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

摘要

In order to make clear the relationship between Δ K_(Ⅱth) and Δ K_(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_(Ⅱth) (9.5 MPa m~(1/2) and 9.7 MPa m~(1/2) ) for crack growth normal to the rolling direction were higher than the values of ΔK_(Ⅱth) (8.8 MPa m~(1/2) and 8.1 MPa m~(1/2) ) 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_(Ⅱth) was approximately equal to ΔK_(Ⅲth), i.e. ΔK_(Ⅱth)≈ΔK_(Ⅲth)≈9.6 MPam~(1/2) 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.