TENSILE FAILURE IN 2-D CARBON CARBON COMPOSITES

A description is given of the mechanisms causing tensile failure in 2-D carbon-carbon composites containing plain-weave fabric reinforcements. The carbon fibers in the composites were made from a rayon precursor and had both a low modulus (6-8 Msi) and a low strength (60 ksi). Two types of carbon-carbon material were studied, with the same fiber content but differing in matrix structure. The first material contained a carbon matrix deposited entirely by chemical vapor deposition (CVD). The second material contained a carbon matrix formed by two steps: initial pyrolysis of a phenolic resin to 4500°F, and then a subsequent CVD deposition. Specimens from both materials were loaded to failure in tension and afterwards microstructural observations were made of crack development. Based upon these observations it was concluded that the mode of failure of the fiber bundles within the composites depended upon the curvature of the bundles. Fiber bundles with small curvatures failed due to tensile stress, or due to a combination of tensile and bending stresses. Fiber bundles with large curvatures failed due to shear stresses, at the point where the local fiber direction was most inclined to the applied load. The shape of the theoretical envelope predicting this shear failure shows good agreement with published data on the ultimate strength of carbon-carbon laminates. © 1990.