Composite Modified Double-base Research Articles

Correlation between microstructural evolution and mechanical properties of CMDB propellant during uniaxial tension

AbstractNumerical simulation of the deformation and damage evolution process of composite modified double base (CMDB) propellant based on the microstructure is of great significance for improving its macro‐mechanical properties. The macro‐mechanical properties and damage evolution of CMDB propellant were studied experimentally and numerically in current work. To establish the correlation between microstructural evolution and the macro‐mechanical response of the propellant, a three‐dimensional microstructure model of the CMDB propellant was constructed utilizing the molecular dynamics particle filling approach. At the interface between RDX particles and the nitrocellulose‐nitroglycerine (NC‐NG) matrix, the conventional cohesive element was replaced by cohesive contact, and the parameters of the cohesive zone model (CZM) at the interface between the particles and the matrix were obtained by parameter inversion based on the Hooke‐Jeeves parameter optimization algorithm. The processes of damage initiation, propagation, convergence, and failure at the cohesive interface were simulated using the bilinear cohesive zone model (BCZM). The results revealed that the fracture strength of NC‐NG propellant with added RDX particles was significantly increased and the fracture elongation was reduced. The three‐dimensional microstructure model can accurately describe the microstructure of CMDB, and dewetting at the interface is the primary cause of propellant degradation when subjected to an external load, which compromises the mechanical properties of the propellant.

Investigation of the Effect of Axial Gap on the Mechanical Response of a Cartridge-Loaded CMDB Propellant Grain under Vibration Loads

In this study, the effect of the axial gap on the mechanical response of a cartridge-loaded propellant grain under vibration loads is investigated. The wide strain rate range of uniaxial compression tests ( 1.7 × 10 − 3 ~ 4 × 10 3 s − 1 ) on the composite modified double base (CMDB) propellant was carried out by using a universal testing machine, a hydraulic testing machine, and a split Hopkinson pressure bar system, respectively. A linear viscoelastic constitutive model of the CMDB propellant was developed by using the experimental measurements. The results show the studied CMDB propellant has a strong strain rate dependence, exhibiting an initial linear elasticity followed by a strain hardening region. The dynamic process of collision between the propellant grain and the motor case in the axial direction induced by vibration loads was simulated with the developed constitutive model by using the finite element method. The effects of the gap size between the propellant grain and the case and the vibration frequency on the mechanical response of the grain were studied. This shows that with a constant vibration frequency, the stress of the grain increases first and then decreases with increasing gap size. Moreover, the stress increases with increasing vibration loads.

Mechanical behaviors and failure mechanisms of CMDB propellant under wide strain rate tension loading

Composite modified double-base (CMDB) is a sort of mature propellant with a stable structure and high performance. Its tensile mechanical properties should be considered in the structural integrity design of solid propellant charges. The tensile tests were performed on the CMDB by a quasi-static measurement machine. The Hopkinson tension bar (SHTB) under strain rates from 0.0001s−1 to 2500s−1 was modified for structural integrity evaluation, and the stress-strain curves were accurately obtained. After comparing under different strain rates, the results show that the initial modulus of elasticity and stress level are significantly affected by the level of strain rate. Furthermore, the strain hardening becomes more distinct with increasing strain rates. Failure specimens were observed by scanning electron microscopy (SEM). It is indicated that the failure under a low strain rate is mainly debonding of filled particles from the matrix. However, it is mainly caused by their embrittlement under dynamic loading.

Preparation of HMX@DHBA-Pb and HMX@NTO-Pb composites via in situ deposition: A way to achieve surface catalysis of HMX

In order to ameliorate the combustion properties of 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX) based composite modified double base (CMDB) propellants, HMX particles with many gullies are first prepared, and then 2,4-dihydroxybenzoic acid (DHBA) and 1,2,4-triazol-5-one (NTO) are used to react with lead via in situ deposition to coat HMX for preparing HMX@DHBA-Pb and HMX@NTO-Pb composites. The structures and properties of HMX@DHBA-Pb and HMX@NTO-Pb composites are characterized in detail. The characterization results show that DHBA-Pb and NTO-Pb shells are uniformly coated on HMX surfaces. The chemical structure of HMX maintains constant during in situ deposition coating process. The mechanical and thermal safeties of HMX are enhanced after coating with DHBA-Pb and NTO-Pb shells. The introduce of DHBA-Pb and NTO-Pb can catalyze the decomposition and combustion of HMX core, and the catalytic effects of in situ coating are better than that of physical mixing.

