Abstract
Fatigue tests of crack propagation from a pre crack in thin walled tubular specimens made of a medium carbon steel were performed under cyclic torsion with and without superposed static tension. The condition of the transition from shear to tensile mode and the propagation behavior during this transition were investigated from the viewpoint of fracture mechanics. The direction of fatigue crack propagation follows the direction of the maximum of the total range of the tangential stress, Δσθ max near the crack tip and then gradually changes to the direction perpendicular to the maximum of the total range of the principal nominal stress. The propagation rate is faster than the uniaxial data when compared at the onset of crack extension and at high stress intensity factors. The former acceleration is caused by crack closure and the latter acceleration is due to the non singular T stress. The negative nonsingular stress induces excessive plasticity ahead of the fatigue crack, and then accelerates the fatigue crack.
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