Residual Stresses And Three Dimensional Stress Intensity Factors Of Subsurface Cracks For Shot-peened-materials

As the most useful method to improve the surface conditions andincrease the fatigue life for materials and structures, shot-peening has beenwidely used in the design of aerospace and automobile components. Themain mechanism of shot-peening is the introduction of residual compressivestress field, which can change the crack initiation locations and decrease thecrack propagation rate. Thus, quantitative study of the residual stresses andthe stress intensity factors after shot-peening has been an important subjectin theoretical and practical field.Firstly, with the comparison between the predictions and test results ofvarious shot-peened metals, the capability of five solutions includingempirical equations, theoretical models and FEM models for predicting thecharacteristic parameters of residual stress field was proved. The empiricalequations of Yao had good predictions for40Cr steel and other similarsteels，and the empirical equations of Wang were capable of various metalswhile theoretical models were suitable for metals with medium strength andhardening rate. The predicting results of single-shot FEM models forcalculating the peening dents diameters and multi-shot FEM models forcalculating the residual stress field were in good agreement with test resultsof various metals with different shot-peening coverage. In practice，combinations of these methods were needed.Secondly, to study the behavior and the three dimensional stressintensity factors of subsurface cracks, the crack body was considered aselliptical crack and decomposed into two kinds of slips, that is the fundamental slips with multi-collinear cracks features and the elastic-springslips with eccentric cracks features. Based on the three dimensional weightfunction theory, the three dimensional weight functions of the subsurfaceelliptical cracks were achieved.Finally, on the basis of the three dimensional subsurface weightfunctions, the stress intensity factors of the40Cr steel after shot-peeningwere achieved. The results showed that the stress intensity factor ofsubsurface cracks under the residual stress was less than that of surfacecracks. Thus，the effect on decreasing the propagation of surface cracksafter shot-peening was better than that of the subsurface cracks. And thefront which was closer to the limited boundary was more sensitive to thedistance from the boundary and loads than remote front of the subsurfacecracks.