脉冲压缩在钢板腐蚀电磁超声SH导波检测中的应用

Application of pulse compression technique in steel plate corrosion detection with SH guided wave EMATs

将Barker码脉冲压缩技术应用于水平剪切(SH)导波电磁超声换能器(EMAT),对提高在役钢板腐蚀检测回波的信噪比(SNR)和空间分辨率,以及实现大范围在线快速扫查具有重要的工程应用价值。建立基于Barker码信号激励的钢板SH导波传播有限元模型,结合实验分析与数值计算,分析了Barker码序列长度、码元长度、EMAT设计参数、提离等因素对脉冲压缩后缺陷回波的信噪比和波包宽度的影响,并与传统单一频率脉冲串激励方式进行对比实验。结果表明:与单一频率脉冲串激励方式相比,采用Barker码脉冲压缩技术,可以将缺陷波的SNR提高5.8 dB;当EMAT提离为3.0 mm时,经过脉冲压缩后的缺陷回波SNR>8.7 dB,而采用单一频率脉冲串激励方式对应的SNR接近于0 dB;当Barker码信号序列长度为13位、码元长度为15μs时,能检测深度为1 mm、直径为20 mm的圆孔,且SNR>25.4 dB。

The application of Barker code pulse compression technique in a shear horizontal(SH)guided wave electromagnetic acoustic transducer(EMAT)has significant value in engineering application as it enhances the signal-to-noise ratio(SNR)and spatial resolution of the corrosion echo detected in in-service steel plates,and helps to realize large-scale and rapid on-line scanning.In this paper,a finite element model of SH-guided wave propagation in a steel plate excited by Barker code signals is established.Then,combined with experimental analysis and numerical calculations,the effects of Barker code sequence length,subpulse length,EMAT design parameters,and lift-off on the SNR and wave packet width of the pulse-compressed echoes are analyzed and compared with those of the traditional tone-burst excitation method.The results show that the SNR of the defect echo processed with the Barker code pulse compression technique is 5.8 dB higher than that with the tone-burst excitation method.When the EMAT liftoff is 3.0 mm,the SNR of the defect echo after Barker code pulse compression is greater than 8.7 dB,while that with the tone-burst excitation method is approximately 0 dB.When the sequence length and subpulse of the Barker code signals are respectively 13 and 15μs,a circular hole with a depth of 1 mm and a diameter of 20 mm can be detected,the SNR of which is greater than 25.4 dB.