Laser-generated Surface Acoustic Waves Research And Its Application To Micro Surface Crack Detection

In the industries such as precision machining, thin film production, microsystems, microelectronics manufacturing and so on, the sprouting and growth of micro surface cracks can greatly affect the performance of materials and the final products. So, the evaluation of surface cracks by Non-Destructive Testing is a very meaningful work. As an emerging ultrasonic testing technology, Laser-Generated Surface Acoustic Wave technique(LSAW) has shown unique advantages in the detection and analysis of the surface features of samples. This dissertation starts from the study of the propagation theory of surface acoustic waves, and its investigation by using the finite element method to simulate the wave propagation in a sample with surface micro cracks. For this purpose, a SAW detection system based on an interferometer with differential detection structure was designed and built, resulting in the precise measurement of micro cracks in both single medium and layered media. The following works are completed in this dissertation:1. Based on the elasticity theory, the wave propagation theory, the frequency-domain signal analysis, and the derivation of the propagation equations in isotropy/anisotropy single medium with different boundary conditions was completed. Moreover, the calculation model in film and layered media material were also derived. The influence of the surface density and Young’s modulus of the material to the dispersion characteristics of waves was studied.2. A finite element simulation model of the Laser-Generated Surface Acoustic Wave propagation was developed, whereby the influence of the laser parameters like laser beam radius, absorption and scattering properties to the excitation waves was analyzed. The SAW envelope and bandwidth mathematical model in the displacement function was redefined to show more realistic propagation in metallic samples with a single crack and multiple cracks, seperately. The simulation results were calculated by programming, and the frequency domain curves were plotted. The results provide guidance for practical experiments, and cross-validation with experimental results.3. The SAW detection system based on the interferometer with differential detection structure was designed and built. Improved from the classical Michelson interferometer, the system can accurately detect weak SAW signals. The detection principle of the system was discussed, and the technical points of the system set-up and the adjustment and calibration of optical path were shown. This new detection system improves the signal to noise ratio and the stability of the entire measurement system, thereby increasing the measuring resolution of the detection of surface micro cracks.4. Applying the experimental LSAW detection system to measure a single crack and multiple cracks on the surface of metallic and non-metallic materials, based on the change of peak to peak of SAW, analysis of the crack parameters like position and width. Proposed dual-beam scanning method to detect the morphology of microcrack. Detection of crack signals and corresponding analysis were conducted in a thin film material. Detection of micro cracks on a layered material, and calculation of its Young’s modulus were also accomplished. Comparing with the results measured by a commercial profiler, the new system was proved with a high reliability.