Abstract:  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.

Key words: magneto-optical（MO） imaging, Faraday rotation effect, weld defect, alternating magnetic field

摘要： 研究焊接缺陷磁光成像检测方法，基于法拉第旋转效应，分析交变磁场下焊接缺陷磁光成像特征与漏磁场之间的关系。建立焊接缺陷的三维有限元模型，对不同类型和宽度的焊接缺陷漏磁场分布进行模拟，并在交变磁场激励下对不同焊接缺陷进行磁光成像无损检测试验，通过试验验证了焊接缺陷检测模型的有效性。研究结果表明，漏磁场分布与缺陷的类型和宽度密切相关，随着宽度增大，缺陷漏磁场的磁感应强度垂直分量亦增大；在相同宽度下，未熔合、表面裂纹、亚表面裂纹和无缺陷磁光图像灰度峰谷差值呈递减趋势，磁光图像灰度值可与漏磁场强度相匹配；所建焊接缺陷模型和磁光成像试验能有效地描述不同焊接缺陷对漏磁信号和图像灰度值分布的影响，有助于提高焊接缺陷检测和质量评估。

关键词: 磁光成像, 法拉第旋转效应, 焊接缺陷, 交变磁场