Eddy Current Testing

Introduction

Eddy current testing is a basic electromagnetic NDT method of testing the conductive material properties without causing any damage. The principle that it works on is known as electromagnetic induction, which was discovered by Michael Faraday. A changing magnetic field is produced by an alternating current flowing through a test coil (probe) placed near a conductive material, and it causes circulating eddy currents in the material. The flow of these eddy currents is interrupted or varied by any discontinuity or change in the material (crack, corrosion, or change in conductivity), thereby changing the impedance of the test coil.

Fundamental Principles

The magnetic field of the probe coil that alternates results in the eddy currents that flow in closed loops and parallel to the magnetic field of the coil. The depth of penetration (skin depth) is very important, and it is determined by the following formula: 0 = 1/sqrt(pi fmu s), where f is the frequency, 0 is the magnetic permeability, and 0 is the electric conductivity. Redder frequencies have more penetrating power and lesser resolution of small surface flaws. The largest disturbance is caused by flaws perpendicular to the flow of the eddy current. The test is used to measure the complex impedance of the coil, which is influenced by conductivity, permeability, geometry, and the occurrence of discontinuities of the test material.

Key Equipment and Probes

The rudimentary ECT system comprises an instrument (eddy current flaw detector or conductivity meter), a probe (coil or array), and a display/recording unit. Differential Probes are created by using two coils that are opposed to isolate local defects by cancelling out the continuous changes, such as a gradual change in thickness. The coils in a circle (bobbin coils) are implemented around tubes, rods, and fasteners to promptly check them. Pancake/Surface Coils are overlaid on the surface to detect cracks and corrosion. Multi-coil Array Probes have wide-area imaging and coverage.

Primary Applications

Flaw Detection: Surface and near-surface cracks, seams, laps, and voids of metals are the most common uses. It finds wide application in the inspection of aircraft skins, aircraft engine parts (blades, discs), weld areas, and heat exchanger tubes. Measurement of Conductivity, Sorting of Alloys: ECT can check the heat treatment, infer the mix-ups in materials, and sort metal alloys (e.g., aluminium grades) without chemical analysis by measuring electrical conductivity.

Advanced ECT Technologies

Remote Field Testing (RFT): This is a specialised low-frequency ECT method of testing ferromagnetic tubes (e.g., carbon steel), in which the magnetic field passes through the wall of the tube twice, so the test method is sensitive to internal and external defects in equal measures. 

Eddy Current Array (ECA): This is a technique that involves the use of more than one coil in a single probe head. Large areas can be covered quickly by electronic multiplexing, producing C-scan images (topographic maps) of the component, which radically increases the speed of inspection and visualisation of the data. Near-Field Testing: It is a high-resolution, high-frequency technique that is well-suited to detecting narrow fatigue cracks in multi-layer aircraft structures. New Innovations and Future Prospects.