Multimillion-to-billion atom molecular dynamics simulations of deformation, damage, nanoindentation, and fracture in silica glass and energetic materials

Multimillion-to-billion molecular dynamics (MD) simulations are carried out to study atomistic mechanisms of deformation, damage and failure in silica glass and energetic materials. The simulations are based on experimentally validated interatomic potentials and employ highly efficiently algorithms for parallel architectures.;The onset of void-void interaction is investigated by performing MD simulations of amorphous silica under hydrostatic tension. The simulations reveal that nanocavities in amorphous silica (a-SiO2), which are linked to Si-O rings, play an important role in void-void coalescence and inter-void ligament failure.;Nanocracks nucleated by the migration of three-fold coordinated Si and nonbridging O on ---Si-O-Si-O--- rings are observed in the multimillion MD simulations of a single void in amorphous silica subjected to a high shear rate. With the increase in shear strain, nanocracks appear on void surfaces and the voids deform into a threadlike structure. At a strain of 40%, the voids break into fragments. The results are similar to experimental and theoretical studies of bubble deformation and breakup under shear.;Defects such as voids are known to be important in the detonation of energetic materials. To investigate deformation of a void in an RDX crystal under high shear rate, we have performed million-atom reactive force field (ReaxFF) MD simulations. Simulations reveal that without breaking a bond, the excess strain energy leads to translational and rotational motion of RDX molecules. At a strain of 13%, molecules with high kinetic energy collapse inward without affecting the rest of the system.;MD simulations of nanoindentation in amorphous silica reveal migration of defects and their recombination in the densified plastic region under and the material pileup region around the indenter. The plastic flow of silica glass is related to the defect transport mechanism where a defect migrates a considerable distance via a chain of bond-switching events[44]. We obtained a hardness value of 7.2 GPa using a sharp indenter and 8.0 GPa for a slightly blunt indenter.;We have also performed nanoindentation simulation on a (100) alpha-RDX crystal surface using ReaxFF. Simulation reveals localized melting and decomposition of RDX molecular fragments. We have found a distinct (210) plane boundary, where molecules above the (210) plane have displaced dramatically and molecules below the plane remain intact. Simulation also shows the fragmented RDX molecules diffuse from the substrate and walk on the indenter surface.