Study Of Test On Thickness Of Material Based On Laser Ultrasonic

The thesis is focused on laser-ultrasonic detection as part of the project supported by the Natural Science Foundation of China (No:51175466).The main work is to investigate laser pulses excited ultrasonic longitudinal wave in thermno-elastic metal material and its application for non-destructive testing of the thickness of material. On the basis of analytical model of thermal laser-ultrasonic mechanism, the ultrasonic longitudinal wave signal were obtained by finite element modeling. The obtained signals were processed using wavelet denoising method and envelope curve algorithm to detect the thickness.A measurement system was proposed with experimental verifications.In chapter one,an intensive literature review was conducted on the background and significance of the development of seamless steel pipe wall thickness testing by using laser-ultrasound. The laser-ultrasonic excitation mechanism, method and technology of detection and its application in non-destructive testing were detailed. The development of ultrasonic testing of seamless steel tubes around the world was reviewed and summarized. Then the thesis framework were given based on the analysis of the literature review and current progress.In chapter two,a one-point source analytical model was proposed based on the study of the characteristics and the basic principle of ultrasonic longitudinal wave and the excitation mechanism of laser-ultrasound.In chapter three,based on the heat conduction theory, the governing equations of laser-ultrasonic excitation were given and solved by finite element method. The simulation of pulsed laser to excite the ultrasonic longitudinal wave was performed on a commercial finite element software ANSYS.The transient temperature field and stress field were obtained as well as the ultrasonic longitudinal wave signals in different materials.In chapter four,a new envelope-based algorithm was proposed based on wavelet transform to denoise the ultrasonic longitudinal wave signal to solve the TOF(time of flight) problem.In chapter five,a laser-ultrasonic detection system including detailed completion of the hardware and software part was designed. Verification experiments were conducted to detect and analyze the plate thickness.In chapter six,the conclusions of the thesis were made as well as the suggestions for further study.