NOTCH-SIZE EFFECTS IN FATIGUE BASED ON SURFACE STRAIN REDISTRIBUTION AND CRACK CLOSURE

A fracture mechanics solution for the stress range at the fatigue limit and the depth of non-propagating cracks in smooth and notched specimens is presented. The solution is based on the non-uniformity of strains at the surface of a specimen and on the development of crack closure. Surface grains oriented for easy slip experience an inherent microstructurally dependent strain concentration, which decays with depth into the material at a rate that is inversely proportional to the grain size. For cracks in smooth specimens the threshold stress range is separated into a crack opening stress component and an intrinsic stress component. The latter is the stress range that is just sufficient to grow a fully open crack and it is estimated by considering the near-surface strain concentration for a variety of crack depths. The crack opening stress range is added to the intrinsic component to obtain the nominal threshold stress range as a function of crack depth. The maximum threshold stress range defines the fatigue limit of the specimen. For notched specimens a geometrical strain concentration is considered in addition to the inherent strain concentration. The predicted fatigue limits agree well with the observed values for a variety of notch sizes and stress ratios in the aluminium alloy 2024-T351. The proposed solution also offers a prediction for the depth of non-propagating cracks in both smooth and notched specimens. In the latter, the depth of non-propagating cracks was found to be dependent on the notch radius and the stress level. The predictions of non-propagating crack depths agree with the experimental data for the aluminium alloy BSL65.