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Informal Seminar by Vesselin Yamakov |
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Date: November 16, 2005
Time: 12:00pm
*NOTE* Location: NIA, Rm 135
Molecular-Dynamics Parameterization of a Cohesive Zone Model for Intergranular Fracture in Aluminum
Vesselin Yamakov, NIA
A traction-displacement relationship that characterizes the load transfer across a growing nanoscopic intergranular crack is extracted from atomistic molecular dynamics simulations and is recast in a form that is suitable for inclusion within continuum finite element models. In this study, a molecular-dynamics model for crack propagation under steady-state conditions is developed to analyze inter-granular fracture along a flat Sigma99 [1 1 0] symmetric tilt grain boundary in aluminum. Under hydrostatic tensile load, the simulation reveals asymmetric crack propagation in the two opposite directions along the grain boundary. In one direction, the crack propagates in a brittle manner by cleavage with no or very little dislocation emission, and in the other direction, the propagation is ductile through the mechanism of deformation twinning. This behavior is well explained by the Rice criterion (Rice, J. R., J. Mech. Phys. Solids 40, 1992, 239-271) for cleavage vs. dislocation blunting transition at the crack tip, and the preference for twinning to dislocation slip is in agreement with the Tadmor and Hai criterion (Tadmor, E. B. and Hai, S., J. Mech. Phys. Solids 51, 2003, 765-793). A comparison with finite element calculations shows that while the stress field around the brittle crack tip follows the expected elastic solution for the given boundary conditions of the model, the stress field around the twinning crack tip has a strong plastic contribution. Through the definition of a cohesive-zone-volume-element – an atomistic analog to a continuum cohesive zone model element - the results from the molecular-dynamics simulation are recast to obtain an averaged continuum traction-displacement relationship that represents the cohesive interactions along a characteristic length of the grain boundary interface for the case of ductile and brittle decohesion.
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