L7 composites - Materials Science and Engineering

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28 Intro Composite Applns. Properties Voigt, Reuss, Hill Anistrpy. CTE Cellular Matls. Wood Voigt, Reuss, Hill • These simple estimates of modulus have names associated with them. • The isostress approach is known as the Reuss modulus. • The isostrain approach is known as the Voigt modulus. • Hill proposed that a reasonable average of the two would be appropriate in materials where the loading is intermediate between the two extreme cases. Hence the average of the Isotress and Isostrain values is known as the Hill Average Modulus. • We can also treat the composite property (for elastic modulus) in terms of an arithmetic mean (isostrain) versus a harmonic mean (isostress), which is the reciprocal of the average of the reciprocal values. • Are there better estimates? Yes, look in the Supplemental slides for a description of the Hashin-Shtrikman estimates of modulus. There are also textbooks by Mura, Nemat-Nasser, Milton and several others.
29 Intro Composite Applns. Properties Voigt, Reuss, Hill Anistrpy. CTE Cellular Matls. Wood Fiber Composites • An important class of composites is that of fiber composites. • The materials involved may be metal, ceramic or polymer. Glass-fiber composite is typical in low-cost structures such as boat hulls. Carbon-fiber composites are used in higher performance structures such as airplanes where their higher cost is justified by the requirements. Ceramic composites are used typically for high temperature service, such as heat exchangers. • The basic idea is to take advantage of high strength and stiffness of the fibers and to obtain damage tolerance (and specific shapes) by embedding them in a suitable matrix. More specifically, the fiber material (e.g. graphite, glass) is a material that would not generally be considered to be a structural material. • Solid mechanics of fiber composites: the key to understanding the mechanical properties of fiber composites (for fibers whose length is short compared to the size of the component) is load transfer between the matrix and the fibers. This means that the stress on each fiber varies along its length. Also, the composite materials are strongly anisotropic (so tensors are useful again). See discussion in the supplemental slides. • Modern developments: carbon nanotubes offer exceptional stiffness and strength, not to mention interesting electrical properties in some cases. If we can figure out how to separate out the various different conformations and how to align the nanotubes, there should be a wide range of exciting materials possible.
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L7 composites - Materials Science and Engineering

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