Laser Scanning Source Method To Stimulate The Far-field And Near Field Of Surface Acoustic Wave For The Experimental Study Of Defect Detection

On the base of the principles of laser ultrasound which is generated by laser under the thermoelastic mechanism, the thesis utilizes the trait of transmit of laser ultrasound during its transmitting, then analyses the interaction between laser generated surface acoustic wave and small surface notch on the base of experiments, providing the theoretical basis of applying surface acoustic wave to test surface defects.Comparing a variety of ultrasound -testing methods. The experiments choose the piezoelectric film— PVDF sensor to receive the surface acoustic wave and. The configuration of this system is simple, and easy to adjust. So it can be used in applying surface acoustic wave to test surface defects. The experiments utilize the technology of scanning laser line source (SLLS) and analyses the interaction between laser generated surface acoustic wave and small surface notch when the line source in different position. From the experiments some very important information are got.In the process of experiments, the laser inspires surface acoustic wave in four areas, which are far field, close field, surface breaking crack, and passed breaking crack. The test reveals the change of surface acoustic wave, when laser gets close to the surface defects. The existence of surface defects results in that the reflected surface acoustic wave gradually gets close to the directly arriving surface acoustic wave. And finally they overlap completely. The surface acoustic wave change from unipolar at the beginning to bipolar after the superposition, at the same time, the highest frequency part of spectral graph sort of increased.The experimental device and the theoretical analysis of wave in the thesis effectively reflect that, the surface defects will influence the surface acoustic wave. So it provided the experimental basis of testing surface defects.