An atomistic investigation into the nature of near threshold fatigue crack growth in aluminum alloys

Despite decades of study, the atomic-scale mechanisms of fatigue crack growth remain elusive. Here we use the coupled atomistic–discrete dislocation method, a multiscale simulation method, to examine the influence of dislocation glide resistance on near-threshold fatigue crack growth in an aluminum alloy. The simulations indicate that the threshold increases with an increase in dislocation glide resistance, and that a transition in the crack growth direction can occur when dislocation nucleation is inhibited by other nucleated dis-locations. Three main mechanisms of fatigue crack propagation are observed: cleavage along the primary slip plane, crack extension by dislocation emission, and crack extension by opening along lattice defects. Aluminum alloys continue to serve as the primary material system for many critical components in aircraft structures. Accordingly, a key aspect of aircraft safety involves the prediction of fatigue crack growth in these materials. This technological motivation has spurred the growth of a vast library of experimental and theoretical studies on fatigue crack growth over the past decades. Nonetheless, one critical aspect of the phenomenon has remained particularly unclear, i.e. the atom-istic mechanism by which the crack tip propagates forward under cyclic subcritical loadings. Considering the atomic nature of crack tip processes, modeling must be atomistic in nature. However, interpreting atom-istic modeling results relative to fatigue crack growth in real alloys involves many significant challenges. One of the largest challenges is the limited spatial domain that is typical of atomistic models. Simulations having a small spatial domain can artificially influence the movement of dislocations away from the crack tip and ultimately bias crack tip behavior [1–8]. Discrete dislocation (DD) dynamics simulations are not generally plagued by this problem as they can accommodate a much larger spatial domain, while still explicitly modeling every dislocation. However, DD models do not explicitly represent the atomic scale complexities that occur at a crack tip [9–13], and thus cannot illuminate the atomic mechanisms by which a crack tip propagates. In this work, a concurrently coupled atomistic–discrete dislocation multiscale method (CADD) is used to resolve the shortcomings of tradition atomistic and DD simulations with the specific goal of illuminating the atomic scale mechanisms that occur at a crack tip during fatigue crack growth. The model consists of an aluminum crystal with a crack, loaded in mode I. Upon loading, dislocations nucleate at the crack tip in the atomistic region of the model. In most cases, the dislocations then glide into the nearby …