Abstract:The crystallization process of 1，3，5-triamino-2，4，6-trinitrobenzene （TATB） will be affected by dioctyl sulfosuccinate sodium salt （AOT） and the molecular dynamics （MD） method was used to study this crystallization process in this work. The crystal morphologies of TATB crystal in vacuum were predicted by Bravis-Friedel-Donnary-Harker （BFDH） and attachment energy （AE） models. Seven important crystal planes of TATB were determined， which are （0 0 1）， （1 0 -1）， （1 -1 0）， （1 0 0）， （1 -1 1）， （0 1 -1） and （0 1 0）. The interface model for TATB with AOT was established and performed the molecular dynamics simulation. The modified AE model was used to analysis simulation data. After calculation， we found that the crystallization rate of TATB was improved under the influence of AOT solution. After analyzing the molecular structure and the intermolecular interaction of TATB， it is considered that because of the special plane structure， the intermolecular interaction between （0 0 1） plane and AOT is weak and the attachment energy （119.832 kJ·mol^-1） of （0 0 1） plane is low. So， the growth rate of （0 0 1） plane is relatively slow. The attachment energies of （1 0 -1）， （1 -1 0）， （1 0 0）， （1 -1 1）， （0 1 -1） and （0 1 0） planes are all higher than （0 0 1） and they all grow faster than （0 0 1）. Therefore， in the experimental process， a leaf-like TATB structure formed first. With the reaction time was further， the （0 0 1） plane gradually grows， the leaves become longer.

Abstract:In order to study the thermal decomposition properties of the characteristic groups of 3，4-Dinitrofurazanylfuroxan （DNTF）， differential scanning calorimetry （DSC） and fast scanning Fourier transform infrared spectroscopy （FTIR）were adopted to study the thermal decomposition properties of DNTF in condensed phase. The changes of characteristic groups of DNTF were studied by using FTIR technology at three different heating rates of 2.5， 5.0， 10 ℃·min^-1. The thermal decomposition kinetic parameters of C─NO₂， furazan ring， and furoxan ring of DNTF were calculated by the Coats-Redfern method. The thermal decomposition mechanism of DNTF was inferred based on the thermal decomposition properties of the characteristic group of DNTF. The results show that the thermal decomposition of DNTF is controlled by the three-dimensional diffusion mechanism. The reactivity of groups in DNTF molecule is nitro>furoxan ring>furazan ring. With the increase of heating rate， the reactivity of each group shows an increasing trend. It is speculated that the thermal decomposition process is that C─NO₂ breaks first， and then the C─C bond connecting furuzan and furoxan ring breaks， and finally the N─O bond in furazan ring and furoxan ring breaks. The furoxan ring decomposes faster than furazan ring.

Abstract:To investigate the influence of the loading density of main propellant charge on the propagation characteristics of ignition charge gas in the granular propellant bed， the test platform for ignition and propagation of a large-diameter dense propellant bed was established， and the flame sequence diagram and the pressure changes of partial pressure gauges were recorded in tests. The porous medium model was used to simulate the granular propellant bed in the charge chamber， and the ignition and propagation model corresponding to the test device was established to numerically simulate the flow process of ignition charge gas in the granular propellant bed. The simulation results were compared with the test results to verify the reliability of the model， and then the propagation characteristics of temperature and pressure fields of gas in the propellant bed with different loading densities were calculated. The results show that the calculated results are in good agreement with the experimental flame propagation sequence process and the experimental pressure histories， which verifies the reliability of the model. Under the condition of any porosity， the axial displacement of flame front develops rapidly and the axial velocity decreases from 25-30 m·s^-1 to 10 m·s^-1 during 0-10 ms， ， and the axial velocity decreases to 2-3 m·s^-1 during 10-40 ms. Similarly， under the condition of any porosity， the development of radial displacement of flame front is concentrated during 2.2-3 ms， and the radial velocity decreases to 20-22 m·s^-1 at 3 ms. However， the radial velocity at the initial time is large for large porosity. When the porosity increases from 0.3 to 0.5， the pressure difference at different positions in the chamber decreases 16.7% from 0.24 MPa to 0.20 MPa， and the uniformity and instantaneity of ignition are improved. With the increase of porosity， the axial and radial resistances of the flame front in propagation process decrease， the axial expansion displacement of the flame front and the initial velocity of flame propagation in the axial and radial directions increase， but the final velocity tends to be the same. The smaller the pressure in the chamber， the smaller the pressure difference in the chamber.

