Abstract:The health condition of solid rocket motors largely depends on the real-time status of the propellant. Therefore， monitoring the propellant status is an important foundation for ensuring the structural integrity and reliability of solid rocket motors. We provides a comprehensive review of research progress in the monitoring of propellant status from four aspects： environmental status monitoring， chemical status monitoring， mechanical status monitoring， and integrated application of monitoring data. The necessity of propellant status monitoring is highlighted， and the research achievements and shortcomings in propellant status monitoring are summarized from multiple perspectives. Furthermore， development ideas are proposed for monitoring technology and the application of monitoring data. The analysis suggests that real-time monitoring technology should focus on embedded sensor compatibility， new sensing technology， and long-life technology. In terms of the application of monitoring data， efforts should be made to establish databases， diagnostic and predictive health management systems， in order to promote the development of digital twin technology for the full life of solid rocket motors using finite element model updating methods.

Abstract:To establish the correlation between the microstructure and mechanical properties of high nitrate plasticized polyethylene glycol （PEG） based polyurethane in nitrate ester plasticized polyether （NEPE） propellant， biuret triisocyanate （N-100） was used as a multifunctional curing agent and mixed with nitrate ester plasticized PEG for curing to prepare PEG elastomers with curing parameters ranging from 1.2 to 1.7. The microstructure of PEG elastomer crosslinked network was studied by uniaxial tension， X-ray diffraction， low field nuclear magnetic resonance and equilibrium swelling test methods. Furthermore， the effects of different network chain structures on the mechanical properties of PEG elastomer were analyzed. The results show that the PEG elastomer is amorphous due to its high plasticized properties. Meanwhile， the total ratio of suspended tail chains to free chains is more than 85%， the structural integrity of the crosslinked network is low， and the elastomer exhibits high elongation， low tensile strength and low initial modulus. Secondly， all the tensile strength and initial modulus of elastomer are positively correlated with the crosslinked chain density. The maximum elongation increases first and then decreases with the increase of physical temporary entanglement chain density. CU-5 elastomer with a curing parameter of 1.6 has the most complete cross-linked network. At the same time， the corresponding tensile strength is 0.80 MPa， and the maximum elongation is 1456%， which indicates that the mechanical properties are the best. Finally， the chain density measured by equilibrium swelling method and low field nuclear magnetic resonance satisfy the magnitude relationship of ν_L，A<ν_s<ν_L，A+B.

Abstract:To study the formation mechanism of residual stress/strain of the nitrate ester plasticized polyether （NEPE） propellant grain during the curing and cooling stages， the temperature field， curing degree field and stress/strain field of the propellant were numerically analyzed via ABAQUS finite element software. The results show that there are temperature gradient and curing rate gradient in the NEPE propellant grain during the curing process at 50 ℃. The temperature and the curing rate are notably higher at the center of the grain， and they eventually reach a consensus at the end of curing. The temperature difference in the propellant does not affect the final residual stress/strain. The total residual stress/strain during curing and cooling obey the principle of stress/strain superposition， and they are mainly composed of the curing shrinkage stress/strain and thermal stress/strain during cooling. For the total residual stress， the proportions of the two stages are approximately 20% and 80%， respectively， and for the total residual strain， the proportions are about 30% and 70%， respectively. Compared with the traditional method， the residual stress/strain calculated in this study have the same distribution characteristics， but the values are smaller.

