Page 103 - Mechanical Behavior of Materials

104 Chapter 3 A Survey of Engineering Materials

Figure 3.27 Microstructure of a graphite reinforced polymer composite, showing fibers
normal to the sectioned surface, and others approximately parallel, as imaged by the
Nomarski DIC technique. The matrix is a thermosetting polymer with a toughening agent.
(Photo courtesy of George F. Vander Voort, Vander Voort Consulting, Wadsworth, IL; used
with permission.)
strength-to-weight ratio and the stiffness-to-weight ratio. On this basis, high-performance fibrous
composites are superior to structural metals in both strength and stiffness. This is illustrated for
strength by Fig. 3.28(b).
Due to the limitations of the matrix, polymer matrix composites have limited resistance to
high temperature. Composites with an aluminum or titanium matrix have reasonable temperature
resistance. These metals are sometimes used with continuous straight fibers of silicon carbide of
fairly large diameter, around 140 μm. Other fiber types and configurations are also used.
For high-temperature applications, ceramic matrix composites have been developed. These
materials have a matrix that is already strong and stiff, but which is brittle and has a low fracture
toughness. Whiskers or fibers of another ceramic can act to retard cracking by bridging across small
cracks that exist and holding them closed so that their growth is retarded. For example, whiskers of
SiC in a matrix of Al 2 O 3 are used in this manner. Continuous fibers can also be used, such as SiC
fibers in a matrix of Si 3 N 4 . Some intermetallic compounds, such as Ti 3 Al and NiAl, have ceramic-
like properties, but also a helpful degree of ductility at high temperature that encourages their use
as matrix materials for temperature-resistant composites.

3.7.3 Laminated Composites
A material made by combining layers is called a laminate. The layers may differ as to the fiber
orientation, or they may consist of different materials. Plywood is a familiar example of a laminate,
the layers differing as to grain direction and perhaps also as to type of wood. As already noted,
unidirectional composite sheets are frequently laminated, as in Fig. 3.26(a). Aramid–aluminum