低温における疲労き裂伝ぱのフラクトグラフィ的検討

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Fatigue crack growth behavior and facture toughness of a structural steel, JIS SM50A, have been investigated at room temperature and 123 K, and the influences of load ratio, R, and frequency on the fatigue crack growth rate were examined. Low temperature was achieved by changing the flow quantity of liquid nitrogen. An electron Fractographic analysis was employed to determine the mechanism of fatigue crack growth. At 123K, the fatigue crack growth rate was considerably affected by the load ratio, and crack growth curves indicated a marked acceleration to high ΔK regions near the final failure, because of cleavage during the fatigue crack growth. On the other hand, the crack growth rate at room temperature was found to be insensitive to the load ratio where the mechanism of growth does not involve a static fracture mode. At 123K, the beltlike traces of cleavage appeared on the fatigue fracture surface as the evidence of cyclic cleavage when the fatigue crack exceeded the value of K_<max> about 25Mpa√<m>. The value of K_<ci> at which the onset of cyclic cleavage occurs was not influenced by the load ratio, frequency or loading condition , and the K_<ci> value was equivalent to about 70% of the value of fatigue fracture toughness, K_<fc>. The cleavage width on the fracture surface tended to increase with increasing K_<max>. Formation of cyclic cleavage may be related to the local embrittlement caused by work-hardening in the plastic zone ahead of the fatigue crack. K_<fc> was slightly decreased as compared with the value of K_c obtained by monotonic tensile loading. Striation spacings were found to be reduced when temperature was decreased. However the striation spacings were closely correlated with the effective stress intensity factor range, ΔK_<eff>, regardless of temperature.
1985-06-15

著者

橘内良雄
産業安全研究所

低温における疲労き裂伝ぱのフラクトグラフィ的検討

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