Stiffness-Thermal Expansion Relationships in High Modulus Carbon Fibers

Abstract

Improved crystallite orientation in carbon fibers increases the axial stiffness while decreasing the axial coefficient of thermal expansion. Measurements of unidirectional Pitch 120 and Pitch 140 fibers, with axial tensile Young's moduli of 828 GPa (120 msi) and 966 GPa (140 msi), respectively, in epoxy and aluminum matrices are described. The axial CTE of these fibers was found to be -1.62 ± 0.04 × 10-6 C-1 in the range 0 ± 60°C. The positive deviation is attributed to high compressive stresses (P140/A1) while the most negative value is achievable by fiber misalignments of a few degrees from unidirectional. The stiffness-expansion relation changes for fiber stiffness above 680 GPa. This suggests a decoupling of the CTE dependence on preferred orientation with possible emphasis on defect structure.