13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Computational prediction of fracture toughness of polycrystalline metals Yan Li, David L. McDowell and Min Zhou* The George W. Woodruff School of Mechanical Engineering, School of Materials Science and Engineering Georgia Institute of Technology, Atlanta, GA 30332-0405, U.S.A. * Corresponding author: min.zhou@me.gatech.edu Abstract A three-dimensional multiscale computational framework based on the cohesive finite element method (CFEM) is developed to establish relations between microstructure and the fracture toughness of ductile polycrystalline materials. This framework provides a means for evaluating fracture toughness through explicit simulation of fracture processes involving arbitrary crack paths, including crack-tip microcracking and branching. Fracture toughness is computed for heterogeneous microstructures using the J-integral, accounting for the effects of grain size, texture, and competing fracture mechanisms. A rate-dependent, finite strain, crystal plasticity constitutive model is used to represent the behavior of the bulk material. Cohesive elements are embedded both within the grains and along the grain boundaries to resolve material separation processes. Initial anisotropy due to crystallographic texture has a strong influence on inelastic crack tip deformation and fracture toughness. Parametric studies are performed to study the effect of different cohesive model parameters, such as interface strength and cohesive energy, on the competition between transgranular and intergranular fracture. The two primary fracture mechanisms are studied in terms of microstructure characteristics, constituent properties and deformation behaviors. The methodology is useful both for the selection of materials and the design of new materials with tailored properties. Keywords cohesive finite element method, crystal plasticity, microstructure-fracture toughness, multiscale framework 1. Introduction Microstructural design is an important approach for enhancing material properties such as fracture toughness for many industries, including aerospace and automotive engineering. It is of great importance to quantify how an advancing crack interacts with the microstructure at multiple length scales and how microstructure determines a material’s fracture toughness through the activation of different fracture mechanisms. A lot of efforts have been made to develop 2D simulation methods to study the crack initiation and propagation in brittle materials (cf. Xu and Needleman [1], Zhai et al. [2] and Li and Zhou [3, 4]) and metallic polycrystalline materials (cf. Guo et al. [5] and Hao et al. [6]). However, fracture is inherently a 3D problem. Most of these 2D models, which assume plane strain conditions, cannot capture the 3D morphologies and orientation of grains, nor do they track crack-material interactions due to non-planar crack extension. This paper aims at expanding the current 2D capabilities to 3D by considering realistic microstructures and incorporating crystalline plasticity in constitutive response modeling. Both intergranular and transgranular fracture modes are
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