Numerical Prediction And Characteristic Research For Residual Stress Relaxation Of Shot Peening

Shot peening process mainly introduces a certain size of the residual compressive stress field which distributes along the depth direction of the workpiece surface. The residual compressive stress field can effectively improve the fatigue resistance of materials. But the residual stress of shot peening isn’t unchanged all the time, The actual work environment of shot-peened workpieces with cyclic load, temperature changes, micro cracks and many other factors could lead to relaxation or redistribution of shot peening residual stress and reduces the improvement of material fatigue resistance consequently. Nowadays, the residual compressive stress field induced by shot peening is considered as the most important factor in many models for fatigue life prediction, so if only considering the initial residual stress field of shot peening will undoubtedly result in an error for the fatigue life prediction. With the help of software ABAQUS and the numerical simulation method, the shot peening residual stress relaxation laws under cyclic loading is studied, including the residual stress in the smooth specimens and three point bending specimens with crack, and the stable distribution of residual stress in different conditions is predicted. On the basis of it, the influence of shot peening on micro-crack closure effect is discussed. The main research contents are as follows:(1) A new numerical method to predict the residual stress static relaxation of the Smooth specimens under the cyclic loads is presented and the stable residual stress fields with the 7075 aluminum alloy specimens under different peening intensities are obtained. There is a good consistency between the simulation results and the experimental results by X-ray diffraction method. Some influence factors of residual stress relaxation is also researched with the method, including the cycle numbers of loading, different stress ratios, initial residual stress field and the equivalent plastic strain caused by shot peening.(2) Established the finite element model of three point bending with micro cracks, the method of DEBOND is adopted to simulate the crack propagation, shot peening residual stress relaxation and distribution laws are studied under the condition of the micro crack propagation. The results show that crack propagation is significant impact on the distribution of shot peening residual stress, the formation of crack tip plastic zone can enhance the residual stress, but crack tip plastic zone is also affected by the plastic strain introduced by shot peening. Based on the crack propagation, the influence of some factors is discussed, such as load intensity, initial conditions and crack length.(3) Based on the finite element model of three point bending with micro cracks, the crack closure effect is analyzed with the help of the node contact pressure. On the basis of it, the effect of shot peening residual stress on crack closure effect is studied and further reveals the mechanism of shot peening can improve the fatigue life of workpieces.