基于方向调制原理的涡流热成像漆层下裂纹检测技术

Eddy current thermography for under paint coating crack detection based on directional modulation

针对传统技术检测漆层下金属表面裂纹信噪比低的问题,提出了一种基于方向调制原理的涡流热成像检测技术。首先介绍了方向调制的原理,通过推导方向调制的数学模型,从理论上证明了方法的可行性,根据理论公式与实际情况选择了调制频率,根据该方法搭建了实验平台;分析了涡流无法完整旋转的问题,并提出了一种基于驱动PWM驱动信号翻转的改进方法;针对加热可能导致的谐振频率漂移问题,提出了一种双通道频率跟踪策略;最后对不同漆层厚度下45#钢的人工裂纹与0.517 mm漆层厚度下20#钢磨平焊缝裂纹进行检测并与磁粉法的结果对比。实验结果表明:方向调制方法可检测出0.902 mm漆层下的人工裂纹与0.517 mm漆层下的模拟自然裂纹;对比0.486 mm漆层下人工裂纹与0.451 mm漆层下模拟自然裂纹的磁粉法检测结果,方向调制方法有效提升了漆层下裂纹的检测能力。

The metal surface crack under paint coating has the problem of low signal-noise ratio using conventional detection methods.To address this issue,an eddy current thermography detection technique based on the directional modulation principle is proposed.Firstly,the principle of direction modulation is introduced.By deducing the mathematical model,the feasibility of the method is proved.The modulation frequency is selected according to the theoretical formula.According to the directional modulation,the experimental platform is established.Then,the reason that the eddy current cannot rotate in full angle is analyzed.One kind of improved method based on PWM drive signals flipping is proposed.To solve the problem of resonance frequency drift caused by heating,a two-channel frequency tracking strategy is proposed.Finally,the artificial cracks of 45#steel under different paint coating thicknesses and the cracks of 20#steel weld under 0.517 mm paint coating thickness are detected and compared with the results of magnetic powder detection.Detection results show that the direction modulation method is competent to detect the artificial cracks under 0.902 mm paint coating and the simulated natural cracks under 0.517 mm paint coating.Compared with the magnetic particle detection results of the artificial cracks under 0.486 mm paint coating and the simulated natural crack under 0.451 mm paint coating,the directional modulation method effectively improves the detection ability of under paint coating cracks.