Page 99 - Mechanical Behavior of Materials

100 Chapter 3 A Survey of Engineering Materials

Figure 3.23 Composites reinforced by (a) particles, (b) chopped fibers or whiskers, and
(c) continuous fibers. (Adapted from [Budinski 96] p. 121; c 1996 by Prentice Hall, Upper
Saddle River, NJ; reprinted with permission.)

3.7 COMPOSITE MATERIALS

A composite material is made by combining two or more materials that are mutually insoluble by
mixing or bonding them in such a way that each maintains its integrity. Some composites have
already been discussed, namely, plastics modified by adding rubber particles, plastics reinforced by
chopped glass fibers, cemented carbides, and concrete. These and many other composite materials
consist of a matrix of one material that surrounds particles or fibers of a second material, as shown
in Fig. 3.23. Some composites involve layers of different materials, and the individual layers
may themselves be composites. Materials that are melted (alloyed) together are not considered
composites, even if a two-phase structure results, nor are solid solutions or precipitate structures
arising from solid solutions. Some representative types and examples of composite materials and
their uses are listed in Table 3.12.
Materials of biological origin are usually composites. Wood contains cellulose fibers sur-
rounded by lignin and hemicellulose, all of which are polymers. Bone is composed of the
fibrous protein collagen in a ceramic-like matrix of the crystalline mineral hydroxylapatite,
Ca 5 (PO 4 ) 3 OH.
Composite materials have a wide range of uses, and their use is rapidly increasing. Man-made
composites can be tailored to meet special needs such as high strength and stiffness combined
with light weight. The resulting high-performance (and expensive) materials are increasingly being
used in aircraft, space, and defense applications, and also for high-grade sports equipment, as in
golf club shafts and fishing rods. More economical composites, such as glass-reinforced plastics,
are continually finding new uses in a wide range of products, such as automotive components,
boat hulls, sports equipment, and furniture. Wood and concrete, of course, continue to be major
construction materials, and new composites involving these and other materials have also come into
recent use in the construction industry.
Various classes of composite materials will now be discussed.

3.7.1 Particulate Composites
Particles can have various effects on a matrix material, depending on the properties of the
two constituents. Ductile particles added to a brittle matrix increase the toughness, as cracks
have difficulty passing through the particles. An example is rubber-modified polystyrene, the