Graphite Fibers from Pitch
Final technical rept. Apr 1975-May 1976
UNION CARBIDE CORP CLEVELAND OH
Pagination or Media Count:
Type P fibers are capable of achieving a tensile strength of 4.0 GPa approximately 600 Kpsi with an elongation to break approacing 2.0 percent. However, such properties have been measured only on monofilaments with equal to or less than 7.5 micro meter diameter, carbonized at 1700 C and prepared from a mesophase pitch derived from a carefully filtered precursor. The modulus of the fibers is easily increased by heating to higher temperatures, but the tensile strength is usually reduced by such treatment. The tensile strength of the fiber shows a strong dependence on the gauge length, down to about .3 mm. Frequency- of-break analysis revealed a bimodal distribution. Fiber failure below 2.0 GPa 300 Kpsi is induced by macro-flaws such as foreign particles, voids and surface cracks, all of which were identified by optical and electron microscopy. The wedge-like crack, very common in yarn filaments may cause early fiber failure, particularly when the crack follows a tight spiral along the fiber axis, as it was observed in isolated cases. The ultimate tensile strength appears to be determined by crystallographic features, such as crystallite size, number of defects, extend of misorientation, as well as by the symmetry of fiber cross section. A cylindrical tree-trunk symmetry always results in higher strength than the radial, wheel-spoke symmetry.
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