A theoretical and experimental study of alternating current stress measurement under different loading modes

This paper presents some experimental studies on the inverse magnetostriction effect that were recently conducted at University College London (UCL) by the authors. A form of alternating current field measurement (ACFM) instrumentation, which was developed to detect and size surface defects in ferromagnetic components, was used in the present study to induce a low alternating magnetic field near the surface of a mild steel specimen and to pick up the influences of applied stresses on that field. The mild steel specimen was tested both in four-point bending and uniaxial tension to investigate the response under different stress modes within the elastic region. Consistent and repeatable results have been obtained under both bending and tension loading modes when different probe types with different inducing frequencies are used. In the present paper, the mathematical model of half-space electromagnetic induction which was recently developed at UCL has been combined by the authors with the previously published fundamental relationships between stress and magnetization. The combination gives a new model, which has been introduced here to demonstrate that stress will alter the magnetic field. This suggests that ACFM technology can be further developed into that of alternating current stress measurement (ACSM) for industrial applications.