Effect of stress-induced magnetization on crack monitoring by self magnetic flux leakage method

Wireless crack monitoring on ships and offshore structures based on Self Magnetic Flux Leakage (SMFL) measurements is a promising method to guarantee the structural integrity in a more effective way, leading to reduced operational costs and increased safety. For accurate crack sizing, the SMFL measurements must be interpreted correctly, also during cyclic loading. Not much research has been done that focus on the effect of high cyclic stresses on the magnetization of ferromagnetic steels in weak magnetic fields. The aim of the research presented in this paper is to investigate the effect of stress-induced magnetization on the SMFL in the stress concentration zone of a structural steel plate, and its implications for crack monitoring by the SMFL method. By means of an experiment, measured stress magnetization curves were obtained in a grid of points around an elliptical hole in a steel plate that was cyclically loaded up to the yield stress. The results show that the stress-induced magnetization causes a maximum variation of the measured signal of 25 μT, which is fully reversible. It is concluded that, depending on the application, this stress-induced variation may need to be taken into account for the interpretation of the measured signals by a crack monitoring system using the SMFL method.