Assessing material properties and defects without contact
Eddy current testing is based on electromagnetic induction in electrically conductive materials. A high-frequency alternating magnetic field is generated via a probe coil. When this coil is brought close to a conductive component, the magnetic field induces annular electric currents in the material – so-called eddy currents.
These eddy currents in turn generate their own magnetic field, which opposes the original excitation. In homogeneous, defect-free material, a stable electromagnetic state forms. If, however, electrical conductivity, permeability or the geometric structure of the component changes – for example due to a crack, porosity or a microstructural change – the distribution of eddy currents is also altered locally.
These local changes immediately affect impedance, amplitude or phase of the measurement signal. The inspection probe detects these signal changes without contact and passes them to the evaluation unit. From the type and intensity of the signal deviation, conclusions can be drawn on the position, extent and character of the irregularity.
The penetration depth of eddy currents depends on the material, electrical conductivity, magnetic permeability and the selected test frequency. As frequency increases, penetration depth decreases (skin effect), which makes the method particularly sensitive to near-surface defects. By suitable frequency selection, the inspection can be tuned to specific edge zones of the component.
Depending on the inspection task, different probe types are used, for example pencil probes for spot inspections or encircling coils for series components. Eddy current testing thus enables fast, non-contact and reproducible examination of conductive materials – both for crack detection and for checking material properties or heat treatment conditions.
