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 dimensionsThe 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.

Governing Laws* Ampere's law* Faraday's law* Lenz's lawProperties 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 depthCoil ImpedanceZ = R + j Xl