Hydroxyl-terminated Polybutadiene Propellant Research Articles

Molecular dynamics simulation of bonding role of cyclic borate ester in improving mechanical properties of the HTPB propellant

Excellent mechanical properties are the fundamental requirement for the practical application of solid propellants. However, solid oxidant fillers such as hexahydro-1,3,5-trinitros triazine (RDX) seriously weaken the mechanical strength of the propellant due to poor adhesion with the binder matrix. In this work, novel cyclic borate ester (CBE) bonding agents with long-chain alkyl moiety were designed by the molecular dynamics (MD) method to improve the unfavorable interfacial bonding effect. The interface models, which consist of RDX substrate and binder matrix where hydroxyl-terminated polybutadiene (HTPB) crosslinked with curing agents, were constructed to reveal the interface enhancement mechanism by CBE bonding agents. The results indicate that the designed CBE bonding agents with appropriate solubility parameters (SPs) can migrate from the binder matrix to the RDX surface. Besides, CBE bonding agents exhibit strong interaction with RDX compared to the binder matrix due to the formation of hydrogen bonds. Finally, the simulated tensile tests with two tensile modes showed that CBE bonding agents can enhance the interfacial strength and reduce stress softening rates by the calculation of debonding work. Moreover, this improvement can be controlled by adjusting the chain length of CBE bonding agents. The results obtained in this work deepen the understanding of the interfacial strengthening mechanism of bonding agents for solid propellants and provide a framework for designing high-performance bonding agents based on the MD method.

High-nitrogen azotetrazole based pyrogen gas generating propellants: Aspects on synthesis, characterization, combustion characteristics and kinetics

Propellant formulations containing a novel high nitrogen azotetrazole salt viz., diammonium 5,5′-azotetrazolate (DAAzT) along with two different binders viz., hydroxyl terminated polybutadiene (HTPB) and glycidyl azide polymer (GAP) were realized as potential pyrogen-based gas generators for use in inflatable structures. The DAAzT-GAP and DAAzT-HTPB propellant systems were investigated for their gas generating capabilities and it was found that the GAP based system offers enhanced gas yield of ∼430 mL/g against ∼320 mL/g for the HTPB based propellant at standard operating pressure of 1 bar and temperature of 298 K. Combustion product analyses of the propellant were undertaken by pyro GC–MS and it was found that the combustion products are benign and constitutes 62% and 53% N2 (mass fraction) as major combustion products from GAP and HTPB propellant respectively. A detailed thermal analysis of DAAzT by isoconversional method of Vzayovkin was undertaken and the dependence of activation energy with conversion was evaluated. The activation energy was also determined using a classical Kissinger method. The impact and friction sensitivity of the promising GAP-DAAzT propellant was found to be 3 Nm and 360 N respectively and the auto ignition temperature is 177.0 °C. Functional evaluation and gas generating capability of DAAzT-GAP propellant were successfully demonstrated in a sub-scale inflation test in a stand-alone configuration.

Uniaxial tensile mechanical behavior research of HTPB propellant under wide temperature and confining pressure

To investigate the effects of confining pressure, strain rates, and temperatures on the mechanical properties of the Hydroxyl-terminated Polybutadiene (HTPB) propellant, uniaxial tensile tests were performed utilizing wide-temperature-confining pressure systems. The resulting damage was subsequently analyzed and characterized through scanning electron microscopy and mirco-ct. The results indicate that the stress–strain curves of HTPB at 20 and 70 °C are comparable, and the propellant damage is primarily attributed to de-wetting at 20 and 70 °C with respect to the mechanism. At −50 °C, the maximum tensile strength and ultimate tensile strain at 8 MPa surpass those at 0 and 2 MPa to a significant degree and the damage shifts from de-wetting and ductile fracture of particles to severe particle breakage with the elevated confining pressure. Ultimately, the primary curve of the HTPB propellant's maximum tensile strength was constructed by the curve fitting analysis based on the time–temperature equivalent superposition principle (TTSP) and time–pressure equivalent superposition principle (TPSP). Comparing to the properties of TPSP, TTSP exhibits a wider range of applicability and greater fitting precision in relation to the HTPB propellant. This study mainly serves to establish a fundamental theory and furnish data support for the enhancement of mechanical properties and structural integrity of solid rocket motors.

