The Theory And Application Of Ultrasonic Nondestructive Detection Of Light-induced Sound Waves

Laser ultrasonic technique is a very active new testing method in the non-destructive testing (NDT) field. .In the dissertation, the principle and the advantages of the optical NDT are discussed in detail. Confocal Fabry-Perot interferometer(CFPI) is employed to receive and extract the laser ultrasonic signal .On the basis of discussing the principles of detecting ultrasonic signal with CFPI , two new methods of laser ultrasonic NDT, the fiber-optical Fabry-Perot interferometer and active interferometer are theoretical designed and experimental studied respectively.In the dissertation, the mechanism of ultrasonic generated by laser pulse are discussed systematically. The law of finite element is utilized to carry on the theoretical research to the laser ultrasound in the tabular aluminium. The ultrasonic signal excited by pulse laser which could be either point source or line source was analyzed. According to acoustic wave equation, the property and propagation of surface acoustic wave(SAW) was described. The influences of the characteristics of pulsed laser source, detecting distance, detecting method and the characteristics of the specimen on the results were detailed analyzed. In conclusion, the ultrasound excited by a laser line source has much more advantages than by a laser point source; the output of signal intensity would increase with decrease of the angle between laser propagating direction and the normal of the scattering surface and the one between scattering direction and the normal; the SNR would increase by reducing CFPI's bandwidth, but increasing cavity's length and reflectivity.Different kinds of applications on laser ultrasonic NDT were demonstrated, including elastic constants measurement, defect recognizing and composite performance testing. A nonconductive method of metallic elastic constants measurement—to combine longitudinal waves with Rayleigh waves—has been verified through comparing my theoretically results with experimental published results . The ultrasound generated by pulse laser has been modeled by the plane strain finite element method. The interaction process between SAW and surface defects with varied structure has been analyzed.