過大過小応力下における疲労き裂伝ぱの加速 平均応力効果とき裂閉口

Abstract

Fatigue crack growth behaviour under intermittent overstresses of different mean stress levels was investigated in dry air with a low carbon steel. A very small number of cycles of overstress applied intermittently between a very large number of cycling of understress below threshold caused significant acceleration in crack growth rate amounting to about one hundred times as compared to the case of steady cyclic stress for all cases of mean stress level. The acceleration in the cases with tensile mean stress was slightly smaller than that in the cases without it, and it was related to a little higher crack closure level in the former case. However, correlation in the acceleration versus understress diagram was not improved appreciably by using effective stress intensity taking account of the crack closure. Stress history in precracking had some effect on the crack closure and, consequently, on the acceleration. The correlation in this case was appreciably improved by using the effective stress intensity. Difference was not recognized in fracture surface morphology among the cases with different mean stress levels, and so the micromechanism of crack growth acceleration would be the same throughout all cases of different mean stress. The acceleration under multi-level intermittent overstresses with mean stress could be predicted from the two-level intermittent overstress test data by linear summation in the same way as the case without mean stress.