疲労 Ｔｉ‐６Ａｌ‐４Ｖ合金ショット材の高温疲労特性

Abstract

The effect of shot peening on the fatigue strength of Ti-6Al-4V alloy was investigated in the temperature range of 20°C to 450°C. In order to evaluate separately the individual effects of compressive residual stress, strain hardening and surface roughness induced by shot peening, the results obtained were discussed by the Murakami's √area parameter model which could predict fatigue strength using two parameters of defect size (√area) and Vicker's hardness (HV), and the effect of static mean stress (σm) could also be considered in the model. Shot peening increased fatigue strength at 20°C-350°C, but decreased at 450°C. The reduction at 450°C was due to surface roughness because compressive residual stress was decreased during fatigue loading. At 450°C, by comparing the fatigue strength of the rough surface specimens with that of the surface polished specimens, the defect size (√area) equivalent to the surface roughness was evaluated to be approximately 76μm. At 20°C-350°C, the fatigue strength observed could be regarded as that obtained by adding the contribution of compressive residual stress (static mean stress) to the fatigue strength of the specimens with the defect size of 76μm. The compressive residual stress predicted by the model showed reasonable values quantitatively. Drilling holes equivalent to √area of 93 and 185μm decreased fatigue strength markedly. The holes, however, were crushed by shot peening. For example, √area of 185μm was decreased down to about 55μm. Therefore, the fatigue strength of the drilled and shot-peened specimens increased drastically and was comparable to that of the shot-peened specimens without drilling holes but with the rough surface of 76μm.