Eddy Current Testing: Basics

Transcription

1 Eddy Current Testing: Basics B.P.C. Rao Non-destructive Evaluation Division Metallurgy and Materials Group Indira Gandhi Center for Atomic Research Kalpakkam , TN, India Eddy current technique is an important electromagnetic non-destructive evaluation technique that is widely used in power, aerospace, petrochemical and other industries for detection of surface cracks and sub-surface damage in components made of metallic materials. Besides, it is also used traditionally for assessing the adequacy of heat treatment of alloys, as eddy currents are sensitive to changes in microstructure and stresses, which alter the electrical conductivity and magnetic permeability of the material. This paper gives a brief account of basic principle, features, applications, limitations of the eddy current technique. It also covers instruments and sensors to enable better appreciation of the technique and its capabilities. Introduction Early detection and quantification of defects, microstructural variations and stresses is utmost important to ensure high quality manufacturing and safe operation of engineering components. The role played various non-destructive evaluation (NDE) techniques is well known from their wide spread use during manufacture and assembly stages and during service life of components. NDE techniques that use some form of electromagnetic excitation are termed as electromagnetic NDE techniques and some of these include eddy current (EC), magnetic particle, magnetic flux leakage, magnetic Barkhausen emission, micromagnetic, potential drop, microwave, AC field measurement techniques etc. [1]. In these techniques, material under investigation is excited electromagnetically and the manifestation of electromagnetic fields due to material discontinuities affecting electrical conductivity or magnetic permeability or dielectric permittivity are measured using a sensor, with the exception of magnetic particle testing in which magnetic particle are used in place of a sensor [2]. EC technique is the most popular and widely used electromagnetic NDE technique. In industrial scenario, among other electromagnetic NDE techniques, this technique finds larger number of applications. This technique finds versatile applications in power, aerospace and petrochemical industries. It is not incorrect to say that worldwide almost all the heat exchangers and aircrafts are inspected using this technique. Two main aspects behind this widespread use are excellent sensitivity to surface as well as sub-surface defects and testing speed of as high as 10 m/s which no other NDE technique can match. This is especially profitable to industries as it enables rapid examination during manufacturing stages, while it drastically reduces the down time of operating plant components. Many developments are taking place in this existing NDE technique incorporating the rapid progress in the fields of microelectronics, instrumentation, sensors, computers, numerical modelling, digital signal & image processing (Fig.1) [2]. The way EC testing is practised now 1

2 is different from that it was four decades ago. These concurrent advances in other fields have enhanced the capabilities of the traditional EC technique enabling detection and sizing of incipient surface defects as well as sub-surface defects, changes in microstructures and accumulated plastic deformation, stress or damage e.g. prior to crack formation etc. Such possibilities allow efficient preventive actions to be taken avoiding catastrophic failure of components. Possibilities to inspection of large areas with automation, elimination of operator fatigue & uncertainty, inspection of inaccessible as well remote areas, and on-line inspection ensuring high probability of detection and accurate sizing, all have further enhanced the acceptability of the EC technique by the industry. This technique richly benefited by the wisdom and knowledge and contributions from scientists, engineers and other experts from physics, electrical engineering, material technology, micro-electronics, computers, automation and robotics domains. For better appreciation of the EC technique and correctly choose it for an application at hand, it is essential to know the basic principles, features, capabilities and limitations of the technique. That forms the objective of this paper. Fig. 1 Recent advances in eddy current testing that are responsible for its enhanced use by the industry. Principle EC testing works on the principles of electromagnetic induction. In this technique, a coil (also called probe or sensor) is excited with sinusoidal alternating current (frequency, f, ~ 50 Hz-5 MHz, ~ 100 ma). Following the Ampere s law, this current generates primary magnetic field in the vicinity of the coil. When an electrically conducting material is brought close to this coil, eddy currents are induced in the material according to the Faraday s law (refer Fig. 2) [3]. 2

