Computer Simulation Studies of Fracture in Vitreous Silica
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Computer Simulation Studies of Fracture in Vitreous Silica Romulo Ochoa, Michael Arief, and Joseph H. Simmons1 Department of Physics, The College of New Jersey Ewing, NJ 08628 1 Department of Materials Science and Engineering, University of Arizona Tucson, AZ 85721 ABSTRACT We conduct molecular dynamics computer simulations of fracture in silica glass using the van Beest, Kramer, and van Santen model. Stress is applied by uniaxial strain at different pulling rates. Comparisons with previous fracture simulations of silica that used the Soules force function are presented. We find that in both models stress is relieved by rotation of the (SiO4)-2 tetrahedrons, increasing Si-O-Si bonding angles, and only small changes in the tetrahedron dimensions and O-Si-O angles. INTRODUCTION Amorphous silica is a common material of particular scientific and technological importance. Over the past two decades numerous computer simulation studies have been conducted to investigate its structural and dynamical properties. A limited number of the numerical studies have concentrated on examining the fundamental mechanisms underlying the brittle fracture of this network forming glass. Previous molecular dynamics (MD) investigations used an interatomic force-model proposed by Soules [1]. The studies showed that when amorphous silica samples are strained, in a vacuum environment, they respond by increasing the Si-O-Si bond angles while maintaining their (SiO4)-2 tetrahedron almost unchanged [2-5]. Vibrational atomic motion coupled with the numerous voids, characteristic of amorphous structures, allowed for this rotation of the tetrahedron in response to the applied strain. The stress was found to increase linearly with strain up to a 10% volume increase with a failure value dependent on the strain rate. The Soules force function is the product of the force expression derived from the Born-MayerHuggins potential and an empirical truncation factor. This last factor effectively cuts off the two-body atomic force interaction at 5.5 Å. The net effect is that of ions interacting through short-range forces. In the past few years a two-body potential proposed by van Beest et al. (BKS) [6] has proven to represent very effectively the structural and thermodynamical properties of crystalline and amorphous silica [7-11]. This potential also has the Born-Mayer-Huggins functional form with an additional dispersion term, included to better simulate the covalent nature of silica. We have set out to find the differences, if any, in the fracture process when two potentials, that have different functional forms, are used to simulate amorphous silica at room temperature. Fracture using the Soules interatomic force function has been reported elsewhere; in this manuscript we present the results of fracturing amorphous silica simulated using the BKS potential.
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