The comparison of compression and impression relaxation tests with different diameter of indenter in salt rock

In rock mechanics, several researches have been contributed to the creep behavior, however, a limited number of studies have been carried out on the relaxation behavior of rocks. Impression test is a novel technique to predict time-dependent behavior of materials. In relaxation impression, a cylindrical flat ended indenter penetrates into a small region of sample and after holding the indenter at a constant position, the decrease in the stress level is recorded. Impression relaxation tests in comparison with compression relaxation tests are much more economical and less time-consuming. In this article, 25 impression relaxation tests using indenter diameters of 2, 2.5, 3, 3.5, 4 mm and 4 compression relaxation tests are done on rock salt. Then, the parameters of viscoelastic behavior are calculated. The results of both techniques of impression and compression tests illustrate that the value of mu in Maxwell model decreases as the stress increases.