Molecular Dynamics Simulations for Effects of Fluoropolymer Binder Content in CL-20/TNT Based Polymer-Bonded Explosives.

In order to better understand the role of binder content, molecular dynamics (MD) simulations were performed to study the interfacial interactions, sensitivity and mechanical properties of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane/2,4,6-trinitrotoluene (CL-20/TNT) based polymer-bonded explosives (PBXs) with fluorine rubber F2311. The binding energy between CL-20/TNT co-crystal (1 0 0) surface and F2311, pair correlation function, the maximum bond length of the N-NO2 trigger bond, and the mechanical properties of the PBXs were reported. From the calculated binding energy, it was found that binding energy increases with increasing F2311 content. Additionally, according to the results of pair correlation function, it turns out that H-O hydrogen bonds and H-F hydrogen bonds exist between F2311 molecules and the molecules in CL-20/TNT. The length of trigger bond in CL-20/TNT were adopted as theoretical criterion of sensitivity. The maximum bond length of the N-NO2 trigger bond decreased very significantly when the F2311 content increased from 0 to 9.2%. This indicated increasing F2311 content can reduce sensitivity and improve thermal stability. However, the maximum bond length of the N-NO2 trigger bond remained essentially unchanged when the F2311 content was further increased. Additionally, the calculated mechanical data indicated that with the increase in F2311 content, the rigidity of CL-20/TNT based PBXs was decrease, the toughness was improved.

Application of Fiber Bragg Grating Acoustic Emission Sensors in Thin Polymer-Bonded Explosives.

Fiber Bragg grating (FBG) acoustic emission (AE) sensors have been used in many applications. In this paper, based on an FBG AE sensor, the sensing principle of the interaction between the AE wave and the sensor is introduced. Then, the directionality of the FBG AE sensor on the surface of a thin polymer-bonded explosive (PBX) material is studied. Finally, the time coefficient location method is proposed to correct the AE time detected by the FBG AE sensor, thereby improving the accuracy of location experiments.

Molecular dynamics simulation of CL20/DNDAP cocrystal-based PBXs.

CONTEXT: CL-20/DNDAP cocrystal is a promising new type of explosive with exceptional energy density and detonation parameters. However, compared to TATB, FOX-7 and other insensitive explosives, it still has higher sensitivity. In order to decrease the sensitivity of CL20/DNDAP cocrystal explosive, in this article, a CL20/DNDAP cocrystal model was established, and six different types of polymers, including butadiene rubber (BR), ethylene-vinyl acetate copolymer (EVA), polyethylene glycol (PEG), hydroxyl-terminated polybutadiene (HTPB), fluoropolymer (F2603), and polyvinylidene difluoride (PVDF), were added to the three cleaved surfaces of (1 0 0), (0 1 0) and (0 0 1) to obtain polymer-bonded explosives (PBXs). Predict the effects of different polymers on the stability, trigger bond length, mechanical properties, and detonation performance of PBXs. Among the six PBX models, CL-20/DNDAP/PEG model exhibited the highest binding energy and the lowest trigger bond length, indicating that CL-20/DNDAP/PEG model had the best stability, compatibility, and the least sensitivity. Furthermore, although the CL-20/DNDAP/F2603 model demonstrated superior detonation capabilities, it should be noted that this model displayed low levels of compatibility. Overall, CL-20/DNDAP/PEG model exhibited the superior comprehensive properties, thereby demonstrating that PEG is a more suitable binder option for PBXs based on the CL20/DNDAP cocrystal. METHODS: The properties of CL-20/DNDAP cocrystal-based PBXs were predicted by molecular dynamics (MD) method under Materials Studio software. The MD simulation time step was set at 1fs and the total MD simulation time was 2ns. The Isothermal-isobaric (NPT) ensemble was used for the 2ns of MD simulation. The COMPASS force field was used, and the temperature was set at 295K.

Comprehensive theoretical study on safety performance and mechanical properties of 3-nitro-1,2,4-triazol-5-one (NTO)-based polymer-bonded explosives (PBXs) via molecular dynamics simulation.

