NATURE OF MATERIALS - Materials for Civil and Construction Engineers - page 64

a 4
3

p r 3 4 * b

APF 0.74

= =

10 - 29

6.636 *

r 3 10 - 29 = 0.293 *

10 - 9 m r = 0.143 * = 0.143 nm

Knowledge of the type of lattice structure is important when determin-
ing the mechanical behavior of a metal. Under elastic behavior, the bonds of
the atoms are stretched, but when the load is removed, the atoms return to
their original position. On the other hand, plastic deformation, by defini-
tion, is a permanent distortion of the materials; therefore, plastic deforma-
tion must be associated with a change in the atomic arrangement of the
metal. Plastic deformation is the result of planes of atoms slipping over each
other due to the action of shear stress. Naturally, the slip will occur on the
planes that are the most susceptible to distortion. Since the basic bonding
mechanism of the various metals is similar, differences in the theoretical
strength of a material are attributed to the differences in the number and ori-
entation of the slip planes that result from the different lattice structures.

■ 2.2.2 Lattice Defects

Even under special circumstances, it is very difficult to grow perfect crys-
talline structures. Generally, pure crystalline structures are limited to 1
micron in diameter. These pure materials have a strength and modulus of
elasticity approaching the maximum theoretical values based on the bond-
ing characteristics. However, the strength and deformation of all practical
materials are limited by defects. There are several causes for the develop-
ment of defects in the crystal structure. These can be classified as follows:

1. point defects or missing atoms
2. line defects or rows of missing atoms, commonly called an edge
dislocation
3. area defects or grain boundaries
4. volume defects or cavities in the material

In the case of point defects, single atoms can be missing in the lattice
structure because the atoms are vibrating as they transition from liquid to
solid. As a result, one atom may vibrate in the area where two atoms should
be in the lattice. Vacancies have little effect on the properties of the material.
In considering the differences between manufactured and theoretical
material behavior, an understanding of line defects becomes important. A
typical line defect is shown in Figure 2.10, where a line of missing atoms
extends back into the illustration (Van Vlack 1989; Guy and Hren 1974).
The atoms above the dislocation are in compression and those below the