18 Geological setting
&
Figure 2.8 Complete stress-strain curve illustrating the stiffness (or modulus, E),
the strength, 0, and brittleness.Ductile
c- ______-- -&
Figure 2.9 Illustration of the difference between a brittle material and a ductile
material.in the form of continuing strain at the same stress level (a ductile material)
and a drop in the stress level to zero at the same strain value (a brittle
material). The brittleness is indicated by the steepness of the curve between
these two limits. In fact, the situation is more complicated than this because
it is possible to have strain-hardening materials (a curve above the ductile
line) and super-brittle materials (to the left of the brittle line). These cases
will be discussed further in Chapter 6.
Possible variation in the three main factors is illustrated for a suite of
rocks in Figs 2.10-2.13. The figure legends describe the features quali-
tatively. As we have mentioned, the form of the complete stress-strain
curve is dictated by the nature of the microstructure. For example, a high
grain strength, fine grain basalt has a high stiffness, high strength and is
very brittle. On the other hand, a limestone rock with a variation in the
grain geometry has a medium stiffness, medium strength and a more gentle
descending part of the curve caused by the gradual deterioration of the
microstructure as it is progressively and increasingly damaged.
There will be variations on this theme for the variety of microstructures
that exist and the influence that they have on the shape of the curve-in