This work investigates the dislocation nucleation processes that occur at the tip of a crack in aluminum under a broad range of crystallographic orientations and temperatures. A concurrent multiscale molecular dynamics - continuum simulation framework is employed. The results are then interpreted using a Peierls continuum model that uses finite temperature material properties derived from molecular dynamics simulation. Under ramped loading, partial dislocation nucleation at the crack tip is found to lead to both full dislocation emission and twinning, depending upon the orientation, temperature, and magnitude of the applied load in the simulation. The origins of the dependencies are made apparent by the Peierls continuum model. The continuum model suggests that in many instances dislocation nucleation from the crack tip can be considered to be a strain rate independent process, yet still temperature dependent through the temperature dependence of the stacking fault energies and elastic constants. (C) 2013 Elsevier Ltd. All rights reserved.