Abstract
The objectives of the present study were: (1) to investigate the influence of phase difference between combined axial and torsional loading on the fatigue strength of steel containing small defects, and (2) to develop a unified method for fatigue strength prediction for such loading conditions. The mat e-rial investigated was an annealed 0.37% carbon steel. As an artificial defect, a drilled hole of either 100μm or 500μm in diameter was introduced into round-bar specimens. Combined axial and torsional fatigue tests were performed at R=-1 under in-phase or 90° out-of-phase loading condition. Non-propagating cracks emanating from holes were observed at fatigue limit. On the assumption that the fatigue limit is determined by the threshold condition for propagation of a small Mode I crack, a single criterion was proposed, which accounted for the determination of the fatigue strength, regardless of phase difference of combined stresses . Under out-of-phase combined loading, a crack initiated at a hole is subjected to Mode I loading as well as Mode II loading with a 90° phase difference. The threshold level was slightly influenced by the Mode II loading.
