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.

Eddy Current Testing 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.

Briefly:

This NDT method consists in generating induced currents in electricity conducting materials via an alternative magnetic field (generated by a solenoid) and variable in time (low or high frequency).

The currents locally induced and generated are called Eddy currents. Their distribution and repartition depend on the magnetic excitation field , as well as on the geometry the electrical conductivity and the magnetic permeability of the controlled structure.

The presence of a defect in the testpiece generates disturbances in the circulation of Eddy currents, entailing a variation in the solenoid impedance.

The method is applicable to all electricity conducting materials.

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.

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.
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.

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.

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