A damage model for fracture prediction of superplastic aluminium composite sheet

A piece of SiC whisker-reinforced aluminium composite sheet has been deformed by using a computer-controlled superplastic bulge tester under a constant strain rate of 0.2s-1and an operation temperature at 808K. The components of fracture strain have been measured from deformed grid circles pre-printed on the surface of the composite sheets. In order to predict the fracture limit of the superplastic composite induced by the initiation and growth of damage for different strain paths, a surface continuity parameter ψs is defined to describe the damage of the material in a large plastic deformation process. The relationship between the classical damage variable D and the continuity variable ψ is derived. A continuity deterioration equation, which takes into account the stress triaxiality, has been established based on continuum damage mechanics and plasticity theory. Hence, a ductile fracture criterion is developed from the continuity deterioration equation. This fracture criterion may be used to determine the fracture strain in the whole range of possible strain ratios for a sheet metal forming process. The predicted fracture strains of the superplastic composite sheet match well with the experimental measurements.