Nonuniqueness of the fatigue threshold

Abstract

The fatigue crack growth prediction near the threshold regime remains one of the most challenging fields in fatigue research. In previous studies, the threshold effects have been incorporated into fatigue crack growth models upon modification of the Paris Law in an empirical fashion. In this work, without a priori assumptions and empirical constants, we derived the threshold levels with a combination of molecular dynamics and continuum calculations. We illustrate that the threshold value is non-unique and depends on the state of the microstructure surrounding the crack. Also, in reality, we envisage very low fatigue crack growth rates near the ‘threshold’ and not a cut-off value, which is consistent with the experimental trends. In the model, the microstructure is characterized by the grain boundary types, the grain size, and the initial dislocation density. We derive friction stresses for the forward and the reverse motion of the crack tip dislocations interacting with the grain boundaries which allow determination of the irreversible crack tip displacements. We illustrate the benefits of sigma-3 grain boundaries, finer grain sizes and the shielding dislocations at the crack tip on improving the near-threshold fatigue crack growth behavior.