Supposition of the Interior Originating Fatigue Fracture Mechanism of Titanium Alloy Based on the Surface Small Crack Growth Properties in High Vacuum

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Recently, some researchers reported that fatigue fracture in Ti alloy occurred from interior of the material in very high cycle over 10^7-10^8. The reason of this phenomenon has not been clarified. This study focused on the environment of the material where crack surface is not exposed to ambient air, and carried out fatigue experiments in high vacuum using specimens with small artificial defect. We investigated the mechanism of interior fatigue fracture by comparing crack propagation processes in air, vacuum and inside environment of the material. As a result, the following were obtained :1. A high vacuum environment improved the fatigue life drastically.2. The threshold of crack propagation was determined by the initiation of the crack in air whereas by the arrest of the crack in vacuum. The latter phenomenon coincided with the dominant factor of fatigue limit of interior fracture.3. Transitional stress intensity factor from stage 2a to stage 2b process decreased in high vacuum as compared with in air. This could be explained by the decrease of the local yield stress at crack tip. The longer fatigue life in high vacuum was considered to result from the lower efficiency of the adsorbed film, which facilitated the plastic deformation at crack tip.4. The tendency of longer fatigue life of interior originating fracture coincided with that in high vacuum. However, fatigue life in high vacuum was much longer than interior fracture. Fracture surfaces of interior fracture were more similar to those of surface fracture in high vacuum than in air, but they did not completely correspond to those in high vacuum. It seemed that the interior environment of material was not equal to high vacuum, and that the amount of gaseous molecule which affected crack propagation, was larger in the interior of the material than in high vacuum.