High fidelity numerical simulations and atomistics-informed continuum mesoscale modeling of heterogenous energetic materials
Creators: name: Okafor, Chukwudubem Obinna
Publisher: name: University of Iowa
Year: 2024
Subjects: Energetic materials
High order schemes
Hotspot ignition and growth
Pore collapse
Rate dependent plasticity
Shear bands

description: Eulerian frameworks for the simulation of shocked heterogeneous energetic materials (EM) typically rely on low order numerical methods and diffuse interfaces to handle the dynamics of shock-induced deformation, and mesoscale isotropic strength models forms traditionally employed for metals to capture key aspects of shock-induced energy localization. However, low-order schemes incur stringent mesh demands which is a bottleneck for computations of large ensembles of microstructures required for closure models. Compared to all-atom molecular dynamics (MD) simulations the currently available continuum strength models for HMX fail to correctly capture pore collapse and hotspot temperatures. This thesis presents a high order accurate, high fidelity numerical framework to accurately model the response of energetic materials to shock/impact loading. First, high-order numerical discretization and sharp interface treatments are combined to produce a high-fidelity modeling and simulation capability to accurately capture the physics of energetic materials without incurring substantial computational time penalty. The methodology is then applied to demonstrate the impact of high-accuracy simulations at the micro-, meso- and macro-scales. Meso-scale simulations of shock-induced hotspot evolution in microstructures of neat-pressed EM show that the accuracy of the techniques significantly impacts the calculation of energy localization rates which are used as closure models at the macro-scale. To address the second problem, continuum material models for HMX are examined in the context of shock-induced pore collapse, treating predictions from molecular dynamics (MD) simulations as ground truth. Insights from MD are employed to advance a continuum strength model, which is shown to capture key aspects of the physics of shock-induced localization in HMX. The MD-guided strength model combined with other atomistics-consistent, thermo-mechanical material properties produces continuum pore-collapse results in better alignment on several aspects with those predicted by MD, including pore-collapse mechanism and rate, shear-band formation in the collapse zone, temperature fields in the hotspot zone and surrounding material. Finally, these updated high-fidelity physical models which show significant differences in hotspot development at pore collapse sites are combined with reaction rate laws which leads to new physics characterized by energy localization at shear bands. Calculation of ignition and growth rates is critical for hydrocodes modeling the shock-to-detonation transition in energetic materials. Previous meso-scale calculations of ignition and growth rates relied on material models that failed to capture key behaviors, including thermophysical properties and material strength, leading to incomplete representations of energy localization at reactive hotspots. By leveraging atomistics-consistent models, this thesis also redefines hotspot characterization, introduces a re-evaluated methodology for ignition and growth rate quantification, and compares these results to previous approaches. The present work therefore leads to advances in both physical understanding and quantification of energy localization in HMX, informing the development of improved meso-informed reactive burn models for prediction of sensitivity of such materials.