HTPB propellant binder supplemented with nitro potato starch: Formulation, characterization, and thermal decomposition behavior

This study conducted a detailed examination of the characteristics of hydroxyl‑terminated polybutadiene (HTPB) doped with potato starch nitrate (NPS) as a high-energy-density binder. The investigation specifically focused on evaluating the impact of NPS doping on the chemical structure, energetic attributes, and thermokinetic behavior of HTPB-NC binder systems. The findings demonstrated advantageous features, including favorable chemical compatibility, improved material density accompanied by remarkable thermal stability, substantial energy release, and high chemical reactivity. These findings contribute to the understanding of HTPB-NPS composite and offer evidence for its remarkable features, indicating promising prospects for its utilization in the development of advanced formulations for high-energy-density propellants in future applications.

Experimental measurement and simulation of the biaxial mechanical properties of solid propellant HTPB

AbstractHydroxyl‐terminated polybutadiene (HTPB) solid propellant was widely used in carrier rocket and guided missiles, owing to its excellent mechanical properties. The material of HTPB subjected to complex load such as in the process of manufacturing, storage and transportation. Therefore, the mechanical properties of HTPB had important implications for the safety of structure and service life. The purpose of this work is to study the effects of different loading ratios and loading speeds on the biaxial tension of hydroxyl‐terminated polybutadiene (HTPB) solid propellant with a proper kind of biaxial tensile specimen and a high volume fraction three dimensional particle filling model of the material. The proper kind of biaxial tensile specimen with which the stresses in its central part could be obtained with the loads acted on each loading direction was designed and used in the study, and the strains in its central parts were obtained with digital image correlation (DIC) method. The stress‐strain relationship at each direction could be obtained by experiments. Experimental results indicated that the loading speed remarkably influenced the biaxial tensile behaviors of HTPB propellant at room temperature and at different loading ratios. The finite element simulation results demonstrated the accuracy of the biaxial experiment. Based on the experimental results, the particle debonding in biaxial experiments were analyzed and an equivalent Poisson's ratio parameter was proposed to evaluate the degree of debonding. The highly volume fraction three dimensional particle filling model provides a good simulation with experimental results and the equivalent Poisson's ratio parameter can conveniently and accurately determine the debonding of particles which is meaningful to the structure integrity assessment of HTPB solid propellant.

Biaxial Tensile Mechanical Properties of HTPB Solid Propellant

The purpose of this work is to study the effects of different loading rate ratios and loading speeds on the biaxial tension of hydroxyl-terminated polybutadiene (HTPB) solid propellant. A proper kind of biaxial tensile specimen with which the stresses in its central part can be obtained with the loads acted on each loading direction is designed and used in the study, and the strains in its central parts are obtained with the digital image correlation (DIC) method. The stress and strain relationship at each direction can be obtained by experiments. The uniaxial stress vs. strain curves and the biaxial stress vs. strain curves were obtained, and it was found that the loading speed remarkably influenced the biaxial tensile behaviors of HTPB propellant. The Mises equivalent stress and strain could be used to describe the biaxial tension stress and strain state, and the exponential constitutive model obtained in the study could be used to predict the stress vs. strain curve under different test conditions.

