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Abstract:This paper is devoted to the building of a viscoelastic model based on actual damage mechanisms for explosive charges filled in projectiles during penetrating concrete targets. The characteristic damage modes present in polymer bonded explosive(PBX)s include intragranular voids, crystal fractures, and interfacial debonding. Quasi-static uniaxial compression tests and split-Hopkinson pressure bar tests were modelled to study mechanical responses and damage evolutions of PBXs at different loading conditions, and the dynamic finite element algorithm was applied to estimate the mechanical responses of PBXs under low intensity stimulus. Analyzing the damage and pressure distributions and the variations of different damages, the position of PBXs that is ready to be ignited, we find that the initial loading of the explosive charge head is suffered from the greater compressive stress. When the stress wave is propagated to the tail, reflected tensile waves are produced. Furthermore, the pressure increases rapidly to 0.25 GPa, leading to the more serious damages on the head and the tail, attributed to the inertia of the projectile body and the collision between the charge tail and the inner surface of the shell. It suggests that the head and the tail could be two key protection areas.

Abstract:The hybrid detonation formulation of three-dimensional condensed phase explosives is presented based on the multiphase compressible fluid equation and the reaction rate equation. The iteration method of thermodynamic equilibrium for the multiphase detonation mixed reaction zone is built and an efficient detonation parallel software is developed. The feasibility of the physical model, the numerical method and the program module are verified by comparative experiments. We focus on the wave front structure, the coupling of flow fields and chemical reactions and the secondary initiation mechanism of condensed phase explosives by numerical simulations of the detonation process at various corners. The numerical results show that the diffraction area of a corner of 135°is larger than that of 90°of PBX9404 explosive, and the wave front structure is affected by the local flow field velocity. The temporary "death zone" is formed because the chemical reactions are decoupled from the leading shock wave when the detonation wave passes the corner. As the corner angle increases, the dead zone of the corner is expanded. The key of secondary initiation in the dead zone is that the acting time of back-detonation wave should be longer than the critical induction time of explosives in the corner zone.

Abstract:The crystal plasticity constitutive model is developed to investigate the role of solid-solid phase transformation on the deformation mechanism of the HMX single crystal, with accounting for nonlinear elasticity, crystalline plasticity and temperature-controlled phase transition (β→δ phase transition). The phase transition of HMX at various heating rates was simulated using a finite element software ABAQUS and a subprogram VUMAT. It shows that the phase transition is reversible, while the paths for β→δ and δ→β transitions are different from each other. Residual strain and stress occur when the crystal converts back into the β phase. In addition, cracks are generated during the β→δ phase transition, and maintain after the δ→β phase transition. Moreover, the temperature homogenization can facilitate the phase transition, and the propagation of the phase transition and the heating proceed in a same direction. Furthermore, the β→δ phase transition can lead to crystal expansion. Because the thermal expansion coefficient of the δ phase is larger than that of the β phase, the crystal volume is expanded more quickly after the β→δ phase transition, causing a rapid increase in internal stress and slipping. Due to the anisotropic thermal expansion of the HMX single crystal, cracks occur most readily along the third principal axis after the phase transition, resulting in ready formation of hot spots. Therefore, the β→δ phase transition makes the HMX crystal more sensitive to cook-off.

Abstract:The thermal conductivity of explosives is an important index to understand the non-shock initiation and the non-ideal detonation. On the basis of experimental thermal conductivity data of polymers, two methods for calculating thermal conductivity, which are proposed by Bicerano and Askadskii, respectively, are compared. By calculating and comparing the relative deviations, Askadskiii group contribution method is verified to be efficient and thus adopted, because of its small relative deviation. Based on the analysis results of the structures, properties and thermal conduction mechanism of conjugated polymers, several energetic conjugated polymers are designed by adding energetic groups —NO₂, —N₃ and —ONO₂ up to the conjugated polymer structures. A coupling between phonon conduction and electron conduction in the conjugate systems of energetic conjugated polymers will lead to an increase in the thermal conductivity. Thermal conductivity calculation shows that the new designed conjugated energetic polymers exhibit the enhanced thermal conductivity, above 1.0 W·mK^-1

