Analysis of Magneto-optical Imaging Characteristics of Weld Defects under Alternating Magnetic Fields

A method of detecting the weld defects was studied based on MO imaging. According to the Faraday rotation effect, the key to detect weld defects under alternating magnetic fields was to accurately describe the relationship between the magneto-optical imaging characteristics of weld defects and the leakage magnetic field. A three-dimensional finite element model of weld defects was established, and the leakage magnetic field distribution of different types and widths of defects was simulated. MO imaging nondestructive testing (NDT) tests were carried out to detect different weld defects under an alternating magnetic field excitation. The validity of a finite element model was verified by the tests. The results show that the leakage magnetic field distribution is closely related to the types and widths of the defects. With the widths increase, the vertical components of magnetic induction intensity of defect leakage magnetic field increase. Under the same width conditions, the peak-to-valley gray values of incomplete fusion, surface crack, subsurface crack and no defect show a decreasing trend. The gray values of a MO image may match the corresponding magnetic field intensities. The proposed weld defect model and MO imaging tests may effectively describe the influences of different weld defects on the distribution of magnetic flux leakage signals and image gray values, which are helpful to improve the weld defect detection and quality assessment. © 2021, China Mechanical Engineering Magazine Office. All right reserved.