Fundamental Studies Relating to the Mechanical Behavior of Solid Propellants, Rocket Grains and Rocket Motors

Abstract

In the previous report the isothermal strain energy function and the associated constitutive stress and strain law for a 47 volume % voided polyurethane foam were presented. This function involves three parameters--the shear modulus, the £-factor, and the Blatz-Poisson ratio, all of which are constants of the material for deformations up to fracture. Based on experimental data obtained in three different stress fields, experimental values were assigned to each of these parameters. Some thought is being given to an analysis which will predict these parameters from a knowledge of the properties of the continuum binder and the void content and void size. In addition, a failure criterion was proposed for this foam. In order to check further the invariance of the proposed criterion over the failure surface, additional data in a triaxial stress field were obtained. On the basis of these data and an inference based on thermodynamic consideration, which suggests that all carpets of the surface must be non-concave, it looks like the failure surface is the frustum of a triangular pyramid, the frust rated facet being the carpet of the hydrostatic plane. The sides of the frustum are then the carpets of the planes of maximum principal strain. Thus it looks like a dual criterion obtains: each criterion controls one of two subsets of failure modes. In order to confirm the universality of the function, law and definition mentioned above, the triaxial tensile test was performed and reported in the previous report. Because of unsatisfactory bonding of the specimens, the test points were scattered a round the theoretical curves. The present report furnishes i) more triaxial data obtained with improved bonding of the specimens, and depicts more clearly the ii) failure surface based on the averaged experimental values of ultimate stresses in the normal stress space.