3 Surface coil Crack Test plate Void Depth Depth Fig. 2 Principle of eddy current testing (left) and distortion of eddy current due to crack, edge-effect, surface crack, and sub-surface void (right). The eddy currents have very unique and interesting properties such as: They are induced currents that exist only in electrically conducting materials They are always in closed loops, usually parallel to the coil winding (Fig. 2) They are distorted by defects such as cracks and corrosion wall loss and by discontinuities such as edge-effect, end-effect as shown in Fig. 2 They attenuate with depth (also axially or laterally) Their intensity depends on material properties, electromagnetic coupling (lift-off/fillfactor) and excitation frequency, but maximum on the surface These eddy currents also generate a secondary magnetic field, but in the opposite direction to the primary magnetic field following the Lenz's law and this field in turn, changes the coil impedance, Z which is a complex quantity with real component, R and imaginary component, X L. Defects such as cracks, voids, inclusions, corrosion wall loss, microstructure degradation, localised stresses alter the local electrical conductivity, σ, and magnetic permeability, µ, of the material and cause distortion of the eddy currents and change the coil impedance. This impedance change, usually of the order of micro ohms, is measured using high-precision bridge circuits, analysed and correlated with defect dimensions. Alternately, the secondary magnetic field can be detected using a separate receiver coil or a solid state field detection sensor. Discontinuities or defects that cause maximum perturbation to eddy currents flow, in other words, distortion produce large change in impedance. The impedance change is also affected by excitation frequency (effect of X L or 2πfL) and electromagnetic coupling. The flow of eddy currents in the test material is not uniform at different depths. The eddy currents are quite denser at the surface as compared to the deep inside, an effect referred to as 3

4 skin effect [4]. Theoretical standard depth of penetration of eddy currents, δ, that describes the skin-effect, can be expressed as δ = 1/ ( π fµ µ σ ) m (1) o r where is f is excitation frequency, Hz, µ 0 is magnetic permeability of free space, 4π10-7 H/m, µ r is relative magnetic permeability, dimensionless, and σ is electrical conductivity, mho/m. δ is the depth at which the surface eddy current density has fallen to 37%. EC technique capability, applicability, selection of test frequency etc. can be readily understood using equation (1). For example, depth of penetration of eddy currents, in other words, interaction of electromagnetic fields, is very low in highly conducting (e.g. Copper) as shown in Fig. 3, as compared to that of austenitic stainless steel which is less conducting. Due to the skin effect, with EC test one can readily detect the surface-breaking defects as compared to the sub-surface defects or buried defects. Fig. 3 Interaction of magnetic fields from a coil at different test conditions. The locus of impedance change during the movement of an EC coil system over the test object is called EC signal. While the amplitude of the EC signal provides information about the defect severity, the phase angle provides information about the defect depth. As depicted in Fig. 4, the impedance change can be displayed in a complex plane (real, X imaginary, Y) as impedance plane trajectory or as a time-domain signal viz. X(t) or Y(t). In the impedance plane, magnitude and phase can be seen; however, the signal extent or defect length cannot be seen. On the contrary, in time-domain signals, phase angle information that is essential for depth estimation is absent. Selection of test frequency is very important in the EC tests and in general, it is chosen such that a maximum amplitude signal is formed for defects and with a decent phase separation from the lift-off axis. A simpler way to determine the working test frequency range involves assuming value of 1 and 2 for δ in equation (1) and calculating the extreme frequencies upon substituting σ, µ 0 and µ r values of the test object. 4

