| The determination of fiber properties often involves the measurement of fiber cross-sectional area or diameter. Unfortunately, the measurement of fiber cross-sectional area or diameter is often the major source of error due to the tiny dimension and non-circular cross-section. Therefore, for accurate measurement of fiber properties, the fiber cross-sectional area or diameter must be measured accurately. Laser diffraction is a convenient technique to measure the fiber diameter. However, the accuracy of the technique has been questioned. Using laser diffraction techniques, fiber diameters were calculated from scattering theory and the Fraunhofer diffraction approximation which were then compared to that measured by image analysis.;The transverse coefficient of thermal expansion (CTE) of a single carbon fiber was measured using laser diffraction techniques and two CCD (charge-coupled device) detectors located at both sides of the diffraction pattern. A small vacuum furnace for heating a single fiber was designed and constructed. Transverse CTEs in two perpendicular directions can be measured on the same fiber in the current experimental setup. Anisotropy of transverse CTE in Amoco P-series pitch-based carbon fibers with an oriented-core transverse microtexture was demonstrated. The measured values of transverse CTE were well correlated to the transverse microstructure. Transverse CTEs were computed for different transverse microstructures from a mechanics approach. The comparison of calculated and experimental transverse CTEs and the effect of transverse microstructure are discussed.;The structure of carbon fibers was studied using X-ray diffraction and scanning electron microscopy. The preferred orientation was measured using an azimuthal transmission stage. The relationship between modulus and preferred orientation was discussed and compared to the uniform stress model and Ruland's elastic unwrinkling model. The nonlinear behavior of stress-strain curves was also investigated and its relationship with shear modulus was discussed. Finally, the effects of microstructure on the mechanical behavior of carbon fibers were discussed. |
