106 Intact rockbehaviour at a strain rate in the laboratory of 1 x and very much more
ductile behaviour when undergoing tectonic movements at strain rates of
1 x Note that this range is through 21 orders of magnitude.
Temperature effects. Only a limited amount of information is available
indicating the effect of temperature on the complete stress-strain curve
and other mechanical properties of intact rock. The limited test data do
however agree with one’s intuition, that an increase in temperature
reduces the elastic modulus and compressive strength, whilst increasing
the ductility in the post-peak region. The complete stress-strain curves
shown in Fig. 6.17 illustrate this trend. Also, very high temperatures can
result in damage to the microstructure. At the other end of the
temperature spectrum, there is increasing interest in the effect of very
low temperatures on rock, within the context of liquified natural gas
storage.
Strain in percent
Figure 6.17 The effect of temperature on the complete stress-strain curve.6.5 Failure criteria
We do not know exactly how a rock fails, either in terms of the precise
details of each microcrack initiation and propagation, or in terms of the total
structural breakdown as many microcracks propagate and coalesce. In both
cases, the process is extremely complex and not subject to convenient
characterization through simplified models. Nevertheless, as engineers we
should like some measure of the failure properties and the ability to predict
when failure will occur. It was mentioned earlier that stress has been