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strength. Even though the mechanism for fiber failure in compression is not fully understood, it is evident that, for a given tensile modulus, round PAN-based carbon fibers have a higher compressive strength than round pitch-based carbon fibers. It is agreed, therefore, that microstructure has a major influence on the compression properties of carbon fibers.
1.0 H. Mesophase carbon fiber P-100 O I I I I I —l 200 300 400 500 600 700 800
Fiber tensile modulus, GPa
Kumar and Helminiak (ref. 24) have shown that, for both pitch-based and PAN-based carbon fibers, the compression strength decreases as the thickness of the graphite crystallites increases. Additionally, increases in the void content within the fiber or in the interlayer spacing correlate with increased compression strength. Therefore, it appears that a carbon fiber with an extended highly graphitic structure is likely to be weak in compression. On the other hand, fibers with microstructures that impede the development of an extended graphitic structure should exhibit higher compressive strengths.
PAN-Based Carbon Fibers
Johnson has used a single-filament recoil test to study the compression failure of PAN-based and pitch-based carbon fibers (ref. 25). In these tests, he found that the first response of high-strength PAN-based and low-modulus pitch-based fibers to a compression load was buckling and the formation of kink bands. As deformation progressed, a tensile crack formed on the tension side of the buckled fiber, and the kink bands propagated inward. Finally, the tensile crack and the kink bands met, and failure occurred. This failure sequence is shown in figure 11(a).
(b) High-modulus pitch-based fiber.
Figure 11. Compressive failure of low-modulus and high-modulus carbon fibers (ref. 25).
Pitch-Based Carbon Fibers
When Johnson studied high-modulus pitch-based carbon fibers using the same technique, he observed a markedly different failure mode (ref. 25). Under initial compression, these fibers developed kink bands across the whole fiber by simple shear deformation. Then, as shown in figure 11(b), the fracture simply propagated along the kink plane. These observations have led Johnson to conclude that the low compressive strengths of high-modulus pitch-based carbon fibers are caused by their high degree of order (graphitization) and sheet-like, flat-layer microstructure. This structure is expected to be less resistant to shear between the basal planes than a more random structure.
This influence of microstructure on compression strength becomes more evident when pitch-based carbon fibers with different microstructures are compared. Figure 12 compares the compression strength of a random microstructure. pitch-based carbon fiber produced by Nippon Steel to that of a layered microstructure, pitchbased carbon fiber produced by Amoco Performance Products, Incorporated.* Because tensile modulus and tensile strength increase simultaneously for pitch-based fibers, this plot (fig. 12) and figure 10 show the same trend. However, as figure 12 shows, the compression strength of the fiber with a random microstructure consistently is higher than that of the fiber with a layered microstructure.
Figure 12. Compressive strength as function of tensile strength for different pitch-based carbon fibers (Nippon Steel data (Sato. K., Nippon Steel Corporation. Tokyo. Japan, personal communication. 1989) and Amoco data (ref 19)).
*Saio. K.. Nippon Steel Corporation. Tok_\o. Japan, personal communication. 1989.
Effect of Fiber Shape on Fiber and Composite Properties
Changing the shape of the fiber as well as the microstructure can change the mechanical properties of a fiber. Often the effect of these two variables is difficult to separate. However, fibers with noncircular cross sections have long been used in the synthetic fiber industry to improve the wetting characteristics and increase the buckling resistance of polymeric fibers. During 1989, Owens-Coming Fiberglas Corporation introduced a variety of glass fiber with a trilobal cross section, claiming the shape resulted in a significant improvement in fiber stiffness as well as tensile strength (ref. 26). This new noncircular fiber is, coincidentally, targeted for composite applications.
Effect of Shape on Tensile Strength of Carbon Fibers
Carbon fibers, similar to other fibers, also appear to display different properties when circular fibers are compared with noncircular fibers. Because they are formed by precipitation, the cross-sectional shape of solution-spun PAN fibers tends to be either circular or dogbone. Figure 13 shows a comparison of the balance between modulus and strength for these two shapes of PAN-based carbon fibers, as determined by Diefendorf and Tokarsky (ref. 14). This same trend also has been found in other studies (refs. 27 and 28). As the modulus of a dogbone fiber increases, so does its strength. For increases in modulus above 250 GPa, however, the strength of a round fiber actually drops (ref. 29). While this decrease may be related to flaw-inducing impurities in the fiber (ref. 12), differences in the flaw sensitivity of the fiber, or differences in residual stresses (ref. 14), the evidence remains that the balance of properties differs between round and dogbone PANbased carbon fibers.
Pitch-Based Carbon Fibers
Because pitch-based carbon fibers are melt-spun, they can be extruded into a variety of cross-sectional shapes merely by changing the shape of the extrusion die. Edie et al. (ref. 22) extruded a solvent-extracted mesophase through the trilobal spinnerette capillary shown in figure 14(a) to produce fibers with a trilobal cross section. When heat-treated at 1900°C, the tensile strength of these noncircular fibers was 39 percent higher than round fibers of equal cross section that were produced for comparison. Because the microstructure of the trilobal fibers (fig. 14(b)) differed from the radial microstructure seen in the round fibers, it is difficult to tell if the increased strength is the result of the change in fiber shape or the change in microstructure. The investigators noted that this increase in strength could be caused by several factors, including the possibility that the
Figure 13. Modulus versus strength for round and dogbone shape PANbased carbon fibers (ref 14).
residual stresses in trilobal fibers differ from those in round fibers and may create fewer surface flaws.
In a later study. Gainey et al. (ref. 30) melt-spun a heat-soaked mesophase into trilobal fibers using a similar extrusion system. Since the capillary used in this study had a more defined trilobal shape (fig. 15). the resulting fibers were more highly noncircular than those produced by Edie. These trilobal fibers, along with round fibers of equal cross section produced as a control, were thermoset and then carbonized at I900°C. The single filaments were tested, using a 40-mm gauge length, to compare the strength of the two fiber shapes. A statistical analysis of the data obtained in 100 single-filament breaks of each fiber shape showed that the tensile strength of these relatively large (27 /mi effective diameter) trilobal fibers was approximately 27 percent greater than that of similar size round fibers. A Weibull analysis was applied to the data to show that even though both the round and the trilobal fibers showed a single-failure mode, the trilobal fibers contained fewer or less severe flaws.
Kffect of Shape on Compressive Strength for Carbon Fibers
As mentioned previously, the compressive strength of carbon fibers, especially that of pitch-based carbon fibers, is usually much less than the tensile strength. Because this weakness appears to be the result of the graphite structure itself.