Anisotropic elastic properties in glass, aramid, and carbon fibers

Fibers used in fiber-reinforced polymer matrix composites (FRPs) exhibit significant anisotropy between radial and axial directions. In this study, six epoxy composites containing different fiber types—E-glass, aramid, and carbon fibers—were prepared. Nanoindentation tests were conducted on three orthogonal cross-sections of each fiber. The Vlassak–Nix (VN) model, based on theoretical implicit equations derived from anisotropic elasticity theory, was applied to extract five independent elastic constants: c 11 , c 12 , c 13 , c 33 , and c 44 . The anisotropic index and its correlation with tensile modulus and extrapolated indentation modulus were systematically evaluated. Results revealed that fibers with higher stiffness exhibited greater anisotropy. These findings highlight the importance of considering both anisotropy and stiffness in fiber selection for composites and additive manufacturing applications.