Time-independent elastic modulus of polymer materials determined by nanoindentation

Abstract

In nanoindentation testing, the elastic modulus is evaluated from contact stiffness measured at the initial unloading stage. Since the measured stiffness of polymer materials generally depends on the unloading rate (so-called viscoelastic deformation behavior), it is difficult to determine the representative elastic modulus of such polymers using conventional nanoindentation theories. If the measured stiffness of a polymer material converges to some value at sufficiently fast or slow unloading rates, that value can be defined as its time-independent stiffness. The contact stiffness values of polymer materials were measured at various unloading rates and their dependency on unloading rate was discussed. Time-independent elastic modulus values were determined by analyzing the trend of stiffness with unloading rate. Uniaxial compression tests were performed to measure directly elastic modulus at various unloading rates. The validity of the time-independent elastic modulus evaluated by nanoindentation tests is assessed by comparing it with elastic modulus values measured in uniaxial compression tests.