1988 Volume 37 Issue 419 Pages 951-956
An experimental study was made on fatigue crack growth in SUS304 stainless steel at 650°C for stress ratios R in the range of -0.2≤R≤0.4. The behaviors of near-threshold fatigue crack growth and the associated crack closure in this range were compared with those obtained previously for R≤-0.4 and 0.5≤R. The crack closure was observed for -0.2≤R≤0.4 under the creep-fatigue conditions employed. The R value had a significant effect on the relationship between the cycle-dependent crack growth rate da/dN and stress intensity range ΔK as well as on the value of fatigue crack growth threshold ΔKth for -0.2≤R≤0.4. When the J-integral range ΔJf, eff, in which the effect of crack closure was taken into account, was used instead of ΔK, the R dependence was eliminated, and the da/dN-ΔJf, eff relationship and the threshold value (ΔJf, eff)th were consistent with those obtained under 0.5≤R and R≤-0.4.
To investigate the character of the fatigue crack growth threshold in the creep regime, crack growth experiments were made using two-step varying loads, in which high load-range blocks were introduced periodically in sub-threshold low load-range sequences. To exclude the effect of crack closure, high stress ratios were employed. It was found that, when the number of load excursions in the high load-range block used in the experiments was small, the threshold was extinguished: cracks grew even under load-ranges lower than the threshold value obtained under constant-amplitude loading conditions. A modified Miner's rule was found to give a reasonable estimate of the crack growth rate under two-step varying loading conditions.
