Seminar Report

Non-Destructive Testing (NDT) Techniques

1 INTRODUCTION Non-destructive testing (NDT) is a wide group of analysis anal ysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage. The terms Non-destructive examination (NDE), Nondestructive inspection (NDI), and Nondestructive evaluation (NDE) are also commonly used to describe this technology. Because NDT does not permanently alter the article being inspected, it is a highly-valuable technique that can save both money and time in product evaluation, troubleshooting, and research. Non-destructive Testing is one part p art of the function of Quality Control and is Complementary to other long established methods. By definition non-destructive testing is the testing of materials, for surface or internal flaws or metallurgical condition, without interfering in any way with the integrity of the material or its suitability for service. The technique can be applied on a sampling basis for individual investigation or may be used for 100% checking of material in a production quality control system. Whilst being a high technology concept, evolution of the equipment has made it robust enough for application in any industrial environment at any stage of manufacture - from steel making to site inspection of components already in service. A certain degree of skill is required to apply the techniques properly in order to obtain the maximum amount of information concerning the product, with consequent feed back to the production facility. Non-destructive Testing is not just a method for rejecting substandard material; it is also an assurance that the supposedly good is good. The technique uses a variety of principles; there is no single method around which a black box may be built to satisfy all requirements in all circumstances. What follows is a brief description of the methods most commonly used in industry, together with details of typical applications, functions and advantages. The methods Covered are: 1) Radiography 2) Magnetic Particle Crack Detection 3) Dye Penetrate Testing 4) Ultrasonic Flaw Detection 5) Eddy Current and Electro-magnetic Testing

Department of Mechanical Mechanical Engineering NIT SRINAGAR | SEMINAR SEMINAR REPORT

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Non-Destructive Testing (NDT) Techniques However, these are by no means the total of the principles available to the N.D.T. Engineer. Electrical potential drop, Sonics, infra-red, acoustic emission and Spectrograph, to name but a few, have been used to provide information that the above techniques have been unable to yield, and development across the board Continues. [8]

Definitions The following definitions apply 1) Testing: Testing or examination of a material or component in accordance with this Classification Note, or a standard, or a specification or a procedure in order to detect, locate, measure and evaluate flaws. 2) Defect: One or more flaws whose aggregate size, shape, orientation, location or properties do not meet specified requirements and are rejectable. 3) Discontinuity: A lack of continuity or cohesion; an intentional or unintentional interruption in the physical structure or configuration of a material or component 4) Flaw: An imperfection or discontinuity that may be detectable by non-destructive testing and is not necessarily reject able. 5) Indication: Evidence of a discontinuity that requires interpretation to determine its significance 6) False indication: An indication that is interpreted to be caused by a discontinuity at a location where no discontinuity exists. 7) Non relevant indication: An indication that is caused by a condition or type of discontinuity that is not reject able. False indications are non-relevant 8) Imperfections: A departure of a quality characteristic from its intended condition. 9) Internal imperfections: Imperfections that are not open to a surface or not directly accessible. 10) Quality level: Fixed limits of imperfections corresponding to the expected quality in a specific object. The Limits are determined with regard to type of imperfection, their amount and their actual dimensions. 11) Acceptance level: Prescribed limits below which a component is accepted. 12) Planar discontinuity: Discontinuity having two measurable dimensions 13) Non-planar discontinuity: Discontinuity having three measurable dimensions.


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Non-Destructive Testing (NDT) Techniques Abbreviations 1) ET Eddy current testing 2) MT Magnetic particle testing 3) PT Penetrant testing 4) RT Radiographic testing 5) UT Ultrasonic testing 6) VT Visual testing 7) HAZ Heat affected zone 8) WPS Welding Procedure Specification 9) TMCP Thermo mechanically controlled processed 10) NDT 10) NDT Non-destructive testing.

Where to apply NDE Methods: There are NDE application at almost any stage in the production or life cycle of a component. 1) To assist in product development 2) To screen or sort incoming materials 3) To monitor, improve or control manufacturing processes 4) To verify proper processing such as heat treating 5) To verify proper assembly 6) To inspect for in-service damage

Uses of NDE Methods: 1) Flaw Detection and Evaluation 2) Leak Detection 3) Location Determination 4) Dimensional Measurements 5) Structure and Microstructure Characterization 6) Estimation of Mechanical and Physical Properties 7) Stress (Strain) and Dynamic Response Measurements 8) Material Sorting and Chemical Composition Determination


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2 HISTORY OF NDT-INSTRUMENTATION After World War II the emerging modern industry needed more and more testing equipment for the production of flawless components. Therefore, instruments for NDT were developed, produced in quantities and continuously improved. The first NDT-method coming into industrial application was the X-Ray Technique.

2.1 X-Ray Technique Already 1895 Wilhelm Conrad Röntgen discovered "An Unknown Kind of Radiation" which were named in all German speaking countries after him. In his first publication he described all effects including possible flaw detection. At that time industry did not yet need this invention but medicine did. So medical equipment was developed, used and produced in quantities. The only effect Röntgen could not foresee was that X-rays harm human health. Before radiation protection became introduced, many persons lost their life. Early technical X-ray applications ap plications in Germany were realized by Richard Seifert around 1930. He improved medical equipment, cooperated with welding-institutes and built up the small company founded by his father to a world-wide respected name: Richard Seifert Hamburg 13. He got competition by Siemens and C.H.F. Muller, part of the Philips-organization, who already worked in the medical field. Seifert died in 1969, but his company kept leadership in technical X-ray-application under the direction of his youngest daughter Elisabeth Samish. Radiation testing can also be carried out with radioactive isotopes. This was discovered by Mme. Curie. She, born as Maria Sklodowska in Warscaw received the Nobel-prize for physics in 1903 together with her husband Pierre Curie and Henri Becquerel. This was the second award after Rontgen’s in 1901. Also radioactive isotopes were initially used for medical applications. In Germany Rudolf Berthold and Otto Vaupel applied them after 1933 to welded joints. After World War II Arturo Gilardoni in Italy, Drenk and Andreasen in Denmark developed X-ray-equipment, Kurt Sauerwein portable isotope-containers in Germany.

2.2 Magnetic particle crack detection Magnetic particle crack detection was executed even earlier than X-ray testing. The Englishman S.M. Saxby already in 1868 and the American William Hoke in 1917 tried to find cracks in gun barrels by magnetic indications. Real industrial application was made by Victor de Forest and Department of Mechanical Mechanical Engineering NIT SRINAGAR | SEMINAR SEMINAR REPORT

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Non-Destructive Testing (NDT) Techniques Foster Doane after 1929. They formed 1934 a company with the name Magnaflux in 1934, famous world-wide until today. The first European who built a magnetic particle crack detector was an Italian in 1932: Giraudi. His machine was named "Metalloscopio". In Germany Berthold and Vaupel applied MP-technique to welded constructions. Their equipment was produced by Ernst Heubach. Bruno Suschyzki sold this equipment. He invented swinging field MP-testing.In Berlin too E.A.W. Müller designed MP-testing machines for Siemens. In Prague the Seifertrepresentative Karasek began with similar production.After World War II Wilhelm Tiede, a former Seifert-employee, started his own company in Southern Germany. Through the Seifert-organisation he had connections to Karasek who emigrated to Brazil in 1948 after the communistic revolution in Cechoslovakia. There he continued production of MPmachines.Starting with dry-powder methods two more companies entered this market in the late Fifties: Karl Deutsch in Germany and CGM (Carlo Gianni Milano) in Italy.[11]

2.3 Penetrate Testing Penetrant Testing started also in second half of 19th century. The first people who applied the "Oil and Whiting"-process for crack detection to railway-components are unknown. The method was replaced by the upcoming MP-technique. Just before and during World War II the fast growing aircraft-industry used more and more nonmagnetic light metals, which could not be tested with MP. So independent of each other Magnaflux together with the brothers Switzer in USA, Brent Chemicals in GB, Adler (Blohm & Voss) and Klumpf (Junkers) in Germany started production of fluorescent and dye penetrants.

