Influences of elasticity on the measurement of power law creep parameters by nanoindentation

Abstract

Indentation testing is a well-known method to measure the rate dependence of the strength of materials at small scales through a variety of different testing methodologies. In many cases, a power-law creep constitutive relationship relating the indentation strain rate to the hardness, or the mean pressure applied to the material by the indenter, is assumed. However, this method of analysis does not explicitly include elastic effects, which for some testing methods may be significant. In this work, a new method of analysis that explicitly considers the elastic effects is presented. The conditions and materials for which the elastic contribution is non-negligible are identified, and simple closed-form analytical expressions are developed to account for the elastic effects. This leads to improvements in the measurement of power law creep parameters such as the stress exponent and creep coefficient. Experimental assessment of the new analysis procedures is presented for amorphous selenium (Se) and calcium fluoride (CaF2). After accounting for the elastic effects, an equivalence of different indentation creep testing methods is observed for Se due to the fact that this material has a very nearly history independent plastic response and no indentation size effect (ISE). On the other hand, significant differences are observed in the case of CaF2 because of indentation size effects. The findings of this work have important implications for several widely used indentation creep measurement methods such as the constant strain rate method (CSR) and the strain rate jump test (SRJ). Based on the results, guidelines for performing indentation creep tests with improved precision and accuracy are presented.