Page 47 - Mechanical Behavior of Materials

46 Chapter 2 Structure and Deformation in Materials

Figure 2.8 Hydrogen-to-chlorine secondary bonds between chain molecules in
polyvinyl chloride.

In polymers, covalent bonds form the chain molecules and attach hydrogen and other atoms
to the carbon backbone. Hydrogen bonds and other secondary bonds occur between the chain
molecules and tend to prevent them from sliding past one another. This is illustrated in Fig. 2.8
for polyvinyl chloride. The relative weakness of the secondary bonds accounts for the low melting
temperatures, and the low strengths and stiffnesses, of these materials.

2.3 STRUCTURE IN CRYSTALLINE MATERIALS

Metals and ceramics are composed of aggregations of small grains, each of which is an individual
crystal. In contrast, glasses have an amorphous or noncrystalline structure. Polymers are composed
of chainlike molecules, which are sometimes arranged in regular arrays in a crystalline manner.

2.3.1 Basic Crystal Structures

The arrangement of atoms (or ions) in crystals can be described in terms of the smallest grouping
that can be considered to be a building block for a perfect crystal. Such a grouping, called a unit
cell, can be classified according to the lengths and angles involved. There are seven basic types of
◦
unit cell, three of which are shown in Fig. 2.9. If all three angles are 90 and all distances are the
same, the crystal is classed as cubic. But if one distance is not equal to the other two, the crystal
◦
is tetragonal. If, in addition, one angle is 120 while the other two remain at 90 , the crystal is
◦
hexagonal. The four additional types are orthorhombic, rhombohedral, monoclinic, and triclinic.
For a given type of unit cell, various arrangements of atoms are possible; each such arrangement
is called a crystal structure. Three crystal structures having a cubic unit cell are the primitive cubic
(PC), body-centered cubic (BCC), and face-centered cubic (FCC) structures. These are illustrated
in Fig. 2.10. Note that the PC structure has atoms only at the corners of the cube, whereas the
BCC structure also has one in the center of the cube. The FCC structure has atoms at the cube
corners and in the center of each surface. The PC structure occurs only rarely, but the BCC structure
is found in a number of common metals, such as chromium, iron, molybdenum, sodium, and