Effects of Porosity on Strength of Carbon-Carbon Composites

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Abstract
Filament wound/CVD (Chemical Vapor Deposition) carbon-carbon composites have received considerable attention and application within the past few years because of their desirable characteristics such as high heat of ablation, thermal shock resistance, high strength at elevated temperatures, and chemical inertness. However, poor mechanical properties in the transverse direction have hampered the total effectiveness of these composites in some applications and a study of the effects of porosity on transverse tensile strength of filament wound/ CVD carbon-carbon composites has been conducted. Two types of filament wound/ CVD carbon-carbon composites were studied. A standard filament wound/ CVD carbon composite, and a composite similar to the standard processed composite; but with short chopped carbon fibers sprayed on the substrate during the winding process. Transverse tensile strength, volume fraction of open cell porosity, pore geometry, size, and orientation were determined. A linear relationship was observed for the logarithm of transverse tensile strength as a function of volume fraction porosity. However, a considerable difference in the slopes of the lines for sprayed and unsprayed materials was observed, indicating the effect of a parameter other than volume fraction porosity. This additional parameter is pore size distribution. The transverse tensile strength of filament wound/CVD carbon-carbon composites is drastically reduced by increasing volume fraction porosity. In addition, parameters such as pore size distribution, orientation, and geometry are also important, and should be considered when studying porosity effects. The theoretical model prepared by Brown et al [6] for predicting the strength of polycrystalline materials as a function of porosity (size, geometry and orientation) appears reliable for predicting transverse tensile strength of filament wound carbon-carbon composites.

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