Magnetic Properties of Structural Steels for Simulation of Crack Monitoring by Finite Element Method

Abstract

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%.

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

“…The stress magnetization measurements for different locations may not be directly compared as the stress magnetization is dependent on the total magnetization, which is most likely non-uniform especially around the machined elliptical defect. Still, it can be seen that the stress magnetization at high stress concentration areas such as location (1, 2) is generally much higher than where the stress is low such as location (1,3). For all the measurement points, the stress magnetization curves for increasing load are slightly different from those for decreasing load, but the way they differ from each other is not consistent for each of the measurement locations, see Figs. 12-14.…”

“…The Earth-induced magnetization from equation (3.1) can be modeled reasonably accurate with a linear magneto-static Finite Element model using the software package COMSOL Multiphysics. The same steel is used as in [3] so the material is modeled linearly with a relative permeability of 225. The geometry from Fig.…”

“…For accurate sizing of the crack, SMFL measurements must be interpreted correctly. Earlier research gave insight in the contributions of Earth-induced magnetization and permanent magnetization to the SMFL signals near a crack in a structural steel plate that is unloaded [2,3]. When applying fatigue loads on a cracked specimen, the SMFL will change due to crack opening [4] and stress-induced magnetization.…”