研究目的
To examine the characteristics of magneto-optical images of weld defects under alternating and rotating magnetic field excitation, compare their nondestructive testing effects, and determine the optimal parameters for detection.
研究成果
The rotating magnetic field excitation provides superior detection effects for weld defects compared to alternating magnetic field excitation, with advantages in detecting multidirection and subsurface defects up to 8 mm depth. Optimal parameters are 120 V excitation voltage and 20 mm lift-off value. Histogram equalization significantly improves image contrast, aiding in defect recognition. This method has practical significance for industrial nondestructive testing applications.
研究不足
The study is limited to specific weld defects (cracks, non-penetration, sag) and materials (steel plates). The rotating magnetic field setup may be complex and require precise control. Detection depth is capped at about 8 mm for subsurface defects. The method may not be applicable to non-magnetic materials or other types of defects not tested.
1：Experimental Design and Method Selection:
The study uses magneto-optical imaging (MOI) based on the Faraday magneto-optical rotation effect. It involves comparing MO images under alternating and rotating magnetic field excitations to assess nondestructive testing effectiveness for weld defects. A rotating magnetic field is generated using two crossed yokes with sinusoidal alternating currents having a phase difference of 90°.
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Sample Selection and Data Sources:
2. Sample Selection and Data Sources: Weld defects are created on steel plates (No. 45) using laser welding. Surface defects include crack 1, crack 2, non-penetration, and sag. Subsurface defects are simulated by adding steel plates of thicknesses 2 mm, 4 mm, 6 mm, and 8 mm over surface defects. A cruciform welding seam is also used for verification.
3：List of Experimental Equipment and Materials:
Equipment includes a motor control cabinet, a three-axis motion platform, a magnetic field generator (with yokes for alternating and rotating fields), an MO sensor (with LED, polarizer, MO medium, analyzer, and CMOS camera), and an MO image acquisition system. Materials include low-carbon steel plates and No. 45 steel plates for defect simulation.
4：Experimental Procedures and Operational Workflow:
The weldment is placed on the motion platform. The magnetic field generator excites the sample with alternating or rotating magnetic fields at 50 Hz. The MO sensor captures dynamic images at 75 Hz sampling rate. Images are processed using histogram equalization. Parameters like excitation voltage (varied from low to high) and lift-off value (varied from 10 mm to 80 mm) are optimized. Data on grayscale values, standard deviation, and maximum difference are extracted and analyzed.
5：Data Analysis Methods:
Image analysis includes grayscale curve extraction, calculation of standard deviation and maximum difference values, and application of histogram equalization to enhance contrast. Three-dimensional grayscale graphs are used for subsurface defect analysis.
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