Abstract:The structures and mechanical and electronic properties of dihydroxylammonium 5, 5′-Bistetrazole-1, 1′-diolate(TKX-50) were examined over a pressure range to clarify its structural and chemical stability at high pressures with a first-principles method based on density functional theory using PBE functional plus vdW corrections(DFT-D). The intermolecular interactions under high pressures were analyzed with the Hirshfeld surfaces and 2D fingerprint plots to clarify its change trend. The calculated lattice constants, axial compressibility, and equation of state are in good accordance with the available experimental results. For the first time, we predict the elastic constants of TKX-50 using a first-principles method. The predicted stiffness constants are ordered as C₃₃(61.9 GPa > C₁₁(58.1 GPa) > C₂₂(36.9 GPa). The electronic energy band structures show that the band gap changes from a direct one (B-B) at 0 GPa to an indirect one at 50 GPa. The band gap at 100 GPa is also indirect (from G to B). The band gap decreases with respect to an increase in pressure, implying that the impact sensitivity increases with compression.

Abstract:Controlling the fluid behaviors of energetic suspensions is one of the crucial technologies for controlling the inner defects of munitions. Considering the lack work on energetic suspensions with nanoparticles, dissipative particle dynamics(DPD) simulation technology was used to investigate the influences of particle shape, size, content and gradation on the viscosity of energetic suspension systems in this paper. It shows that the influence of the particle shape can be neglected in the case of low contents. With the increase of the content of nanoparticles, the viscosity of the suspension rises accordingly. For a same amount of nanoparticles, the smaller the size, the greater the viscosity of the suspension. The addition of nanoparticles does not change the dependence of the system viscosity on the temperature. For the same content, the particle gradation of two different sizes of nanoparticles can effectively adjust the viscosity of the system. By introducing the solvation effect of nanoparticles, the traditional Einstein viscosity calculation formula is corrected to be more efficient. The particle size and content are both introduced to a model to effectively predict the viscosity of fluids ranging from nano-to micro-scale.

Abstract:Ab initio molecular dynamics simulations were used to study the decomposition mechanism of 2, 4, 6, 8, 10, 12-hexanitro-2, 4, 6, 8, 10, 12-hexaazaisowurtzitane(CL-20) crystal, an excellent high energy density cage compound, under extreme conditions. It is found that the initiation and subsequent decomposition mechanisms of the CL-20 crystal are diverse at different conditions, and CL-20 is sensitive to both high temperature and pressure. Comparing the numbers of corresponding main products, it is found that the high pressure decelerates the decomposition. While, the appearance of special intermediates R-C_xO_y (x>2, y>5) indicates that the high pressure makes the decomposition much more complex. Among these intermediates, C₃O₆ is proved to be a high energy density compound.

Abstract:Starting with the molecular structure of α-RDX, the non-empirical full atom self-consistent force field (SCFF) for the explosive was derived via the first principles including electronic structure theory, intermolecular perturbation theory, and based on the SCFF-phonon approach to calculating the anharmonic free energy, the isentropic locus for α-RDX was obtained, covering a pressure range from the ambient pressure to 55 GPa. The relationships between p(GPa)and V(cm³·mol^-1), and between T(K) and V(cm³·mol^-1) of the isentropic locus are: p=66.59633+4.52375exp(-V/0.68117)+16.56087exp(-V/4.31486)+35.94539exp(-V/32.26906) and lnT=5.06548+0.74774lnV-0.13886(lnV)², respectively. The SCFF and SCFF-phonon methods can be generalized toward other high energetic materials and thus provides an effective theoretical tool for constructing their precise EOSs and computing thermodynamic properties.

Abstract:Recently, a new nitrogen-rich compound, dihydroxylanmonium 5, 5′-bistetrazole-1, 1′-diolate(TKX-50) was synthesized. It possesses low impact sensitivity and a high energy content, and is readily synthetized. We use ab initio molecular dynamics method to simulate the decomposition process of TKX-50 under various pressure and temperature. The formation mechanism of the main product N₂ is then analyzed. There are three main paths for the N₂ generation: two paths are derived from the break of the tetrazole ring and the remaining is related to the interaction between the ammonium ion and the triazole ring. The rate of the N₂ generation is affected by the temperature and pressure. That is, the higher the temperature, the lower the pressure, and the faster the reaction rate. The N₂ generation rate by the intermolecular interactions between the ammonium ion and ammonium ring depends on diffusion, because the diffusion rate is observed positively correlated with temperature while negatively with pressure. The reaction rate of the TKX-50 thus increases with temperature increasing and pressure decreasing.

