The metal magnetic memory method is a novel technique for monitoring fatigue cracks in steel structures, which can reduce operational expenses and increase safety by minimizing inspections. The crack geometry can be identified by measuring the self magnetic flux leakage, which is induced by the Earth’s magnetic field and the permanent magnetization. The finite element method can be used to simulate the induced magnetic field around cracks to help interpret the self magnetic flux leakage measurements, but it is unclear what material properties to use. This study aims to determine the magnetic permeability of structural steel for accurate simulation of the induced magnetic field around cracks by the finite element method. The induced magnetic field was extracted from measurements above two square steel plates, one without defect and one with a straight slit, and compared with finite element results in function of the relative permeability. For both plates, a uniform relative permeability could be found for which experimental and numerical results were in good agreement. For the plate without defect and a relative permeability of 350, errors were within 20% and were concentrated around the plate’s edges. For the plate with the slit and a relative permeability of 225, errors were within 5%.@en

Ship and offshore structures are prone to fatigue damage as they are cyclically loaded by waves. Therefore, regular inspection is needed in order to confirm adequate structural integrity throughout the entire service life of the structure. Detected fatigue cracks that are too long for safe operation need to be repaired. Detected cracks of acceptable length need to be at least inspected more frequently. These inspections are costly, time consuming, and hazardous, so additional inspections on top of the periodical class approval surveys are to be avoided if possible.@en

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.@en