Modeling damage accumulation to fracture in a magnesium-rare earth alloy

A micromechanics-based continuum damage model is employed to rationalize key experimental trends in the fracture of a Mg-Y-RE alloy. The model has three components: (i) a phenomenological law of strain-controlled void nucleation; (ii) a void growth part that accounts for microcrack blunting; and (iii) a void coalescence part that determines fracture. Plastic flow in the undamaged state is represented by J2J2 theory with isotropic hardening. The material parameters involved in the model consist of (i) the hardening parameters, (ii) the initial values of microstructural variables associated with damage, and (iii) one adjustable parameter entering the microcrack opening rate equation. An experimental procedure for determining the material parameters is outlined. The model enables several hypotheses to be explored. It is found that when the volume fraction, aspect ratio and spatial arrangement of initially nucleated microcracks are associated with second-phase particles on the grain boundaries, the fracture strains are well predicted over a range of stress triaxialities using a single heuristic parameter that governs the rate of microcrack blunting. Implications are discussed in the broader context of high-strength materials failing after limited void growth.

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