Multiscale interface delamination, fracture, and reaction initiation of oriented HMX/HTPB interfaces using steered molecular dynamics informed simulations*

The property, performance, and sensitivity of faceted molecular crystals are hard to determine without understanding the post-shock microstructure. Experimental investigations reveal only a subset of properties that are hard to connect to their observed behavior. Interfaces play a unique and outsized role in modifying properties and responses in materials. A rigorous thermodynamic and kinetic analysis is length-scale and time-scale dependent. In this thesis, we extend first-principles-informed molecular simulations to quantify non-equilibrium properties and responses of faceted molecular crystals. We explore energetic crystals and their oriented surfaces; in particular, we explore surface free energy (SFE) in place of surface energy (SE) as a better descriptor for determining their stability and response. Moreover, molecular crystals are often stabilized using polymeric binders (HTPB) and plasticizers such as DOA. The binders by themselves can modify the response of energetic composites to shock compression but can get delaminated. We calculate the work of adhesion for HTPB along different beta-HMX facets. We explore the time-dependent response to thermal and mechanical stimuli in the form of shock and shear responses needed for debonding of HMX crystals. The molecular scale information was then used in the multiphysics finite element simulations of oriented HMX crystals with binders. The propensity for crack formation and the free energy-dependent crack propagation rates along different crystal orientations will be discussed. The onset of chemical reactions is also calculated using reactive molecular dynamics (RMD) simulations. A delaminated versus fully bonded crystal interface can provide a very different hot spot formation environment.  The orientation dependence of thermal and mechanical response using multiscale methods along with reactive crack propagation can provide a much better understanding of the safe handling and performance of differently manufactured energetic composites.