Ameliorating the sensitivities, thermal and combustion properties of RDX by in situ self-assembly TA-Pb/Cu shells to RDX surface

In order to ameliorate the sensitivities, thermal and combustion properties of cyclotrimethylenetrinitramine (RDX), tannic acid (TA) is used to react with lead and copper via in situ self-assembly to coat RDX for preparing RDX@TA-Pb/Cu microcapsules. The structures of RDX@TA-Pb/Cu microcapsules are characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and Fourier-transform infrared spectra (FT-IR). The surface topography of RDX@TA-Pb/Cu microcapsules are characterized by scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The mechanical sensitivities and explosion points of RDX@TA-Pb/Cu microcapsules are measured to study the influence of TA-Pb/Cu shells on mechanical and thermal safeties of RDX. The non-isothermal properties of RDX@TA-Pb/Cu microcapsules are characterized by differential scanning calorimetry (DSC). The catalytic effects of TA-Pb/Cu shells on RDX are characterized by accelerating rate calorimeter (ARC). The residues of RDX@TA-Pb/Cu microcapsules after combustion in air are collected and characterized by SEM and XRD to further study the catalytic effect of TA-Pb/Cu shells. The study results show that a 150 nm TA-Pb/Cu shells are uniformly coated on RDX surfaces. The chemical structure of RDX maintains constant during in situ self-assembly coating process. The mechanical and thermal safeties of RDX are enhanced after coating with TA-Pb/Cu shells. The decomposition and combustion property of RDX can be catalyzed by TA-Pb/Cu, and the catalytic effects of in situ self-assembly coating are better than that of physical mixing. The RDX@TA-Pb/Cu microcapsules can be used in RDX based composite modified double base (CMDB) propellants.

ReaxFF molecular dynamics simulations on thermal decomposition of RDX-based CMDB propellants.

ReaxFF molecular dynamics (ReaxFF MD) simulations were performed to study the thermal decomposition property of cyclotrimethylenetrinitramine (RDX)-based composite modified double base (CMDB) propellants. The intermediate products and final products of the decomposition of RDX-based CMDB propellants at 2000K, 2500K, and 3000K are obtained. The simulation results show that the decomposition of RDX and RDX/HTPB/Al is primary triggered by N-NO2 rupture, and then, the intermedia products undergo a series of complex interactions to form final products. The final products of RDX/HTPB/Al are H2 and N2, while the final products of pure RDX are H2, N2, and H2O. In addition, the abundance of the main intermediate products generated by RDX/HTPB/Al is lower than that of RDX, in that the reaction between intermediate products is more complex for RDX/HTPB/Al. Moreover, the decomposition rate of RDX/HTPB/Al increases with the increasing temperature.

Quickly determination of resorcinol in composite modified double-base propellants based on near-infrared reflectance spectroscopy

Resorcinol, as a stabilizer component, plays an important role in maintaining the stability of composite modified double-base (CMDB) propellants. Near-infrared reflectance spectroscopy (NIRS) was used to quickly detect the resorcinol content of CMDB propellant. The optimal spectral interval was determined to be 4126.92–6927.05 cm−1 by analyzing the useful characteristic peaks in sample spectra. The partial least squares (PLS) algorithm was used to establish the resorcinol prediction models. By comparing the performance of the developed models with different preprocessing, the best method of spectral preprocessing was determined as standard normal variate transformation (SNV) + first-order derivative (1ST). The optimal number of factors was 7 for resorcinol model. For the developed model, the determination coefficients of calibration and cross-validation (Rc2, Rcv2) were 0.997 and 0.987, respectively. The root mean square errors of calibration and cross-validation (RMSEC, RMSECV) were 0.013 %, 0.024 %, respectively. The determination coefficients (Rp2) and the root mean square errors of prediction (RMSEP) of prediction were 0.991 and 0.018 %. The average relative error of the proposed and reference methods was only 0.253 %. The developed method can be used for quickly determining the content of resorcinol in CMDB propellants. In addition, methyl violet test showed that the stability of CMDB propellant became worse with the decrease of resorcinol content. In terms of stability, qualified CMDB propellant should contain resorcinol content not less than 1.285 %. Thus, the prediction model of resorcinol is available for judging the stability of CMDB propellants qualified or unqualified.