Rheological, Ballistic, and Mechanical Properties of 3D Printed, Photocured Composite Propellants

This study explores the rheological, mechanical, and ballistic properties of printed ammonium perchlorate composite propellant at 82.5% solids loading with binders curable with ultraviolet light of wavelength from 215 to 400 nm (UV). A polybutadiene urethane acrylate and two polyester urethane acrylate propellants are printed by an in-house-fabricated fused deposition molding printer. Propellants are all shear-thinning and have significantly lower viscosity than similar hydroxyl-terminated polybutadiene (HTPB) propellants. Uniaxial stress–strain measurements indicate that ultimate tensile strength and ultimate tensile strain of all photocurable propellants are found to be greater than HTPB propellant. In particular, the ultimate tensile strain of polyester urethane acrylate propellant is six times that of HTPB propellant, demonstrating high compliance. Ballistic properties are measured from combustion of printed propellant articles in a windowed Crawford combustion bomb at inert gas pressures of up to 12.1 MPa. The burning characteristics were found to be relatively planar, though strong burning rate anisotropy, expected as a result of print layer inhomogeneities, was observed in two of the three formulations. Overall, pressure exponents of the propellants were mild and ranged from 0.17 to 0.33. These results are compared and contrasted to those of other printed propellants. These results provide valuable insight into the selection of a safe binder system for printing of photocurable composite propellants.

Investigation of hydroxyl-terminated polybutadiene propellant breaking characteristics and mechanism impacted by submerged cavitation water jet

A submerged cavitation water jet (SCWJ) is an effective method to recycle solid propellant from obsolete solid engines by the breaking method. Solid propellant’s breaking modes and mechanical process under SCWJ impact are unclear. This study aims to understand those impact breaking mechanisms. The hydroxyl-terminated polybutadiene (HTPB) propellant was chosen as the research material, and a self-designed test system was used to conduct impact tests at four different working pressures. The high-speed camera characterized crack propagation, and the DIC method calculated strain change during the impact process. Besides, micro and macro fracture morphologies were characterized by scanning electron microscope (SEM) and computed tomography (CT) scanning. The results reveal that the compressive strain concentration region locates right below the nozzle, and the shear strain region distributes symmetrically with the jet axis, which increases to 4% at first 16th ms, the compressive strain rises to 2% and 6% in the axial and transverse direction, respectively. The two tensile cracks formed first at the compression strain concentrate region, and there generate many shear cracks around the tensile cracks, and those shear cracks that develop and aggregate cause the cracks to become wider and cut through the tensile cracks, forming the tensile-shear cracks and the impact parts eventually fail. The HTPB propellant forms a breaking hole shaped conical after impact 10 s. The mass loss increases by 17 times at maximum, with the working pressure increasing by three times. Meanwhile, the damage value of the breaking hole remaining on the surface increases by 7.8 times while 2.9 times in the depth of the breaking hole. The breaking efficiency is closely affected by working pressures. The failure modes of HTPB impacted by SCWJ are classified as tensile crack-dominated and tensile-shear crack-dominated damage mechanisms.

Effect of Mechanical Heterogeneity on the Structural Integrity of HTPB Propellant Grain.

To investigate the structural effects of the mechanical heterogeneity of Hydroxyl-terminated polybutadiene (HTPB) propellant grain under ignition pressurization, a gradient finite element method was proposed to evaluate its structural integrity. The heterogeneous mechanical properties of the propellant grain were constructed and assessed. The results demonstrate that the mechanical properties of the propellant grain are spatially variable when taking into account the effect of the load. The range of variation in the mechanical properties is related to the size of the load and its effect on the mechanical properties of the propellant. Two key parameters that affect the mechanical response of the grain are the non-uniform distribution of the modulus and the damage strain threshold. An increase in the propellant modulus leads to an increase in the stress response and a decrease in the strain response of the propellant grain under ignition pressurization. Meanwhile, an increase in the damage strain threshold improves the propellant's modulus in the linear elastic stage in a disguised form. This also leads to an increase in the stress response and a decrease in the strain response when the strain response exceeds the damage strain threshold. The safety factor, based on the equivalent strain failure criterion of the grain, directly depends on both the strain response of the propellant grain and the maximum elongation of the propellant. Furthermore, the change in the safety factor of two propellant grains is primarily affected by the maximum elongation of the propellant.

