Experimental Quantification of Fracture and Crack Growth in a Porous Carbon Fiber Material

Porous materials display various attractive functional properties, such as low density and low thermal conductivity, that have led to their use in aerospace applications. To fully realize the potential of these materials though a thorough understanding of their mechanical reliability and damage tolerance is necessary. Current efforts have been made attempting to effectively model the mechanical behavior of 3D random fibrous network materials, but experimental validation is still required. Additionally, little is currently known about the mechanisms resulting in catastrophic mechanical failure of these materials, especially as it relates to various loading modalities and orientations. To this end, this study investigates the mechanistic reasoning for the overall damage tolerance and fracture behavior of Fiber Form(a substrate for Phenolic-Impregnated Carbon Ablator [PICA]) at both the macro scale and mesoscale. Compression and tensile tests were completed to obtain macro scale mechanical properties and were subsequently tied to strain localization and damage propagation leading to fracture through digital image correlation (DIC). From this work loading perpendicular to the build direction under compression resulted in a toughness that was between 1-3 orders of magnitude larger than any other loading modality and orientation combination. Additionally, a preferred orientation of approximately 20° was observed for initial fracture during compressive loading. These results will provide specific experimental data relating to the catastrophic failure of porous fibrous materials which can be used to improve the fidelity of current computational models and tailor the reliability of parts under service conditions.