:
1 = fj1r~l (Jd? 5066
~ f or f ~ (1.98 )' P .
8'
Equation 28
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 dimillil tion. The aluminum plate has a resistivity of 5 ).IQcm. By establishing a depth of penetration at 2.54 mrn (0.1 in. ), the far surface current will be less than 10% of the available current, thus reducing response to the steel PiUt. The frequency required for this can be calculated by applying:
Limit frequency (fg) is then established in terms of conductivity, permeability, some dimensional property and a constant (5066). Because limit frequency is haSRd on these parameters, a technique of frequency detennination using a test frequency to limit frequency ratio flfg can be accomplished. High flfg ratios are used for near surface tests and lower flfg ratios are used for subsurface tests. Often results of such tests are represented graphically by diagrams. These diagrams are called impedance diagrams (4). Impedance illustrated by vector diagrams in Chapter 3 shows inductive reactance represented un the Y axis and resistance on the X axis. The vector sum of the reactive and resistive components is impedance, This impedance is a quantity w ith magnitude and direction that is d irectly proportional to frequency. To construct a univerSill impedance diilgram valid for all frequencies, the impedance m ust be normalized (4). Figure 5.1 illustrates a normalization process. Figure 5.1 (a) shows the effect on primary impedance Zp w ith changes in frequency (00 = 2xl ). Figure S. l (a) rep resents primary impedance without a secondary circuit or test object. Figure S.l (b) illustrates the effect of frequency on primary impedance w ith a secondary circuit or test object present. The prim il ry resistance R, in Figure 5.1 (a) has been subtracted in Figure S. l (b) because resistance is not affected by frequency. The term wLsG in Figure 5.1 (b) represents a reference qUilntity~ for the secondary impedance. The units arc secondary conductance (G ) and wLs (secondary reactance). Furth er normalization is accomplished by dividing the reacti ve and resistive components by the term wLo or the primary inducti ve reactance w ithout a secondary circuit present.
f ~ (1.98 )'(5) ~ 19.6 (0. 1)'
0.01
f ~ 1960Hz
Equation 25
If detection of the presence of the steel part w as required, the depth of penetration could be reestablished at 7.6 O1m (0.3 in. ) in the alumimlm plate and a ll CW frequ ency could be calculated.
f ~
(1.98 )' (5) 19.6 (OJ)'
~ 0.09
f ~ 21gHz
Eq uation 26
Another approach to frequency selection uses argument A of the Bessel function (1 ) ·where iUgument A is equal to unity or 1.
Equation 27
28
Figure 5.2 shows a typical normalized impedance d iagram (1 5). The terms roLl roLo and R/ roLo represent the relative impedance of the test coil as affected by the test object. Signals generated by changes in roL or R caused by test object conditions such as surface and subsurface discontinuities may be noted by .6roL or LlR. The !lwLo and LlR notation indicates a change in the impedance. Figure 5.3 shows the impedance variation in a nonferromagnetic cylinder caused by surface and subsurface discontinuities.
Figure 5.1: Effect of frequency change: (a) primary impedance without secondary circuil; (b) primary impedance with secondary circuit (a) Zp (10 ttl,)
A
Zp (8 (,)1)
Figure 5.2: Normalized impedance diagram for long coil encircling solid cylindrical nonferromagnetic bar and for thin-wall tube, Coil fill factor 1.0
=
>.0
.b
Zplw,) 0.9
0"'------';;_ _ _ _ __ R, Resistance R (relative scale)
(b)
§:
0.8
'-I
0.7
~
'
. 1.2 1.4 -'''''~=t--+-~ 0.6 >.6
kr = r J(W\.l(f) '" 2.
Solid CYlindrJI bar
~
0.6
2.'
28'_t-_ 3.0 ·/' 1
05
,. ,.,,"il---I--/,- .
0.4
Er...._f
'.0
-/-+-_ 4-+ >.2
0.'
I
Or-Nl, 0.2
0.'
c
o
1'.4 n
H)()
'~=---.L2.0 o
0.'
0.2
," 0.3
'6[-1-1 0.4
0.5
0.6
Normalized resistance H{UlLo)-1 (0) B
k = v(t~.J(f) = electromagnetic wave propagation constant for conducting material r = radius of conducting cylinder (m) J1 = magnetiC permeability of bar (4n x 10-7 H'm- I if bar is nonmagnetic) o = electrical conductivity of bar (S'm- I ) (J) angular freQuency = 2n'where , freQuency (Hz) v(UlLoG) = equivalent of v(~o) for simplified electrical circuits, where G = conductance (8) and inductance in air (H)
o Resistance R (relative scale)
=
B. C, 0, E, F ",loci for selected values of Zp G = secolldary conductance Zp = primary impedance w = angular frequency = 'lit, where' = frequency (Hz) UlLS = secondary reactance
=
Lo '"
29
Figure 5.3 also ill ustrates a sensitivity ratio for surface and subsurface d iscontinuities. Notice with an fllg ratio of 50, a relatively high frequency, the response to subsurface discontinuities is not very pronounced. Figure 5.4 shows responses to the same discontinuities with an flig ratio of 15. This lower frequency allows better detection of subsurface d iscontinuities as shown in Figure 5.4.
frequency arc called I1Il1ltifreqllellcy or mll/tiparalllt>fer systems. It is conunon for a test coil to be driven with three or more frequencies. Although several frequenci es may be applied Simultaneously or sequentially to a test coil, each of the individual frequencies follows rules established by single frequency tI...'C hniques. Signals generated at the various frequencies are often combined or mixed in electron ic circuits that algebraically add or subtract signals to obtain a desired result . One multurequency approach is to apply a broadband signal, with many frequency components, to the test coil (4). The information transmitted by this signal is p roportional to its bandwidth and the logarithm of 1 plus the signal-to-noise po'wer ratio, Thi s relationship is stated by the eq'llation:
Multifrequency Systems It becomes obvious that the technician must have a good working knowledge of current density and phase relationships to make intell igent frequency choices. The frequency chOice discussed to da te deals with coil systems driven by only one frequency. Test systems driven by more than one
Figure 5.3: Impedance variations caused by surface and subsurface cracks
Figure 5.4: Impedance variations caused by surface and subsurface cracks
0.14
0.1
-g" -" og~'I:: 2 0 '·
0.08
ciJ&
~"',g
0
6£'0
0.06
Distance of crack from surface in %
0.04
of diameter
0.02 0 6R 6R
0 .02
0 .04 0,06 wLo
"'Lo
Impedance variations caused by surface and subsurface cracks in nonferromagnetic cylinders. at a freque ncy ratio flfg = 50.
Impedance variations caused by surface and subsurface cracks in nonferromagnetic cylinders, at a frequency ratio flfg '" 15,
30
First, a frequency is selected to give optimum phase and amplitude information about the tube wall. This is ca lled 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 ha\'e about the same phase angle. A second freque ncy called the sub/rador 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 I multiparameter testing. Optimization of a test frequency (or frequencies) will therefore depend on the desired measurement or parameter(s) of interest (11, 12, 4).
Equation 29 where:
C
=
B
=
S/N =
rate of information transmitted in bits per second bandwidth of the signal signal-ta-noise power ratio
This is known as the Shannon-Hartley theorem. Another approach to multiparameter techniques is to use a multiplexing process (12). The multiplexing process places one frequency at a time on the test coil. This results in zero crosstalk between freque ncies and eliminates the need for channel specific bandpass filters. The major advantages of a m ultiplex system, in addition to the crosstalk reduction issues, are lower cost and greater flexibility in frequency selection. If the multiplex switching rate is sufficiently high, both broadband and multiplex systems have essentially the same results. The characterization of ed dy current signals by their phase angle and ampliru de is a common practice and provides a basis for signal mixing to suppress unwanted signals from test data (12). Two frequencies are required to remove each unwanted variable. Practica l multipara meter freq uency selection can be demonstrated by the following example: Problem: Eddy current inspection of installed thin-wall non ferromagnetic heat exchanger tubing. Tubing is structu rally supported by ferromagnetic tube supports at several locations. It is desired to remove the tu be su pport response signal from tube wall data. Solution: Apply a ffiultiparameter technique to supprt.'Ss the tube support signal response.
31
Chapter 5 Review Questions Q.'s.l
Wh
Q.5.2
To reduce effects of far surface indications, the test freque ncy: A. must be m ixed . B. must be raised . C. must bt:' lowered. D. has no effect.
Q.5.3
The frequency required to establish the Bessel function argument A cqUelJ to 1 is
Q.5.6
In Figure 3.1(b) the value roLsG equaling 1.4 'w ould be indicative of: A. a high resisti\' ity material. B. a high conductivity material. C. a low conductivity material. D. a nonconductor.