5 Fig. 4 Two types of eddy current signals viz. Impedance plane (X-Y) and time-domain (right) from three surface cracks (a, b, c) in a steel plate. The electrical conductivity is usually expressed as percentage IACS (International Annealed Copper Standard) in which the electrical conductivity of pure copper at 25 C is taken as 5.8 x10 7 Siemens/meter. For example, the IACS% value of austenitic stainless steel (type 304) is 2.5 with an absolute electrical conductivity of 1.45x10 6 and that of Admiralty brass is 24% with a conductivity of 1.392x10 7 [5]. The electromagnetic coupling between coil and test object is very important. For reliable detection of defects, it is always preferable to minimise and maintain uniform lift-off or fillfactor which will be discussed later in this paper. Failing to do so will result in degradation of signal-to-noise ratio (SNR). Instead of continuous A.C. if the exciter is driven with a repetitive broadband pulse, such as a square wave which induces transient eddy currents associated with highly attenuated magnetic pulses propagating through the material, a new technique called pulsed eddy current (PEC) technique is formed. The signals reflected from defects in the object are picked up by a sensor. At each probe location, a series of voltage-time data pairs are produced as the induced field decays, analogous to ultrasonic A-scan data. Defects close to the surface will affect the eddy current response earlier in time than deep surface defects. PEC technique is useful for detection of hidden corrosion in layered structures such as aircraft lap-splices and corrosion under insulation in insulated components. Features EC technique is a preferred technique for material sorting, determination of hardness, heat treatment inadequacies, coating thickness measurements, and detection of defects in tubes, rods, bars, multi-layer structures, discs, welds, blades and other regular as well as irregular geometries [6]. Some of the attractive features of EC technique include the following: Ability to distinguish metallic materials from non-metallic ones and sorting of materials based on difference in heat treatment, microstructure and material properties (refer Fig. 5) Ability to easily detect tight hairline cracks which cannot be seen by naked eye 5

6 Ability to perform tests on regular as well as irregular geometries without the need for using any couplant Ability to carry out tests at more than 10 m/s speed Ability to measure coating thickness as small as 5µ Possibility to carry out high temperature testing, even up to 1000 C Computer based automated testing, data storage, analysis and interpretation without the need for an operator Possibility to perform numerical modelling for optimisation of the technique, probes and test parameters [7] X L Magnetic Material µ 2 µ 2 > µ 1 Lift-off µ 1 Air θ Lift-off ρ 1 VT-14 R ρ 2 ρ 1 > ρ 2 Non-magnetic material Fig. 5 Response on the impedance plane for different metallic materials, enabling material sorting and determination of conductivity and permeability. Instruments In EC technique the alternating current through the coil is kept constant (~ few hundred ma) and the changes in the coil impedance are measured. Since the impedance change is very small (< micro-ohms), high precision A.C. bridge (refer Fig. 6) circuits are employed. The bridge imbalance is correlated with the defect or material attribute responsible. Typical analogue EC instrument consists of an oscillator (excitation frequency, ~ 50 Hz-5 MHz), constant current supply (step down from 230 V AC), a bridge circuit, amplifier, filters, oscilloscope (to display the impedance changes in a 2-D graph or as a vector) or meter display unit or decision making unit. With the micro-electronic revolution digital EC instruments have replaced the analogue EC instruments. These instruments are smart, high-sensitive, low-cost, automated, modular and 6

7 efficient (Fig. 7). They are, in many instances, interfaced to personal computers, industrial computers, and laptops with possibility for easy measurements, adjustments, controls, data storage, analysis and management, all performed by suitable software. Fig. 6 A.C. bridge circuit used to measure small changes in EC coil impedance. Probes EC probe forms the basic link between EC instrument and the test material. Depending on the geometry of the test material, different probes such as surface probes (for plates), encircling probes (for rods and tubes) and bobbin probes (for tubes) with coil configurations shown in Fig. 8 are used. Appropriate selection of probe coil is important in eddy current testing, as even an efficient EC instrument cannot achieve much if it doesn t get the right (desired) information from the coils. Fig. 7 Advanced general purpose digital instruments for static as well as dynamic EC tests and for multi-frequency EC inspection of non-ferromagnetic and ferromagnetic heat exchanger tubes (Courtesy: M/s. Technofour, Pune). 7

