Nitrate Ester Plasticized Polyether Propellant Research Articles

Research on Crack Propagation of Nitrate Ester Plasticized Polyether Propellant: Experiments and Simulation.

To understand the fracture properties of the nitrate ester plasticized polyether (NEPE) propellant, single-edge notched tension (SENT) tests were carried out at room temperature (20 °C) under different tensile rates (10-500 mm/min). The mechanical response, crack morphology, evolution path, and crack propagation velocity during the fracture process were studied using a combination of a drawing machine and a high-speed camera. The mode I critical stress intensity factor KIc was calculated to analyze the tensile fracture toughness of the NEPE propellant, and a criterion related to KIc was proposed as a means of determining whether the solid rocket motors can normally work. The experimental results demonstrated that the NEPE propellant exhibited blunting fracture phenomena during crack propagation, resulting in fluctuating crack propagation velocity. The fracture toughness of the NEPE propellant exhibited clear rate dependence. When the tensile rate increased from 10 mm/min to 500 mm/min, the magnitude of the critical stress intensity factor increased by 62.3%. Moreover, numerical studies based on bond-based peridynamic (BBPD) were performed by modeling the fracture process of the NEPE propellant, including the crack propagation speed and the load-displacement curve of the NEPE propellant. The simulation results were then compared with the experiments.

NEPE Propellant Mesoscopic Modeling and Damage Mechanism Study Based on Inversion Algorithm.

To accurately characterize the mesoscopic properties of NEPE (Nitrate Ester Plasticized Polyether) propellant, the mechanical contraction method was used to construct a representative volume element (RVE) model. Based on this model, the macroscopic mechanical response of NEPE propellant at a strain rate of 0.0047575 s-1 was simulated and calculated, and the parameters of the cohesive zone model (CZM) were inversely optimized using the Hooke-Jeeves algorithm by comparing the simulation results with the results of the uniaxial tensile test of NEPE propellants. Additionally, the macroscopic mechanical behavior of NEPE composite solid propellants at strain rates of 0.00023776 s-1 and 0.023776 s-1 was also predicted. The mesoscopic damage evolution process of NEPE propellants was investigated by the established model. The study results indicate that the predicted curves are relatively consistent with the basic features and change trends of the test curves. Therefore, the established model can effectively simulate the mesoscopic damage process of NEPE composite solid propellants and their macroscopic mechanical properties.

Study on the constitutive model of NEPE propellant based on indentation creep

AbstractThe significance of the mechanical properties of solid propellant during the curing process lies in their contribution to stress‐strain calculations of viscoelastic engineering materials and the analysis of structural integrity of solid rocket motor charges. In order to study the Constitutive equation of the nitrate ester plasticized polyether (NEPE) propellant during the curing process, indentation creep tests were carried out on the NEPE propellant, and the viscoelastic constitutive model of propellant with curing reaction at 50 °C and atmospheric pressure was studied. Firstly, based on the viscoelastic theory, the experimental principle of indentation creep test is deduced, and indentation creep tests are carried out for propellants with different curing degrees. Then, based on 2S2P1D (2 Springs, 2 Parabolic elements, 1 Dashpot) model and time‐temperature equivalence principle, the indentation test data are processed, and the complex modulus master curve of propellant with different curing degrees is established, and the influence of curing reaction on viscoelastic properties of propellant is analyzed, so as to better understand the response characteristics of propellant system during curing. The results show that the 2S2P1D model has a good fitting effect for the master curve in low frequency period, the fitting error is controlled within 10 %, and the correlation coefficients are all above 0.95. With the curing reaction, the complex modulus gradually increases, while the change rate gradually decreases. And it tends to be stable on the seventh day.

Structural Integrity Assessment of an NEPE Propellant Grain Considering the Tension-Compression Asymmetry in Its Mechanical Property.

