Dear Abdur-Rasheed, I am not an expert in nanoindentation, but it seems to me that this is a matter of choice. In fact the true strain rate (as well as the true strain) will be quite inhomogeneous, both in space and time, below the indentation and I believe that either of the two options will be a measure of some sort of average. My suggestion is that you review the literature (as you did), define one of the two measures as your "strain rate", and use it accordingly.

Dear Abdur-Rasheed, as you know, nanoindentation is commonly applied for calculating material properties of hard thin films. The majority of the related articles focus on the material characterization of coatings. If the material you are testing is applied as a film, then go on. Otherwise, I cannot propose nanoindentation as a testing method for material characterization.

Regarding strain rate calculation, I agree with Cláudio that both calculations will give you an average strain rate. Due to indenter's geometry, strain rate will not have a constant value but a gradient concentrated around the indenter. If you want to calculate this gradient then I propose that you should use FEA.

Also, pay attention that you may render you results invalid if you contact nanoindentations with a high load velocity, since the machine's structural stiffness (a crucial parameter for keeping the obtained results valid) is calculated for low velocity loading.

Thanks to all who have responded to my inquiry. Another suggestion was given by Prof. C.A. Schuh of MIT in USA through private communication is copied below. "I believe the best approach is to use the instantaneous depth and displacement rate; the “indentation strain rate” changes over the course of the experiment. I hope this is helpful."

Georgios... I'm very interested in your assertion that nanoindentation is not to be recommended for testing anything that isn't a film. Can you explain that? I work with researchers who regularly use nanoindentation for testing of bulk materials (including many single crystals), site-specific indentation of precipitates in a matrix, site-specific testing around a braze or weld, site-specific testing around damage sites, etc...

Hi Phillip. Nanoindentation cannot be used as a general method for material characterization. But it is the perfect method for special cases as the ones you refer to. In order to test or characterize the materials of such cases, the employed method should be able to cause a tiny stress field, but in many cases quiet intense, in order to induce plastic deformation. Thin hard films fall into the same category. In these materials it is of a great interest to extract the material behavior at micro scale (using nanoindentations) and not at the macro scale.

Thanks for the reply. Are you saying that nanoindentation is not suitable for recovering bulk properties in materials with structure on a larger scale than the indents? In that case, I agree with you. For single-phase bulk materials, comparison of nanoindentation and micro- or macroindentation results can provide some very interesting information about material structure and behaviour.

Dear Phillip, it seems that we agree on the fact that nanoindentation is well fitted for micro scale material characterization. I also agree with you with the fact that the combination of results at different scales can provide valuable insight regarding material behavior.

Georgios, thank you for your replies. Please don't think I was trying to be argumentative; I was just trying to understand your position!

Dear Phillip, I apologize if I sounded like being 'attacked'. This is not the case!

I think the paper by Oliver and Pharr attached will help provide insight into whether or not nanoindentation can be used to characterize bulk material properties.

Thank you, Abdur-Rasheed... I think anyone who has worked in the field of nanoindentation is familiar with the excellent work of Oliver and Pharr!

I agree that Oliver-Pharr is a good start for everyone that wants to understand nanoindentation methodology. I aslo agree with Cláudio that FE analysis is crucial for getting in depth infermation for the examined material. SInce I conducted my PhD in a lab experienced in nanoindentations I can propose the following papers for further reading:

Seems I pushed enter too quickly!

Article Indenter surface area and hardness determination by means of...

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Article An accurate and fast approach for determining materials stre...

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Article Coatings' and other materials' mechanical properties conside...

That's a nice discussion, thanks to Abdur-Rasheed for nucleating it. I am not an expert on nanoindentation myself, but I already used it and I work with a research group which actively researches the method. I participated in the thesis commission of Dr. Sara Aida Pulecio Rodrigues, supervised by my colleague Prof. Dr. Roberto Martins de Souza, from the Mechanical Engineering Department of my School. She investigated the nanoindentation technique (actually, they use somewhat larger loads and a Vickers indenter, so they call it microindentation, but it is about the same thing). They investigated the method using FEM simulations and physical tests (amazingly no one had done this before). They were interested in investigating the transferibility of the different algorithms suggested to extract macroscopic mechanical properties from the indentation curves. The results showed that the answer to this question is not as simple as one would think at first. Depending on the parameters of the indentation curve these algorithms may provide a precise prediction of bulk macroscopic properties (e.g. yield stress) or no answer at all. The link below allows to download her thesis (in Portuguese, unfortunately). I don't know whether they already published papers based on this thesis. Personally, I believe extracting the Young modulus from the unloading curve is safe, since the hypotheses used for Sneddon's solution are met. For other properties the method should be used with care. I'm trying to get Prof Martins de Souza to join this discussion so he can contribute with his knowledge.

http://www.teses.usp.br/teses/disponiveis/3/3151/tde-16082010-135510/en.php

Thanks to all the responses given including those that have suggested some references. Anyway, another useful material that can also provide useful insight into the discussion is the paper by Lawn B.R. and Cook R.F. (probing material properties with sharp indenters: a retrospective) which is attached here. Thanks while I await further comments after reading the paper.

Thanks to all the responses given including those that have suggested some references. Anyway, another useful material that can also provide useful insight into the discussion is the paper by Lawn B.R. and Cook R.F. (probing material properties with sharp indenters: a retrospective) which is attached here. Thanks while I await further comments after reading the paper.