Fatigue and Fracture behaviour of AZ31b Mg alloy plastically deformed by Constrained Groove Pressing in the Presence of Overloads

Within the class of lightweight metallic materials, magnesium alloys are gaining popularity mainly thanks to abundant supply and high specific strength. A weakness of Mg alloys is their poor formability at room temperature. For this reason, in recent years thermo-mechanical treatments have often been sought that would improve this aspect. One particular way towards the combination of better formability (ductility) and strength is through grain refinement by means of Constrained Groove Pressing (CGP). The use of a pair of matching dice having groove-like geometry, coupled with controlled process temperature, allows promoting microstructural refinement and achieving sub-micron grain size in the processed plates. The purpose of the present study was to characterise the resulting fine-grained material in plate form in terms its fatigue fracture behaviour. Compact tension samples were machined and subjected to cyclic tensile loading to monitor fracture propagation and extract their Fatigue Crack Growth Rate (FCGR) behaviour as a function of the applied crack tip Stress Intensity Factor range. Firstly, as a reference sample an as-received material plate was tested at the loading ratio R=0.1. Subsequently, CGP-processed fine-grained material was tested at R=0.1 and R=0.7, and subjected to anomalous load that included overload (50% increase in the maximum load) and, in the specific case of R=0.1, underload (reversal of the sign of minimum load from tension to compression). These studies provided the critically needed input for the development of new approaches to the evaluation of the apparent fracture resistance of the material processed by CGP under variable amplitude loading (i.e. overload and underload) essential for accurate fatigue life prediction for a broad variety of applications. By combining the Walker model accounting for the mean stress effect and a modified Wheeler model for crack growth retardation due to the application of a single overload, a new predictive approach was formulated.