3-nitro-1,2,4-triazol-5-one (NTO)-based polymer-bonded explosives (PBXs) have been widely used in insensitive munitions, but the main properties of NTO-based PBXs such as compatibility, safety performance, and mechanical properties are rarely reported. In this work, molecular dynamics simulation was carried out to study interface interactions of NTO-based PBXs, in which hydroxy-terminated polybutadiene (HTPB), ethylene-vinyl acetate copolymer (EVA), glycidyl azide polymer (GAP), poly-3-nitratomethyl-3-methyl oxetane (Poly-NIMMO), and ester urethane (Estane5703) are selected as binders. The binding energy analysis indicates that the order of compatibility is NTO/GAP > NTO/Estane5703 > NTO/HTPB > NTO/Poly-NIMMO > NTO/EVA. Radial distribution function analysis results show that the interface interaction is mainly the hydrogen bond between H atoms of NTO and O atoms of Estane5703, HTPB, EVA, and Poly-NIMMO or N atoms of GAP. The values of cohesive energy density verify that the safety is NTO/GAP > NTO/Poly-NIMMO > NTO/HTPB > NTO/EVA > NTO/Estane5703. Mechanical properties results show that GAP and EVA would improve the plasticity of the systems effectively. Furthermore, it can be found that the most favorable interactions occur between the NTO (1 0 0) crystal face and binders.

Theoretical research on performances of CL-20/HMX cocrystal explosive and its based polymer bonded explosives (PBXs) by molecular dynamics method.

In this article, the CL-20/HMX cocrystal model was established and its based polymer bonded explosives (PBXs) were designed. The static performances, including mechanical properties, stability and detonation performance of CL-20/HMX cocrystal model and PBXs models, were predicted by molecular dynamics (MD) method. The mechanical parameters, binding energy, and detonation parameters of PBXs models were calculated and compared with that of pure CL-20/HMX cocrystal model. The influence of polymer binders on performances of CL-20/HMX cocrystal explosive was evaluated. Results show that the polymer binders make the engineering moduli (tensile modulus, shear modulus, and bulk modulus) of PBXs declined and Cauchy pressure increased, meaning that the polymer binder can obviously improve mechanical properties of CL-20/HMX cocrystal explosive, and the PBXs model with fluorine rubber (F2311) has the best mechanical properties. In different PBXs models, the binding energy between CL-20, HMX molecules and F2311 is higher than other polymer binders, indicating that the CL-20/HMX/F2311 model is more stable. The PBXs models have lower value of crystal density and detonation parameters compared with pure CL-20/HMX cocrystal and the energetic performance of PBXs is weakened. The PBXs model with fluorine resin (F2314) has the highest energetic performance and it is higher than pure HMX. Therefore, the CL-20/HMX/F2311 and CL-20/HMX/F2314 models have more favorable comprehensive properties, proving that F2311 and F2314 are more preferable and promising to design CL-20/HMX cocrystal based PBXs.

Computational analysis the relationships of energy and mechanical properties with sensitivity for FOX-7 based PBXs via MD simulation.

Molecular dynamics (MD) simulations have been applied to investigate 1, 1-diamino-2, 2-dinitroethene (FOX-7) crystal and FOX-7 (011)-based polymer-bonded explosives (PBXs) with four typical polymers, polyethylene glycol (PEG), fluorine-polymer (F2603), ethylene-vinyl acetate copolymer (EVA) and ester urethane (ESTANE5703) under COMPASS force field. Binding energy (E bind), cohesive energy density (CED), initiation bond length distribution, RDG analysis and isotropic mechanical properties of FOX-7 and its PBXs at different temperatures were reported for the first time, and the relationship between them and sensitivity. Using quantum chemistry, FOX-7 was optimized with the four polymers at the B3LYP/6-311++G(d,p) level, and the structure and RDG of the optimized composite system were analysed. The results indicated that the binding energy presented irregular changes with the increase in temperature. The order of binding ability of different binders to the FOX-7 (011) crystal surface is PEG > ESTANE5703 > EVA > F2603. When the temperature increases, the maximum bond length (L max) of the induced bond increases and the CED decreases. This result is achieved in agreement with the known experimental fact that the sensitivity of explosives increases with temperature, and they can be used as the criterion to predict the sensitivity of explosives. The descending order of L max is FOX-7 > F2603 > ESTANE5703≈EVA > PEG. The intermolecular interactions between FOX-7 and the four polymers were mainly weak hydrogen bonding and van der Waals interactions, and these interactions helped to reduce the bond length of C-NO2, leading to a decrease in the sensitivity of FOX-7. The addition of polymers can effectively improve the mechanical properties of explosives. Among the four polymers, EVA has the best effect on improving the mechanical properties of FOX-7 (011). At the same temperature, the modulus can be used to predict the sensitivity of high-energy materials. Cauchy pressure can predict the sensitivity of non-brittle energetic materials. The nature of the interaction between FOX-7 and the four polymers is hydrogen bonding and van der Waals force, of which hydrogen bonding is the main one. These studies are meaningful for the formulation design and sensitivity prediction of FOX-7 and its PBXs.