2.4 Eddy Current Testing Eddy Current Testing has old roots as well. The French Dominique Arago discovered the th

phenomenon during the first half of 19 century. The principle was explained by and named after Leon Foucault. Many proposals for application were made until Friedrich Förster brought this method to industrial use. He worked since 1933 for the Kaiser-Wilhelm-Institute and there he developed instruments for measuring conductivity and to sort out mixed-up ferrous components. In 1948 he founded his own company in Reutlingen which grew up with eddy current testing (ET). Institut Dr. Förster became market-leader worldwide for many decades. In Sweden Anders Arnelo started similar developments at Svenska Metalverken (SM). He solved


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Non-Destructive Testing (NDT) Techniques the problem to test hot wires and invented the pre-magnetization for ET of ferritic bars.Other companies followed later: Magnaflux, Hentschel, Law and Zetec in the USA, Rohmann and Prüftechnik Busch & Partner in Germany, Bergstrand in Sweden and Hocking in Britain.[17], [8]

2.5 Ultrasonic Testing Came latest into industrial use. The methods of exciting ultrasound were discovered already in 1847 by James Precott Joule and in 1880 by Pierre Curie and his brother Paul Jacques. Not earlier than 1912 a first application was proposed after the "Titanic" had sunk. The Englishman Richardson claimed the identification of icebergs by ultrasound in his patent applications. In France Chilowski and Langevin started their development to detect submarines by ultrasound during World War I. In 1929 the Russian Sokolov proposed to use ultrasound for testing castings. In Berlin Pohlman realized in 1937 an image-cell to indicate the differences of ultrasound-energy similar to a X-ray image-screen. The detection of laminations in plates and fine non-metallic inclusions in hot-rolled profiles became necessary during World War II. The already existing NDT-methods - X-rays, MP, PT and ET - were unable to solve these problems.Industrial uses of ultrasonic testing started simultaniously in three countries: USA, GB and Germany. The key-persons, Floyd Firestone, Donald O. Sproule and Adolf Trost had no knowledge of each other as they worked strictly in secret. Not even their patent-applications were published. Sproule and Trost used transmissiontechnique with seperate transmitter- and receiver-probes. Trost invented the so-called "TrostTonge". The 2 probes were contacted on opposite sides of a plate, held in same axis by a mechanical device - the tonge - and coupled to both surfaces by continuously flowing water. Sproule placed the 2 probes on the same side of the workpiece. So he invented double-crystal probes. But it has to be mentioned that he used this combination also with variing distance from each other. Firestone was the first to realize the reflection-technique. He modified a radar instrument and developed a transmitter with short pulses and an amplifier with short deadzone.Sproule and Firestone found industrial partners for their instruments: Kelvin-Hughes and Sperry Inc.In Germany 1949 two persons received information about the Firestone-SperryReflectoscope by publications in technical papers: Josef Krautkrämer in Cologne and Karl Deutsch in Wuppertal. Both started developments - without knowledge of each other. Josef Krautkrämer and his brother Herbert were physicists, working in the field of oscilloscopes. They


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Non-Destructive Testing (NDT) Techniques could develop ultrasonic instruments alone. Karl Deutsch, a mechanical engineer needed a partner for the electronics and found him with Hans-Werner Branscheid who had got some technical experience in radar-technique during the war. Within only one year both young and tiny companies could present their UT-flaw-detectors, starting a competition still existing today.Later on more UT-units came on the international markets: Siemens and Lehfeldt in Germany, Kretztechnik in Austria, Ultrasonique in France and Ultrasonoscope in Britain. They all stopped their production before the 70-ies, Kelvin-Hughes also stopped at the same time, Sperry was later renamed Automation Ind., around 1995. Krautkrämer became world-wide market-leader in the early 60-ies and has kept this position until today. Besides Karl Deutsch new names came up: Nukem in Germany, Panametrics and Stavely (after Sonic and Harisonic) in USA, Sonatest and Sonomatic in GB, Gilardoni in Italy and Mitsubishi in Japan.Today more than 50 companies are active in industrial NDT. They are still working under strong competition to the benefit of their customers on their way to improve the quality of industrially produced parts.

3 A BRIEF DESCRIPTION OF NDT TECHNIQUES 3.1 Radiography This technique is suitable for the detection of internal defects in ferrous and non-ferrous metals and other materials. X-rays, generated electrically, and Gamma rays emitted from radio-active isotopes, are penetrating radiation which is differentially absorbed by the material through which it passes; the greater the thickness, the greater the absorption. Furthermore, the denser the material the greater the absorption. X and Gamma rays also have the property, like light, of partially converting silver halide crystals cr ystals in a photographic film to metallic silver, in proportion to the intensity of the radiation reaching the film, and therefore forming a latent image. This can be developed and fixed in a similar way to normal photographic film. Material with internal voids is tested by placing the subject between the source of radiation and the film. The voids show as darkened areas, where more radiation has reached the film, on a clear background. The principles are the same for both X and Gamma radiography. In X-radiography the penetrating power is determined by the number of volts applied to the XRay tube - in steel approximately 1000 volts per inch thickness is necessary. In Gamma


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Non-Destructive Testing (NDT) Techniques radiography the isotope governs the penetrating power and is unalterable in each isotope. Thus Iridium 192 is used for 1/2" to 1" steel and Caesium 134 is used for 3/4" to 21/2" steel. In Xradiography the intensity, and therefore the exposure time, is governed by the amperage of the cathode in the tube. Exposure time is usually expressed in terms of milliampere minutes. With Gamma rays the intensity of the radiation is set at the time of supply of the isotope. The intensity of radiation from isotopes is measured in Becquerel’s and reduces over a period of time. The time taken to decay to half the amount of curies is the half life and is characteristic of each isotope. For example, the half life of Iridium 192 is 74 days, and Caesium 134 is 2.1 years. The exposure factor is a product of the number of curies and time, usually expressed in curie hours. The time of exposure must be increased as the isotope decays - when the exposure period becomes uneconomical the isotope must be renewed. As the isotope is continuously emitting radiation it must be housed in a container of depleted uranium or similar dense shielding material, whilst not exposed to protect the environment and personnel. To produce an X or Gamma radiograph, the film package (comprising film and intensifying screens - the latter being required to reduce the exposure time – enclosed in a light tight cassette) is placed close to the surface of the subject. The source of radiation is positioned on the other side of the subject some distance away, so that the radiation passes through the subject and on to the film. After the exposure period the film is removed, processed, dried, and then viewed by transmitted light on a special viewer. Various radiographic and photographic accessories are necessary, including such items as radiation monitors, film markers, image quality indicators, darkroom equipment, etc. Where the last is concerned there are many degrees of sophistication, including fully automatic processing units. These accessories are the same for both X and Gamma radiography systems. Also required are such consumable items as radiographic film and processing chemicals. In X-radiography the intensity, and therefore the exposure time, is governed by the amperage of the cathode in the tube. Exposure time is usually expressed in terms of milliampere minutes. With Gamma rays the intensit y of the radiation is set at the time of supply of the isotope. The intensity of radiation from isotopes is measured in Becquerel’s and reduces over a period of time.


