Influence of Sampling Frequency on Magneto-Optical Imaging Under Alternating Magnetic Field Excitation

This paper studies the magneto-optical image (obtained by different sampling frequencies) features of weld defects under alternating magnetic field excitation. Weld cracks with different gaps—0.03 mm, 0.05 mm, 0.07 mm, and 0.1 mm—were detected through the magneto-optical imaging method. In order to study the effect of sampling frequency on magneto-optical imaging, different sampling frequencies, such as 5 fps, 15 fps, 25 fps, 35 fps, 45 fps, 55 fps, 65 fps, and 75 fps, were used under the excitation of an alternating magnetic field. The distribution range of the gray-scale value for the magneto-optical image obtained at each sampling frequency was analyzed. The effect of the sampling frequency on the magneto-optical imaging was analyzed by comparing the magneto-optical images of weld defects and their gray-value curves at different sampling frequencies. Experimental results show that the sampling frequency of the magneto-optic sensor affects the distribution range of the gray-scale value of the magneto-optic image. Under the same experimental conditions, as the sampling frequency increases, the whole magneto-optical image becomes dark, and the span of the gray value becomes larger.