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

Abstract

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.

Use of Different Types of Magnetic Field Sensors in Diagnosing the State of Ferromagnetic Elements Based on Residual Magnetic Field Measurements

“…The authors of [ 40 ] emphasized that a significant threat to this method is the influence of the magnetic force of the neighboring elements of the tested object, which can effectively suppress the diagnostic signal. The researchers of [ 41 ] rightly noted that, in the case of passive crack monitoring, it is essential to study and quantify the effect of large and cyclic stresses in weak magnetic fields. Magnetic signal inspection based on the self-magnetic flux leakage (SMFL) effect can effectively identify the location of defects.…”

A Review of Magnetic Flux Leakage Nondestructive Testing

“…The stress effect has been studied by many researchers [ 80 , 81 , 82 , 83 , 84 , 85 , 86 , 87 , 88 , 89 , 90 , 91 ]. The properties of ferromagnetic materials change with the loading stress due to the magneto-mechanical coupling, thus the MFL signal also changes with the stress.…”

Quantitative Study on MFL Signal of Pipeline Composite Defect Based on Improved Magnetic Charge Model

“…The magnetic memory method is used to detect materials in the geomagnetic environment without external excitation magnetic field [ 13 ]. The detected magnetic signal is the self-magnetic flux leakage (SFML) signal of the material [ 14 , 15 ]. Compared with traditional MFL detection signals, the MMM signals are unstable and easy to disturb, so the MMM technology has not been widely used in the online internal detection of long-distance oil and gas pipelines.…”