Study of the Mechanical Properties of a CMDB Propellant Over a Wide Range of Strain Rates Using a Group Interaction Model

Composite modified double base (CMDB) propellants are heterogeneous propellants in which properties are significantly improved by adding solid particles into the polymer matrix. A molecular group interaction model that can predict the mechanical properties of polymers through a molecular structure is used to predict the viscoelastic behavior of the CMDB propellant. Considering that the addition of solid particles will improve the crosslinking degree between polymer molecules and reduce its secondary loss peak, the input parameters of the model are modified through dynamic mechanical analysis (DMA) experimental data. By introducing the strain rate into the expression of model glass transition temperature, the mechanical properties of propellant over a wide strain range ( 1.7 × 10 − 4 s-1 ~ 3000 s-1) are obtained. The reliability of the model is verified by comparison with uniaxial compression test data. By modifying the input parameters of the model, the effects of different mass ratios of nitrocellulose (NC)/nitroglycerin (NG) on the mechanical properties of the CMDB propellant were analyzed. The results show that the glass transition loss increases with increasing mass ratio of NC/NG, while Young’s modulus and yield stress decrease.

Effect of Metal Nanopowders on the Performance of Solid Rocket Propellants: A Review.

The effects of different types of nano-sized metal particles, such as aluminum (nAl), zirconium (nZr), titanium (nTi), and nickel (nNi), on the properties of a variety of solid rocket propellants (composite, fuel-rich, and composite modified double base (CMDB)) were analyzed and compared with those of propellants loaded with micro-sized Al (mAl) powder. Emphasis was placed on the investigation of burning rate, pressure exponent (n), and hazardous properties, which control whether a propellant can be adopted in solid rocket motors. It was found that nano-sized additives can affect the combustion behavior and increase the burning rate of propellants. Compared with the corresponding micro-sized ones, the nano-sized particles promote higher impact sensitivity and friction sensitivity. In this paper, 101 references are enclosed.

The Auto‐Ignition Behaviors and Thermal Safety of the Composite Modified Double Base Propellants under Rapid Heating

AbstractThe safety issue of energetic materials (EMs) such as propellants or explosives has been a high concern. When EMs are subjected to rapid thermal stimulus, they will ignite or explode, which may cause personal injury and property loss. Studies on auto‐ignition behaviors and thermal safety of EMs under rapid thermal stimulation provide important guidelines for EMs in their production, transportation, storage, and application. In this work, auto‐ignition experiments of the composite modified double base (CMDB) propellants, which include 1,1‐diamino‐2,2‐dinitroethylene (FOX‐7) and hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine (RDX) mixtures under high initial temperature condition, have been conducted by using a rapid compression machine (RCM). The temperature rise rate can reach up to 2×104 K/s. Pressure evolution recording and high speed‐visualization were synchronized to reveal the auto‐ignition behaviors of CMDB samples. Results show that the end of compression (EOC) temperature and pressure have a significant impact on the ignition delay time (IDT). The IDT increases with a decrease in temperature and pressure. In addition, through tuning the EOC temperature and pressure, the critical condition that separates the ignition region and non‐ignition region is obtained. The critical ignition temperature of CMDB decreases with an increase in pressure. When the pressure is higher than 2.0 MPa, the critical temperature is about 650 K.

