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

Published online by Cambridge University Press:  31 January 2011

N. Fujisawa  and
M.V. Swain
N. Fujisawa*
Affiliation:
Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, Australian National University, Canberra, Australian Capital Territory 0200, Australia
M.V. Swain
Affiliation:
Biomaterials Science Research Unit, Faculty of Dentistry, University of Sydney, Sydney Dental Hospital, Surry Hills, New South Wales 2010, Australia
*
a)Address all correspondence to this author. e-mail: naoki.fujisawa@anu.edu.au
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Abstract

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.

Keywords

Elastic propertiesPolymerViscoelasticity

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Type
Articles
Information
Journal of Materials Research , Volume 23 , Issue 3 , March 2008 , pp. 637 - 641
Copyright
Copyright © Materials Research Society 2008

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References

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