Cone indentation of time-dependent materials: The effects of the indentation strain rate

The method of instrumented indentation is well suited to the mechanical characterization of materials on a micro- and nanoscale. Conical indentation is specially interesting to characterize elastoplastic materials which take advantage of the principle of geometric similarity. But materials can also exhibit time-dependent behaviours and their characterization requires special loading conditions that are discussed in the first part of this paper. Then the influence of the indentation strain rate on the hardness of linear viscoelastic solids, power law creep solids and elastic–viscoplastic solids is investigated. The function describing the evolution of the mean pressure with the mechanical properties and the indentation strain rate has been analytically determined for generalized Maxwell’s linear viscoelastic solids and an approximation is given for elastic–viscoplastic solids with the help of finite elements simulations. Finally, the determination of the strain rate sensitivity using the viscoplasticity index is discussed and a procedure is given to check its relevance a posteriori.