Eddy Current Non Destructive Testing

What is Eddy Current Testing?

Non-destructive testing (NDT) aims detection and characterisation of defects /flaws / discontinuities in a material without impairing the intended use of the material. Eddy Current Testing (ECT) is an electromagnetic NDT technique widely used in nuclear, aerospace, power, petrochemical and other industries to examine metallic plates, sheets, tubes, rods and bars etc. for detection and sizing of cracks, corrosion and other material discontinuities during manufacturing as well as in-service.

This is not a volumetric (radiography and ultrasonic) technique. Like liquid penetrant and magnetic particle techniques, this is a surface technique and can readily detect very shallow surface defects (fatigue cracks, intergranular stress corrosion cracks etc.) and sub-surface defects (inclusions, voids etc.) within a depth of, say 6 mm. Eddy curent testing is a simple, high-speed, high-sensitive, versatile and reliable NDT technique and is popularly used in many engineering industries. Theory and principle of eddy current testing, advantages, limitations, applications and standards are covered briefly in this page.

Eddy Current Testing Introduction / Principles / Theory

Eddy current testing works on the principles of electromagnetic induction (recall Maxwell's equations, electrical transformers, induction furnace, skin-effect, Ohm's law, Wheatstone bridge etc.). In eddy current (EC) technique, a coil (also called probe or sensor) is excited with sinusoidal alternating current (frequency, f, ~ 50 Hz-5 MHz) to induce what are called eddy currents (swrling or closed loops of currents that exist only in metallic materials) in an electrically conducting material such as stainless steel, aluminium etc. being tested. The change in coil impedance, Z that arises due to distortion of eddy currents at regions of discontinuities (defects, material property variations, surface characteristics etc,) and associated magnetic flux linkages, is measured and correlated with the cause producing it i.e. discontinuities. Eddy currents are a problem in electircal engineering systems such as transformers, as they cause severe heating losses. However, they are used to advantage in eddy current non-destructive testing. An eddy current coil can be considered to be having resistance and inductance in series in an AC circuit. According to Ohm's law, the circuit impedance Z (Voltage/Current) is a vector quantity with resistance R and inductive reactance Xl as the real and imaginary components (Z = R + jXl).

Briefly in eddy current testing, the following sequential things happen:
* Eddy current coil generates primary magnetic field (Ampere's law) * Primary magnetic field induces eddy currents in the material (Faraday's law) * Eddy currents generate secondary magnetic field in the opposite direction (Lenz's law) * Coil impedance changes, as a result * Impedance change is measured, analyzed and correlated with defect dimensions

The locus of impedance change formed during the movement of an eddy current probe coil over a test material having a defect is called an eddy current signal. The peak-to-peak amplitude of the eddy current signal provides information about the defect severity. The phase angle of the eddy current signal with respect to a known reference (lift-off) provides information about the defect location or depth. Defects that cause maximum perturbation to eddy current flow produce large eddy current response (signal amplitude) and hence detected with high sensitivity (see distortion figure below). Similarly, defects that are parallel to eddy current flow may not produce a significant change in coil impedance and as a result they produce a weak reponse i.e. detected with poor sensitivity.

Electromagnetic Interactions in Eddy Current Testing
Governing Laws
* Ampere's law
* Faraday's law
* Lenz's law

Properties of Eddy Currents
* They are closed loops
* They flow in a plane that is parallel to coil winding or material surface.
* They attenuate and lag in phase with depth

Coil Impedance
Z = R + j Xl


Skin Effect / Standard Depth of Penetration (SDP)

Eddy current density in a material is not uniform in the thickness (depth) direction. It is greatest on the material surface and decreases monotonously with depth (skin effect) and the eddy currents lag in phase with depth, allowing employ phase discrimination method to locate, size and differentiate defects and disturbing variables. "Standard depth of penetration" (SDP) equation given above can be used to explain the capability of eddy current testing. For an uniform, isotropic and very thick material, SDP is the depth at which the eddy current density is 37% of its surface value. From the SDP equation, one can easily interpret that depth of penetration (delta) decreases with increasing frequency, conductivity, permeability (see flux line contours below). Thus, in order to detect very shallow defects (cracks, flaws) in a material and also to measure thickness of thin sheets, very high frequencies are to be used (see flux line contours below). Similarly, in order to detect sub-surface buried defects and to test highly conductive/ magnetic/ thick materials, low frequencies are to be employed.
Theoretical isomagnetic fluxline contours demonstrating the skin-effect

