Compressive response of lightweight ceramic ablators: Phenolic impregnated carbon ablator

Lightweight ceramic ablators are porous fibrous ceramic substrates impregnated with organic resins, The materials were developed for use as heat shields for planetary entry. The compressive response and hardness of a phenolic impregnated carbon ablator and its fibrous substrates are reported. Results are also given for charred specimens of the ablator, Fibers in the substrate tend to be randomly and uniformly aligned parallel to a preferred plane. Two characteristic compressive responses were observed, depending on the orientation of the load with respect to the preferred fiber-alignment plane. When the load was parallel to this plane, the stress increased approximately linearly with increasing strain to strains of about 2%, The stress then followed an erratic "horizontal" path with increasing strain as the materials apparently failed by an internal separation and buckling mechanism, In contrast, during compression transverse to the fiber-alignment plane, the materials underwent significant, nonlinear, plastic deformation, wherein the stress increased with increasing strain to strains of over 50% without indications of fracture. Considerable energy was absorbed during compression, particularly for materials subjected to the transverse loading condition. Resin impregnation substantially strengthens the ablator material; this effect was prominent for specimens loaded transverse to the fiber-alignment plane.