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Figure 1(a) - Illustration Of Radiography [3] The part is placed between the radiation radiation source and a piece of film. The part will stop some of the radiation. Thicker and denser area will will stop more of the radiation Advantages of Radiography

1. Information is presented pictorially. 2. A permanent record is provided which may be viewed at a time and place 3. Distant from the test. 4. Useful for thin sections. 5. Sensitivity declared on each film. 6. Suitable for any material.

Disadvantages of Radiography

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Generally an inability to cope with thick sections.

2. Possible health hazard. 3. Need to direct the beam accurately for two-dimensional defects. 4. Film processing and viewing facilities are necessary, as is an exposure compound. 5. Not suitable for automation, unless the system incorporates fluoroscopy with 6. an image intensifier or other electronic aids 7. Not suitable for surface defects. 8. No indication of depth of a defect below the surface


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3.2 Magnetic Particle Inspection This method is suitable for the detection of surface and near surface discontinuities in magnetic material, mainly ferrite steel and iron. An Illustration of the Principle of Magnetic Particle Inspection

Figure 1(b)-Magnetic Particle Inspection [3] The principle is to generate magnetic flux in the article to be examined, with the flux lines running along the surface at right angles to the suspected defect. Where the flux lines approach a discontinuity they will stay out in to the air at the mouth of the crack. The crack edge becomes magnetic attractive poles North and South. These have the power to attract finely divided particles of magnetic material such as iron fillings. Usually these particles are of an oxide of iron in the size range 20 to 30 microns, and are suspended in a liquid which provides mobility for the particles on the surface of the test piece, assisting their migration to the crack edges. However, in some instances they can be applied in a dry powder form. The particles can be red or black oxide, or they can be coated with a substance, which fluoresces brilliantly under ultra-violet illumination (black light). The object is to present as great a contrast as possible between the crack indication and the material background. The technique not only detects those defects which are not normally visible to the unaided eye, but also renders easily visible those defects which would otherwise require close scrutiny of the surface. There are many methods of generating magnetic flux in the test piece, the simplest one being the application of a permanent magnet to the surface, but this method cannot be controlled accurately because of indifferent surface contact and deterioration in magnetic strength. Modern equipments generate the magnetic field electrically either directly or indirectly.


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Non-Destructive Testing (NDT) Techniques In the direct method a high amperage current is passed through the subject and magnetic flux is generated at right angles to the current flow. Therefore the current flow should be in the same line as the suspected defect. If it is not possible to carry out this method because of the orientation of the defect, then the indirect method must be used. This can be one of two forms: 1. Passing a high current through a coil that encircles the subject. 2. Making the test piece form part of a yoke, this is wound with a current carrying coil. The

effect is to pass magnetic flux along the part to reveal transverse and circumferential defects. If a bar with a length much greater than its diameter is considered, then longitudinal defects would be detected by current flow and transverse and circumferential defects by the indirect method of an encircling coil or magnetic flux flow. Subjects in which cracks radiating from a hole are suspected can be tested by means of the threading bar technique, whereby a current carrying conductor is passed through the hole and the field induced is cut by any defects. Detection of longitudinal defects in hollow shafts is a typical application of the threader bar technique. The electricity used to generate the magnetic flux in any of these methods can be alternating current, half wave rectified direct current or full wave rectified direct current. A.C. generated magnetic flux, because of the skin effect, preferentially follows the contours of the surface and does not penetrate deeply into the material. H.W.D.C. penetrates more deeply but is inclined not to follow sharp changes in section. H.W.D.C. is useful for the detection of slightly subsurface defects. The pulsing effect of A.C. and H.W.D.C. gives additional mobility to the indicating particles. D.C. penetrates even more deeply but does not have this facility. Furthermore, demagnetizing of the material after D.C. magnetizing is far more difficult than after A.C. magnetizing. Normally, to ensure that a test piece has no cracks, it is necessary to magnetize it in at least two directions and after each magnetizing - and ink application – visually examine the piece for crack indications. Since this double process, which would include adjustment of the magnetizing equipment controls in between each magnetizing takes time it is obviously advantageous to have the facility to reduce the time required. The recent development of the Swinging Field method of multi-directional magnetizing will indicate all defects, regardless of their orientation on the surface, with one magnetizing shot and therefore requires only one inspection. (Please refer to our paper entitled Faster Magnetic Crack Detection using the Multi-directional Swinging Field Method). Department of Mechanical Mechanical Engineering NIT SRINAGAR | SEMINAR SEMINAR REPORT

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Non-Destructive Testing (NDT) Techniques Basically magnetic crack detection equipment takes two forms. Firstly, for test pieces which are part of a large structure, or pipes, heavy castings, etc. which cannot be moved easily, the equipment takes the form of just a power pack to generate a high current. This current is applied to the subject either by contact prods on flexible cables or by an encircling coil of cable. These power packs can have variable amperages up to a maximum of 2000 Amps for portable units, and up to 10,000 Amps for mobile equipments. Both A.C. and H.W.D.C. magnetising current is available. The indicating material is applied by means of a spray and generally the surplus runs to waste. For factory applications on smaller more manageable test pieces the bench type of equipment, as represented by our EUROMAG range, is normally preferred. This consists of a power pack similar to those described above, an indicating ink system which recirculates the fluid, and facilities to grip the work piece and apply the current flow or magnetic flux flow in a more methodical, controlled manner. The work pieces are brought to the equipment and can be individually tested. Subjects up to approximately 100" long can be accommodated is such equipments and can be loaded by crane if necessary. This type of universal equipment is ideally suited to either investigative work or routine quality control testing. These bench type equipments often incorporate a canopy to prevent direct light falling on the subject so that ultraviolet fluorescent material can be used to the best effect. The indicating particles may be suspended in very thin oil (kerosene) or water. In some circumstances the indicating medium can be applied dry. These equipments are suited to production work and in certain circumstances can be automated to the extent of loading, magnetising, inking and unloading. The work pieces still have to be viewed by eye for defect indications. Specialised equipments are also frequently manufactured to test a particular size and type of test piece.

