Comparison of Various Damage Models in Modelling 3D Crack Propagation

It is of interest to model crack propagation in irradiated Zr-2.5Nb nuclear pressure tubes and X70 pipeline steel. These materials can undergo a range of conditions leading to fracture with operating temperatures between room temperature and 300 °C for Zr-2.5Nb and strain-rates ranging from quasi-static to dynamic in the case of pipeline steel. In the case of the hexagonal closed-packed zirconium alloy, the influence of plastic anisotropy is also of interest. When trying to capture the fracture response under a wide variety of conditions, limitations of traditional Linear Elastic and Elastic Plastic Fracture Mechanics become apparent such as trying to capture effects of crack tunnelling, the transition from flat-to-slant fracture, and anisotropy. Various damage mechanics based approaches to model 3D crack propagation will be presented and discussed including the crack tip opening angle, a Gurson-Tvergaard-Needleman type damage model based on void nucleation, growth, and coalescence, and the Xue and Wierzbicki model based on the relationship between failure strain with stress triaxiliaty and lode angle dependence. A non-local damage scheme, which mitigates the mesh-dependence of results, will also be presented. For Zr-2.5Nb pressure tube, simulations were performed using Hill’48 and Cazacu-Barlat-Plunkett 2006 anisotropy yield functions. Experimental data from compact tension and rising pressure burst tests on irradiated Zr-2.5Nb pressure tube and both static and dynamic drop weight tear tests on X70 pipeline steel will be compared to predictions from finite element simulations. It is shown that simulations using the various damage models can capture the measured crack propagation behaviour, including crack tunnelling, to varying degrees of accuracy. The Xue and Wierzbicki fracture model was shown to capture the transition from a flat, tunnelling (ductile fracture) crack to a slanted (shear fracture) crack during propagation that was observed in both Zr-2.5Nb and pipeline steel.