The Study And Application Of The Laser-ultrasonic Testing System Based On Two-wave Mixing Interferometer

The laser ultrasonic testing technology is one of the active frontier topics in the field of non-destructive testing. Laser ultrasonic testing technology is an emerging technology, using laser to generate and detect ultrasound, which is widely used in non-destructive field.Theoretically, this paper introduced the ways of ultrasonic generated by laser in thermoelastic and ablative regime, and summarized technologies of optical detection of ultrasound which base on the interference and non-interference method. This paper discussed the deficiencies of traditional laser ultrasonic system.A new laser ultrasonic testing device using two-wave mixing interferometer was proposed by the author, the wave-front of signal and reference beams are compensated by dynamic holographic gratings where two coherent optical beams interact within a photorefractive crystal. This device can eliminate the low frequency noises from turbulence, work-piece translation and mechanical noise. Emphatically analyzed the working principle of the system which using two-wave mixing interferometer, and the components of the device are described in detail. Based on what mentioned above, the traditional two-wave mixing interferometer was improved by using a linearly polarized light as the light source. Comparing with the traditional two-wave mixing interferometer, the testing system in this paper can prevent the interferences of the feedback lights and eliminate influences of optic noises, and fully utilized most of the intensity of light.In the experimental section, another system using heterodyne interferometer was built. A sample was detected by the above-mentioned two systems defectively and nondefectively. From the analysis of experiment results that error rate of system using two-wave mixing interferometer was 1.67%, while using heterodyne interferometer was 11.5%. The laser ultrasound testing system using two-wave mixing interferometer has some merits, such as high accuracy, high sensitivity and anti-noise ability. And the structure of the system is simpler, carrying conveniently, which will be the optimal device in the field of non-destructive testing in future.In this paper, the propagation characteristic of surface wave in 20# steel was studied based on the system using two-wave mixing interferometer. Because the surface wave detecting is very complex, this paper analysis the characteristics of surface wave through combination FEM numerical simulation and experimental study. Firstly, straight crack and oblique crack have been simulated, the relation between the propagation of reflection wave and the cracks different types has been obtained. From what we have done, the straight crack of different depths, and the model of same depth different detect points were built. Then, we detected the surface wave of different types cracks, different depths cracks by the laser-ultrasonic system, and we changed the position of probe laser, the experimental results are very similar to the simulation. Finally,we conclude that the amplitude of reflected wave decreases with the angle of crack increases, the cut-off frequency of transmitted wave decreases with the depth of crack increases. The relation between the position of probe laser and the time of reflected wave was obtained. We can do qualitative and quantitative test the surface defects use the laser-ultrasonic system, and can locate the defects.