Abstract:To investigate the influence of temperature-induced phase transition of polytetrafluoroethylene （PTFE） on the fracture toughness of Al-PTFE reactive material， the quasi-static tensile test and fracture toughness test were performed， and the normalization data reduction technique with single specimen in ASTM E1820 was applied for analyzing the elastic-plastic fracture toughness of Al-PTFE by J-integral method. Combined with the microstructures analysis of the fracture surface， the effect of temperature on the fracture toughness of Al-PTFE was revealed. The results show that the strength of Al-PTFE reactive material decreases with the increase of temperature， while the fracture toughness increases. Moreover， the yield strength and fracture toughness of this material change obviously after crossing the phase transition temperature. The crack propagation pattern changes from brittle fracture to ductile fracture. Furthermore， when PTFE is in phase Ⅱ， less PTFE fibrils can be formed by stretching. However， increasing temperature can lead to the transform of the crystal phase for PTFE to phase Ⅳ and Ⅰ. Besides， the stable formed PTFE fibrils can effectively dissipate the external energy in the form of local plastic deformation. The crack tip is passivated by winding bridging， so as to prevent the crack propagation and improve the fracture toughness of this material.

Abstract:In order to clarify the influence mechanism of H₂O and H₂O₂ molecules on the thermal stability of energetic cocrystals， molecular dynamics （MD） simulation method was employed to analyze the diffusion behavior and thermal decomposition mechanism of solvent molecules in α-CL-20 and CL-20/H₂O₂ （orthogonal/monoclinic）. The results show that both H₂O and H₂O₂ will diffuse out of the cell as the temperature rises， among which H₂O molecules diffuse faster； when the temperature is lower than 500 K， the monoclinic CL-20/H₂O₂ lattice framework has the ability to hinder the diffusion of H₂O₂ molecules. When the temperature rises above 500 K， this hindering effect no longer exists. In the process of thermal decomposition， α-CL-20 releases energy the slowest， and the decomposition of CL-20 also proceeds the slowest； when the temperature is lower than 1500 K， the solvent exhibits a certain stabilizing effect on the thermal decomposition of energetic components， but this effect disappears as the temperature rises. In addition， the presence of solvents can increase the lattice energy significantly.

Abstract:Aiming at the problem of the irreversible deformation mechanism caused by thermal cycling in PBX-9502 （a type of TATB based polymer bonded explosive）， a three-phase microstructure model and an extended finite element method （XFEM） are used to establish a calculation model. The model takes into account the anisotropy of the TATB grains， the local differences in the thermodynamic properties of the bonding agent and its interface. Using the established model， the phenomenon and mechanism of irreversible deformation caused by thermal cycling of PBX-9502 are numerically analyzed. The results show that due to the severe anisotropy of TATB grains in PBX-9502 and the differences in the thermodynamic properties of TATB grains and binders， PBX-9502 specimens have internal deformation and stress concentration during thermal cycling. Resulting in the destruction of the bonding agent and the debonding of the interface and other internal damage to the material， which in turn led to the irreversible deformation of the PBX-9502 specimen under thermal cycling. At the end of the calculation， the axial strain of the PBX-9502 specimen reached 0.2%.

Abstract:In order to get the phase diagrams of 3，4-dinitropyrazole （DNP）/3，4-bis（3-nitrofurazan-4-yl） furoxan （DNTF） binary mixture system and understand the eutectic melting process， DSC was used to investigate the liquefaction and melting process of DNP/DNTF mixed system with different proportions. The T-x phase diagram and H-x phase diagram were established， and the effects of different heating rates and additives on melting process of eutectic were studied. Finally， the kinetic parameters E_a and A of eutectic melting process were calculated by Kissinger equation and Šatava-Šestak equation. Results show that the mass percentage of DNP/ DNTF eutectic is 70.38/29.62 and the eutectic temperature is 76.38 ℃ based on T-x phase diagram， which is good consistent with that from H-x phase diagram as 70.57/29.43. With the increase of heating rate， the initial temperature and peak temperature of melting process are delayed correspondingly. After the addition of Octogen （HMX） and nitroguanidine （NQ）， the melting point of the low eutectic is significantly delayed， while ammonium perchlorate （AP） has little effect on the melting point. The melting kinetic parameters E_a and A of DNP/DNTF eutectic are 19.13 kJ·mol^-1 and 10^9.74 s^-1 respectively， and the integral form of the most probable mechanism function is： G（α）=（1-α）^-1-1.

Abstract:Hollow charges usually adopt compression molding process. Using the method of continuum mechanics， a finite element simulation of the compression process model was established. First， the relative density， displacement and equivalent stress change laws of the pressing process of the JO-9159 explosive were simulated and analyzed. Then， the influences of pressing rate， initial relative density and friction coefficient on the pressing quality of JO-9159 explosive are simulated and analyzed. Results show that the JO-9159 explosive powder flows mainly in the axial direction during the pressing process， and the explosive powder flows slowly near the female mold area； when the pressing rate is 0.5 mm·s^-1 and the friction coefficient is 0.25， the relative density of the charge after molding is more uniform and the amount of rebound is smaller.