Effect of hexanitroethane (HNE) and hydrazinium nitroformate (HNF) on energy characteristics of composite solid propellants

This work deals with energy performance calculate results of new solid composite propellants based on Hexanitroethane (HNE) and Hydrazinium nitroformate (HNF). The energy characteristics of solid propellants with HNE and HNF are estimated using NASA Chemical Equilibrium with Applications (CEA). The effect of HNE and HNF as energetic oxidizers on the energy characteristics of composite modified double-base (CMDB) propellants, hydroxyl-terminated polybutadiene (HTPB) propellants, nitrate ester plasticized polyether (NEPE) propellants, glycidyl azide polymer (GAP) propellants has been carried out. Characteristics of ingredients and propellant formulations with HNE and HNF are presented together with the calculate results which demonstrate the increased performance compared to conventional high-performance propellants based on Ammonium Perchlorate (AP). The results show that HNE and HNF can improve the standard theoretical specific impulse of the four propellants in different degrees, and HTPB propellant has the most significant improvement, which is 12.26 s for HNE-based propellant and 14.12 s for HNF-based propellant. At the same time, both HNE and HNF reduce the infrared emission of propellant exhaust plumes when they replace AP, and reduce or even completely eliminate HCl gas in exhaust products. Therefore, it is expected that HNE-based and HNF-based propellants may provide a promising chlorine-free alternative to existing AP-based formulations.

Simulation of the plasticizing behavior of composite modified double-base (CMDB) propellant in grooved calendar based on adaptive grid technology

The frequent occurrence of safety accidents during the calendering process is caused by the flammable and explosive properties of composite modified double-base (CMDB) propellant. Optimization of process parameters with the aid of fluid simulation technology could effectively ensure the safety of the calendering process. To improve the accuracy of the simulation results, material parameters and model structure were corrected based on actual conditions, and adaptive grid technology was applied in the local mesh refinement. In addition, the rheological behavior, motion trajectories and heat transfer mechanisms of CMDB propellant slurry were studied with different gaps, rotational rates and temperatures of two rollers. The results indicated that the refined mesh could significantly improve the contour clarity of boundaries and simulate the characteristics of CMDB propellant slurry reflux movement caused by the convergent flow near the outlet. Compared with the gap, the increased rotational rate of roller could promote the reflux movement and intensify the shear flow of slurry inside the flow region by viscous shear dragging. Meanwhile, under the synergistic effect of contact heat transfer as well as convective heat exchange, heat accumulated near the outlet and diffused along the reflux movement, which led to the countercurrent heat dissipation behavior of CMDB propellant slurry. The plasticizing mechanism of slurry and the safety of calendering under different conditions were explored, which provided theoretical guidance and reference data for the optimization of calendering process conditions. Based on the simulation results, the safety of the CMDB propellant calendering process could be significantly improved with a few tests conducted during a short research and development cycle.

Real‐Time Plume Velocity Measurement of Solid Propellant Rocket Motors Using TDLAS Technique

AbstractAccurate measurement of plume velocity plays an important role in investigation of rocket motors, and the plume velocity is an indicator of energy level of solid propellants. A method based on laser Doppler effect using tunable diode laser absorption spectroscopy (TDLAS) is employed in this study. The absorption spectrum of water vapor feature at 1391.7 nm is utilized to measure the plume velocity. Three solid propellants, double‐base (DB), composite modified double‐base (CMDB), and nitrate ester plasticized polyether (NEPE) i. e., are burned in a motor with an internal diameter of 50 mm and their plume velocities are measured with scanned‐WMS‐2f technique. Afterburning reactions in the plume are numerically simulated. The simulations can provide insight into the nuances in measured TDLAS results and give confidence in the result. The results show that TDLAS is a good method to measure plume velocity. The relative standard deviation of the measurements is less than 4 %. The difference between the measured velocity and the simulated one is less than 8.25 %. Additionally, it is found that NEPE has the largest plume velocity, and the velocity of DB is the lowest. Uncertainties of velocity measurement using TDLAS are analyzed. Measurement uncertainty comes from velocity non‐uniformity in the direction perpendicular to the plume direction. Furthermore, it can be found that it is better to put the measurement position close to the nozzle exit to reduce measurement errors.

