Properties of carbon fibers

In spite of the better alignment of basal planes in the skin region, the surface of carbon fibers can show extremely fine-scale roughness. A scanning electron micrograph of AS4 carbon fibers is shown in Fig. 8.9a, while an atom-force microscope picture of the same fibers is shown in Fig. 8.9b. Note the surface striations and the roughness at a microscopic scale. 

As described above, a carbon fiber with a perfectly graphitic structure will have a theoretical Young s modulus of slightly over 1000 GPa. In practice, however. 

Following Huttinger (1990), we can correlate the modulus and strength of carbon fiber to its diameter. We make use of Weibull statistics to describe the mechanical properties of brittle materials (see Chapter 10). Brittle materials show a size effect, i.e. the experimental strength decreases with increasing sample size. This is demonstrated in Fig.8.10 which shows a log-log plot of Young s modulus as a function of carbon fiber diameter for three different commercially available carbon fibers. The curves in Fig. 8.10 are based on the following expression  

Now the theoretical strength of a crystalline solid, o- is expected to be about 

li (Meyers and Chawla, 1984), i.e. in this case a- =100GPa. For this value of the exponent n in Eq. (8.3) turns out to be 1.65 and 2 (Huttinger, 1990). This means that in order to obtain a strength of lOOGPa, the diameter of the carbon fiber must be reduced from d to dg p,m. Note that this value corresponding to theoretical strength is less than the d value corresponding to the 

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Table 1. Mechanical and Physical Properties of Carbon Fibers ...

Most recent studies (69) on elevated temperature performance of carbon fiber-based composites show that the oxidation resistance and elevated temperature mechanical properties of carbon fiber reinforced composites are complex and not always direcdy related to the oxidation resistance of the fiber. To some extent, the matrix acts as a protective barrier limiting the diffusion of oxygen to the encased fibers. It is therefore critical to maintain interfacial bonding between the fiber and the matrix, and limit any microcracking that may serve as a diffusion path for oxygen intmsion. Since interfacial performance typically deteriorates with higher modulus carbon fibers it is important to balance fiber oxidative stabiHty with interfacial performance. 

Hamada, T., Nishida, T., Sajiki, Y. and Matsumoto, M., Structures and physical properties of carbon fibers from coal tar mesophase pitch, J Mat Res, 1987, 2(6), 850 857. 

Low density, carbon fiber-carbon binder composites are fabricated from a variety of carbon fibers, including fibers derived from rayon, polyacrylonitrile (PAN), isotropic pitch, and mesophase pitch. The manufacture, structure, and properties of carbon fibers have been thoroughly reviewed elsewhere [3] and. therefore, are... 

Donnet JB, Bansal RC (1990) Mechanical properties of carbon fibers. In Donnet JB, Bansal RC (eds) Carbon fibers. Marcel Dekker, New York, p 267... 

Fig. 3.27. Effect of the interface shear strength on mechanical properties of carbon fiber-epoxy matrix composites ( ) tran.sverse tensile strength (A) maximum transverse tensile strain (O) transverse tensile modiilns. After Madhukar and Drzal (1991),...

Interface properties of carbon fiber-epoxy matrix composites and Weibull parameters of carbon fibers"... 

The surface properties of carbon fibers are intimately related to the internal structure of the fiber itself, which needs to be understood if the surface properties are to be modified for specific end applications. Carbon fibers have been made from a number of different precursors, including polyacrylonitrile (PAN), rayon (cellulose) and mesophase pitch. The majority of commercial carbon fibers currently produced are based on PAN, while those based on rayon and pitch are produced in very limited quantities for special applications. Therefore, the discussion of fiber surface treatments in this section is mostly related to PAN-based carbon fibers, unless otherwise specified. 

Summary of olT-axis properties of carbon fiber-828 mPDA epoxy matrix composites with different fiber surface treatments"... 

Bascom. W.D. and Drzal, L.T. (1987). The surface properties of carbon fibers and their adhesion to organic polymers. NASA contract Report 4084. 

Goan, J.C., Marlin, T.W., Prescott, R. (1973). The influence of inlerfacial bonding on the properties of carbon fiber composites. In Proc. 2Sth. Annual Tech. Conf. Reinf. Plast. Composites Inst., SPI. Paper 21B. 

Mechanical properties of carbon fiber-epoxy matrix composites . 

Bader M.G., Bailey J.E. and Bell 1. (1973). The effect of fiber-matrix interface strength on the impact and fracture properties of carbon fiber-reinforced epoxy resin composites. J. Phys. D Appi. Phvs. 6, 572-586. 

Lam P.K. and Piggott M.R. (1989b). The durability of controlled matrix shrinkage composites Part 2, Properties of carbon fiber-epoxy copolymer pultrusion, J. Mater. Sci. 24, 4427-4431,... 

Tomlinson, W.J. and Barnes, J.A. (1992). Fiber eoatings and the meehanical properties of carbon fiber/ nylon 6 eomposites. J. Mater. Sci. Lett. 11, 440-442. 

Reynolds WN. Structure and physical properties of carbon fibers. In Walker Jr. PL, Thrower PA,. Chemistry and Physics of Carbon, vol. 11, New York Dekker. 1973 pp. 1-67. 

Fig. 9. Effect of sub-Tg annealing on the ultimate mechanical properties of carbon-fiber-reinforced epoxies...

Recent tests have revealed surprisingly good fatigue and creep resistance for carbon/carbon composites. Figure 29 presents some results of torsion and flexure tests in which the fatigue properties of carbon-fiber-reinforced carbon (CFRC) 3D composites are compared with those of carbon-fiber-reinforced polymer (CFRP) 3D composites (53). 

In order to improve the surface properties of carbon fibers without affecting the mechanical properties of the reinforcing fibers, various monomers, including pyrrole and carbazole, were used to electrograft conjugated copolymers and produce homogeneous and continuously coated carbon fiber surfaces <2001SM391>. 

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