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

点击翻译按钮获取中文摘要

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.