Interactions of fatigue cracks with elastic obstacles

To quantify the growth behaviour of fatigue cracks growing towards microstr uctural barriers or elastic obstacles, parametric solutions are obtained fo r crack-tip opening displacement and plasticity-induced crack closure of a mode I fatigue crack growing towards elastic obstacles. Three common bi-mat erial systems are analysed using the finite element method, in which both c onstituent materials have identical elastic properties but only the phase t hat contains the crack can deform plastically. It has been found that under monotonic loading the crack-tip opening displacement decreases as the crac k-tip approaches the interface boundary, but reaching a non-zero value when the crack-tip terminates at the boundary. For a fatigue crack growing unde r constant amplitude loading, the crack-closure stress has been found to in crease as the crack grows towards the barrier. Based on these results a mec hanistic model is proposed to quantify the influence of stress level on the fatigue threshold of microstructurally small fatigue cracks, with predicti ons being in close agreement with experimental data.