Multidirectional magneto-optical imaging system for weld defects inspection

Abstract

Magneto-optical (MO) imaging can be used to detect weld defects under alternating magnetic field excitation. However, in an alternating magnetic field, an MO sensor is more sensitive to defects perpendicular to the excitation direction than defects with other directions. This paper introduces a rotating magnetic field generated by an orthogonal inducer and proposes a multidirectional MO imaging nondestructive testing (NDT) system using rotating magnetic field excitation for the detection of natural weld defects. The relationship between imaging characteristics of MO images and magnetic field strength is analyzed based on the Faraday rotation effect. A finite element analysis (FEA) model of cruciform weld cracks is established to study the distribution of rotating magnetic fields at different transient time. MO imaging NDT experiments were performed on weld defects including surface crack, subsurface crack, and non-penetration. The finite element model is proved by MO imaging experiments of cruciform weld cracks. Defect information of the MO images is extracted through fusion image technology. The MO imaging effects of weld defects under alternating magnetic field excitation and rotating magnetic field excitation are compared. The maximum difference of image gray value is analyzed to determine the effect difference between the two magnetic fields. Research results show that MO imaging under rotating magnetic field excitation overcomes the limitation of directional detection of MO imaging under alternating magnetic field excitation and effectively realizes the detection of multidirectional invisible weld defects.