Q.5.7
Primary resistance is subtracted from Figure 3.1 (b ) because: A. rcsistJIlCe is always constan t B. resistance is not frequency dependent. C. resistance does not add to the impedance. D. None of the above.
Q.5.8
called: A. an optimum frequency. B. a fesonant frequency.
C. "limit f
The reference qu.,n tity is d ifferent for solid cy linder and thin-wall tube in Figure 5.2 b ecause: A. the frequency is different. B. the conductivity is different. e. the skin effect is no longer negligible. D. the thin-w
Q.5.4
Calculate the limit frequency for a copper bar (q = 50.6 meter /ohm mm 2) 1 em in diameter. The correct limit freq uency is: A. 50 kHz. B. 50.6 Hz. C.
Q.5.9
A 25% dcep crack open to the ncaT su rface gives a n,-,sponse times greater
than the same crack 3.3% of diameter under the surface. (Refer to Figure 5.4. ) A. 10
100 Hz .
D. loa kHz.
B.
3
C. 2 D. 5
Q.S.5
Using the example in Question 5.4, what is
fifg
the ratio if the test frequency is 60kHz? A. 1.2
Q.5.10 When using multifrequency systems, low subtractor frequencies are used to suppress:
B.
110 C. 60 D. 600
A. conducti vity changes. B. far surf
32
Chapter 6 Instrument Systems Most eddy current instrumentation is categorized
The demodulation and analysis section is made up of detectors, analyzers, discriminators, filters and sampling circuits. Detectors can be a simple ampl itude type or a more sophisticated instrumentation. phase / amplitude or coheren t type. Five different elements arE' usually required to The signal display section is the key link between produce a viable eddy current instrument (4). These the test equipment and its intended purpose. The functions are excitatiOIl, modulatioll, Sigllll/ preparation, signals generated can be displayed many different sis"a/ analysis and sig/wl display. An optional sixth ways. The type of display or readout depends on component would be test object halldling equipment . the test requirements (4). In some tests, a simple Figure 1 illustrates how these components CO / NO-GO indicator circuit may be all that is interrelate. The generator provides excitation signals to the required. However, some applications may require test coil. The signa l modulation occurs in the recording of 100% of all raw data generated during electromagnetic field of the test coil assembly. Next, a test. This data may be imported into other digital the signal preparation section, usually a balancing devices that allow sophis ticated data analysis or network, prepares the signal for demodulation and engineering statistics to be generated. One example analysis. In the signa l preparation stage, balance of thiS is the inspection of large inservice nuclear networks are used to I1UIl out steady value components so that discontinuity growth can be alternating current Signals. Amplifiers and filters are monitored for determining potential failure rates or also part of this section 10 improve signal-Io-noise replacement cycles. Signal display processes will be r,1tio <'l nd raise signal levels for the subsequent discussed more in Chapler 7. demodu lation and analysis stage. A series of simple eddy current ins truments is shown in Figure 6.2 (15). In Figure 6.2(a), the voltage across the Figure 6.1 : Internal functions of the electromagnetic inspection coil is monitored by an nondestructive test alternating current voltmeter. This type of instrument could be used to measure large Excitation ----.. Generator lift off variations where accuracy was not critical. Figure 6.2(b) shows an impedance bridge circuit. This instrument consists of an Coil Object Modulation alternating current exciting source, dropping ,, resistors and a balancing impedance. ,, ,, Figure 6.2{c) is similar to Figure 6.2(b). In ,, Balancing Figure 6.2(c) a balance coil similar to the Signal preparation - - - - . , network ,, ins pection coil is used to provide a balanced bridge. Figure 6.2(d) illustrates a balancing Object handling coil affected by a reference sample. This is equipment Detectors commonly used in ex ternal reference Analyzers Signal analysis ----.. Discriminators differential coil tests. In all cases, because Filters Sampling only the voltage change or magnitude is cirCUits monitored, these systems can al1 be grouped as impedance magnitllde Iypes (5). Eddy current testing can be divided into Oscilloscope Meters Ihree broad groups (2). The groups arc Recorders Signal display - - - - . impedance testing, phase analysb h:::ting Alarms ----------------Relays and modulation analysis testing. Jmpt·.i.lI:~-~' Automatic testing is based on g ross changes in coil Mechanisms impedance when the coil is placed near the test object. Phase al/alysis k:.'tillS is ba...-.ed 00
by its final output or display mode. There are basic requi rements common to all types of eddy current
----..1
,
,
I
r--:
, ,
•
33
Figure 6.2: Four types 01 simple eddy current instruments
..
•
• YOlTM!lf.~
V
oC
CiIlOUNO
(a)
(b)
Q~OU NO '"
(e)
(d)
subgroups depending on the type of data display. Some of the earlier test system outputs were called vector point, ellipse and linear time base (2).
phase changes occurring in the test coil and the test object's effect on those phase changes. Modulation a1wiysi$ 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 tra nsit time of the discontinuity through the coil's fiel d. 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.
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 the Y aris and X axis voltages present in the test coil (2). By proper selection of frequency and phase adjustment a response in the vertical pla ne might represent dimensional changes while a voltage shift in the horizontal plane could represent changes in conductivity.
Impedance Testing Ellipse
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 reactallce magnitude illstrll/nellt (5). In reactance magnitude tests, the test coil is part of the fundament al 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 Mit is, therefore, an
As with the vector point technique, the test object and reference standard are used to provide a balanced output. A nonnal balanced output is a straight horizonlallinc. Figure 6.4 shows typical ellipse responses (2). Linear TIme Base
An early test system that was better suited to compensate fo r harmonic distortions present in the fundamental waveform used the linear time base technique (5).
indica tion of test object I~;;-;;:;-~~;-;;;;;;-;;;;;-;;;;--------------l condition. I display logic Reactance magnitude D 11, j.J = permeability systems have many of the same limitations as impedance 0:: dimensional properties 90 0 ::: conductivity magnitude systems. 0
Phase Analysis Testing Phase analysis processes can be divided into many
a 34
Figure 6.4: Cathode ray tube displays for dimension and conductivity
M "" slit value A = amplitude of the measurement in the slit e = angle behveen base Signal and measurement effect In Figure 6.5, the angle difference A to B is abou t 90 degrees.
Dimension
Impedance Plane Testing
Small change
Conductivity
Small change
Both dimension and conductivity
Small change
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 ~nergy applied to the coil. This allows one complete cycle of the sine wave voltage applied to the coil to appear on the CRT. Figu re 6.5 illustrates a linear time base display. A slit or narrow vertical scale is provided to measure the ampli tude of signals present in the slit (5). The base voltage is normally adjusted to cross the slit at 0 volts, the 180 degree point on the sine wave. The slit value M is used to analyze results. The slit value M is described by the equation:
M=Asi nB
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 circu its 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 changes that they did d etect. As the field of electronics advanced, more sophisticated Large change components became available. In today's marketplace many eddy current test systems have the capability to display data in multiple modes. The classic X-V type display mode is a simple way of showing what is meant by an impedance plane test system. In Chapter 4 impedmlce 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 d irection (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 tvvo-dimensional output device.
Equation 30
where:
35
A clac;sic example of the ad vantage of this X-Y screen presentation in surface scanning applications is to put !ift off responses 011 the horizon with discontinuities responding lip on the screen. Mode 3 systems are phase sensiti ve systems although th(!y have only amplitude detectors. They achieve phase sensitivity by operating in a manually selected off balance condition. Based on this st'lection, the off mIll signal change can be set so thilt it may appear larger than the inherent impedance change due to test object variabJes.
Figure 6.5: Screen image of a linear time base instrument with sinusoidal signals Slit
Screen
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Test Coil Excitation The second consideration that was previously mentioned for defining the mode of operation o f a test unit could be the way the probe is being energized. Figure 6.7 shows a typical surface riding pancake coil response to an
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Mode of Operation Test instruments m ay also be cl
Figure 6.6 : Null balance instrument with amplitude phase detectors
Signal Compensation Mode 1. Null balance with amplitude detector, Mode 2. Null balance with amplitude phase detectors, (Figure 6.6) and Mode 3. ~lecte d off null ba lance w ith amplitude detector. Mode 1. responds to any signal irrespective of phase angle. These would typically be meter-based instrumentation capablc of shOWing only the volt
36
Figure 6.7: Typical surface riding pancake coil response to an array of EDM notches on a calibration standard
are impressed across the coil at tile SlIme time. You will recall from earlier
Figure 6.8: Single frequency selectable instrument
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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. Multiple circuits are used throughout the instrument (4). The test coil output carrier freque ncies 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 Inseroice Inspection of Steam
Generator Tubing Usil1g Multiple Frequency Eddy Currellt Techniques (12).
