October 4-9, 2015
Miniaturisation of devices and a paradigm shift toward using compliant material require small scale characterisation techniques such as nanoindentation . Initial non-conformity of contact and delayed elasticity on the unloading curve are not currently taking into account in nanoindentation methods [2, 3], where the unloading curve is seen fully elastic. A different approach has been taken which considers actual localised deformation during nanoindentation, thus the proposed method  is able to acquire untainted elastic or viscoelastic response data. The method, validated for both viscous and non-viscous materials, takes into account the correction of the stiffness associated with the delayed elasticity at initial unloading and determines modulus with less variability even if testing conditions are non-quasi-static. The key step in the methodology is to account for initial conformity of the contact, the nose-out phenomena and delayed elasticity. Thus a full elastic point (FEP) is determined as seen in the figure 1. Traditionally to eliminate the initial delayed elastic response a multi-cycle approach was needed, whereas in our method any materials, even viscous, can be tested under any test conditions in a single loading-unloading cycle. The algorithm is universally applicable. Experiments were conducted on six different viscous materials under single and multi cycle loading conditions to validate our method to existing ones [2–3]. Multi-cycle test on PET & PEN are reported in figure 2. It was found that except for rubber all the different materials studied by using just a single-cycle, our method determined values equally well as previous Oliver and Pharr method  with less variation. For multi-cycle tests our method is capable of producing results as good as the fourth-cycle of Feng’s method , thus time and cost of experimentation can be reduced. So this method becomes appropriate as a standardised technique, and also for the characterisation of polymers which have been an issue in the past.
1) BS EN ISO 14577-1:2015 2) Oliver, W.C., Pharr, G.M.:J. Mater. Res., 1992, 7, (06), pp. 1564–1583. 3) G. Feng, A.H.W.N.:J. Mater. Res., 2002, 17, (03), pp. 660 – 668. 4) Shah A, Renevier N, Sherrington I., UK Patent Application Number 1513480