Damage-based initiation and growth of cracks in compacted graphite iron: Comparison of numerical strategies for realistic morphology

Compacted graphite iron (CGI) is widely used in automotive engines thanks to its excellent castability and thermal conductivity. Despite extensive research, the influence of its microstructure on the fracture behaviour has not been fully elucidated. In this work, four different damage models with realistic and simplified morphologies are compared. The developed models consider the effect of graphite-particle morphology and the domain’s boundary conditions. The crack path and morphology were characterised with in situ tensile tests inside a scanning electron microscope. Then, finite-element models capturing the actual microstructure morphology were generated, assuming isotropic and ductile properties for the matrix and graphite. Crack initiation was simulated employing the Johnson-Cook damage scheme and cohesive-zone elements. It was found that cracks tended to initiate at the ends of vermicular graphite particles. Besides, small matrix bridges between the neighbouring graphite inclusions facilitated the concentration of high stress, with its level increasing as the spacing decreased. Validation of simulations was based on in situ experimental data. The developed model could assist in the understanding of the mechanical and fracture behaviours of CGI.