Investigation on Threshold Behavior of High Temperature Fatigue Crack Propagation and Its Mechanisms Using Macroscopic Experiments and Microscopic Molecular Dynamic Analyses

本项目研究了近阈值疲劳裂纹扩展行为及阈值扩展机理，为此开展了宏观高温疲劳裂纹扩展试验。考察了环境、加载频率、温度和加载历史的影响。另一方面，对近阈值疲劳裂纹扩展进行了微观分子动力学分析。所得结果总结如下：1。对sus304不锈钢和镍基高温合金在空气和真空条件下进行了高温疲劳裂纹扩展试验。结果表明，环境、加载频率和温度对近阈值疲劳裂纹扩展速率da/dN和阈值影响较大。裂纹扩展阻力小于空气中裂纹扩展阻力。对时效Cr-Mo-V钢的高温裂纹扩展行为进行了研究。时效对裂纹扩展的影响较小。对剩余寿命进行了估计。确定了许多参数对寿命的影响。采用分子动力学方法对铁的近阈值疲劳裂纹扩展进行了模拟。提出了一种考虑新裂纹表面氧化导致原子间力下降的方案。研究发现，用分子动力学方法可以模拟近阈值疲劳裂纹扩展行为。考察了取向和晶界的影响。将线弹性奇异应力场中的裂纹扩展行为与Hutchinson-Rice-Rosengren塑性奇异应力场中的裂纹扩展行为进行了比较。

The present project investigated the near-threshold fatigue crack propagation behavior and mechanisms of the threshold For this purpose macroscopic high temperature fatigue crack propagation experiments were conducted. Effects of environment, loading frequency, temperature and loading history were examined. On the other hand microscopic molecular dynamic analyses on near-threshold fatigue crack propagation were made. The results obtained are summarized as follows.1.For SUS 304 stainless steel and Ni-base super alloy, high temperature fatigue crack propagation experiments were conducted in air and in vacuum. It was found that environment, loading frequency, and temperature had major influences on the near-threshold fatigue crack propagation rate da/dN and the threshold. The crack propagation resistance was lower that in air.2.For aged Cr-Mo-V steel, high temperature crack propagation behavior was investigated. The effect of aging on the crack propagation was small. The estimation of remaining life was made. The influences of many parameters on the life were identified.3.The simulations of the near-threshold fatigue crack propagation in iron were made using the molecular dynamics. A scheme was proposed which considered the decrease in the inter-atomic force due to oxidation of the new crack surface. It was found that near-threshold fatigue crack propagation behavior can be simulated by using the molecular dynamics. The effects of the orientation and grain boundary were examined. The crack propagation behavior contained in the linear elastic singular stress field was compared with that in the Hutchinson-Rice-Rosengren plastic singular field.