Developing a viscoelastic relaxation model for AP-HTPB composite solid propellant based on experimental data

The aim of this study is to develop an effective and accurate relaxation model for ammonium perchlorate hydroxyl-terminated polybutadiene (AP-HTPB) composite solid propellant considering the nonlinear viscous effects and time-temperature dependent characteristic. Uniaxial tensile tests at a constant strain rate and different temperatures, and stress relaxation tests at different strain levels and different temperatures have been carried out on standard specimens of solid propellant to measure the material properties. The material’s Prony series coefficients were determined numerically based on the experimental relaxation test data by using the least square method. Furthermore, in order to simulate the relaxation behavior, a 3D finite element model was established to identify and simulate the complexity nature of viscoelasticity and time-dependent behavior of the solid propellant specimen. The finite element analysis by using ANSYS APDL software gives satisfactory results with a maximum deviation of 2.85% from the corresponding experimental stress relaxation data at different conditions. The relaxation model and the relative material subroutine can be applied to perform a structural integrity analysis for solid rocket motors.