弯曲波对板上裂纹的检测成像及其机理研究

Detection and imaging of the crack in a thin plate are of critical importance for structural health monitoring of high-pressure vessels, aircrafts, reactors and other plate-like structures. Ultrasonic echo and thickness-measuring techniques on using to detect the crack in the plate are powerless. Flexural wave, as one of the modes propagating in the plate, is a main tool to detect defects on a plate. However, since the waveform of flexural wave will be expanded due to its dispersion when propagating in the plate, it is difficult to detect and further image the defects on the plate by the flexural wave. To overcome the difficulties, a modified reverse time migration (RTM) method which is suit to the dispersed flexural wave is presented to image the crack in the thin plate. The key of the method is to conduct the time reversal (TR) operation for the backward wave in the process of RTM, which make the expanded flexural wave recompressed. Both the forward wave and backward wave satisfy the cross-correlation imaging condition of the RTM method, and then the image of the crack in the plate is obtained by the flexural wave. In the experiment, the transmitted and received waveforms of flexural wave are recorded with high fidelity by a laser vibrometer, that is, the accurate experimental datas of the flexural wave are obtained, which ensures the quality of image of the crack in the plate by the modified RTM method. Finally, the scattering mechanism of the flexural wave on the crack is studied, which will provide the physical basis for the image of the crack in the plate by the flexural wave.

薄板上裂纹的检测成像对高压容器、航空器、核反应堆及其他板状结构的健康监测是非常重要的。对于薄板上的裂纹，采用传统的超声体波回波法和超声测厚术等方法是不适用的，而板上弯曲波可望成为板上裂纹等缺陷检测的一个主要工具。但由于板中弯曲波是频散的，在传播过程中，发射的弯曲波脉冲会因频散效应而逐渐扩展变宽，直接用它去检测板上裂纹等缺陷是困难的。为此，本项目提出了一种改进的逆时偏移方法，利用频散的弯曲波实现板上裂纹的检测成像。这个改进方法的关键是对逆时偏移中的逆向波进行时间反转操作，使得已扩展的弯曲波被重新压缩，从而使前向波与逆向波满足逆时偏移的互相关成像条件，实现板上裂纹的弯曲波成像。同时，在实验上利用激光测振仪高保真地记录了弯曲波的信号，提供了精准的弯曲波波形的实验数据，保证了用这种改进逆时偏移方法对板上裂纹成像的质量。最后，还研究了弯曲波在裂纹上的散射机理，为板上裂纹的弯曲波成像技术提供物理基础。

薄板上裂纹的检测和成像对高压容器、航空器、核反应堆及其他板状结构的健康监测是非常重要的。弯曲波可望成为薄板上裂纹等缺陷检测的一种重要工具。但由于薄板中弯曲波传播具有频散特性，即在传播过程中发射的弯曲波脉冲会因频散效应而逐渐扩展变宽，直接用它去检测薄板上的裂纹等缺陷是困难的。为此，本项目提出了一种改进的逆时偏移方法，利用频散的弯曲波实现了对薄板上裂纹的检测和成像。该方法的关键是对接收元处的裂纹反射弯曲波进行时间反转处理，使得已扩展的弯曲波在逆向传播过程中被重新压缩；当传播到板平面某点时，对其再进行一次时间反转处理，然后与从发射元传播到该点的前向波进行相关处理；通过相干叠加所有发射-接收对互相关的结果即可实现薄板上裂纹的检测和成像。在实验中采用激光测振仪高保真地记录了自由场中发射元发射的弯曲波波形和接收元接收的裂纹反射弯曲波波形，采用改进的逆时偏移方法处理这些实验波形数据得到铝板背面不同形状裂纹（平直裂纹、弧形裂纹、斜直裂纹、反Z形裂纹、V形裂纹和W形裂纹）的声学图像，其与实际裂纹在位置和形状上高度一致，从而实现了薄板上裂纹的高质量地原位成像。最后，采用表面微扰技术和有限元ANSYS软件研究了薄板表面矩形槽状裂纹对弯曲波的散射作用，结果表明弯曲波经该裂纹作用后的反射波中A0模式的Lamb波占主要成分，这与实验结果相一致。此外，超出原计划目标之外，运用我们提出的快照的时间反转-逆时偏移方法实现了对分层介质中目标的检测和定位，抑制了界面反射信号的干扰。本项目的研究将为薄板上缺陷的超声检测和成像提供理论和应用依据。