8 Fig. 8 Basic configuration of coils in eddy current probes. EC probes are induction based and are made up of a few turns of copper wire usually wound around a ferrite core with or without shielding. Every probe has an operating frequency range and impedance value matching the bridge circuit of the instrument. It is desirable to operate the probe within that range. It is essential to avoid operating near the resonance frequencies. The probes are operated in absolute (single coil), differential (two coils wound opposite) or send-receive (separate coils for excitation and detection) modes. Their design is dependent on the object geometry viz. tube, plate, bar etc. As shown in Fig. 8 and the expected type and location of discontinuity. Absolute probes are good for detection of cracks (long or short) as well as gradual variations. However, absolute probes are sensitive also to lift-off, probe tilt, temperature changes etc. Differential probes have two sensing coils wound in opposite direction investigating two different regions of a material. They are good for high sensitive detection of small defects. They are reasonably immune to changes in temperature and the operator-induced probe wobble [5]. The most simple and widely used probe types are: Surface or pancake or pencil probes (with the probe axis normal to the surface), are chosen for testing plates and bolt-holes either as a single sensing element or an array - in both absolute and differential (split-d) modes. Encircling probes for inspection of rods, bars and tubes with outside access and Bobbin probes for pre-and in-service inspection of heat exchanger, steam generator, condenser tubes & others with inside access. Phased array receivers also possible for enhanced detection and sizing. While pancake or surface probes are used for testing plates and regular geometries, encircling or bobbin type probes are employed for testing tubes, rods, and other cylindrical objects. The EC probes possess directional properties i.e. regions of high and low sensitivity (impedance change). Defects that cause maximum perturbation to eddy currents are detected with high sensitivity. For good sensitivity to small shallow defects, a small probe should be used. Similarly, in order to detect sub-surface and buried defects, large diameter high throughput probes operating at lower frequencies are necessary. As a general rule, the probe diameter should be less than or equal to the expected defect length and also comparable to the thickness of the test object. The sensing area of a probe is the physical diameter of the coil plus an extended area of 4δ due to the magnetic field spread. Hence, it is common to use 8

9 ferrite cores as well as shields with high µ and low σ, to contain the field without affecting the depth of penetration. Figure 9 shows some typical EC probes developed for specific applications [3, 8]. Fig. 9 Different types of EC probes. Coupling of magnetic field to the material surface is important in EC testing. For surface probes, it is called "lift-off" which is the distance between the probe coil and the material surface. In general, uniform and very small lift-off is preferred for achieving better detection sensitivity to defects. The electromagnetic coupling in the case of tubes/bars/rods is referred to as "fill-factor". It is the ratio of square of tube diameter to square of coil diameter for encircling coils and is expressed as percentage (dimensionless) Fill factor = ( D 2 t / D 2 p )* 100 (2) where D p is the probe inner diameter and D t is the tube outer diameter [3,5]. Usually, 70-90% "fill-factor" is targeted for reliable inspection. The encircling probes exhibit reduced sensitivity for shallow and localised defects and for such applications, motorised rotating probe coil (MRPC), phased-array, plus-point etc. are used. The inspection data from these probes can be displayed as images which allow easy identification of circumferential location of defects. For inspection of irregular and inaccessible regions, flexible and conformal sensors are employed. The pancake type probes show reduced sensitivity for sub-surface and buried defects and for such needs, integrated probes with coils for excitation and solid state sensors for reception are very attractive [2]. 9

10 Such integrated probes are useful for inspection of rivets and multi-layer structures in aircrafts and for detection of deeply located (> 10 mm) defects in steel components. Test Procedure General EC test procedure for detection of defects involves calibration of EC instrument using reference standard defects in a material with similar chemical composition and geometry as that of the actual component. Artificial defects such as saw cuts, flat bottom holes, electro-discharge machining (EDM) notches are used while well characterised natural defects, cracks in failed or withdrawn components are always preferred. Instrument test parameters such as excitation frequency, gain, phase angle etc. are optimised for a desired performance. In general, signal phase is rotated such that it is parallel to lift-off or wobble axis and phase separation between ID and OD defects is nearly 90 degrees. A suitable EC signal parameter, e.g. signal peak-to-peak amplitude or phase angle is identified and an appropriate threshold is determined for incorporating accept/reject criterion. When defect sizing is required, a calibration graph between signal parameter and defect size is generated and used [3]. During actual testing, any region that produces EC signals with parameter greater than the threshold is recorded defective, while its equivalent size is determined using the calibration graph. Similar procedure is followed for material sorting, conductivity measurement, microstructure characterisation, and coating thickness measurement. Testing non-ferromagnetic tubes For periodic monitoring of corrosion of tubes in heat exchangers, steam generators and condensers in power, petrochemicals, fertiliser and other industries, EC technique is employed because of its ease of operation, sensitivity, versatility, speed (~ 10 m/s) and repeatability. This technique can detect wall thinning, cracks, pitting, stress corrosion cracking, hydrogen embrittlement, carburization, denting and crude deposits etc. Typical EC signals from an ID defect, OD defect and hole in a heat exchanger tube and a surrounding steel support plate are shown in Fig. 10 for absolute and differential bobbin probes. Fig. 10 Typical absolute (left) and differential probe (right) EC signals for an ID defect (A), OD defect (B) and hole (C) in a heat exchanger tube and for a steel support plate (D). 10

