Whether or not a material can deform plastically at low applied stresses depends on its lattice structure. It is easier for planes of atoms to slide by each other if those planes are closely packed. Therefore lattice structures with closely packed planes allow more plastic deformation than those that are not closely packed. Also, cubic lattice structures allow slippage to occur more easily than non-cubic lattices. This is because of their symmetry which provides closely packed planes in several directions. Most metals are made of the body-centered cubic (BCC), face-centered cubic (FCC), or hexagonal close-packed (HCP) crystals, discussed in more detail in the Module 1, Structure of Metals. A face-centered cubic crystal structure will deform more readily under load before breaking than a body-centered cubic structure.

Figure 2 Change of Shape of Cylinder Under Stress

The BCC lattice, although cubic, is not closely packed and forms strong metals. (x-iron and tungsten have the BCC form. The FCC lattice is both cubic and closely packed and forms more ductile materials. y--iron, silver, gold, and lead are FCC structured. Finally, HCP lattices are closely packed, but not cubic. HCP metals like cobalt and zinc are not as ductile as the FCC metals.

Summary

The important information in this chapter is summarized below.

Strain Summary

Strain is the proportional dimensional change, or the intensity or degree of distortion, in a material under stress.

Plastic deformation is the dimensional change that does not disappear when the initiating stress is removed.

Proportional limit is the amount of stress just before the point (threshold) at which plastic strain begins to appear or the stress level and the corresponding value of elastic strain.

Two types of strain:

Elastic strain is a transitory dimensional change that exists only while the initiating stress is applied and disappears immediately upon removal of the stress.

Plastic strain (plastic deformation) is a dimensional change that does not disappear when the initiating stress is removed.

y--iron face-centered cubic crystal structures deform more readily under load before breaking than (x-iron body-centered cubic structures.