Overview of research in energetic materials

This paper examines the influence of microstructure on the quasi–static failure of PBX 9501, a polymer–bonded explosive (PBX) manufactured for the Los Alamos National Laboratory in America. Optical microscopy has been used to examine qualitatively cracked and pristine material. Consequent on the manufacturing process, the explosive crystals display angular features and natural facets. In addition, considerable growth twinning, internal defects and voidage has been observed. These defects are found significantly to alter the failure path. In common with other PBXs, failure paths tend to run around the long straight edges of the explosive filler and avoid regions of fine filler and binder. Explosive crystals were found to fracture due either to cracks propagating from another region or internal defects. These observations are confirmed by the use of high–resolution moire interferometry. This sensitive optical technique allows the deformation of the sample to be measured up to and including the point of failure. By taking white–light micrographs that are in exact registration with the measured displacement maps, the influence of the underlying microstructure can be seen.

The behavior of energetic materials is significantly influenced by the spatial distributions of microstructure heterogeneities and voids. We pursue the concept of Functionally Graded Energetic Materials whose microstructure features (e.g., grain size, grain volume fraction, void size, and void volume fraction) change spatially such that they may allow the behavior of the materials to be tailored. We explore using gradients in the density of voids to alter the detonation behavior of a polymer-bonded explosive (PBX) echoing PBX9501 with HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) grains and Estane binder. Five cases, two graded void distributions from 1% to 10% and 10% to 1% by volume along the length of the sample, and three uniform distributions matching the lowest (1%), average (5.5%), and highest (10%) void densities are considered. An Arrhenius reaction burn model is used to account for the chemical kinetics of HMX. Different detonation behaviors are obtained from the same graded sample when impact loading is from 1% void end and from the 10% void end as well as from the uniform cases. The SDT (shock to detonation transition) behaviors are analyzed in terms of the run distance, the time duration and shock velocity changes over the SDT process. The computational results are presented in the context of available experimental data for PBX9501 with which agreement is obtained through a parametric study. Overall, it is shown that gradients in microstructures of PBX can lead to SDT behaviors different or not obtainable from microstructures without gradients, thereby offering a mechanism for designing and tailoring new materials.

Coupling effect of high temperature and pressure on the decomposition mechanism of crystalline HMX

: The research supported by AFOSR-AASERT grant F49620-95-L-0411 (parent grant F49620-95-L-0310) for the period 1 June 1995 to 31 May 1999 is described. The purpose of the research is the formulation of methods and realistic models for studying the fundamental properties and behavior of energetic materials. The work performed during this grant focused on the development of intramolecular potentials to describe the vibrational dynamics of the insensitive energetic materials NTO (5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one) and ANTA (5-amino-3-nitro-1H-1,2,4-triazole). In addition, the applicability of intramolecular dynamics diffusion theory (IDDT) for computing reaction rates in large, complex systems such as energetic molecules and solids was demonstrated.

An environmental issue has arisen with M-72 malfunction on anti-tank ranges because many of these rockets break into pieces without exploding on impact, dispersing their energetic materials content on the ground surface and exposing them to transport by infiltration of rainfall and snowmelt. A case study (1998--2005) at Arnhem Anti-Tank Range (Garrison Valcartier, Canada, in operation since the 1970s) revealed octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) contamination and traces of 1,3,5-trinitro-1,3,5-triazine (RDX) and 2,4,6-trinitrotoluene (TNT) in ground water at varying concentrations, with all detected HMX concentrations below the USEPA guideline for drinking water of 400 microg L(-1). An HMX mass balance on a transect perpendicular to ground water flow, about 300 m downgradient of the impact area, indicated an HMX flux of about 3 g d(-1) (0.7-1 kg yr(-1), 2005). The HMX mass in the impact area on the sand terrace was estimated at 7 to 10 kg (in 2005). The annual dissolved HMX flux represents about 10% of the source. The dissolved HMX plume in ground water consisted of a series of slugs, generated at each significant infiltration event. HMX is weakly retarded by sorption and is neither biotransformed nor mineralized under the aerobic conditions of the aquifer. TNT and RDX exceeded the USEPA guideline (2 microg L(-1) RDX and 1 microg L(-1) TNT) in three and two samples, respectively. The TNT plume was discontinuous because this compound was not always present at the ground surface. TNT is biotransformed, weakly sorbed, and not mineralized. In two wells, perchlorate associated with the propellant was found at concentrations above the Health Canada preliminary guideline of 6 microg L(-1) near the firing position.