11 Using phase discrimination, it is possible to readily distinguish various defects as well as support plates. However, single frequency EC technique is inadequate for detection of defects under support plates, baffle plates and in the presence of probe wobble. Many a time, it is under the support plates corrosion damage takes place. To eliminate signals from support plates, multi-frequency technique is employed. This technique involves simultaneous excitation of two or more frequencies in an EC coil and processing of the corresponding signals to suppress the contributions from disturbing sources, similar to solving a system of linear equations [4, 6, 8]. The multi-frequency test procedure is described in detail in ASME Section V, Article 8, Appendix 1. Testing ferromagnetic tubes Examination of ferromagnetic tubes is difficult using conventional EC test procedures due to high relative magnetic permeability which restricts penetration of eddy currents and produces disturbing signals due to continuously varying permeability. This disturbance can be eliminated by employing bias or saturation direct current (D.C) which saturates the material magnetically and makes the tube material behave non-ferromagnetic, thus, allows conventional EC testing after which the tubes are to be demagnetised. Typical test set up is shown in Fig. 11. However, in the case of installed tubes of smaller diameters, saturation units cannot be accommodated due to limited access. For mildly magnetic materials, partial saturation using high-energy permanent magnets such Nd-Fe-B is a possibility. The other technique possible is remote field eddy current (RFEC) technique [2]. DC Power Supply Steel tube Mechanical Rollers Magnetizing coil EC Instrument Encircling Differential Coil Fig. 11 D.C. Saturation based EC testing of ferromagnetic steel tubes. This technique uses low frequency excitation and a separate receiver coil kept at two to three tube diameters away from exciter coil. The phase lag of induced voltage in the receiver coil with respect to the exciter is measured using lock-in amplifiers and correlated with wall loss or defect depth. The advantages of RFEC technique include ability to test tubes with equal sensitivity to internal and external wall loss and linear relationship between wall loss and measured phase lag. RFEC technique can achieve inspection speeds of 1 m/s. It is used for inspection of carbon steel and other ferromagnetic tubes in process industry. One recent application of this technique is in-service inspection of steam generator tubes of sodium 11

