Nonlinear Ultrasonic Surface Wave Testing Of Residual Stress In Metal

Residual stress is generated by the uneven thermal expansion or asymmetrical plastic deformation during the metal components manufacturing process and in the work.Residual stress will make a great influence on the properties of materials and mechanical components,such as fatigue strength,deformation,dimensional stability,corrosion resistance,and brittleness.Therefore,timely and accurate detection of residual stress is significant for reducing the operational risk of equipment and ensuring the reliability of materials.Nonlinear ultrasonic testing technology is a method to evaluate material damage by using harmonic signal.Compared with the linear ultrasonic technology,the nonlinear ultrasonic testing technology is more sensitive to the micro-structural changes of the material.In this paper,the effects of different residual stresses on material nonlinearity are studied by finite element simulation method and experiment.The main work is as follows:Based on the theory of elastic wave,the basic principle of acoustic nonlinearity phenomena is analyzed.According to the nonlinear constitutive equation of stress and strain,the nonlinear ultrasonic wave equation with one-dimensional longitudinal wave is resolved by the perturbation method.Based on this,the propagation characteristic of surface wave in the solid was studied and its numerical solutions for fundamental and second harmonic are solved.The relative nonlinearity parameter of the surface wave is defined.The research makes a foundation for studying the residual stress detection by ultrasonic nonlinear surface wave.According to the theory of nonlinear ultrasound,the propagation model of nonlinear surface waves in solid medium was established.In order to study the influence of ultrasonic excitation frequency on the measured nonlinearity parameter,the nonlinear surface wave model was simulated under different ultrasonic excitation frequencies.Studies have shown that the higher the frequency of the excitation,the shallower the penetration depth of the surface wave,the more concentrated the energy on the surface and the larger the measured nonlinearity parameter,but the faster the attenuation rate and the shorter the effective detection distance.In order to study the influence of different residual stresses on the nonlinearity of the material,the residual stress distribution caused by the shot peening was applied to the model and the different residual stresses were produced into specimen by adding internal stress.The change of residual stress was characterized by the ultrasonic nonlinearity parameter.Studies have shown that the nonlinearity of the material increases monotonously with the increase of residual stress.At the same time,according to the stress testing simulation of LCR wave,the sound velocity changes of different residual stress specimens are studied.Compared with the sensitivity of the ultrasonic nonlinearity parameter,the sensitivity of ultrasonic wave velocity to residual stress is much less.Finally,7075 aluminum alloy treated with different shot peening intensity were selected as the specimens to verify the nonlinear ultrasonic testing of residual stress.The effect of coupling agent on nonlinear parameter testing was considered and the nonlinear stability of the experimental system was verified.The ultrasonic nonlinearity parameters of the specimens with different residual stresses were detected by the nonlinear ultrasonic testing system and the effects of residual stress on the ultrasonic nonlinearity parameter was studied.The experimental results show that the larger residual stress,the larger the ultrasonic nonlinearity parameter is,which is consistent with the simulation results.Simulation and experimental results show that the nonlinear ultrasonic testing technology is feasible in residual stress detection.