A unified model of ductile fracture considering strain rate and temperature under the complex stress states

Abstract

In the thermoforming process of complex components, such as the multi-directional loading forming of multi-cavity parts, complex stress states always exist and change. Meanwhile ductile fracture is prone to occurring, which is sensitive to stress state, as well as deformation temperature and strain rate. This makes fracture behavior very complex and prediction for it difficult. In this paper, a new type of fracture test device was developed, also a trapezoidal groove shape (TGS) specimen was ingeniously designed to change the stress state from tension-shear to pure shear and compression-shear only by increasing the groove bridge angle β. The fracture behaviors of AA7075-H112 under strain states 0.04s−1∼1s−1 and temperatures 20 °C∼450 °C were studied, and the fracture mechanism was identified as a mixture of surface shear and core tensile fracture, but an increasing β promoted the formation and proportion of shear plane and finally led to shear fracture. Based on this, a shear-dominated DF2014 model was determined as the basic framework, then the temperature and strain rate terms of Johnson-Cook fracture model were referenced and modified to be introduced, and finally a unified thermoforming fracture model under various complex stress states was established. The full set of model parameters was determined by a hybrid experimental-numerical method and the predictions of model were verified to match the experimental ones. Then the distribution of critical equivalent strains to fracture was presented in space of stress triaxiality and Lode parameter, which showed a good predictive capability over a wide stress state range with low stress triaxialities.