Growth Mechanism and Threshold of Mode II and Mode III Fatigue Crack

摘要

In order to make clear the relationship between ΔK{sub}(IIth) and ΔK{sub}(IIIth) and the influence of the rolling texture, Mode II and Mode III fatigue crack growth tests were carried out for annealed 0.47% carbon steels (JIS S45C). The values of ΔK{sub}(IIth) (9.5 Mpa√m and 9.7 Mpa√m) for crack growth normal to the rolling direction were higher than the values of ΔK{sub}(IIth) (8.8 Mpa√m and 8.1 Mpa√m) for crack growth parallel to the rolling direction. Mode II and Mode III fatigue crack growth produced darkly etched microstructural layers in the vicinity of Mode II and Mode III fatigue crack. Vickers hardness of the layers was much higher than that of the original matrix (Mode II: HV 164 → HV 468. Mode III: HV 175 → HV 678). The layers consisted of very fine grains with the size ranging from 100 to 200 nm. The value of ΔK{sub}(IIth) was approximately equal to ΔK{sub}(IIIth), i.e. ΔK{sub}(IIth) ≌ ΔK{sub}(IIIth) ≌ 9.6 Mpa√m for crack growth perpendicular to rolling direction. Therefore, the mechanism of Mode II fatigue crack growth is presumed to be essentially identical to that of Mode III fatigue crack growth.