Specimen geometry, loading conditions and environmental effects 95Feedback
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Beam test
CompressionFFeedback
bit penetrationTIFeedbackTensionFeedbackBrazilian test
Figure 6.10 Rock testing geometries and optimal feedback for closed-loop control
of failure.today, the full potential of closed-loop control as described here has not
been fully exploited in rock mechanics, particularly with respect to
discontinuity testing and field testing.
Perhaps the key reason why servo-control is so successful is that the
overall response time of the control system is of the order of 5 ms, which
is faster than the speed at which any significant cracking can develop.
Although terminal crack velocity in rock is about one-third of the acoustic
velocity (i.e. very high), it should be remembered that crack propagation
must accelerate from zero: the high response time of the control system
ensures that the loads applied to a specimen are only sufficient to permit
slow crack propagation.
6.4 Specimen geometry, loading conditions
and environmental effects
Having described how the complete stress-strain curve can be obtained
experimentally, let us now consider the effects of the specimen size and
shape, loading conditions, and time and temperature effects.