Constitutive Properties of Silicone-Impregnated Reusable Ceramic Ablator in Compression: Poisson's Ratios

摘要

THIS paper is the fifth in a series [1–4] of papers that presentnresults of experimental studies, of the mechanical properties ofnlightweight ceramic ablator (LCA) materials, carried out at thenMechanical Engineering Department at the University of California,nSanta Barbara, in support of NASA’s space exploration programs.nHere we report on measurements of 1) the intrinsic linear elasticncompressive response of silicone-impregnated reusable ceramicnablator (SIRCA), from which the Poisson ratios are calculated, andn2) the extrinsic (or global) nonlinear compressive response, whichnmay be reproduced accurately by a theoretical model. The motivationnfor (and importance of) these mechanical properties studies isndiscussed in [1,2]. Briefly, LCA materials were developed at thenThermal Protection Materials and Systems Branch at the NASAnAmes Research Center for use as heat shields on spacecraft fornplanetary entry [5–11]. The LCA materials consist of fibrous ceramicnsubstrates that are partially impregnated with organic resins. Thenfibers in the substrate tend to lie randomly and uniformly parallel to anpreferred (unique) plane, hence the LCA materials have complexnanisotropic mechanical properties that may be approximated asntransversely isotropic, with the unique axis of symmetry perpendicularnto the unique plane. The principal types of LCA materialsnare SIRCA and phenolic impregnated carbon ablator. SIRCAnhas been used in a number of thermal protection system (TPS)napplications, including the successful Mars Pathfinder and MarsnExploration Rover (MER) missions. In particular, SIRCA was usednfor protection of the parachute deceleration system and the backshellninterface plate during entry into the atmosphere of Mars, as well asnthe transverse impulse rocket system motors, which were employednto reduce lateral landing velocities on Mars.nParmenter et al. [2] present results of tests of the compressivenresponse of SIRCA; [3] contains results of Iosipescu shear testing ofnSIRCA and finite element analysis (FEA) modeling of the Iosipescuntests. More recently, while carrying out tests of the mechanicalnbehavior of SIRCA in support of the MER missions, it was foundnthat SIRCA possess a relatively thin, soft, surface layer that cannhave a profound influence on the overall compressive response; thisnbehavior has been documented in [4]. In particular, Gray andnMilstein [4] employed a video microscopy technique to make numerousnmeasurements of stress vs strain in SIRCA samples, overnvarious microscopic gauge lengths L, thereby demonstrating experimentallynthe existence of SIRCA’s soft surface layer. Their experimentalnstress–strain (u0003-") responses were recorded over gaugenlengths L that both resided within and spanned the softer surfacenlayer. They developed a relatively simple, physically meaningful,nanalytical model (with but one adjustable parameter LS, the effectiventhickness of the soft surface layer) to express the nonuniform strainneS, at a point located at a depth x in the soft surface layer, as a functionnof depth x and level of stress u0003, for 0