Relaxation impression technique is a novel relaxation test in which a cylindrical flat ended indenter penetrates into a small region of sample and after keeping the displacement at a certain point, the decrease in the stress level will be recorded. Conventional relaxation test is a time consuming and expensive technique when a number of samples are needed to be tested. In contrast, impression relaxation test requires only a small amount of sample which makes this method cheaper. In this study, the impression relaxation test will be carried out on salt rock using an apparatus, which is designed and fabricated for impression relaxation test, and the results will be compared to the results of conventional compression relaxation test.
There is a strong demand for constructing deep underground spaces in many fields of rock engineering such as mining activities and petroleum production. As a result, defining characteristics of time-dependent deformations and failure around the underground openings are a major concern for stability and safety of structures. Creep under constant stress and relaxation under constant strain are two typical time-dependent phenomena of materials. In natural geological systems true creep or relaxation is, however, hard to be implemented, because a combination of creep and relaxation is commonly observed . When a constant strain is applied to a viscoelastic material, which behaves in a nonlinear, and non-Hokean fashion, the force necessary to maintain that strain is not constant but decreases with time. This behavior is called .stress relaxation.. Conversely, when a viscoelastic material is subjected to a constant stress, an increase in the deformation takes place with time; this behavior is called .creep.. The processes causing stress relaxation may be physical or chemical in nature, and under all normal conditions, both types of processes will occur simultaneously.