EXAMINATION OF A ROCK FAILURE CRITERION BASED ON CIRCUMFERENTIAL TENSILE STRAIN

Uniaxial compression, triaxial compression and Brazialian tests were c onducted on several kinds of rock, with particular attention directed to the principal tensile strain. In this paper we aim to clarify the e ffects of the experimental environment-such as confining pressure, loa ding rate, water content and anisotropy-on the critical tensile strain , i.e., the measured principal tensile strain at peak load. It was det ermined that the chain-type extensometer is a most suitable method for measuring the critical tensile strain in uniaxial compression tests. It is also shown that the paper-based strain gage, whose effective len gth is less than or equal to a tenth of the specimen's diameter and gl ued on with a rubber-type adhesive, can be effectively used in the Bra zilian tests. The effect of confining pressure P-C on the critical ten sile strain epsilon(TC) in the brittle failure region was between -0.0 2 x 10(-10) Pa-1 and 0.77 x 10(-10) Pa-1. This pressure sensitivity is small compared to the critical tensile strain values of around -0.5 x 10(-2). The strain rate sensitivities partial derivative epsilon(TC)/ partial derivative{log(d\epsilon\/dt)} were observed in the same way a s the strength constants in other failure criteria. They were found to be from -0.10 x 10(-3) to -0.52 x 10-3 per order of magnitude in stra in rate in the triaxial tests. The average magnitude of the critical t ensile strain epsilon(TC) increased due to the presence of water by 4% to 20% for some rocks, and decreased by 22% for sandstone. It can at least be said that the critical tensile strain is less sensitive to wa ter content than the uniaxial compressive strength under the experimen tal conditions reported here. An obvious anisotropy was observed in th e P-wave velocity and in the uniaxial compressive strength of Pombetsu sandstone. It was not observed, however, in the critical tensile stra in, although the data do show some variation. A ''tensiie strain crite rion'' was proposed, based on the above experimental results. This cri terion signifies that stress begins to drop when the principal tensile strain reaches the critical tensile strain. The criterion is limited to use within the brittle failure region. The critical tensile strain contains an inelastic strain component as well as an elastic one. It i s affected by the strain rate, however, it is relatively insensitive t o the confining pressure, the presence of water and anisotropy.