Stiffness glass/epoxy composite

Advanced composites have been used most extensively in helicopters. Sikorsky s S-75 helicopter, for example, is about 25% composite by weight, mostly graphite-epoxy and aramid-epoxy composite materials. Composites are used in rotors, blades, and tail assemblies. Future military helicopters are likely to comprise up to 80% advanced composites by structural weight. Graphite-epoxy composites are likely to be used in the airframe, bulk-heads, tail bones, and vertical fins, while the less stiff glass-epoxy composites will be used in rotor systems. 

Several experiments will now be described from which the foregoing basic stiffness and strength information can be obtained. For many, but not all, composite materials, the stress-strain behavior is linear from zero load to the ultimate or fracture load. Such linear behavior is typical for glass-epoxy composite materials and is quite reasonable for boron-epoxy and graphite-epoxy composite materials except for the shear behavior that is very nonlinear to fracture. 

Studies with the hybrid pultrusion of glass and carbon at the University of Mississippi have shown that the flexural stiffness of a glass/epoxy composite could be significantly improved by the addition of carbon fiber, symmetrically distributed at the outer surfaces of the composite beams, but perhaps this is not surprising, since the glass occupies the neutral axis. 

Low-cost and soft materials are used for low volume or prototype manufacturing, or when the dimensional accuraqr and surface roughness are not critical. Shell composite molds (glass fiber reinforced polyester, vinly ester or epoxy) with an epoxy gel-coated inner surface are commonly used for these cases. To increase the mold stiffness, the shell composite may be supported by a steel frame. 

Another approach employing concepts of continuum damage mechanics (e.g., Lemaitre and Chaboche 1994) was presented by Perreux and Suri (1997). Studying the effects of cyclic fatigue on [ 55°]3 glass fiber/poly epoxy composite pipes they noticed that in the absence of stress moisture uptake data could be modeled as a process of a two-phase diffusion (see Sect. 3.6). Damage in the axial direction was related to the decrease in the axial stiffness, namely and its effect on... 

Most successful composites combine the stiffness and hardness of a ceramic (like glass, carbon, or tungsten carbide) with the ductility and toughness of a polymer (like epoxy) or a metal (like cobalt). You will find all you need to know about them in Chapter 25. 

Polymer networks such as epoxies play an increasing role as adhesives in industry. Two properties are of special importance for their application (a) a strong adhesive bond is required between the solidified adhesive and the bonded object, which is often a metal (b) the mechanical stiffness of the adhesive has to be adapted to the desired level. As a consequence, the adhesive has to be selected according to its adhesion properties as well as its mechanical properties. Several studies have shown that both properties are linked as soon as the epoxy polymer layer is sufficiently thin the contact of the polymer with the substrate may induce in the polymer a broad interphase where the morphology is different from the bulk. Roche et al. indirectly deduced such interphases, for example from the dependence of the glass transition temperature on the thickness of the polymer bonded to a metal substrate [1]. Moreover, secondary-ion mass spectroscopy or Auger spectroscopy provided depth profiles of interphases in terms of chemical composition, which showed chemical variations at up to 1 pm distance from the substrate. 

Carbon itself has been successfully used as a biomaterial. Carbon based fibers used in composites are known to be inert in aqueous (even seawater) environments, however they do not have a track record in the biomaterials setting. In vitro studies by Kovacs [1993] disclose substantial electrochemical activity of carbon fiber composites in an aqueous environment. If such composites are placed near a metaUic implant, galvanic corrosion is a possibility. Composite materials with a polymer matrix absorb water when placed in a hydrated environment such as the body. Moisture acts as a plasticizer of the matrix and shifts the glass transition temperature towards lower values [Delasi and Whiteside, 1978], hence a reduction in stiffness and an increase in mechanical damping. Water immersion of a graphite epoxy... 

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