No abstract

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.

Ceramic polycrystals are prone to slow crack growth (SCG) which is stress and environmentally assisted, similarly to observations reported for silica glasses. The kinetics of fracture are known to be dependent on the load level, the temperature and the relative humidity. In addition, evidence is available on the influence of the microstructure on the SCG rate with an increase in the crack velocity with decreasing the grain size. Crack propagation takes place beyond a load threshold, which is grain size dependent. We present a cohesive zone model for the intergranular failure process. The methodology accounts for an intrinsic opening that governs the length of the cohesive zone and allows the investigation of grain size effects. A rate and temperature-dependent cohesive model is proposed (Romero de la Osa M, Estevez R et al 2009 J. Mech. Adv. Mater. Struct. 16 623–31) to mimic the reaction–rupture mechanism. The formulation is inspired by Michalske and Freiman's picture (Michalske and Freiman 1983 J. Am. Ceram. Soc. 66 284–8) together with a recent study by Zhu et al (2005 J. Mech. Phys. Solids 53 1597–623) of the reaction–rupture mechanism. The present investigation extends a previous work (Romero de la Osa et al 2009 Int. J. Fracture 158 157–67) in which the problem is formulated. Here, we explore the influence of the microstructure in terms of grain size, their elastic properties and residual thermal stresses originating from the cooling from the sintering temperature down to ambient conditions. Their influence on SCG for static loadings is reported and the predictions compared with experimental trends. We show that the initial stress state is responsible for the grain size dependence reported experimentally for SCG. Furthermore, the account for the initial stresses enables the prediction of a load threshold below which no crack growth is observed: a crack arrest takes place when the crack path meets a region in compression.