The effect of strain rate on compressive behavior and failure mechanism of CMDB propellant

The compressive mechanical behavior of composite modified double base (CMDB) propellant was investigated across a wide scope of strain rates ranging from 10−3 s−1 to 4210 s−1 at room temperature, by applying a conventional universal testing machine and a split Hopkinson tension bar (SHPB), respectively. The derived stress-strain curves at different strain rates show a strong rate dependence, indicated that yield stress, ultimate stress and strain energy density of CMDB propellant all increase with strain rate by following a power law function, while the amplification of increase are different. The deformation and damage modes of CMDB propellant has changed from a typical ductile manner (cracking along the axial direction) to a brittle manner (maximum shear failure) with increasing of strain rate. Scanning electron microscopy (SEM) was employed to explore the microscopic failure characteristics of CMDB propellant. Under quasi-static loading, the nearly parallel micro-cracks propagating along the axial direction and the debonding of RDX particle without particle crushing can be observed. While under dynamic loading, the micro-crack is 45° angle to the axial direction, and multiple cracking modes of RDX particles appeared. Finally, the correlation between strain energy density and failure mechanisms of CMDB propellant was revealed by developing four characteristic failure modes. The findings of this study is very important to evaluate the structural integrity of CMDB propellant.

Effect of Al content and particle size on the combustion of HMX-CMDB propellant

In solid propellants, aluminum plays a key role in energy performance enhancement, but it also induces combustion problems such as low combustion efficiency and agglomeration. Herein, we study the effects of Al on the combustion properties of low-signal composite-modified double-base (CMDB) propellants. Experiments were conducted to determine the heat of explosion, the burning rates, and the combustion wave structures. The flame structure, the burning surface phenomenon and the morphology of the extinguished surface layer were observed. In addition, distribution of Al particles was calculated. It was found that using small Al particles or lowering the Al content will reduce the burning rate. This result was mainly cause by the “thermal sink” effect of Al. The Al agglomeration and ignition behaviors in the dark zone and the flame zone were observed at 1 and 3 MPa. It was found that propellants containing more than 10 wt.% Al or using Al particles smaller than 5 µm tended to generate large Al agglomerates. The extinguished burning surface presented “metal skeleton” structure under low pressures. The Al particles smaller than the HMX would be locally concentrated, so that it was easier for them to form large agglomerates.

Application and Properties of CL‐20/HMX Cocrystal in Composite Modified Double Base Propellants

AbstractThe composite modified double base (CMDB) propellants containing 2,4,6,8,10,12‐hexanitro‐2,4,6,8,10,12‐hexaazatetracyclo[5.5.0.05,9.03,11] dodecane (CL‐20), 1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocane (HMX) or CL‐20/HMX cocrystal were prepared by the solventless extrusion formation technique. The performances of CMDB propellants were evaluated by means of stability, sensitivity, combustion and mechanical experiments. Raman spectra analysis indicated the stability of CL‐20/HMX cocrystal is partially destroyed in the rolling process under 90 °C, 10 MPa. Methyl violet tests revealed that the colored time of CMDB propellants containing CL‐20/HMX cocrystal, CL‐20 or HMX are 77, 60, and 75 min, respectively. Burning rate tests shown that the burning rates of CMDB propellants containing CL‐20/HMX cocrystal, CL‐20 or HMX in the pressure range of 8 to 18 MPa, increased 1.82, 4.67, and 4.67 mm/s, with the pressure exponents 0.095, 0.306, and 0.346, respectively. Mechanical property tests revealed that the mechanical properties of CMDB propellant containing CL‐20/HMX cocrystal are inconsistent, but the tensile strength has improved compare with CMDB propellant containing CL‐20. These results suggest that CL‐20/HMX cocrystal is a potential material to solve the application problem of the safety performance and stability of the propellants containing CL‐20.

Study on Comparative Performance of CL‐20/RDX‐based CMDB Propellants

AbstractThe energy, combustion and combustion residues properties of composite modified double‐base (CMDB)propellants with CL‐20 were compared to those propellants with RDX. The energy characteristic of CL‐20/RDX‐CMDB propellants had also been calculated theoretically based on the principle of minimum free energy. The energy property of propellants with CL‐20 was found to be evidently enhanced in comparison to those propellants containing RDX. The findings on combustion properties revealed that the combustion properties of CL‐20‐CMDB propellants were contrary to the of RDX‐CMDB propellants. With the mass fraction of CL‐20 increasing in the propellants, the burning rates of propellants can be enhanced significantly, but the burning rates of propellants containing RDX decreased. Analysis of the combustion residues for CL‐20/RDX‐CMDB propellants revealed that the C, Cu and Pb elements aggregated on combustion surface, which may be useful for guiding the regulation of combustion performance of high‐performance CMDB propellants containing CL‐20.

