Composite fibers

Composites may also be examined by transmission EM methods, such as by bright field, dark field and electron diffraction of ultrathin sections. Sections of carbon fiber composites are quite difficult to obtain, but the technique is possible and has been described by Oberlin [246,247]. Important information that can be obtained relates to the fiber-resin interface which is known to be critical to composite properties and is often adversely affected by environmental conditions. 

The strength of a short fiber reinforced composite depends upon the fiber length and orien- 

Contact microradiography is a method that permits assessment of the length and orientation of glass fibers in composites, although it cannot be used for aramid or graphite fibers. Sections are cut 50 150 L(m thick with a low speed diamond 

Scanning electron microscopy is the most widely used imaging technique for the study of both short and continuous fiber composites. Fracture 

The microscopy techniques described for evaluation of glass fiber composites are widely used to determine the microstructure of carbon and graphite fiber composites. Microscopy of crack propagation in carbon fiber reinforced composites is also very important in understanding 

Matrix Fiber Fabrication method and conditions E GPa Flexure strength MPa MPami Investigator 

AI0O3 (dry mixed with C (18volZ chopped, 1 5 HP 1600 C (20 hrs) - Yoshikawa, Asaeda 

Figur 16i Shapes niade of continuous fiber ccanposlte utilizing glass-based matrices. Photos courtesy of Drs Karl Preuo and John Brennan, United 

An Important factor In obtaining the highest strength and toughness 1n many of these composites has been the use of CVD fiber coatings, initially 172-176,177) - 7j later multilayer coatiogs. In 

Almost all of the composites described in the references above have been hot pressed from bodies in which the tows were infiltrated wiUi a slurry of the powder to produce the matrix. Some have also been processed with sol- 

Waddon et al [315] have extensively studied the crystal texture of PEEK, PEK and PPS, including melt grown spherulites using polarized light microscopy. They formed thin films by heating the polymer and the fibers on microscope slides and pressing them or smearing between the slides and coverslips. Polymers were chosen 

The microscopy techniques described for the evaluation of glass fiber composites are widely used to determine the microstructure of carbon and graphite fiber composites. Microscopy of crack propagation in carbon fiber reinforced composites is also very important in understanding mechanical properties. Test specimens and actual composite products are often evaluated to determine the distribution of the fibers in the resin, typically epoxy, and the degree of resin wetting of the fibers. Voids in the composite can be the locus of failure, and their identification and cause are quite important to mechanical property evaluation. 

An example of reflected light microscopy of a polished specimen is shown in Fig. 5.66 which elucidates the complex engineering involved in 

Hybrid composites are formed from a mixture of fibers and a bonding matrix [327], and these include carbon-glass and carbon-aramid fibers. Hybrid composites have economic, physical and mechanical property advantages that are 

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J. Dehnonte, Technology of Carbon and Graphite Fiber Composites, Van Nostrand Reinhold Company, New York, 1981, p. 398. 

Poly(ethylene terephthalate), the predominant commercial polyester, has been sold under trademark names including Dacron (Du Pont), Terylene (ICI), Eortrel (Wellman), Trevira (Hoechst-Celanese), and others (17). Other commercially produced homopolyester textile fiber compositions iaclude p oly (1,4-cyc1 oh exa n e- dim ethyl en e terephthalate) [24936-69-4] (Kodel II, Eastman), poly(butylene terephthalate) [26062-94-2] (PBT) (Trevira, Hoechst-Celanese), and poly(ethylene 4-oxyben2oate) [25248-22-0] (A-Tell, Unitika). Other polyester homopolymer fibers available for specialty uses iaclude polyglycoHde [26124-68-5] polypivalolactone [24937-51-7] and polylactide [26100-51-6],... 

In the late 1980s, new fully aromatic polyester fibers were iatroduced for use ia composites and stmctural materials (18,19). In general, these materials are thermotropic Hquid crystal polymers that are melt-processible to give fibers with tensile properties and temperature resistance considerably higher than conventional polyester textile fibers. Vectran (Hoechst-Celanese and Kuraray) is a thermotropic Hquid crystal aromatic copolyester fiber composed of -hydroxyben2oic acid [99-96-7] and 6-hydroxy-2-naphthoic acid. Other fully aromatic polyester fiber composites have been iatroduced under various tradenames (19). 

