Page 111 - Mechanical Behavior of Materials

112 Chapter 3 A Survey of Engineering Materials

cross-links, or a network structure, to form during processing. Once these covalent bonds are
formed, the material cannot later be melted—this explains the thermosetting behavior. Vulcanizing
of rubber is also a thermosetting process, in which case sulfur atoms form bonds that link chain
molecules.
Ceramics are nonmetallic and inorganic crystalline solids that are generally chemical com-
pounds. Clay products, porcelain, natural stone, and concrete are fairly complex combinations of
crystalline phases, primarily silica (SiO 2 ) and metal oxides, and CaCO 3 in the case of some natural
stones, bound together by various means. High-strength engineering ceramics tend to be fairly
simple chemical compounds, such as metal oxides, carbides, or nitrides. Cermets, such as cemented
carbides, are ceramic materials sintered with a metal phase that acts as a binder. Glasses are amor-
phous (noncrystalline) materials consisting of SiO 2 combined with varying amounts of metal oxides.
All ceramics and glasses tend to be brittle, compared with metals. However, many have
advantages, such as light weight, high stiffness, high compressive strength, and temperature
resistance, that cause them to be the most suitable materials in certain situations.
Composites are combinations of two or more materials, with one generally acting as a matrix
and the other as reinforcement. The reinforcement may be in the form of particles, short fibers,
or continuous fibers. Composites include many common man-made materials, such as concrete,
cemented carbides, and fiberglass, and other reinforced plastics, as well as biological materials,
notably wood and bone. High-performance composites, as used in aerospace applications, generally
employ high-strength fibers in a ductile matrix. The fibers are often a ceramic or glass, and the
matrix is typically a polymer or a lightweight metal. However, even a ceramic matrix is made
stronger and less brittle by the presence of reinforcing fibers.
It is often useful to combine layers to make a laminated composite. The layers may differ as to
fiber direction, or they may consist of more than one type of material, or both. High-performance
composite laminates may be advantageous for use in situations such as aerospace structure, as their
strength and stiffness are both quite high compared with those of metals on a unit-weight basis.
Materials selection for engineering design requires an understanding of materials and their
behavior, and also detailed information as found in handbooks or provided by materials suppliers.
Systematic analysis as described in Section 3.8 may be useful.
The survey of engineering materials given in this chapter should be considered to be only a
summary. Numerous sources of more detailed information exist, some of which are given in the
References section of this chapter. Companies that supply materials are also often a useful source
of information on their particular products.

NEW TERMS AND SYMBOLS

alpha–beta titanium alloy ceramic
annealing cermet
austenite coherent precipitate
casting cold work
cast iron composite material
cemented carbide copolymer
cementite, Fe 3 C cross-linking