Elastic constant of graphite

Table 1 Elastic constants of graphite single crystal (23)...

The level of the tensile modulus o/carbon fibers is controlled by the degree of preferred orientation of the layer planes in the direction parallel to the fiber axis. Cjj, the elastic constant of graphite single crystals in the direction of the layer planes is 1060 20 GPa. In general, the ratio of tensile strength to tensile modulus is smaller than 1-lCf for carbon fibers type HM (but the tensile strength is influenced by flaws in the fibers and may be improved in the future). 

Comparison of the elastic constants for graphite and diamond, as given in Table 3.1, shows that although the elastic constants are comparable for uniaxial strains in the x, and directions (strongly covalent), the value of Cjj for graphite... 

Blackslee GL, Proctor DG, Seldin EJ, Spence GB, Weng T, Elastic constants of compression-annealed pyrolytic graphite, J Appl Phys, 41 8, 3389, 1970. 

From these considerations on elastic constants of the graphite single crystal we can derive the structural precondition for this technically interesting anisotropic polymer carbon. 

Carbon fibers type HM (high modulus) are carbon fibers with a value of Young s modulus (tensile modulus) larger than 300 GPa (nearly 30 of the Cn elastic constant of a graphite single crystal). 

Mechanical Properties. The hexagonal symmetry of a graphite crystal causes the elastic properties to be transversely isotropic ia the layer plane only five independent constants are necessary to define the complete set. The self-consistent set of elastic constants given ia Table 2 has been measured ia air at room temperature for highly ordered pyrolytic graphite (20). With the exception of these values are expected to be representative of... 

As mentioned in Sec. 1.2, the bond between eitoms within the basal plane of a graphite crystal is considerably stronger than the bond between the planes with an anisotropy ratio of approximately 75. This means that, while the strength in the ab directions is considerable, that in the c direction (interlaminar strength) is low and graphite shears easily between basal planes. The elastic constants are l ... 

The yield stress showed a similar trend to the elastic modulus of significantly decreasing with development of graphitic structure of GCs. The constants of power law fitting, n and k, for the stress-strain curves by the indentation on the different GCs decreased with the elevation of HTT (Table 3). From the reduction of these... 

Another class of inhomogeneous materials of increasing practical importance is fiber-reinforced composites. Usually the matrix is a polymer, such as epoxy. The reinforcement may be polymeric, such as Kevlar, or nonpolymeric, ie, glass, steel, or graphite. For a unidirectional composite with transverse isotropy, there are five independent elastic constants or moduli Cn, C13, C33, C44, and Cee, (28,29). Absorption measurements are particularly difficult to make on such composites. Frequently, the measured attenuation has a significant contribution as a result of scattering from the reinforcement this is sometimes referred to as geometric dispersion. 

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