Page 110 - Mechanical Behavior of Materials

Section 3.9 Summary 111

If cost is indeed important, the previous choice of graphite–epoxy composite would probably
have to be eliminated as it is the most costly. Wood is now the highest ranking material, and mild
steel is the second highest. If both light weight and cost are important, then wood is the clear
choice. If wood is unsuitable for some reason, then either glass–epoxy composite or an aluminum
alloy might be chosen as representing a reasonable compromise.

3.9 SUMMARY
A number of metals have combinations of properties and availability that result in their use as load-
resisting engineering materials. These include irons and steels, and aluminum, titanium, copper, and
magnesium. Pure metals in bulk form yield at quite low stresses, but useful levels of strength can
be obtained by introducing obstacles to dislocation motion through such means as cold work, solid-
solution strengthening, precipitation hardening, and the introduction of multiple phases. Alloying
with various amounts of one or more additional metals or nonmetals is usually needed to achieve
this strengthening and to otherwise tailor the properties to obtain a useful engineering metal.
In steels, small amounts of carbon and other elements in solid solution provide some
strengthening without heat treatment. For carbon contents above about 0.3%, substantially greater
strengthening can be obtained from heat treating by the quenching and tempering process. Small
percentages of alloying elements, such as Ni, Cr, and Mo, enhance the strengthening effect. Special
steels, such as stainless steels and tool steels, typically include fairly substantial percentages of
various alloying elements.
Considering aluminum alloys, the highest strengths in this lightweight metal are obtained by
alloying and heat treatment that causes precipitation hardening (aging) to be effective. Magnesium
is strengthened in a similar manner and is noteworthy as being the lightest engineering metal.
Titanium alloys are somewhat heavier than aluminum, but have greater temperature resistance.
They are strengthened by a combination of the various methods, including multiple phase effects in
the alpha–beta alloys. Superalloys are corrosion- and temperature-resistant metals that have large
percentages of two or more of the metals nickel, cobalt, and iron.
Polymers have long chainlike molecules, or a network structure, based on carbon. Compared
to metals, they lack strength, stiffness, and temperature resistance. However, these disadvantages
are offset to an extent by light weight and corrosion resistance, leading to their use in numerous
low-stress applications. Polymers are classified as thermoplastics if they can be repeatedly melted
and solidified. Some examples of thermoplastics are polyethylene, polymethyl methacrylate, and
nylon. Contrasting behavior occurs for thermosetting plastics, which change chemically during
processing and thereafter cannot be melted. Examples include phenolics and epoxies. Elastomers,
such as natural and synthetic rubbers, are distinguished by being capable of deformations of at least
100% to 200%, and of then recovering most of this deformation after removal of the stress.
A given thermoplastic is usually glassy and brittle below its glass transition temperature, T g .
Above the T g of a given polymer, the stiffness (E) is likely to be very low unless the material has
a substantially crystalline structure. Stiffness and strength in polymers is also enhanced by longer
lengths of the chain molecules, by chain branching in amorphous polymers, and by cross-linking
between chains. Thermosetting plastics have a molecular structure that causes a large number of