Investigation On Near-Threshold Fatigue Crack Growth Behavior by Using Nano-and Mesoscale Simulation and Observation

本研究将纳米尺度分子动力学、宏观尺度实验和细观尺度观察相结合，对近门槛值疲劳裂纹扩展行为进行了研究。主要研究结果如下：1.对SUS 304不锈钢进行了高温疲劳裂纹扩展试验，分别在空气和真空中进行。结果发现，在真空中的疲劳裂纹扩展阻力高于在空气中。在真空和空气中，疲劳裂纹扩展门槛值在550℃左右达到最大值。在降低K值的试验中观察到门槛值后，发现在空气和真空中，当最大K值增加时，裂纹再次扩展。通过重复这个过程，达到了所谓的真实阈值。温度对裂纹扩展速率da/dN与Δ K的关系影响不大，对镍基高温合金和时效材料进行了疲劳裂纹扩展实验，并对滑移、塑性变形与裂纹扩展的关系进行了细观观察。2.采用分子动力学方法对铁的近门槛疲劳裂纹扩展进行了纳米尺度模拟。在模式II加载以及模式I加载下观察到裂纹扩展。采用分子动力学方法研究了倾斜晶界和扭转晶界对疲劳裂纹近门槛值扩展的影响。当裂纹有时跨越73和99小角度晶界时，对于99和11大角度晶界，势垒较高。对于垂直于裂纹面的扭转晶界，晶界阻碍了疲劳裂纹的扩展。

In the present research, nanoscale molecular dynamics, macroscale experiment, and mesoscale observations were applied to the near-threshold fatigue crack growth behavior. The results obtained are summarized as follows.1.High temperature fatigue crack propagation experiments were conducted for SUS 304 stainless steel, in air and in vacuum. It was found that the fatigue crack growth resistance was higher in a vacuum than in air. The threshold value of fatigue crack growth attained maximum around 550℃ in a vacuum and in air. After the threshold behavior was observed in the K decreasing tests, the crack was found to grow again when the maximum K value was increased in air and in a vacuum. By repeating this process a so-called true threshold was attained. The effect of temperature on relationship between crack growth rate da/dN and Δ K was small.Fatigue crack growth experiments were made also for Ni base super alloy and aged material.Mesoscale observations were made to find relationship between slip, plastic deformation and crack growth.2.Nanoscale simulation of the near-threshold fatigue crack propagation in iron were made by using the molecular dynamics. Crack growth was observed under mode II loading as well as mode I loading. Crack growth direction under mixed mode loading agreed well with that predicted by the maximum circumferential stress criterion.Effect of tilt grain boundary and twist grain boundary on the near-threshold fatigue crack growth was examined using the molecular dynamics. While cracks sometimes grew across Σ73 and Σ99 small angle grain boundaries, the barrier was higher for, Σ9 and Σ11 largel angle grain boundaries. For the twist grain boundary perpendicular to crack plane, the grain boundary was found to impede the fatigue crack growth.