Figure 1(c)- An Illustration of Magnetic Particle Inspection [1] Advantages of Magnetic Particle Crack Detection Department of Mechanical Mechanical Engineering NIT SRINAGAR | SEMINAR SEMINAR REPORT

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Non-Destructive Testing (NDT) Techniques 1. Simplicity of operation and application. 2. Quantitative. 3. Can be automated, apart from viewing. (Though modern developments in automatic defect recognition can be used in parts of simple geometry e.g. billets and bars. In this case a special camera captures the defect indication image and processes it for further display and action)

Disadvantages of Magnetic Particle Crack Detection

1. Restricted to ferromagnetic materials. 2. Restricted to surface or near surface flaws. 3. Not fail safe in that lack of indication could mean no defects or process not carried out properly.

3.3 Dye Penetrate Testing This method is frequently used for the detection of surface breaking flaws in non ferromagnetic materials. The subject to be examined is first of all chemically cleaned, usually by vapour phase, to remove all traces of foreign material, grease, dirt, etc. from the surface generally, and also from within the cracks. Next the penetrate (which is a very fine thin oil usually dyed bright red or ultra-violet fluorescent) is applied and allowed to remain in contact with the surface for approximately fifteen minutes. Capillary action draws the penetrate into the crack during this period.

Figure1(d)- An Illustration of Dye Penetrate Testing [1] Department of Mechanical Mechanical Engineering NIT SRINAGAR | SEMINAR SEMINAR REPORT

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Non-Destructive Testing (NDT) Techniques The surplus penetrate on the surface is then removed completely and thin coating of powdered chalk is applied. After a further period (development time) the chalk draws the dye out of the crack, rather like blotting paper, to form a visual, magnified in width, indication in good contrast to the background. The process is purely a mechanical/chemical one and the various substances used may be applied in a large variety of ways, from aerosol spray cans at the most simple end to dipping in large tanks on an automatic basis at the other end. The latter system requires sophisticated tanks, spraying and drying equipment but the principle remains the same.

Advantages of Dye Penetrate Testing

1. Simplicity of operation. 2. Best method for surface breaking cracks in non-ferrous metals. 3. Suitable for automatic testing, with reservation con cerning viewing. (See automatic defect recognition in Magnetic Particle Inspection)

Disadvantages of Dye Penetrate Testing

1. Restricted to surface breaking defects only. 2. Decreased sensitivity. 3. Uses a considerable amount of consumables.

3.4 Ultrasonic Flaw Detection This technique is used for the detection of internal and surface (particularly distant surface) defects in sound conducting materials. The principle is in some respects similar to echo sounding. A short pulse of ultrasound is generated by means of an electric charge applied to a piezoelectric crystal, which vibrates for a very short period at a frequenc y related to the thickness of the crystal. In flaw detection this frequency is usually in the range of one million to six million times per second (1 MHz to 6 MHz). Vibrations or sound waves at this frequency have the ability to travel a considerable distance in homogeneous elastic material, such as many metals with little attenuation. The velocity at which these waves propagate is related to the Young’s Modulus for the material and is characteristic of that material. For example the velocity in steel is 5900 meters per second, and in water 1400 meters per second. Ultrasonic energy is considerably attenuated in air, and a beam propagated through a solid will, on reaching an


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Non-Destructive Testing (NDT) Techniques interface (e.g. a defect, or intended hole, or the back wall) between that material and air reflect a considerable amount of energy in the direction equal to the angle of incidence. For contact testing the oscillating crystal is incorporated in a hand held probe, which is applied to the surface of the material to be tested. To facilitate the transfer of energy across the small air gap between the crystal and the test piece, a layer of liquid (referred to as ‘coolant’), usually oil, water or grease, is applied to the surface. As mentioned previously, the crystal does not oscillate continuously but in short pulses, between each of which it is quiescent. Piezo electric materials not only convert electrical pulses to mechanical oscillations, but will also transducer mechanical oscillations into electrical pulses; thus we have not only a generator of sound waves but also a detector of returned pulses. The crystal is in a state to detect returned pulses when it is quiescent. The pulse takes a finite time to travel through the material to the interface and to be reflected back to the probe. The standard method of presenting information in ultrasonic testing is by means of a cathode ray tube, in which horizontal movement of the spot from left to right represents time elapsed. The principle is not greatly different in digitized instruments that have a LCD flat screen. The rate at which the spot moves is such that it gives the appearance of a horizontal line on the screen. The system is synchronized electronically so that at the instant the probe receives its electrical pulse the spot begins to traverse the screen. An upward deflection (peak) of the line on the left hand side of the screen is an indication of this occurrence. This peak is usually termed the initial pulse. While the base line is perfectly level the crystal is quiescent. Any peaks to the right of the initial pulse indicate that the crystal has received an incoming pulse reflected from one or more interfaces in the material. Since the spot moves at a very even speed across the tube face, and the pulse of ultrasonic waves moves at a very even velocity through the material, it is possible to calibrate the horizontal line on the screen in terms of absolute measurement. The use of a calibration block, which produces a reflection from the back wall a known distance away from the crystal together with variable controls on the flaw detector, allows the screen to be calibrated in units of distance, and therefore determination of origins of returned pulses obtained from a test piece. It is therefore possible not only to discover a defect between the surface and the back wall, but also to measure its distance di stance below the surface. s urface. It is important that the equipment is properly calibrated and, since it is in itself not able to discriminate between intended boundaries of the object under test and unintended discontinuities, the operator must be able to identify the origin Department of Mechanical Mechanical Engineering NIT SRINAGAR | SEMINAR SEMINAR REPORT

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Non-Destructive Testing (NDT) Techniques of each peak. Further as the pulses form a beam it is also possible to determine the plan position of a flaw. The height of the peak (echo) is roughly proportional to the area of the reflector, though there is on all instruments a control, which can reduce or increase the size of an indication - variable sensitivity in fact. Not only is party of the beam reflected at a material/air interface but also at any junction where there is a velocity change, for example steel/slag interface in a weld. Probing all faces of a test piece not only discovers the three-dimensional defect and measures its depth, but can also determine its size. Two-dimensional (planar) defects can also be found but, unlike radiography, it is best that the incident beam impinges on the defect as near to right angles to the plane as possible.

Figure 1(e)- An Illustration of Ultrasonic Flaw Detection [1], [3]


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Non-Destructive Testing (NDT) Techniques To achieve this some probes introduce the beam at an angle to the surface. In this manner longitudinal defects in tubes (inner or outer surface) are detected. Interpretation of the indications on the screen requires a certain amount of skill, particularly when testing with hand held probes. The technique is, however, admirably suited to automatic testing of regular shapes by means of a monitor – an electronic device that fits into the main equipment to provide an electrical signal when an echo occurs in a particular position on the trace. The trigger level of this signal is variable and it can be made to operate a variety of mechanical gates and flaw warnings. Furthermore, improvements in computer technology allow test data and results to be displayed and out-putted in a wide variety of formats. Modern ultrasonic flaw detectors are fully solid state and can be battery powered, and are robustly built to withstand site conditions. Since the velocity of sound in any material is characteristic of that material, it follows that some materials can be identified by the determination of the velocity. This can be applied, for example in S.G. cast irons to determine the percentage of graphite Modularity.

