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Ultrasonic velocity measurements have been made to obtain the dynamic elastic stiffnesses necessary to determine fully the elastic properties of a unidirectional glass-reinforced epoxy-fiber composite. In units of 106 psi, these stiffnesses are C11=6.01, C22=C33=2.58, C12=C13=0.70, C23=1.42, and C44=0.49, where the subscript 1 refers to the fiber direction. Since more velocities were measured than were necessary to obtain the five constants required by the symmetry of this composite, the extra measurements were used to check on the experimental method. Analysis shows the ultrasonic technique to be satisfactory for measurement of the elastic stiffnesses of a fiber composite. The experimental results are compared with the elastic constants predicted for this composite from expressions based upon several theoretical models. Good agreement is obtained when the theoretical calculations are made using the dynamic (as opposed to the static) modulus of the epoxy matrix.

When the organic precursors to coke and manufactured graphite (petroleum and coal-tar pitches) are carbonized, large, flat, polynuclear aromatic molecules are formed by the reactions of aromatic polymerization. As the molecular weights approach 1500, usually near 450 C, the molecules condense to form a liquid crystal which has been termed the carbonaceous mesophase. The distinctive feature of the carbonaceous mesophase is the plate-like molecule, in contrast to the rod-like molecule of nematic liquid crystals. The carbonaceous mesophase thus appears to be a lamellar liquid crystal in which space is filled by plate-like molecules which may vary in size and shape but tend to pack in parallel arrays. These arrays are free to bend and twist as they trace out a lamelliform or Mobius morphology with topological discontinuities which bear similarities to the disclination structures found in conventional liquid crystals. Two types of lamellar disclinations may be conceived: layer-stacking disclinations resulting from long-range discontinuities in parallel stacking, and layer disclinations that may exist within a given layer. Layer-stacking disclinations exist in profusion in most cokes and fabricated graphites and appear to play significant roles in determining such properties as graphitizability, thermal expansion, and fracture toughness.

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