Dynamic Mechanical Damage and Non-Shock initiation of a New Polymer Bonded Explosive during Penetration.

Complexities of heating mechanisms make it difficult to investigate the safety of a polymer bonded explosive (PBX) charge of earth-penetrating-weapons (EPWs) during penetration. In this paper, the dynamic damage and non-shock initiation of PBX1314 (60 wt % hexahydro-1, 3, 5-trinitro-1, 3, 5-s-triazine (RDX), 16 wt % aluminum, 24 wt % hydroxy-terminated polybutadiene (HTPB)) during penetration is investigated through experiments and simulations. In the experiments, steel projectiles filled with PBX1314 are launched to penetrate concrete targets. In the results, non-shock initiations occur on the tail surface of PBX1314 along with mechanical damage of the tail and middle part of PBX1314. A dynamic damage and initiation model is proposed to characterize the effects of microcracks on the mechanical and thermal responses of PBX1314. Investigation based on the model suggests that microcrack interfacial friction plays significant roles in damage, heat generation and localization in PBX1314. A non-shock initiation criterion is developed based on macroscale variables in PBX1314. Numerical simulations of the penetration experiments are performed by using the proposed model and criterion. The mechanical damage and non-shock initiation of PBX1314 in the experiments are successfully predicted. The simulation results indicate that the tail of PBX1314 impacts the projectile repeatedly during penetration. Finally, the initiation criterion is satisfied because of frictional heat localization near microcrack surfaces and initiation is activated in the tail of PBX1314.

Modelling Microstructural Deformation and the Failure Process of Plastic Bonded Explosives Using the Cohesive Zone Model.

A microstructure finite element method combining the cohesive zone model (CZM) is used to simulate the mechanical behavior, deformation, and failure of polymer-bonded explosive (PBX) 9501 under quasi-static loading. PBX 9501 consists of Cyclotetramethylene tetranitramine (HMX) filler particles with a random distribution packaged in a polymeric binder. The particle is treated as elastic and the binder as viscoelastic. Cohesive elements with a bilinear softening law are inserted into the particle/binder interface, the HMX particle, and the binder to study the interface's debonding and failure evolution. Macroscopic stress-strain curves homogenized across the microstructure under tension and compression with different strain rates are basically consistent with the experimental data. The interface debonding approximately vertical to the loading direction is the primary failure mechanism under tension, while shear failure along the interfaces and particle fracture plays a significant role under compression. The effects of interface strengths and strain rates on the performance of PBX 9501 are also evaluated. The tensile and compressive strengths are dependent on the interface strength and strain rate, but the failure paths are insensitive. This model is shown to accurately predict macroscopic responses and improve our understanding of the relationship between the mechanical behavior and microstructure of PBX 9501.

In-Situ X-ray Tomography Observation of Structure Evolution in 1,3,5-Triamino-2,4,6-Trinitrobenzene Based Polymer Bonded Explosive (TATB-PBX) under Thermo-Mechanical Loading.

In order to study the fracture behavior and structure evolution of 1,3,5-Triamino-2,4,6-Trinitrobenzene (TATB)-based polymer bonded explosive in thermal-mechanical loading, in-situ studies were performed on X-ray computed tomography system using quasi-static Brazilian test. The experiment temperature was set from −20 °C to 70 °C. Three-dimensional morphology of cracks at different temperatures was obtained through digital image process. The various fracture modes were compared by scanning electron microscopy. Fracture degree and complexity were defined to quantitatively characterize the different types of fractures. Fractal dimension was used to characterize the roughness of the crack surface. The displacement field of particles in polymer bonded explosive (PBX) was used to analyze the interior structure evolution during the process of thermal-mechanical loading. It was found that the brittleness of PBX reduced, the fracture got more tortuous, and the crack surface got smoother as the temperature rose. At lower temperatures, especially lower than glass transition temperature of binders, there were slipping and shear among particles, and particles tended to displace and disperse; while at higher temperatures, especially above the glass transition temperature of binders, there was reorganization of particles and particles tended to merge, disperse, and reduce sizes, rather than displacing.

