One of the principal difficulties in predicting properties of CC composite materials is in defining the constituents to be used as input to the models. This difficulty is a problem because the constituents that one starts with are modified by the high temperatures during processing. Consequently, the process history becomes a major influence upon the final in situ constituent properties.
Previous chapters described processing approaches and indicated how processing can affect the constituents. The analyst must use previous experience or similar processed material properties to define a starting point for constituents. The following points should be considered when beginning to define input properties:
- High graphitization temperatures plus tensile stresses during processing will generally increase the in situ fiber modulus and lower the fiber expansion characteristics. These properties are highly dependent upon the fiber type used in the material.
- First cycle pitch impregnation will promote high alignment of the matrix material surrounding the fibers, leading to an increasingly effective composite modulus.
- Pyrolytic graphite will result locally in very anisotropic matrix properties, which can influence the matrix-dominated composite properties.
- Cooldown from graphitization cycles will result in cracks developing between subcell regions, thus reducing the composite shear modulus and expansion coefficients. Whether a final graphitization cycle is or is not used will have a large effect on the properties during the first subsequent heat-up of the material. If multiple temperature cycles must be sustained by the composite, then final graphitization should be taken into account when deciding on the final processing conditions.