Slit induced self magnetic flux leakage in a square steel plate

Marine structures, continuously subjected to action of waves, suffer from fatigue cracks developing predominantly in welded connections. For this reason, marine structures have to be regularly inspected in order to confirm sufficient level of their structural integrity. A crack monitoring system can help keeping this level with less frequent inspections, because detected fatigue cracks need time to reach unacceptable sizes. This study is part of a research and development program aiming at developing a wireless crack monitoring system which is based on the Self Magnetic Flux Leakage (SMFL) phenomenon. Although this phenomenon is very attractive, still many knowledge gaps exist that prevent a successful application to marine structures. The goal of this research was to investigate different sources of magnetization and their effects on slit induced SMFL in a square steel plate. A slit in the plate was representing a through thickness fatigue crack in a marine structure. The combined effects were measured experimentally, whereas effects induced by the Earth’s magnetic field were determined numerically using steel magnetic properties defined in a separate experiment. The difference between the experimental and numerical SMFL was caused by permanent magnetization. The numerical simulation shows that the SMFL induced by the Earth’s magnetic field is constant over the slit length. The SMFL induced by the permanent magnetization is one order of magnitude higher and varies linearly over the slit length for the investigated plate. The study concludes that it is feasible to develop a monitoring system for detected cracks in marine structures based on the SMFL.