Laser Array Sources For Finite Element Simulation Of Metal Defect Detection

In this paper, the finite element method (FEM) has been applied to simulate the physical process, which includes the generation of the surface acoustic wave (SAW) and the interactions of the SAW with the surface breaking crack and the subsuface defect, while the single laser line source and the laser array source irradiate on the aluminum plates respectively. Based on this, the thermoelastic laser array has been employed to detect the internal flaws in the aluminum material.From the excitation mechanisms of the laser ultrasonics, the property and the detection methods common used in laser-generated SAW are introduced successively. Subsequently, three main analytical models of the laser-generated ultrasonics are given, constructed by the point source, line source and laser array irradiating on the sample respectivily. The features of each model are also analyzed. All these works are preparations for the further theory study.Based on the elastic theory of plane strain, in order to simulate the generation of laser-generated SAW, the finite element models of the line source and laser array source irradiating on the aluminum plate have been established. The results of the simulation show that the amplitute of the SAW increases over a single-element source (single line source) in proportion with the number of elements in laser array. And a 10-element array can obtain about 10×the amplitude. Laser array parameters influencing on the SAW have been discussed, which shows that the amplitute increases and the frequency width becomes wider as the array element half-width decreases, and as time-delays reduces a narrow bandwidth acoustic signal could be obtained.The finite element method has been employed to simulate the interactions of the laser-generated SAW with the surface defect and the subsuface defect, while the line source and laser array source irradiate respectively on the aluminum plates. The varied depth or width of the rectangular shape flaws impacting on the laser-generated SAW have been discussed. On this basis, the spread direction of the longitudinal wave is controled, by the means of the modulation of the appropriate time-delays. The laser array has been applied to detect the internal flaws, and the variation of the signal received at each inspection points have been analyzed. Briefly, the laser array is designed to improve the SNR of the laser ultrasonic systems in the thermoelastic regime, which can also achieve the sectorial scanning of the sample with the ultrasonic beams. The results of this paper can provide some hints for the utilization of the thermoelastic laser array to improve the overall signal-to-noise ratio of laser ultrasonic systems and detect the internal flaws in the material.