Study On Nondestructive Evaluation Of Residual Stress On Welded Metal By Laser Ultrasonic Technology

This paper present a new method based on the measurement of velocity of laser-generated ultrasound to determinate the SOEC and TOEC, after that, both the numerical simulation and experiments are carried out to obtain the residual stresses distribution on the welded plates.The process form melting to forming after cooling of weld area is been simulated by finite element method (FEM). A very detailed and localized three-dimensional transient thermal model is initially established, which simulates the mechanisms of melting formation, calculates the temperature distribution in the local weld area from the beginning of welding to the final cooling. Subsequently, using the preceding welding model, thermo-mechanical analysis of the welding process is performed, from which residual stresses distribution around the joint are calculated.Experiments based on laser-generated surface acoustic wave (SAW) method were carried out to determine the velocity distribution of surface acoustic wave (SAW) around the joint. The waves are detected by self-made PZT transducer and the generated laser and sample is moved on the2D directions, which realize the2D scanning on the welding line area.According to the Christoffel theory of elasticity, a measurement system based on laser-generated ultrasound is set up to calculate the elastic constants of metal sample. A serial of ultrasonic pulses is detected by self-made PZT transducer fixed in the detection point when scanning the line source focused from Nd:YAG laser with electronic control translation stage. Then the velocity of Rayleigh waves, longitudinal and shear wave are obtained by different traveling time with their propagation distance. At the end, the SOEC and density of the metal are determinate.A new method of measurement of TOEC is proposed through equivalent micro strain which caused by linear thermal expansion. The isotropous sample is placed in steady stress field, then the experiment system is built up to measure the velocity of Rayleigh wave, longitudinal and shear wave accurately. At the same time, the thermal machine analysis (TMA) test between stress and strain is carried out on the sample in order to get the linear thermal expansion coefficient (LTEC). Finally, according the Rayleigh Equation as well as the relation between equivalent elastic coefficients and the velocity of ultrasound, the value of TOEC are calculated.The velocity of SAW on different position near welding line then can be calculated by waveform related algorithm, from which the residual stresses distribution are calculated due to acoustic-elastic theory. By comparing the thermal-structure model answers with the measurement results, it is found that the numerical simulation results are in good agreement with the experimental data.The results in this paper will provide the theoretical and experimental evidence for the determination of elastic constants and residual stresses on metal by laser-generated ultrasound; it’s also helpful for the further development and application of nondestructive testing and evaluation (NDT&E) using laser ultrasonic technology.