There can be one or more of several reasons as mentioned below and steps you can take:

Material inhomogeneity like inclusions or grain boundaries may cause irregular hardening.

Surface roughness or uneven polishing can affect load-displacement curves.

Indentation size effect (ISE) could make hardness appear load-dependent.

Work hardening or pile-up might alter curve slopes at higher loads.

Instrument compliance or thermal drift may introduce measurement artifacts.

Localized plasticity or phase transformations could influence material response.

Edge effects or grain boundary proximity may affect indentation behavior.

Calibration or vibration issues in the equipment could distort results.

Microstructure analysis (e.g., SEM/EBSD) can reveal site-specific factors.

Repeat tests and FEM simulations can help verify and analyze anomalies.

Dear Viswanathan Ramamoorthy. What you have observed is the well-known hysteresis!

Dear Veniamin Nazarov

Thank you very much for the lead, its very helpful. I will look it up extensively online. Could you please suggest any recommended resources to read on the hysteresis behavior during materials indentation? Thank you once again :)

Dear Viswanathan Ramamoorthy,

Check out at the first time the book "Dislocations and mechanical properties of crystals",

An international conference, Held at Lake Plasid, September 6-8, 1956// New York, John Willey and Sons, Inc London, Chapman and Hall, Limited.

In paper H.Mott, The theory of dislocations, Fig.1.zaaa (On Russian - 323 p., in English about nearly this page.)

V.N.

Also Veniamin Nazarov, the hysteresis loop becomes more evident at generally higher peak loads. Could it be occurring from the possible strain-induced phase transformation like martensite formation? I would greatly appreciate to have an explanation for this behavior. Thank you!

I am sorry, the paper (PDF) only Russian an now time.

I am tried in Inglish to morrow.

V.N.

I am sorry, the paper (PDF) only Russian an now time.

I am tried in Inglish to morrow.

V.N.

…

• 4.56 MBФТТ-Головин,50395.pdf

Contact mechanics. K.L. Johnson, 1985 (Second - 1987) Cambridge university press

Dear Viswanathan,

in my opinion what your curve shows is what is normally called "nose" of the unloding curve. Nose is a common way to call a negative Stiffness (Stiffness is S=dP/dH) and is normally a result of creep.

The creep can result from :

1) not exhausted creep during the hold period (hold period too short hence creep still happening during the unloading curve)

2) unloading rate too slow so creep occuring during the unloading

3) compliance of the set-up too high especially (please check this accurately ) the way in which you mount your sample to the sample holder offers too much compliance and(the glue used to fix the sample on the holder is too much/too thick layer and such glue creeps during the test resulting in a "nose" behaviour).

What Veniamin said might be correct but first of all you have to be sure that your system is well set-up and creep is not affecting your measurements in the way i have just explained especially because you are testing the same sample in different location but whith same parameters I would reccomend that you check if there is an uneven layer of glue used to fix the sample to the sample holder (thicker layer under test number 3)

Regards enrico

Your sample is highly heterogeneous in elastic and plastic properties at different points on the surface and at different depths of the subsurface layer of the sample.

Dear Enrico Corniani

Actually, the variation starts from the later stage of loading curve itself and proceeds to the initial stage of unloading segment. In other words, they occur only at the higher indentation depths. I am not sure if this is related to the 'nose' behavior. Thank you.