In Situ Monitoring of Curing Reaction in Solid Composite Propellant with Fiber-Optic Sensors.

Curing activity in the preparation of solid composite propellants determines the performance of solid rocket motors in operation. Limited by the lack of effective monitoring tools, the complete curing behavior and thermal-induced curing kinetics are rarely disclosed. It is still a challenge to monitor in situ and in real-time the physical and chemical cross-linking reaction during the curing of propellant. Herein, we demonstrate a promising approach based on optical fiber capable of being implanted inside the propellant to monitor the internal stress evolution during the curing process, by taking hydroxyl-terminated polybutadiene propellant as an example. Attributed to the strain and temperature sensitivity of a pair of optical fiber gratings, the thermal-assisted physico-chemical cross-linking states of curing process have been demonstrated in detail. By tracking the stress-induced wavelength shifts of fiber gratings and calculating the curing mechanism function, the complete curing roadmap, including the viscous flow stage, gel stage, hardening stage can be clearly revealed, and the curing completion times are obtained as 154, 81, and 40 h, at the curing temperatures of 60, 70, and 80 °C, respectively. The apparent activation energy of this curing system obtained by calculation is 73.88 kJ/mol. This flexible fiber-based sensor provides an effective tool for unraveling the cure kinetic mechanism, and paves a universal pathway to guide the preparation and applications of versatile composite materials for solid rocket motors.

Mechanical response of HTPB propellant grain under ultra-high overload

In this study, the mechanical response of HTPB propellant grain under ultra-high loads is investigated. The low, intermediate and high strain rate of uniaxial compression tastings (1.7×10−4~3×103s−1) of the hydroxyl terminated polybutadiene (HTPB) propellant at room temperature were carried out by using the universal testing machine and the split Hopkinson pressure bar (SHPB) system, respectively. The uniaxial compression tests of the cellular rubber, the EPDM rubber and felted wool were carried out. A linear viscoelastic constitutive model of the HTPB propellant was developed by using the test data. The mechanical property of the cellular rubber, the EPDM rubber and the felted wool were studied by the test data. The dynamic mechanical response of the propellant grain under high overloads was simulated with the developed constitutive model by using the finite element method. The deformation of the grain under high overload was studied. The result shows when the overload value is maximum, the stress value of the grain is maximum. The location where the stress value of the grain is maximum is the center of the inner hole of the cushion. The maximum stress value is 20.17MPa.

Low strain rate yield characteristics and failure modes of solid composite propellant

Solid motor propellant grain is in low strain rate creep state under long-term storage condition. To analyze the yield characteristics and failure modes of HTPB (hydroxyl-terminated polybutadiene) propellant under different strain rate conditions, the constant speed tensile test of 2.38 × 10−5 s−1～2.38 × 10−2 s−1, 12-week long-term relaxation test and 4-week long-term creep test were carried out. The dynamic yield function of elastic-viscoplastic model was proposed, and the function parameters were fitted and verified by experimental data. The results show that the mechanical properties of the propellant show obvious rate dependence under lower strain rate, and the breaking strength and maximum elongation decrease significantly with the decrease of strain rate. The propellant starts to yield after strain reaching 15.0%, and the meso-structure enters an unstable state. The matrix and particle interface dewetting gradually becomes serious, developing into large particle shedding and matrix tearing, and finally causing macroscopic failure. The dynamic yield function can well describe the relationship between the dynamic yield strength, viscoplastic strain rate and static yield strength. The static yield strength of the propellant was analyzed and obtained as 0.06 MPa. The obtained yield strain and static yield strength can support the establishment of structural integrity failure criteria for long-term storage solid rocket motor.

