Sensitivity of polymer nanoindentation creep measurements to experimental variables

Abstract Spherical indentation creep experiments were used to characterize the viscoelastic response of three polymeric materials with different elastic and viscoelastic responses. A hereditary integral formulation for creep following ramp loading was used to analyze experimental indentation data. A wide range of conditions, including different load levels and ramp times, were employed to examine sensitivity of results to the selected experimental variables. Modulus values for materials exhibiting minimal time-dependent responses were found to agree well with known values and to be invariant of experimental conditions. However, a material with substantial time-dependent deformation in the experimental time frame was found to exhibit some effect of both load level and ramp time on modulus values. Time constants measured in the experiment varied with loading rate for two of three polymeric materials examined, but time constants did not vary with peak load. The examination of the shape of experimental load–displacement responses demonstrated no appreciable plasticity in two of three materials – the same two materials with both time- and level-independent creep responses.