A MULTIAXIAL HIGH CYCLE FATIGUE LIFE PREDICTION MODEL CONSIDERING THE EFFECT OF MEAN STRESS FOR TENSION-TORSION LOADINGS WITH SAME FREQUENCY

The influence of normal mean stress on fatigue life prediction has been well reflected for most of the critical plane-based high cycle fatigue life prediction models, whereas the effect of shear mean stress isn't well considered in these models. It is found that the fatigue life of 7075-T651 aluminum alloy is substantially reduced due to the existing of the shear mean stress by analyzing the experimental data of this aluminum alloy, which is similar to the effect of tensile mean stress. Therefore, nonconservative predictions maybe obtained under the loading paths with substantial shear mean stresses for these life prediction models ignoring the effect of mean shear stress. In order to estimate the fatigue life better, a new critical plane-based multiaxial high cycle fatigue life prediction model is proposed to take into account the effects of both normal and shear mean stresses. In the proposed model, the strain-based Fatemi-Socie criterion is first extended to the high cycle fatigue life prediction. And then a stress-based Fatemi-Socie criterion is developed. The shear and normal Walker factors are introduced in the developed criterion to consider the effects of shear and normal mean stresses, respectively. Both the shear and normal Walker factors vary from 0 to 1, which reflects the sensitivity of the material to shear and normal mean stresses. Procedures to determine the damage parameters acting on the critical plane and to calculate the constants contained in the proposed model are all presented. The proposed model is valid for the metallic materials with the ratio 0.5<\tau _-1 / \sigma _-1 <0.8. Comparisons between test results of 5 kinds of metallic materials and model predictions under 12 types of loading paths with different mean stress levels showed that the proposed model presents relatively accurate predictions. Most of the predictions are fell within a life factor of \pm 3.