Another approach to the multifrequency technique uses a sequential coil For deep subsurface crack detection [more than 0.5 em (0.2 in.)] the lower frequency range would be drive called multiplexillg (12). The frequencies are required. This test might also be performed w ith changed in a step·by·step sequence with such hybrid (driver/pick up) coils to improve detection rapidity that the test parameters remain unchanged. of the low amplitude responses from smaller The multiplex technique has the advantages of discontinuities deeper in a product. lower cost, continuously variable frequencies and For detection of very small stress or fatigue little or no crosstalk between channels. cracks in a near surface inspection process the Figure 6. 10 illustrates a multifreq uency higher frequency range could improve sensitivity to instrument capable of generating up to 16 channels of data sequentially. Each channel or time slot may smaller cracks. The compromise at very high frequencies is the issue of skin effect or surface Figure 6.9: Multifrequency instrument operating at three frequencies lIoise. Special probes or simultaneously scanning processes may be required for this type Generator of test also. Figure 6.9 shows a Generator L block diagram for a multifrequency instrument operating at Generator L three frequencies simultaneollsly. In modern systems this is -----, referred to as
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si/llu/tam'Qus injection . This diagram shows three dedicated frequcncy modules but mort" rerent adaptations uS(.' multiple variable frequency circu its. In Figure 6.9, excitation currents at each selected frequency
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-
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37
be adjusted over a wide range of frequencies. In
light.., audio alarms, meters, digital displays, CRTs, recorders and computer interfaces.
add ition, this digital system provides for the creation of mixed channel combinations for suppression of unwanted te!it variables. Results of such suppression arc described in Mliltifrequency Eddy Cllrrellt Method alld the Separation of Test Specimen Variables (12). 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 reca lled and used by semi -skilled personnel. Systems can be created with programs having supenrisory code interlocks that prevent reprogramming by other than authori7..ed 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 (4). A pu lse is applit.-d to the test coil, compensating networks and analyzers simu ltaneously. Systems having analyzers with one or two sampling points perform similar to a single frequency tester using sinusoidal excitation. Pulsed eddy current systems (7) haVing multiple sampling points perform more like the multi frequency tester shown in Figure 6.10.
Indicator Lights A si mple use of the indicator light is to monitor the eddy current signal amplitude with an amplitude gate circuit. 'vVhen the signal reaches a preset amplitude limit, the amplitude gate switches a relay that applies 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.
Audio Alanns Audio alarms can be used in much the same manner as alarm lights. Usually an audible alarm would be used to indicate an abnorma l condition . These types of alarms are commonly incorporated into online eddy current k-st I.·quipment that might be found in a manufacturing plant. These alarms give only qualitative information about the tested item. The degree or amount of the condition that exceeded the preset threshold calUlat nonnally be determined with the!iC devices. Indicator lights and audible alarm.. arc relatively inexpensive. Both can easily be incorporated into inspection systems found in ma nufacturing inspection applications where processes may be monitored by uncertified or semi-skilled tabor. Audible alarms are also very useful in handheld portable testers when the in... pcctor may be doing manual scanning. Often these inspectors have to pay very close attention to the probe position and speed and they may not be able to continuously monitor a visual display.
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 (4). Some common readout mechanisms are indkator
Figure 6.10: Commercial multifrequency instrument
Meters Meters operate on the d' Arsonval galvanometer principle. The prindple is based on the action between two magnetic fields. A common meter uses a permanent magnet to produce one magnetic field while the other magnetic field is produced by a movable coil wound on a core. The coil and core are suspended on jeweled bearings and attached to a pointer or ueedle. The instrument output current is passed through the coil and produces a magnetic field about the coil that reacts to the permanent magnetic field su rrounding the assembly. The measuring coil is ddlected, moving the meter pointer. The amount of pointer movement can sometimes be related to spechc test object Variables. Even with the aVailability of digital electronics that have many advanced features some inspectors
38
Figure 6.11 : Pulsed waveform excitation
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Balance circuits
Amplifiers
Analyzers
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d iscontinuity 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.
Digital Displays are still more comfortable with analog technology. As long as it can be demonstrated that these units Numerical digital displays can also be used to are still fu nctional and can meet the inspection provide qual itative information. These might have sensitivity requirements then they will continue to be used. Good maintenance and electronic , _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ calibration checks are essential with vintage Figure 6.12: A quantitative meter response indicating a test units. specific conductivity (in percent of the lACS) 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 CONDUCTIVITY system lIsed fo r measuring conductivity % IAC.S. (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 lACS value if the system has been properly set up. Some meter-based devices (Figure 6.13) Figure 6.13: A qualitative analog meter response showing that might be used for simple discontinuity only percent of fu ll scale detection do not give the opera tor a numerical value other than a percent of full scale. A given crack could generate either a small amplitude voltage at a low ga in setting or a larger amplitude response at a higher ga in setting. This would be a qual itative type response. These systems are not used for d iscontinuity sizing. An qualitative meter response could be used in a test situation where a minimum discontinuity amplitude response can be accurately defined. 111is might be an EDM notch of a spedfied depth in a ca libration block. As long as the meter stays below the preset voltage level from the selected 39
sl;!veral applications but thl;! most common wou ld be for measuring conducti vity values.
been used. To display nonrecurrent or single events, a Iliglt persistt1lce 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 systems provide the additional flexibilit~, for the selection of various color and contrast conditions (Figure 6.14). This allows the operator a choice of color options that can be established on the same system to compensate for use in different lighting conditions. Because the data are output to the 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 operator to choose how long the digital images created stay on the screen for viewing.
Cathod e Ray Tubes Cathode ray tubL'1i, or CRT type displays, play an important role in the display of eddy current infOlTntltion. In more recent times many eddy current ~)'stems have become available with digital representations of CRT type screens. In the original ana log system there were three main element'>: the electron gun. 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 w('re situated between the electron gun and the screen, arranged in two pairs, usually called II01"iZOIt/n/ and vertiml 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 tha t produce photochemical reactions when struck by the electron beam. The photochemical action appears in two sta,(;cs . Fluorescellce occurs as the electron beam strikes the screen. Phosphorescellce 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 persistellce. Persistence can be grouped as either low, medium or high persistence. To display repetitive signals, a low or medil/m persisfellCt! CRT may have
Recorders Da ta 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 digita l recording media can retain the files created for offl ine analysis and long term historical use. Early d igital systems were write ollce - read mallY devices. The more recent recording med ia can be erased and reused. The advantage of digital systems is that all of the raw data created by a muItifrequency test system can be viewed in multiple display formats at the same time. Tubing exam d" ta are often reviewed using both the X-Y and strip chart modes to optimize d iscontinuity ddechon and sizing. The strip 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 ind icates various materia l condi tions. The ampl itude of the X-Y lissajous response in Figure 6.15 (6.66 V) is an indicator o f the volume of the d iscontinuity. The p hase angle with respect to the X axis (114 degrees) represents discontinuity depth (in this case, 41%) and d iscontinuity origin (tube outside diameter), indicating whether the discontinuity originated on the inside or outside surface of the tube (13). Many computer-based systems have multiple display modes available for the same raw data set.
Figure 6.14: Num erical readouts/digital conductivity tester
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One way of displaying the data are in a top or plan view of the specimen. These are commonly called C-scans. This is sometimes a composite view of repetitive mechanized scans of a coil over a large area in multiple passes. Each time the coil travels over the surface of the part the coil is offset by about 0.5 coil diameters to ensure 100% coverage of critical areas. This same type of information can also be generated by scanning an array of coils over a region of the test specimen in one pass. These digital images can be colorized to indicate specific conditions. They can also be processed to create a three-d imensional view that can be rotated in multiple dimensions or planes.
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 functio ns as well as adjusting tester operating parameters, such as frequen cy, 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.
