Nitrate Ester Plasticized Polyether 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.

Investigation of aluminum particle ignition dynamics in various propellant environments

The ignition delay of aluminum particles can be profoundly influenced by the agglomeration and combustion characteristics of solid propellants. In this experimental study, the ignition of individual aluminum particles in nitrate ester plasticized polyether (NEPE) and hydroxyl‑terminated polybutadiene (HTPB) propellant atmospheres was examined using laser ignition tests and high-speed photography. Focus was directed at the effects of the atmospheric composition, pressure, and aluminum particle size (500, 800 and 1000 µm). It was observed that the non-homogeneous reaction heat generation and convective heat exchange rates during particle ignition could be increased by increasing the CO2 content or ambient temperature, resulting in a shorter ignition delay time. This explains why aluminum particles tended to burn more efficiently in nitrate ester plasticized polyether than in hydroxyl‑terminated polybutadiene. In addition, it was found that the ignition delay time decreased with increasing pressure and was inversely proportional to the particle size squared. An ignition model for aluminum particle ignition in a multi-component atmosphere was developed and validated to describe the temperature change and energy transfer during ignition. The calculated ignition delay times were found to be within 5% of the experimental values. The model also showed that increasing the ambient temperature was more effective than increasing the CO2 content at enhancing ignition, and that CO2 was more effective than O2, which was itself more effective than H2O. Overall, this study provides useful insight into the ignition characteristics of aluminum particles in different propellant environments. The results could be used to develop more effective propellant formulations, optimize combustion processes, and improve safety in the handling and use of aluminum-based propellants.

Mesoscale mechanics investigation of multi-component solid propellant systems

Abstract To enhance the mechanical properties of the Nitrate Ester Plasticized Polyether solid propellant matrix, the uniaxial tension of multi-component systems is simulated and the factors influencing the mechanical properties of the propellant matrix are investigated. First, mesoscale models of five types of systems include poly alpha olefin (PAO(3)), polyethylene glycol (PEG200, PEG400, PEG600), and 1,4-butanediol (BDO) are established, followed by uniaxial tensile simulations. The results show PEG600, PEG400, PEG200, BDO, and PAO(3) in order of enhancing the mechanical performance of the matrix. Second, the diffusion behavior of nitroglycerin (NG) and butanetriol trinitrate (BTTN) in various systems is investigated. The results show that NG exhibits higher diffusion capacity than BTTN, and the diffusion coefficient increases with an increment in the molecular weight of PEG. Additionally, the influence of different plasticizer ratios (2.8–3.0), curing parameters (1.58–1.62), and chain extension parameters (0.08–0.10) on the mechanical properties of the PEG600 system are investigated. The results demonstrate that as the plasticizer ratio increases, there is a gradual decrease in the modulus of the matrix. Additionally, an increase in the curing parameter leads to a substantial enhancement in the tensile strength of the matrix, while increasing the chain extension parameter significantly expands the maximum tensile length of the matrix. Finally, employing the Slip-Spring model, the effects of the physical and chemical cross-linked network of the propellant are simulated. The result shows that increasing the content of a chemical cross-linked network significantly improves the tensile strength of the matrix.

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.

A simple and highly accurate method for fuzzy edge-based 3D morphometry

As the level of engineering technology increases, the mechanical problems posed in the area of mechanics become complex and diverse. The traditional measurement methods and content of past measurements no longer meet the needs, so more advanced experimental methods are needed to accurately measure the three-dimensional shape of objects and their deformations. Nitrate ester plasticized polyether (NEPE) propellants, are the highest energy solid propellant that have been applied in public reports in the world. Due to the material properties of the material itself, its mechanical properties cannot be accurately measured by conventional methods. This paper proposes a simple and highly accurate optical measurement method to study the volume change rate, Poisson’s ratio and true stress-strain curves of NEPE at different displacement loading rates of NEPE. In this paper, the strain of solid propellant NEPE in three directions during unidirectional stretching was measured by defocusing method and digital image correlation(DIC) method and the volume change rate, Poisson’s ratio and true stress-strain curves of NEPE under different stretching rates were obtained. The measured damage initiation engineering strain of NEPE at different tensile strain rates is 0.4687.

