Ceramic crack deflection

A partial answer to the first question has been provided by a theoretical treatment (1,2) that examines the conditions under which a matrix crack will deflect along the iaterface betweea the matrix and the reinforcement. This fracture—mechanics analysis links the condition for crack deflection to both the relative fracture resistance of the iaterface and the bridge and to the relative elastic mismatch between the reinforcement and the matrix. The calculations iadicate that, for any elastic mismatch, iaterface failure will occur whea the fracture resistance of the bridge is at least four times greater than that of the iaterface. For specific degrees of elastic mismatch, this coaditioa can be a conservative lower estimate. This condition provides a guide for iaterfacial desiga of ceramic matrix composites. 

The toughness induced in ceramic matrices reinforced with the various types of reinforcements, that is, particles, platelets, whiskers, or fibers, derives from two phenomena crack deflection and crack-tip shielding. These phenomena usually operate in synergism in composite systems to give the resultant toughness and noncatastrophic mode of failure. 

Kerans, R.J., and Parthasarathy, T.A. (1999), Crack deflection in ceramic composites and fiber coating design criteria , Composites, A30, 521-524. 

Kovar, D., Thouless, M.D., and Halloran, J.W. (1998), Crack deflection and propagation in layered silicon nitride/boron nitride ceramics , J. Am. Ceram. Soc., 81, 1004-1012. 

Vandeperre, L.J., Kristofferson, A., Carlstrom, E., Clegg, W.J. (2001), Thermal shock of layered ceramic structures with crack-deflecting interfaces , J. Am. Ceram. Soc., 84( 1), 104-110. 

Ceramics and the matrix in ceramic composites exhibit cleavage fracture at room temperature as well as at elevated temperatures. This is fortunate since most of the theoretical developments in dynamic fracture are confined to linear elastic fracture mechanics which is then applicable to fracture of the ceramic matrix. However, the additional complexities of crack deflection and fiber-matrix interface cracking, as well as fiber/whisker/particulate pull-outs, are at this time yet to be addressed. 

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