Relation between the Rate of Fatigue Crack Propagation and the Size of Plastic Zone in an Fe-3%Si Alloy

Abstract

Relation between the rate of fatigue crack propagation and the size of plastic zone was investigated with an Fe-3%Si alloy. The effect of the stress ratio on the relation was also examined. Main results obtained are as follows.
(1) When the conditions of small scale yielding are not satisfied at crack tip region, the relation between the rate of fatigue crack propagation and the stress intensity factor range is affected markedly by stress amplitude.
(2) Regardless of the scale of yielding, the rate of fatigue crack propagation, da⁄dN, is related to the size of plastic zone, Z_p^s, by a power function,
da⁄dN=D(Z_p^s)ⁿ,
where the coefficient D and the exponent n are constants independent of stress amplitude.
(3) The coefficient D in the da⁄dN−Z_p^s relation depends on the stress ratio, R, as D=D₀(1−R)^β, D₀ and β being constants independent of R. The da⁄dN−Z_p^s relation for various R-values is expressed by a simple relation,
da⁄dN=D₀{(1−R)^β⁄nZ_p^s}ⁿ, β⁄n\simeq1.
(4) For all R values examined, Z_p^s can be predicted with reasonable accuracy regardless of the scale of yielding by using B-C-S theory modified for fatigue cracks.