A model for ductile damage prediction at low stress triaxialities incorporating void shape change and void rotation

Ductile fracture at the high triaxiality regime is well-known to be controlled by void nucleation, growth and coalescence. However, under low stress triaxiality conditions and general three dimensional finite deformations, damage is still poorly predicted due to the complex loading state and microstructural changes under such a condition. Experimental results have revealed not only void growth, but also important void shape change and void rotation under shear-dominated loading. The ability of ductile damage models to predict both void growth with shape change and void rotation is thus crucial for complex loading applications. In the present study, a Gurson-like nonlinear homogenization-based model (namely GVAR) is proposed and compared with the constitutive models for elasto-plastic porous materials developed in Kailasam and Ponte Castañeda (1998) (VAR model) and Danas and Aravas (2012) (MVAR model). The proposed model is based on ad hoc modifications of the VAR model, to give sufficiently accurate results for void growth at both low and high stress triaxialities and keeping the functional form of the original Gurson model. The VAR and MVAR models were based on rigorous linear comparison composite (LCC) homogenization methods, which can describe the evolution of microstructure of porous materials, represented by the void volume fraction, the aspect ratios and the orientations of general ellipsoidal voids. The proposed GVAR model thus inherits these characteristics and provides a sufficiently accurate void growth formulation (and simple at the same time). In addition, the loading direction is not necessary aligned with the ellipsoidal void axes. These models are implemented in an object-oriented finite element (FE) code. The identification of model parameters and the assessment of the proposed model are then carried out via 3D periodic unit-cell computations subjected to different stress states. Comparative results show that the present model predicts relatively accurately the evolution of void volume fraction, void aspect ratios and void rotation for different initial void shapes, void volume fractions and under different stress triaxiality levels. A qualitative application to a tensile test on a notched round bar shows the efficiency of the model to predict microstructure evolution (i.e. voids volume, shape and orientation) in a real-scale model simulation. This model with few parameters to be identified is thus promising to predict damage under complex loading paths and ready to be applied to complex FE simulations.