Silica-based glass may possess paradoxically high resistance to hypervelocity impact due to the experimentally observed phase change emanating from high pressure characteristic to hypervelocity impact combined with irreversible densification of the material, which leads to a highly efficient kinetic energy-absorption mechanism. In order to capture this extraordinary behavior of silica glass in hypervelocity impact, a coupled thermo-mechanical model is developed in the framework of thermodynamics with internal state variables. In addition to pressure induced densification (phase change), the proposed model is aimed at capturing the effects of dramatic increase in temperature, strain rate sensitivity and fragmentation/comminution of the material. The proposed model is based on the multiplicative decomposition of the deformation gradient into thermoelastic and plastic parts. The irreversible densification of silica glass is characterized by the plastic volumetric strain which is a basic internal state variable associated with a molecular structure rearrangement due to phase change. Evolution of the plastic deformation is described by a critical state plasticity model combined with damage evolution. In the absence of damage, the elastic domain is fully informed by molecular dynamics simulations of perfectly intact silica glass. With evolving damage, the atomistically informed elastic domain shrinks smoothly to another critical state plasticity elastic domain which serves as a granular description of the fragmented/comminuted state of the material. Thermo-mechanical coupling is considered where temperature rises as a result of mechanical dissipation while mechanical behavior depends on temperature through thermal softening. In addition, the model is capable of capturing both the material’s ductile behavior (featuring significant densification due to high pressure) in the vicinity of projectile-target contact interface and its characteristically brittle behavior exhibited elsewhere. This is achieved by introducing a brittle damage initial criterion based on the thermodynamic driving force for damage that is analogous to the energy release rate-based criterion for crack growth. Constitutive functions and material parameters in the model are determined from the molecular dynamics simulations. The proposed model has been implemented in the explicit coupled thermo-mechanical finite element code and validated against hypervelocity impact experiments.
Rear surface of the fused silica target plate impacted by a steel projectile with the incident velocity of : (a) snapshot from experiment; (b) snapshot from the present simulation.