Bulletin of the American Physical Society
22nd Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 67, Number 8
Monday–Friday, July 11–15, 2022; Anaheim, California
Session 1H: Energetic Materials: Experiments, Modeling, TheoryStudent Symposium
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Room: Anaheim Marriott Platinum 5-6 |
Sunday, July 10, 2022 2:15PM - 2:30PM |
1H.00001: High-order schemes for simulation of shock-interface interactions in micro-structured materials Chukwudubem O Okafor The response of materials to high strain rate loading is important in applications such as high-speed impact and penetration, high speed flows with particles and shocked flows in multi-material media. For example, the thermomechanical response of energetic materials (EMs) to shock loading is used to characterize their sensitivity. The initiation of EMs depends on shock interactions with their complex microstructure (void spaces/defects and crystal-crystal interfaces). Previous studies have employed at best 3rd-order accurate numerical schemes for shock simulations in EMs, requiring well-resolved simulations to obtain grid-independent solutions. High-order accurate methods can provide an improved balance between computational time and accuracy of calculations. Here, a non-characteristic 5th-order WENO scheme is used to study the response of materials with complex internal structure under shock loading using an Eulerian framework. The high-order scheme is combined with levelsets to define interfaces and a HLLC (Harten,-Lax-van Leer-Contact) approximate Riemann solver is employed to eliminate numerical oscillations and maintain high-order reconstruction across the sharp interface. Test problems involving high speed impact, shocks, elastic-plastic flows and interfaces are used to evaluate the accuracy, computational cost, and performance of the high-order methods. The new techniques provide unprecedented resolution of both embedded interfaces and their dynamics, as well as resolution of shock systems and reaction fronts. |
Sunday, July 10, 2022 2:30PM - 2:45PM |
1H.00002: Multiple time scale simulations of hotspot development due to dielectric breakdown driven by piezo- and flexo-electricity in energetic materials Ju Hwan Shin We demonstrate using multiphysics/multi-timescale simulations that dielectric breakdown due to charge accumulation can lead to sufficient hotspot development leading to the initiation of chemical reactions in P(VDF-TrFE)/nAl films comprising a poly(vinylidene fluoride-co-trifluoroethylene) binder and nano-aluminum particles. The development of electric field in the material is driven by flexoelectric and piezoelectric responses of the binder to mechanical loading which has a time scale of hundreds of microseconds. The breakdown process leading to hotspots has a time scale of nanoseconds. A two-step framework for explicit microscale simulations is used. First, the mechanically driven electric field is analyzed using a mechanical-electrostatic model. Next, the transient breakdown is analyzed using a thermal-electrodynamic model. The temperature field resulting from the breakdown is used to establish the hotspot conditions for the onset of self-sustained chemical reactions. The results demonstrate that temperatures well above the ignition temperatures can be attained. Flexoelectricity plays a primary role and piezoelectricity plays a secondary role. The time to reaction initiation and the time to ignition of the poled films are ~10% shorter than those of the unpoled films. |
Sunday, July 10, 2022 2:45PM - 3:00PM |
1H.00003: Study of energy localization in shocked PBXs through interface-resolved reactive simulations on imaged profiles of HMX crystal aggregates Shobhan Roy The formation of hot spots is known to be the primary mechanism for shock to detonation transition in energetic materials such as plastic-bonded explosives (PBXs). However, the mechanistic details of shock initiation are yet to be fully understood and quantified into closure models for energy localization. In recent literature, an experimental procedure was presented to observe hot spots in deconstructed PBX samples of HMX crystals embedded in a polymeric binder, dynamically compressed using laser-launched flyer plates. We present a computational framework to perform mesoscale continuum mechanics simulations for head-to-head comparison with the experimental tests. The crystal profiles were extracted from CT scan images and imported into the computational domain. The computations leveraged interface-resolved reactive simulations using a sharp-interface Eulerian framework, along with vetted reaction and strength models for HMX and binder (Estane). The study assesses the relative importance of several experimentally observed and simulated mechanisms for hot spot initiation in PBXs, such as intra-crystal void collapse, shock-focusing at surface asperities, pores in the binder, delamination zones at crystal-binder interface, and crystal-crystal interactions. |
Sunday, July 10, 2022 3:00PM - 3:15PM |
1H.00004: Investigating the dynamic response of heterogeneous mixtures with variable geometric complexity Rafee Mahbub The dynamic response of an energetic surrogate mixture of sugar and Polydimethylsiloxane (PDMS) / UV curing resin, subjected to uniaxial loading, is investigated. Of particular interest in this investigation are the fluctuations observed at the Hugoniot states of heterogenous mixtures. This research seeks to better understand what portion of these fluctuations can be attributed to the simple mechanical response of the meso-structure of the mixture. The approach is to build samples with increasing complexity, experimentally load them and simulate them, so that direct comparisons can be made. Samples were fabricated with a single sugar cube, and two sugar cubes with varying distances between them embedded within the binder material. After samples were built, the 3D geometry was obtained using XCT scans. PDV probes were focused at specified locations within the heterogeneous mixture so that a more direct comparison between experiments and simulations could be obtained. Experiments were performed in a one-dimensional plain strain configuration with shot velocities ranging between 79 m/s and 573 m/s. Velocity and some temperature measurements were made on the target. Numerical simulations are performed utilizing one-dimensional Lagrangian (KO) and three-dimensional Eulerian (CTH) approaches. The results indicate a systematic response signature superimposed on the Hugoniot plateau which is correlated to the geometry. |
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