Krishan K. Chawla - Composite Materials

Springer Science+Business Media New York 2012

Krishan K. Chawla 1

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It is a truism that technological development depends on advances in the field of materials. One does not have to be an expert to realize that the most advanced turbine or aircraft design is of no use if adequate materials to bear the service loads and conditions are not available. Whatever the field may be, the final limitation on advancement depends on materials. Composite materials in this regard represent nothing but a giant step in the ever-constant endeavor of optimization in materials.

Strictly speaking, the idea of composite materials is not a new or recent one. Nature is full of examples wherein the idea of composite materials is used. The coconut palm leaf, for example, is essentially a cantilever using the concept of fiber reinforcement. Wood is a fibrous composite: cellulose fibers in a lignin matrix. The cellulose fibers have high tensile strength but are very flexible (i.e., low stiffness), while the lignin matrix joins the fibers and furnishes the stiffness. Bone is yet another example of a natural composite that supports the weight of various members of the body. It consists of short and soft collagen fibers embedded in a mineral matrix called apatite. Weiner and Wagner () surveyed the design impact of composites on fighter aircraft. According to these authors, composites have introduced an extraordinary fluidity to design engineering, in effect forcing the designer-analyst to create a different material for each application as he pursues savings in weight and cost.

Yet another conspicuous development has been the integration of the materials science and engineering input with the manufacturing and design inputs at all levels, from conception to commissioning of an item, through the inspection during the lifetime, as well as failure analysis. More down-to-earth, however, is the fact that our society has become very energy conscious. This has led to an increasing demand for lightweight yet strong and stiff structures in all walks of life. And composite materials are increasingly providing the answers. Figure ). These fibers have been used for reinforcement of resin, metal, and ceramic matrices. Fiber reinforced composites have been more prominent than other types of composites for the simple reason that most materials are stronger and stiffer in the fibrous form than in any other form. By the same token, it must be recognized that a fibrous form results in reinforcement mainly in fiber direction. Transverse to the fiber direction, there is little or no reinforcement. Of course, one can arrange fibers in two-dimensional or even three-dimensional arrays, but this still does not gainsay the fact that one is not getting the full reinforcement effect in directions other than the fiber axis. Thus, if a less anisotropic behavior is the objective, then perhaps laminate or sandwich composites made of, say, two different materials would be more effective. A particle reinforced composite will also be reasonably isotropic. There may also be specific nonmechanical objectives for making a fibrous composite. For example, an abrasion- or corrosion-resistant surface would require the use of a laminate (sandwich) form, while in superconductors the problem of flux-pinning requires the use of extremely fine filaments embedded in a conductive matrix. In what follows, we discuss the various aspects of composites, mostly fiber reinforced composites, in greater detail, but first let us agree on an acceptable definition of a composite material. Practically everything in this world is a composite material. Thus, a common piece of metal is a composite (polycrystal) of many grains (or single crystals). Such a definition would make things quite unwieldy. Therefore, we must agree on an operational definition of composite material for our purposes in this text. We shall call a material that satisfies the following conditions a composite material:

Springer Science+Business Media New York 2012

Krishan K. Chawla 1

(1)

Reinforcements need not necessarily be in the form of long fibers. One can have them in the form of particles, flakes, whiskers, short fibers, continuous fibers, or sheets. It turns out that most reinforcements used in composites have a fibrous form because materials are stronger and stiffer in the fibrous form than in any other form. Specifically, in this category, we are most interested in the so-called advanced fibers, which possess very high strength and very high stiffness coupled with a very low density. The reader should realize that many naturally occurring fibers can be and are used in situations involving not very high stresses (Chawla ). In this chapter, we confine ourselves to a variety of man-made reinforcements. Glass fiber, in its various forms, has been the most common reinforcement for polymer matrices. Aramid fiber, launched in the 1960s, is much stiffer and lighter than glass fiber. Kevlar is Du Ponts trade name for aramid fiber while Twaron is the trade name of aramid fiber made by Teijin Aramid. Gel-spun polyethylene fiber, which has a stiffness comparable to that of aramid fiber, was commercialized in the 1980s. Other high-performance fibers that combine high strength with high stiffness are boron, silicon carbide, carbon, and alumina. These were all developed in the second part of the twentieth century. In particular, some ceramic fibers were developed in the last quarter of the twentieth century by some very novel processing techniques, namely, sol-gel processing and controlled pyrolysis of organic precursors.