12 cooled prototype fast breeder reactors with 23 m long modified 9Cr-1Mo ferromagnetic steel tubes For this application, a comprehensive RFEC technology comprising of instrumentation, probes, robotic device has been developed as a solution to the problem of smaller diameter, expansion bends, support plates, one-side access and electrically conducting sodium deposits. Characterisation of Microstructures During manufacturing stages of components made of alloys, heat treatment is given to ensure required levels of mechanical and physical properties and desired microstructures. Likewise, during service life of components, it is essential to ensure that there is no undesirable degradation in microstructures. EC technique is useful for assessing these two situations as it exploits measurement of changes in electrical conductivity and magnetic permeability. Changes in microstructure, precipitate size and distribution, cold work, deformation, dislocation pile-up etc. alter the coil impedance or induced voltage in a pick-up coil. The magnitude and phase of induced voltage or impedance change are used for quantitative characterisation of microstructures and to estimate the volume fraction of various phases present. Cold worked and annealed conditions, e.g. in stainless steel 316 or 304 effect electrical conductivity in opposite directions and strain-induced martensite, being a magnetic phase, increases the magnetic permeability. This phase can be detected using EC technique. Specimens subjected to heat treatments are used to simulate the service exposed conditions and the expected microstructures. Usually, EC measurements for microstructure characterisation are location-based. In general, absolute probes are used and analysis is based on impedance plane signal interpretation. Reference standards with known electrical conductivity and magnetic permeability are used also for establishing calibration graph, apart from specimens heat treated to different ageing conditions through measurement of conductivity and permeability. EC technique has been used to characterise microstructures in titanium alloy (VT 14 alloy Ti- 4.5Al-3Mo-1V) subjected to a series of heat treatments consisting of solutionizing for 1 h at selected temperatures in the range of K at an interval of 50 K, followed by water quenching. This treatment produces variety of microstructures due to controlled α-β transformation and formation of various phases. The experimentally measured EC response for various specimens is shown in Fig. 12 along with that of the reference standards viz. Hastelloy-X, Hastelloy-B and Ti-6Al-4V. It has been found that, both magnitude and phase angle of impedance change decrease with increasing solutionizing temperature up to 1123 K and this is attributed to decrease in α phase (reduction in electrical conductivity). Beyond 1123K, formation of α martensite dominates the interactions and results in increase in effective electrical conductivity and hence, the impedance change. Beyond 1273K, magnitude and phase angle reach a constant maximum, due to 100% formation of α martensite. Comparison of impedance magnitude and phase angle with hardness measurements has established that EC technique can be implemented in production line to quickly assess the adequacy of heat treatment. 12

13 Air Ti- 6Al-4V θ X L, Volts Hastalloy B Hastalloy X 1123K 1073K 923K 1173K 1223K 1273K 1323K ρ ρ R, Volts Fig. 12 Impedance plane response for various heat treated specimens at 150 khz. Applications A few specific practical applications of EC technique are given below for better appreciation of the technique. Quality assurance of austenitic stainless steel tubes, plates and welds. Inspection of installed heat exchanger/steam generator/condenser tubes (single and multi-frequency) Detection of surface as well as sub-surface defects in multi-layer aircraft structures (single frequency, multi-frequency & pulsed techniques) On-line automated saturation based quality assurance of steel (ferromagnetic) tubes. Location of garter springs in PHWRs and measurement of gap in coolant channels Detection of intergranular corrosion (IGC) in stainless steels (316, 316L and 304 L) Detection of weld centre line in austenetic stainless steel welds at high temperature Measurement of coating thickness of SiC on carbon-carbon composites Sorting of materials based on electrical conductivity and magnetic permeability Characterisation of heat treated as well as degraded microstructures in alloys Non-contact detection of metallic objects, land mines, security metal detectors Monitoring of liquid levels and for position encoding Reference Standards Reference standards are used for adjusting the eddy current instrument sensitivity to enable detection of desired size of defects and quantification of conductivity, permeability and material thickness etc. They are also used for sizing defects [2]. Some commonly used 13