In order to investigate the effect of tension-compression asymmetry of propellant mechanical properties on the structural integrity of a Nitrate Ester Plasticized Polyether (NEPE) propellant grain, the unified constitutive equations under tension and compression were established, a new method for grain structural integrity assessment was proposed and the structural integrity of the NEPE propellant grain under the combined axial and transverse overloads was evaluated. The results indicate that the mechanical state of the NEPE propellant grain is in the coexistence of tension and compression under the combined axial and transverse overloads, and the tension and compression regions in the propellant grain is independent of the propellant constitutive behavior. The tension-compression asymmetry of the propellant mechanical properties has a certain impact on its mechanical response. The maximum equivalent stress and strain considering the tension-compression asymmetry falls between that obtained through the tension and compression constitutive model, and is the same as damage coefficient. The safety factor of the NEPE propellant grain considering the tension-compression asymmetry of its mechanical properties is larger than that non-considering, and the traditional method of structural integrity assessment is conservative.

In situ X-ray tomography study on internal damage evolution of solid propellant for carrier rockets

Nitrate Ester Plasticized Polyether (NEPE) propellant has been widely used for carrier rockets. However, the propellant is usually subjected to complex loads, changing the microstructures, which eventually affects its structural integrity. To elucidate the damage mechanism of the propellant, an in situ tensile testing rig with a maximum strain rate of 0.1 s−1 was developed with high-resolution synchrotron X-ray tomography. The internal microstructure and the resultant damage evolution have been characterized quantitatively. The mechanical behaviour of sampled propellant was extracted from the dynamic mechanical analysis. In addition, numerical simulation in terms of the spatial geometry of filler particles was performed in the framework of the micromechanical approach. To visualize the debonding process, a cohesive-zone model was employed to trace the debonding at the filler/matrix interface and then the damage evolution was characterized according to the cohesive behaviour. The results can assist in correlating the cohesive behaviour due to local heterogeneity with the macroscopic mechanical response for the NEPE propellant.

Effect of neutral polymeric bonding agent on tensile mechanical properties and damage evolution of NEPE propellant

Introducing Neutral Polymeric bonding agents (NPBA) into the Nitrate Ester Plasticized Polyether (NEPE) propellant could improve the adhesion between filler/matrix interface, thereby contributing to the development of new generations of the NEPE propellant with better mechanical properties. Therefore, understanding the effects of NPBA on the deformation and damage evolution of the NEPE propellant is fundamental to material design and applications. This paper studies the uniaxial tensile and stress relaxation responses of the NEPE propellant with different amounts of NPBA. The damage evolution in terms of interface debonding is further investigated using a cohesive-zone model (CZM). Experimental results show that the initial modulus and strength of the NEPE propellant increase with the increasing amount of NPBA while the elongation decreases. Meanwhile, the relaxation rate slows down and a higher long-term equilibrium modulus is reached. Experimental and numerical analyses indicate that interface debonding and crack propagation along filler–matrix interface are the dominant damage mechanism for the samples with a low amount of NPBA, while damage localization and crack advancement through the matrix are predominant for the ones with a high amount of NPBA. Finally, crosslinking density tests and simulation results also show that the effect of the bonding agent is interfacial rather than due to the overall crosslinking density change of the binder.

Research on NEPE Propellant at High Strain Rates

In order to study the mechanical properties of Nitrate Ester Plasticized Polyether (NEPE) propellant at high strain rates, the impact tests of NEPE propellant were carried out by the split Hopkinson bar, and the stress-strain curves of NEPE propellant under different strain rates ( 1500s−1-4500s−1 ) were obtained. The test results show that NEPE propellant has obvious nonlinear elasticity and strain rate sensitivity. The microstructure of NEPE propellant was observed by scanning electron microscope, and the fracture mechanism of NEPE propellant under high strain rate was obtained by analyzing the microstructure of propellant. In this paper, a nonlinear visco-hyperelastic constitutive model is adopted, in which Rivlin model and the viscoelastic model are used to describe the mechanical properties of materials. The constitutive parameters of the material were obtained by fitting the test data, and it was proved that the model could well describe the mechanical properties of NEPE propellant at high strain rates by comparing the test curve under different strain rates.

