Conventional creep test is a time consuming procedure which becomes prohibitive when a number of samples need to be tested. Some methods such as indentation creep test have been used for measuring creep parameters in metals, alloys, glasses, ceramics and polymers. Actually, this kind of creep test has not been carried out on rocks before. This paper describes the impression creep apparatus which was designed and fabricated for rock samples. Results obtained from testing salt rock are discussed in the paper. Previous indentation creep tests using a conical, pyramidal or spherical indenter did not show a steady state condition at constant load. It turned out that a cylindrical indenter with a flat end gives a steady state penetration velocity at constant load. This special kind of indentation creep is called 'impression creep. This new creep test can be applied successfully to the soft rocks which provide a fast insight about the creep behavior of soft rocks.
The impression creep technique is a modified indentation creep test wherein a cylindrical indenter with a flat end is impressed in a specimen to apply a steady state load through a small area, and the resulting penetration velocity is recorded. [1, 2, 3] At a constant load (and sometimes an elevated temperature), the indenter penetration velocity reaches a steady rate after a transient period, which is related to the plastic flow properties of a relatively small volume of materials underneath the indenter. [4] The main advantage on this method is that many tests can be performed on the same samples which avoid the sample-to-sample variations and the use of many samples. Also when the micro structures of samples differ from each other, the investigations on models and mechanism becomes very difficult [2, 4]. In addition, it enables probing very small material volume; it is particularly attractive for testing of modern micro-components. [1, 5] Other advantages of the impression creep method are the minimum amount of tools needed in the laboratory and capability of measuring the effect of temperature and stress on the creep of a crystal. Considering the small diameter of the punch, deformation without the need to go through the tertiary creep stage is achieved and the variation of the creep mechanisms, with the effects of the punch size, stress and temperature is also avoidable. Other than these, by using this method, several uni-axial creep curves can be obtained from one sample. It has been shown that the stress and temperature dependencies obtained from the impression creep test, display a good agreement with the results of the conventional creep test on the metals. [1] Lucas 1983 reviewed some creep testing techniques of metals that require only small volumes of material. Impression creep was shown to agree well with the results from the conventional creep tests. He studied the effect of specimen size to indenter size ratio and also the effect of temperature in a stress relaxation test.
