Abstract:Hot spots are critical initiators of both shock-induced detonation and non-shock ignition in explosives， with pore collapse as a primary formation mechanism. Since energetic materials are routinely subjected to complex mechanical loading across a broad intensity range during service， a mechanistic understanding of pore evolution and hot-spot generation under moderate shock pressures （1-10 GPa） is essential for reliable safety assessment. In this study， HMX （octahydro-1，3，5，7-tetranitro-1，3，5，7-tetrazocine） crystals containing a 300 μm prefabricated pore were investigated. High-speed imaging combining X-pinch and visible-light diagnostics captured the dynamic pore collapse process， while thermomechanically coupled numerical simulations accounted for the conversion of plastic work into thermal energy. The results reveal distinct pore collapse modes and associated hot-spot formation mechanisms. At 2.5 GPa， the isotropic pore collapse mechanism was observed， with hot spot intensity correlating positively with the extent of pore collapse. The temperature rise occurs in two stages： an initial gradual increase due to upstream viscoplastic deformation， followed by a sharp temperature rise after pore closure， resulting from the thermal conversion of kinetic energy during the impact of high-velocity upstream material on the downstream pore wall. When the shock pressure is increased to 3.5 GPa， pore collapse initiates earlier， the crescent-shaped deformation becomes more pronounced， and the mechanical response exhibits hydrodynamic behavior， indicating a transition toward jetting-type collapse.

Abstract:To address the application potential and demand of propellant/supercritical carbon dioxide （SC-CO₂） in ejection work， thermal analysis， static combustion and closed combustion tests were carried out in comparison with a N₂ environment， to investigate the thermal decomposition and combustion characteristics of single-based propellant in CO₂. The results show that the CO₂ atmosphere significantly suppresses the pyrolysis process of single-based propellant， and its maximum activation energy is 15.53 kJ·mol^-1 higher than that in N₂ atmosphere. The combustion time of single-based propellant in CO₂ is remarkably prolonged， increasing by 1729 ms at 7 MPa compared with the N₂ environment. Moreover， the combustion flame of single-based propellant in SC-CO₂ exhibits a special morphology with an inner white core and outer pale red periphery. During closed combustion， the phase change and heat absorption of liquid CO₂ lead to an ignition delay of single-based propellant up to 12.20 ms， and the action duration of single-based propellant in SC-CO₂ is on the order of 10^-2 s. Increasing the initial pressure helps to improve the energy release rate of single-based propellant in the SC-CO₂ environment.

Abstract:To quantitatively investigate the evolution of surface mesoscopic damagein nitrate ester plasticized polyether （NEPE） propellant during tension before and after aging， thermal accelerated aging tests were conducted with different aging days （0， 7 d， 40 d， 80 d）. An in-situ tensile machine combined with field emission scanning electron microscopy （FE-SEM） was employed for in-situ tensile testing， after which the captured images underwent digital processing. The fractal dimension was applied to quantitatively characterize and analyze the evolution of mesoscopic damage in the propellant at different aging stages. The differences between porosity method and fractal dimension method in characterizing mesoscopic damage before and after aging were studied. Results indicate that fractal dimension successfully captures the apparent damage evolution of NEPE propellant over different aging stages. During tension， for elongation below 20%， the fractal dimension rises relatively quickly with elongation. In the 20%-60% elongation range， the fractal dimension still increase with elongation， though at a reduced rate. Moreover， longer aging times were found to enhance the linear correlation between fractal dimension and elongation in this stage. Beyond 60% elongation， the fractal dimension shows irregular variation with elongation for all aging stages. Differences emerged between the porosity method and the fractal dimension method when characterizing the tensile process， especially after aging. While the porosity method mainly measuresthe area fraction of pores， the fractal dimension method characterizes the roughness and complexity of the surface morphology.

Abstract:To investigate the diffusion behavior of hydrazine hydrate in nitrocellulose （NC）–based propellant and its influence on the gradient distribution of nitrate ester groups on the propellant surface， molecular dynamics （MD） simulation combined with experimental characterization was employed to study the diffusion process during the construction of a nitro gradiently distributed propellant （NGDP）. An NC/N₂H₄/H₂O ternary system was established to examine the effects of NC nitrogen content， reaction temperature， and hydrazine hydrate concentration on diffusion behavior. Meanwhile， NGDP samples were prepared through a one-step green synthesis route， and their structural and compositional distributions were characterized using extended-depth-of-field microscopy， Raman line-scanning， and X-ray photoelectron spectroscopy （XPS）. The results show that an increase in the NC nitrogen content significantly enhances the diffusion coefficient and free-volume fraction， and increases the radial distribution function （RDF） peak intensity， indicating that higher nitrogen content facilitates hydrazine migration and strengthens local interactions. Higher temperature and hydrazine concentration also promote diffusion and enlarge the free-volume fraction， while decrease the RDF peak intensity， reflecting a more dispersed local configurations under intensified molecular motion and weakened short-range interactions. The thickness of the reaction layer increases with increasing temperature and concentration， and both the characteristic peaks of nitrate ester groups and the N 1s content gradually increase from the exterior toward the interior， exhibiting a graded distribution.

