150. Relationships betxeen three-dimensional structural models and properties in carbon fibers
E. W. Tokarsky and R. J. Diefendorf (~~~u~ D&&on, Rme&r Polytechnic Z~~itu~, Troy, New York). In carbon fibers two kinds of preferred o~entation of the basal plane have first-order effects on mechanical properties. The axial elastic modulus is determined primarily by the axial preferred orientation, while strength is determined not only by axial, but aiso radial preferred orientations. Fibers studied were of PAN and rayon base, and represented a modulus range of 6-116 Msi. With increasing modulus, the average axial preferred orientation and crystallite perfection increased; in several of the higher modulus PAN fibers, modulation of the (1010) into six spots in electron diffraction was commonly obseyved. The arrangement of these spots with respect to the (0002) arcs suggests a tendency of one of the [lOlO] to lie parallel to the fiber axis within the rippled ribbon structural units. As the modulus increases the amplitude to wavelength ratio of the rippled ribbons decreases along with fibril-fibril bonding; as a consequence, the fracture mode of the fiber becomes analogous to a macro-composite. Optically determined radial and axial preferred orientations indicate that radial preferred orientations develop along with axial. Measurements also indicate that the outer surfaces of most fibers have a higher axial preferred orientation than the core-as a result, when fibers are loaded in tension, the surface sustains most of the load. On the other hand, the poorer axial preferred orientation of the core will place the surfaces of fibers in longitudinal compression on cool down from final heat-treatment temperatures, thus lowering flaw sensitivity. The tendency toward the development of centrosymmetric radial textures will cause circumferential cracks to form on cool down from final heat-treatment temperatures exceeding about 1300X-this will have a deleterious effect on fiber strength. A model of a 60 Msi fiber illustrating basic preferred textures is presented.