Near-Threshold Fatigue Crack Growth Characteristics and Quantitative Evaluation of Relevant Fracture Surface of Directionally Solidified Superalloy in a Vacuum at Elevated Temperatures

对定向凝固镍基高温合金在真空和空气中进行了近门槛区疲劳裂纹扩展试验。随着K_（？）的增加，裂纹扩展出现“表观门槛值”现象<max>，低于该值时，裂纹扩展行为与温度、加载频率和加载波形有关。通过AFM观察这些实验后的断裂表面。分形分析应用于数据和定量评价这些断裂表面的尝试。结果表明，在真空条件下，分形维数随K的增大而增大<max>.

Fatigue crack growth experiments in the near-threshold region were conducted in a vacuum and in the air for directionally solidified Ni-based superalloy at elevated temperature. The so-called "apparent threshold" behavior occurred by K_<max> increased, and it was found that the crack growth behaviors below that had the temperature, loading frequency, and loading waveform dependences. Observations of fracture surfaces after those experiments were made by AFM. Fractal analysis was applied to the data and quantitative evaluations for those fracture surfaces were attempted. It was found that fractal dimensions increased as K_<max> increased in a vacuum.