On the estimation of microstructural effects in the near-threshold fatigue of small cracks

The growth of small fatigue cracks is strongly influenced by the microstructure of the material concerned. Particularly in the early stages, the crystallographic orientation of the grains through which the crack must propagate plays a fundamental role. Cracks are generally initiated in well-oriented grains, but afterwards they are forced to grow through less favourably oriented grains. This work examines the influence of this crystallographic resistance to small crack growth in micromechanical terms. It is argued that the crystallographic orientation alone cannot explain the difference between small- and long-crack growth thresholds found in metals. Other phenomena must be called upon to account for this difference. For instance, crack closure exerts a resistance to crack growth whose evolution with crack length is similar to that due to the crystallographic orientation. Finally, it is shown that, when microstructural and mechanical thresholds are interpreted within the context of micromechanical models, a number of classical parameters and expressions usually employed in engineering practice can be naturally obtained and understood.