Release of 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) from polymer-bonded explosives (PBXN-109) into water by artificial weathering.

Polymer-bonded explosives (PBX) fulfil the need for insensitive munitions. However, the environmental impacts of PBX are unclear, even though it is likely that PBX residues from low-order detonations and unexploded ordnance are deposited on military training ranges. The release of high explosives from the polymer matrix into the environment has not been studied in detail, although as polymers degrade slowly in the environment we anticipate high explosives to be released into the environment. In this study, PBXN-109 (nominally 64% RDX) samples were exposed to variable UK climatic conditions reproduced in the laboratory to determine the effects of temperature, UV irradiation and rainfall on the release of RDX from the polymer binder. The most extreme conditions for spring, summer and winter in the UK were artificially reproduced. We found that up to 0.03% of RDX was consistently released from PBXN-109. The rate of RDX release was highest in samples exposed to the summer simulation, which had the lowest rainfall, but the highest temperatures and longest UV exposure. This was confirmed by additional experiments simulating an extreme summer month with consistently high temperatures and long periods of sunlight. These results probably reflect the combination of polymer swelling and degradation when samples are exposed to higher temperatures and prolonged UV irradiation.

Experimental Investigation of the Acoustic Nonlinear Behavior in Granular Polymer Bonded Explosives with Progressive Fatigue Damage.

The measurement of acoustic nonlinear response is known as a promising technique to characterize material micro-damages. In this paper, nonlinear ultrasonic approach is used to characterize the evolution of fatigue induced micro-cracks in polymer bonded explosives. The variations of acoustic nonlinearity with respect to fatigue cycles in the specimens are obtained in this investigation. The present results show a significant increase of acoustic nonlinearity with respect to fatigue cycles. The experimental observation of the correlation between the acoustic nonlinearity and fatigue cycles in carbon/epoxy laminates, verifies that an acoustic nonlinear response can be used to evaluate the progressive fatigue damage in the granular polymer bonded explosives. The sensitivity comparison of nonlinear and linear parameters of ultrasonic waves in the specimens shows that nonlinear acoustic parameters are more promising indicators to fatigue induced micro-damage than linear ones. The feasibility study of the micro-damage assessment of polymer bonded explosives by nonlinear ultrasonic technique in this work can be applied to damage identification, material degradation monitoring, and lifetime prediction of the explosive parts.

Research on structures, mechanical properties, and mechanical responses of TKX-50 and TKX-50 based PBX with molecular dynamics.

To improve the practicality and safety of a novel explosive dihydroxylamm onium 5,5'-bis (tetrazole)-1,1'-diolate (TKX-50), polyvinylidene difluoride (PVDF) and polychlorotrifluoroe-thylene (PCTFE) were respectively added to the TKX-50, forming the polymer-bonded explosives (PBX). Interfacial and mechanical properties of PBX were investigated through molecular dynamics (MD) method, desensitizing mechanisms of fluorine-polymers for TKX-50 were researched by compression and bulk shear simulations. Results show that the binding energies (E bind ) between polymers (PVDF or PCTFE) and TKX-50 surfaces all rank in order of (011) > (100) > (010), shorter interatomic distance and the resulted higher potentials lead to higher E bind on TKX-50/PVDF interfaces than that on PCTFE/TKX-50 interfaces. Compared with TKX-50, the ductility of PBX is improved due to the isotropic mechanical property and flexibility of fluorine-polymers especially the PCTFE. Desensitizing effect of fluorine-polymers for TKX-50 is found under loading condition, which is attributed to the enhanced compressibility and buffer capacity against external pressure in compression, as well as the improved lubricity to reduce the sliding potentials in bulk shear process. Graphical Abstract Comparisons of the internal stress and slide potentials of the novel explosive,TKX-50 and its based PBX. Desensitizing effects can be found by the adding of fluorine-polymers, it owes to their better flexibility and lubricity as well as the amorphous nature.