Thermal Safety of a Gun‐Launched Missile's Solid Rocket Motor Under Conditions of High Environm. Temp. and Overloaded Forces

AbstractTo verify a hypothetical failure cause of a gun‐launched missile's solid rocket motor (SRM) under conditions of high environmental temperatures and overloaded forces of up to 6000∼12000 g, experiments and simulations were conducted and a new criterion variable was proposed. Uniaxial compression tests of the composite modified double base (CMDB) propellant used in the gun‐launched missile were carried out at low and intermediate strain rates at 50 °C to characterize the CMDB propellant's dynamic properties. The Schapery nonlinear viscoelastic constitutive model was employed and applied in finite element analyses. Contact and an oscillating frictional direction between the grain and the case were observed when the overload curve oscillated with time. Three key variables of the process were researched: the contact pressure, the relative velocity, and the total contact time. Based on these, a new criterion variable q is proposed for judgements regarding the SRM's thermal safety. By comparing the results with those from friction sensitivity tests, it was discovered that the grain had a possibility of self‐ignition that cannot be ignored. Meanwhile, the gap width between the case and the grain was determined to be a key influencing factor.

Applying Mechanically Activated Al/PTFE in CMDB Propellant

AbstractA novel reactive powder (aluminum/polytetrafluoroethylene, Al/PTFE 75/25 mass %) containing two kinds of PTFE (pre‐sintering PTFE and fresh PTFE, P‐PTFE and F‐PTFE) was prepared by a two‐step mechanical activation (MA) method. By introducing it into composite modified double base (CMDB) propellant, an attempt to improve the propellant performance of this mechanically activated Al/PTFE was made. Scanning electron microscopy (SEM) characterization indicated that F‐PTFE in activated powders deformed to abundant fibers under uniform shear forces. These fibers significantly enhanced propellant tensile properties and show improved safety performance. Compared to blank CMDB propellant (without PTFE), the elongation at maximum strength for propellant with activated Al/PTFE inclusions was increased to 3.7 times at −40 °C and to 3.9 times at −70 °C, the friction sensitivity and impact sensitivity were reduced by 88.9 % and 20.4 %, respectively. P‐PTFE in activated powders promoted the Al reaction activity, thereby reducing the propellant combustion agglomeration. The average diameter (D50) of coarse products for propellant with activated Al/PTFE 75/25 mass‐% was reduced by 82 % compared to that of blank CMDB propellant with spherical aluminum. This novel activated Al/PTFE shows increased propellant mechanical performance, decreased sensitivity and improved combustion efficiency of Al, which might expand the application field of CMDB propellants.

Experimental and theoretical studies on stability of new stabilizers for N-methyl-P-nitroaniline derivative in CMDB propellants

Although N-methyl-P-nitroaniline (MNA) was a quite effective stabilizer in composite modified double base (CMDB) propellants, it undergoes crystallization easily from nitroglycerin (NG) during storage. In order to improve its solubility in nitroglycerin (NG) and the stability in propellants, several new stabilizers including N-ethyl-p-nitroaniline (ENA), N-n-propyl-p-nitroaniline (n-PNA), N-i-propyl-p-nitroaniline (i-PNA), N-n-butyl-p-nitroaniline (n-BNA) and N-t-butyl-p-nitroaniline (t-BNA) were designed and synthesized to replace MNA by increasing the carbon chain length. The interaction between NG and different stabilizers was simulation by Materials Studio 5.5 and the stability and the high temperature stability performance of those new stabilizers in propellants were calculated by Gaussian 09. It was found that both the solubility of new stabilizers in NG and the stability and the high temperature stability performance of those in propellants were improved when the carbon chain length of substitution groups on nitrogen atom was increased. Thus, the n-BNA was a most potential stabilizer. Then all properties of the stabilizers were studied experimentally, which was agreement well with the theoretical analysis.