Hafnium tetrafluoride [13709-52-9] is one component in the cladding layer of a proposed zirconium fluoride glass optical waveguide fiber composition which is expected to have a lower intrinsic light absorption than fused quart2 optical fiber (see Glass Fiber optics Fluorine compounds, inorganic-zirconium). 

Composites. Various composite materials have evolved over the years as a significant class of high performance textile products. The prototype composite is carbon fiber with an epoxy resin matrix for stmctural akcraft components and other aerospace and military appHcations. Carbon fiber composites ate also used in various leisure and spotting items such as golf clubs, tennis rackets, and lightweight bicycle frames. However, other types of appHcations and composites ate also entering the marketplace. For example, short ceUulose fiber/mbbet composites ate used for hoses, belting, and pneumatic tire components. 

Carbon-Fiber Composites. Cured laminates of phenoHc resins and carbon-fiber reinforcement provide superior flammabiHty resistance and thermal resistance compared to unsaturated polyester and epoxy. Table 15 shows the dependence of flexural strength and modulus on phenoHc—carbon-fiber composites at 30—40% phenoHc resin (91). These composites also exhibit long-term elevated temperature stabiHty up to 230°C. 

Table 15. Strength Properties of Phenolic—Carbon-Fiber Composites...

Carbon—Carbon Composites. Above 300°C, even such polymers as phenoHcs and polyimides are not stable as binders for carbon-fiber composites. Carbon—carbon composites are used at elevated temperatures and are prepared by impregnating the fibers with pitch or synthetic resin, foUowed by carbonization, further impregnation, and pyrolysis (91). 

The water hberated during the cure has no apparent effect on the composite properties. Glass-filled composites prepared in this manner retain mechanical properties at elevated temperatures as well as solvent and flammabiUty resistance (88). PhenoHc-graphite-fiber composites that exhibit superior mechanical properties have also been prepared by this process. 

Sophisticated stmctural analysis techniques make it possible to determine both the amount and exact orientation of reinforcement that the product wQl need to meet the critical stresses in actual service. Hybrid reinforcement systems containing different fiber compositions with different properties are being increasingly used. For example, hybrid carbon and glass fiber automotive drive shafts are in commercial use. 

If this unit of twist measurement is substituted for the horizontal axis in Figure 1, then it is possible to determine the optimum twist levels for maximum yam strength for any size yam of a given fiber composition. 

Table 2. Effect of Peak Carbonization Temperature on PAN Carbon Fiber Composition, wt %...

Whisker and Short Fiber Composites. Whiskers and short fibers tend to align during forming, lea ding to anisotropic properties and large... 

Fig. 5. Interlaminar fracture toughness, for a number of thermosetting and thermoplastic composites (36,37). Open white bars represent glass-fiber composites shaded bars are for carbon fibers. The materials are A, polyester (unidirectional) B, vinyl ester (CSM = chopped strand mat) C, epoxy (R/BR1424) D, epoxy (T300/914) E, PPS F, PES and G, PEEK. To convert J/m to fdbf/in. multiply by 2100.

PMR-15—carbon fiber composites include jet-engine cowlings, ducts, compressor blades, and flaps and fairings (24,38). 

A number of amorphous thermoplastics are presently employed as matrices in long fiber composites, including polyethersulfone (PES), polysulfone (PSU), and polyetherimide (PEI). AH offer superior resistance to impact loading and higher interlaminar fracture toughnesses than do most epoxies. However, the amorphous nature of such polymers results in a lower solvent resistance, clearly a limitation if composites based on such polymers are to be used in aggressive environments. 

Fig. 7. Micrographic cross section of an autoclaved carbon fiber composite.

J. Aveston, G. A. Cooper, and A. Kelly, ia The Properties of Fiber Composites, IPC Science Technology Press, The National Physical Laboratory, 1971, pp. 15-26. 

Chung, D. D. L., Carbon Fiber Composites, Butterworth-Heinemann, Boston, 1994. 

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