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Eddy Current Testing: Basics

B.P.C. Rao

Non-destructive Evaluation Division

Metallurgy and Materials Group

Indira Gandhi Center for Atomic Research Kalpakkam – 603 102, TN, India

e-mail: bpcrao@igcar.gov.in

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

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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].

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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/fill-

factor) 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,

XL. 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 XL 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

Crack

Surface coil

Test plate

Void

DepthDepth

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skin effect [4]. Theoretical standard depth of penetration of eddy currents, δ, that describes the

skin-effect, can be expressed as

)(/1 σµµπδ rof= m (1)

where is f is excitation frequency, Hz, µ0 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 , µ0 and µ r values of the test object.

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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

x107 Siemens/meter. For example, the IACS% value of austenitic stainless steel (type 304) is

2.5 with an absolute electrical conductivity of 1.45x106 and that of Admiralty brass is 24%

with a conductivity of 1.392x107 [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 fill-

factor 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

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Air

µ1

XL

Lift-off

Lift-off

Magnetic Material

Non-magnetic material

VT-14

µ2

µ2 > µ1

ρ1 > ρ2

ρ1

ρ2

θ

R

� 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]

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

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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).

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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

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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 = ( D2

t / D2

p )* 100 (2)

where Dp is the probe inner diameter and Dt 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].

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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).

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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].

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

EC Instrument

DC Power Supply

Mechanical

Rollers

Steel

tube

Encircling

Differential Coil

Magnetizing coil

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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 923-1303 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.

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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

-0.2 0.0 0.2 0.4 0.6 0.8 1.0-0.8

-0.6

-0.4

-0.2

0.0

0.2

1273K

ρρρρ

Ti- 6Al-4V

θθθθ

Air

1173K

1323K

Hastalloy Xρρρρ

Hastalloy B

1223K

1123K1073K923KX

L,

Volts

R, Volts

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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 heat-

exchanger 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.

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• 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

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methods for non-destructive testing and material characterisation, Ed. C.H. Chen, World Scientific Publishing co. (Singapore), June 2007, pp.247-278.

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1990.

7. W. Lord, “Electromagnetic methods of Non-destructive Testing”, Gordon and Breach,

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