I'm doing some research on mechanical properties of materials using indentation method. On the basis of load-indentation depth curve I would like to obtain stress-strain curve. Equations how to calculate projected area of contact Ac for all indenters are well known; there is no problem to calculate strain for spherical indenter - r/R, where r stands for contact radius and R stands for indenter radius. How to calculate strain for Vickers, Berkovich and conical tip? Contact radius can be found from Ac, how to calculate R for Vickers, Berkovich and conical tip?
I think this link to a lecture may help.
Also I found the paper in the second link which looks useful, especially Fig 4, 5, & 6.
http://szft.elte.hu/oktat/www/mikronano/Nano_indentation.pdf
Article Comparison between Berkovich, Vickers and conical indentatio...
Due to the shape of the indenter, there is strain field and stress field (as mentioned by Jeff Le Due) around the indenter rather than a uni-axial strain.
The stress-strain curves (both engineering and true) are determined from uni-axial tensile or compression tests, where the stress and strain in almost uniform throughout the cross-section till necking/buckling occurs.
I find that it is very difficult to determine the stress-strain curves from the load-displacement curves from indentation experiments.
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