Page 101 - Mechanical Behavior of Materials

102 Chapter 3 A Survey of Engineering Materials

Figure 3.24 Fracture surface of polyphenylene oxide (PPO) modified with high-impact
polystyrene particles. (Photo courtesy of General Electric Co., Pittsfield, MA.)

amounts of oxide ceramics, such as ThO 2 ,Al 2 O 3 ,SiO 2 , and K 2 O, so that it has sufficient creep
resistance for use in lightbulb filaments. Note that particles introduced in this manner will not have
a coherent crystal structure with the parent material.

3.7.2 Fibrous Composites

Strong and stiff fibers can be made from ceramic materials that are difficult to use as structural
materials in bulk form, such as glass, graphite (carbon), boron, and silicon carbide (SiC). When
these are embedded in a matrix of a ductile material, such as a polymer or a metal, the resulting
composite can be strong, stiff, and tough. The fibers carry most of the stress, whereas the matrix
holds them in place. Fibers and matrix can be seen in the photograph of a broken open composite
material in Fig. 3.25. Good adhesion between fibers and matrix is important, as this allows the
matrix to carry the stress from one fiber to another where a fiber breaks or where one simply ends
because of its limited length. Fiber diameters are typically in the size range 1 to 100 μm.
Fibers are used in composites in a variety of different configurations, two of which are shown
in Fig. 3.23. Short, randomly oriented fibers result in a composite that has similar properties in all
directions. Chopped glass fibers used to reinforce thermoplastics are of this type. Whiskers are a
special class of short fiber that consist of tiny, elongated, single crystals that are very strong because
they are dislocation free. Diameters are 1 to 10 μm or smaller, and lengths are 10 to 100 times
larger than the diameter. For example, randomly oriented SiC whiskers can be used to strengthen
and stiffen aluminum alloys.
Long fibers can be woven into a cloth or made into a mat of intertwined strands. Glass fibers
in both of these configurations are used with polyester resins to make common fiberglass. High-
performance composites are often made by using long, straight, continuous fibers. Continuous fibers
all oriented in a single direction provide maximum strength and stiffness parallel to the fibers. Since
such a material is weak if stressed in the transverse direction, several thin layers with different fiber
orientations are usually stacked into a laminate, as shown in Fig. 3.26. For example, composites
with a thermosetting plastic matrix, often epoxy, are assembled in this manner by using partially