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 steel plate caused by a fatigue or a stress are a very serious problem. We have measured residual magnetic field in order to estimate a reverse-side machined 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. The gradio-MI sensor is suitable for detection of localized micro magnetic field. From results of experiments, a distribution 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 results of a defect detection utilizing the distribution of residual differential magnetic field.