Detection And Imaging Of Metal Internal Porous Defects Based On Laser Ultrasound

Plate structure is a common structure in machinery.Because of its thin volume,once there is damage,the safety performance of the whole machinery will be greatly affected.Therefore,how to effectively detect the existence of defects is extremely important.When ultrasonic body waves propagate in the plate,they interfere with each other greatly.So the longitudinal wave with less interference is selected as the main research object.The 45 steel plates with pore internal defects were tested by laser ultrasonic detection technology.Two kinds of models with and without defects were established by using finite element method,and the propagation process of ultrasonic body wave was numerically simulated.At the same time,the influence of defects on the propagation process of waveform was analyzed by comparing the experimental results,and the propagation path of waveform was summarized.In order to realize the quantitative analysis of the defects,the variation law of the different eigenvalues of the longitudinal wave in the time domain and frequency domain is analyzed from the aspects of simulation and experiment.The curve fitting method was used to fit the variation rules of each characteristic value,and the quantitative results were inversed by using the experimental data.The analysis showed that the error of defect diameter could be controlled within 5% by using the P-wave center energy,P-wave amplitude and frequency-domain center power.Combined with the relationship between the wave propagation and the arrival time of the wave in the scanning process,the defect boundary location algorithm was deduced by using the idea of the transit time method.Based on the simulation data,the algorithm is implemented,and it is found that the algorithm can only describe the upper and lower sections of the defect very well,so it is necessary to supplement the left and right boundaries by means of fitting.Finally,on the basis of depicting the boundary of the defect,the three-dimensional imaging of the defect was completed by ray projection method.