中文摘要：

为了探究超声导波在90°弯头所引起的模态转换对检测造成的影响,采用试验和模拟的方法来研究导波在90°弯头中的传播特性,利用周向均布的长度伸缩型压电片激发导波L（0,2）模态,对弯头及弯头两端直管道上的周向裂纹缺陷进行位置识别;并对比直管道检测结果,研究弯头对导波模态转换的影响。采用小波分析对试验信号进行降噪处理,试验结果表明,L（0,2）模态导波穿过弯头后发生模态转换,产生弯曲模态F（1,2）。弯头处裂纹缺陷的检测敏感性与其所在位置有关,弯头外侧的检测敏感性最高;且检测敏感性与激发频率有一定关系,在较高的激发频率（120~130 k Hz）下,弯头两端直管道上裂纹缺陷检测的敏感性最高;当激发频率处于较低频率（80 k Hz）时,导波对弯头内侧缺陷检测的敏感度最高。因此提出采用不同激发频率的导波对带有弯头的管道系统进行综合检测的方法。利用数值模拟的方法对导波在弯头处的传播特性进行研究,模拟结果与试验结论相吻合。

英文摘要：

The 90＆#176; elbow can generate complex mode conversions when ultrasonic guided wave passing through, thus makes it difficult to inspect defects in elbow and beyond elbow. The detection of circumferential cracks of different areas in elbow pipes are investigated in experiments and simulations using the guided waves in mode of L（0, 2） which are excited by a piezoelectric transducer. The effect of elbow to guided wave inspection is researched by comparing the result of straight pipe inspection. The signal is denoised by using three-level discrete wavelet decomposition and reconstruction which mother wavelet is db1. The experimental results show that the circumferential cracks can be determined in axial position for both elbow and bend straight side by the reflected echo of defects in the pipes. The detection sensitivity depends on the location of the cracks in bend areas of the pipes. It is easier to detect cracks in the extradose of the bend while harder to detect those in the other locations of the bend. There is a certain relationship between the detection sensitivity and the frequency of guided waves. The signal-to-noise ratio of detecting the crack before and beyond elbows is highest in the frequency ranges from 120 kHz to 130 kHz; the crack in the intradose of elbow can be inspected accurately in the frequency nearby 80 kHz. Therefore, the combination of high-frequency and low frequency is used to detect the bended pipes. The propagation behaviors in elbows are investigated by using numerical simulation. The results of simulation intuitively explain the experimental phenomenon.