涡流探头提离效应的理论分析与实验研究

Theoretical and experimental analysis of lift-off effects on probe impedance in eddy-current testing

推导了线圈置于有限厚度的金属平板上方时其阻抗计算的级数表达式,并以此为基础分析了由线圈的提离改变所引起的线圈阻抗的变化特征。同时采用有限元法对导体平板上有缺陷存在时的复杂三维涡流问题进行计算求解,进一步分析了提离变化对缺陷所引起的线圈阻抗变化的影响。结果表明提离增加会显著降低缺陷信号的幅值,但并不影响缺陷信号和提离轨迹之间的相位差,通过分析相位差随检测频率的变化规律发现:存在一个合适的频率可使缺陷信号和提离干扰之间出现90°的相位差。实验数据说明了阻抗计算模型及其理论分析的正确性,其结论为实际检测中利用相位旋转及信号增强技术消除提离干扰提供了有价值的理论参考。

The impedance of a probe-coil above a conductive plate with finite thickness was derived and expressed in series expansion, and then the impedance characteristics of the coil due to lift-off variation were analyzed. Furthermore, three-dimension eddy-current problem for a coil above the plate with a defect was calculated by finite element method. Effects of lift-off on coil impedance change produced by a defect were investigated. The results show that amplitude of defect signal is adversely affected by lift-off variations, but phase difference between defect signal and lift-off loci is almost unchangeable. After analyzing the characteristics of the phase difference with the chang of frequency, a conclusion that defect signal is approximately vertical to lift-off noise at a well-selected frequency is obtained. The impedance calculation model and theoretical analy- sis are confirmed to be correct by experiments, which are valuable for removing lift-off disturbance by using phase rotation and signal enhancement technique in a practical eddy-current testing.