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

近年来，一些研究人员报道了钛合金在10^7-10^8以上的极高循环中从材料内部发生疲劳断裂。造成这种现象的原因还没有弄清楚。本研究针对材料裂纹面不暴露于环境空气的环境，采用人工缺陷小的试样在高真空条件下进行疲劳试验。通过对比材料在空气、真空和内部环境中的裂纹扩展过程，研究了材料内部疲劳断裂的机理。结果表明：1。高真空环境大大提高了疲劳寿命。裂纹扩展的阈值取决于裂纹在空气中的起裂，而取决于裂纹在真空中的止裂。后一种现象与内部断裂疲劳极限的主导因素相吻合。与在空气中相比，高真空条件下从2a阶段到2b阶段的过渡应力强度因子降低。这可以解释为裂纹尖端局部屈服应力的减小。在高真空条件下，较长的疲劳寿命被认为是由于吸附膜的效率较低，有利于裂纹尖端的塑性变形。内始发断口延长疲劳寿命的趋势与高真空条件下一致。而在高真空条件下的疲劳寿命远长于内部断裂。内部断口断面与高真空条件下的表面断口断面比空气条件下的更接近，但与高真空条件下的断口断面不完全对应。材料内部环境似乎不等于高真空，材料内部影响裂纹扩展的气体分子量比高真空时要大。

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.