The composite propellant is a type of particle-included composite material and serves as the power source of solid rocket motors. During service, the composite solid propellant sustains complex mechanical loads, which would cause internal damage and ultimately lead to material failure. To investigate the rate-dependent nonlinear mechanical behaviors of nitrate ester plasticized polyether (NEPE) composite propellants under tensile loading, this study focused on the mesoscopic damage mechanisms via observation of mesostructures. The key damage modes were particle-matrix interfacial debonding and expansion of dewetting pores, with porosity evolution showing significant rate dependence. Based on the mesoscopic damage mechanisms and the rate-dependent porosity prediction model, a viscoelastic damage constitutive model was established by introducing an internal state variable (characterizing both damage modes) and a softening function (reflecting modulus degradation). The model effectively predicted the propellant’s nonlinear responses under different strain rates, including the typical three-stage stress trend at high strain rates. Numerical analysis confirmed rate-dependent damage accumulation: higher strain rates delay damage initiation and fracture, with consistent failure modes across strain rates. This work links mesoscopic damage to macroscopic properties, providing a reliable model for propellant design.
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