Instrument / Instrumentation for Eddy Current Testing

Usually, current through the eddy current coils is kept constant ~ few hundred mA and changes in the coil impedance that occur due to perturbation of eddy currents at defect regions are measured. Since these impedance changes are very small

Probes / Sensors for Eddy Current Testing
Appropriate selection of probe coil is important in eddy current testing, as even an efficient eddy current testing instrument can not achieve much if it doesn’t get the right (desired) information from the coils. The most popular coil designs are: * Surface probes or pancake 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 genertor, condenser tubes & others with inside access. Phased array receivers also possible for enhanced detection and sizing.

											Click for more details on Eddy Current Probe / Sensor Design, Development and Selection

These three types of probes can be operated in absolute or differential (left, last). They can also operated in send-receive mode (separate coils for sending and receiving [again absolute or differential]). The EC probes consisting of a single sensing coil for excitation and reception are called absolute probes. Such probes are good for detection of cracks (long as well as 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 and investigating two different regions of the material. They are good for high sensitive detection of small defects and they are reasonably immune to changes in temperature and probe wobble.



Eddy Current Testing Signals
An eddy current signal is the trajectory of coil impedance formed upon scanning the coil over a material surface. Eddy current (impedance change) signal / data is analysed in time-domain (strip-chart) and also in impedance plane (CRT or computer screen). Typical time-domain and impedance plane signals for a plate tested using a surface probe (absolute mode) are given on the right hand side. CRT Screen or Impedance Plane Display

Electromagnetic Coupling (Lift-off / Fill-factor)

Coupling of magnetic field to the material surface is important in ECT. 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. Similarly, the electromagnetic coupling in the case of tubes/bars/rods is referred to as "fill-factor". It is the ratio of square of coil diameter to square of tube diameter, in the case of encircling coils and is expressed as percentage (dimensionless). Usually, 70-90% "fill-factor" is targeted for reliable inspection.

Eddy Current Testing Procedure

Usual EC test procedure involves first calibration. Artificial defects such as saw cuts, flat bottom holes, and electro-discharge machining (EDM) notches are produced in a material with similar chemical composition and geometry as that of the actual component. Well-characterised natural defects such as service induced fatigue cracks and stress corrosion cracks are preferred, if available. The test frequency, instrument gain and other instrument functions are optimised so that all specified artificial defects are detected, e.g by thresholding of appropriate EC signal parameters such as signal peak-to-peak amplitude and phase angle. With optimised instrument settings, actual testing is carried out and any indication that is greater than the threshold level is recorded defective. For quantification (characterisation) master calibration graphs, e.g. between eddy current signal parameters and defect sizes are generated. In the case of heat exchanger tube ECT, calibration graph is between depth of ASME calibration defects (20%, 40%, 60%, 80% and 100% wall loss flat-bottom holes) and the signal phase angle. In order to detect and characterise defects under support plates multi-frequency EC testing which involves mixing of signals from different frequencies is followed and separate calibration graph is generated for quantification of wall loss.
Magnetic flux line contours of an eddy current probe in air, in an Inconel tube and in the tube surrounded by a carbon steel support plate. Freedom-loving flux lines are constrained by the tube wall and the support plate. This constraint (manifested as distortion / perturbation of eddy currents and associated impedance change) is what is measured to advantage in eddy current testing !