11R11. Mechanical Behavior of Engineering Materials, Volume 2: Dynamic Loading and Intelligent Material Systems. - YM Haddad (Mech Eng, Univ of Ottawa, Canada). Kluwer Acad Publ, Dordrecht, Netherlands. 2000. 484 pp. ISBN 0-7923-6355-8. $225.00.Reviewed by YA Rossikhin (Dept of Theor Mech, Voronezh State Univ of Architec and Civil Eng, ul Kirova 3-75, Voronezh, 394018, Russia).The book under review is the second volume of the two-volume monograph Mechanical Behavior of Engineering Materials. This volume is devoted to the evaluation of the engineering materials properties under dynamic loading. However, due to the wide scope of topics covered and its popular presentation, this book is not a monograph, it can be classified as a tutorial on dynamic behavior of engineering materials. In each chapter, a review of well-known results in particular aspects of dynamic loading is given, which paves the way for a person inexperienced in the field for perceiving the current state of the art in the development of mechanics of materials. Having read this book, a reader will be ready to embark on in-depth study of the subject. The second volume begins with Chapter 9. The previous eight chapters, devoted to static and quasi-static loading of linear and nonlinear elastic, viscoelastic, and elastic-plastic continuum media, are contained in Volume 1 (an interested reader is referred to the review of Volume 1 by J-C Roegiers published in AMR, July 2002, p B61). Chapter 9 introduces the subject of the response of metallic materials to dynamic loading both under high and low rates. Behavior of the materials within the plastic range, with due account for plastic instability and localization effects, is reviewed in Chapter 10. Chapter 11 presents brief information concerning elastic bulk and plane wave propagation in unbounded and semi-infinite elastic media, as well as surface waves. It is worth noting that aspects of dynamic behavior of rods and plates are discussed rather weakly and superficially. Dynamic plastic behavior of engineering materials, as well as plastic shock wave propagation, are presented in Chapter 12. Linear viscoelastic properties of materials under dynamic loading are discussed in Chapter 13, and linear and nonlinear viscoelastic wave propagation is considered in Chapter 14. The dynamic behavior of fiber-reinforced composite materials is presented in Chapter 16. Chapter 17 gives an introduction into the concept of intelligence in engineering materials, as well as a review of different models, control algorithms, and analyses developed by various researchers. Pattern recognition and classification methodology for the characterization of material response states is discussed in the last chapter, 18. Each chapter ends with a list of main references, as well as the references for further reading, but this reviewer should note that only a few of them, excluding those of the author, were published during the last 10 years. Chapters 10, 11, 14, and 15 contain problems for student self-checking. Volume 2 concludes with a subject index and cumulative subject index. This reviewer can recommend Mechanical Behavior of Engineering Materials, Volume 2 only for undergraduate students of civil engineering and mechanical engineering departments as an introductory course in dynamic behavior of engineering materials.

The paper begins with a review of the literature on high pressure and shear properties of materials. This is an area in which further interest was recently stimulated by published Russian work on s...

The mechanical behavior of energetic aggregate materials (polymer bonded explosives, propellants, etc.) at low to intermediate strain rates is a complex function of the material constituents, manufacturing methods, and microstructure morphology. Shear-induced volumetric dilatation, similar to the characteristic behavior of granular materials and soils, can play a key role to open up porosity in the microstructure, particularly when the material is under some degree of confinement. We present a continuum modelling approach that accounts for this phenomena via a pressure-dependent viscoplastic extension to the classical ViscoSCRAM [1] constitutive model and use it to study the role of shear-induced dilatation in confined and unconfined material at a variety of strain rates. Modeling results are compared with intermediate strain rate Split Hopkinson Pressure Bar experiments and show good agreement between predicted and measured stresses.

