Indentation creep vs. indentation relaxation: A matter of strain rate definition?

An extensive study of the different methods able to measure material creep properties in indentation is proposed. A systematic comparison of the fast creep, long-term creep, constant strain rate and long-term relaxation tests is performed on amorphous selenium. Even though measured apparent activation energy and strain rate sensitivities are very similar for all tests, a discrepancy is highlighted between the creep/constant strain rate tests and the relaxation tests. This effect seems to be related to the strain rate definition which should be a function of the strain rate sensitivity m. In this paper, it is proposed to use a representative strain rate (epsilon)over dot(r) as proposed by Kermouche et al. (2008). An excellent agreement is found between all the methods, including uniaxial compression when expressing the creep behavior as a function of representative stress and strain rate.