RETARDATION OF FATIGUE-CRACK GROWTH IN CERAMICS BY GLASSY LIGAMENTS -A RATIONALIZATION

In high-temperature fatigue crack growth (FCG) experiments on ceramic materials containing amorphous grain boundary phases, the crack growth rates under cyclic loads were observed to be lower than those predict ed solely on the basis of crack growth velocities measured under stati c loads, In this paper, a rationalization was offered for such a behav ior by means of a phenomenological glass-bridging model which takes th e relaxation behavior of glass into account, In ceramics which exhibit subcritical crack growth through cavitation ahead of the crack tip, t he maximum stress intensity factor of the fatigue cycle required to in itiate FCG was observed to be always greater than or equal to the thre shold stress intensity factor for crack growth under sustained far-fie ld loads. This trend was also explained with the aid of the glass-brid ging model and invoking the equivalence between bridging and damage zo nes, The elevated temperature FCG behavior of nitride-based ceramics w hich exhibit grain bridging in the wake during crack propagation was d iscussed and contrasted with oxide-based ceramics which show glass bri dging.