Abstract:To improve the accuracy of long-term storage performance evaluation for solid rocket motor grains and solve the critical limitation of existing models being difficult to simultaneously couple the effects of temperature and aging time， this investigation aims to achieve high-precision prediction of relaxation modulus over a wide range of temperature and the full aging cycle. Based on the stress relaxation data obtained from accelerated aging tests of Composite Modified Double-Base（CMDB） propellant at 343.15 K， a temperature-dependent relaxation modulus model that accounts for aging effects was developed by applying the time-temperature equivalence principle and introducing aging time as an internal variable. The results show that the predicted curves are in good agreement with the experimental data under test temperatures ranging from 233.15 K to 323.15 K and after accelerated aging at 343.15 K for 0-100 days. Thus， it indicates that the developed model can effectively describe the variation of the relaxation modulus for CMDB propellant over a wide range of temperature and the full aging cycle.

Abstract:Solid propellants optimization significantly enhances combustion efficiency of the rocket engine. Aluminum （Al） particles are widely used as metallic additives due to the high reactivity and energy density. A two-dimensional homogenized steady-state combustion model with a sandwich structure was developed for AP/HTPB/Al propellants based on a five-step gas-phase reaction mechanism. The reliability of the model was validated by comparing simulation and experimental data. The results show that as the pressure increases from 0.2 MPa to 6.5 MPa transits flame structures from premixed to diffusion combustion， while the peak area of heat release gradually approaches the burn surface. At constant pressure， the burn rates rise significantly with higher Al content， however， the pressure index first decreases and then increases with the increase of Al content. This indicates that there is a trade-off between high burn rates and flame stability.

Abstract:Polypropylene （PP） asa low-density polymer material， forms jets with considerable lethality. Micro UAVs with advantages such as low cost， strong adaptability， and rapid response， can significantly enhance combat effectiveness， requiring their warheads to balance lightweighting and destructive performance dual demands. This study explores the application potential of 3D-printed polypropylene as a shaped charge material in the damage field of micro UAV warheads. Micro thin-walled polypropylene shaped charges were prepared using Selective Laser Sintering （SLS） technology， and mechanical property tests were conducted to obtain the mechanical properties of 3D printed PP materials. The Johnson-Cook constitutive model parameters were fitted， and the results indicate that the material features excellent ductility and strain rate sensitivity， with dynamic yield strength significantly increasing as the strain rate rises. The polymer jet formation process was analyzed by combining PER theory and viscoplastic theory， revealing its head expansion characteristics. Static shaped charge penetration tests and numerical simulation were adopted to verify the damage lethality of 3D-printed PP jets at 3-5 CD （CD represents the caliber of the shaped charge）. The experimental results show that the effect is optimal at 4 CD， with a penetration depth of 17.10 mm and significant hole expansion. The numerical simulation results indicate a penetration depth of 16.04 mm and an open hole diameter of 7.986 mm， which are highly consistent with the experimental data. The performance of four polymer jets and a copper jet was further analyzed across five distinct dimensions. The research demonstrates that the designed 3D-printed polypropylene shaped charge liner can satisfy the carriage requirements of miniature UAVs， providing a theoretical basis and novel insights for the design of high-lethality micro warhead destruction.

Abstract:The mechanical sensitivity of energetic materials severely restricts their safe application. How to achieve low sensitivity while ensuring high energy density remains the core challenge in current energetic materials research. This paper focuses on the surface coating desensitization technology for energetic materials and reviews recent research progress of mainstream coating technologies and material systems. It emphasizes the analysis of the principles by which the coating layer inhibits the formation and propagation of “hot spots” during mechanical stimulation through three major mechanisms： “filling and buffering”， “energy absorption and isolation”， and “lubrication”. The characteristics and applicability of key technologies such as water suspension， emulsion suspension， in-situ polymerization， spray and microfluidics are summarized. A comprehensive review is conducted on the desensitization effects and mechanism differences of seven types of coating systems， including polymer adhesives， carbon materials， waxes， energetic materials， salts， biomimetic materials， and composite materials. By evaluating the overall performance and development potential of different coating systems， it is suggested that future research should focus on the in-depth revelation of desensitization mechanisms， intelligent design of coating structures， precise process control， and the creation of new multifunctional integrated materials. These efforts are directed toward driving technological innovation， facilitating the development new material systems， and ultimately enhancing the synergy between energy density and safety of energetic materials.

Abstract:To better understand the thermal decomposition behavior of energetic materials， recent advances in key theoretical methods and experimental techniques， including quantum chemical calculations， molecular dynamics simulations， coupled thermal analysis techniques， and spectroscopic and structural characterization methods were summarized. The capabilities and limitations of these approaches in identification of reaction pathways and characterization of energy release and structural evolution were discussed. Quantum chemical calculations provide detailed insights into potential energy surfaces and initial bond cleavage process， but due to the limitations of computational scale， they are primarily applicable to the small molecular systems. Molecular dynamics simulations enable tracking of atomic motion and energy transfer， making them suitable for analyzing typical decomposition pathways， but with limited time scales. Coupled thermal analysis techniques allow simultaneous characterization of thermal effects and gaseous products but have limited ability to identify transient intermediates. Spectroscopic and structural characterization methods can probe the processes of phase transitions， bond rearrangements， and valence state evolution， yet their resolution often hampers the observation of rapid structural changes. Based on the comparative analysis of different techniques， future studies should emphasize integrating theoretical simulations with in situ high-resolution diagnostics， combined with machine-learning potentials， high-throughput computation， and multiscale modeling to promote the research on the thermal decomposition mechanism of energetic materials towards multi-scale coupling and predictable modeling， and enhance the ability of characterization and prediction of complex thermal decomposition behaviors.