CYCLIC FATIGUE CRACK GROWTH IN SILICON NITRIDE: INFLUENCES OF STRESS RATIO AND CRACK CLOSURE

摘要

The characteristics of cyclic fatigue crack growth was investigated in silicon nitride which hardly shows rising R-curve behavior, using single edge precracked beam specimens. Crack growth behavior is known to be explicable by a power law relationship including both the stress intensity range ΔK and the maximum stress intensity K_(max); i.e. da/dN = C(K_(max))~p(ΔK)~q. However, when K_(max) is a constant, the crack growth rate da/dN is found to be not a unique power law function of ΔK. The curve in a double-logarithmic plot of da/dN and ΔK is composed of two regions. The slope q in the lower ΔK region is larger than that in the higher ΔK one. The difference of the q value between the two regions is primarily responsible for crack closure. Namely, the crack growth rate for a constant K_(max) does correlate with the range of the effective stress intensity ΔK_(eff) defined as ΔK_(eff) = K_(max) — K_(cl), where K_(cl) is the crack closure stress intensity. It is, therefore, suggested that the crack growth behavior is expressed by a crack growth law containing the effect of crack closure, i.e. da/dN = C(K_(max))~p(ΔK_(eff))~q. On the basis of these results and other data available, cyclic fatigue mechanisms are discussed.