A critical assessment of the "stable indenter velocity" method for obtaining the creep stress exponent from indentation data
A technique for evaluating the (steady-state) creep stress exponent (n) from indentation data has come into common use over recent years. It involves monitoring the indenter displacement history under constant load and assuming that, once its velocity has stabilised, the system is in a quasisteady state, with Stage II creep dominating the behaviour. The stress field under the indenter, and the way in which the creep strain field is changing there, are then represented by "equivalent stress" and "equivalent strain rate" values. These are manipulated in a similar manner to that conventionally employed with (uniaxial) creep test data, allowing the stress exponent, n, to be obtained as the gradient of a plot of the logarithm of the equivalent strain rate against the logarithm of the equivalent stress. The procedure is therefore a very simple one, often carried out over relatively short timescales (of the order of an hour or less). However, concerns have been expressed about its reliability, regarding the neglect of primary creep (after a very short initial transient) and about the validity of representing the stress and strain rate via these "equivalent" values. In this paper, comprehensive experimental data (both from a conventional, uniaxial loading set-up and from instrumented indentation over a range of conditions) are presented for two materials, focussing entirely on ambient temperature testing. This is supplemented by predictions from numerical (FEM) modelling. It is shown that the methodology is fundamentally flawed, commonly giving unreliable (and often very high) values for n. The reasons for this are outlined in some detail. An attempt is made to identify measures that might improve the reliability of the procedure, although it is concluded that there is no simple analysis of this type that can be recommended.