Applications of Eddy Current Testing

Sorting of materials with different heat treatment, microstructure etc. (metal detectors)

Detection of flaws / defects in metallic plates, tubes, rods and bars (as small as 0.2 mm deep)

Measurement of non-conductive and conductive coating thickness (upto 10 microns)

Measurement of electrical conductivity and magnetic permeability (0.5% IACS)

Advantages of Eddy Current Testing

Eddy current test can nearly all metallic materials

High inspection speeds possible ( ~ 5 m/s)

Eddy current test can readily detect very shallow and tight surface fatigue cracks and stress corrosion cracks (~ 5 microns width and 50 microns depth)

High temperature and on-line testing is possible, even in shop floors

Non-contact / remote / inaccessible testing is possible (Couplant is not required unlike in ultrasonics)

Recording and analysis of inspection data is possible (Computer based instruments / systems available with data acquisition, storage, analysis and database management)

Limitations of Eddy Current Testing

Like any other NDT technique ECT too has certain limitations, which are overcome to a large extent by the recent advances in the technique. A few key limitations are:

Only electrically conducting (metallic) materials can be tested

Maximum inspectable thickness is ~ 6 mm
(12 mm possible by tuning frequency, probes, instrumentation etc.)

Inspection of ferromagnetic materials is difficult using conventional eddy current tests
(Saturation ECT and Remote field ECT are possible for tubes)

Use of calibration standards necessary

Operator skill is necessary for meaningful testing and evaluation

Recent Trends / Advances in Eddy Current Testing
* Pulsed EC testing for sub-surface defect detection

* Remote field EC testing for ferromagnetic tubes

* Eddy current imaging to produce images or pictures of defects and to automate inspection

* Signal and image processing methods to extract more useful information of defects for enhanced detection and characterisation of defects

* Low-frequency eddy current testing

* Numerical modelling (finite element, boundary element / volume integral, hybrid etc.) for
Simulation of inspection technique / situation
Prediction of ECT signals for inversion
Optimisation of probes / test parameters

* Design of Phased-array and special focused probes

* Realization of expert systems and data-base systems
Eddy Current Image of a Stainless Steel Weld

Standards in Eddy Current Testing

Reference standards are used for adjusting the eddy current instrument’s sensitivity detection of cracks, conductivity, permeability and material thickness etc. and also for sizing. Some commonly used standards in eddy current testing are:

ASME, Section V, Article 8, Appendix 1 and 2), Electromagnetic (eddy current) testing of heat exchanger tubes

BS 3889 (part 2A): 1986 (1991) Automatic eddy current testing of wrought steel tubes
BS 3889 (part 213): 1966 (1987) Eddy current testing of non-ferrous tubes

ASTM B 244 Method for measurement of thickness of anodic coatings of aluminum and other nonconductive coatings on nonmagnetic base materials with eddy current 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 (eddy current) testing of seamless copper and copper alloy tubes
ASTM E 309 Eddy current examination of steel tubular products using magnetic saturation
ASTM E 376 Measuring coating thickness by magnetic field or eddy current (electromagnetic) test methods
ASTM E 426 Electromagnetic (eddy current) testing of seamless and welded tubular products austenitic stainless steel and similar alloys
ASTM E 566 Electromagnetic (eddy current) sorting of ferrous metals
ASTM E 571 Electromagnetic (eddy current) examination of nickel and nickel alloy tubular products
ASTM E 690 In-situ electromagnetic (eddy current) examination of non-magnetic heat-exchanger tubes
ASTM E 703 Electromagnetic (eddy current) sorting of nonferrous metals

Specific Applications of Eddy Current Testing

Quality assurance and in-service inspection of austinetic stainless steel tubes, plates and welds.

In-service inspection of heat exchangers, steam generators and condensers for
• Detection and sizing of defects in tubes (single frequency)
• Detection and sizing of defects near support plates (multi-frequency)

Detection and sizing of defects in multi-layer aircraft structures (multi-frequency & pulsed eddy current tests)

Quality assurance and in-service inspection of ferromagnetic tubes.

Detection and characterisation of intergranular corrosion (IGC) in stainless steel 316L / 304 L

Detection of weld centre line in austenetic stainless steel welds using eddy current C-scan imaging

Measurement of thickness of plates as well as thickness of coatings using eddy currents

Sorting of materials based on electrical conductivity and magnetic permeability

On-line eddy current detection of defects in materials

High temperature and non-contact testing of materials