Stress intensity factors for two critical angular positions of semi-elliptical surface cracks in shafts

Many power transmission devices run under load bending combined with steady torsion. After crack initiation the crack front profile usually grows with a semielliptical surface shape. During the rotation of a shaft, the crack growth of a semi-elliptical surface crack depends on the two crack angular positions: one is when the load bending direction is perpendicular to semi-elliptical crack front profile; another one is an oblique position whenever one of two external points are on the load bending direction. The present study is based on 3D finite elements in order to obtain Stress Intensity Factors (SIF) for semi-elliptical surface cracks, taking into account the two critical angular positions of the crack front when a round bar or a shaft rotate in operating conditions. Results have shown that SIF present the maximum values for these two critical positions and at each rotation. On the other hand there is an increasing or a decreasing on KI values due to the effect of the torsion when it is superimposed to bending, depending on the direction of the applied torsion. When the analysis is performed for combined bending and torsion, different SIF are obtained at both sides where the crack front intersects the free shaft surface. This result explains clearly the rotation of the crack front experimentally observed whenever a steady torsion is superimposed to the reversed or rotary bending.