Fatigue crack propagation from a hole in tubular specimens under axial and torsional loading

Fatigue tests of crack propagation from a circular notch in thin-walled tubular specimens made of low-carbon steel were performed under cyclic uniaxial tension-compression, and combined loading of cyclic torsion with and without superposed static and cyclic axial loading. The propagation path of fatigue cracks followed the plane on which the total range of the normal stress including the compressive component of the stress was maximum for the cases of combined mode loading as well as uniaxial loading. For all cases examined, when compared at the same stress intensity range, the propagation rate became faster than that predicted from the crack propagation law obtained for carbon steels under uniaxial loading in small-scale yielding condition. Even when compared at the same value of the effective stress intensity range, the crack propagation rate was faster,because of excessive plasticity ahead of the fatigue crack. The J-integral range determined from the relation between load and displacement was proved to be an appropriate parameter for crack propagation with excessive plasticity. The J-integral range was correlated to the stress amplitude and the crack length for each loading condition.