Cohesive zone model and slow crack growth in ceramic polycrystals

Ceramics polycrystals are subjected to slow crack growth (SCG) and also environmentally assisted failure, similarly to what is observed for glasses. The kinetics of fracture are known to be dependent on the load level, the temperature and also on the Relative Humidity (RH). However, evidences are available on the influence of the microstructure on the SCG rate with a marked increase in the resistance to the crack advance when comparing the response of a single crystal to that of a polycrystal. The clarification of the origin of the latter observation motivates the development of a local approach of fracture. We propose a physically based cohesive zone description that mimics the reaction-rupture process underlying SCG. We present how the parameters involved in the cohesive zone formulation can be determined to capture SCG in a single crystal. We then present simulations of intergranular failure in a 2D plane strain polycrystal. The triple junctions are shown to be key in the resistance to crack growth. The presence of defects at these junctions and their influence on the crack propagation is then investigated. We also show that the description is able to capture the damage induced by the process cooling, thus providing insight of the appropriate temperature for the sintering, for instance. The present numerical tool is then thought valuable to perform virtual tests on ceramics fracture in order to estimate thermal as well as mechanical loads limits for a safe use as well as proper the process conditions.