Evaluation of a reverse-side defect on stainless steel plates by the residual magnetic field method

Defects on a reverse-side of a metallic plate such as a thick stainless ste el plate caused by a fatigue or a stress are a very serious problem. We hav e measured residual magnetic field in order to estimate a reverse-side mach ined model defect existing on a 5 mm thick austenitic stainless steel plate . A differential type magneto-impedance effect sensor (the gradio-MI sensor ) is applied to measure it. Because the gradio-MI sensor is suitable for de tection of localized micro magnetic field. From results of experiments, a d istribution of the residual differential magnetic field is clearly changed near a reverse-side machined model defect on an austenitic stainless steel plate. In this paper, we describe the residual differential magnetic field over a 5 mm thick austenitic stainless steel plate, and show experimental r esults of a defect detection utilizing the distribution of residual differe ntial magnetic field.