Weld cracks nondestructive testing based on magneto-optical imaging under alternating magnetic field excitation

•A method for weld crack detection based on magneto-optical (MO) imaging technology is proposed.•Inspection and identification of weld surface cracks and sub-surface cracks are discussed by a magnetic dipole model.•A novel experimental system of MO image acquisition of weld cracks under alternating magnetic field excitation is built.•The MO image of weld crack can reflect the magnitude of the magnetic field intensity through varied brightness. This paper researches the magneto-optical (MO) imaging law of weld cracks under alternating magnetic field excitation. Weld surface and subsurface cracks are detected by a MO sensor, and the relationship between the MO images’ characteristics and the magnetic field strength is analyzed based on the Faraday MO effect. A magnetic dipole model is proposed to study the magnetic field distribution over the weld crack. A finite element analysis (FEA) model of the weld crack is established, and the relationship between the magnetic flux leakage signal and the crack width is analyzed, which is useful for identifying cracks either on the surface or on the subsurface of the weld. A MO imaging nondestructive testing (NDT) experiment is carried out to detect weld cracks under alternating magnetic field excitation, and the difference among weld cracks is obtained by analyzing the gray values of the weld cracks’ MO images. Research results show that the magnetic flux leakage signals of weld surface and subsurface cracks can be clearly distinguished, the magnetic field intensity of the surface cracks is larger than that of the subsurface cracks at the same width, and the MO image of the weld cracks can reflect the intensity of the magnetic field through varied brightness, that is, the gray value of the MO image can match the corresponding magnetic field intensity.