Initial crack ceramic

Crack branching is a common feature in failure of ceramic components. Cracks branch at a critical velocity, which is of the order of half the speed of sound in the specific glass under study. The acceleration of crack initiation to the critical velocity depends on the energy dissipation available from the release of stored energy. The energy source can be applied stress, prestressing or residual stress. 

Finally, Palmqvist cracks are short, shallow cracks, initiated at the end of the indent diagonals at relatively low loads. Fig. 9d. The plane of a Palmqvist crack coincides with the plane of the radial cracks, and there is some evidence that the Palmqvist cracks may, in some ceramics, initiate before the radial cracks, at the same time as the median crack. 

Uniform microstractuie is cracial to the superior performance of advanced ceramics. In a cerantic material, atoms are held in place by strong chentical bonds that ate impervious to attack by corrosive materials or heat. At the same time, these bonds are not capable of much "give." When a ceramic material is subjected to mechanical stresses, these stresses concentrate at minute imperfections in the microstmcture, initiating a crack. The stresses at the top of the crack exceed the threshold for breaking the adjacent atomic bonds, and the crack propagates throughout the material causing a catastrophic brittle failure of the ceramic body. The rehability of a ceramic component is directly related to the number and type of imperfections in its microstmcture. 

Lankford, J., and Davidson, D. L., The Crack-Initiation Threshold in Ceramic Materials Subject to Elastic/Plastic Indentation, /. Mater. Sen, Vol. 14,1979,pp. 1662-1668. 

Lankford J., Davidson D. L., 1979a, The crack-initiation threshold in ceramic materials subject to elastic-plastic indentation, J. Mater. Sci., 14, 1662-1668. 

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