Different Methods for Developing Relaxation Modulus Master Curves of AP-HTPB Solid Propellant

A comparative assessment of three different methods used for estimating and generating the relaxation modulus master curves of ammonium perchlorate-hydroxyl-terminated polybutadiene (AP-HTPB) solid propellant was carried out. These methods are the Williams-Landel-Ferry (WLF) method, the Arrhenius method, and the basic time-temperature superposition (TTS) method. The experimental data of the relaxation modulus for composite solid propellant were gathered by performing stress relaxation tests at constant strain level 10% for 1380 s at several temperature(-40, +20, +76℃). The procedures and steps for evaluating the three methods was based on the same experimental data obtained for relaxation modulus to estimate the shift factors and then to generate the master curves of relaxation modulus and finally comparing the best fit method through the determination coefficient (R²). The corresponding results indicate that the basic TTS method generated the best fit curve relative to the experimental data because this method is independent of external material constants and empirical equations in its application and it can be generally applicable to any viscoelastic material. However, in the applications of finite element software, the material constants are usually required to define the non-linear viscoelastic material model, in this case, the result of the present work demonstrates that both WLF and Arrhenius methods can produce satisfactory results, when appropriate constants are used, and the WLF method has proven to be more accurate and would be preferred in finite element analysis of AP-HTPB solid propellant.