Graphite fiber reinforced composites structural applications

Fiber-reinforced composite materials such as boron-epoxy and graphite-epoxy are usually treated as linear elastic materials because the essentially linear elastic fibers provide the majority of the strength and stiffness. Refinement of that approximation requires consideration of some form of plasticity, viscoelasticity, or both (viscoplasticity). Very little work has been done to implement those models or idealizations of composite material behavior in structural applications. 

Samples were cut from 3-mm-thick sheets of the following 15-ply uniaxial fiber-reinforced composites 0.14-mm boron/6061 aluminum S-901 glass/NASA Resin 2 type AS graphite/NASA Resin 2 and 0.14-mm boron/5505 epoxy. The mechanical properties of these materials, which had been selected by the National Bureau of Standards, Boulder, Colorado, as candidates for low-temperature structural applications, have been discussed in detail by Schramm and Kasen [ ]. NBS supplied the samples for the thermal expansion measurement [" ] and calorimetric studies conducted at Battelle. 

The second stage involves heating the fibers for a further period at 1770 K to eliminate all elements other than carbon. This carbonization is believed to involve cross-linking of the chains to form the hexagonal graphite structure, and this final heat treatment can affect the mechanical properties to a marked extent, as shown in Figure 15.17. The major application so far is in composite structures where they act as extremely effective reinforcing fibers. These reinforced plastic composites find uses in the aircraft industry, in the small-boat trade, and as ablative composites. 

Boron/tungsten fiber applications include the use of filaments and of boron/tungsten fiber reinforced prepreg tape, aluminum matrix composites, and boron/graphite structures. The major applications for these structures are found in the aerospace market and about 25% in sporting goods markets [36]. SiC/carbon fiber reinforced products include aluminum, titanium, and ceramic matrix composites. Major applications for these structures are also found in the aerospace market, minor uses in the industrial market [37]. 

It has been shown that substituted boron atoms in the carbon lattice can accelerate the graphitization and suppress the oxidation of carbon materials [149-152]. Furtha-, electromagnetic properties of graphite are reported to be strongly affected by a small amount of substituted boron [153]. An example of application is the use of boron-doped carbon fibers, BCx, as a reinforcement of C-C composites in the aerospace industry, as an alternative to C-C structural materials that oxidizes at temperatures in excess of673 K [154—155]. 

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