Test Object Handling Equipment Test object handling equipment is often a necessary component of an online test system (4). 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 coil(s) is usually transported at a constant speed. The transport speed needs to be adjusted to allow adequate time for testing an d for the reject, cutting or marking systems to perform their tasks. Should product centering or speed
Computers Most eddy current testers use an integral visual output device of some sort. Advanced eddy current testers may include such options as an eddy current card that extends the functionality of a standard PC with eddy current testing capability. Field hardened eddy current testing systems may just be a box of
Figure 6.15: Computerized system response - heat exchanger tubing exam R(I(239
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41
change during the examination system performance cou Id be limited. Automatic sorting devices are very common in online inspection systems used in a manufacturing t!nvironment. When a volumetric test is requi red for heat treatment or hardness verification the probe assembly may interrogate the entire test specimen (or some criti cal region of the specimen) in one vit?1v. For Small speci mens like ball bearings this could take just fractions of a second per sample. In larger ~pecimens the volumetric test may take a few !>eConds 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 critica l areas in one test. The eddy current technique can often demon.<;trate much higher discontinuity sensitivity and more rapid economica l testing for surface discontinuities in parl'> than any of the other nondestructive testing processes. 1£ unacceptable material cond itions were encountered at any inspection station the part
would be dropped into a reject bin. A digital counter and / or remote sensors can be used to track the number of reject<; and to alert the plant staff of potential problems in the manufacturing process.
Probe Delivery Systems In.<;tead of moving the p
Figure 6.16: Multimode output responses: rotating pancake coil inspection in a batt hole application. The same crack response can be seen in all four display formats.
Ii 42
As technology has been Figure 6.17: Multiple online eddy current test stations for detection of improved it has been possible unacceptable material conditions in a manufacturing plant to create other types of spinning probe possibilities. 1. Demag coil There are now many situations 2. Hardness test stallon ~~~ ~;;;:'!:~~ \,·here Spilming probes can be 3. Crack detection ""[;00 : t;! 1 ~ used. High speed probe guOll are used to perform bolt hole inspections after the removal of metal fasteners in aerospace structures. Small motors can also be used to perform a motorized rotating p;;lncake coil (MRPC) inspection from the inside surface of thinwalled tubes. Multiple coils of different designs can be used at the same time to enhance both discontinuity detection and characterization. In the case of large heat exchangers, a probe positioning device or robot might be used to posi tion a result.. arc monitored in real time for data quality bobbin, array or MRPC type test probe on the centerline of each tube to be inspected. Tubes to be but the data are al50 recorded for tater analysis. lfISpected are identified ;;lnd their coordinates are Remotely Operated Vehicles (ROVs) can
43
Chapter 6 Review Questions Q.6.1
Signal preparation is usually accomplished by, A. detectors. B. samplers. C. balance networks. D. discriminators.
Q.6.2
Most ed dy CllITent instruments have -,-_ _ _ __ coil excitation. A. square wave 13. triangulilI wave
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
Q.6.8
Alarms and light~ offer only: A. qualitative information. B. quantitative information. C. reject information. D. accept information.
Q.6.9
The length of a strip chart presentation C
C. sine wave
O. sawtooth wave
Q.6.3
Eddy current systems can be grouped by: A. output characteristics. 6 . excitation mode. C. phase ana lysis extent. O. both A and B.
Q .6.4
A multifrcquency instrument that excites the test coil with several frequencies sequentially uses the _ _ __ _ _ _ concept. A. multiplexing 8. time base C. b roadband D. cartesian
Q.fi.5
Q.6.10 A top view display of the test results from a specimen can be referred to as: A. an X-V display. B. a C scan. C. a crosshatch presentati on. D. a sweep dL~pla y.
Reject limits should always be adjtL'>ted to: A. one-half the screen height. B. 5 volts. C. ensure unacceptable components are properly identified. D. reduce operator training costs.
44
Chapter 7 Eddy Current Applications A problem common to the chemica l and electric power industries is the corrosion of heat exchanger tubing. This tubing is installed in closed vessels in a h igh density array. It is not lUlcommon for a nuclear steOlm generator or main condenser to contain many thousands of tubes. This high density and limited access to the inspection Olrcas oft en precludes the use of other nondestl1lctive testing methods. A bobbin coil inspection provides a volu metric inspection of the tube wall in a cost effective process. Heat exchanger inspection systems and results are described by Libby (8), Dodd, Sagar and Davis
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 (4). With the discontinuity, or the detection group, we are concerned \vith locating cracks, corrosion, erosion and mechanical damage. The m.aterial properties group includes measurements of conductivity, penncability, hardness, alloy sorting OT chemical composition and degree of heat treatment. Dimensional meas urements commonly made are thickness, profilomctry, spacing or location and coating or cladding thickness. Special applications indude measurements of temperature, flow metering of liquid metals, somc \'ibrations and ilnisotropic conditions. Regardless of the specific application, once the test system has been properly calibrated there should not be any fundamental changes made to it du ring the testing process. If it has been determined tha t the instrument has bcen set up incorrectly or is not working as specified in the operationa 1 procedures being used, all material should be retested since the last time the correct setup and proper system operation was verified.
(1 2).
Phase angle and amplitude relationships are usually established by using reference standards with artificial d iscontinuities of known and documented values. These d iscontinuities should refl ect expected damage modes as close as possible. III most thin-walled tubing cases the severity of the discontinuity can be determined by analyzing the eddy current sign
Discontinuity Det ection The theoreticill response to d iscontinuities 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.
Figure 7.1: American Society of Mechanical Engineers (ASM E) thin-walled tubing standard TSP
100%
80%
60%
40%
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45
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The geometry of real discontinuities may differ from reference standard discontinuities. This difference produ ces interpretation errors as discussed by Sagar (12). 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 si:.::e of the discontinuity and its resultant eddy current signal in relation to the tube support signal. This follows the basic princip le of signal-tcrnoise ratio. The signal-to-noise ratio can be improved at tube-to-tube support intersections by the use of multifrcq uenc), techniques (12, 11). In lI1ultifreqllclIc!I app/imtiolls, an optimum (or prime) frequency is chosen for response to d iscontinuities within the tube walL A lower than optimum or suppressioll 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 Figu res 7.4 and 7.5.) Both channels mo."t be able to detcct 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. A common problem with turbines is fatigue cracking of the compressor blades or disks in the root areas (13). Given the potential sa fety risks if these components fa il, the inspection criteria thre~holds are set to detect extremely small artifacts. Special probe designs and inspection techniques are Figure 7.2: P hase-to-depth calibration curve required to deal with the difficult sample geometries and smail 100% 1 1 discontinuity detection limits. Prime frequency (fa) 1 Many other aircraft , 80% ,---,Good phase spread inspections are designed to deal I ')-.... 1 , I with cracking o r corrosion / I proccsses that may not lead to 60% immediate catastrophic fai lu res , but that do need to be handled in ~ 40% a timely manner. Portable inspection devices are often used to perform these tests. Careful 20% test system calibration using appropriate procedures and ,\ 1 0 reference specimens is I\."quired 20 40 eo 80 100 120 140 160 180 o to maintain aircra ft fleet Degrees serviceability. The reference specimen and its 00 flaw plane associated discontinuities are very critical to the success of the test. Often models are Figure 7.3: Volts-to-depth calibration curve constructed with artificial 100% discontinuities that are exact I I duplicates of the item being i i I inspected. Field degraded 80% , specimens arc also used to verify , test discontinuity sensitivity. 60% D.J. Hagemaier discussed low freq uency eddy current ~ li inspection of aircraft structures ~ 40% I for subsurface d iscontinuity I- Idetection in an article published ,I 20% in Materials Evaluation in 1982. II I 1 (14) A low frequency (100 Hz to i 1000 Hz) technique can be used 0 4 7 o 2 5 6 8 9 to locate cracks in thick or multiple Ii'lyer, bolted or ri veted
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C1ircraft struchlres. 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. Pu lsed eddy current systems, if available, might also be used for crack detection in thick structures.
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47
Dimensional measurements, such as thickness, shape and position, or proximity of one item to another, are importClnt uses of the eddy current technique. Materials are often clad with other materials to present a resistance to chemicals or to provide 'w ear resistanct'. Cladding or plating thickness then becomes an important variable to the serviceability of the unit (6). For nonconductive coatings on conductive bases, the probe-ta-specimen spacing (6), or lift off technique can be applied. The case of conductive p la ting or cladding on conducti ve bases requires more refinement. The thickl1ess loci respond in a complex manner on the impedance plane (4). The loci for multilayered objects with each layer consisting of a material 'w ith a different conductivity follow a spiral pattern. In certain cases, two frequency or multifrequency systems (6) 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 s tai.nless steel (11). 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 achlai metallographic results. Eddy current profilometry is another common way to measure
The secondary standards are usually certified accurate to within ±O.35% or ±l ~o of value, whichever is less. Temperature is an important variable when making conductivity measurement... Most instruments and standards are certified at 20° e. Primary conductivity standards are maintained at a constant temperature by oil bath
Figure 7.6: A single frequency hardness tester output presentation
cur OR
BR... TO RESTART
.~ .
