Yasuyuki Horie

Energy localization in HMX-Estane polymer-bonded explosives during impact loading

We report the results of a mechanistic study of energy localization in aHMX (High Melting point eXplosive octahydro-1,3,5,7-tetranitro-1,2,3,5-tetrazocine)/Estane PBX system during dynamic loading. The focus is on the thermal-mechanical response over the strain rate range of 104 – 105 s−1 under different confinement conditions. A recently developed cohesive finite element method is used to track and analyze the contributions to heating from different constituents, interfaces, deformation and fracture mechanisms, and internal friction. In particular, energy dissipations due to viscoelastic deformation, grain fracture, interfacial debonding, and friction along crack faces are quantified as functions of time and overall deformation. The materials analyzed have HMX volume fractions between 0.69 and 0.82. Calculations show that variation in strain rate can significantly affect the spatial distribution but not the overall number of hot spots. Higher confining stresses lead to more intense heating in the binder ...

Microstructural level response of HMX-Estane polymer-bonded explosive under effects of transient stress waves

The effect of transient stress waves on the microstructure of HMX–Estane, a polymer-bonded explosive (PBX), is studied. Calculations carried out concern microstructures with HMX grain sizes on the order of 200 μm and grain volume fractions in the range of 0.50–0.82. The microstructural samples analysed have an aspect ratio of 5:1 (15×3 mm), allowing the transient wave propagation process resulting from normal impact to be resolved. Boundary loading is effected by the imposition of impact face velocities of 50–200 m s−1. Different levels of grain–binder interface strength are considered. The analysis uses a recently developed cohesive finite element framework that accounts for coupled thermal–mechanical processes involving deformation, heat generation and conduction, failure in the forms of microcracks in both bulk constituents and along grain/matrix interfaces, and frictional heating along crack faces. Results show that the overall wave speed through the microstructures depends on both the grain volume fraction and interface bonding strength between the constituents and that the distance traversed by the stress wave before the initiation of frictional dissipation is independent of the grain volume fraction but increases with impact velocity. Energy dissipated per unit volume owing to fracture is highest near the impact surface and deceases to zero at the stress wavefront. On the other hand, the peak temperature rises are noted to occur approximately 2–3 mm from the impact surface. Scaling laws are developed for the maximum dissipation rate and the highest temperature rise as functions of impact velocity, grain volume fraction and grain–binder interfacial bonding strength.

Prediction of probabilistic ignition behavior of polymer-bonded explosives from microstructural stochasticity

Random variations in constituent properties, constituent distribution, microstructural morphology, and loading cause the ignition of explosives to be inherently stochastic. An approach is developed to computationally predict and quantify the stochasticity of the ignition process in polymer-bonded explosives (PBXs) under impact loading. The method, the computational equivalent of carrying out multiple experiments under the same conditions, involves subjecting sets of statistically similar microstructure samples to identical overall loading and characterizing the statistical distribution of the ignition response of the samples. Specific quantities predicted based on basic material properties and microstructure attributes include the critical time to ignition at given load intensity and the critical impact velocity below which no ignition occurs. The analyses carried out focus on the influence of random microstructure geometry variations on the critical time to ignition at given load intensity and the critic...

Ignition probability of polymer-bonded explosives accounting for multiple sources of material stochasticity

Accounting for the combined effect of multiple sources of stochasticity in material attributes, we develop an approach that computationally predicts the probability of ignition of polymer-bonded explosives (PBXs) under impact loading. The probabilistic nature of the specific ignition processes is assumed to arise from two sources of stochasticity. The first source involves random variations in material microstructural morphology; the second source involves random fluctuations in grain-binder interfacial bonding strength. The effect of the first source of stochasticity is analyzed with multiple sets of statistically similar microstructures and constant interfacial bonding strength. Subsequently, each of the microstructures in the multiple sets is assigned multiple instantiations of randomly varying grain-binder interfacial strengths to analyze the effect of the second source of stochasticity. Critical hotspot size-temperature states reaching the threshold for ignition are calculated through finite element ...

Modeling and simulation of pressure waves generated by nano-thermite reactions

This paper reports the modeling of pressure waves from the explosive reaction of nano-thermites consisting of mixtures of nanosized aluminum and oxidizer granules. Such nanostructured thermites have higher energy density (up to 26 kJ/cm3) and can generate a transient pressure pulse four times larger than that from trinitrotoluene (TNT) based on volume equivalence. A plausible explanation for the high pressure generation is that the reaction times are much shorter than the time for a shock wave to propagate away from the reagents region so that all the reaction energy is dumped into the gaseous products almost instantaneously and thereby a strong shock wave is generated. The goal of the modeling is to characterize the gas dynamic behavior for thermite reactions in a cylindrical reaction chamber and to model the experimentally measured pressure histories. To simplify the details of the initial stage of the explosive reaction, it is assumed that the reaction generates a one dimensional shock wave into an air...

