方管中超声导波的传播特性及缺陷检测

Propagation characteristics of ultrasonic guided waves in square tubes and its application to the detection of defects

针对基于点对点检测方式的传统体波方法在检测长距离方管时效率低下的问题,利用超声导波高效检测的优势,提出基于超声导波的方管检测方法。首先利用半解析有限元法求解出方管的相速度和群速度频散曲线,选择群速度频散曲线上群速度最大、相对平坦,频率范围在54~74 kHz的L(0,6)作为激励模态。其次分别采用有限元仿真和实验的方法,在方管中激励出中心频率为64 kHz的超声导波,研究导波在正常、面上存在圆孔缺陷和边缘存在槽缺陷的方管中的散射特性。在实验过程中,通过对压电晶片进行阻抗匹配来提高信噪比。仿真和实验结果表明,能够在方管中有效激励出L(0,6)模态超声导波,而且利用L(0,6)模态的缺陷反射回波,能够快速地检测出方管中的圆孔和槽缺陷,并定位缺陷的轴向位置。同时,阻抗匹配后信号的幅值是未阻抗匹配的2~3倍。纵向L(0,6)模态超声导波能够对方管中缺陷进行高效检测,为方管的检测提供了一种新的方法。

It is inefficient to inspect long square tubes by using traditional ultrasonic bulk waves based on point-to-point inspection.In order to improve the efficiency of inspecting square tubes,ultrasonic guided wave method is proposed.Firstly,the phase and group velocity dispersion curves in square tubes are derived by semi-analytical finite element method.The longitudinal L(0,6)mode is selected as the excitation mode with the group velocity value reaching the maximum,the group velocity dispersion curve being flat and the frequency ranging from 54 to 74 kHz.Secondly,numerical and experimental studies are conducted respectively to investigate the excitation of longitudinal L(0,6)mode at the center frequency of 64 kHz in square tubes and to explore the propagation characteristics of ultrasonic L(0,6)mode guided waves in normal square tube,and in square tubes with circular through-hole damages located in surfaces and slot damages at edges.During the experiments,impedance matching is applied to piezoelectric transducers to improve the signal-to-noise ratio.Numerical and experimental results show that L(0,6)modal ultrasonic guided wave can be effectively excited in square tubes.Furthermore,the flaw reflection echo of L(0,6)mode can be used to effectively detect circular through-hole damages and slot damages at edges in the square tube,and to locate the axial position of the defects.Meanwhile,the amplitudes of the signals after impedance matching is improved by 2~3 times.The use of longitudinal ultrasonic guided wave of L(0,6)mode to detect damages in square tubes is quite efficient.It provides a new effective alternative for the inspection of square tubes.