Investigation of high rate mechanical flow followed by ignition for high-energy propellant under dynamic extrusion loading

Investigating the ignition response of nitrate ester plasticized polyether (NEPE) propellant under dynamic extrusion loading is of great significant at least for two cases. Firstly, it helps to understand the mechanism and conditions of unwanted ignition inside charged propellant under accident stimulus. Secondly, evaluates the risk of a shell crevice in a solid rocket motor (SRM) under a falling or overturning scene. In the present study, an innovative visual crevice extrusion experiment is designed using a drop-weight apparatus. The dynamic responses of NEPE propellant during extrusion loading, including compaction and compression, rapid shear flow into the crevice, stress concentration, and ignition reaction, have been firstly observed using a high-performance high-speed camera. The ignition reaction is observed in the triangular region of the NEPE propellant sample above the crevice when the drop weight velocity was 1.90 m/s. Based on the user material subroutine interface UMAT provided by finite element software LS-DYNA, a viscoelastic-plastic model and dual ignition criterion related to plastic shear dissipation are developed and applied to the local ignition response analysis under crevice extrusion conditions. The stress concentration occurs in the crevice location of the propellant sample, the shear stress is relatively large, the effective plastic work is relatively large, and the ignition reaction is easy to occur. When the sample thickness decreases from 5 mm to 2.5 mm, the shear stress increases from 22.3 MPa to 28.6 MPa, the critical value of effective plastic work required for ignition is shortened from 1280 μs to 730 μs, and the triangular area is easily triggering an ignition reaction. The propellant sample with a small thickness is more likely to stress concentration, resulting in large shear stress and effective work, triggering an ignition reaction.

Numerical Analysis of Curing Residual Stress and Strain in NEPE Propellant Grain.

In order to investigate the formation mechanism of the residual stress and residual strain in a nitrate ester plasticized polyether (NEPE) propellant grain during the curing and cooling process, the temperature, curing degree and stress/strain of the NEPE propellant grain during the curing and cooling process were analyzed via ABAQUS finite element software. The results indicate that there is a temperature gradient in the NEPE propellant grain during curing at 50 °C. The maximum temperature difference is about 5 °C and the maximum temperature is located on the center of propellant grain. At the end of curing, the temperature in the interior of the grain tends to be uniform. The curing degree in the NEPE propellant grain during the curing process has the same trend as temperature. The residual stress/strain of the NEPE propellant grain during the curing and cooling down processes are mainly composed of curing shrinkage stress/strain in the curing process and thermal stress/strain in the cooling down process. The curing shrinkage stress and strain in the curing process account for 19% and 31% of the whole process, respectively. The thermal stress and thermal strain in cooling down process account for 75% and 69% of the whole process, respectively. The thermal stress and thermal strain in the curing process can nearly be ignored. The residual stress and residual strain calculated by the traditional method is larger than that obtained in this paper. The maximum deviation of the residual stress and residual strain are about 8% and 17%, respectively.

Molecular Simulation Study on the Aging Mechanism of NEPE Propellant Matrix

Polyethylene glycols (PEG) and toluene diisocyanate (TDI) are often used as the main components of binders and curing agents in solid propellants, and their aging is an important issue in the storage and use of propellants. To study the aging behavior and aging mechanism of nitrate ester plasticized polyether propellant (NEPE) matrix during storage, the transition states of aging reactions of binder and curing agent were optimized at the (U)B3LYP/6-311G(d,p) level of theory, and the rate coefficients over the temperature range of 298-1000 K were calculated by CVT theory. The results showed that there were five kinds of aging reactions for binder, which included decomposition, nitration, H abstraction, oxidation, and crosslinking reactions. Among them, theenergy barriers of oxidation and H abstraction reactions were relatively low (79.3-91.2 kJ·mol-1) and the main reaction types of binder aging. The main aging reaction of curing agent was decomposition. Compared with the aging reactions of binder, the energy barriers of curing agent are higher (196.6-282.7 kJ·mol-1) and the reaction is more difficult to occur. By comparing the energy barriers and rate constants of different reactions, it is found that the aging of NEPE propellant matrix can be divided into two stages. In the first stage, the propellant matrix mainly undergoes H abstraction and oxidation reaction, and as the reaction proceeds, the products crosslink to form -O-O-, -C-C-, and -C-O-C- bonds. At this time, the long chain molecules of the propellant matrix crosslink, and the molecular weight increases. This stage corresponds to the rising stage of mechanical properties in the aging process of the propellant. In the second stage, the propellant matrix mainly undergoes decomposition and nitration, resulting in degradation, the reduction of molecular weights, and the appearance of holes and microcracks in the matrix. This stage corresponds to the decline of mechanical properties in the aging process of the propellant. The above simulation results are in good agreement with the aging experimental phenomena, revealing the microscopic mechanism of the changes in the macroscopic properties of NEPE propellant during the aging process, and providing a theoretical basis for the related research on the aging properties and anti-aging technology of NEPE propellant.

