Transitional flaw size sensitivity of amorphous silica nanostructures analyzed by ReaxFF/SiO based molecular dynamics

J Park and K Kirane, JOURNAL OF APPLIED PHYSICS, 129, 175103 (2021).

DOI: 10.1063/5.0044840

This paper presents an investigation aimed at understanding the flaw size sensitivity in amorphous silica nanostructures. The investigation is carried out in LAMMPS via reactive molecular dynamics analyses by employing ReaxFF-SiO, a bond order-based force field. First, a validated procedure is developed to build the amorphous silica nanostructures via a melt, quench, and equilibration process. This procedure is seen to correctly reproduce the molecular structure as well as mechanical properties of silica. The best agreement to experimental data is obtained by using non-periodic boundary conditions with the isothermal- isobaric ensemble. The validated model is then used to analyze crack propagation in amorphous silica samples with varying flaw sizes subjected to mode I tensile fracture. The analyses reveal a marked transition from flaw sensitive to insensitive behavior with decreasing flaw size. The transition flaw size is found to be 20-25 angstrom. Fracture propagation is found to be accompanied by the formation of several single atom thick strands near the crack tip, previously reported as "stress fibers." This is proposed as a viable mechanism causing blunting of an initially sharp crack, analogous to blunting of a macroscale crack by an inelastic damage zone. The nanoscale fracture process zone estimated by probing near crack tip stresses is found to nearly equal the transition flaw size, providing an explanation for the transitional behavior. A semi-empirical, transitional flaw size effect law rooted in quasibrittle fracture mechanics is derived based on asymptotic matching and is found to capture well the nanoscale transitional behavior.

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