A Study of Indentation Cracking in Brittle Materials Using Cohesive Zone Finite Elements

Cohesive zone finite element simulations of pyramidal indentation cracking in brittle materials have been carried out in order to: (1) critically examine indentation cracking models that relate fracture toughness to indentation data; (2) determine the underlying physical mechanisms of indentation crack growth from a continuum view and their relationship to material properties; (3) explore the influence of indenter geometry on crack extension; and (4) provide a platform from which future simulations can add more complex material behavior as well as guidance for experimental measurements of fracture toughness. Standard fracture toughness geometries in addition to simplified indentation geometries were simulated in order to assess the advantages and limitations of using cohesive zone finite element simulations to study indentation cracking in brittle materials. Simulation results were found to be consistent with linear-elastic fracture mechanics when crack lengths approximately 10 times larger than process zone sizes. Results from Vickers indentation cracking simulations showed deviations from standard models and additional material dependencies not considered in therein. A transition in cracking behavior from median type cracks to Palmqvist type cracks was observed as the ratio of elastic modulus to hardness increased and plasticity played a more prominent role in the deformation response. Separate stress intensity factor solutions were derived for the two cracking regimes by applying simple scaling relationships and observations from the finite elements. Simulations of different indenter geometries were found to correlate well with the stress intensity factors. In addition, the indentation cracking response could be tailored to a specific behavior by changing the indenter centerline-to-face angle. Cohesive zone finite element simulations were found to be well suited to exploring, improving, and studying the materials science of indentation cracking.