Quantifying plasticity-independent creep compliance and relaxation of viscoelastoplastic materials under contact loading

Here we quantify the time-dependent mechanical properties of a linear viscoelastoplastic material under contact loading. For contact load relaxation, we showed that the relaxation modulus can be measured independently of concurrent plasticity exhibited during the loading phase. For indentation creep, we showed that the rate of change of the contact creep compliance L̇(t) can be measured independently of any plastic deformation exhibited during loading through L̇(t) = 2a(t)ḣ(t)/P _max, where a(t) is the contact radius, h(t) is the displacement of the contact probe, and P _max is the constant applied load during the creep phase. These analytical relations were compared with numerical simulations of conical indentation creep for a viscoelastoplastic material and validated against sharp indentation creep experiments conducted on polystyrene. The derived relations enable extraction of viscoelastic material characteristics, even if sharp probes confer concurrent plasticity, applicable for a general axisymmetric contact probe geometry and a general time-independent plasticity.