管道超声导波检测中信号选取的实验研究

为了研究信号选取对缺陷检测的影响,激励4个不同周期数但中心频率均为120kHz的单音频信号,对长7．12m的管道中的单缺陷进行了检测．检测结果表明,20个周期的单音频信号适合检测该管道中的缺陷．当单音频信号周期数较小时,激励的L(0,2)模态波形较窄,频带较宽,信噪比较低,并且当该模态遇到缺陷后,反射的一些转换模态(如弯曲模态)频散严重,不利于模态识别和缺陷检测．当周期数逐渐增加至一定数基时,L(0,2)模态频带变窄,频散程度变小,信噪比更高,有利于管道的缺陷检测．周期数过多,尽管模态控制较好,但各缺陷回波波形持续时间较长,彼此之间容易相互叠加,反而影响缺陷定位和识别．

Experimental Study on Signal Choice for Ultrasonic Guided Wave Inspection in Pipes

To investigate the influence of signal choice on flaw detection,detection is done on single flaw by using 4 tone bursts with different cycles at 120 kHz in a 7.12 m long pipe.The tone burst of 20 cycles is suit- able for flaw detection in the pipe by comparing and analyzing corresponding experimental results.When its cycle number becomes smaller,the tone burst is unsuitable to detect flaws because the waveform of excited L(0,2)mode becomes narrower,and its frequency spectrums become wider,signal-to-noise ratio(SNR)of flaw signals become smaller,and the dispersion of converted modes,such as flexural modes,becomes more se- rious when the incident mode reflects from the flaw.When its cycle number gradually increases to a certain value,frequency spectrum of excited L(0,2)mode becomes narrower,its dispersion becomes more inconspic- uous,and the SNR of flaw signals become larger,thus,the tone burst with this number of cycles is suitable for flaw detection.However,the tone burst becomes improper when its cycle number increases largely.Be- cause flaw echoes are inclined to superpose mutually when durations become longer.