Computational prediction of probabilistic ignition threshold of pressed granular octahydro-1,3,5,7-tetranitro-1,2,3,5-tetrazocine (HMX) under shock loading

The probabilistic ignition thresholds of pressed granular octahydro-1,3,5,7-tetranitro-1,2,3,5-tetrazocine explosives with average grain sizes between 70 μm and 220 μm are computationally predicted. The prediction uses material microstructure and basic constituent properties and does not involve curve fitting with respect to or prior knowledge of the attributes being predicted. The specific thresholds predicted are James-type relations between the energy flux and energy fluence for given probabilities of ignition. Statistically similar microstructure sample sets are computationally generated and used based on the features of micrographs of materials used in actual experiments. The predicted thresholds are in general agreement with measurements from shock experiments in terms of trends. In particular, it is found that grain size significantly affects the ignition sensitivity of the materials, with smaller sizes leading to lower energy thresholds required for ignition. For example, 50% ignition threshold of...

Computational Analysis of Ignition in Heterogeneous Energetic Materials

This paper focuses on the ignition of polymer-bonded explosives (PBXs) under conditions of non-shock loading. The analysis uses a recently developed ignition criterion [ which is based on the quantification of the distributions of the sizes and temperatures of hotspots in loading events. This quantification is achieved by using a cohesive finite element method (CFEM) developed recently and the characterization by Tarver et al. [ of the critical size-temperature threshold of hotspots required for chemical ignition of solid explosives. Calculations are performed on PBXs having monomodal grain size distributions with grain volume fractions varying between 0.72 and 0.90. The impact velocities considered vary between 100 and 200 ms-1. Results show that the average distance between the hotspots is dependent on the grain volume fraction. As the grain volume fraction increases, the time to criticality (tc) decreases, signifying increases in the ignition sensitivity of PBX to impact loading. The microstructure-performance relations obtained can be used to design PBXs with tailored performance characteristics and safety envelopes.

CYLINDRICAL EXPLOSIVE DISPERSAL OF METAL PARTICLES

The explosive dispersal of densely‐packed metal particles in cylindrical RDX‐based charges is studied numerically in support of experimental trials. Simulations are conducted using a reactive multiphase fluid dynamic code. Spherical tungsten particles are applied in high metal mass fraction cylindrical and spherical charges in two configurations: a particle matrix uniformly embedded in a solid explosive versus an annular shell of particles surrounding a high‐explosive core. The effect of particle number density is investigated by varying the nominal particle diameter from 27 to 120 □m while maintaining a constant metal mass fraction. Results are compared with steel particles to evaluate the influence of material density on dispersal. The dispersal dynamics are recorded on wave diagrams and are observed at radial locations in terms of arrival time, velocity and particle concentration.

Fluid dynamic modeling of nano-thermite reactions

This paper presents a direct numerical method based on gas dynamic equations to predict pressure evolution during the discharge of nanoenergetic materials. The direct numerical method provides for modeling reflections of the shock waves from the reactor walls that generates pressure-time fluctuations. The results of gas pressure prediction are consistent with the experimental evidence and estimates based on the self-similar solution. Artificial viscosity provides sufficient smoothing of shock wave discontinuity for the numerical procedure. The direct numerical method is more computationally demanding and flexible than self-similar solution, in particular it allows study of a shock wave in its early stage of reaction and allows the investigation of “slower” reactions, which may produce weaker shock waves. Moreover, numerical results indicate that peak pressure is not very sensitive to initial density and reaction time, providing that all the material reacts well before the shock wave arrives at the end of the reactor.

Hot Spots, High Explosives Ignition, and Material Microstructure

This paper reviews the subject of high explosives ignition with focus on impact and shock loadings from the view point of modeling and identifying scientific issues that need to be addressed to establish a science basis on which to build a better predictive methodology for explosives safety. A motivation is to move explosives safety from empiricism to an advanced computation based analytic scientific and engineering basis, facilitating innovation. But because of the vastness of the subject, the scope of coverage is limited primarily to select aspects of shock and impact loadings that are relevant to the project the author has been involved in and presented in the companion paper by Min Zhou et al. of Georgia Institute of Technology.* (*Min Zhou et al., Materials Science Forum, in this volume)