Bulletin of the American Physical Society
21st Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 64, Number 8
Sunday–Friday, June 16–21, 2019; Portland, Oregon
Session Y2: TMS: Mesoscale Explosive Initiation IV |
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Chair: Brian Henson, LANL Room: Grand Ballroom II |
Friday, June 21, 2019 9:15AM - 9:30AM |
Y2.00001: Computational investigation of preshock desensitization of liquid nitromethane with air-filled cavities XiaoCheng Mi, Louisa Michael, Nikolaos Nikiforakis, Andrew J. Higgins The phenomenon of preshock desensitization of heterogeneous explosives has been experimentally investigated for decades. Two governing mechanisms have been speculated: 1) Elimination of hot spots due to the removal of heterogeneities (e.g., a weak preshock closes the pores); 2) a smaller increase in temperature and entropy due to multiple input shock waves. To determine the dominant desensitization mechanism, two-dimensional, meso-resolved simulations are performed to capture the dynamics of double-shock initiation in mixtures of liquid nitromethane with air-filled cavities. Without invoking any phenomenological reaction models to account for the meso-scale effects, Arrhenius chemical kinetics and a statistically significant amount of heterogeneities are explicitly considered in these simulations. Uniformly random, random, and clustered distribution of cavities are considered in order to probe the response of heterogeneities to double shocks. Statistical analysis are performed on the simulation results to gain further insight into the mechanisms underlying shock desensitization. [Preview Abstract] |
Friday, June 21, 2019 9:30AM - 9:45AM |
Y2.00002: Direct comparison of computations and experiments on void collapse in PMMA over a range of loading conditions. Nirmal Rai, Emilio Escuariza, Dan Eakins, H.S. Udaykumar Pore collapse in energetic materials under shock loading leads to hotspot formation and reaction initiation. Depending on the strength of the shock, the collapse mode can vary from weak to strong jet formation. For shock strengths comparable to the yield stress of the material, the strength of the material (strain rate dependency, anisotropy etc.) affects the collapse mode and energy localization collocates with adiabatic shear bands. For high strength shocks, the collapse proceeds with a strong material jet formation. In the previous work, this shift in collapse mechanism has been studied in HMX using meso-scale simulations. However, this transition in collapse modes has not been analyzed/observed using physical experiments. In the present work, a coupled simulation and experiment approach has been pursued to explain the transition in collapse mechanisms in PMMA that exhibits rate dependent strength behavior similar to energetic crystals. It is observed that PMMA shows similar transition behavior as HMX, varying from a shear-band localized collapse mode to jet collapse with increasing shock strength. Using scaling analysis, the effect of various collapse modes on the hotspot shape and temperature is quantified. Direct, head-to-head comparison of pore collapse characteristics are demonstrated across a wide range of loading conditions. This work therefore provides crucial insights for modelers of energetic material response to shock/impact loading. [Preview Abstract] |
Friday, June 21, 2019 9:45AM - 10:00AM |
Y2.00003: Approximating Reaction Chemistry of Energetics for Critical Velocity Predictions Sushilkumar Koundinyan, D. Barrett Hardin The ability to accurately predict temperature during simulations of explosives is essential as the chemistry driving the initiation process is highly dependent on local temperature. The current major chemical reaction models are all Arrhenius functions of temperature. Yet, temperature is the least understood and most volatile parameter in many simulations. Hence, we analyze the effect of mesh resolution on temperature for 2-D void collapse simulations with widely used models for HMX. The incorporation of chemical kinetics in thermo-mechanical simulation is computationally expensive. Yet, the effect of chemical kinetics is required to predict ignition behavior in energetic materials. The analysis presented here is our first step at finding an approximation of reaction chemistry using non-reactive simulations. To that effort, a series of non-reactive HMX single void collapse simulations are performed at various shock loading conditions and Tarver critical hot spot curve is used to approximate the critical velocity. The results are compared to simulations performed with reactive chemistry at similar loading conditions. [Preview Abstract] |
Friday, June 21, 2019 10:00AM - 10:15AM |
Y2.00004: Single crystal plasticity model with deformation twinning for the high rate deformation of $\beta $-HMX Milovan Zecevic, Darby Luscher, Marc Cawkwell, Francis Addessio, Kyle Ramos Deformation twinning is an important deformation mechanism during loading along certain crystallographic directions of $\beta $-cyclotetramethylene tetranitramine ($\beta $-HMX). In this work, a finite strain thermomechanical model developed by Luscher et al. is extended to include twinning as a deformation mechanism in addition to plastic slip. The stress is derived from the free energy expression including a term representing equation of state. The crystal plasticity framework is used to divide the total strain into inelastic and elastic, where elastic part is used in expression for free energy. The twin systems are treated as pseudo slip systems and the shear rate on twin systems is evaluated in terms of the twin resistance and appropriate projections of the stress tensor.~ The model parameters were calibrated against a set of plate impact experiments performed on $\beta $-HMX by Dick et al. The remaining plate impact experiments are used to evaluate the predictive capability of the model. The quality of the model fits and predictions is discussed from physical and modeling perspective. Particularly, the role of twin modeling on results is highlighted. The model is used to explore relationship between propensity for twinning and crystal orientation, experimentally studied in Gallagher et al. [Preview Abstract] |
Friday, June 21, 2019 10:15AM - 10:30AM |
Y2.00005: Mechanistic Modeling of Shock to Detonation Transition in High Explosives at Mesoscale Ahmed Hamed, Marisol Koslowski Understanding energetic materials response to different types of stimuli is of critical importance for efficient performance and safety of their applications. In this regard, developing high fidelity models for shock to detonation transition is a key challenge. Reliable prediction of this phenomenon dictates capturing the heterogeneous nature of hot-spot formation and initiation as well as the interplay between the underpinning mechanisms---spanning different time- and length-scales. We present a novel mesoscale model for the shock to detonation transition in high explosives. The model solves reactive flow equations in Lagrangian formulation with explicit consideration of the underlying mechanical, chemical, and thermal processes. Supplementary models are used to account for dissipative heating mechanisms, namely, viscoplasticity, fracture, and thermoelastic effects. For thermodynamic consistency, energy conservation equation incorporates two distinct equations of state for solid reactant and gaseous products to account for the two modes of energy transport. Statistical approach is sought for microstructure features representation. Informed by a separate investigation, hot spots critical temperature is size-dependent. All model parameters are calibrated by MD simulations. [Preview Abstract] |
Friday, June 21, 2019 10:30AM - 10:45AM |
Y2.00006: Modeling the effect of plasticity and damage in $\beta $-HMX single crystals under shock loading Camilo Duarte, Marisol Koslowski, Nicolo Grilli $\beta -$HMX is an energetic crystal commonly used in polymer-bonded explosives (PBX). In PBXs, ignition may occur due to the formation ``hot-spots'' in the material, which are regions of localized thermal energy. Several mechanisms of hot spot formation have been proposed such as void collapse, plastic flow, crack propagation, and crack surface/interfacial friction. Additionally, it is believed that regions of high stress concentration in the energetic crystal such as micro-cracks and voids are preferential nucleation sites of hot spots. However, experimental observation of hot-spots in energetic materials remains difficult due to the short time and length scale of the reactions. In order to understand which mechanisms may mitigate or not the formation of hot spots we study the effect of plasticity and fracture evolution in shock-loaded $\beta $-HMX single crystals. A thermodynamically consistent finite strain model is used with a crystal plasticity model for the energetic crystal. Fracture evolution is modeled using a phase field model of damage. Numerical results are compared with gas gun experiments on $\beta $-HMX crystals containing engineered defects.~ [Preview Abstract] |
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