Nanoindentation-derived elastic modulus of an amorphous polymer and its sensitivity to load-hold period and unloading strain rate

An amorphous polymer was contacted by a Berkovich indenter using the same loading history but with four different unloading rates following a wide range of load-hold time periods. The strain-rate sensitivity index of the creeping solid was determined at each load-hold period based on two readily determinable parameters, which are the effective contact stiffness and strain rate at the onset of unloading. The measured strain-rate sensitivity index was found to increase with decreasing load-hold period, suggesting that the elastic moduli of the amorphous polymers determined by nanoindentation (together with the true contact area) depends significantly on the selection of the load-hold period. The rheological condition of the creeping solid under constant load changes substantially with time to affect the subsequent unloading recovery process. It is therefore advisable to control not only the unloading strain rate but also the load-hold period when testing time-dependent materials.