Mode II fatigue crack growth mechanism and threshold in a vacuum and air

In order to make dear the effect of air on the Mode U fatigue crack growth mechanism and threshold, the Mode II fatigue crack growth test was carried out in a vacuum. Fibrous pattern on the Mode II fatigue fracture surface of the specimen tested in a vacuum was clearer than that tested in air. The side view of the Mode II fatigue crack of the specimen tested in a vacuum was thinner than that tested in air due to the lack of oxidation in a vacuum. The layer with the microstructural change was observed in the vicinity of the Mode II fatigue crack as well as that in air. The Vickers hardness of the layer of the specimen tested in a vacuum was 770 (HV= 630 in air). These values were much higher than that of the original matrix (HV= 190). The layer consisted of very fine grains (∼130nm in a vacuum, ∼100nm in air). The Mode II threshold stress intensity factor ranges, ΔK _IIth = 10.2MPa √m (Longitudinal) and 12.5MPa √m (Transverse) in a vacuum were higher than that in air ΔK _IIth = 9.4MPa √m (Longitudinal) and 10.8MPa √m (Transverse). Both in a vacuum and air, the values of ΔK _IIth for crack growth perpendicular to the rolling direction were higher than the values of ΔK _IIth for crack growth parallel to the rolling direction.