Variation of the strain rate during CSM nanoindentation of glassy polymers and its implication on indentation size effect

Abstract

The indentation strain rate is currently assumed to remain unvaried during continuous stiffness measurement (CSM) nanoindentation where P/P is imposed to remain constant. To probe the validity of this assumption for the nanoindentation of glassy polymers, a series of experiments have been performed at different set P/P values on poly(methyl methacrylate) and polycarbonate using CSM technique. It is firstly shown that the actual P/P value changes drastically at shallow indentation depths and it takes a considerable depth, which is material independent, for this parameter to attain a stabilized value. Furthermore, the strain rate is measured directly as the descent rate of the indenter divided by its instantaneous depth ( h/h), and indirectly via considering the variations of the load and hardness during the test. Both of these approaches reveal that the strain rate is considerably larger at shallow depths, and the depth beyond which it becomes constant is material and P/P ratio dependent. Finally, by considering the relationship between the hardness and strain rate, it is observed that although the strain rate variation alters the hardness, its contribution is not able to justify the observed indentation size effect; hence, other contributing factors for this phenomenon are discussed for their possible effects. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 2179–2187