Advantages of Ultrasonic Flaw Detection

1. Thickness and lengths up to 30 ft can be tested. 2. Position, size and type of defect can be determined. 3. Instant test results. 4. Portable. 5. Extremely sensitive if required. 6. Capable of being fully automated.

Disadvantages of Ultrasonic Flaw Detection

1. Access to only one side necessary. 2. No consumables. 3. Disadvantages of Ultrasonic Flaw Detection 4. No permanent record available unless one of the more sophisticated sop histicated test results and an d data collection systems is used. 5. The operator can decide whether the test piece is defective or not whilst the test is in progress. 6. Indications require interpretation (except for digital wall thickness gauges). Department of Mechanical Mechanical Engineering NIT SRINAGAR | SEMINAR SEMINAR REPORT

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Non-Destructive Testing (NDT) Techniques 7. Considerable degree of skill necessary to obtain the fullest information from the test.

8. Very thin sections can prove difficult. 3.5 Eddy Current and Electro-Magnetic Methods The main applications of the eddy current technique are for the detection of surface or subsurface flaws, conductivity measurement and coating thickness measurement. The technique is sensitive to the material conductivity, permeability and dimensions of a product. Eddy currents can be produced in any an y electrically conducting material that is subjected to an alternating magnetic field (typically 10Hz to 10MHz). The alternating magnetic field is normally generated by passing an alternating current through a coil. The coil can have many shapes and can between 10 and 500 turns of wire. The most simple coil comprises a ferrite rod with several turns of wire wound at one end and which is positioned close to the surface of the product to be tested. When a crack, for example, occurs in the product surface the eddy currents must travel farther around the crack and this is detected by the impedance change

. Figure 1(f)-Coil with single winding [1], [3] Department of Mechanical Mechanical Engineering NIT SRINAGAR | SEMINAR SEMINAR REPORT

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Coils can also be used in pairs, generally called a driven pair, and this arrangement can be used with the coils connected differentially. In this way ‘lift off’ (distance of the Probe from the surface) signals can be enhanced.

Figure 1(g)-Coil with double winding winding [1], [3] [3] Coils can also be used in a transformer type configuration where one coil winding is a Primary and one (or two) coil windings are used for the secondary’s.


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Figure 1(h)-Coil used as transformer [1], [3]

The detected eddy current signals contain amplitude and phase information and which can be displayed on CRT type displays – non digital displays. Signals can be displayed as the actual, i.e. absolute signal, or with appropriate electronics, only a signal change is displayed. The best results are obtained where only one product parameter is changes, e.g. the presence of a crack. In practice changes in eddy current signals are caused by differences in composition, hardness, texture, shape, conductivity, permeability and geometry. In some cases the effects of the crack can be hidden by changes in other parameters and unnecessary rejection can occur. However, the coils can be selected for configuration, size and test frequency in order to enhance detection of cracks, conductivity, metal loss etc. as required. The depth to which the eddy currents penetrate a material can be changed by adjusting the test frequency – the higher the frequency, the lower the penetration; however, the lower the frequency, the lower sensitivity to small defects. Larger coils are less sensitive to surface roughness and vice versa. The latest electronic units are able to operate a wide range of coil configurations in absolute or differential modes and at a wide range of frequencies. For surface testing for cracks in single or complex shaped components, coils with a single ferrite cored winding are normally used. The probe is placed on the component and ‘balanced’ by use of the electronic unit controls. As the probe is scanned across the surface of the component the cracks can be detected. See Figure (1) Where surfaces are to be scanned automatically the single coil windings are suitable only if the lift off distance is accurately maintained. Generally differential coil configurations are used with higher speed scanning systems where lift off effects, vibration


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Non-Destructive Testing (NDT) Techniques effects, etc. can be cancelled out to an acceptable extent. See Figure2. Tubes, bar and wire can be inspected using an encircling coil and these usually have a coil configuration with one primary and two secondary’s connected differentially. See Figure3. Most eddy current electronics have a phase display and this gives an operator the ability to identify defect d efect conditions. In many man y cases signals from cracks, lift off and other parameters can be clearly identified. Units are also available which can inspect a product simultaneously at two or more different test frequencies. These units allow specific unwanted effects to be electronically cancelled in order to give improved defect detection. The eddy current test is purely electrical. The coil units do not need to contact the product surface and thus the technique can be easily automated. Most automated Systems are for components of simple geometry where mechanical handling is simplified.

Figure 1(i)-An Illustration of Eddy Current Testing Equipment [1], [3]

Advantages of Eddy Current Testing

1. Suitable for the determination of a wide range of conditions of conducting material, such as defect detection, composition, hardness, conductivity, permeability etc. in a wide variety of engineering metals. 2. Information can be provided in simple terms: often go/no go. Phase display electronic units can be used to obtain much greater product information. 3. Extremely compact and portable units are available. 4. No consumables (except probes – which can some times be repaired). 5. Flexibility in selection of probes and test frequencies to suit different applications. 6. Suitable for total automation. Department of Mechanical Mechanical Engineering NIT SRINAGAR | SEMINAR SEMINAR REPORT

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Disadvantages of Eddy Current Testing

1. The wide range of parameters which affect the eddy current responses means that the signal from a desired material characteristic, e.g. a crack, can be masked by an unwanted parameter, e.g. hardness change. Careful selection of probe and electronics will be needed in some applications. 2. Generally tests restricted to surface breaking conditions and slightly subsurface flaws.

3.6 Visual Inspection 1. Thermo graphic surveys 2. Bore scope and fibre optic inspections 3. Miniature fiberscope inspections form HGP components with complex geometries.

Figure 1(j)-Visual Inspection [1], [3]


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Non-Destructive Testing (NDT) Techniques Table (1) Non-Destructive Testing Methods & Applications


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4 The Eddy Current Inspection Method: 4.1 Basic electrical theory:

When a voltage is applied to a circuit containing only resistive elements, current flows according to Ohm’s Law: I = V/R or V = I.R If a circuit consists of more than one element, the overall voltages, resistance and capacitance can be calculated by simple algebra, for example, with two resistors in series, as show in Figure 1, current (I) must be the same for both resistors, so: V 1 =I.R1 , V 2=I.R2 , V total total = V 1+V 2 = I.R1+ I.R2 = I (R 1+R2 ) = I.Rtotal So, Rtotal = R1+ R2 4.2 Electromagnetic induction:

In 1824 Oersted discovered that current current passing though though a coil created a magnetic field capable of shifting a compass needle. Seven years later, Faraday and Henry discovered the opposite: that a moving magnetic field would induce a voltage in an electrical conductor.

Figure 2. Circuit containing only resistive elements [1], [5] The two effects can be shown in a simple transformer connected to a DC supply as shown in Figure 2.The meter needle will deflect one way when current is applied, then back the other way when it is removed. A voltage is only induced when the magnetic field is changing.


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Non-Destructive Testing (NDT) Techniques Such a voltage is also induced in the first winding, and will tend to oppose the change in the applied voltage.

