Fracture mechanics and fractography of fatigue and creep fracture under service conditions.

疲劳和蠕变裂纹扩展研究是在变应力条件下进行的，其中<about>在低于大量循环（大于&lt;10^5&gt;循环）或长时间（10小时）的阈值的欠应力之间间歇地施加非常少量循环（1- 2循环）或非常短时间（10秒）<about>的过应力。繁殖速度显着加快与在空气和惰性气体（氮气）环境中的稳定循环应力的情况相比，这种可变应力可使空气中的温度升高（在某些情况下超过100倍）。当欠应力较小时，空气中的加速度小于氮气中的加速度。俄歇电子能谱分析表明，这是由于断裂面氧化导致裂纹闭合的结果。在高温蠕变裂纹扩展的情况下，厚板试样中变应力引起的加速比薄板试样中的小得多，这表明厚度方向的变形对加速度有重要影响。在蠕变情况下，在高温交变应力下的疲劳裂纹扩展试验表明，在空气和真空中，裂纹扩展都能很好地用J表示<DELTA>。由于没有氧化，在真空中的传播速率略低于在空气中的传播速率。

Fatigue and creep crack propagation studies were carried out under variable stress conditions where overstresses of a very small number of cycles(1--2cycles) or of very short time( <about> 10sec) were applied intermittently between understress below the threshold of a large number of cycles (more than <10^5> cycles) or of long time( <about> 10hr).In the case of fatigue crack propagation at room temperature, Propagation rates were accelerated significantly (more than one hundred times in some cases) by such variable stresses as compared with the case of steady cyclic stresses both in air and in inert gas(nitrogen) environment. When magnitude of the understress is low, the acceleration was less in air than in nitrogen gas. This is shown to be a result of crack closure due to oxidation of fracture surface by means of Auger electron spectroscopy.In the case of creep crack propagation at elevated temperature, the acceleration by variable stresses was considerably less in thick plate specimens than in thin plate specimens, indicating an important effect of the deformation in thickness direction on the acceleration.In the case of creep-fatigue crack propagation under alternating stresses at elevated temperature, it was shown that the crack propagation is correleted well in terms of <DELTA> J both in air and in vacuum. The propagation rates in vacuum were slightly less than those in air due to absence of oxidation.