クリープ温度領域における疲労き裂伝ぱとその下限界

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An experimental study was made on fatigue crack growth in SUS 304 stainless steel at 650℃ 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 ΔK_<th> for -0.2≦R≦0.4. When the J-integral range ΔJ_<f, 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 ΔJ_<f, eff> relationship and the threshold value (ΔJ_<f, 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.
社団法人日本材料学会の論文
1988-08-15

クリープ温度領域における疲労き裂伝ぱとその下限界

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