On objects made of electrically conductive materials eddy current testing can be used to test for integrity, composition and tempering quality, or also for geometric dimensions. In the process it exploits the physics of electromagnetic fields. Its application is thus contactless and requires no coupling media.
A coil traversed by an alternating current forms a (primary) magnetic field in its surroundings. As a result of this, currents form on the surface of the electrically conductive product to be tested. Known as “eddy currents”, they flow parallel to the coil turns, but in the opposite direction of the coil current. That is why they generate a (secondary) magnetic field, in the opposite direction of the magnetic field of the coil. This ultimately produces a weakening of the magnetic field of the coil. This can be measured as change to the alternating current resistance of the coil (impedance).
Should the product to be tested have any local defects (e.g. cracks, non-metallic inclusions, pores, corrosion pits), the eddy currents won’t be able flow unimpeded anymore. With that, the value measured on the coil changes in comparison with a product that is defect-free. As the probe slides along the product to be tested the high-frequency carrier signal of the eddy current coil (HF, excitation frequency) is modulated, which means the signal level varies. For the displayed example of a crack, the signal level increases. With the help of demodulation a low-frequency signal (LF) is acquired, which contains the information about the product to be tested, including any possible defects.
Based on the variations in coil impedance it is thus possible to record and characterize the properties of the product to be tested (insofar as they have an influence on the nature of the eddy currents), including any possible defects. Specific analysis methods are required for this, such as amplitude or phase analysis, signal form analysis or harmonic analysis. Before testing begins the eddy current instrument must be configured (excitation frequency, amplification, phase settings, filter settings, etc.) with the help of reference objects, which must possess certain properties in terms of geometry (dimensions, shape), material parameters (electrical conductivity, permeability, hardness), and material defects. To ensure reliable test results the test conditions must be adhered to while testing is in progress (e.g. constant probe distance and uniform testing speed). All perturbations (e.g. mechanical vibrations, temperature fluctuations, or electromagnetic fields) should also be ruled out, or minimized as much as possible.
Another important requirement for successful eddy current testing is the choice or development of a suitable probe; this includes the number and arrangement of the coils used, the type of electrical wiring, their dimensions, number of turns, a magnetic core if necessary, or shielding.
