A VISCOELASTIC MODEL FOR PBX BINDERS

Abstract. Stress-strain measurements done at different rates and temperatures along with measurementsof the rate- and temperature-dependent dynamic storage modulus have allowed us to construct a generalizedMaxwell model for the linear viscoelastic response of plasticized estane. A theoretical analysis ispresented to include effects of impurites.INTRODUCTIONComplete knowledge of the thermo-mechanicalbehavior of the constituents of PBX-9501 is requiredfor any micromechanics method to be a useful toolfor modeling its behavior. The primary constituentsof PBX 9501 are the explosive cyclotetramethylene-tetranitramine (HMX) crystals and the inertplasticized estane binder matrix. Estane 5703 is apolyester polyurethane elastomer manufactured bythe B.F. Goodrich Company with a density of 1.19gm/cm 3 . The polymeric binder shows dramaticsensitivity to changes in strain rates andtemperatures. For example, a change in thetemperature from -50 C to 50 C will have anassociated change in the shear modulus of five ordersof magnitude. Obviously, a successful theory forPBX 9501 must account for this behavior. Becauseof recent experimental effort, much high-qualitystress-strain data has become available for theplasticised binder. A primary goal was to use thisdata to formulate a generalized Maxwell model(GMM) thermo-mechanical constitutive law for thebinder. While a GMM constitutive law hasimmediate applications for PBX 9501, ourtheoretical analysis used to obtain the constitutivelaw has interest to the general community involvedwith plastic bonded high explosives.The aforementioned stress-strain data was measuredby the LANL's Material Structure/Property Group(MST-8) and was obtained by several differentexperimental methods. An Intron 5567 testingmachine was used for measuring uniaxial stress-strain data for rates in the range of 1