Mechanical properties and influence mechanism of hydroxyl‐terminated polybutadiene propellant under confining pressure

AbstractTo better understand the mechanical properties and influence mechanism of hydroxyl‐terminated polybutadiene (HTPB) propellant under various confining pressures, the uniaxial tension tests with low to medium strain rate and various confining pressures were carried out. The coupling effects of strain rate and confining pressure on mechanical properties were presented. Meanwhile, the confining pressure influence mechanism was analyzed and revealed using the meso numerical simulation method and scanning electron microscopy images of fracture surfaces. The meso damage evolution processes under various confining pressures were presented. The results show that strain rate and confining pressure affect mechanical responses of HTPB propellant significantly. As confining pressure increases, the maximum tension strength and fracture strength increase. The maximum tension strain and failure strain vary slightly with confining pressure under low strain rate, while they increase significantly with confining pressure at medium strain rate. The fracture energy density increases with the increase of strain rate and confining pressure. Meanwhile, the value of saturation pressure depends on strain rate. The confining pressure influence mechanism is to suppress dewetting damage initiation and evolution. Besides, the failure mode of HTPB propellant under different confining pressures is still particles dewetting.

Crack propagation velocity and fracture toughness of hydroxyl-terminated polybutadiene propellants with consideration of a thermo-viscoelastic constitutive model: Experimental and numerical study

Solid rocket motors are subject to numerous temperature loads during their service life, so it is essential to investigate the sensitivity of propellant mechanical properties to temperature, particularly fracture properties. This paper presents fracture characteristics of hydroxyl-terminated polybutadiene (HTPB) propellant with mechanical-thermal coupled responses. This work investigated the fracture morphology of the HTPB propellant at different temperatures (233.15 K–333.15 K) using a self-developed thermostatic in-situ video imaging. The crack propagation velocity and J-integral were measured using single-edge notched tension (SENT) specimens. In the numerical study, a thermo-viscoelastic constitutive model combined with 1/r singular elements was used to perform calculations on J-integral. The thermo-viscoelastic constitutive model combined with a cohesive zone model was used to simulate crack propagation; meanwhile, load–displacement curves and crack propagation velocity were obtained. All simulation results were analyzed in comparison to the experimental results. The results revealed that temperature could significantly affect the fracture properties of the HTPB propellant. The fracture morphology of the HTPB propellant changed significantly at different temperatures. As the temperature decreases, the strength of the material gradually increases, with a consequent increase in the critical J-integral and a decrease in the average crack propagation velocity. The time ratio governed the regularity of average instantaneous crack propagation velocity with temperature. Furthermore, three simulation methods for calculating the J-integral were analyzed and discussed.

Research on the vacuum aging performance of HTPB propellant based on the DSC method

The space environment has extreme conditions such as high vacuum and rapid temperature changes, which will lead to problems such as energy drop, irregular burning rate, and abnormal ignition of solid propellants. To obtain the space aging law of HTPB propellant, the aging performance of HTPB (hydroxyl-terminated polybutadiene) propellant in a vacuum environment was studied and compared with that under the atmosphere. According to the DSC test results of the propellant before and after aging, the behaviors of oxidative crosslinking, chain-scission degradation, and oxidant decomposition during the aging process of solid propellant were investigated. The research showed that the low-temperature endothermic peak and “V-type” high-temperature exothermic peak temperature of HTPB propellant increased after aging, and the high-temperature peak bifurcated into “W-type” double peaks. The right peak caused by the “decarboxylation” reaction was more obvious after atmospheric aging while the left peak caused by AP decomposition was remarkable after vacuum aging. In addition, the oxidative crosslinking reaction occurred after aging, and the activation energy characterized by low-temperature peaks continued to increase. In the later stage of atmospheric aging, chain-scission degradation occurred due to OA moisture absorption, and the activation energy decreased. It firstly increased by 126% within 93 days, and then decreased by 23.4%. No degradation and chain-scission reaction occurred in a vacuum environment, the crosslinking reaction rate was high, and the activation energy increased by 40.4% within 49 days. After the aging of the HTPB propellant, the internal AP decomposed, and the activation energy characterized by the high-temperature exothermic right peak continued to increase. The activation energy increased by 40.5% within 49 days after vacuum aging, and increased by 29.9% within 93 days after atmospheric aging, indicating that the decomposition of AP in the HTPB propellant was more obvious under a vacuum environment.

