Finer-Scale Extraction of Viscoelastic Properties from Nanoindentation Characterised by Viscoelastic–Plastic Response

Motivated by recent progress in viscoelastic indentation analysis, the identification of viscoelastic properties from materials exhibiting elastic, viscous and plastic material behaviour by means of nanoindentation is dealt with in this paper. Based on existing solutions for pure viscoelastic material behaviour, two methods allowing us to consider the effect of plastic deformation are presented: (i) the so-called double-indentation technique, with the second indentation characterised by pure viscoelastic material response and (ii) the use of spherical indenter geometries instead of commonly used pyramidal indenters avoiding plastic deformation at all. Both methods are applied to three different polymers, giving access to the model parameters of the fractional dash-pot which is used to describe the viscoelastic behaviour. The results obtained are compared with results from standard (single) indentation tests using a Berkovich indenter. Moreover, the influence of the maximum load, determining the amount of plastic material response, on the identified model parameters is investigated. Finally, the creep-compliance functions identified by nanoindentation are compared with the respective macroscopic creep-compliance functions obtained from bendingbeam rheometer tests.