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R4.6.1

Displacement Modulation Based Dynamic Nanoindentation for Viscoelastic Material Characterization Sehaj P. Singh1, Raman P. Singh1 and James F. Smith2 1 Mechanics of Advanced Materials Laboratory, Department of Mechanical Engineering Stony Brook University, Stony Brook, NY 11794-2300, USA. 2 Micro Materials Limited Wrexham Technology Park, Wrexham LL13 7YP, UK. ABSTRACT This paper demonstrates a new displacement modulation technique for using a depth sensing nanoindentation instrument to measure the dynamic mechanical properties of viscoelastic materials. For testing low modulus, high damping polymeric materials, dynamic nanoindentation offers several advantages over quasi-static testing. In this research, a model for the dynamic response of the system is proposed and shown to match well with experimental observations. A new calibration procedure, which involves the use of a variable cantilever spring, is employed to determine the damping characteristics of the testing frame as a function of excitation frequency. Using the proposed procedure dynamic nanoindentation tests are carried out on a viscoelastic material to determine the storage and loss moduli as functions of excitation frequency. Finally, a comparison with results from conventional testing (DMA) is provided. INTRODUCTION Depth sensing indentation, especially nanoindentation, is one of the most advanced techniques available today for measuring a wide range of mechanical properties of engineering materials [1-3]. Both quasi-static and dynamic mechanical properties can be measured using this technique. The techniques for measuring quasi-static properties like hardness and Young’s modulus are well developed for elastic materials. Hardness can be obtained from the peak applied load and the contact area at that load, and Young’s modulus can be obtained from the slope of the P-
unloading curve using the Oliver-Pharr method [2]. However, the testing and analysis for polymers is complicated due to their time dependent material properties. This complicates the measurement of elastic modulus from the slope of the unloading curve which can even turn out to be negative in some cases preventing any meaningful interpretation of data. Although several researchers have come up with corrections [4-6] to be applied to the P-
curve to obtain the quasi-static modulus of polymers, there is still no definite method available for analysis. Moreover, these methods are intrinsically limited because of the fact that they attempt quasi-static testing of a time dependent material. Dynamic nanoindentation offers several advantages over quasi-static testing in this respect as it not only provides the storage and loss modulus of the material but also offers significantly decreased testing time by examining properties over a range of frequencies rather than over an extended period of time. Another advantage of dynamic testing is that temperature dependent properties can be extracted using the time-temperature superposition principle. Dynamic nanoindentation has been used by some