Continuous Stiffness Measurement Nanoindentation Experiments on Polymeric Glasses: Strain Rate Alteration

Abstract

In many studies using continuous stiffness measurement (CSM) nanoindentation technique, it is assumed that the strain rate remains constant during the whole experiment since the loading rate divided by the load ($\dot{P}/P$) is considered as a constant input parameter. Using the CSM method, the soundness of this assumption in nanoindentation of polymeric glasses is investigated by conducting a series of experiments on annealed poly(methyl methacrylate) (PMMA) and polycarbonate (PC) at different set $\dot{P}/P$ values. Evaluating the variation of the actual $\dot{P}/P$ value during the course of a single test shows that this parameter varies intensely at shallow indentation depths, and it reaches a stabilized value after a significant depth which is not material dependent. In addition, the strain rate variation is examined through two methods: first, using the definition of the strain rate as the descent rate of the indenter divided by its instantaneous depth ($\dot{h}/h$) and second, considering the relationship between the strain rate and the load and hardness variations during the test. Based on the findings, the strain rate is greatly larger at shallow indentations, and the depth beyond which it attains the constant value depends on the material and the set $\dot{P}/P$ ratio. Lastly, incorporating the relationship between the hardness and strain rate, it is revealed that although the strain rate variation changes the material hardness, its effect does not give a justification for the observed indentation size effect (ISE); therefore, other contributing parameters are discussed for their possible effects on this phenomenon.