Stress evolution and process optimization in pressure-cured NEPE solid rocket motor grains: A thermo–chemo–viscoelastic analysis

Manufacturing-induced residual stress can compromise the structural integrity and reliability of solid rocket motor grains. This study investigates the evolution and mitigation of residual stress in nitrate ester plasticized polyether (NEPE) solid rocket motor grains during pressurized curing. A coupled thermo–chemo–viscoelastic finite-element framework was developed to predict temperature, degree of cure, deformation, and stress throughout curing, cooling, and depressurization. The model was validated using pressure-induced motor-case deformation, storage-modulus and degree-of-cure data, and inner-bore diameter measurements from a 480-mm-diameter motor grain. A four-factor Box–Behnken design combined with response surface methodology was then employed to evaluate the effects of curing temperature, curing pressure, cooling rate, and depressurization rate. The results show that curing temperature and pressure dominate residual-stress evolution and exhibit a significant nonlinear interaction, whereas cooling and depressurization rates have secondary effects within the investigated ranges. Moderate pressurization enables the elastic recovery of the motor case to partially compensate for cure shrinkage and thermal contraction, while excessive temperature–pressure coupling limits stress relaxation and increases stress concentration. The optimized parameters were 50.0 °C, 3.33 MPa, 0.064 °C/min, and 0.007 MPa/min, reducing the maximum residual stress to 0.018 MPa, 53.6% lower than that under atmospheric curing. The proposed framework provides a practical basis for process design and structural-integrity control in the manufacture of large solid rocket motor grains.