A Rate‐Dependent Constitutive Model of HTPB Propellant Based on Parallel Rheological Framework

AbstractNumerous studies have shown that highly strain‐rate dependent behavior is present in the mechanical properties of Hydroxyl‐terminated polybutadiene(HTPB)propellants, and many experts and scholars have attempted to develop constitutive models to describe such rate‐dependent properties of propellants. From the linear viscoelastic model to the Zhu‐Wang‐Tang model, the strain‐rate dependent properties of viscoelastic materials have been described to some extent. But due to the highly nonlinear mechanical behavior exhibited in propellant materials, the nonlinear behavior of propellants at different strain rates is difficult to be captured in the above constitutive models which are all based on linear elasticity. Therefore, in this paper, a nonlinear viscoelastic constitutive model was proposed based on the parallel rheological framework, in which the hyperelastic and viscoelastic flow laws were embedded. Because of the excellent ability to capture the nonlinear behavior of the material, the parallel rheological framework was used to predict the mechanical response at six sets of experiments with different rates. Surprisingly, this constitutive structure model performed good prediction of the mechanical properties of the material by comparison between theoretical calculations and experimental data. The constitutive model was verified by finite element method with numerical simulation software, and the robustness of the constitutive model was checked by a new set of experimental data. A good foundation was established for the accurate description of the propellant constitutive model strain‐rate dependent characteristics by the results of this paper, which could provide a new method for the rapid prediction and evaluation of the propellant mechanical response.

Establishment of a NIR-based methodology for tracking the blend homogeneity of HTPB propellant slurry in the mixing process

The hydroxyl-terminated polybutadiene (HTPB) propellants with high level of solid loadings from 80 wt% to 90 wt% consist of aluminum (Al) powder, ammonium perchlorate (AP) and HTPB. The Al/AP/HTPB adhesive system full of solid grains appears high viscosity against flow. Therefore, the mixing is a crucial procedure in the production as it directly affects the structural integrity of the finished product. This work focused on the feasibility of tracking the blend homogeneity of Al/AP/HTPB adhesive system in the mixing process through using the near-infrared (NIR) spectroscopy and orthogonal partial least squares discrimination analysis (OPLS-DA). The OPLS-DA classification models were created by variable selection, spectral pretreatment and latent variables (LVs) optimization. It had been demonstrated that the developed models presented an excellent predictability with the root mean square error of cross-validation (RMSECV) for slurries in Ⅰ, Ⅱ groups of 0.1261 and 0.0789, respectively. Meanwhile, the well-fitted models for slurries in Ⅰ, Ⅱ groups with the squared correlation coefficient (R2) of 0.806 and 0.980, exhibited separately an acceptable predictive capability with the predictive squared correlation coefficient (Q2) > 0.5. Furthermore, Euclidean distance and move block standard deviation (MBSD) as reference methods were used to validate the predictive performance of the developed models with respect to the blend homogeneity of HTPB propellant slurry. The experimental results showed that the terminal time for each batch of slurry reaching to ideal uniformity predicted by Euclidean distance/MBSD and OPLS-DA were both at 26–30 min. Therefore, it had been proved that the method we proposed was a potential tool to monitor the variation of the uniform state of HTPB propellant slurry in the mixing process.

