Detection model of invisible weld defects by magneto-optical imaging at rotating magnetic field directions

Magneto-optical (MO) imaging non-destructive testing (NDT) system excited by rotating magnetic field is proposed for feature extraction and detection classification of invisible arbitrary-angle weld defects. The magnetic field direction of the rotating magnetic field changes periodically with time, and the magnitude and direction of the leakage magnetic field on the weldment also changes periodically. A finite element analysis (FEA) model of weldment is established to study the distribution of rotating alternating current field at different transient time. The correctness of the rotating alternating current field theory is verified by FEA. Based on the Faraday rotation effect, the relationship between the imaging characteristics of weld defect MO image and the leakage magnetic field intensity is analyzed. The gray value of MO image can match the corresponding leakage magnetic field intensity. MO imaging NDT experiments are performed on invisible weld defects, including surface crack, subsurface crack, and non-penetration. The gray-level co-occurrence matrix (GLCM) method is used to extract texture features of the MO image of weld defects, and the texture features of the images can reflect the leakage magnetic field characteristics of the defects. These texture features of MO images are used as input data for the defect classification model established using support vector machine (SVM). This model is evaluated by weld defect detection experiments and shows that it can effectively and accurately classify invisible arbitrary-angle weld defects.