回折线圈导线长度对表面波EMAT性能及缺陷检测的影响

Effect of Wire Length in Meander-line-coil on Performance of Surface Wave EMAT and Defect Detection

为了改善表面波电磁声传感器(electromagnetic acoustic transducer,EMAT)的激励及接收性能,并提高缺陷定量检测的准确性,试验研究了EMAT中回折线圈导线长度对传感器频率特性、所激励表面波声场指向性、衰减特性的影响.采用不同长度回折线圈对人工裂纹进行检测,得到了对应的反射和透射系数,分析了导线长度和裂纹长度相互关系对缺陷定量检测的影响.试验结果表明,表面波EMAT的频率特性与回折线圈导线长度无关,但其所激励表面波的声场指向性及衰减速率与导线长度密切相关,回折线圈导线长度越大,声束辐射半张角越小,表面波衰减越慢;作为接收传感器,回折线圈导线长度越大,EMAT检测时空间分辨率越低,但接收信号信噪比越高.导线长度小于裂纹长度时,才能得到准确的反射和透射系数.

To improve the excitation and reception performance of surface wave EMAT and increase the accuracy for quantitative defect detection,the influence of wire length in meander-line-coil on the frequency characteristic,acoustic beam directivity and attenuation were studied experimentally. Meanderline-coils with different wire lengths were used to detect the artificial cracks, the reflection and transmission coefficients of surface wave were obtained,and the influence of the relation between wire length and crack length on quantitative detection of defect was analyzed. Results show that the frequency characteristic is independent of wire length,while the beam directivity and attenuation of surface waves are closely related to it. With the increase of wire length,the radiation half angle of surface waves beam decreases and the attenuation slows down. The space resolution of EMAT is reduced and the SNR of received signal increases with the increase of wire length when the EMAT is used as a receiver. The reflection and transmission coefficients can be measured accurately only when the wire length of meanderline-coils is smaller than the crack length.