ABSTRACT

Impression creep test, which involves loading on a small area of the specimen with a flat-tipped cylindrical punch, is a new method used for rocks in this research. This uniaxial compression method enables testing of very small volumes of material with minimal required sample preparation. One of the attractive features of the impression creep test is its ability to establish steady-state creep behavior within a very short time. The tests are carried out under constant stress (in the range of 30- 65 MPa) and at temperatures in the range of 250 -350° c. Assuming a power law relation between the creep rate and applied stress, values of 3.28 and 4.72 are obtained for stress exponent (n) at low and high temperatures whereas the range for Q value (activation energy) was obtained as 40.7 and 27.0 kJ/mol again at low and high temperature.

1 INTRODUCTION

A comprehensive conventional creep test requires many samples which in turn necessitates a lot of time for right sample preparation. It is also difficult to build models and study mechanisms when there are sample-to-sample variations in the microstructure level. Hence, a localized creep testing technique has always had its own merits where many tests can be done in a small sample. Previous research has addressed an indentation creep test using a conical, pyramidal or spherical indenter which is performed on a small specimen. The main advantage on this method is that many tests can be performed on the same sample which avoids the sample-to-sample variations and necessity for many samples. Also when the micro structures of samples differ from each other, the investigations on models and mechanisms becomes very difficult (Chu and Li (1997), Pen and Dutta (2004)). It is particularly attractive for testing on modern micro-components (Sastry (2005), Yang and Li (1995)).

The problem with aforementioned indenters is that they do not show a steady state creep behavior at constant load. It turned out that a cylindrical indenter with a flat end results in a steady state penetration velocity at constant load on ceramic samples. This special kind of indentation creep is called ‘impression creep’. In this kind of test, 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 (Pen and Dutta (2004)).

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 uniaxial creep curves can be obtained from one sample.

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