Session H6: EM.1 Ignition I

Dissipative Heating in Porous Solid Explosives: Correlation of Thermomechanical Fluctuations and Microstructure

Impact induced heating of porous solid explosives is locally influenced by variations in microstructure. Materials having similar effective (or average) porosities and composition, and particle size and shape distributions, may have different impact responses due to spatial fluctuations in these quantities at the particle scale. In this study, a combined finite and discrete element technique is used to computationally examine the inert impact response of aluminized HMX for different effective porosities and metal mass fractions, and explosive and metal particle sizes. Emphasis is placed on examining the statistical correlation between predicted fluctuations in thermo-mechanical fields within and behind compaction waves and the local microstructure. To this end, predicted fields are mapped onto the initial material configuration and analyzed using multivariate Principal Component Analysis (PCA). Preliminary predictions will be given that identify microstructural features, or combinations of features, that result in a high probability of hot-spot formation and their dependence on impact speed.   

HNS EOS Refinements and Qualitative Experimental Parameter Space Modeling

There remains great interest in predicting the response of energetic materials to impact stimulus from a performance standpoint. In most conventional condensed-phase energetic materials, this response is governed by characteristics of the microstructure below the macroscale. To better understand the response of energetic materials to impact, a mesoscale model has been developed. The objective of this work is to show the capability of modeling the response of heterogeneous energetic materials without relying on data fitting routines, and to use these models to investigate the physics of shock response and how they lead to shock to detonation transition. To this end, a first principles EOS was used to parameterize a variable heat capacity Mie-Gruneisen model for the condensed phase EOS. Using the realistic hot-spot states made possible by this EOS, trends from the historical parameter space of flyer impact experiments were reproduced. These results highlight the importance of the EOS in grain-scale reactive burn models, and showcase the predictive capabilities of these models when coupled with a valid unreacted EOS.   

Mesoscale Simulations of Reaction Initiation and Growth in HE Composites using PBRB model

Two-dimensional (2D) finite element based mesoscale simulation results are presented to predict reaction initiation and growth in high energy (HE) composites using the physics based reactive urn (PBRB) model. The HE composites are modeled as an ensemble of grains with statistically-distributed second phase particles. Their shock response is modeled with elastic-inelastic deformation coupled with the PBRB equation-of-state model. The inter-grain response is described by the contact-cohesive model that allows grain boundary failure and creation of free surface with friction characteristics during compressive shock loading. The heat generation due to the non-linear elastic, inelastic, cohesive, and friction energy dissipation into pre-assumed statistically-distributed hot spots, surface sublimation, and gas phase reaction are described as coupled mechanisms by the PBRB model yielding the mean stress as a function of the reaction at any given time. The simulations predict the time and run to detonation with reasonable agreement with data. The relative merits of 1D hot spot idealization, embodied in the PBRB model, for generic mesoscale simulations will be discussed.   

Thermomechanical Response of HMX Polycrystals to Simulated Impact Loading

A framework for analyzing the thermo-mechanical response of ensembles of HMX crystals to impact loading is presented. The effects of material microstructure and anisotropy on heating and stress evolution are investigated. The model accounts for anisotropic elasticity, crystalline plasticity, and thermal conduction. Simulations carried out concern the response of fully dense HMX polycrystalline ensembles under loading at impact velocities from 50 - 400 m/s. Herein, the effect of the inherent anisotropies on the energy and stress localization in an HMX based PBX is quantified. The results show that when local stress and temperature states are critical, such as energetic composites, modeling the crystalline anisotropy of the constituents is essential to capturing the whole range of states experienced by the material.   

Microstructural Effects on the Ignition Behavior of Various HMX Materials

The detonation physics community has embraced the idea that initiation of high explosives proceeds from an ignition event through subsequent growth to steady detonation. This construct is the basis for the well-known Lee-Tarver reactive flow model. A weakness of all the commonly used ignition and growth models is that microstructural characteristics are not explicitly incorporated in their ignition terms. This is the case in spite of a demonstrated, but not well-understood, empirical link between morphology and initiation of energetic materials. Morphological effects have been parametrically studied in many ways, with the majority of efforts focused on establishing a tie between bulk powder metrics and ignition of the consolidated material. More recently, there has been a shift toward characterizing the microstructure of consolidated materials in order to understand the underlying mechanisms governing performance. We have assessed the utility of using the James' Ignition model as a tool to quantify effects of bed microstructure on ignition behavior. We have studied the ignition behavior of four types of HMX materials ranging from fine particle fluid energy milled to course particle material. We will also report characterization of the pressed microstructure of each of the various materials and discuss how the measured ignition behavior may have been influenced. DISTRIBUTION A. Approved for public release, distribution unlimited. (96ABW-2013-0063)