The mechanical behavior of energetic aggregate materials (polymer bonded explosives, propellants, etc.) at low to intermediate strain rates is a complex function of the material constituents, manufacturing methods, and microstructure morphology. Shear-induced volumetric dilatation, similar to the characteristic behavior of granular materials and soils, can play a key role to open up porosity in the microstructure, particularly when the material is under some degree of confinement. We present a continuum modelling approach that accounts for this phenomena via a pressure-dependent viscoplastic extension to the classical ViscoSCRAM [1] constitutive model and use it to study the role of shear-induced dilatation in confined and unconfined material at a variety of strain rates. Modeling results are compared with intermediate strain rate Split Hopkinson Pressure Bar experiments and show good agreement between predicted and measured stresses.

Energetic materials - explosives, thermites, populsive powders - are used in a variety of military and civilian applications. Their mechanical and electrostatic sensitivity is high in many cases, which can lead to accidents during handling and transport. These considerations limit the practical use of some energetic materials despite their good performance. For industrial applications, safety is one of the main criteria for selecting energetic materials. The sensitivity has been regarded as an intrinsic property of a substance for a long time. However, in recent years, several approaches to lower the sensitivity of a given substance, using nanotechnology and materials engineering, have been described. This feature article gives an overview over ways to prepare energetic (nano-)materials with a lower sensitivity.

Embracing Anisotropy – Opportunities and Challenges to the Functional Application of Advanced Manufacturing of Energetic Materials

Particle shapes significantly affect viscosity and flow behavior of energetic materials, and therefore affect their packability and processability. This study presents a computational geometry framework for automatically quantifying two-dimensional (2D) and three-dimensional (3D) particle shapes of energetic materials. A specimen by mixing three typical energetic materials including HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), RDX (1,3,5-Trinitroperhydro-1,3,5-triazine) and AP (Ammonium Perchlorate) particles is used in this study. This specimen is scanned by high-resolution X-ray computed tomography (X-ray CT), yielding a volumetric image. An improved watershed analysis algorithm is used to process the volumetric image to identify individual 3D particles. The stereology sampling method is used to obtain 2D projections of 3D particles. Computational geometry techniques are developed by this study to analyze 2D particle projections and 3D particle geometries to compute seven commonly used shape descriptors, including convexity, circularity, intercept sphericity, area sphericity, diameter sphericity, circle ratio sphericity, and surface area sphericity. Results show that those different shape descriptors of energetic materials can be divided into three groups based on their numerical ranges. This study also evaluates the effectiveness and accuracy of 2D shape descriptors for quantifying the true 3D shapes. The inconsistent characterization results between 2D and 3D shape descriptors suggest that researchers should be cautious when using 2D images to characterize 3D particle shapes of energetic materials. The computational geometry framework and particle shape analysis results presented in this study can be potentially useful in numerical modeling, experimental analysis, and theoretical investigation for energetic materials.

This project strives to improve our understanding of the environmental behavior of energetic materials (EM). A thorough understanding of how these EM interact with soil and water is expected to ultimately lead to improved remediation strategies. The immediate goals of the project are to predict a priori the chemical interactions of energetic materials with model soils; and to predict a priori the decomposition reactions of EM and resultant breakdown products. Adsorption research will involve periodic density functional theory (DFT) calculations, using plane-wave/pseudopotential codes. Decomposition studies will use the Specular Reflection Isopotential Searching algorithm of Irikura et al.

Professor Dr. Thomas M. Klapötke

Energetic materials may transit to different phases or decompose directly under compression. Their reactivity in the explosions can be evaluated by their high-pressure induced behaviors, including polymorphism or phase transition. Here, we applied DFT methods to understand high-pressure behaviors of four typical tetrazole derivate crystals, including 5-aminotetrazole (ATZ), 1,5-aminotetrazole (DAT), 5-hydrazinotetrazole (HTZ), and 5-azidotetrazole (ADT), under the gradually increased pressure from ambient pressure to 200 GPa. In response to the extreme-high pressures, the performances are dominated by compressibility of crystals, reflected by compressive symbols on the basis of the molecular orientation in crystals. The crystal with weak compressibility (large symbol) generally dissociates, triggered by cleavage of weak bonds. However, the crystal with low compressive symbol is generally corresponding to a pressure-induced structural transformation or phase transition.