Abstract
The fatigue crack growth behavior of SUS 304 stainless steel at 650°C was investigated under low stress ratios R on the basis of the non-linear fracture mechanics. Emphasis was placed upon the near-threshold fatigue crack growth behavior and crack closure under creep conditions. It was found that in the very low da/dN region and the high da/dN region the relation between the cycle-dependent crack growth rate da/dN and stress intensity factor range ΔK obtained for -1≤R<0.2 deviated from the same correlation obtained under 0.5≤R. The values of fatigue threshold ΔKth for low R were much greater than those obtained for high R. Such a strong R dependence of the crack growth behavior found in the low R region was eliminated by the use of J-integral range ΔJf, eff, in which the effect of crack closure as well as that of material non-linearity were taken into account. The near-threshold crack growth behavior was found to be very sensitive to the loading history including frequency. This was rationalized in terms of the effect of oxide-induced crack closure enhanced by fretting as well as that of time-dependent creep deformation.
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