| || Assessment of Poissons Ratio for Hydroxy-terminated Polybutadine-based Solid Rocket Propellants
Author : Shekhar, Himanshu;Sahasrabudhe, Anil D.
Source : Defence Science Journal ; Vol:60(5) ; 2010 ; pp 497-501
Subject : 623 Military Science and Engineering;Defence Science Journal
Keywords : Composite propellants;Poissons ratio;tensile test;incompressible materials;filled polymers;solid rocket-propellants
Abstract : Poissons ratio of hydroxy-terminated polybutadine (HTPB)-based composite propellant is estimated from uni-axial tensile testing. Double dumbbell specimens as per ASTM D638 type IV standard were used and Poissons ratio at break, obtained by change in volume of specimen, was calculated as approximately 0.25. It was also observed that Poissons ratio is different along different lateral directions of the propellant specimen. Poissons ratios in two orthogonal directions perpendicular to longitudinal axis were calculated as 0.17 and 0.30. As ASTM specimen has rectangular cross-section of approximate size 6 mm x 4 mm, the directional behaviour of Poissons ratio may be attributed to initial dimensions. Prismatic propellant specimen with square cross-section and of 115 mm x 6 mm x 6 mm dimension do not show any variation wrt Youngs modulus, tensile strength, and percentage elongation as compared to ASTM specimen. Directional behaviour of Poissons ratio with almost similar numerical value was again observed, thus ruling out dependence of this behaviour on different initial dimensions of propellant cross-section. Further, Poissons ratio varies linearly with strain even in linear portion of stress-strain curve in uni-axial tensile testing. The rate of reduction of Poissons ratio with increase in strain is slower in linear region and it accelerates after dewetting due to formation of vacuoles. Variation of Poissons ratio with strain has two different slopes in linear (slope = 0.3165) and nonlinear regions (slope = 0.61364). Numerical value of slope for variation of Poissons ratio with strain almost doubles after dewetting. It must be noted that no change in volume does not necessarily indicate constant Poissons ratio equal to 0.5. Composite propellants behave as compressible material in most of the regions and near-failure region or at higher strains; Poissons ratio is not anywhere near to 0.5, instead it is near 0.25.