Molecular Dynamics Simulations of Fracture in Amorphous Silica
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ABSTRACT Fracture in amorphous silica is studied using million-atom molecular dynamics simulations. The dynamics of crack propagation, internal stress fields, and the morphology of fracture surfaces are examined as a function of temperature and strain rate. At 300K and 600K we observe brittle fracture: internal stress increases to a critical value (typically 2 - 3 GPa) and then turns over when the crack reaches a terminal speed on the order of half the Rayleigh wave speed. At 900K crack propagation slows down dramatically due to plastic deformation and the material becomes ductile.
INTRODUCTION Silica is one of the most interesting materials found in nature. It occurs in various crystalline forms such as quartz, cristobalite, tridymite, and the high-pressure phase known as stishovite. Silica also exists in the vitreous form, which can be viewed as a frozen-in supercooled liquid. Vitreous silica is a network glass of corner-sharing SiO 4 tetrahedra. It displays mediumrange order, whose fingerprint is the first sharp diffraction peak (FSDP) in the static structure factor.'2 Crystalline and vitreous forms of silica have unique mechanical, thermal, electrical, and optical properties. As a result, this material has numerous technological applications': in sensors and accelerometers and in microelectronic devices. Porous silica is an excellent material for thermal insulation and passive solar energy collection devices. In this paper, we investigate mechanical failure in vitreous silica at the microscopic level. Despite a great deal of experimental, theoretical, and computer simulation work, there is very little knowledge about the microscopics of crack-front propagation in amorphous SiO,. Five years ago, Ochoa, Swiler and Simmons4 carried out molecular-dynamics (MD) simulations to investigate brittle failure in vitreous silica. Their simulations were based on a simple Born-Mayer-Huggins potential and the system they simulated was of a modest size. Using large scale molecular-dynamics simulations, we investigate dynamic fracture in vitreous SiO2 at temperatures between 300 and 900K. These fracture simulations, involving 1. 18 million particle systems, are based on interatomic potentials 5 which include effects of charge transfer, electronic polarizabilities, steric repulsion, and covalency. From these simulations we find that the amorphous silica systems at 300 and 600K undergo cleavage fracture. The crack tips reach terminal speeds of 1.40 km/s at 300K and 0.85 km/s at 600K. At 300K the terminal speed is close to half the Rayleigh wave speed in this material (VR = 2.84 km/s). The critical strains at
267 Mat. Res. Soc. Symp. Proc. Vol. 455 ©1997 Materials Research Society
which the amorphous systems at 300 and 600K fracture are 8% and 9%, respectively. In contrast, at 900K the amorphous silica system shows plastic behavior up to 17% strain.
MOLECULAR DYNAMICS SIMULATIONS MD simulations of silica are performed using an empirical interatomic potential' which includes two- and three-body terms. The two-body part contain
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