Microstructure-elastic property relationships in carbon fibers: A nanoindentation study

Carbon fibers with high axial stiffness and strength have been used to reinforce polymer-matrix materials in advanced composites. However, these fibers exhibit large anisotropies in the radial and axial directions. In addition, limited data has been reported on the elastic properties of fibers in other directions than longitudinal and transversal directions, and on the relationship between elastic properties and fiber structures. In this study, we prepared five epoxy composites each containing a different type of carbon fiber and used a nanoindentation method in combination with finite element analysis to investigate five elastic constants of the carbon fibers. The results demonstrate that the crystallite size and the orientation of its graphite-crystal inclusions affect the elastic constants c₁₁, c₁₂, c₁₃ and c₃₃, while the carbon fibers with well-aligned large-sized crystallite possess a large mechanical anisotropy. In contrast, the shear modulus (c₄₄) is less affected by the alignment of the crystallite. Instead, the fibers with larger interlayer spacing of (002) basal planes and large crystallite size exhibit a smaller c₄₄.