Help needed with interpretation of received P h curves during NANOINDENTATION

Dear iMechanicas,

I just started doing nanoindentation tests on dry biological samples (wood) with a flat punch tip and I have problems analyzing the results! I would be happy if anybody could give me some suggestions.

While running a test I receive quiet good force-displacement curves (no adhesion, good surface find, no time-depend behavior) but the values for the recieved elastic modulus are not stable! The value scatters over the whole range of displacement.

And sometimes the curve for the Phase angle is not like usual. Sometimes it just shows a wild zick-zack pattern.

Unfortunaltely I have no one to ask at my institute :(

So I would be very happy about comments, suggestions!

Thanks!!

Cheers Anna