Tensile strength of carbon fibers

A.3.2 Factors affecting tensile strength of carbon fibers... 

Figure 5.7 Tensile strength of carbon fiber monofilaments as function of fiber diameter. Source Reprinted with permission from Fitzer E, PAN-based carbon fibers-present state and trend of the technology from the viewpoint of possibilities and limits to influence and control the fiber properties by the process parameters, Carbon, 27(5), 621-645, 1989. Copyright 1999, Elsevier.

Table 5.11 The effect of gage length on the tensile strength of carbon fibers heat treated to 2500°C.

Figure 5.43 The tensile strength of carbon fibers as a function of the density of stabilized fibers from an AN/ MA precursor at stabilization temperatures of O 240°C A 255°C 270°C. Source Reprinted with permission from Takaku A, Hashimoto T, Miyoshi T, J AppI Polym Sci, 30, 1565, 1985. Copyright 1985, John Wiley Sons Ltd.

Figure 15.5 Ultimate tensile strength of carbon fiber reinforced cement. Source Reprinted from Aveston J, Mercer RA, Sillwood JM, Conference Proceedings Composites Standards Testing and Design, NPL 1974, IPC Science Technology Press, 93-103, 1974.

Manders PW, Kowalski IM, The effect of small angular fibre misalignments and tabbing techniques on the tensile strength of carbon fiber composites, 32nd International SAMPE Symposium, 985-996, Apr 6, 1987. 

If we perform a similar analysis with respect to the tensile strength of carbon fibers, we can write ... 

Table 9.9. Tensile Strength of Carbon Fiber, SiC- and B4C-Matrix Composites...

The disposition of boundaries between the fiber types is somewhat arbitrary. For carbon fibers type HT, the values of the strength-to-stiffitess ratio are typically larger than 1.5-10-2, The tensile strength of carbon fibers is flaw controlled, however, and therefore the measured values increase strongly as the diameter of the filaments is decreased. 

A number of the polyamides have achieved commercial importance because of the very high tensile strengths of the fibers that can be spun from the nematic solutions. The additional chain orientation in the direction of the fiber long axis, obtained from the nematic self-ordering in the system, leads to a dramatic enhancement of the properties and makes them attractive alternatives to metal or carbon fibers for use in composites as reinforcing material. 

The tensile strength of UHMWPE fiber (Spectra 1000) is in the range of 2.9-3.7 GPa. This compares well with that of high-carbon steel (1.2 GPa). Multiwalled carbon nanotubes, however, have even higher tensile strength of over 60GPa ... 

Figure 7.6 Effect of final heat treatment on tensile strength of mesophase fibers. Source Reprinted with permission from Bright AA, Singer LS, Carbon 17, 59, 1979. Copyright 1979, Elsevier.

The SEM observations revealed that PAN based fibers failed by a buckling mechanism, whereas MP based fibers failed by shear. The TIOOO fiber exhibited the highest compressive failure strength and tensile strain to break. Compressive strengths of carbon fibers are related to the ability to absorb energy, as shown in Figures 20.20 and 20.21. 

The tensile modulus E and strength of carbon fibers are shown as a function of carbonization temperature in Figure 8 [21]. The carbon fiber modulus increases with increasing carbonization temperature. This increase in modulus is caused by increased graphitization of the carbon at higher temperatures, since the more perfect graphite has a higher modulus than the less ordered carbon sheets. 

The tensile breaking strength of carbon fibers is dominated by flaws within the fiber and on its surface. Some of the flaws that can affect the fiber strength include ... 

Surface treatment has been observed to change the breaking strength of carbon fibers. Bahl et al. [40] and Fitzer and Weiss [41] have observed that treatment of carbon fibers in nitric acid initially increases the fiber tensile strength. Continued anodization results in a loss in strength caused by fiber damage. This initial increase in strength can be explained by removal from the fiber surface of defects, which can initiate fracture. 

It should be mentioned that compressive strength of carbon fiber is low compared to its tensile strength. The ratio of compressive strength to tensile strength for carbon fibers may vary anywhere between 0.2 and 1 (Kumar, 1989). High-modulus PAN-based carbon fibers buckle on compression, forming kink bands at thinner surface of the fiber. A crack initiates on the tensile side and propagates across the fiber (Johnson,... 

In spite of the relatively poor tensile strengths of the fibers, carbonized Kevlar possesses characteristics that make it an ideal candidate as a substrate for chemical vapor deposition of silicon carbide including a compatible coefficient of thermal expansion, sufficient tensile strength to survive the coating process, availability from a reliable domestic supplier, and a smooth, even diameter of less than 7 fim (78). 

Nonoxide fibers, such as carbides, nitrides, and carbons, are produced by high temperature chemical processes that often result in fiber lengths shorter than those of oxide fibers. Mechanical properties such as high elastic modulus and tensile strength of these materials make them excellent as reinforcements for plastics, glass, metals, and ceramics. Because these products oxidize at high temperatures, they are primarily suited for use in vacuum or inert atmospheres, but may also be used for relatively short exposures in oxidizing atmospheres above 1000°C. 

Fig. 12. (a) The variation of the tensile strength of unidirectional carbon-fiber-reinforced epoxy resin as a function of the fiber volume fraction, (b) The variation of the tensile strength of unidirectional carbon-fiber-reinforced epoxy resin as a function of the fiber volume fraction for low fiber volume... 

Since PAN-based carbon fibers tend to be fibrillar in texture, they are unable to develop any extended graphitic structure. Hence, the modulus of a PAN-based fiber is considerably less than the theoretical value (a limit which is nearly achieved by mesophase fibers), as shown in Fig. 9. On the other hand, most commercial PAN-based fibers exhibit higher tensile strengths than mesophase-based fibers. This can be attributed to the fact that the tensile strength of a brittle material is eontrolled by struetural flaws [58]. Their extended graphitic structure makes mesophase fibers more prone to this type of flaw. The impure nature of the pitch preciusor also contributes to their lower strengths. 

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