CYCLIC FATIGUE-CRACK GROWTH FROM INDENTATION FLAW IN SILICON-NITRIDE INFLUENCE OF EFFECTIVE STRESS RATIO

The characteristics of cyclic fatigue crack growth in a silicon nitride which hardly shows rising R-curve behavior were investigated by using specimens having an indentation-induced flaw. V-shaped behavior was observed in the relation between the maximum stress intensity factor K(max) and crack growth rate. When a crack growth rate is plotted as a function of the effective maximum stress intensity factor K(eff,max) which takes account of the residual crack opening stress component K(r) in addition to K(max), crack growth behavior is expressed by a power law relationship for the region above a crack length of about 0.37 mm in this experiment. However, it deviates from this relation below the critical length. Such crack growth behavior is explicable by introducing the term of the effective stress ratio R(eff) defined as (K(min) + K(r))/(K(max) + K(r)). Crack growth behavior in the terms of K(eff,max) and R(eff) is expressed by a unique growth law, irrespective of crack length, i.e. da/dN = C(K(eff,max))30(DELTAK(eff))5, where DELTAK(eff) = (1 - R(eff))K(eff,max). Such behavior is consistent with that in the specimen containing a through-thickness crack. This equation indicates that fatigue crack growth rate in the present material is particularly sensitive to K(eff,max), and less sensitive to DELTAK(eff).