脉冲压缩在铝薄板电磁超声导波检测中的应用

Application of pulse compression in electromagnetic ultrasonic guided wave detection of aluminum sheet

针对电磁超声换能器(EMAT)换能效率低导致超声回波信噪比(SNR)差这一问题,基于相位编码脉冲压缩技术,讨论了匹配滤波器及旁瓣抑制滤波器的设计及其在电磁超声检测中的实现。选用典型二相编码信号-巴克码作为激励信号,分析了不同码长、载波周期数的巴克码信号对电磁超声导波检测脉冲压缩效果的影响,并研究了旁瓣抑制算法的效果。以含人工预制裂纹的7075航空铝合金薄板为检测对象,选用SH;模态电磁超声导波换能器对比了单频正弦信号激励方式与巴克码脉冲压缩技术获得的缺陷回波信号信噪比。结果表明,采用巴克码脉冲压缩后超声回波的信噪比较单频正弦激励方式至少提高了7.2 dB,且巴克码脉冲压缩技术能探测出单频正弦激励方式而不能可靠检出的小裂纹(长×宽×深为10.0 mm×1.0 mm×0.25 mm)。将相位编码压缩技术应用于电磁超声导波检测,可达到增加EMAT提离和减小同步平均次数的目的,能满足在线快速检测的需求。

Aiming at the problem of poor Signal-to-Noise Ratio(SNR)of ultrasonic echoes caused by low energy conversion efficiency of Electromagnetic Ultrasonic Transducer(EMAT),the design of matching filter and sidelobe suppression filter and their implementation in electromagnetic ultrasonic detection were discussed based on phase-coded pulse compression technology.A typical two-phase coded signal,Barker code,was selected as the excitation signal.The effect of barker code signals with different lengths and carrier cycles on the pulse compression effect of guided wave EMAT detection was analyzed,and the effect of sidelobe suppression algorithm was also studied.We selected a 7075 aluminum alloy sheet with four manually prefabricated defects as the testing object,a SH;mode guided wave EMAT was used to compare the SNRs of defect echo signals obtained by single-frequency sinusoidal signal excitation method and Barker code pulse compression technology.The results show that compared with single-frequency signal excitation,the SNR of the ultrasonic signal after Barker code pulse compression was increased by at least 7.2 dB.With Barker code pulse compression technology,a defect of small size(long×wide×deep is 10.0 mm×1.0 mm×0.25 mm)can be effectively detected,which cannot be detected with the single-frequency sinusoidal signal excitation method.With the application of Barker code pulse compression technology to guided wave EMAT detection,the increase in EMAT lift-off and the reduction in the number of synchronous averages can be realized,and requirements of online and rapid inspection can be met.