Atomistic simulations can illuminate detailed mechanisms of brittle and ductile fracture and plasticity. However, there are many limitations to these simulations like short timescales, small spatial scales, and limitations of the discretization. Using molecular dynamics (MD) and multiscale methods, adaptations can be made to allow MD to answer problems relevant to engineers. In the first of three examples, MD is adapted to simulate brittle fracture by changing the discretization and allowing permanent damage between particles. By changing the discretization, specific mechanisms inherent to MD can be suppressed to allow accurate, macroscopic simulations of dynamic fragmentation of brittle materials. Second, the timescale available to MD is extended in a concurrent multiscale method (CADD) combined with accelerated MD. This combined approach allows for microseconds of simulation time at experimentally achievable loading rates. The method is applied to crack opening in aluminum alloys, and the effect of the loading rate on crack growth mechanisms is observed. From the results, it is clear that crack growth mechanisms depend greatly on the rate of the far-field loading. Third, the effect of aging on fatigue crack growth is studied by varying the resistance to dislocation motion in the dislocation dynamics region of CADD. Only in a multiscale simulation like CADD, can dislocation pileups reaching microns into the material interact with the atomic-scale mechanisms at a crack tip. The results of the simulations indicated that increasing the friction force raises the fatigue crack threshold. Also, a transition from stage I fatigue crack growth to stage II fatigue crack growth occurs by dislocations shielding dislocation nucleation on the primary slip plane. These observations support the conclusion that the fatigue crack growth threshold is controlled by the spacing between obstacles to dislocation glide, which is consistent with experimental observations.
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