Relationship between axial compression strength and longitudinal microvoid size for PAN-based carbon fibers

Single fiber axial compression strength has been determined on a series of polyacrylonitrile-based carbon fibers using micro compression test at a gage length of about 10 μm and a detailed analysis has been made on the distribution and the fiber diameter dependence of the axial compression strength. The length of the unsupported region of carbon layer stack calculated from the axial compression strength was well comparable with the longitudinal length of the microvoids determined with small-angle X-ray scattering. This indicated that the fracture of carbon fibers is initiated from the buckling of the unsupported region of carbon layer stack which is formed where the microvoid adjoins it. The tensile and the axial compression strengths of the carbon fibers showed different relationships between the coefficient of variation and the average. This indicated that the tensile and the axial compression strengths are governed by different factors although they showed a correlation. Comparison has also been made between the longitudinal length of the microvoid and the microvoid sizes perpendicular to the fiber axis.