Abstract
We present the results of free 3D molecular dynamics (MD) simulations, focused on the influence of temperature on the ductile-brittle behavior of a pre-existing central Griffith through microcrack (1¯10)[110] (crack plane/crack front) under biaxial loading σA and σB in tension mode I. At temperatures of 300 K and 600 K, the MD results provide new information on the threshold values of the stress intensity factor K and the energy release rate G, needed for the emission of <111>{112} blunting dislocations that support crack stability. A simple procedure for the evaluation of thermal activation from MD results is proposed in the paper. 3D atomistic results are compared with continuum predictions on thermal activation of the crack induced dislocation generation. At elevated temperature T and biaxiality ratios σB/σA ≤ 0.8 dislocation emission in MD is observed, supported by thermal activation energy of about ~30 kBT. With increasing temperature, the ductile-brittle transition moves to a higher biaxiality ratios in comparison with the situation at temperature of ~0 K. Near the transition, dislocation emission occurs at lower loadings than expected by continuum predictions. For the ratios σB/σA ≥ 1, the elevated temperature facilitates (surprisingly) the microcrack growth below Griffith level.
Highlights
It is known that temperature significantly influences the brittle or ductile behavior of metallic materials since the dislocation motion and the plastic deformation are thermally- activated processes [1]
A simple procedure is proposed in this paper to evaluate the thermal activation energy for the observed crack tip process. 3D thermal molecular dynamics (MD) simulations confirm that the activation energy for dislocation emission from (110)[110] microcrack under biaxial loading in mode I corresponds well to
The level and the sign of the T-stress in MD are controlled by the external biaxial loading σA and σB
Summary
It is known that temperature significantly influences the brittle or ductile behavior of metallic materials since the dislocation motion and the plastic deformation are thermally- activated processes [1]. Temperature affects the ability of cracks to emit blunting dislocations. It determines ductile vs brittle behavior at the crack front, i.e., crack stability vs fracture. The ductile vs brittle behavior of cracks is extremely important for engineering applications and beside the experiments, it is studied via continuum analysis and atomistic models. Rice [2] was the first one who proposed an analysis and a crack stability criterion based on the Peirls-Nabarro model for dislocations and on the self-similar concept of linear fracture mechanics (LFM), utilizing the stress intensity factor K or the energy release rate G = C·K2 , where C is an effective elastic coefficient for a given crack orientation
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