A new testing method for investigating the loading rate dependency of peak and residual rock strength

Abstract

One of the important methods for investigating viscoelasticity is to measure the loading-rate dependency of peak strength; however, no experimental method has been established for accurately measuring the loading-rate dependency of peak strength from a small number of samples. In this study we propose such a method. A single sample is loaded at alternating strain rates to obtain stress–strain curves for both strain rates. The loading-rate dependency of peak strength obtained via this method was compared with the findings of conventional methods. The loading-rate dependency indicated for Tage tuff, Sanjome andesite and Akiyoshi marble was nearly identical to previous results obtained using conventional methods, including results obtained under confining pressure. The loading-rate dependency of peak strength in these experiments shows a close relation with the creep stress-dependency of creep life. We also investigated the loading-rate dependency of the stress–strain curve for the post-failure region for which few results have been published. Under confining pressure, the corrected stress–strain curve, obtained by multiplying the stress of the complete stress–strain curve obtained at the fast strain rate by a constant determined by the ratio between the fast strain rate and slow strain rate, is nearly coincident with the stress–strain curve for the slow strain rate. This is an interesting result and represents new knowledge that may help elucidate failure mechanisms in the post-failure region. The loading-rate dependency of stress in the alternating strain rate experiment proposed here was most clearly observed when the stress–strain curve becomes flat, parallel to the strain axis. Some improvements to the proposed method are required to enable accurate investigations of loading-rate dependency during low stresses immediately after initiation of loading or during the abrupt decreases in stress that occur following peak strength.