In this paper we analyze growth of small cracks initiated from microscopic notches and loaded near the growth threshold under different stress ratios. The results of in-situ optical measurements during high cycle fatigue testing show that small cracks initiate and grow quickly after which there is a long period of slow growth until the large crack growth threshold is reached. The crack growth rate data from different loading ratios indicates that the positive portion of the stress amplitude or the maximum stress intensity factor can be used to compare crack growth rates. The test data also shows that increasing only the compressive stress portion of the loading will cause an arrested small crack to grow again.
Roiko, A., Solin, J., & Hänninen, H. (2017). Behavior of small cracks under negative stress ratio fatigue loading. International Journal of Fatigue, 104, 379-388. https://doi.org/10.1016/j.ijfatigue.2017.07.006