Methodology for determining the influence of soft surface layers in porous solids on compressive behavior: Application to NASA’s lightweight ceramic ablator

Abstract

A methodology is presented for determining the compressive response of a porous material’s surface layer. Microscopic strain measurements are combined with an analytical model that describes the strain in the surface layer, at a given compressive stress, as a nonlinear function of surface depth. The model yields the effective thickness of the surface layer, explains quantitatively an increase in apparent material stiffness with increasing sample thickness, and yields accurate stress-strain curves (prior to yielding) for samples of arbitrary thickness. The methodology is applied successfully to silicone impregnated reusable ceramic ablator, a thermal protection system material developed and used by NASA for planetary missions, including the Mars rovers.