.• •,,' -,. 10.. ' •
•
. :/ . .. ~J ../ ...
sy~tems .
: '. t... t"oIII"~
Primary standards are measured with precision maxwell bridge type instruments. This circuit design increases measurement accuracy and minimizes frequency dependencc of the measurement (12). The secondary standards used for fie ld t(;'!;ter set up and calibration are often required to have their listed values recertified on an annual basis.
Hardness Measurements dimen:;ions. One example i:; the measurement of the inside diameter~ of tubes using a lift off technique (11). For this measurement, several small pancake coils are mounted radially in a coil form. The coil form is inserted into the tube ilnd each coil':; 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 lL'iefu l when the amount of hlbe wall deformation due to either manufacturing or operational conditions may require corrective action. An obvious problem encountered 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 locali7.ed dimension
Hardness measurements can be performed on both ferritic and nonferritic materials. Some hardn~ mea..-;urcments are performed with a two coil comparative process but this is not a strict requirement. When using a two-coil system the reference and test coils are both b
Conductivity Measurements Alloy Sorting Conducti vi ty is an important measured variable. 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 inform
In the aircra ft indu stry, aluminum is used
extenSively. Aluminum conductivity varies not only with alloy but also with hardne~s and tensile strength. Eddy current instruments scaled in percent LACS arc normally used to inspect for conductivity variations. Secondary conductivity standards (12)
In addition, it is advisable to h ave more than one reference specimen for backup in case of loss or damage. In the case of steel parts, they should be completely demagnetized to remove the effects of residual magnetism on instrument readings. As in most com parative tes ts, temperature of specimen and test object should be the same or compensated. Many o ther measurements can be made using eddy curre nt techniques. The electromagnetic technique produces so much information about a material that its application is only limited by the ability to decipher this information (13). With the right equipment, probes, techniques and training, the experienced operator should be capable of making the required distinctions between relevant and nonrelevant indications.
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 aluminu m 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 cor rect and accurate reference specimens. Because most eddy current instruments respond to a w ide range of variables, the reference specimen parameters must be controlled carefully. Test object and reference specime ns must be the same or very similar in the following characteristics: t. geometry, 2. heat treatment, 3. surface fin ish, 4. residual s tresses, 5. metallurgical structure.
49
Chapter 7 Review Questions Q.7.1
Conductivity, hardness and composition are
part of the
Q.7.6
Subsurface discontinuities located in thick or multilayered aircraft structures could be detected by: A. low frequency sinusoidal continuous wave instruments. 13. high frequency sinusoidal continuous wave instruments. C. pulsed systems. D. Aore
Q.7.7
Response to multilayer varying conductivity structures fo llow _ _ _ loci. A. orthogonal B. spiral C. linear D. stepped
Q.7.8
Nitride case thickness variations can be detected in stainJess steel cylinders by measuring: A. conducti vity. B. dimensions. C. permeability. O. none of the above.
Q.7.9
Conductivity is not affected. by temperature. A. True B. False
group.
A. discontinuity detection
6. material properties C. dimensional
D. special
Q.7.2
Using an inside diilmeter coil on tubing and applying the ph
O. a bulge.
Q .7.3
Q.7.4
Discontinuities in heat exchangers at rube support locations are easier to detect because the support plate concentrates the electromagnetic field at that point. A. True B. False
Using multifrequency techniques on installed heat exchanger tubing, a rube support plate signal can be suppres.<;ed by
subtracting a frequency signal from the optimum frequency Signal. A. low B. high C. AorB D. None of the abOve. Q.7.5
Q.7.1 0 Residual stresses in the test part produce such a small effect that they are usually ignored when selecting reference specimens. A. True B. False
In the aircraft industry, a common problem in gas turbine engines is: A. corrosion. S. fatigue cracking. C. vibration damage. D. erosion.
50
Chapter 8 Other Electromagnetic Techniques Eddy current testing is just one of a group of techniques that as a whole
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 m(.:'asurement, flux leakage te<;ting and remote field testing are all special elcctromagnetics testing techniques that, if used properly, can provide useful nondcsmlCtive testing information about ferromagne tic components. The deciding factor of one over the other is the type of material, part size or geometry and the type and size of d iscontinuities 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 ma terial thicknesses over 5.08 mm (O.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.
Method : Electromagnetic Testing Techniques: • alternating current field measurement • eddy current testing • flux leakage testing • remote field testing The borders are sometimes a little gray between one p rocess and another. These techniques have been grouped in this fashion more on the basis of their specifi c market area or specialized applications in the field testing environment rather than on a purely scientific basis. Electromagnctics 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~.
Alternating Current Field Measurement
"Now accepted a$ il $l
Primary application: Inspection of weldments Power source: Alternating current
u'e ASNT Elech"omagnetics Committee, at the time of this revision, h as selected the first four techniques because they are currently available and fairly well established to perform specific nondestructive testing inspections ill the field. In this chapter the generic differences beh-veen 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
Advantages Compared to Magnetic Particle and Dye Penetrant Inspection
•
•
• •
51
Work..<; 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, 50 it is ideal for in specting at welds. Relatively insensitive to p robe lift off, allowing deployment through coatings and on rough surfaces.
•
Allows depth sizing of d iscontinuities up to about 25 mm (1 in.), depending on probe type.
Alternating current field measurement has its origins in alternating current potential drop but instead of using a contact type probe the curn.·n! is induced in the test specimen. The contact probes prcviously used in alternating current potential drop have been changed to (noncontact) magnetic field sensitive coils. The models developed in alternating current potcn ti
Figure 8.1 shows the ba~ic principles of the technique. With no discontinuity present and a uniform current flmving in the Y direction, the magnetic field is lmiform in the X direction perpendicular to the current flow, while the other components are O. The presence of a discontinuity d iverlc; curren t 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 ampli tu de rela ted to the depth. Alternating current field measurement has been developed from the alternating cu rrent potential drop technique. Alternating current potential drop uses cu rrent injection and contact potential drop probes. This technique required extensive sUJface p repa ration of the weld under examination. It could be used to produce crack depth measurements. Figure B.1: Alternating current field measurement qualitative explanation of the magnetic forces above a notch 0;
~."~ ~~
." 0;
I
V
B,
~
"'--T - Clockwise flow
/'\..
I
I
gives Bz peak Unlfo,m input current
Counterclockwise flow gives Bz trough Current lines close together gives
Bx :ak\
." =w E:.= uo , m ~>
.,,-"
~
\
Current lines far apart gives ~ BxtrOUgh
Bx
+-- - --
T
Legend magnetic flux component normal to eleck ~ field and parallel to test surface Bz '" magnetic flu x component normal to test surface T :time or scan distance (retative scale)
Bx :
52
Flu x Leakage Testing
Figure 8.2: Equipment for magnelic flux leakage testing of pipes and lubes: (a) pig tool ; and (b) dala acquisition from pig sensors.
Primary App lication; Ferromagnetic Materia ls: p ipe, p late, wire, oil fiel d lubulars and pipelines Power Source: Permanent magnets or direct cu rrent coils Flux leakage testing has been extensively used in the pipe inspection industry. This entails the introduction of a moving d irect current magnetic field into a ferromagnetic test piece. Any localized (normally surface breaking) discontinuities that lie w ithin the inspection zone will cause the field to l1el/d or leak and extend above the surface at that point. These flux lines cut across a moving coil, or other magnetic sensor, and are lIsed to detect this direct current leakage fi eld. In pipe inspection, flu x leakage testing is u sed to look for corrosion pits and cracks. The locally thinned area puts a hi gher magnetic flux distriblltion in the space nearer to the flux detection d evice. This relative increase in fiel d strength can be measu red. Any d iscontinui ty with its major axis pa rallel to the direction of the flux flowing in the material has little ch ance of being detected using Ihis method. The pull speed of the flux leakage testing probe must be maintained at a fa irly constant rate o r the accuracy of the test is d ecreased even further. Pipeline inspections are performed with what art' called smart pigs (Figure 8.2). These devices can simultaneously carry out multiple nondestructive testing tests. The most common is flu x leakage testing. The most commonly used inservice inspection tools utilize flu x leakage testing to detect internal or external corrosion. The fl ux leakage testing inspection pig uses a circumferential array o f detectors positioned between the poles of strong permanent ma gnets to magnetize the pipe wall to nea r saturation flux density. Abnormalities in the pipe wall, i>uch as corrosion pits, result in fl ux leak.,ge 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 magnetjc inspection tools are sh ifting from the mere detection, IOc.ltion and classification of pipeline discontinuities, to the accurate measurements of discontinuity size and geometry. Modern, high resolution fl u x leakage testing inspection tools are capable of giving very detailed signals. However, converting these signals to accu rate estimates of size requires considerable expertise, as well as a detailed unde rstanding of the effects of inspection conditions and the magnetic behavior of the type of steel used.
