Mechanical properties of silicone reusable ceramic ablator.

摘要

Silicone impregnated reusable ceramic ablators are highly porous fibrous ceramic substrates that are partially impregnated with silicone. The ablators are currently used as heat shields for planetary entry and countless other thermal protection systems (TPS) applications. The fibers in the materials tend to be randomly and uniformly aligned parallel to a preferred plane, resulting in an (anisotropic) orthotropic, transversely isotropic mechanical response, characterized by five (5) independent elastic constants. This thesis reports the anisotropic mechanical response of Silicone Reusable Ceramic Ablator (SIRCA) in tension, compression, and shear. Compressive, shearing, and tensile loads were applied either parallel or transverse to the preferred plane.; Low cycle elastic fatigue ("preflexing") data, for compression and shear, shows the material to exhibit a stress stiffening effect, which is most pronounced from the initial loading to the second consecutive loading. In contrast, low cycle inelastic fatigue (for compression and shear) imparts a stress softening effect to the material, as demonstrated by a reduction in elastic moduli and mechanical strength. Mechanical test data reported herein also includes Poisson ratio measurements, which, for a transversely isotropic material, dictates three (3) independent Poisson ratios.; The present work also examined experimentally the influence of sample geometry and density upon the compressive response of SIRCA; compressive tests were made with rectangular samples of height to width (h/w) ratios equal to 0.5, 1.0, 1.5, 2.0, 3.0, and 6.0. A particularly interesting result of the h/w study is that, for both parallel and transverse compression, compressive moduli were noted to increase considerably with increasing h/w, with the most pronounced increase occurring among samples in the smaller h/w groupings. This result is explained on the basis of a hypothesis that there exists a soft surface layer, the thickness of which is independent of h/w (i.e., a surface layer of a given thickness with a much smaller module than that of the interior bulk material).