Figure 3. Simple transformer. [1], [5] The induced voltage is proportional to the rate of change of current: di/dt A property of the coil called inductance (L) is defined, such that: Induced voltage= L di/dt If an AC current flows through an inductor, the voltage across the inductor will be at maximum when the rate of change of current is greatest. For a sinusoidal waveform, this is at the point where the actual current is zero. See Figure 3.[7],[13] Thus the voltage applied to an inductor reaches its maximum value a quarter-cycle before the current does - the voltage is said to lead the current by 90 degrees.The value of the voltage and current can be calculated from the formula: V = I.XL where XL is the inductive reactance, defined by the formula: XL = 2πf L where f is the frequency in Hz. As we saw above, for series DC circuits calculation of total resistance is simply a matter of adding the individual resistance values.


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Figure 4. Sinusoidal waveforms of voltage and current in an inductive circuit. . [1], [5] For an AC circuit it is not so simple, but the same basic principles apply: the current through both elements must be the same, and at any instant the total voltage across the circuit is the sum of the values across the elements (see Figure 4). However, the maximum voltage across the resistance coincides with zero voltage across the inductor and vice versa - see Figure 5.

Figure 5. Sinusoidal waveforms of voltage and current in an AC circuit containing resistive and inductive elements. . [1], [5]


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Non-Destructive Testing (NDT) Techniques We can represent this graphically using a vector diagram, as shown in Figure 6.

Figure 6. Vector diagram. [1], [5] The impedance of the circuit is therefore given by the formula: Z=√ XL +R 2

2

Total resistance in AC circuits – Impedance. The phase angle between voltage and current is given by:

ൌ sinିଵ XL/ 4.3 Theory: Simple coil above a metal surface:

When an AC current flows in a coil in close proximity to a conducting surface (see Figure 7) the magnetic field of the coil will induce circulating (edd y) currents in that surface. The magnitude and phase of the eddy currents will affect the loading on the coil and thus its impedance. As an example, assume that there is a deep crack in the surface immediately underneath the coil (see Figure 8). This will interrupt or reduce the eddy current flow, thus decreasing the loading on the coil and increasing its effective impedance. This is the basis of eddy current testing. By monitoring the voltage across the coil (Figure 9) in such an arrangement we can detect changes in the material of interest. Department of Mechanical Mechanical Engineering NIT SRINAGAR | SEMINAR SEMINAR REPORT

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Figure 7. Induction of eddy currents. [1], [5]

Figure 8. Eddy currents are affected by the presence of a crack. [1], [5]


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Figure 10. Cracks must interrupt the surface eddy current flow to be detected. [1], [5] Note that cracks must interrupt the surface eddy current flow to be detected. Cracks lying parallel to the current path will not cause any significant interruption and may not be detected (see Figure 10).

5 Factors affecting eddy current response: A number of factors, apart from flaws, will affect the eddy current response from a probe. Successful assessment of flaws or any of these factors relies on holding the others constant, or somehow eliminating their effect on the results. It is this elimination of undesired response that forms the basis of much of the technology of eddy current inspection. The main factors are: 5.1 Material conductivity:

The conductivity of a material has a very direct effect on the eddy current flow: the greater the conductivity of a material, the greater the flow of eddy currents on the surface. Conductivity is often measured by an eddy current technique, and inferences can then be drawn about the different factors affecting conductivity, such as material composition, heat treatment, work hardening etc.[1],[9]


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Non-Destructive Testing (NDT) Techniques 5.2 Permeability:

This may be described as the ease with which a material can be magnetised. For non-ferrous metals such as copper, brass, aluminium etc., and for austenitic stainless steels the permeability is the same as that of ‘free space’, ie the relative permeability (µr ) is one. For ferrous metals however the value of µr may be several hundred, and this has a very significant influence on the eddy current response, in addition it is not uncommon for the permeability to vary greatly within a metal part due to localised stresses, heating effects etc. 5.3 Frequency:

As we will discuss, eddy current response is greatly affected by the test frequency chosen, fortunately this is one property we can control. 5.4 Geometry:

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

Figure 11. Standard depth of penetration. [1], [5] Department of Mechanical Mechanical Engineering NIT SRINAGAR | SEMINAR SEMINAR REPORT

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Non-Destructive Testing (NDT) Techniques 5.5 Proximity/lift-off:

The closer a probe coil is to the surface the greater will be the effect on that coil. This has two main effects: 1) The ‘lift-off’ signal as the probe is moved mov ed on and off the surface. s urface. 2) A reduction in sensitivity as the coil to product spacing increases. 5.6 Depth of penetration:

The eddy current density, and thus the strength of the response from a flaw, is greatest on the surface of the metal being tested and declines with depth. It is mathematically convenient to define the ‘standard depth of penetration’ where the eddy current is 1/e (37%) of its surface value (see Figure 11).[12],[7] The standard depth of penetration in mm is given by the formula:

Where: ρ is resistivity in mΩ.cm (ρ = 172.41/ 172 .41/ material conductivity) f is frequency in Hz µr is the relative permeability of the material, for Non-Ferrous = 1 from this it can be seen that depth of penetration: 1) Decreases with an increase in frequency 2) Decreases with an increase in conductivity 3) Decreases with an increase in permeability – this can be very significant – penetration into ferrous materials at practical frequencies is very small.


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Non-Destructive Testing (NDT) Techniques The graph in Figure 12 shows the effect of frequency on standard depth of penetration.

Figure 12. The effect of frequency on standard depth of penetration. . [1], [5] It is also common to talk about the ‘effective depth of penetration’ usually defined as three times the standard depth, where eddy current density has fallen to around 5% of its surface value. This is the depth at which there is considered to be no influence on the eddy current field.

6 The impedance plane and probes: 6.1 The impedance plane: Eddy current responses of a single coil may be conveniently described by reference to the ‘impedance plane’. This is a graphical representation of the complex probe impedance where the abscissa (X value) represents the resistance and the ordinate (Y value) represents the inductive reactance – see Figure 13.


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Figure 13. Impedance plane diagram. . [1], [5] Note that, while the general form of the impedance plane remains the same, the details are unique for a particular probe and frequency.The display of a typical CRT eddy current instrument represents a ‘window’ into the impedance plane, which can be rotated and ‘zoomed’ to suit the needs of the application.For example, in the impedance plane diagram in Figure 13 a rotated detail of the ‘probe on aluminium’ area would appear as Figure 14.[6]


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Figure 14. Typical eddy current instrument and its display. [1], [5] This shows the display when moving over a series of simulated cracks of varying depths. Note that in the example shown, both the amplitude and the phase of response from the different-sized cracks varies.

6.2 Eddy Current Generation and Detection: Coils:

A coil will increase the intensity of the magnetic field produced from an electric current.The field from adjacent wires in a coil add to provide a new total magnetic field dependent on the current and the number of turns in the coil. Coils are necessary in eddy current testing to produce


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Non-Destructive Testing (NDT) Techniques a sufficient magnetic field from limited current or a sufficient current from a limited magnetic field. The shape of the magnetic field from a coil is similar to that from a permanent magnet.This can be represented as a series of lines or, for simplicity a single arrow. For D.C. current the arrowhead is at the North Pole; for A.C. this only occurs at a certain point in time but is related to the directions of currents flowing at the same point in time. The magnetic field varies at the same frequency as the current in the coil. The coil windings are also sometimes shown collectively. In practical eddy current probes a ferrite material is often used to further concentrate and control the magnetic field. The ferrite is usually in the centre of the coil, and in some applications (shielded probes) may also surround the coil.