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.

Investigation on the creep models of the NEPE solid propellant

In this paper, the creep experiments of Nitrate Ester Plasticized Polyether (NEPE) propellant under eight different stress levels are carried out. The creep experimental data is discussed with different constitutive models, and it is found that the four parameters fluid constitutive model and the composite time-hardening constitutive model can be used to fit the experiment data better. The composite time-hardening constitutive model is more suitable to predict the long-term creep behavior based on the short-term creep behavior. The accuracy of the constitutive model is proved by comparing the predicted results with the experimental results.

Multiscale viscoelastic constitutive modeling of solid propellants subjected to large deformation

This paper develops a multiscale nonlinear viscoelastic constitutive model for the Nitrate Ester Plasticized Polyether (NEPE) propellant with high elongation. The proposed model considers viscoelasticity, strain rate, confining pressure, and softening effects. The macroscopic damage due to interface debonding or failure in the matrix itself is investigated by capturing the microstructural changes in a representative volume element (RVE). To account for the evolution of interface debonding, a physics-based normalized debonding ratio is introduced and modeled using a cohesive zone model (CZM). As the damage functions are obtained from the RVE, decoupled calibration of the material parameters is possible. The model is then programmed into ABAQUS via a subroutine for numerical analysis. The effects of strain rate, multi-step relaxation, creep, and confining pressure on the stress response are presented. Overall, the predicted results are in good agreement with the experiments.

Viscoelastic debonding criterion-based interface for modeling the mechanical behavior of solid propellants subjected to large deformation

Introducing new energetic particles into the solid propellants for improving their power performance may result in unsatisfactory mechanical responses. The current contribution aims to understand better the link between macroscopic mechanical behaviours and microstructural changes of a novel high elongation (>100%) Nitrate Ester Plasticized Polyether (NEPE) propellant using a computational representative volume element (RVE) containing a viscoelastic debonding criterion-based interface. The stress-strain responses and microstructural damage evolutions predicted at different strain rates agree reasonably well with the experiments. Results indicate that strain rate influences the local damage, and the particle size is not more critical than the interfacial properties for the appearance of interface debonding in the propellant with various types of fillers. Damage mechanisms and mechanical response under tensile and compressive loadings are discussed together with the impacts of damage in the matrix itself. While the damage in tension is characterized by amounts of interface debonding, fibrillar matrix, cavities and catastrophic cracks orthogonal to the loading direction, the microstructural damage in compression features in-layer tearing fracture, which is almost inclined at 45° to the loading direction, with relatively few damaged interfaces. Besides, the stress-strain response is slightly affected by matrix damage in the case of tensile loading but significantly influenced under compression.

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.

Experimental and numerical investigation on dynamic deformation and ignition response of NEPE-based propellants

An experimental and numerical study has been conducted to investigate the effects of strain rates on the mechanical responses and ignition mechanism of NEPE-based propellants (Nitrate Ester Plasticized Polyether). The modified split Hopkinson pressure bar (SHPB) equipped with a high-speed camera was used to capture the mechanical-chemical feature images during the whole impact process. The macro-mesoscopic thermomechanical model was employed to describe the viscous shear flow hotspot formation at the mesoscale. The results showed that the mechanical and ignition responses of the NEPE-based propellants are heavily dependent on strain rates. It is observed that the impact processes can be summarized into four stages: homogeneous deformation, shear flowing, initiation of local shear region and ultimate fracture (or ignition). Selected image sequence and SEM images indicate that viscous shear flow is important to the ignition of propellants. The calculated axial stress-strain curves indicate that propellant exhibits a remarkable rate-dependence and viscous effect. The predicted features of hot spot-ignition evolution are consistent with the experimental phenomenon and prove the critical strain rate of ignition is 4500 s−1.

