The tangent modulus method was applied to pre-loaded cylindrical specimens of Shikotsu welded tuff, Inada granite and Kimachi sandstone. It was clarified that the tangent modulus method could be applied to the rocks. A positive correlation was observed between the preloading duration and the maximum delay time at which the preloaded stress was determined with the error less than 20% for Shikotsu welded tuff and Inada granite although the correlation was not confirmed for Kimachi sandstone due to small number of specimens. The confirmed maximum delay time for each rock was 42 days for Shikotsu welded tuff, 21 days for Inada granite and 28 days for Kimachi sandstone based on clip gage.
The stress at bending point in uniaxial cyclic loading for triaxially pre-loaded Kimachi sandstone decreased as confining pressure in the preloading increased. Kimachi sandstone and Shirahama sandstone were triaxially preloaded and triaxially cyclic loaded under decreased or increased confining pressure. The stress at bending point decreased with decrease of confining pressure for Shirahama sandstone. No apparent change was observed for Kimachi sandstone. Bending points were not observed for Shirahama sandstone under increased confining pressure. On the other hand, the stress at bending point decreased with increase of confining pressure for Kimachi sandtsone.
The physical phenomena utilized to detect stress memory in tangent modulus method was explained based on a model which consisted of two springs and a slider. One spring represents the elasticity of rock. Another spring and the slider are connected in parallel and represent inelasticity due to irreversible closure of voids. Decrease of tangent modulus at the preloaded stress was well simulated by the model.
