Crack stability in rate sensitive ceramics and their rate dependence

The crack stability condition in rate sensitive brittle materials is analysed. The analysis shows that a crack begins to extend before a maximum load ( P , , ) appears in fracture toughness (XI) tests and the extension depends on the n-value in the relation. This suggests that the evaluation of a true KI;value is quite difficult. An analytical method for using the KT. -value (the lnltlal stress intensity evaluated by the Pma, and initial crack length) is proposed. Experimental data for an alumina ceramlc and a glass are analysed by the method. The K I ~ T ~ relation of the glass consists of 4 regions over the range s < ~ ~ 1 0 ~ s where Tf is the time to failure.