Effect of Stress Frequency on Surface Crack Propagation

Many machine and structural materials have strain rate dependence at room temperature. In such material, fatigue strength, that is, fatigue crack initiation life and propagation rate more or less depend on stress frequency. However, in many cases, the effect of stress frequency was almost all ignored and fatigue strength was evaluated based on the results of fatigue test under a constant loading frequency. Such evaluation is likely to over/under estimate fatigue strength. So, in strain rate dependence materials, the study made clear the effect of stress frequency on surface crack propagation by the following experiments and analysis. (1) Evaluation of strain rate dependence of material. (2) Making clear the effect of loading frequency on surface crack propagation by fatigue test and observation of fracture surface by SEM. (3) 3D-elasto/visco-plastic FEM simulation of surface crack propagation. (4) Comparison of the experimental results with the calculation ones by FEM, and establishment of an evaluation's parameter describing the effect of stress frequency on surface crack propagation. The results obtained are summarized as follows; (1) In strain rate dependence materials, the surface crack propagation rate and striation space depend on stress frequency. (2) The effect of stress frequency on surface crack propagation may be explained by the equivalent elasto/visco-plastic strain range at crack front based on the visco-plasticity of the materials.