Nano structural analysis on stiffening phenomena of PAN-based carbon fibers during tensile deformation

The effects of inner microstructures, including crystallite size, orientation and defects on the stiffening phenomena of PAN-based T300 carbon fibers under tension were explored. Single-fiber tensile test was conducted on three types of fiber: as received, 1400 and 1600 °C heat treated. The distribution of crystallite orientation in a fiber from core to skin was obtained by using transmission electron microscopy. The observations indicate the load-transfer between crystallites depends on the crystallite entanglement. The slide-lock of the entangled junctions among the loosely compacted crystallites is responsible for the increasing elastic modulus during tension. The sharp drop of tensile strength (−36%) of the fiber after heat treated at 1600 °C was attributed to the increasing of crystallite size, nano-pore defects and a higher misalignment of crystallites in the core region.