Ti-6Al-4V合金の超高真空環境における疲労き裂進展特性(<小特集>M&M2009材料力学カンファレンス)

Abstract

To determine the effects of the vacuum environment on fatigue crack propagations, K-decreasing tests with compact tension (CT) specimens were conducted in air and vacuum environments. The test data clarified the fatigue crack propagation rate (da/dN) and threshold stress intensity factor range (ΔK_<th>) for both environments. The da/dN becomes lower and ΔK_<th> becomes larger with decreasing vacuum pressure. However, this tendency was not explained only by the crack closure. Based on fracture surface observations, the crack propagation properties in vacuum were compared with those in air. A few micrometer size granular region was observed on the fracture surface only in the high vacuum (〜10^<-6>Pa) and ultra high vacuum (〜10^<-7>Pa), but not in the air and medium vacuum (〜10^<-1>Pa). The high vacuum environment is one of the necessary conditions for the formation of the granular region, and the fraction of surface coverage of fracture surfaces probably relates to the phenomenon. The formation of the granular region represents the difference of the crack propagation mechanism between in vacuum and in air environments, and the tendency observed in da/dN and ΔK_<th> can be accounted for by the different mechanism.