Abstract:In order to investigate the uniaxial compression mechanical behavior of four-component HTPB propellant under wide temperature and wide strain rate ranges， uniaxial compressive mechanical performance tests of propellant were conducted under wide temperature and strain rate ranges based on universal material testing machine， high-speed hydraulic servo testing machine， split Hopkinson pressure bar， and programmatic constant temperature and humidity testing machine. Stress-strain curves of HTPB propellant under 10^-4-10³ s^-1 at -40， -25， -10， 20 ℃ and 50 ℃ were obtained， and the segmented uniaxial compression rate-temperature constitutive relationship of HTPB propellant was established. The results indicate that the mechanical response of HTPB propellant exhibits a significant rate-temperature correlation. At any strain rate， its mechanical response undergoes staged changes， i.e.， linear elastic stage-nonlinear yield stage-strain softening or strain hardening stage. Moreover， at high strain rates， the strain softening phenomenon after the nonlinear yield behavior is significantly weaker than that at low and medium strain rates. In addition， at high strain rates， as the temperature decreases， the changing rate of the stress-strain curve gradually slows down； while at low and medium strain rates， the changing rate of the stress-strain curve gradually increases as the temperature decreases. The mechanical strength of HTPB propellant increases significantly with decreasing temperature. When the temperature drops from 50 ℃ to -40 ℃， the maximum stress under wide strain rate increases from about 2.2-8.8 MPa to 11-22 MPa. The segmented rate-temperature constitutive relationship constructed based on the experimental data has a better fitting effect at higher temperatures， which can better predict the mechanical behavior of HTPB propellant.

Abstract:To analyze the mesostructure evolution behavior of nitrate ester plasticized polyether （NEPE） propellants under uniaxial quasi-static tensile load， in-situ observation test was conducted on NEPE propellants during the tensile process using Micro CT. The size and an shape of ammonium perchlorate （AP） particles and initial defects in NEPE propellants were characterized. The failure process of the mesostructure in the propellant during uniaxial tensile process was obtained， and the quantitative analysis of the changes in meso-damage of NEPE propellants was carried out using porosity. The reason for the changes of macro-mechanical properties is explained based on the evolution laws of the structure of NEPE propellant on the mesoscale. The results indicate that the initial defects size of NEPE propellant is small and the volume ratio is low， with an average value of 0.12%. In the process of uniaxial quasi-static tension， the meso-failure process of NEPE propellant mainly includes three stages， pores nucleation， growth and convergence. Although the volume fraction of AP particles is low， it is often the start of meso-damage because of easy debonding. HMX particles will also debonding after a certain degree of debonding of AP. The influence of HMX debonding behavior should be considered when analyzing the macro-mechanical properties of NEPE propellant. The nucleation and growth of a large number of mesoscopic defects is the reason why the macro-mechanical properties properties of NEPE propellant enter the nonlinear section. The phenomenon that the increase of macroscopic stress lags behind the increase of strain is more and more obvious because of the continuous convergence of mesoscopic defects. During the loading process， the porosity shows a change trend of slowly increasing first， then sharply increasing and finally increasing tends to be gentle. The change law of porosity can not only quantitatively reflect the evolution stages of the mesoscopic defects of NEPE propellant， but also have a certain corresponding relationship with the changes of the macroscopic mechanical properties of NEPE propellant.

Abstract:This research investigates the mesoscopic damage mechanisms in hydroxyl-terminated polybutadiene （HTPB） propellants under thermal-mechanical coupling. Experimental characterization and theoretical analysis were employed to analyze these mechanisms at different environmental temperatures （50， 70， and 90 ℃） and loading cycles. At 50 ℃， the propellants underwent approximately 3000 and 10800 loading cycles； at 70 ℃， 1800， 3600， and 7030 cycles； and at 90 ℃， around 1800 cycles. The findings indicate that the mesoscopic damage in HTPB propellants， exacerbated by thermomechanical coupling， is more pronounced than that caused by a single aging factor. This damage primarily results from two processes： thermal degradation of the matrix， diminishing both its load-bearing capacity and adhesion to particles， leading to particle dewetting； and the subsequent exacerbation of the matrix"s thermal degradation by this dewetting. The above interaction makes the mesoscopic damage more severe. Moreover， the damage intensifies with increasing aging temperature， but extremely high temperatures modify the mesoscopic damage mechanism. Furthermore， the study emphasizes the importance of selecting an appropriate number of loading cycles to assess significant mesoscopic damage in HTPB propellants. Notably， substantial damage occurs when the loading cycle ratio （） at 50 ℃ and 70 ℃ exceeds 0.281 and 0.330， respectively.