(a)
Pickup coils (b)
Remote Field Testing Remote fie ld testing shou ld 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 appl ications 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 w ith in 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 tube wall for some distance in both directions from an exciter coil. The distribution of the primary field is dependent on the magnetic properties of the tube, the tube wall thickness and the p resence of surrounding support structures. The transmitted field may be affected by d iscontinu ities w ithin the tube wa ll or support structures on the
53
tube outside diameter. 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 penneability 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 usc of strong magnets or direct current driven satu ration coils. The remote field testing process requires no magnetic saturation. Instead it makes use of the natural tendency of ferromagne tic 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 reluctflllce. They will flow down the tube wall. which acts as a wave guide, for a considerable distance. 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 Figure 8.3: Remote field testing probe types Detector
Standard probe - rigid
Double exciter
EXCiter
, f5li 'I
3
•
•
1111
the exciter has been almost completely attenuated, or absorbed by the tube walt and the external field is actually stronger than the fi eld inside the tube. Through transmission is a teon that is often used to describe the remote field testing process. This term normally implies that there is a source of energy that transmits throllgll a medium. For example fruough 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 fiel d 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 W(1l/ tech lliqlle may be hard to visualize, but the energy path is actuaUy twice through the wall; once Ollt at the exciter and then ill at the detector. It is for this reason that short discontinuities show hvo distinct signals when the exciter and detector pass the discontinuity at different moments in time. The short discontinuity has interruptcd the through transmission path twice. In remote field testing inspt."Ction of tubing it is probably marc 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 8.5), mixing wi th incoming exciter energy in a transition zone (one to hvo diameters) and finally becoming homogenous in the remote field zone (two to three diameters) where the detector should be l ocated. The main
Flexible (small·bore)
Flexible (large-bore)
Detector { configurations (Available for all probe types)
a-OJ
Centralizer brushes
o-w i@ - W
From top to bottom: Larger diameter tubing with either Single or dual exciters, smaller diameter tubing and boiler tubing.
54
concern is to determine where along the length of the tube thc primary magnetic flux lines will reverse their direction and start their retum 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 Figure 8.4: Remote field testing energy di stribution supplies a low frequency alternating current magnetic field which couples to the tube wall. 360 Electromagnetic induction occurs I/) 10.2 ~ h \'ice. In the near field or direct r~ 10. __________ Transition zone 3 coup/I'd LOne, eddy currents are 270 ~ o created in the tube wall. These actually decrease the efficiency of the process. Eddy currents are --also created through induction as \.' --- --1 the field flux lines cut across the 10.6 ...= ". = 90 pickup coils on reentering the tube inside diameter. 10.7 B}' making careful measurements it is possible to 2 3 4 5 6 map the strength and d istribution Tube Diameters from Exciter Coil of the driver coil's flux density as it travels down the tube wall. A Inner wall phase graph can be generated, such as Outer diameter amplitude Figure 8.4, using experimental Inner diameter amplilude data that shows there are three distinct areas of interest. In an attempt to define the variations in the an area that is currently not considered to con tain alternating current energy distributions that are reliable data because the location of the transition present in the tube wall the following terminology zone changes with changes in wall thickness, has been developed: permeability and conductivity. In this zone there is Near Field (direct coupled) Zone - (0-1.5 tube a great deal of interaction between the flux of one diameters from the d ri ver coil) field that is diffusing outward from the exciter and Transition Zone - (1 .5-2 tube diameters from the the flux of the returning energy that is diffUSing driver coil) inward from the outside surface of the tube. Remote Field Zone - (2-3 tube diameters from the The total or resultant fi eld strength in this area driver coil) tends to be weaker because of the negative interaction of fields with differing directional Near Field Zone - Within the near fil::'ld zone characteristics. When the two opposing fields meet, the eddy currents generated in the tube w"ll by the the result is a cancellation of some of their alternating current driven exciter coil cre"te a respective energy. shieldillg effect of the exciter's flux . As eddy curTCnts Remote Field Zone - The third definable region propagate through the material's inner wall, an sta rts to occur at about hvo tube diameters from the opposing secondary magnetic fl ux is developed in exciter coil. The detector coil's signal amplitude the miJterial tha t attenuates the primary field bottoms out at the base of the logarithmic curve and strength and limits its extension. starts a linear decay. Notice that the curves LOgically, the near zone wou ld bc the area (Figure8.4) describing signal amplitudes of the where there is the greatest sensitivity to inner and outer walls parallel each other and are discontinuities because of the high concentration of linear after peaking at maximum values. magnetic flux. However, the field tends to be Considering the rate of attenuation of the inner ·wall field strength, the result is that in the area concentrated near the iImer surface of the tube, next to the exciter and this strong field tends to mask where the remote field zone starts, the outer wall field strength can be 10 to 100 times the strength of any signals from the tube outside diameter, which are much weaker. In remote field testing the pickup the inner wall field . coils are placed at some distance awa y from the exciter coil in an effort to get ou tside the high Phase - The phase change of the signals detected internal field area of the nea r fi eld zonc. at the pick up coil can be used to estimate the loss of walL A thinner wall allo\vs the fl ux traversing the \-vall to arrive at the detector sooner (similar to the Transition Zone - The region just outside the near field zone is known as the transition zone. It is time of pigllt of ultrasonic testing signals).
g
10"'1\
~
g.
----
"
~
55
---
-=== ----=J---
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 out<;ide diameter events; however, in remote field testing data inside diameter and outside diameter discontinuities of the same depth will have about the $ame phase angle.
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 fi eld testing is capable of detecting both small and large volume discontinuities in most ferroma gnetic 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 fa n tubing found in s a guide to the types of min imum detection capability that should be demonstrated by inspection personnel \vhen they apply the proper tools and techniques while performing remote field testing examinations.
Amplitude (voltage) - The remote fie ld 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 am plitude of the signal while sma ller 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 ill 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 ex tends out beyond the tube outside diameter, tube support plates or baffles interfere with the magnetic field distribu tion. Any metallic material on the tube outside diameter will tend to block the energy
56
Chapter 8 Review Questions Q.8.1
Which of the following electromagnetic testing techniques does not usc an
Q.8.6
The most common electromagnetic testing technique used to loca te corrosion thinning in large d iameter cross country piping systems would be: A. alternating current field measurement. B. eddy current testing. C. flux leakage testing. D. remote field testing.
Q.S.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 fi eld measurement B. eddy current testing C. flu x leakage testing D. remote field testing E. All of the above.
Q.S.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. d irect 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 perfoml remote Held te!>ting inspections are: A. usually higher than thoS€ used in conventiona l eddy current tests. B. usually lower than those uS€d in conventional eddy current tests. C. identical to those used in conventional eddy current tests. D. about one half of those used in conventional ed dy current tests.
alternating current coil excitution process? A. alternating (\lTrent field measurement
B. eddy current testing flux leakage testing D. remote field testing
C.
Q.S.2
Which of the following electromagnetic testing techniques ShOll Id provide the best discontinuity depth and length sizing capabi lity for cracks in ferromagnetic weldments? A. alternating current fi eld measurement
B. eddy current testing C. flux leakage testing
O. 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.S.4
Q.8.5
A generally accepted definition oi 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 o f 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. When a nonferromagnetic tube is inspected w ith a self-comparison differential encircling coil arrangement a nondetection could occur when a discontinuity is:
Q.8.10 The amplitude or voltage of the detected response from a discontinu ity is most often rel ated to: A. the width of the d iscontinuity. B. the location of the discontinuity. C. the depth of the discontinuity. D. the volume of the discontinuity.
