Carbon fibers tensile strength

The energy stored in a flywheel depends on the strength of the rotor material. Carbon fiber tensile strength remains well below theoretical limits. Expected increases in strength along with reduction in cost as the use of this material expands will translate into more energy dense, less expensive rotors. 

Fig. 18. Correlation between carbon fiber tensile strength and stabilized fiber tenacity. = AN/VBr optimized (cf. Sea. 5)...

In brittle materials such as carbon fibers, tensile strength (a") is controlled by defects introduced in the materials during processing or/and handling. For a defect of size a, it is given by the Griffith relationship ... 

Can new PAN precursors be developed which are totally free from Na, cheaper and have definitive rate determining stages However, such introductions would involve expensive re-qualification procedures. The reduction of flaws by more effective dope filtration would improve the attainable carbon fiber tensile strength, but there is probably no more latitude for improvement via this route. 

Figure 8. Dependence of PAN-based carbon fiber tensile strength and modulus E on carbonization temperature. (Reprinted with permission from Ref. 21. Copyright 1985 Elsevier.)...

Soltes and Abbot of USA during 1955 developed processes for converting both natural cellulose and rayon into fibrous carbon. Essentially the carbon fibers were produced by heat-treating the precursors to temperatures about 1,000C (1,832F) in inert atmosphere. Fiber tensile strengths were as high as 40 ksi (275 MPa). 

Figure 5.58 Fiber tensile strength as a function of heat treatment, measured by single filament test with 30 mm gage length at a crosshead speed of 1 mm min Source Reprinted with permission from Fitzer E, Frohs W, The influence of carbonization and post heat treatment conditions on the properties of PAN-based carbon fibres, Presented at Carbon 88, Newcastle upon Tyne, 298-300, 1988. Copyright 1988, The Insitute of Physics Publishing.

Adherent Technologies Inc. [8] has developed a process for the reclamation of carbon fibers from carbon/epoxy composites. It has studied the depolymerization of thermoset carbon fiber reinforced epoxy matrix composites using a low temperature (20 min at 325°Q catalytic tertiary recycling reclamation process and has been able to obtain a product with 99.8% carbon and 0.2% residual resin, with only a loss of about 8.6% in fiber tensile strength. The process can be economically viable, provided sufficient scrap feedstock is available. Possible applications for the recovered fiber include thermoplastic and thermoset molding compounds. 

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. 

In order to achieve the desired fiber properties, the two monomers were copolymerized so the final product was a block copolymer of the ABA type, where A was pure polyglycoHde and B, a random copolymer of mostly poly (trimethylene carbonate). The selected composition was about 30—40% poly (trimethylene carbonate). This suture reportedly has exceUent flexibiHty and superior in vivo tensile strength retention compared to polyglycoHde. It has been absorbed without adverse reaction ia about seven months (43). MetaboHsm studies show that the route of excretion for the trimethylene carbonate moiety is somewhat different from the glycolate moiety. Most of the glycolate is excreted by urine whereas most of the carbonate is excreted by expired CO2 and uriae. 

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. 

Carbon fibers are generally typed by precursor such as PAN, pitch, or rayon and classified by tensile modulus and strength. Tensile modulus classes range from low (<240 GPa), to standard (240 GPa), intermediate (280—300 GPa), high (350—500 GPa), and ultrahigh (500—1000 GPa). Typical mechanical and physical properties of commercially available carbon fibers are presented in Table 1. 

Sihcon carbide fibers exhibit high temperature stabiUty and, therefore, find use as reinforcements in certain metal matrix composites (24). SiUcon fibers have also been considered for use with high temperature polymeric matrices, such as phenoHc resins, capable of operating at temperatures up to 300°C. Sihcon carbide fibers can be made in a number of ways, for example, by vapor deposition on carbon fibers. The fibers manufactured in this way have large diameters (up to 150 P-m), and relatively high Young s modulus and tensile strength, typically as much as 430 GPa (6.2 x 10 psi) and 3.5 GPa (507,500 psi), respectively (24,34) (see Refractory fibers). 

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... 

Alcaniz-Monge, J., Cazorla-Amoros, D., Linares-Solano, A., Yoshida, S. and Oya, A., Effect of the activating gas on tensile strength and pore structure of pitch-based carbon fibers. Carbon, 1994, 32(7), 1277 1283. 

In addition to their exceptional tensile strengths, PAN-based carbon fibers are far more resistant to compressive failure than are their pitch-based counterparts or polymeric high-performance fibers. However, because the PAN precursor is not... 

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. 

Fig. 9. Tensile strength versus modulus for some commercial carbon fibers (adapted from [57]).

Applied Sciences, Inc. has, in the past few years, used the fixed catalyst fiber to fabricate and analyze VGCF-reinforced composites which could be candidate materials for thermal management substrates in high density, high power electronic devices and space power system radiator fins and high performance applications such as plasma facing components in experimental nuclear fusion reactors. These composites include carbon/carbon (CC) composites, polymer matrix composites, and metal matrix composites (MMC). Measurements have been made of thermal conductivity, coefficient of thermal expansion (CTE), tensile strength, and tensile modulus. Representative results are described below. 

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