Stress Dependence of Anhysteretic Permeability and Its Application to Nondestructive Residual Stress Measurement.

Abstract

Stress dependence of anhysteretic permeability was used for nondestructive measurement of residual stress in a welded plate. Magnetic flux density in a ferromagnetic material depends on its magnetic history, which is well known as the hysteresis. If a decaying AC magnetic field is superimposed on a certain DC field, the flux density converges at the anhysteretic value, which is independent of the history. The relationship between these values and the magnetic field is called an anhysteretic curve. We measured the initial permeability of the anhysteretic curve as a function of stress for mild steel. We found that the anhysteretic permeability decreases with the stress when the specimen is magnetized parallel to the compressive stress or perpendicular to the tensile stress. The stress dependence was used for estimating residual stresses in a butt-welded plate. Anisotropy of the anhysteretic permeability was defined to be used for the evaluation of the principal stress difference. The results showed good agreement with the conventional acoustoelastic measurements and the straln gauges.