Structure change of carbon fibers during axial compression

Abstract

In situ wide-angle X-ray diffraction and small-angle X-ray scattering measurements have been performed during axial compression tests of composite strands of polyacrylonitrile-based carbon fibers with varying carbon layer stacking height and the changes in the crystallite and the microvoid structure have been analyzed. The longitudinal length of microvoids, the orientation parameter of the carbon layer stacks and the orientation parameter of the microvoids decreased almost linearly with increasing axial compression stress of the fiber. The decrease in the longitudinal length of microvoids up to the compression fracture of the fiber was in the range of about 4–10%, which was larger than the macroscopic compression fracture strain of the fiber. This indicated that a deformation larger than the macroscopic fiber strain arose in a local region of the fiber structure reflecting the deformability of the local microtexture. The axial compression strength of carbon fibers was calculated based on the equation proposed in a previous study using the measured values of the microvoid length. As compared with the strengths calculated using the initial longitudinal lengths of microvoids, the strengths calculated using the lengths at the compression fracture of the fibers showed better agreement with the measured strengths.