Investigation of electromagnetic nondestructive evaluation of residual strain in low carbon steels using the eddy current magnetic signature (EC-MS) method

A novel magnetic nondestructive testing method, that is named eddy current magnetic signature (EC-MS) method, is proposed to evaluate the residual strain in low carbon steels. This method relies on characterization of eddy current signals in the impedance plane if low frequency major magnetization is superimposed. To investigate the mechanism of the proposed method, the eddy current magnetic signatures of a set of tensile test specimens are measured, and their relationship to the residual strain is surveyed. The trajectories of the eddy current signals show significant dependences on the residual strain; the EC-MS changes depending on the three residual strain stages, the region just after elastic limit before yield point, the Luders-strain region, and the region after start of work hardening. The EC-MS under elastic strain/stress is measured to investigate the influence of stress on the EC-MS. Numerical calculation is performed considering micro-eddy current field associated with moving domain walls and compared with experimental results to interpret the EC-MS phenomena.