From the general shape of the E(r) curve, one can easily sketch the shape of a typical force versus distance curve, as shown in Fig. 4.6. The following salient features are noteworthy:
o The net force between the atoms or ions is zero at equilibrium, i.e., at r = ro
o Pulling the atoms apart results in the development of an attractive restoring force between them that tends to pull them back together. The opposite is true if one tries to push the atoms together.
o In the region around r = ro the response can be considered, to a very good approximation, linear (inset in Fig. 4.6). In other words, the atoms act as if T
hey are tied together by miniature springs. It is in this region that Hooke's law (see below) applies.
o The force pulling the atoms apart cannot be increased indefinitely. Beyond some separation rail, the bond will fail. The force at which this occurs represents the maximum force F that the bond can withstand before failing.
In the remainder of this section, the relationships between stiffness and theoretical strength, on one hand, and E(r) and F(r), on the other hand, are
developed.