13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Influence of the residual stresses on the crack deflection in ceramic laminates Oldrich Sevecek1,*, Michal Kotoul1, Tomas Profant1, Raul Bermejo2 1 Institute of SMMB, Brno University of Technology, Brno 616 69, Czech Republic 2 ISFK, Montanuniversitaet Leoben, Leoben 8700, Austria * Corresponding author: sevecek@fme.vutbr.cz Abstract The main goal of ceramic laminates designed with residual stresses is to increase the fracture energy of the system during fracture through energy dissipating mechanisms such as crack deflection or crack bifurcation. A computational tool based on Finite Fracture Mechanics (FFM) is implemented in this work to predict the propagation of cracks in ceramic laminates. The crack path is defined by the direction where maximum rate of the potential energy is released during fracture. Laminates studied here consist of two materials alternated in a layered structure designed with high compressive residual stresses, which are developed during cooling phase after sintering process. Different volume ratios of the material components are chosen in order to demonstrate the influence of the level of compressive residual stresses on the type of propagation (single crack deflection / crack bifurcation) and direction of the crack advance. Using the model a single crack deflection or crack bifurcation can be predicted for a given laminate. According to the calculations, the higher compressive stress in the layer is, the more the crack deflects from the straight direction and the more prone to the bifurcation. Results are in good agreement with experimental observations. Keywords Ceramic laminates, Finite Fracture Mechanics, Crack propagation, Fracture criterion 1. Introduction Ceramic laminates have become an alternative choice for the design of structural ceramics with improved fracture toughness and mechanical reliability. The brittle fracture of monolithic ceramics has been overcome by introducing layered architectures of different kind, i.e. geometry, composition of layers, weak interfaces, strong interfaces with residual stresses, etc. The main goal of such layered ceramics has been to increase the fracture energy of the system through energy dissipating mechanisms such as crack deflection, crack bifurcation, interface delamination, or crack shielding. Among the various laminate designs reported in literature, two main approaches regarding the fracture energy of the interfaces must be highlighted. On the one hand, laminates designed with weak interfaces have been reported to yield significant enhanced failure resistance through interface delamination [1-5]; the fracture of the first layer is followed by crack propagation along the interface, the so-called “graceful failure”, preventing the material from catastrophic failure. On the other hand, laminates designed with strong interfaces have shown significant crack growth resistance (R-curve) behavior through microstructural design (e.g. grain size, layer composition) [6-8] and/or due to the presence of compressive residual stresses, acting as a barrier (“flaw tolerant”) to crack propagation [2, 9-14]. The increase in fracture energy in these laminates is associated with energy dissipating mechanisms such as crack deflection/bifurcation phenomena, which act during crack propagation. The optimization of the layered design is based on the capability of the layers to deviate the crack from straight propagation. Experimental observations have shown the tendency of a crack to propagate with an angle through the compressive layer and even cause delamination of the interface [15] (see Fig. 1). It seems that the magnitude of compressive stresses may influence the angle of propagation
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