Circumferential strain behavior during creep tests of brittle rocks

Creep tests were carried out on Inada granite under confining pressure and on dry and wet specimens of Kamisunagawa sandstone. Distinctive points Q_A (the point where the axial strain rate reaches a minimum), Q_C (the point where the magnitude of circumferential strain rate reaches a minimum) and R_C (the point where the magnitude of circumferential strain acceleration begins to increase) were defined. Dependency of the strain values at the distinctive points on creep stress, confining pressure and water presence was examined. Circumferential strain at Q_C or R_C was not affected by creep stress for both rocks. Peak load points were observed in reloading stress–strain curves when reloading was carried out before Q_C for Kamisunagawa sandstone. Circumferential strain values at peak load during reloading coincided with critical extensile strain, which is defined as circumferential strain value at peak load during constant strain rate tests and is insensitive to confining pressure, water presence, and anisotropy, although slightly affected by strain rate. Load increased slightly or did not increase when reloading was carried out after R_C for both rocks. R_C in creep tests scattered around unloading stress–strain curves from peak load during constant strain rate tests for Kamisunagawa sandstone. These observations suggest that Q_C and R_C in creep tests are closely related to the peak load points during constant strain rate tests. Circumferential strain would be used as a condition-insensitive damage indicator of rock in creep tests as well as in constant strain rate tests.