A. filled , .... ith water. B. deep but very narrow. C. long " 'ith slowly v"'ying depth. D. short and wide. 57
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 de5ired end result and are acceptable to industry, are the basis of procedures, specifications and standards. Many publica tions are available to guide or instruct us. Some of the most frequently u sed references are the American Society for Testing and \Iatcnals (ASTh1), American Society o f Mechanical Engineers (ASME), American National Standards Institute (ANSI) and Military Standards ('.1lL -STD-)()()()(). These publications arc labariou!>])' p roduced by committees made up o f scien tific 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 £1316 to provide Sta ndard Terminology for Nondestructive Testillg . The military standard, M1L-STD-1537C Electrical
terms specific to the equipmen t or examination covered by the standard. SiglIifica w:e alld Use is a more detailed discussion of test TeSt1lt~ 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 u sing or referencing the standard such as persOlUlel qualification, qualification of nondestructive testing agencies, procedures and techniques, sUIface 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 includ ing instrumentation, coils, position ing 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 thi s section . Detailed in structions to process the inspection appears under procedure. These instructions may include acceptance limits and the handling of components that arc not acceptable. ASTM publishes severa l standards pertaining to the eddy current method. These standurds arc numbered; for example, E 571-98 . "E 571" refers to the standard and "98" refers to the vear of revision. Some ASTM standards that pert;in to the eddy current method are: £ 215 Standard Pmctice for Standardizing Equipment
Conductivity Test for Veri/icatiol! of Heat Trelltmf.'llt of Alumillum Alloys, Eddy Current Method, references _-\STM B193 Resistivity of Electrical COllductor Jlla terials and ASTM E18 Rockwell Hard/less a/1d Rockwell SIIpe1ficiai 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 ·w ritten 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 termillology section usually may contain definitions of unique
for Electromagnetic Examination of Seamless Aluminum-Alloy Tube E 24,'J Standard Practice for Electromagnetic (EddyCurrent) E.mmillatiOIl of Copper and Copper-Alloy Tubes E 426 Electromagnetic (Eddy-Cllrrel1t)Jesting of Seamless mid Welded Tubulal" Products, Austenitic Stainless Steel and Similar Alloys
59
American Society of Mechanical Engineers
E 571 Stal/dard Practice for Electromaglletic (EddyCllrrCllt) hamillation of Nickel and Nickel Alloy Tubular Products E 690 Standard Practice for III Situ Electromagnetic (Edd y-Cllrrmt) Examination of Nonmagnetic Heat ExcJ lUlJger Tubes E 1316 Standard Terminology for Nondestrllctil'e Testing
In 1911 the American Society of Mechanical Engineers (ASME) set up a committee to establish rules o f safety for design, fabrication and inspection of boilers and pressure vessels. These rules ha v(' 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 Vlll) has {'wo working groups and five s ubgroups reporting to it. The purpose of these groups is to interface with industry to keep pace with changing requirements and needs of indus try and public safety. The ASME Boiler and Pressure Vessel Code is divided into 11 sections. ASME Section V, NOlldestrnctive Examination, is divided into two subsections, A and B. Subsection A deals with Nondestructive MetllOds of Examination. Article 8 is
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-STO-XXXXX document<;. Special requirements are speci fied by the Military Specification system. For example, MIL-STD-1537C refers to Electrical
Conductivity Test for Verification of Heat Treatm ent of AlumilIllm Alloys, Eddy Currellt Method. The ClllibrntiOIl System RequiremCllts for MIL-STO-1537C arc contained in Military Specification MIL-C-45662. The MIL-SID usually conta ins several part<; and is very descriptive. These parts normally include Scope, Applicable Documents, Definitions, General Reqlliremcllts, Detail Requirements and Notes. The Scope contains a general statement o f applicability and intent of the Standard. Applicable Docllments pertains to other reference or controlling document<; such as other MIL-SID, Military Specification or ASTM publications. Definitioll contains precise defi nitions of key words and p hrases used in the Standard. Under General Requirements, equipment:,. reference specimen an d personnel requiremen ts 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 nO'quirements. Detail Requirements describes the specific procedure to implement the Standard. Notes contains pertinent statements about the process and guidelines fo r reporting results.
Eddy Current Examinatioll 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 SE21S. ASTvfE Section V, Article 8, Appendix I gives detailed p rocedrne requirements for Eddy Cumnt
Examination Method for Installed NOllferromngnetic Heat Exchnnger Tubing. A procedure designed to meet this requirement can be illustrated by the fo llowing example, Docu men t QA 3.
60
Procedure No. QA 3
11-1
EDDY CURRENT INSPECTION OF NONFERROUS TUBING BY SINGLE FREQUENCY TECHNI QUES 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 ASM E Section V, Article 8 of the ASME Boiler and Pressure Vesse l 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 circu late air through the generator to dry the tube sheet. 2.
Equipment
The examinations shall be performed utilizing an XXXXIXX 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. ii. iii. iv. v.
XXXX/XX eddy current instru ment. Bobbin coil probes capab le of operation in the differential and absolute modes. Digital recording device(s). Communications system . 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. fl. 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 by 360 degrees on the outer tube wal l 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.
61
Procedure No. QA 3
11·2
b.
Probe positioning and feeding shall be accomplished remotely for inservice 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 I or higher in accordance with Document QA 101. Personnel interpreting data collected in accordance with procedure shall be qualified to Level llA 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. PR ECAUTIDNS 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. PERFDR MANCE 1.
Preparation a.
Establ ish 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. 2.
Install remote digital data acquisition computers and recording devices at the data acquisition contro l ce nter.
Eq uipment Calibration a.
Prior 10 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:
62
Procedure No. QA 3
11-3
Insert the reference bobbin coil probe into a reference standard . i. ii. iii. iv. v.
vi. vii.
viii.
ix.
3.
Insert the test bobbin coil probe into a section of the reference standard, which is free 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 Procedu re. Perform a hardware null. Remotely pu ll 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. Set the display sensitivity setting for each channel per the site specific calibration procedures. Set the rotation (phase) val ue 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 dril l hole going down first as the probe is withd rawn from the standard. Set the rotation (phase) value so that the probe motion signals in the discontinu ity 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 . Complete the digital calibration summary form, update it with all pertinent information and store this information to the selected digital storage device .
Tube Inspection General (Refer to Site Specific Catibration Procedure QA 2) a.
Eddy current inspection activi1ies 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 eq uipment calibration shall be verified and recorded at the beginn ing and end of each calibration cycle. At a minimum, the calibration will be verified at 4 h intervals and after any equipment change. e.
4.
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 reinspect any of the tubes.
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 duri ng probe insertion, the speed may be as rapid as possible. c.
Due to rad iation exposure probe pusher/pullers shou ld be used to facilitate the inspection .
63
5,
F
Procedure No. QA 3 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.
11·4
a.
Install remotely operated probe feeder local to sleam 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 pos itioner 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.
ff performing manually or automatically ensure that the tube alphanumeric identifier has been properly updated. Monitor the withdrawal at 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 recordi ng 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 untif all the tubes selected for inspection have been examined .
INSPECT ION RESULTS AN D DOCUMENTATION 1.
Requirements a.
The data interpreter shall be certified to Level itA or IliA as per Procedure OA 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, equ ipment serial numbers and certification dates, software revisions and probes design and serial number shall be recorded at the start of each calibration cycle.
e. The data collection station shall be set up and calibrated as per Procedure OA 3.
64
Procedure No. QA 3
2.
11-5
Perlormance a. The data interpreter shall: Determine that all tubes selected for inspection have been tested. Report tubes whose data are incomplete or uninterpretable. Requ ire a retest of any tubes exhibiting excessive noise or unusual responses . InselVice inspections aa. Report all discontinuities> 19%. bb. Report all other indications that appear to be relevant. cc. Identify the axial position of all indications with respect to a known structural member. v. PreselVice inspections aa. Report all indications obselVed . Include the axial position of the indication with respect to a known structural member. 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 channe ls 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 obselVed shall be reported.
i. ii. iii. IV.
b.
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 perlormed and extent of completion. c.
Final Reports including all
i ndic~tions
resolved by the Data Resolution Analyst.
65
Chapter 9 Review Questions Q.9.1
A precise statement of a set of requirements to be s£ltbfied by a material, product,
Q.9.-7
The prime artificia l discontinuity used to calibrate the system described in QA 3 is: A. 20% inside diameter. 8. 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. e. every 4 h. D. every 8 h.
Q.9.9
QA 3 specifies a maximum probe traverse rate of: A. 305 mml s (12 in ./ s ). B. 355.6 mml s (14 in./s). C 152.4 mm l s (6 in./s). D . not speCified.
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 8. scope C. significance
D. procedure Q .9.10 The system in QA 3 is calibrated with an
Q.9.4
Military Standards are designated by MIL-C-(number}, A. True 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
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 m ust be: A. reinspected. B. reported . C. reevaluated. D. removed from service.
In example QA 3, personnel interpreting results must be: A. Level J or higher. B. Level II or higher. C. Level IIA or higher. D. Level III.
66
67
Answers to Review Questions Numbers in parenthe:o;es indicate where an swers may be checked and verified . For the majority o f questions 1.1 through 9.5, numbers in parentheses are keyed to the references on page v of this Study Guide. For questions 9.6 through 9.11, numbers in parentheses refer to the reprint procedure No. QA 3, starting on page 61 of this St udy Guide. Numbers m arked w ith asterisks indicate w here the answer can be found in this Study Guide.
