Epoxy composites, graphite-reinforced

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. 

Bradley W.L. and Cohen R.N. (1987). Matrix deformation and fracture in graphite-reinforced epoxies. In Toughened Composites, ASTM STP 937 (N.J. Johnston ed.), ASTM, Philadelphia, PA, pp. 389-410. 

Consider the same unidirectional lamina with the stresses now applied perpendicular to the fiber axis as shown in Fig. 12. The local stress at the fiber matrix interface can be calculated and compared to the nominally applied stress on the whole lamina to give K, the stress concentration factor. The plot of the results of this analysis shows that the interfacial stresses at the point of maximum principal stress can range up to 2.6 times the applied stress depending on the moduli of the constituents and the volume fraction of the reinforcement. For a typical graphite-epoxy composite, with a modulus ratio of 70 and a volume fraction of 70 % the stress concentration factor at the interface is about 2.4. That is, the local stresses at the interface are a factor of 2.4 times greater than the applied stress. 

Like other composite properties, the wear resistance is influenced by the orientation of reinforcing fibres. Sung and Suh found that with biaxially-oriented glass fibre and molybdenum disulphide in a PTFE matrix (Duroid 5813), the wear resistance was greatest when the highest proportion of fibre was normal to the sliding surface. The same effect was found with a graphite fibre/epoxy composite and a Kevlar fibre/epoxy composite. 

Epoxy resin systems with fiber reinforcements are called "reinforced systems" or "composites." Composites are made by impregnating reinforcing fibers such as glass, synthetic polymer, or graphite fibers, by one of several processes with the desired epoxy resin system, and then curing in a heated mold or die. Epoxy composite systems are formulated with either liquid or solid resins with selection of the type of system dependent on the fabrication process, the cure temperature, and the final part application. 

Polymer composite materials are substances that have a polymer matrix with reinforcing fibers. Glass-fiber-reinforced polyesters are used for boat hulls and car body panels. Graphite fiber-epoxy composites are used in fishing rods and tennis racquets. 

Broughton WR, Kumosa M, Hull D, Analysis of the losipescu shear test as applied to unidirectional carbon-fibre reinforced composites. Composite Sci Technol, 38(4), 299-325, 1990. Browning CE, Abrams FL, Whitney JM, A four point shear test for graphite/epoxy composites. Browning CE ed.. Composite Materials, Quality Assurance and Processing, ASTM STP 797, American Society for Testing and Materials, Philadelphia, 54- 74, 1983. 

Jarmon, D.C., Weeks, C.A., Naik, R.A., Kogstrom, C.L., Logan, C.P., Braun, P.F., 1998. Mechanical property comparison of 3-D and 2-D graphite reinforced epoxy composites fabricated by resin transfer molding. In Proceedings of 43rd International SAMPE Symposium, Anaheim, CA, May 31-June 4, 1998. 

Important fiber materials are surface-treated glass, boron, graphite (carbon), and aromatic polyaramides (eg, DuPont s Kevlar). In most composites the reinforcement constitutes ca 65% of the final mass. Orientation of the fibers is important in establishing the properties of the laminate. Unidirectional, bidirectional, and random orientations are possible. The characteristics of the cured resin system are extremely important since it must transmit the applied stresses to each fiber. A critical region in a composite is the resin-fiber interface. The adhesive properties of epoxy resins make them especially suited for composite applications. 

Tracking resistance measurements have been reported on graphite-reinforced epoxy composites [17]. 

Today, thermosets such as epoxy resins are frequently reinforced with either glass fibers or carbon fibers (also known as graphite fibers) (8,9). As an example calculation, the modulus of a unidirectional glass fiber-epoxy composite will be calculated in both the longitudinal and transverse directions. The properties of the materials are... 

Loos AC, Springer GS (1979) Moisture absorption of graphite-epoxy composites immersed in liquids and in humid air. J Compos Mater 13(2) 131-147 Lou A, Murtha T (1988) Enviromnental effects on glass fiber reinforced PPS stampable composites. J Mater Eng 10(2) 109-116... 

Hamada H, Maekawa Z,MoriiT,Gotoh A, TanimotoT (1991) Durability of adhesive bonded FRP joints iittmmsed in hot water. In Cardon AH, Verchery G (eds) Durability of polymer based composite systems for structural applications, vol 46. Elsevier, Amsterdam, pp 418-427 Haque A, Mahmood S, Walker L, Jeelani S (1991) Moisture and temperature induced degradation in tmrsile properties of kevlar-graphite/epoxy hybrid composites. J Reinforced Plast Compos 10(2) 132-145... 

He showed how the use of double logarithmic plots of the recovery strain against time, obtained for different stress levels, could be related to one another by shift factors the shift factors could then be simply related to gi and Oa. The technique of step loading combined with Equation (11.23) has also been used by Crook [45] and Lai and Bakker [46]. Schapery s model has been applied to nitrocellulose, fibre-reinforced phenolic resin and polyisobutylene [44] polycarbonate [45] high-density polyethylene [46] and graphite-epoxy composites [10]. 

---