On the criterion for compensation to avoid elastic–plastic transients during strain rate change tests

Determination of the apparent activation volume and the instant strain rate sensitivity from strain rate change tests is a non-trivial task due to the short elastic–plastic transient. The flow stress can be back-extrapolated towards the incipient value, or the transient can be eliminated by a compensation technique consisting of an instant displacement introduced exactly at the moment of strain rate change. The criterion for determination of the correct amount of compensation is derived analytically. It was found that the established “experimental” criterion for the level of compensation gives an error of up to 20% in activation volume for soft tensile test rigs and affects the Haasen plot. It is shown that back-extrapolation of the stress gives the correct compensation. However, undesired changes in dislocation density may occur and modify the transients. A combination of slightly under-compensated strain rate jumps and back-extrapolation is recommended.