Dynamic Fracture Mechanisms in Nanostructured and Amorphous Silica Glasses Million-Atom Molecular Dynamics Simulations
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Dynamic Fracture Mechanisms in Nanostructured and Amorphous Silica Glasses Million-Atom Molecular Dynamics Simulations. Laurent Van Brutzel, Cindy L. Rountree, Rajiv K. Kalia, Aiichiro Nakano, and Priya Vashishta Concurrent Computing Laboratory for Materials Simulations, Biological Computation and Visualization Center, Department of Physics and Astronomy, Department of Computer Science, Louisiana State University, Baton Rouge, Louisiana 70803 ABSTRACT Parallel molecular dynamics simulations are performed to investigate dynamic fracture in bulk and nanostructured silica glasses at room temperature and 1000 K. In bulk silica the crack front develops multiple branches and nanoscale pores open up ahead of the crack tip. Pores coalesce and then they merge with the advancing crack-front to cause cleavage fracture. The calculated fracture toughness is in good agreement with experiments. In nanostrucutred silica the crack-front meanders along intercluster boundaries, merging with nanoscale pores in these regions to cause intergranular fracture. The failure strain in nanostructured silica is significantly larger than in the bulk systems.
INTRODUCTION Amorphous silica (a-SiO2) is widely used in various technological applications because of its unique chemical and physical properties. However, the brittle nature and poor shock resistance of silica have precluded its use as a structural material. With the synthesis of “ductile” nanophase ceramics, there is renewed hope that novel amorphous nanostructured silica systems that fracture more gracefully than conventional bulk a-SiO2 will find use in structural applications. However, any hope to enhance mechanical properties rests on understanding crack initiation and propagation at the atomic scale. Ten years ago Simmons and al. carried out the first Molecular Dynamics simulations to investigate brittle fracture in a-SiO2 [1-3]. They showed that the crack in a-SiO2 is not only initiated by the surface defects but also has an origin in the intrinsic structure. Nevertheless, these simulations were not large enough to study the propagation on a largest scale. We present here results of Molecular Dynamics (MD) simulations on crack propagation and fracture in both bulk a-SiO2 and nanostructured a-SiO2 at low and high temperatures. These simulations, involving a million-atom each, are performed with reliable inter-atomic potentials on parallel computers using highly efficient algorithms. In the bulk system at room temperature (300 K), we find that the crack-front propagates by merging with the cavities that open up just ahead of it. In contrast, the bulk system at 1000 K many more cavities both close and far from the crack-tip appear. Those far from the crack-tip coalesce before merging with the main crack forming a secondary crack. In the nanostructured a-SiO2 at room and high temperature, we observe intergranular fracture with the crack meandering along the nanoparticle boundaries. The observed crack propagation and fracture behavior in both bulk and nanostructured silica are related t
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