A discrete lattice model of quasi-brittle fracture in porous graphite

Lattice models allow the incorporation of length scale dependent micro-structural features and damage mechanisms into analyses of the mechanical behaviour of materials. We describe our 3D lattice implementation and its use in fracture simulations. The method is particularly suitable for modelling fracture of nuclear graphite. This is a quasi-brittle material in which there is considerable non-linearity prior to final fracture due to the inherent porosity which triggers a field of local distributed failures upon mechanical and thermal loading. Microstructure representative models are generated with experimentally measured particle and pore size distributions and volume densities in two graphite grades. The results illustrate the effect of distributed porosity on the emerging stress-strain response and damage evolution. It is shown how the failure mode shifts from graceful, plastic-like, behaviour associated with substantial energy dissipation via distributed damage at lower porosities, to glass-like behaviour with negligible energy dissipation at higher porosities. Thus the work proposes a microstructure-informed methodology for integrity assessment of aging structures, where porosity increase is driven by environmental factors, such as radiation of nuclear graphite components.