Research Ultrasonic Residual Stress Measurement Method Considering Orthotropic Property Of Thin-walled Parts

Residual stress is not only a key factor that affects fatigue,fracture,and deformation of high-performance parts,but also a key parameter that characterizes manufacturing quality and service status.Residual stress distribution is of great significance for manufacturing processes planning and optimization,such as forming,machining and assembling.Among the residual stress measurement methods,the ultrasonic method has the advantages of rapid and non-destructive detection of residual stress,which makes it possible for residual stress measurement in the manufacturing environment.Focusing on the ultrasonic quantitative detection of residual stress,it is urgent to solve the problems such as ultrasound propagation modeling considering residual stress and in-plane orthotropy,decoupling of plane stress components.The mechanical mechanism of acoustoelastic effect and methods of residual stress measurement are mainly studied in this dissertation.To analyze acoustoelastic effect for in-plane orthotropic metal materials,a novel elastic constitutive model is proposed.Based on the strain energy density theory,the strain energy polynomial related to the residual stress is obtained,and the relationship between the polynomial coefficient and the second and third-order elastic constants is analyzed.In-plane Orthotropic Factors(IOFs)are proposed,and a strain energy density function modified by the in-plane orthotropic factors(IOFs)is established.The strain energy polynomials related to the IOFs and the residual stress tensor are introduced into Murnaghan's strain energy density function as correction terms,and hence,an elastic constitutive model considering residual stresses and in-plane orthotropic property is established.To simulate and analyze the influence of residual stress on the ultrasonic propagation process and ultrasonic sound velocity,an approach to simulate elastic wave in metal material is proposed.A middle node is introduced into the time element to interpolate the acceleration quadratically.The shape functions of displacements,velocities and accelerations in the time element are constructed.The integration scheme ISQA-1 based on the quadratic acceleration is established.The acoustoelastic effects of transverse waves,longitudinal waves and critical refracted longitudinal(LCR)waves are simulated precisely.The problem of period enlongation and calculation error caused by Newmark integration scheme is solved.To decouple plane residual stress components in thin-walled parts with ultrasonic velocities,the relation between ultrasonic velocities and plane residual stress components is established.According to the proposed elastic constitutive model,the characteristic equation relating to IOFs and residual stress is obtained.Hence,a relation model between LCR wave velocity and plane residual stress,and a relation model between shear wave,longitudinal wave velocity and plane residual stress are established.A wave velocity calculation method based on time delay estimation is proposed.The transfer function of the time delay system is solved by the variation principle accurately.Finally,the velocity-plane-residual-stress models are verified experimentally.To ensure the reliability and accuracy of ultrasonic stress measurement,two ultrasonic probes are designed to generate and receive proper ultrasonic waves.For the method of LCR waves,a filling coupling technique is proposed,and the waveform distortion and attenuation of the received signal are suppressed.For the method of transverse and longitudinal waves,an enhancement technology of electromagnetic ultrasonic transducer(EMAT)with a permanent magnetic array is proposed.The enhanced EMAT can generate and receive transverse and longitudinal waves simultaneously and increases the SNR of the receiving signals.Finally,taking the aluminum alloy sheet as an example,using the proposed method and the developed ultrasonic probe,the ultrasonic measurement of unidirectional tensile prestress,sheet bending residual stress,and friction stir welding residual stress can be completed with an accuracy of ±20 MPa.