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With the expansion of use of carbon fiber reinforced plastics (CFRPs), it becomes important to obtain precise knowledge of elastic properties carbon fibers in radial direction as well as axial direction. Although the elastic property in radial direction of polyacrylonitrile (PAN)-based carbon fibers have been investigated using ultrasound scatter measurements, compression tests of single fibers and nanoindentation, there is no experimental evaluation for the pitch-based carbon fibers which possess higher crystallinity and tensile modulus than PAN-based carbon fibers. Here, we investigate the mechanical anisotropy of PANand pitch-based carbon fibers by the nanoindentation technique. Nanoindentation tests are carried out on longitudinal (0°), 45° and transversal (90°) cross sections of carbon fibers by a Hysitron TriboScope (Minneapolis, MN) using a diamond conical indenter with a nominal tip radius of 10 μm. We demonstrate that the indentation modulus of both carbon fibers decreases with a decreasing orientation angle from axial to radial direction, but this tendency is more significant in the pitch-based carbon fibers. Supposing that the indentation modulus in the radial direction is same as the transverse elastic modulus, the anisotropy of elastic modulus (Ef/ET) of the pitch-based carbon fibers is calculated to be 165 which is approximately 15 times as high as that of PAN-based carbon fiber (Ef/ET = 11). This result suggests that the pitch-based carbon fiber possesses a large mechanical anisotropy. The higher mechanical anisotropy observed in the pitch-based carbon fiber is mainly due to the existence of the parallel arrangement carbon crystallite microtexture with high crystallinity in the axial direction.

Mechanical anisotropy of PAN-based and pitch-based carbon fibers