In a mechanically loaded composite, it is usually assumed that the fibers and the matrix are bonded together and that no differential movement exists between them. The strain in each part of the carbon composite can then be considered identical and equal to the total strain on the carbon composite. For an elastic response, the stress on a unidirectionally reinforced composite loaded parallel to the fiber axis Oc can be computed by multiplying the elastic modulus in that direction Ec by the measured strain Ec, i.e.,
(3)
Or, combining (1) with (3)
(4)
The failure strain of a polymer or a metal Em is substantially larger than that of the reinforcing fibers Ef, and failure of this type composite occurs when the fibers break. Failure occurs rapidly and results from the catastrophic propagation of a crack initiated at some weak point in the carbon composite.
The failure of CC composites can again be discussed with reference to equation (4). If it is assumed that the maximum failure strain of carbon matrices is approximately 0.3 percent, then the composite will fail at that strain. Also, if it is assumed, for simplicity, that the matrix carries no load, the strength of a carbon matrix reinforced with 50 volume percent of 75Msi high modulus fibers would be about 112ksi. Conversely, the strength of the same matrix reinforced with 30Msi low modulus fibers would be about 45 ksi. Since higher modulus fibers are usually weaker, we note the curious result discussed by Fitzer and Huttner that for composites that fail at the matrix failure strain, the strengths can be larger when stiffer, usually weaker fibers are used as the reinforcement. The failure of a composite whose failure is matrix-dominated can occur at lower strains when differences in thermal expansion between the fiber and matrix cause the matrix to be prestressed in tension on cooling form carbonization temperatures. Since the matrix and fiber must be bonded together in order to prestress the matrix, very low strengths might be expected from well-bonded materials. Thomas and Walker studied phenolic resin precursors reinforced with three types of commercially available carbon fibers. Some of the fibers were surface treated to promote bonding; some were not. In every case, the matrix-dominated properties of well-bonded phenolic char-matrix composites were superior to those exhibited by less well-bonded composites. Unfortunately, the longitudinal strengths of the well-bonded material were extremely poor and the work of fracture was low. In contrast, the matrix-dominated properties were poor, and the longitudinal strengths of fully processed CC composites reinforced with nonsurface treated fibers was superior to those containing surface-treated fibers.
An additional series of experiments were performed by Fitzer at al who studied the effect of the degree of fiber oxidation on the mechanical properties of carbonized resin materials reinforced with carbon fibers. After initial carbonization, but before subsequent densification, they found, relative to material reinforced with nonoxidized fibers, that increasing oxidation improved the strength of the material reinforced with the HM fibers, but decreased the strength of the material containing the high-strength fiber. cfccarbon.com
Manocha et al. performed a similar experiment using surface-treated Toray M40 fibers and a matrix of furfural alcohol condensate. It was found that after the initial carbonization, the strength of the composite containing the surface-treated fibers was poorer than the containing untreated fibers. The effect was similar to that observed by Fitzer et al. on HF materials. Manocha et al. graphitized their first carbonized material without densifying it. The strength of the surface-treated fiber composites increased by a factor of 3 or more while the strength of the nonsurface-treated material decreased by nearly the same amount. In effect, Manocha showed that the composite reinforced with the surface-treated fibers exhibited the highest strength when graphitized.
It is reasonable to conclude from the discussed data that matrix precursor-fiber combinations that promote strong bonds will not produce a high-strength composite when measured parallel to the direction of the fibers.
The addition of graphite powder is often used to decrease the shrinkage tendency of phenolic during carbonization and to improve the carbon yield. Since the decrease in the thermal contraction should also produce a corresponding decrease in the internal stress generated in the matrix on cooling from carbonization temperatures, the strength of composites whose failure is matrix dominated should be improved. The results of Fitzer et al. confirmed this when they demonstrated that the strength translation of HF fiber-reinforced material increased from 40 to 60 percent when 50-percent graphite powders were added to the resin matrix before carbonization. We have shown in a previous section that weak interfaces will occur naturally in some matrix-fiber combinations, while strong interfaces will occur in others. Specifically, the nature and strength of the fiber-matrix bond appear dependent on the reactivity of the fiber, the reactivity of the matrix, and the type and degree of fiber-surface treatment. Fitzer has shown that and improvement in properties is achieved with densification. He found that the strength of composites containing nonsurface-treated HM fibers could be increased from 10 percent after the initial carbonization to 90 percent of the theoretical strength after four densification steps, while the strength of the composite containing nonsurface-treated HF fibers increased only about 40 percent. These data suggest that a reaction occurred naturally between the fiber and matrix in the case of HF fibers and a limited reaction occurred throughout the densification process for the HM materials. Perry and others working with phenolics and different fibers found a maximum of only 40-50 percent translation of strength indicating some degree of reaction; however, since their preparation technique involved a number of graphitizations, other factors may have contributed to the low strength values.
Fitzer also found the resins which form strong fiber-matrix bonds and also exhibit the highest shrinkage should be avoided if high translation of fiber strengths is required. He suggests the use of high carbon-yielding precursors that do not form strong bonds and that exhibit minimum shrinkage. Data presented indicate that pitches will produce better translation of fiber strengths for fewer densification cycles.