Reliable assessment of the long-term structural integrity of solid rocket motors requires quantitative understanding of the stress- and temperature-dependent viscoelastic response of composite propellants subjected to sustained thermo-mechanical loading. In highly filled energetic formulations, minor variations in formulation architecture and energetic filler chemistry (e.g., HMX and RDX) can induce disproportionate changes in creep–recovery behavior, thereby influencing dimensional stability, stress redistribution, and service life. In this study, the creep–recovery response of three high-energetic composite propellants (C-I, C-II, and C-III) is systematically investigated using dynamic mechanical analysis over a broad stress range (0.1–3 MPa) and temperature domain (−20°C to 55°C), representing one of the first comprehensive evaluations encompassing both HMX- and RDX-based fillers under combined thermo-mechanical conditions. Viscoelastic constitutive modeling reveals that the Burgers model provides the most robust and physically interpretable representation of the strain–time response, resolving instantaneous elastic deformation, time-dependent viscoelastic creep, and irreversible viscous strain with high fidelity. All formulations exhibit primary and secondary creep without onset of tertiary creep within the investigated regime. Among the compositions, C-III demonstrates the highest resistance to time-dependent deformation and superior load-bearing capacity, whereas C-II shows elevated compliance and accelerated strain accumulation at increased stress and temperature, with C-I exhibiting intermediate behavior. Post-DMA scanning electron microscopy corroborates these rheological trends, revealing formulation-dependent interfacial debonding, void nucleation, cavitation, and matrix plasticization, thereby establishing a direct structure–property correlation governing the sustained-load viscoelastic stability of high-energetic composite propellants.
Rao et al. (Sun,) studied this question.