Figure 15. Magnetic Field produced in a coil. . [1], [5] Eddy Current Generation:

If a coil is brought in close clo se proximity with a conductive material the alternating magnetic field (primary field) will pass through the material. As discussed above eddy currents will be induced in to the material. The eddy currents generated will normally have circular paths at right angles to the primary field. The flow of the eddy currents in terms of magnitude, phase and distribution depend on several factors.[1]


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Figure 16. Eddy Currents flowing in a material. [1], [5] These electrical Eddy Currents will induce a secondary magnetic field to flow inopposition to the original primary field.

Figure 17. Secondary Field produced by the Primary Magnetic Field. [1], [5]


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Non-Destructive Testing (NDT) Techniques 6.3 Eddy Current Detection:

This situation can be balanced and so the display can be set to read zero in the normal set of circumstances, (no crack) but if there is a change in the Eddy Current flowing in the material this will then alter the secondary field, which in turn will affect the characteristics of the primary coil. It is this change that will be monitored and so displayed, normally, on either a meter or cathode ray tube monitor.

Figure 18. Advanced Eddy Current instrument. . [1], [5]

Figure 19. General Crack Detection image on instrument shown in picture above.


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Figure 20. Corrosion Detection image on instrument shown in picture above.

Figure 21. Coating Thickness Detection image on instrument shown in picture above.

7 Applications and practical testing: Applications Eddy current applications encompass a wide range of capabilities. Here is given a summary of some of these and a discussion about their practical requirements. If we refer to part one of this series we will find the standard equation for calculating the effective depth of penetration. This equation has parameters for resistivity, test frequency, relative magnetic permeability and depth of penetration. The change in any one of these parameters gives the basis for an eddy current. Figure 24 illustrates some of the applications possible.[8],[13]


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Figure 22. Eddy current applications. . [1], [5]

Defect detection: Surface crack detection:

This is normally carried out with pencil probes or ‘pancake’ type probes on ferrous or nonferrous metals. Frequencies from 100 kHz to a few MHz are commonly used. Depending on surface condition it is usually possible to find cracks 0.1 mm or less in depth. Shielded probes, with their focused field, add the ability to test very close to edges or dissimilar materials such as ferrous fasteners in an aluminium structure. Differential probes are sometimes used, particularly in automated applications, but care must be taken to ensure that the orientation of flaws is correct for detection. Sub-surface crack/corrosion detection:

This is primarily used in airframe inspection. By using a low frequency and a suitable probe, eddy currents can penetrate aluminium or similar structures to a depth of 10 mm or so, allowing the detection of second and third layer cracking, which is invisible from the surface, or thinning of any of the different layers making up the structure. Test frequencies are generally in the range


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Non-Destructive Testing (NDT) Techniques 100 Hz to 10 kHz. Probe size should also be two or more times wider than the depth of penetration required.

Material sorting: Non-ferrous metal sorting:

This is conductivity testing, and for dedicated applications a conductivity meter may be a better choice. From the impedance plane diagram one can observe that the indication from a conductivity change is essentially the same as from a crack, and both meter and impedance plane type crack detectors can be successfully used to sort similar metals using a suitable absolute probe. It should be remembered that: 1) Widely different metals may be a similar s imilar conductivity; 2) The allowable values for similar alloys may overlap; 3) An alloy of one material can, in vastly different states of heat treatment, have the same electrical conductivity; and 4) There is no direct relationship between conductivity and hardness. However, once these caveats are understood then conductivity measurement can be used as part of a quality control system. Suitable test frequencies used are in the range 10 kHz to 2 MHz, although account should be taken of the material thickness to ensure the depth of penetration is less than one third of the material thickness. Ferrous metal sorting:

Ferrous material may be sorted using eddy current impedance plane equipment. Unfortunately it is not possible to produce quantitative values due to the reading obtained being related to electrical conductivity, magnetic permeability and the depth of the change in material properties. Frequencies to use are 100 Hz to 10 kHz. The use of two or more frequencies gives additional information about the depth of the material properties such as in induction hardening.


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Non-Destructive Testing (NDT) Techniques Coating thickness assessment:

In the simple case of a non-conductive coating (for example paint) on a conductive material, then the eddy current lift of signal can be used. The probe type to be used should have an absolute response with reflection spot face probes offering some advantage in temperature stability and frequency range. Higher frequencies are preferred (100 kHz and higher) and for non-ferrous materials it should be checked that the frequency is sufficiently high so as not to be influenced by material thickness thickn ess (say 10 times that to make the wall thickness equal the effective depth d epth of penetration) .To obtain quantitative q uantitative readings, a calibration piece with several different thickness of coating in the range of interest is essential, and a calibration curve created. Some instruments have an intrinsic function as part of conductivity measurement for obtaining direct readings. For the more complex case where the coating is conductive, then the following needs to be taken into account. The two materials must have different conductivities and/or relative permeabilities and the top coating must be non-magnetic. Choose a frequency that will make the effective depth of penetration equal the nominal wall thicknes s. If the surface coating has higher resistivity than the lower coating, then by using a frequency that is sufficiently low to penetrate the surface coating, results will be similar to that for non-conductive coatings. Wall thickness assessment:

This is possible in the same way as it is possible to determine non-ferrous conductive coating thickness and the same rules apply about choice of frequency.

Tube inspection: Tubes may be inspected from the outer diameter (OD), usually at the time of manufacture and from the inner diameter (ID), usually for in-service inspection, particularly for heat exchanger inspection. ID heat exchanger tube testing:

Heat exchangers used for petrochemical or power generation applications may have many thousands of tubes, each up to 20 m long. Using a differential Internal Diameter (ID or ‘bobbin’) probe, these tubes can be tested at high speed sp eed (up to 1 m/s with computerised data analysis) and


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Non-Destructive Testing (NDT) Techniques by using phase analysis, defects such as pitting can be assessed to an accuracy of about 5% of tube wall thickness. This allows accurate estimation of the remaining life of the tube, allowing operators to decide on appropriate action such as tube plugging, tube replacement or replacement of the complete heat exchanger. The operating frequency is determined by the tube material and wall thickness, ranging from a few kHz for thick-walled copper tube, up to around 600 kHz for thin-walled titanium. Tubes up to around 50 mm diameter are commonly inspected with this technique. Inspection of ferrous or magnetic stainless steel tubes is not possible using standard eddy current inspection equipment. Dual or multiple frequency inspections are commonly used for tubing inspection, in particular for suppression of unwanted responses due to tube support plates. By subtracting the result of a lower frequency test (which gives a proportionately greater response from the support) a mixed signal is produced showing little or no support plate indication, thus allowing the assessment of small defects in this area. Further frequencies may be mixed to reduce noise from the internal surface. Remote field:

Remote field is a branch of eddy current testing that has evolved over the last decade or so. By using specially designed equipment and ID probes it is possible to obtain indications of wall thickness changes on magnetic material. In-line inspection of tubing:

External eddy current encircling test coils are commonly used for inspecting high quality metal tubing of wall thicknesses less than 6 mm. When the tube is made of a magnetic material there are two main problems: 1) Because of the high permeability, there is little or no penetration of the eddy current field into the tube at practical test frequencies. 2) Variations in permeability (from many causes) cause eddy current responses which are orders of magnitude greater than those from defects.


