Stress intensity factors for semi-elliptical surface cracks in round bars subjected to mode I (bending) and mode III (torsion) loading

Abstract

Crack growth under mixed-mode loading of rotor shafts is the reason for many failures occurring under cyclic Mode I (ΔΚ I ) combined with steady Mode III (K III )- This study presents Stress Intensity Factors calculations of semi-elliptical surface cracks in round bars subjected to bending, torsion and bending/torsion using a three- dimensional finite element model. The configuration of the semi-ellipse follows the experimentally determined equation: b=(2s)/π, where b is the crack depth and s is the semi-arc crack length. Stress Intensity Factors are obtained for bending loading and compared with available literature results in order to validate the proposed model. For Mode III, Stress Intensity Factors are obtained with the full shaft geometry due to non-symmetrical torsion loading. When the analysis is performed for combined bending and torsion, different Stress Intensity Factors are obtained at both sides where the crack front intersects the shaft surface. This result explains clearly the rotation of the crack front experimentally observed whenever a steady torsion is superimposed on the reversed or rotary bending. Results are compared with experimental crack growth measurements.