Measurement of the distribution of cross sectional shape along the axis of carbon fibers and its effect on the mechanical properties

Mechanical tests of a single carbon fiber are difficult because of its small size. The diameter or the shape of the cross section is very important for a precise evaluation of the properties. However, most size evaluations use a measurement of fiber diameter in only one direction and its effect on the results has not been studied sufficiently. In order to investigate the apparent diameter variation of carbon fibers, we have developed a new apparatus with two functions. One is that we can accurately control the measurement position along the fiber axis and the other is that we can rotate a single fiber around its axis. Measurements were carried out for two types of carbon fibers. Though the cross section of one of them was far from circular, its shape was almost the same along the fiber axis and its orientation was rotating randomly. For both fibers, an assumption of an elliptical cross section gave a smaller coefficient of variation for torsional modulus and tensile properties.