Page 62 - Mechanical Behavior of Materials

Section 2.6 Summary 61

grain boundaries and the formation of cavities along grain boundaries. Creep behavior in crystalline
materials is strongly temperature dependent, typically becoming an important engineering consid-
eration around 0.3 to 0.6T m , where T m is the absolute melting temperature.
Different creep mechanisms operate in amorphous (noncrystalline) glasses and in polymers.
One of these is viscous flow in the manner of a very thick liquid. This occurs in polymers at
temperatures substantially above the glass transition temperature T g and approaching T m .The
chainlike molecules simply slide past one another in a time-dependent manner. Around and below
T g , more complex behavior involving segments of chains and obstacles to chain sliding become
important. In this case, much of the creep deformation may disappear slowly (recover) with time
after removal of an applied stress, as illustrated in Fig. 2.25. Creep is a major limitation on the
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engineering application of any polymer above its T g , which is generally in the range −100 to 200 C
for common polymers.
Additional discussion on mechanisms of creep deformation is given in Chapter 15.

2.6 SUMMARY

Atoms and molecules in solids are held together by primary chemical bonds of three kinds: ionic,
covalent, and metallic. Secondary bonds, especially hydrogen bonds, also influence the behavior.
Covalent bonds are strong and directional and therefore resist deformation. This contributes to the
high strength and brittleness of ceramics and glasses, as these materials are bound by covalent or
mixed ionic–covalent bonds. Metallic bonds in metals do not have such a directionality and therefore
deform more easily. Polymers are composed of carbon-chain molecules formed by covalent bonds.
However, they may deform easily by relative sliding between the chain molecules where this is
prevented only by secondary bonds.
A variety of crystal structures exist in solid materials. Three of particular importance for metals
are the body-centered cubic (BCC), face-centered cubic (FCC), and hexagonal close-packed (HCP)
structures. The crystal structures of ceramics are often elaborations of these simple structures, but
greater complexity exists due to the necessity in these compounds of accommodating more than
one type of atom. Crystalline materials (metals and ceramics) are composed of aggregations of
small crystal grains. Numerous defects, such as vacancies, interstitials, and dislocations are usually
present in these grains.
Elastic deformation, caused by the stretching of chemical bonds, disappears if the stress is
removed. The elastic modulus E is therefore higher if the bonding is stronger and is highest in
covalent solids such as diamond. Metals have a value of E about 10 times lower than that for highly
covalent solids, and polymers have a value of E that is generally lower by an additional factor of 10
or more, due to the influence of the chain-molecule structure and secondary bonds. Above the glass-
transition temperature for a given polymer, E is further lowered by a large amount, then becoming
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smaller than for diamond by as much as a factor of 10 .
Estimates of the theoretical tensile strength to break chemical bonds in perfect crystals give
values on the order of E/10. However, strengths approaching such a high value are realized only in
tiny, perfect single crystals and in fine wires with an aligned structure. Strengths in large samples of
material are much lower, as these are weakened by defects. In ceramics, the defects of importance
are small cracks and pores that contribute to brittle behavior.