Solid propellant is the main part of solid rocket motor (SRM). To further investigate the service life of SRM, aging of solid propellant should be concerned. The aim of this study is to propose a new aging constitutive model of solid propellant using the degree of crosslinking. The aging development equation of viscoelastic Poisson’s ratio (VPR) and relaxation modulus are considered in establishing the model. A series accelerated aging experiments accompanied with some tensile tests are designed to verify the new model. Aging experiments show that degree of crosslinking (DCL) can be used to act as a link to facilitate the communication between chemical aging and aged mechanical parameters well.

A direct numerical method is introduced herein to investigate time-dependent Poisson’s ratio of solid propellant based on a representative volume element (RVE) model. Time-dependent longitudinal and transverse strains are considered in the calculation of time-dependent Poisson’s ratio under the relaxation test. The molecular dynamics (MD) packing algorithm is used to generate the high area fraction RVE model of solid propellants consisting of ammonium perchlorate (AP) particles whose radius follows lognormal distribution. In order to simulate the dewetting response of the interface between particles and matrix, the PPR model is modified and utilized during the analysis. Time-dependent Poisson’s ratio is measured under different cohesive parameters, loading conditions (loading temperature, loading rate, and fixed strain), and area fraction. Numerical results reveal that time-dependent Poisson’s ratio can be nonmonotonic or monotonic according to the different cohesive parameters. A concept of critical cohesive parameters is proposed to judge whether the monotonic property of time-dependent Poisson’s ratio appears or not. According to the numerical analysis, the cohesive contact and the shrinkage of the bulk element are two main factors which will control the change of monotonic property. All time-dependent Poisson’s ratios will increase at the beginning of the relaxation stage because the effects of cohesive contact can be ignored compared with the large shrinkage of the bulk element. However, with the increased shrinkage of the bulk element, the increased cohesive contact will defend further shrinkage at the same time. Although the shrink of the bulk element never changes its direction, the ratio of the transverse strain to longitudinal strain may decrease or keep increasing in this stage. When transverse and longitudinal strains stop to change, all time-dependent Poisson’s ratios will achieve their equilibrium values.

High-quality interface between propellant and insulation is the strict requirement difficult to quantify in solid rocket motor. In this study, the mixed mode delamination process of propellant and insulation interface is considered in double cantilever sandwich beam (DCSB) and single lap-joint (SLJ) test. The PPR cohesive zone model (CZM) and bilinear CZM in ABAQUS are introduced in this mixed fracture progress. In order to implement the PPR model in ABAQUS, user subroutine user element (UEL) is programmed for the novel CZM. Two simple pure mode I and mode II fracture problems are designed to check the accuracy of the UEL, and the result of verification is excellent. DCSB and SLJ test and their corresponding results are used again in the same inverse analysis with the two typical effective displacement-based and potential-based CZM. Base on the results, a series discussion and some conclusions are made. The debonding progress of the propellant and insulation interface in DCSB and SLJ test are mixed mode. The PPR CZM is prior in simulation than the bilinear CZM in ABAQUS because the PPR CZM is much more flexible with changeable traction-separation shape. The real normal and tangential displacement at damage initiation shows the unreasonable change in bilinear CZM in ABAQUS under mixed mode fracture. The PPR CZM and bilinear CZM in ABAQUS are all thickness-dependent model. The real initial stiffness and the critical displacement in the bilinear CZM and the real maximum traction in PPR model are dependent on the thickness of cohesive element. The different thickness dependence of the two model is caused by the implementation method.

Reliable prediction of the macromechanical properties of composite solid propellants in the microscale can accelerate the development of propellants with high mechanical properties. According to the characteristics of the composition ratio of a four-component hydroxyl-terminated polybutadiene (HTPB) propellant with the component ammonium perchlorate (AP), hydroxyl-terminated polybutadiene, aluminum powder (AL), and cyclotrimethylenetrinitramine (or RDX for short), an improved random delivery algorithm was developed to separately model filler particles with the different sizes. A step-by-step equivalent representative volume element (RVE) model was generated to reflect the microstructures of the propellant. The isotropy and uniformity of the RVE model were also tested using a two-point probability function. The Park-Paulino-Roesler (PPR) cohesive model was introduced to simulate the particle debonding (or dewetting) in solid propellant. The stress-strain curves of the propellant were obtained by the macroscopic test with the extension rate 200 mm/min at different temperatures. Based on these experimental data, the 8 characteristic parameters suitable for the microinterface of the propellant were obtained by using an inversion optimization method. A microscale finite element prediction model of the propellant considering dewetting damage was constructed to study the evolution process of the microdamage of the propellant. The predicted stress-strain curves of the propellant under different loading conditions were in good agreement with the test results.