As already observed, a straight bar will change in length when loaded
axially, becoming longer when in tension and shorter when in compression.
For instance, consider again the prismatic bar of Fig. 1-2. The elongation
d
of this bar (Fig. 1-2c) is the cumulative result of the stretching of all
elements of the material throughout the volume of the bar. Let us assume
that the material is the same everywhere in the bar. Then, if we consider half
of the bar (length
L
/2), it will have an elongation equal to
d
/2, and if we
consider one-fourth of the bar, it will have an elongation equal to
d
/4.