Engineering Rock Mechanics

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This situation can be considered as simply shortening the sample and
measuring the associated load. The resultant curve, known as the strain-
controlled complete stress-strain curve, was first obtained in 1966. A list of
the developments in materials science testing leading up to this time is
given in Hudson et al. (1972).
Because rock often has a higher stiffness than standard testing
machines, even under strain control, the complete curve often cannot be
obtained without modifying the machine. The testing techniques required
for rock mechanics are thus unique, both in their requirements and their
methodologies. For example, in soil mechanics testing, the soil usually has
a low enough stiffness to allow the complete curve to be obtained as a
matter of course. Also, in concrete testing there is not so much importance
attached to obtaining the complete curve, because the peak strength is
defined as failure. Therefore, we need to consider very carefully the
consequences of different methods of testing and different stiffnesses of the
applied loading device, whether in the laboratory or in the field.
In Fig. 6.6, we illustrate both schematically and conceptually the testing
machine and associated stiffnesses. The specimen has a certain stiffness and
the machine has a certain stiffness. Whatever the load in the specimen, an
equal and opposite load is applied to the machine. Thus, in the lower dia-
gram in Fig. 6.6, not only can we plot the axial force versus the axial
displacement for the specimen, we can also plot the same parameters for the
machine. Note that these two curves are drawn on adjacent sides of the axial
force axis: this is because compression of the sample is regarded as positive,
and the corresponding extension of the machine is negative (another way

Non-linear

Testing machine Spring model

As the jack compresses the specimen, it alsn

stretches the machine

Machine AA Specimen

Extension Axial displacement Compression

While the specimen is compressed to peak strength,

the machine stretches. As the specimen is compressed

beyond the peak, the machine returns to normal length

Figure 6.6 Schematic and conceptual illustration of specimen and testing machine

stiffnesses.