Loading-rate dependency of rocks in the post-failure region has been investigated by many researchers, but knowledge obtained in the previous studies is insufficient compared to that for the peak strength. In this study, loading-rate dependency of stress-strain curve at very low stress level in the post-failure region was investigated with three testing methods.
A testing method, in which a single rock sample is loaded at alternating loading-rate, was applied to andesite and granite. Loading-rate dependency of stress-strain curve was clearly observed up to the very low stress level in the post-failure region, where failure progressed considerably. Next, unloading-reloading cycles were repeated in constant loading-rate test with these two rocks, and unloading curves at many points along stress-strain curve were obtained. Stress-strain curve at slow loading-rate obtained in alternating loading-rate test was shifted along the unloading curves, and then it overlapped on stress-strain curve at fast loading-rate. And percentage of change of stress in post-failure region was almost equal to that of the peak strength.
A new testing method, in which alternating two loading-rates and unloading-reloading cycles were repeated with a single rock sample, was proposed and applied to andesite, granite and tuff. It was found that a new testing method is easy to conduct and suitable for practical use.