Page 85 - Mechanical Behavior of Materials

84 Chapter 3 A Survey of Engineering Materials

This accounts for their use in high-temperature applications, despite the high cost due to the
relative scarcity of nickel, chromium, and cobalt. Superalloys are often produced in wrought form,
and Ni-base and Co-base alloys are also often cast. Strengthening is primarily by solid-solution
effects and by various heat treatments, resulting in precipitation of intermetallic compounds or metal
carbides.

3.5 POLYMERS

Polymers are materials consisting of long-chain molecules formed primarily by carbon-to-carbon
bonds. Examples include all materials commonly referred to as plastics, most familiar natural
and synthetic fibers, rubbers, and cellulose and lignin in wood. Polymers that are produced or
modified by man for use as engineering materials can be classified into three groups: thermoplastics,
thermosetting plastics, and elastomers.
When heated, a thermoplastic softens and usually melts; then, if cooled, it returns to its original
solid condition. The process can be repeated a number of times. However, a thermosetting plastic
changes chemically during processing, which is often done at elevated temperature. It will not melt
upon reheating, but will instead decompose, as by charring or burning. Elastomers are distinguished
from plastics by being capable of rubbery behavior. In particular, they can be deformed by large
amounts, say 100% to 200% strain or more, with most of this deformation being recovered after
removal of the stress. Examples of polymers in each of these groups are listed, along with typical
uses, in Table 3.8.
After chemical synthesis based primarily on petroleum products, polymers are made into useful
shapes by various molding and extrusion processes, two of which are illustrated in Fig. 3.11. For
thermosetting plastics and elastomers that behave in a similar manner, the final stage of chemical
reaction is often accomplished by the application of temperature and/or pressure, and this must
occur while the material is being molded into its final shape.
Polymers are named according to the conventions of organic chemistry. These sometimes
lengthy names are often abbreviated by acronyms, such as PMMA for polymethyl methacrylate.
In addition, various trade names and popular names, such as Plexiglas, Teflon, and nylon, are often
used in addition to, or in place of, the chemical names.
An important characteristic of polymers is their light weight. Most have a mass density similar
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to that of water, around ρ = 1 g/cm , and few exceed ρ = 2 g/cm . Hence, polymers are typically half
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as heavy as aluminum (ρ = 2.7 g/cm ) and much lighter than steel (ρ ≈ 7.9 g/cm ). Most polymers
in unmodified form are relatively weak, with ultimate tensile strengths typically in the range 10 to
200 MPa.
In the discussion that follows, we first consider the basic molecular structure of typical polymers
in each group. This provides the background for later discussion of how the details of molecular
structure affect the mechanical properties.

3.5.1 Molecular Structure of Thermoplastics
Many thermoplastics have a molecular structure related to that of the hydrocarbon gas ethylene,
C 2 H 4 . In particular, the repeating unit in the chain molecule is similar to an ethylene molecule,
except that the carbon-to-carbon bond is rearranged as illustrated previously in Fig. 2.6. The