Experimental and Theoretical Study of the Stress and Strain Fields Around Fatigue Cracks and Plasticity-Induced Crack Closure

Abstract

This work investigates experimentally and theoretically the local crack tip stress and strain fields, and plasticity-induced crack closure acting on the crack flanks behind the crack tip, in fine-grained aluminium alloy 5091 (Al-Mg-Li-C-O) compact tension fatigued specimens. Compressive residual stresses ahead of the crack tip and plasticity-induced crack closure are often attributed to be the two main mechanisms responsible for crack growth retardation. Specimens investigated were prepared: with constant amplitude fatigue; with constant amplitude fatigue followed by a single overload; and finally with constant amplitude fatigue followed by a single overload followed by further constant amplitude fatigue. Experimental crack tip strain (and hence stress) measurements have been carried out along the midthickness of the specimens using energy dispersive X-ray diffraction (EDXRD) at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. The very small grain size of aluminium alloy 5091 allows minimization of the sampling volume in order to maximize the spatial resolution achievable with EDXRD. High spatial resolution is essential for measuring highly localized crack tip stresses. For theoretical investigations of the crack tip stress and strain fields, existing analytical solution (Westergaard, 1939) and the finite element code ABAQUS (ABAQUS Inc., 2004) have been used. Crack closure has been investigated by finite element analysis and compliance experimental technique. Finite element analyses for both stress fields and crack closure have been carried out in plane stress and plane strain. A possible link between the stresses ahead of the crack tip and the plasticity-induced crack closure on the crack flanks behind the crack tip has been investigated