Soft, stiff and servo-controlled testing machines 89all costs, or to be encouraged, is a function of the engineering objective, the
form of the complete stress-strain curve for the rock (or rock mass), and
the characteristics of the loading conditions. These features are crucial in
the design and analysis of underground excavations.
At this stage, we will consider whether the specimen is being loaded at
a constant stress rate or a constant strain rate: in other words, is stress the
cause and strain the effect, or vice versa? It is customary in science to plot
the independent (i.e. controlled) variable along the x-axis and the
dependent (or measured) variable along the y-axis. Stress-strain curves are
usually plotted with strain on the x-axis, with the implication that the test
is strain controlled. Yet, very often, it is the stress rate (say, expressed as
MPdminute) which is specified in standardized testing. It can clearly be
seen from the diagrams of the complete stressstrain curve in this section
that the inevitable consequence of conducting a test at a constant stress rate
will be violent uncontrolled failure at the point of peak strength, when the
machine tries to apply more stress than the specimen can sustain.
Furthermore, the descending portion of the complete stress-strain curve
is difficult to intrepret when stress is considered as the cause of strain,
because a reduction in stress apparently causes an increase in strain.
Conversely, if strain is considered as the cause of stress, the response of the
material in the post-peak region can be interpreted simply as the fact that
beyond a certain strain value (corresponding to the maximum stress) the
rock continues to suffer further mechanical breakdown with an attendant
loss of load-bearing capacity. This concept is amplified in Section 6.3.
6.3 Soft, stiff and servo-controlled testing
machines
The effects of the two extreme options for loading, i.e. stress control and
strain control, are illustrated in Fig. 6.5. Note that in this figure we have
chosen the axes such that the independent variable is plotted along the
x-axis. The first curve represents the application of an increasing load (for
example, a series of weights) to the specimen. When the peak strength is
reached, the deadweight causes a continous increase in strain at this peak
stress level, i.e. the specimen is uncontrollably crushed. The second curve
represents the continual compression of the specimen as the ends are
moved together (for example, in a screw-controlled press); the stress
associated with this movement can rise or fall without uncontrolled failure.
Figure 6.5 Stress- and strain-controlled stress-strain curves.