中文摘要：

理论研究了带黏弹性包覆层管道中扭转模态的传播特性。在实验中利用研制的传感器激励出50kHz和75kHz的扭转模态T（0，1），对带环氧树脂包覆层钢管中纵向和周向缺陷等两种不同类型人工缺陷进行了检测。利用小波包分析方法对接收到的信号进行消噪处理。实验结果表明，低频的T（0，1）模态超声导波可用来检测带黏弹性包覆层管道中的缺陷，并且利用小波包分析对信号进行消噪后，可以有效提高缺陷回波信号的信噪比，便于管道中的缺陷定位。

英文摘要：

Propagation characteristics of torsional modes in pipes with viscoelastic coating were investigated theoretically. T（0,1）, the lowest torsional mode which attenuates and disperses low, is suitable for defect detection in pipes with viscoelastic coating according to theoretical analysis. T（0,1） modes at 50kHz and 75kHz were excited to detect two completely different kinds of defects, artificial longitudinal and circumferential notches,in 4m tong epoxy coated pipe using developed transducers in the experiment. In order to make the defects easilier identified, the received signals were denoised using wavelet packet analysis. Experimental results show that it is feasible for defect detection in pipes with viscoelastic coating using T（0, 1） mode at low frequency of ultrasonic guided waves. Furthermore, signal processing which is wavelet packet analysis denoises the received signals and can improve their signal/noise （S/N） ratio of defect echoes efficiently, implementation convenience of locating defects is dramatically improved.