A Strain-Rate-Dependent Hyperelastic Constitutive Model for Tensile Mechanical Behaviour of HTPB Propellant

To investigate the effects of strain rate on the tensile mechanical behaviour of hydroxyl-terminated polybutadiene (HTPB) propellant, the uniaxial tensile stress-strain curves at room temperature over strain rates ranging from 1.2×10^-4 to 80 s^-1 have been determined on a universal testing machine and a hydraulic testing machine. Experimental results indicate that the stress at a given strain level bilinearly increases with the logarithm of strain rate with a sharp increase in gradient at a strain rate of 1 s^-1, the strain-rate sensitivity linearly enhances with decreasing the strain level. Based on the previous model developed by Mohotti, a modified rate-dependent hyperelastic constitutive model was proposed by combining the mechanical characteristics of both the bilinear strain-rate dependence and the difference in strain-rate sensitivity at different levels of strain in this HTPB propellant. The model comprises a hyperelastic component and a rate-dependent component. The hyperelastic component is considered as a base model to characterise the mechanical behaviour at the selected reference strain rate. The rate-dependent component is multiplied into the hyperelastic component to define strain-rate sensitivity. Comparison of model curves with experimental data shows that it is able to reasonably characterise the uniaxial tensile mechanical properties of HTPB propellant under strain rates ranging from 1.2×10^-4 to 80 s^-1 within the strain of 30%.