Thermal Decomposition, Ignition Process and Combustion Behavior of Nitrate Ester Plasticized Polyether Propellant at 0.1–3.0 MPa

Nitrate ester plasticized polyether (NEPE) propellant is widely used in solid rocket motors, having both good mechanical properties and high specific impulse. However, its ignition and combustion process are complex and need to be better understood. In this study, a high-pressure sealed combustion chamber was constructed, a thermogravimetry-differential scanning calorimetry was used to investigate the thermal decomposition process, and a high-speed camera was used to capture the ignition process and combustion behavior of the propellant. The results showed that the thermal decomposition process of this propellant could be divided into two stages. The first stage (50–350°C) was the major mass loss stage and exhibited typical features of BTTN, RDX, and AP decomposition. The second stage (350–500°C) was mainly accompanied by decomposition of the remaining components as well as slight oxidation of aluminum particles. The ignition process of NEPE propellant was divided into four stages, including the inert heating period stage, thermal decomposition stage, initial flame stage, and stable combustion stage. After the propellant absorbed heat, the propellant started to pyrolyze and gasify to generate flammable gas. When the temperature of the propellant surface reached the flammable gas ignition point, an initial flame was generated on the surface, which spread rapidly, covering the surface. The ignition delay time of the propellant was measured by a signal acquisition system, and a mathematical model was then established for the ignition delay time. The results showed that the ignition delay time decreased with increased laser heat flux and ambient pressure. Finally, the Vielle burning rate empirical formula was used to fit the burning rate data for the propellant. The resulting good fit was consistent with experimental measurements and showed that the formula was valid for predicting the NEPE propellant burning rate under 0.1–3.0 MPa nitrogen.

Molecular simulation study of the stabilization process of NEPE propellant

In this reported study, the density functional theory (DFT) was used at the (U)B3LYP/6-311G(d,p) level to investigate the stabilization process of the nitrate ester plasticized polyether propellant (NEPE). Molecular simulations were conducted of the reaction that generates NO2, the autocatalytic and aging reaction triggered by the NO2, and the nitrogen dioxide absorption reaction of the stabilizers during the propellent stabilization process. These simulations were derived using the transition-state theory (TST) and variational transition-state theory (VTST). The simulation results suggested that the stabilization of the NEPE propellant consisted of three stages. First, heat and NO2 were generated during the denitrification reaction of nitroglycerine (NG) and 1,2,4-butanetriol trinitrate(BTTN) in the NEPE propellant. Second, nitroso products were generated by the reactions of N-Methyl-4-nitroaniline (MNA) and 2-nitrodiphenylamine (2NDPA) with NO2. Third, the stabilizers were exhausted and the autocatalytic reaction of NG and BTTN and the aging reaction of polyethylene glycol (PEG) were triggered by the heat and NO2 generated in the first stage. By comparing the energy barriers of the various reactions, it was found that the NO2 generated from the denitrification reaction significantly reduced the reaction energy barrier to 105.56–126.32 kJ/mol, also increased the reaction rate constant, and decreased the thermal stability and energetic properties of the NEPE propellant. In addition, the NO2 also weakened the mechanical properties of the NEPE propellant by attacking the –CH2 groups and the O atoms in the PEG molecular chain. The energy barriers of the reactions of MNA and 2NDPA with NO2 (94.61–133.61 kJ/mol) were lower than those of the autocatalytic and decomposition reactions of NG, BTTN, and the aging reactions of PEG (160.30–279.46 kJ/mol). This indicated that, by eliminating NO2, the stabilizer in the NEPE propellant can effectively prevent NO2 from reacting with the NG, BTTN, and PEG in the NEPE propellant. Consequently, this would help maintain the energy and mechanical properties of the NEPE propellant, thereby improving its thermal stability.