Investigations on the thermal response of a solid rocket motor with complex charge structure using CL-20/GAP propellant

The thermal safety of a solid rocket motor with a complex charge structure using CL-20 (Hexanitrohexaazaisowurtzitane)/GAP (Glycidyl Azide Polymer) propellant was evaluated. The morphology and thermal decomposition properties of the propellant were first studied by differential thermal analysis, thermogravimetric analysis and scanning electron microscopy experiments. A numerical model of cook-off for a solid rocket motor using GAP/CL-20 propellant was developed. This model takes into account the entire process of cooking off, i.e., the thermal decomposition before ignition and the combustion reaction after ignition. The model is verified, and the kinetic parameters are determined by bomb cook-off experiments. Comparisons between experiments and simulations show that the developed model can accurately predict the thermal response of the propellant. The cook-off processes of a 3D-solid rocket motor with complex charge structures are then simulated with different heating rates. It shows that the charge structure and the annular area of the motor casing play an import role in the ignition location of the propellant. Meanwhile, the heating rate has a dramatic effect on the ignition time. The reaction intensity after the ignition of the solid rocket motor using the GAP/CL-20 propellant is more intense than those using HTPE (hydroxyl-terminated polyether)/HTPB (Hydroxyl-terminated polybutadiene) propellant.

A Nonlinear Viscoelastic Constitutive Model for Solid Propellant with Rate-Dependent Cumulative Damage.

Solid propellant is a composite material exhibiting classic nonlinear viscoelastic mechanical characteristic, which is due in a large part to a cumulative damage process caused by the formation and growth of microflaws inside. The standard relaxation tests and uniaxial tension tests under different velocities of hydroxyl-terminated polybutadiene (HTPB) propellant are carried out in this paper, where Digital Image Correlation (DIC) technique is applied to record deformation. The experimental results show that the material mechanical behavior is rate-dependent. It is also observed that the yield stress and failure stress are significantly rate-dependent on the tensile velocity. Based on these experimental results, it can be inferred that the stiffness degradation and damage evolution of HTPB propellant are a rate-dependent processes. Therefore, the damage accumulation of HTPB propellant is considered rate-dependent in this research. In order to describe the mechanical characteristic precisely, a nonlinear viscoelastic constitutive model with rate-dependent cumulative damage is developed. The damage model is developed based on the concept of pseudo strain, in which a Prony series representation of viscoelastic material functions is applied. Besides, a rate-dependent damage variable is introduced into the model through considering the rate-dependent characteristics of cumulative damage process. In addition, a new normalized failure criterion is derived on the basis of the proposed damage model, which is independent of strain-rate after normalization. Finally, it is implemented in commercial finite element software for stress analysis to verify the predictive capacities of the damage model. The accuracy of the constitutive model and failure criterion is validated under uniaxial tensile tests of various strain rates.

Experimental Investigation on Fracture Properties of HTPB Propellant with Circumferentially Notched Cylinder Sample

AbstractTo comprehensively understand the fracture properties of solid propellants, the mode I tensile fracture toughness tests at different temperatures (253.15–343.15 K) and loading rates (2–500 mm/min) were conducted on the newly designed circumferentially notched cylinder sample with different initial crack sizes (4.5 and 6.5 mm) for hydroxyl‐terminated polybutadiene (HTPB) propellant. Test results reveal that the shape of the tensile fracture stress‐strain curves was not significantly influenced by temperature, loading rate and the initial crack size. Higher loading rate and lower temperature can lead to a rise in the tensile fracture toughness of HTPB propellant described by the stress intensity factor, however, continuously increasing loading rate cannot dramatically improve this fracture toughness beyond the rate of 250 mm/min. In addition, the fracture toughness is more sensitive to temperature. Furthermore, the variation of the initial crack size causes obvious changes in the fracture toughness with the coupled effects of low temperature and higher loading rates. At these test conditions, a higher tensile fracture toughness can be obtained with the shorter initial crack. For all the test conditions, there is a linear rise in the strain corresponding to the fracture toughness as temperature increases. Meanwhile, this strain increases with the shorter initial crack. Whereas, the effect of loading rate on this strain is complex. Based on the time‐temperature superposition principle (TTSP), the master curves with a log‐curvilinear form were constructed to predict the mode I tensile fracture toughness of the propellant at different initial crack sizes in a wide range of loading conditions.