14 standards in EC testing by ASME (American Society for Mechanical Engineers), BS (British Standards), ASTM (American Society for Testing of Materials) and IS (Bureau of Indian Standards) are: ASME, Section V, Article 8, Appendix 1 and 2), Electromagnetic (EC) testing of heat exchanger tubes ASTM B 244 Method for measurement of thickness of anodic coatings of aluminum and other nonconductive coatings on nonmagnetic base materials with EC instruments ASTM B 659 Recommended practice for measurement of thickness of metallic coatings on nonmetallic substrates ASTM E 215 Standardising equipment for electromagnetic testing of seamless aluminium alloy tube ASTM E 243 Electromagnetic (EC) testing of seamless copper and copper alloy tubes ASTM E 309 EC examination of steel tubular products using magnetic saturation ASTM E 376 Measuring coating thickness by magnetic field or EC (electromagnetic) test methods ASTM E 426 Electromagnetic (EC) testing of seamless and welded tubular products austenitic stainless steel and similar alloys ASTM E 566 Electromagnetic (EC) sorting of ferrous metals ASTM E 571 Electromagnetic (EC) examination of nickel and nickel alloy tubular products ASTM E 690 In-situ electromagnetic (EC) examination of non-magnetic heatexchanger tubes ASTM E 703 Electromagnetic (EC) sorting of nonferrous metals BS 3889 (part 2A): 1986 (1991) Automatic EC testing of wrought steel tubes BS 3889 (part 213): 1966 (1987) EC testing of non-ferrous tubes IS 6398:1983 Code of practice for EC testing of ferrous seamless pipes and tubes IS 11612: 2004 Code of practice for EC testing of non-ferrous seamless pipes and tubes IS 13190: 1991 Recommended practice for EC examination by rotating probe method of round steel bars. IS15540:2004 Recommended practice for EC testing of installed non-ferromagnetic heat exchanger tubing using duel frequency method. Limitations Like other NDT technique EC technique has certain limitations too. But interestingly, most of the original limitations of the technique in 60s and 70s have been overcome by the advances in instrumentation, sensors, computer and signal and image processing techniques. Some of the important limitations include: Applicability to only electrically conducting (metallic) materials Inspection of installed ferromagnetic components with the exception of tubes can be inspected by remote field EC technique Difficulty to separate the influence of one desired variable in the combined presence (at the same location beneath the probe) of several other disturbing variables such as stress, microstructure, texture, anisotropy etc. that simultaneously change conductivity and permeability. 14

15 Inability to identify circumferential location of a defect when encircling or bobbin coils are used. Difficulty in detection of a small defect under a large defect Inability to detect defects at the centre of rods using encircling coils Need for skilled personnel for interpretation of signals and results Summary Working on the principle of electromagnetic induction, eddy current technique is a widely used NDE technique for detection of surface and sub-surface damage. The attractive features of this technique include ease of operation, high sensitivity to tight cracks, versatility, extremely high testing speeds (up to 10 m/s), repeatability and reliability. This technique can detect wall thinning, cracks, pitting, stress corrosion cracking, hydrogen embrittlement, carburization, denting and crud deposits etc. This technique finds a lot of applications in engineering industry including material sorting, determination of hardness, heat treatment adequacy assessment, material property determination, coating thickness measurements, and detection of defects in tubes, rods, bars, multi-layer structures, discs, welds, blades and other regular as well as irregular geometries. Successful testing requires selection of proper instrument and probes, optimisation test frequency and use of reference calibration standards. When appropriate standards are used, not only detection of defects but also their sizing is possible using eddy current technique. Acknowledgements Author expresses special thanks to Dr. T. Jayakumar, Mr. S. Thirunavukkarasu, Mrs. B. Sasi of NDE Division, India Gandhi Center for Atomic Research, Kalpakkam, India. References 1. B.P.C. Rao and T. Jayakumar, Discontinuity characterisation using electromagnetic methods, J of Non-Destructive Testing & Evaluation, Vo.2, No.2, 2002, pp B.P.C. Rao, T. Jayakumar, Baldev Raj, Electromagnetic NDE Techniques for Defect and Microstructural Characterization, in Electromagnetic NDE Techniques for Materials Characterization, B.P.C. Rao, T. Jayakumar and Baldev Raj, Ultrasonic and advanced methods for non-destructive testing and material characterisation, Ed. C.H. Chen, World Scientific Publishing co. (Singapore), June 2007, pp B.P.C. Rao, Introduction to Eddy Current Testing, Narosa Publishing, New Delhi, India, February, H.L. Libby, Introduction to Electromagnetic Non-destructive Test Methods, Wiley- Interscience, New York, V.S. Cecco V.S, G. Van Drunnen and F.L. Sharp, Eddy current manual: test method, Vol.1, AECL-7523, Chalk River, Ontario, Nov., D.J. Hagemaier, Fundamentals of Eddy Current Testing, ASNT, Columbus, OH, USA, W. Lord, Electromagnetic methods of Non-destructive Testing, Gordon and Breach, New York, Moore Patrik, Udpa S.S, Non-destructive testing handbook. 3rd edition. Vol.5: Electromagnetic testing, ASNT, Columbus, OH, USA,