Investigation of acoustic characterization of ultrasonic air coupling testing of NEPE propellant under strain control

To explore the acoustic characterization of composite solid propellant materials, the wave equation of the viscoelastic body was derived based on the linear viscoelastic constitutive model, and the theoretical relationship between ultrasonic sound velocity and attenuation coefficient, which are closely related to mechanical properties, such as elastic modulus, viscosity coefficient, and complex modulus, was obtained. Ultrasonic air-coupled testing experiments of the NEPE (Nitrate Ester Plasticized Polyether) propellant under different tensile strain states were carried out to obtain the variation law of ultrasonic transmission wave peak amplitude, sound velocity, and attenuation coefficient with strain variables in combination with the variation in propellant porosity under tensile strain. Furthermore, the acoustic parameters with strain and porosity were fitted by the regression equation, and the effect of mesoscopic damage on macroscopic mechanical properties was analyzed. The results show that with the increase in NEPE propellant tensile strain, the porosity increases exponentially. With the increase in porosity, the sound velocity decreases by first polynomials, the wave amplitude decreases by second polynomials, and the attenuation coefficient increases by third polynomials.

Strain-rate correlation of biaxial tension and compression mechanical properties of HTPB and NEPE propellants

An effective biaxial tension and compression test method is proposed based on the shortcomings of current research for the mechanical properties of solid propellants under complex stress states. The equal proportion biaxial tension and compression test of HTPB (Hydroxyl-terminated polybutadiene) and NEPE (NitrateEster Plasticized Polyether) solid propellants is performed at different rates while at room temperature, and the damage morphology of the tension–compression zone is analyzed using micro-CT. The results show that the failure mode of the solid propellant under biaxial tension and compression loading is similar to that under uniaxial tension. Meanwhile, the compressive strength is much greater than the tensile strength, which will eventually cause tensile failure. With an increased loading rate, the growth trend of the initial modulus, ultimate strength, and maximum elongation of the propellant is gradually flattened, and the damage degree is gradually reduced. Additionally, damage that forms in the HTPB propellant is from dewetting and particle fracture while that for the NEPE propellant is from matrix tearing. The porosity can be used as the meso-damage parameter of the propellant.

Study on the Viscoelastic Damage Properties of NEPE Solid Propellant with Different Cyclic Stress Ratios

AbstractViscoelastic damage properties of NEPE (Nitrate Ester Plasticized Polyether) solid propellant have been studied in this work. This work focuses on the creep and fatigue damage evolution. Cyclic loading tests for NEPE solid propellants are conducted with different stress ratios. Then a modified model incorporating viscoelasticity and damage evolution is presented and used to fit the experimental data. The fitting results show that the model is suitable to characterize the viscoelastic damage properties of NEPE propellants with different stress ratios. Based on the model with parameters for NEPE propellants, the damage parameter D are obtained. The damage evolution curves under different loading conditions are given. It is shown that, from the predicted damage evolution curves, the critical damage parameter Dcr at the fracture points can be used to predict the fatigue lives under different loading conditions.

Molecular Dynamics Simulation of the Influence of PEG‐TDI Aging Degradation on Diffusibility and Compatibility of NG and BTTN

AbstractIn this study, the molecular dynamics method was used to explore the effects of the degradation of an adhesion agent (PEG‐TDI) in a nitrate ester plasticized polyether propellant on the compatibility and diffusibility of energetic plasticizers (NG and BTTN) and the glass transition temperature and mechanical properties of the blend system (PEG‐TDI/NG/BTTN). The radial distribution function, the mean square radius of gyration, and fractional free volume of the PEG‐TDI/NG/BTTN system under different degrees of degradation conditions were calculated. The diffusibility and compatibility of NG and BTTN, the glass transition temperature, and the mechanical properties of the blended system were then discussed. The results showed that, with aggravation of the degree of PEG‐TDI degradation, the fractional free volume in the system decreased, intermolecular hydrogen bonding and van der Waals interactions enhanced, and the size of the polymeric nematic decreased. These changes eventually caused the diffusion ability of NG and BTTN to weaken, compatibility to be enhanced, and glass transition temperature, stiffness, and yield strength of the PEG‐TDI/NG/BTTN system to increase.

