Factors influencing shape instability life in steel 40Cr

Shape instability life, a new standard to evaluate the capacity of materials working at high stress instead of the failure fracture life in the mechanical elements design, is defined as the cyclic number where the plastic strain in specimen reaches 0·2% in the fatigue process. The stress controlled experiments are performed to research the factors influencing shape instability life of steel 40Cr, including stress ratio, maximum stress, load rate, peak maintaining time and load sequence. According to the experimental results, the maximum stress and the stress ratio both show linear-logarithmic relations with the shape instability life. However, load rate and peak maintaining time have no obvious effect on shape instability life. Stress sequence tests indicated that the damage accumulation corresponds with the Miner rule in the shape instability stage. The research will be the foundation for fatigue strength design of disposable mechanical elements.