Relaxation Effects of Viscoelasticity and Time–Dependent Poisson’s Ratio on Nanoindentation Measurements of Solid Polymers

Nanoindentation has proven itself to be a powerful technique to measure mechanical properties of polymers. Determining of material properties of solid polymers is an important aspect ofstress analysis. The conventional tensile and compression tests cannot be appropriately used for measurement of relaxationmodulus and creep compliance of polymeric thin films deposited on substrates, new gradient materials with modulusvarying spatially, and nanocomposites such as carbon nanotubes embedded in polymer matrix. On the other hand, most polymers exhibit viscoelastic behavior even at roomtemperature. In recent years, several works have been made on measurements of some viscoelastic properties of solidpolymers using nanoindentation. Lu et al. [1] developed a new method to measure creep function of polymers in time domain. Huang and Lu [2] measured relaxation functions ofpolymers by nanoindentation. Maxwell et al. [3] showed how creep data can be obtained from indentation experiments andhow these tests can be accelerated by performing short-term indentation tests at elevated temperatures to predict long-term creep at ambient temperature. Ashrafi and Shariyat [4] developed an approach for measurement of the viscoelastic Poisson’s ratios of biological solids. In this study, theviscoelastic and time-dependent Poisson’s ratio effects on the nanoindentation measurement of solid polymers wereinvestigated. The aim of this work is to develop a model capable of determining the viscoelastic shear and bulk modulus, and time-dependent Poisson’s ratios based on thestress relaxation of polymers. A new approach is represented to derive the relaxation Poisson s ratio of solid polymers from nanoindentation tests based on a generalized Maxwell model.