B B C A B D B B C C
(4, p. 19) (4, p. 19) (4, p. 20) (4, p. 20) (4, p. 23) (13, p. 4 ) (4, p. 25) (4, p. 26) (4, p. 26) (4, p. 45)
2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.1 0
D B D D C B B D
(5, (4, (4, (5,
3.1 3.2 3.3 3.4
D
1.1 1.2
1.3
1.4 1.5 1.6 1.7
1.8 1.9 1.10
B E
C
A B
35
c
3.6 3.7 3.8 3 .9 3.10
D
c
D D D
p. 38.25) p. 194) p. 71)
p. 40.1 ) (4, p. 19.5 ) (6, p. 353 ) (4, p. 69) (4, p. 210) (4, p. 198) (4, p. 211)
(4, p. 328) (2, p. 36) (4, p. 332)
(Chapter 3, p. 16)* (2, p. 38) (19, p. 78) (4, p. 212) (4, p. 195) (4, p. 173) (4, p. 211)
4 .1 4.2 4.3 44 4.5 4.6 4.7 4.8 4.9 4.1 0
5.1 5.2 5.3 5.4 5.5 56 5.7 5.8 5.9 5.10 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6. 10
B C D D B B D
c
B A
A B
c
C D B B
c
B B
C
c
D A C D A A B B
(2, p. 8) (1 2, p. 95) (9, p. 56) (2, p. 13) (19, p. 78) (4, p. 171 ) (2, p. 26) (5, p. 36.17) (19, p. 88) (4, p. 27)
7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10
(19, p. 79) (5, p. 37.20) (5, p. 36.13) (5, p. 36.13) (5, p. 36.13) (4, p. 37) (4, p. 37) (19, p. 82) (5, p. 37.20) (12, p. 289)
8 .1 8 .2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 8.1 0
(4, p. 60) (4, p. 60) (4, p. 64) (12, p. 21 9) (1, p. 276) (4, p. 76) (4, p. 76)
(Cha pter 6, p. 38)* (12, p. 450)
(Chapter 6, p . 41 )*
6B
9.1 9.2 9.3 9.4 9.5 9.6 9.7 9 .8 9 .9 9 .10 9 .11
B B B A B D B C B
B
c A B
B C
c E D B D
B C B B B
c C
c B
c
B
(4, p. 270) (13, p. 59) (12, p. 282) (12, p. 256) (13, p. 47) (12, p. 129) (4, p. 51) (11, p. 631) (12, p. 121) (19, p. 102)
(1, (1, (1, (1, (1, (1, (1, (1, (1, (1,
p. p. p. p. p. p. p.
383) 248) 383) 211 ) 364) 386) 235) p. 212) p. 216) p. 403)
(18, Part II, p. iii) (18, Part II, p. iii) (18, Part II, p. 288) (15, p. 1)
(17, Section V, p. X) (QA 3, (DA 3, (QA 3, (QA 3, (QA3, (QA3,
p. p. p. p. p. p.
2) 3) 4) 4) 5) 5)
References 1.
9. Metals Halldbook, Properties mId Selection of Materials, 8th ed. Metal~ Park, OH: American Society for Metals. 1961.
Udpa, Satish S., technica l ed itor, Patrick O. Moore, editor, NondestructizlC Testillg Handbook, Third Edition: Volume 5, Electromagnetic Testing. Columbus, OH: American Society for Nondestructive Testing. 2004.
10. Cecco, V.s., G. Van Drunen, and EL. Sharp, Eddy Current Tesfing, U.s. Edition . Colum bia, MD: GP Courseware. 1987.
2. Cox, J.E. editor, ET-CT-6-5 Eddy Current Testing, Classroom Training Book, General Dyn am ics (Revised Edition). Harrisburg, NC: PH Diversified. 1997.
11 . Nondestructive EvalllatiOIl in the Nuclear Illdustry (1980). Metals Park, OH: American Society fo r Metals. 1981.
3. Hagernaier, OJ, FUlldmflelltals of Eddy Current Testillg. Colu mbus, OH : American Society for Nondesbuctive Testing. t 990. 4.
3.
6.
12. ASTM STP 722 Eddy Current Characterization of Materials and Structures. Philadelphia, PA: American Society for Testing and Ma terial~.
Libby. H .L., Introduction to Electromtlglletic Nondestructive Test Methods. New York, NY: lohn Wi ley & Sons, Inc. 1979.
1981.
13. Eddy Current Nondestructive Testing NBS Special Publication 589. Washington, D .C.: National Bureau of Standards. 1981.
McMaster, R.e., editor, Nondestructive Testing Handbook. Columbus, OH : American Society fo r Nondestructive Testing. 1959 .
14. Hagemaier, DJ, and A.P. Steinberg. " Low Frequency Ed dy Cu rrent Inspection of Aircraft Structure." Materials Evaluation, VoL 40, No.2, Feb. 1982. Columbus, O H: American Society for Nondestructive Testing. pp. 206--210.
McConnagle, W.J., Nondestructive Testing, 2d ed . New York,. NY: Gordon and Breach Publishing Company. 1975.
7. Sharpe, R.S., Resenrch Tec1miqlle5 in Nondestructive Testing, Volume 1, New York, NY: Academic Press. 1970. 8.
15. Metals Handbook, 9th Edition, Vol. 17,
Nondestruc tive EvaluatiOIl and Quality Control. Metals Park, OH: American SoCiety for Meta ls. 1989.
Harvey, O.E., ASNT Referellce Manual - Eddy Currellt Testing Theory and Practice. Columbus, O H: American Society fo r Nondestructive Testing. 1995.
16. Sadek, Hussein, Electromagnetic Testing Classroom Trainillg Book. Colu mbus, OH: American Society for Nondestructive Testing.
2006.
69
Figure Sources Chapter 1
Chapter 6
Figure Figure Figure Figure
Figure ti.I - From Hugo L. l.ibby./lllrotluctif>l' fa £tuum/wsn"';e ;\',mdt'JmIClj"e Te.<1 M ..tluxis. COP)'righl 1979. John Wi l~y &
I.I - A51\1 U - ASi'iT l.3-ASNT 1.<1-AS:-lT
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Fi.!!ure 6.2~Fmm ASM Commiu«.111 F.ddy c.:ummlln~pection. ··Eddy C~nt In~~clion:' '\/('101.1 HwulbcoJ:. Vol. I I. 9th Ed .. Iloward E. Royer. Editor. Aml!rican SotiCly far ~t.;lals. 1989. p. 176. Figul"l' 63 - ASNT I'i~ul"l' 6.4 - ASl'o'T Figure 6.5_ASm Figure 6.6 _ ul«.lnc. Figure 6.7 - Zclec.lnc. Figure 6.8 ~ I'rom Hugt) I.. l.ibby.llllrot/m',iun 10 f;lutrurrwgn~lit: Nond(,llruclir~ Test Mcrllods. copyright 1979. John Wiley & Son, . Inc. Fi£ur~ 6.9- From Hugo I .. Ubby.lrurotill,·timl/C> t.{«lf"OITUIgneric ,\'Ol/dCSIr/Kli"1e Te.I't Methods. copyright 1979. John Will!y & Sons. Inc. Figure 6.IO- ooec.lnc. figure 6.II - From lIugo L. Uhhy.lmrotiuclifHl tfJ £Iurromtl;,:nelic Nondl'.rm.criw! TeJI M~lhndJ. ~vpyri~lu 1979. John Wiley &. Sons. Inc. Figul"l' 6.I2-AS!\"T Figure 6.13-AS:\:"T Figuro: 6.14 ~ Zc(cc. hK:. Figure 6.1S-ZclC(:.lnc. Figun' 6.16- Zclc(:. In.:. Figure 6.17- Zelec.lnc.
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Suns. Inc. figun:: IJI-ASI\'T figure 1.9_AS}.."T
Chapter 2 Figure 2.J - Zclcc . Inc. Figure 2.2-Ze!cc . lnc. Fisure 2J-L.efCC . Inc. Figure 2.4-ASNT
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Chapter 3 Figure 3.I-ASl'oT Figure 3.2-ASNT
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Chapter 4 Fil/ure -4.I _ ASt\T
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Chapter 8 Figure !:I.I ~AS;"T Figure 8.2 -AS~T Figure 8.3 ~ Ru~11l\"DF. Figure 8.4 ~ ASNT
Dr. Foerster
70
S}'~lems
Inc,
The American Society fOI' Nondestructive Testing 1711 Arlingate Lane Columbus, OH 43228-0518
Order #2257R
