Study on a non-destructive evaluation technique based on laser measurements and fast multipole method

本课组进行了超声无损检测的基础研究，利用激光测速法测量由换能器诱导超声产生的结构构件中未知缺陷的散射波。在实际问题中使用该技术的障碍之一是缺乏实用的数值工具来解决时域的大规模弹性动力学问题，并且需要增加实验中使用的频率。由于实际无损检测问题需要求解具有Ο（10^4）或更大自由度空间数的大规模问题，因此在使用边界型方法的方法中，必须使用快速bim而不是传统的bim。鉴于此，我们研究了快速多极子方法的应用，该方法被认为在时域三维弹性动力学中是实用的。在对代码进行并行化处理后，我们建立了一个快速求解器。在实验中，我们开发了一种激光超声无损检测系统，该系统使用的频率比我们早期研究的要高。我们结合这些技术来研究一个在现实世界问题中具有潜力的无损检测系统。我们还考虑了使用YAG激光诱导超声来照亮未知缺陷。根据本研究的结果，我们可以提到以下几点：我们建立了一种简单的方法来识别YAG激光产生的超声波。我们已经看到，使用这样确定的入射波可以准确地确定表面裂纹。此外，在使用FMM的快速方法中，我们可以使用并行FMM代码和改进的向下路径算法来解决数百万空间自由度的问题。

Our group has done fundamental studies on a ultrasonic NDT which utilizes the laser velocimetry for measuring the scattered wave from unknown defects in structural members produced by a transducer induced ultrasound. Among the remaining obstacles for using this technique in real problems are the lack of practical numerical tools for solving large scale elastodynamic problems in time domain and the need to increase the frequencies used in the experiments. Since the practical NDT problems require the solution of large scale problems with Ο(10^4 spatial number of the degrees of freedom or larger, one has to use fast BIEMs rather than conventional ones in approaches which use methods of the boundary type. In view of this we have investigated the use of fast multipole method, which is considered to be practical in 3 dimensional elastodynamics in time domain. We have established a fast solver for this problem after parallelizing the code. In experiments, we have developed a laser-ultrasonic NDT system which uses higher frequencies than in our earlier studies. We have combined these techniques to investigate an NDT system which has potentials in real world problems. We have also considered the use of the YAG laser induced ultrasound in illuminating the unknown defects. As the findings of this research, we can mention the following : We have established an easy way of identifying the ultrasound produced by YAG laser. We have seen that the use of the incident wave thus identified allows the determination of surface cracks accurately. Also, in the fast method using FMM we could solve problems with millions of spatial DOFs using the parallelized FMM code with improved algorithms for the downward path.