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

本课题组对超声无损检测进行了基础性研究，该方法利用激光测速技术测量由换能器感应超声产生的结构构件中未知缺陷的散射波。在真实的问题中使用这种技术的剩余障碍是缺乏用于在时域中解决大规模弹性动力学问题的实用数值工具以及需要增加实验中使用的频率。由于实际无损检测问题需要解决的大规模问题的自由度的空间数量为O（10^4或更大，人们必须使用快速BIEM，而不是传统的方法，使用边界类型的方法。有鉴于此，我们研究了快速多极方法的使用，这被认为是实用的三维弹性动力学的时域。我们已经建立了一个快速解决这个问题后，并行化的代码。在实验中，我们已经开发了一个激光超声无损检测系统，它使用更高的频率比我们以前的研究。我们已经结合这些技术来调查的NDT系统，有潜力在真实的世界的问题。我们还考虑了使用YAG激光诱导超声照射未知的缺陷。作为本研究的结果，我们可以提到以下几点：我们已经建立了一个简单的方法来识别YAG激光产生的超声。我们已经看到，使用这样识别的入射波可以精确地确定表面裂纹。此外，在使用FMM的快速方法中，我们可以使用并行化的FMM代码和向下路径的改进算法来解决具有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.