Relationships between initial unloading slope, contact depth, and mechanical properties for conical indentation in linear viscoelastic solids

Using analytical and finite element modeling, we studied conical indentation in linear viscoelastic solids with either displacement or load as the independent variable. We examine the relationships between initial unloading slope, contact depth, and viscoelastic properties for various loading conditions such as constant displacement rate, constant loading rate, and constant indentation strain rate. We then discuss whether the Oliver-Pharr method for determining contact depth, originally proposed for indentation in elastic and elastic-plastic solids, is applicable to indentation in viscoelastic solids. We conclude with a few comments about two commonly used experimental procedures for indentation measurements in viscoelastic solids: the "hold-at-peak-load" technique and the constant indentation strain-rate method.