Abstract:In order to study the effect of loading Angle on the failure mechanism of HTPB propellant bonding interface， microCT was used to scan and reconstruct the bonding interface in-situ during uniaxial tensile process， and the damage evolution process was characterized. Then， the meso-structural parameters and damage variables were introduced into the cohesive force model， and the meso-damage evolution process of the adhesive interface under different loading angles was obtained. The results show that the initial dehumidification of AP particles at the bonding interface mainly starts from the weak interface layer near the interface， and the direction is along the shear component direction of the interface. The fracture pattern of the interface is related to the shear Angle. The smaller the resultant force and the Angle of the interface， the easier the crack propagation to the propellant/liner interface， whereas the crack propagation is more likely to occur between AP particles. Finally， compared with the experimental results， the accuracy of the calculated results is verified， and the damage evolution law of the propellant bonding interface structure under different loading angles is revealed.

Abstract:To accurately predict the cracking of propellant grain during ignition at low temperatures， a cross-scale analysis method coupled with global-local unidirectional contracting was proposed. For Nitrate Ester Plasticized Polyether （NEPE） propellant， uniaxial tensile tests at low temperature and intermediate strain rate were carried out， and the typical failure modes of the propellant were obtained. The results show that the ignition of solid rocket motor grain at low temperature can be analyzed successfully with macroscope method based on the developed nonlinear viscoelastic constitutive model， and the location of the dangerous point for propellant grain was obtained. Meanwhile， a mesoscopic particle filling model considering the particle-matrix interfacial debonding and particle fracture was established， and the analysis results on a macro scale were applied to the corresponding mesoscopic representative volume element （RVE）. Finally， based on the established mesoscopic failure criteria of the propellant， it is shown that the structural integrity of the propellant grain meets the requirements under low-temperature ignition conditions. Furthermore， the proposed contracting cross-scale analysis method can be used as an effective method to predict the cracking behaviors of the propellant grain during ignition at low temperature.

Abstract:In order to study the damage evolution and mechanical properties of solid propellants， uniaxial tensile and stress relaxation tests were performed on NEPE propellant. The resulting stress-strain curves and relaxation master modulus curves were obtained. A nonlinear viscoelastic constitutive model considering microscopic damage was developed under finite deformation. This model enables multiscale analysis of the mechanical response of propellants by incorporating the evolution of microvoids with various factors， including temperature， strain rate， confining pressure， and cyclic stress softening. The model was then implemented into ABAQUS with the parameters determined based on experimental data. Subsequently， the model was employed to predict the mechanical response of NEPE propellant under different loading conditions. The results demonstrate that the model accurately predicts the uniaxial tensile response of propellants under wide temperature ranges （223-333 K） and loading rates （1-200 mm·min^-1）. Moreover， the model exhibits reasonable predictability in cyclic loading， confining pressure tests， and biaxial tensile tests， thereby validating its effectiveness under complex stress conditions. Notably， the model necessitates only a small set of model parameters and can be easily programmed into commercial software， providing theoretical guidance for the multiscale analysis of the structural integrity of solid rocket motors.

Abstract:As a composite material composed of fillers and matrix， the damage of hydroxylated polybutadiene （HTPB） propellant mainly involves particle breakage， matrix fracture， and debonding of the bonding interface layer. To further explore its structural damage and mechanical performance evolution under external loading， a combination of micro CT， high-speed CCD camera， and all atom molecular dynamics simulation was used to analyze the multi-scale damage of the propellant under in-situ loading. The results indicate that the typical damage process of the propellant begins with the failure of the bonding interface layer， extends to the growth of debonding pores， evolves through the merging of pores， accelerates the collection of local large deformations， and terminates at the fracture of the matrix. Meanwhile， the interface binding energy and stress concentration degree cause the large ammonium perchlorate （AP） particles to debond first， and the porosity and strain exhibit an exponential function relationship. Furthermore， the traction separation curve of the micro interface layer conforms to an exponential cohesive force model， where the initiation and expansion of micro voids disrupt their cohesion， while the molecular spacing affects the evolution of stress.

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