Stress has been traditionally regarded as the ‘cause’ and
strain as the ‘effect’ in materials testing: as a consequence, early
testing and standards utilized a constant stress rate application.
It was then natural to express the strength of a material in terms
of the stress present in the test specimen at failure.
Since uniaxial and triaxial testing of rock are by far the
most common laboratory procedures in rock mechanics and rock
enpeering, the most obvious means of expressing a failure
criterion is