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Non-Destructive Testing (NDT) Techniques These problems can be overcome by magnetising the tube using a strong DC field. This reduces the effective permeability to a low value, thus increasing the depth of penetration and masking the permeability variations, hence allowing effective testing. Ferromagnetic tubing up to around 170 mm diameter are commonly tested using magnetic saturation and encircling coils. Testing may be in-line during manufacture or off line on cut length tube. When tubes are welded (usually by the ERW method) the weld area is the usual site of defects and as the weld position is well controlled, it is more efficient to inspect the weld area only by means of a sector (or saddle) probe. Ferrous weld inspection:

The geometry and heat-induced material variations around welds in steel would normally prevent inspection with a conventional eddy current probe, however a special purpose ‘WeldScan’ probe has been developed which allows inspection of welded steel structures for fatigue-induced cracking. The technique is particularly useful as it may be used in adverse conditions, or even underwater, and will operate through paint and other corrosion-prevention coatings. Cracks around 1 mm deep and 6 mm long can be found in typical welds both in the root area and the cap. Dynamic hole inspection:

Here, differential probes are used attached to high-speed rotary scanners with test speeds as high as 3000 rev/min then the inner bore of holes may be inspected rapidly and reliably with the eddy current technique. Probes may be as small as 1 mm diameter and test frequencies used follow the same rules as for surface defect detection. The use of high- and low-pass filters (so called band pass filters) is essential to ensure optimum signal to noise. Target calibration notch is usually a 0.5 mm corner notch at 45º.[5] Practical testing:

1) Any practical eddy current test will require the following: 2) A suitable probe. 3) An instrument with the necessary capabilities. 4) A good idea of size, location and type of the flaws it is desired to find.


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Non-Destructive Testing (NDT) Techniques 5) A knowledge of the material conductivity and whether it is magnetic or not. 6) A suitable test standard to set up the equipment and verify correct operation. 7) A procedure or accept/reject criteria based on the above. 8) The necessary operator expertise to understand and interpret the results. Operating frequency:

Selection of operating frequency is the primary eddy current test parameter under operator control. Frequency selection affects both the relative strength of response from different flaws and the phase relationship. Thus, selection of operating frequency is very important in obtaining good resolution of flaw signals in the presence of other variables which may affect the test. [1],[4] Instrument set-up:

While the precise details of setting up an instrument will vary depending on the type and application, the general procedure is usually the same. Once the application has been tested the required values for many test parameters will be known, at least approximately. 1) Connect up the appropriate probe and set any instrument configuration parameters (mode of operation, display type etc.). 2) Set the frequency as required for the test. 3) Set gain to an intermediate value, for example 40 dB. 4) Move the probe on/over the calibration test-piece and set phase rotation as desired (for example lift-off or wobble horizontal on a phase plane display). It may help the stability of the readings to attenuate the horizontal (x axis) gain by 12 dB (1⁄4 of the vertical gain). 5) Move over the defects and adjust gain (and horizontal/vertical gain ratio if fitted) to obtain the desired trace size/meter indication. It may be necessary to re-balance after changing gain. 6) Further optimise phase rotation as required by setting the dominant source of noise whilst scanning the probe in the horizontal axis. 7) Use filters etc. to further optimise signal-to-noise ratio (see b elow). 8) Set alarms etc. as required.


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Non-Destructive Testing (NDT) Techniques 9) Run over the calibration test-piece again and verify that all flaws are clearly detected. 10) Perform the test, verifying correct operation at regular intervals using the calibration test piece. Use of filtering:

Searching for defects in an eddy current test conventionally implies probe movement. So when indications are detected then, due to the probe size, these will vary with time in a way which is fairly consistent (assuming that the probe movement speed is reasonably constant). As a result of this speed and the probe size, defects have a characteristic frequency of response (probe width divided by probe speed). For example, if an absolute probe with diameter 2 mm moves over a narrow crack at a speed of 1 m/s the resulting indication will last for approximately 2 ms. If the material composition, thickness or probe lift-off is also varying gradually, the indication from this will change much more slowly. Therefore, a high-pass filter set to a frequency around 100 Hz or so will pass the rapidly changing signal from the defect but not the slowly varying changes. Further rapidly varying signals such as electronic noise or noise caused by surface roughness may be reduced by low-pass filtering. It is good practice to ensure that the low-pass filter is set sufficiently low to ensure the test signal displays the lowest amount of high-frequency noise but high enough to ensure that the smallest target defect is not attenuated by the filter.[12]


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Non-Destructive Testing (NDT) Techniques 8 CONCLUSION : Eddy current NDT is a mature technology with widespread availability of user-friendly, affordable, and commercial, off-the shelf equipment. It can be used on conducting materials and can detect many types of discontinuities. Eddy current testing has enjoyed considerable success in a number of applications including, for example, inspection of nuclear reactor heat exchanger tubes, aircraft engine and metal skin components, and in the manufacturing plant inspection of a variety of metallic components. In addition, eddy current NDT is widely used to inspect welds along with X-ray radiography and ultrasonic testing.


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Non-Destructive Testing (NDT) Techniques REFERENCES

[1] The American Society for Nondestructive Tes ting, Introduction ting, Introduction to Nondestructive Testing . [Online]. Available: http://www.asnt.org/ndt/primer5.htm pp.1-5. [2] Dufour, M. L.; Lamouche, G.; Detalle, V.; Gauthier, B.; Sammut, P. (April 2005). "Low-Coherence Interferometry, an Advanced Technique for Optical Metrology in Industry". Insight Industry". Insight - Non-Destructive Testing and Condition Monitoring 47 (4): 216–219. DOI:10.1784/insi.47.4.216.63149. ISSN 1354-2575. [3] Losert, Robert. (March 31, 2009). "Solution for NDT Inspection". NDT Magazine. Retrieved December 15, 2010. [4] Blitz, Jack; G. Simpson (1991). Ultrasonic Methods of Non-Destructive Testing . Springer-Verlag New York, LLC. ISBN 978-0-412-60470-6. [5] SGS S.A, Non-Destructive Testing BIBLIOGRAPHY

[6] ASTM International, ASTM International, ASTM Volume 03.03 Nondestructive Testing [7] ASNT, Nondestructive ASNT, Nondestructive Testing Handbook [8] Bray, D.E. and R.K. Stanley, 1997, Nondestructive 1997, Nondestructive Evaluation: A Tool for Design, Manufacturing and Service; Service; CRC Press, 1996. [9] Charles Hellier (2003). Handbook (2003). Handbook of Nondestructive Evaluation. Evaluation. McGrawHill. ISBN 0-07-028121-1. [10] Shull, P.J., Nondestructive P.J., Nondestructive Evaluation: Theory, Techniques, and Applications, Applications, Marcel Dekker Inc., 2002. EXTERNAL LINKS

[11] www.ndt-ed.org


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