Crack detection in magnetic material with a flexible coil using partial saturation pulsed Eddy current technique

Eddy current testing has become one of the most widely applied non-destructive inspection methods in the evaluation of metallic materials. Flaw detection in magnetic materials presents a unique problem. The large value of magnetic permeability limits the depth of penetration of eddy currents, which is known as the skin effect. Also, test coil impedance variations due to material permeability are usually much greater than impedance changes due to the interruption of induced eddy currents by defects. To achieve a better depth of penetration and to reduce the random variations in material permeability, a strong steady-state (DC) field generated by neodymium magnets was applied. Because the sample was too thick to be fully saturated, only a partial saturation was obtained. This technique monitors changes in permeability caused by a crack or by a variation in the thickness of the sample. This study shows that it is possible to detect surface breaking cracks and non-surface breaking cracks in a magnetic material using partial saturation and Pulsed Eddy Current technique. Quantification of thickness loss and quantification of crack depth were achieved using both conventional coils and flexible coils.