Resistance-curve method for predicting fatigue thresholds in holed specimens under combined torsional-axial loading

The R-curve method for predicting the fatigue thresholds of notched components was applied to the fatigue thresholds of notched components under combined loading of cyclic torsion and tensioncompression. The prediction was compared with the experimental data obtained from thin-walled tubular specimen with a hole under the combination of cyclic torsion and axial loading. The effect of the hole size on the fatigue thresholds was calculated from the A-curve method for three cases : cyclic torsional stress, cyclic torsional stress with superposition of a in-phase cyclic tension-compression stress with the same amplitude, cyclic axial tension-compression stress. The stress ratio was K = -1. The experimental data agreed well with the prediction both for crack initiation and fracture. The measured length of nonpropagating cracks had some scatter and the maximum length agreed fairly well with the predicted line. The nonpropagating crack length normalized with the hole radius at the threshold stress for fracture was predicted fairly constant without respect to the hole size, while it varied slightly with the loading condition. The effect of the in-phase combination of axial and torsional stress loadings on the fatigue threshold was predicted by assuming the crack direction perpendicular to the maximum principal stress. A good agreement between predicted and experimental results was obtained.