THE EFFECT OF STRAIN RATE ON ROCK STRENGTH

The effect of the strain rate on strength has been evaluated for two widely different rock types, a brittle limestone (Tyndallstone) and a ductile salt rock (Lanigan potash rock). Results of static and dynamic fatigue tests on Tyndallstone, a dolomitic limestone, show an increase in strength with increasing strain or stressing rate although the rate effect is very small. Although the static and dynamic fatigue tests are expected to yield the same stress corrosion parameter, no such agreement has been observed. Dynamic fatigue tests of the more ductile salt rock showed a substantial rate effect. The usual strength criteria, that consider the influence of confining pressure alone, are no longer adequate to describe the strength of Lanigan potash. A general strength criterion, that incorporates the effect of both the confining pressure and the strain rate, is proposed.