Bulletin of the American Physical Society
15th APS Topical Conference on Shock Compression of Condensed Matter
Volume 52, Number 8
Sunday–Friday, June 24–29, 2007; Kohala Coast, Hawaii
Session C6: Structural Reactive Materials |
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Chair: John Aidun, Sandia National Laboratories Room: Fairmont Orchid Hotel Promenade I/II |
Monday, June 25, 2007 1:45PM - 2:15PM |
C6.00001: Mechanochemical Aspects of Shock-Induced Reactions: Time-Resolved Experiments and Meso-Scale Simulations Invited Speaker: Mechanochemical aspects of shock-compression of reactive powder mixtures leading to ``shock-induced'' reactions will be reviewed. Time-resolved experiments performed on intermetallic-forming mixtures of powders of varying configurations have shown evidence of shock-initiation of reactions inferred on the basis of compressibility changes, shock-velocity increases, or excess pressures. The influence of the characteristics of starting powder mixtures on their densification response, crush-up stress, and type and extent of configurational changes between reactants has also been revealed by experiments. However, the mechanisms responsible for reaction initiation and product formation, extent of reaction and type(s) and amount(s) of reaction products formed, and their correlation with measured changes in shock velocity and/or shock compressibility have not been conclusively demonstrated. Furthermore, while changes in compressibility associated with reactions have been calculated based on an assumed kinetics, and constant volume or constant pressure approximations to account for the heat of reaction, the certainty of high-pressure states and their associated kinetics is a function of the actual reaction product and its amount, which has been lacking. Meso-scale numerical simulations of shock-wave propagation through discretely represented powder mixtures can be used to approximate the configuration of reactants participating in the reaction based on a pre-defined stress, strain, or temperature criterion, and therefore, determine the product and its extent under a given set of loading conditions. Meso-scale numerical analysis can also be used to probe configurational changes and reaction mechanisms, considering processes such as forced or turbulent flow and/or fracture and dispersion of reactant powders of dissimilar properties or morphologies. The understanding can be used for controlling energy release characteristics for design of microstructure or materials with controlled energetics. [Preview Abstract] |
Monday, June 25, 2007 2:15PM - 2:30PM |
C6.00002: A study of reactant interfaces in Ni+Al particle systems during shock wave propagation Ryan A. Austin, David L. McDowell, Yasuyuki Horie, David J. Benson Macro-scale responses of energetic materials during shock compression are influenced strongly by thermo-mechano-chemical processes occurring at the level of the microstructure. For example, it is believed that the propagation of chemical reactions in reactive particle systems is intimately linked to conditions at reactant interfaces such as surface temperature, phase changes, defect density, and mass mixing due to inelastic deformation. To provide explicit resolution of such interfacial conditions, numerical models are constructed. The finite element method is used to numerically solve the differential equations that govern the coupled thermomechanical response of micron-size particle mixtures of Ni and Al during shock wave propagation (interface chemistry is not yet modeled). The size and temperature distributions of contiguous reactant contact surfaces are quantified for a range of shock strengths. A parametric study of mixture attributes is undertaken to assess the sensitivity of the aforementioned distributions to variations of the microstructure. [Preview Abstract] |
Monday, June 25, 2007 2:30PM - 2:45PM |
C6.00003: Mechanics driven Chemical Reactions in Structural Energetic Materials. Vindhya Narayanan, Derek Redding, Sathya Hanagud Fundamental mechanisms that are responsible for shock-initiation of chemical reactions, are dominated by non-equilibrium processes including changes in reactant particle configurations caused by plastic deformation or by fracture, mixing of constituents in and around the voids, and rapid increases in temperature from mechanical work. Mechanics driven chemical reactions occur in structural energetic mixtures, during the high-pressure shock state in time scales of mechanical equilibration. These shock-induced reactions represent a unique class of chemical behavior that is not clearly understood. To understand the observed results a model is presented, in this paper, in a hybrid non-equilibrium thermodynamic framework that combines the concepts of internal variables and thermodynamic fluxes. The governing system of partial differential equations is formulated in the framework of extended irreversible thermodynamics. This represents the intimate mixing of the reactants, which is important in the reaction initiation process. The model is developed to distinguish induced or assisted chemical reactions with uniformly blended mixture theories. A yield condition that represents an increase of yield stress behind the shock front is considered. A method of determining the transition states and paths to reach the transition state due to plastic work and void collapse are also discussed. The formulated partial differential equations are integrated and results are discussed. [Preview Abstract] |
Monday, June 25, 2007 2:45PM - 3:00PM |
C6.00004: Mechanistic Aspects of Shock-induced Reactions in Ni+Al Powder Mixtures Dan Eakins, Naresh Thadhani A combination of parallel-plate impact experiments utilizing stress measurements and meso-scale numerical simulations are used to investigate the effect of particle morphology on the mechanical and chemical response of Ni+Al powder mixtures. The instrumented gas-gun impact experiments were performed at pressures up to 6 GPa. Based on measured shock velocity increases and shock compressibility changes consistent with the Ballotechnic model, the flake-based powder mixture was found to exhibit shock-induced reaction. The particle-level mechanistic details of deformation, mass-flow, and mixing, were explored through discrete particle continuum simulations, validated against the experimental results. The micron-scale spherical and flake mixtures were found to display widely varying configurational changes at several length scales, which give insight into why the flake-Ni morphology is more susceptible to shock-induced reactions under the imposed conditions. [Preview Abstract] |
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