Fatigue properties of shot-peened Ti-6Al-4V alloy at elevated temperatures
Kato Yozo; Takafuji Shinzaburo
Abstract:The effect of shot peening on the fatigue strength of Ti-6Al-4V alloy was investigated in the temperature range of 20 degree to 450 degree. 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 root area parameter model which could predict fatigue strength using two parameters of defect size ( root area) and Vicker's hardness (HV), and the effect of static mean stress (sm) could also be considered in the model. Shot peening increased fatigue strength at 20 degree - 350 degree, but decreased at 450 degree. The reduction at 450 degree was due to surface roughness because compressive residual stress was decreased during fatigue loading. At 450 degree, by comparing the fatigue strength of the rough surface specimens with that of the surface polished specimens, the defect size ( root area) equivalent to the surface roughness was evaluated to be approximately 76 mm. At 20 degree - 350 degree, 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 76 mm. The compressive residual stress predicted by the model showed reasonable values quantitatively. Drilling holes equivalent to root area of 93 and 185 mm decreased fatigue strength markedly. The holes, however, were crushed by shot peening. For example, root area of 185 mm was decreased down to about 55 mm. 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 76 mm. Key Words:fatigue strength, Ti-6Al-4V alloy, shot peening, elevated temperature, root area parameter model