Longitudinal Strain Dependent Variation of Poisson’s Ratio for HTPB Based Solid Rocket Propellants in Uni‐axial Tensile Testing

Abstract

AbstractPoisson’s ratio of HTPB based composite propellant is estimated at break using double dumbbell specimens as per ASTM D638 Type IV standard and its value obtained by change in the volume of specimens is calculated as approximately 0.25. This major finding contradicts the behaviour of solid rocket propellants in respect of Poisson’s ratio, which is reported to be 0.5. Further, Poisson’s ratio varies almost linearly with strain even in linear portion of stress‐strain curve in uni‐axial tensile testing as per theoretical calculations. It must be noted that no change in volume does not necessarily indicate constant Poisson’s ratio equal to 0.5. SEM scan indicates that the rate of reduction of Poisson’s ratio with longitudinal strain accelerates after dewetting due to the formation of vacuoles. Bilinear variation of Poisson’s ratio with longitudinal strain is observed. One slope is valid in pre‐dewetting region, calculated from close form solution and other slope is valid for post‐dewetting region, which is measured at break. Measurement of Poisson’s ratio at various longitudinal strains indicates uni‐linear variation and not a bilinear variation with a kink.It is also observed that Poisson’s ratio is different along different lateral directions of the propellant specimen. Poisson’s ratio in two orthogonal directions perpendicular to longitudinal axis is calculated as 0.17 and 0.30. As ASTM Specimen has rectangular cross‐section of approximate size 6×4 mm, the directional behavior of Poisson’s ratio may be attributed to initial dimensions. Prismatic propellant specimen with square cross‐section of 115×6×6 mm dimension do not show any variation in respect of Young’s modulus, tensile strength and percentage elongation as compared to ASTM specimen. Directional behavior of Poisson’s ratio with almost similar numerical value is again observed, thus ruling out dependence of this behavior on different initial dimensions of propellant cross‐section. The propellant slurry flow during vacuum casting, directional curing and orientation of specimen with respect to web of the cast propellant are mainly responsible for this directional behaviour of Poisson’s ratio for the composite propellants. Composite propellants behave as compressible material in most of the region and near failure region or at higher strains; Poisson’s ratio is not anywhere close to 0.5, instead it is close to 0.25.