Page 153 - Mechanical Behavior of Materials

154 Chapter 4 Mechanical Testing: Tension Test and Other Basic Tests

Figure 4.24 Untested and tested 150 mm diameter compression specimens of concrete with
Hokie limestone aggregate. (Photo by R. A. Simonds.)

The ultimate strength behavior in compression differs in a qualitative way from that in tension.
Note that the decrease in force prior to final fracture in tension is associated with the phenomenon
of necking. This, of course, does not occur in compression. In fact, an opposite effect occurs, in that
the increasing cross-sectional area causes the stress–strain curve to rise rapidly rather than showing
a maximum. As a result, there is no force maximum in compression prior to fracture, and the
engineering ultimate strength is the same as the engineering fracture strength. Brittle and moderately
ductile materials will fracture in compression. But many ductile metals and polymers simply never
fracture. Instead, the specimen deforms into an increasingly larger and thinner pancake shape until
the force required for further deformation becomes so large that the test must be suspended.
Ductility measurements for compression are analogous to those for tension. Such measures
include percentage changes in length and area, as well as engineering and true fracture strain. The
same measures of energy capacity may also be employed, as can constants for true stress–strain
curves of the form of Eq. 4.23.

4.6.3 Trends in Compressive Behavior
Ductile engineering metals often have nearly identical initial portions of stress–strain curves in
tension and compression; an example of this is shown in Fig. 4.25. After large amounts of
deformation, the curves may still agree if true stresses and strains are plotted.
Many materials that are brittle in tension have this behavior because they contain cracks or pores
that grow and combine to cause failures along planes of maximum tension—that is, perpendicular
to the specimen axis. Examples are the graphite flakes in gray cast iron, cracks at the aggregate
boundaries in concrete, and porosity in sintered ceramics. Such flaws have much less effect in
compression, so materials that behave in a brittle manner in tension usually have considerably higher
compressive strengths. For example, compare the strengths in tension and compression given for
various ceramics in Table 3.10. Quite ductile behavior can occur even for materials that are brittle
in tension, as for the polymer in Fig. 4.26.