Detection and Analysis of Electric-Current Perturbation Caused by Defects

The electric-current perturbation technique consists of establishing an electric-current flow in the material to be examined and then detecting localized perturbations of this current flow at inhomogeneities such as inclusions or cracks. The current perturbation is sensed by using a small noncontacting probe to detect the associated magnetic-flux perturbation at the surface of the specimen. In the work reported here, analysis of the electric-current perturbation signals was based on an analytic solution for the change in current density caused by a slot of infinite length and finite depth. Comparisons were made with experimental data for electric-discharge machining (EDM) slots of various depths in Incology 901 where the electric current was introduced by ohmic contact. Excellent agreement between theory and experiment was obtained for deep slots, although the model predicts a more rapid decrease in signal amplitude with decreasing slot depth than is experimentally observed. Additional measurements made with fatigue cracks show that cracks as small as 0.45 mm in length are easily detected. It was concluded that the technique is not only a sensitive method for flaw detection, but also offers the possibility of determining flaw characteristics through analysis of signal shapes and amplitudes.