Effect of Mean Stress on Low-Cycle Fatigue Strength of a Medium Carbon Steel S45C -True Stress-Controlled Cycling-

Nominal stress-controlled low-cycle fatigue life under imposed mean stress is affected by elongation or contraction due to cyclic stress-induced creep. Therefore, the effect of mean stress was examined by a microcomputer-assisted testing device which provides true stress controlled loading. Through investigation of a medium carbon steel (S 45 C), the following results, were obtained:(1) Cyclic plastic strain amplitude at the steady cyclic stress-strain state decreased with an increase of the imposed tensile mean stress. (2) The cyclic stress-induced creep proceeded so far as the mean stress was imposed. The increase in axial cyclic creep strain per cycle was proportional to the nth power of the maximum cyclic stress. (3) For the same cyclic stress amplitude, the application of tensile or compressive mean stress prolonged the fatigue life as compared with the fully reversed stressing. (4) A modified Manson-Coffin relation including such two parameters as the cyclic stress amplitude and the mean stress was proposed for evaluating the fatigue strength under cyclic stress-induced creep condition.