Studies of poisson's ratio variation for solid propellant grains under ignition pressure loading

Abstract

For structural integrity evaluation of solid propellant grains, an accurate estimation of the stress and strain response is essential, because of the viscoelastic nature of the polymer material. The results of an investigation on solid propellant grains considering the effect of Poisson's ratio υ variation under ignition pressurization are presented. Traditionally the υ value of solid propellant grains is assumed to be a constant (e.g. 0.4999) to simplify the experimental task, but the real υ value is from 0.47 to 0.4999 depending on the chemical design of the solid propellant grains. In order to simulate the time–temperature-dependent behaviour of viscoelastic polymer materials for a range of Poisson's ratio values, the concepts of a time–temperature shift principle and reduced integration were used. In addition, eight different Poisson's ratio values from 0.47 to 0.4999 were assumed and compared using the finite element method. Results show that the Poisson's ratio variation effect is very important for structural integrity of solid propellant grains, and the stress and strain responses versus Poisson's ratio variation are nonlinear because the type of polymer material is changed from incompressible to compressible. Under ignition pressure loading, the stress and strain responses in the compressible case ( υ≠0.5) are much higher than those of the incompressible case ( υ≒0.5). Therefore unlike metallic structures, an exact value of Poisson's ratio for polymer materials is very important, and an improper assumption ( υ≒0.5) may cause the structural integrity of a missile system subjected to pressure loading to be wrongly evaluated.