Influences of Bu‐NENA and BDNPA/F Plasticizers on the Properties of Binder for High‐Energy NEPE Propellants

AbstractDesensitized energetic plasticizer (DEP) plays a crucial role in the properties of the binder, and thus can further influence the performances of desensitive Nitrate Ester Plasticized Polyether (NEPE) propellants, which represents the development direction of the next generation solid propellant composites. However, at present, a study on influencing mechanisms for curing properties of propellant binders at the atomic and molecular level was rarely reported. In the current study, based on the curing kinetics and mechanical properties of PET/N‐100 binder, the influencing mechanism of the DEPs, BDNPA/F, and Bu‐NENA, on the binder was determined by the Gaussian quantum chemical calculation and MD simulation at the chemical bond and polymer chain levels, as well as the characterizations of the curing network structure, glass transition temperature, and microphase separation of the binder system. The consistent results at both micro and macro levels offer important theoretical guidance for studying the structure‐mechanism‐performance relationship of propellant binder, as well as for exploring the strategies that can reveal the curing mechanism of the binder system in depth.

Experimental Investigation and Modeling the Compressive Behavior of NEPE Propellant under Confining Pressure

AbstractIn order to study the effects of confining pressure and strain rate on the mechanical properties of the Nitrate Ester Plasticized Polyether (NEPE) propellant, compressive tests were conducted under various confining pressure conditions and strain rates using a new‐designed active gas confining pressure testing machine. The stress‐strain curves and mechanical properties of the NEPE propellant under varying confining pressure conditions and strain rates were obtained. The results show that the compressive mechanical properties of the NEPE propellant are remarkably influenced by the confining pressure and strain rate. The compressive strength under confining pressure is obviously larger than those without applied confining pressure. With the coupled effects of the confining pressure and strain rate, the compressive strength under 5.4 MPa and 6.67×10−2 s−1 is 2.17 times of its value at room condition and 6.67×10−4 s−1. Then, based on previous tensile results, the quantified comparisons of tensile and compressive mechanical properties under different experimental conditions were presented. Finally, a nonlinear constitutive model with damage was constructed to model the effect of pressure. The statistical damage model parameters were defined as ExpDec1 function of pressure P. The calibrated model can accurately predict the mechanical behaviour of the NEPE propellant under different confining pressure conditions and strain rates.

Experimental and simulation study of the phase separation of neutral polymeric bonding agent in nitrate ester plasticized polyether propellant and its application

Solid propellant is the key energetic composite material for weapons. Nitrate ester plasticized polyether (NEPE) propellant has shown the best energetic property among all propellants practically used at present, and neutral polymeric bonding agent (NPBA) is usually applied to improve its mechanical properties. However, the complex composition of propellant and explosive properties of nitrate plasticizers make the in-depth study of the cooling phase separation behavior and bonding performance of NPBA very challenging. In the present study, the phase separation behaviors of NPBA in different solution systems were studied by the turbidity method, and its adhesion degree onto solid filler caused by the phase separation was evaluated. The results show that high contents of AN in NPBA, high contents of inert solvents and high concentrations of NPBA are conducive to the phase separation. Based on the experimental results, a molecular dynamics simulation model for the cooling phase separation of NPBA in inert solvents is established, and applied to the phase separation of NPBA in the explosive nitrate plasticizer solution systems. The influences of the ratio of nitrate plasticizers, the ratio between plasticizer and binder, the concentration and molecular weight of NPBA, and the terpolymer composition of NPBA on the cooling phase separation were determined by the molecular dynamics simulation and verified with the experimental application of NPBA in the actual NEPE propellant system. Our work has provided a new strategy for the structural design and bonding performance evaluation of NPBA as the bonding agent for NEPE propellants, and constructed a novel structure-activity model to further guide its application study.

Comprehensive Evaluation of the Accelerated Aging Law of NEPE Propellants

In order to study the accelerated aging law of nitrate ester plasticized polyether (NEPE) propellants, the mechanical properties, weight loss, adhesive network structure fracture, and stability of NEPE propellants during storage were analyzed. The results show that the maximum tensile strength σm shows good change law with the increase of storage time, and the failure of NEPE propellants is mainly strength failure. The content of the stabilizer decreased with the increase of storage time. With the prolongation of storage time, the weight loss ratio increases gradually with good regularity. The decomposition of the components in the propellant leads to the degradation of the polyethylene glycol (PEG) network structure. The thermal decomposition of the nitrate plasticizer in NEPE propellants will occur, and the products will cause the adhesive network structure to decompose and break the chain, leading to the destruction of the matrix structure. The thermal decomposition of the propellant has temperature-increasing rate dependency.