Stress-induced microstructural changes and crystallite modulus of carbon fiber as measured by X-ray scattering

Apparent crystallite modulus (Ecapp) has been evaluated under the assumption of homogeneous stress distribution for carbon fiber (CF) bundles of different bulk moduli by measuring the X-ray 1 0 0/1 0 1 reflections under tensile stress using a synchrotron X-ray beam. The Ecapp was found to be different among the CF fibers with different mechanical properties: 732 GPa for the sample with bulk Young’s modulus 445 GPa and 359 GPa for the sample of 252 GPa, indicating that the assumption of homogenous stress distribution cannot be applied. The heterogeneous stress distribution in the CF has been evaluated quantitatively on the basis of the complex mechanical model consisting of crystalline and amorphous phases.