Abstract
The characteristics of cyclic fatigue crack growth was investigated in silicon nitride which hardly shows rising R-curve behavior, using single edge precraked 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 Kmax; i.e. da/dN = C (Kmax)p(ΔK)q. However, when Kmax 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 Kmax does correlate with the range of the effective stress intensity ΔKeff defined as ΔKeff = Kmax − Kcl, where Kcl 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 (Kmax)p(ΔKeff)q. On the basis of these results and other data available, cyclic fatigue mechanisms are discussed.
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