Study on Tensile Mechanical Properties of GAP/CL‐20/HMX Propellant

AbstractTo study the tensile properties of GAP/CL‐20/HMX (GCH) propellant, a new generation high energy solid propellant, uniaxial tensile test and dynamic mechanical analysis (DMA) test were performed at different temperatures (233.15–333.15 K) and strain rates (0.000476–0.9524 s−1). The experimental results show that, unlike Hydroxyl‐terminated Polybutadiene (HTPB) and Nitrate Ester Plasticized Polyether (NEPE) propellants, GCH propellant have special arc section in the initial tensile stage at room temperature and high temperature. This phenomenon may be caused by the stretching of the curled molecular chains. The variation of mechanical properties shows that the maximum tensile strength and initial elastic modulus increase with the decrease of temperature and the increase of strain rate. The variation of maximum elongation is complex, which increases with the increase of strain rate at room temperature and high temperature, and first increases then decreases with the increase of strain at low temperature. The constitutive model of the GCH propellant was established by introducing the chain extension and the debonding damage thresholds. Both of which show a linear double logarithmic increasing trend with the increase of strain rate at room and high temperature. The latter shows a linear double logarithmic decrease trend with the increase of strain rate at the low temperature.

Narrow-band infrared radiation characteristics of rocket exhaust plume by using correction function related to thermodynamic state

Modeling of plume infrared radiation requires the accurate calculation of gas radiation properties of the plume non-uniform flow field. Aiming at the problem that the Curtis-Godson approximation (CGA) method can give the exact result in the limits of weak lines and strong lines, but overestimates the absorption of the optical intermediates path. This study develops an accurate model to study the infrared radiation of rocket plume. This model calculates narrow-band radiation properties using an equivalent half-width corrected by a correction function, which relates to the thermodynamic state of the no-uniform plume and is followed by radiative transfer calculations. Several Lorentz single lines were adopted to evaluate this correction function by the equivalent width of a fictitious non-uniform gas column, results showed high accuracy, especially at optical intermediates. Simulation of plume reaction flow was implemented by a refined 11-species, 17-reaction chemistry scheme, which can describe the afterburning between the multi-component fuel injected from the nozzle and ambient air. Then, adopting the temperature, pressure and radiating species distribution of the plume flow field, the model can predict accurate spectra that fit the experimental spectrometer data well. Finally, the effects of propellant formulation and flight conditions on plume infrared radiation were investigated: (a) hydroxyl-terminated polybutadiene (HTPB) and nitrate ester plasticized polyether (NEPE); (b) flight altitude; (c) flight Mach number. Results show that the composition of the propellant has a significant effect on the plume afterburning, NEPE plume afterburning effect is stronger, and the total radiation intensity is 61.5% higher than HTPB in 1.5–6.0 μm. The increase of flight altitude weakens the radiation, while due to the expansion of the radiant area, the spectral radiation intensity increases significantly. With the increase of Mach number, the band intensity decreases exponentially, and the plume structure gradually develops into a slender shape, the radiation decreases significantly. The research has significance for the application of target early warning, detection and identification.

Mechanical behavior and constitutive model of NEPE solid propellant in finite deformation

Nitrate ester plasticized polyether (NEPE) solid propellant was widely used in carrier rocket and guided missiles, owing to its excellent mechanical properties, high specific impulse and good performance of low temperature storage. The material of NEPE suffered from thermal cycle, high pressure impulse and vibration during service life. Thus, the mechanical properties of NEPE had important implications for the safety of structure and service life. Experimental results indicated that the material presents significant time-dependence and nonlinear characteristics. In order to clarify the damage mechanism of NEPE solid propellant, the mesostructure evolution was investigated with the 3D in-situ observations by using synchrotron radiation computed tomography (SR-CT). The voids initiated at the interface between matrix and particles and propagated along the loading direction. With the increasing of loading, matrix breaking, particles breaking and debonded particles were observed in the near-surface regions. Furthermore, a visco-hyperelastic constitutive model coupling damage was proposed to describe the mechanical behavior of NEPE under finite deformation. The constitutive model provides a good simulation with experimental results, which is meaningful to the structure integrity assessment of NEPE solid propellant.