The strain rate dependency of materials' failure has been widely observed in experiments and simulations, yet its microscopic mechanism is still elusive due to the complexity of failure processes. In this work, modified molecular dynamics simulations are carried out to investigate the strain rate effect over a wide strain rate range. The results demonstrate three typical failure modes induced by the competition of two timescales involved. The transition of mechanisms underlying these failure modes is discussed with a simplified model. The corresponding analysis indicates that the thermal activation model offers a good prediction for the variation of failure strain with respect to applied strain rate for failure mode I; the coupled evolution of atomic motions and potential landscapes governs the failure mode II; and the failure mode III is a result of the rapid separation between loading and deformation parts of the sample. (C) 2014 Elsevier B.V. All rights reserved.