Mode UFatigue Crack Growth Mechanism and Threshold in a Vacuum snd Air
Kyohei TOYAMA, Yoshihiro FUKUSHIMA and Yukitaka MURAKAMI
Abstract:In order to make clear the effect of air on the Mode U fatigue crack growth mechanism and threshold, the Mode U fatigue crack growth test was carried out in a vacuum. Fibrous pattern on the Mode U fatigue fracture surface of the specimen tested in a vacuum was clearer than that tested in air. The side view of the Mode U 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 U 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 (`130 nm in a vacuum, `100 nm in air). The Mode U threshold stress intensity factor ranges, ˘KIIth = 10.2 Mpaăm (Longitudinal) and 12.5 Mpaăm (Transverse) in a vacuum were higher than that in air ˘KIIth = 9.4 Mpaăm (Longitudinal) and 10.8 Mpaăm (Transverse). Both in a vacuum and air, the values of ˘KIIth for crack growth perpendicular to the rolling direction were higher than the values of ˘KIIth for crack growth parallel to the rolling direction. Key Words:Mode U fatigue crack growth, Mode U threshold stress intensity factor range ˘KIIth, Mode U fatigue fracture surface, Microstructural change, Vacuum