Abstract:We summarized detailedly in this review the basic theory of quantum chemistry, and discussed the methods for calculating key parameters of energetic materials (density, heat of formation, detonation heat, detonation velocity, detonation pressure and impact sensitivity). We also compared the characteristics and applicable scope of these methods. In addition, the application of CHEETA, EXPLO5 and other computing software in the field of energetic materials were briefed. Finally, in order to satisfy the comprehensive requirements for new generation materials, high energy, good stability and environmental friendliness, we designed 20 new high nitrogen molecules, and estimated their physicochemical properties and energetic parameters using the quantum chemistry methods as mentioned above. The design concept and principle for the new energetic compounds was put forward. It was expected that it would promote the development of energetic materials. Seventy-six references were contained.

Abstract:We reviewed the methods usually applied to the phase transition of energetic materials. Meanwhile, we introduced the commonly used experimental techniques of thermal analysis, ultrahigh pressure and impact loading, as well as many first-principle methods and molecular dynamics theoretical simulation methods, with their application scopes. Furthermore, the multiphase structures of nitramines, nitrides, nitrates, azoles and furazanes, as well as cage-like energetic materials under high temperature and pressure were summarized. The molecular configurations and crystal structures of these energetic materials (including partially mixed explosives) in different phases are introduced. Moreover, the phase transition characteristics and corresponding phase distributions of some materials under static and shock loading conditions are summarized too. The phase transition mechanisms revealed by high-precision theoretical simulations of some common energetic materials were reviewed. It should be noted that some results of phase transition points of energetic materials with several complicated phase transition mechanisms are inconsistent with one another. For example, the crystal and molecular structures of some phases of PETN differ one another in different reports. Finally, we found that the profound theoretical mechanism of large amounts of phase transitions is not sufficiently revealed, and the microscopic mechanism of the phase transition of mixed explosives remains lack.

Abstract:The hot-spot theory of energetic materials is of great significance in understanding the mechanisms of sensitivity and detonation, in which modeling and calculation methods are widely applied. We reviewed the recent progress in the hot-spot theories by modeling and calculations. According to the multi-scale feature of the hot-spots, the research progress of Alienation Finite Element and Hydrocodes on the mechanism of hot-spot formation induced by micron defects and friction is analyzed on the mesoscopic scale. Meanwhile, that of molecular dynamics simulations and ab initio calculations on the mechanism of hot-spot formation by nanoscale defects and the issues of chemical reactions leading to hot-spot formation is analyzed on the microscopic scale. Thereby, the challenges of current hot-spot theoretical simulations are raised. The supplement of force fields and composite material's constitutive equations, and the elucidation of thermo-mechanical-chemical coupling mechanism in hot-spot evolution will be the research trends in the future.

Abstract:Energy and safety are the two most important concerns in the field of energetic materials (EMs). And it is universally believed that there exists an inevitable and inherent contradiction between them: the higher energy goes with the lower safety, i.e., the so-called energy and safety(E& S) contradiction. This article discusses the contradiction in combination with many counterexamples showing that the contradiction is not always the case. It is found that the contradiction is structure-level dependent: only on the level of molecule, the E& S contradiction appears most necessarily and remarkably as an inherent one; while, on the higher levels of crystal and mixture, the E& S contradiction can certainly be alleviated. Moreover, we think that the energy and safety of EMs are originated from a sense of more thermodynamics and a sense of more kinetics of chemical reactions, respectively, and thus the E& S contradiction is a thermodynamic-kinetic one. Accordingly, we stress again a strategy for developing EMs, i.e., the nature of increasing the energy is to increase the chemical energy stored in molecules and to enhance the molecular packing compactness; while, the principle of enhancing safety, or reducing sensitivity, is to make external stimuli more and more insufficient to ignite EMs, through crystal engineering and mixing technologies.

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