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

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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.