Abstract:In order to study the safety of processing explosives with femtosecond laser， a calculation model of femtsecond laser sequence machining on explosives was established， which took into account the autothermal reaction of the explosives. The processes of femtosecond laser sequence machining on three different explosives （TNT， TATB and HMX） were calculated. The safety of these processes was analyzed. Results show that the frequencies of the femtosecond laser sequence， the heat release of the autothermal reaction and the thermal diffusion coefficient of explosives will affect processing safety. Among these three explosives， the heat release of the autothermal reaction of HMX is the largest， and its thermal diffusion coefficient is the smallest， so its heat accumulation effect is the most significant. Therefore， HMX is ignited when processing with all the three femtosecond sequences with different frequencies （1×10³ Hz， 1×10⁵ Hz and 2×10⁵ Hz）. On the contrary， the heat accumulation effect of TATB is the weakest， so no ignition occurs when processing with the three femtosecond sequence with different frequencies. The heat accumulation effect of TNT is between those of HMX and TATB， so ignition only occurs when using the femtosecond laser sequence with higher frequencies. In the actual machining process， especially for the explosives with greater heat release of the autothermal reaction and smaller thermal diffusion coefficient， femtosecond laser sequence with low frequency should be selected to ensure safety.

Abstract:To investigate the shock initiation characteristics of the insensitive aluminized melt-cast explosives， a one-dimensional Lagrangian test system was established with the manganin piezoresistive pressure gauges and the loading technique of chemical explosion. The shock initiation process of an aluminized DNAN-based melt-cast explosive was measured， and the growth histories of pressure were obtained under different loading pressures. By virtue of the mesoscopic model of reaction rate of the aluminized melt-cast Duan-Zhang-Kim （DZK） ， the parameters of the aluminized explosive were determined and then the shock initiation process was simulated numerically. It is found that the higher the loading pressure is， the faster the detonation grows inside the aluminized explosive. In the shock initiation of the aluminized melt-cast explosive， the reaction degree and reaction rate are low near the leading wave front. However， the reaction rate of explosive after wave increases continuously and reaches the peak value in a while with the ignition reaction and the accumulation of chemical reaction Furthermore， compared with the growth history of particle velocity， those of pressure involve more detail of the growth of reaction rate， which are more suitable to be used in validating the reaction rate models and determining the parameters of reaction flow models.

Abstract:The thermal stress fracture of polymer bonded explosive （PBX） was studied by a combined simulated and experimental method using notched PBX cylinders. A 2D axisymmetric finite element model， which contained temperature related material properties， was employed to calculate the thermal elastic-plastic response of PBX cylinders under the condition of the initial cooling temperature of 50 ℃ and cooling rate of 10 ℃·min^-1. Temperature， strain and acoustic emission （AE） measurements were used in an air-cooling test for verification. Simulation results show that there is a temperature boundary layer near the surface and a temperature difference in the cylinder， resulting in tension stress greater than the strength of the PBX. A circular notch in the cylindrical surface distinctly amplifies the thermal stress and the stress gradient. The factor of stress concentration reaches a maximum of 1.6 when the notch radius is 2 mm. The PBX endures brittle broken during thermal shock when thermal stress exceeded its tension strength， accompanied with strong AE signals and a sharply decline on strain-time response. The critical fracture temperature of cylinders with and without a notch determined by simulation are respectively 8.3 ℃ and 12.6 ℃， while the value determined by experiment are respectively 9.2 ℃ and 12.5 ℃.

Abstract:In order to explore the influence of the complexity of crack distribution and shape on the particularity of ultrasonic propagation in curved surface components of polymer bonded explosives （PBX）， and optimize the parameters of ultrasonic oblique incidence testing， a numerical model of ultrasonic nondestructive testing of curved PBX components was established， based on the finite element method and typical testing conditions. The surface P under the excitation of transient displacement was calculated and analyzed by COMSOL commercial software. The propagation law of the internal sound field of PBX component was simulated， the ultrasonic detection signals of different angles and cracks were calculated， and the influence of the incident angle and other parameters on the detection results of small angle oblique incidence of ultrasonic water immersion crack of curved PBX component was studied. The numerical simulation results show that when the angle between the beam and the crack is 8°~10°， the small angle oblique incidence ultrasonic detection method has the best detection performance for PBX internal crack defects， and is not affected by the crack depth. At the same time， the curved PBX simulation specimen was designed and manufactured， the small angle ultrasonic testing experiment system was built， and the ultrasonic testing experiments under different incident angles and crack parameters were carried out. The optimal testing angle （8°） obtained from the experiment was consistent with the simulation results （8°~10°）， which verified the rationality of the numerical simulation results and the effectiveness of small angle ultrasonic testing.

Abstract:To characterize the structure of TATB-based polymer bonded explosives（PBX） on the meso-scale， an automatic granule boundary extraction method based on the two-dimensional（2D） reconstructed CT images with low quality is presented. Binarization was first performed according to the gray level distribution of the original CT images of TATB-based PBX. Then a proposed elastic capsule algorithm was applied to the binary images and an approximate location of the granule boundary was obtained. With a combination of morphological operations and the marker-based watershed algorithm， a more accurate segmentation of TATB-based PBX granules was realized. The present granule boundary extraction method for 2D CT images of TATB-based PBX shows improved accuracy and portability， as compared to Canny algorithm， iterative phase congruency， SLIC superpixels and other feature extraction techniques.