Hello Kathiravan Subramaniam:

From Load-displacement curve, I dont understand one thing. Why the displacement is huge where the sheets are only .5,1,2 mm. How can the indenter travel more than the thickness? Am I missing something? 

To make more sense out of your indention curve, you will need to identify different regimes during your rather too wide range of stroke(displacement). Each regime represents at least one dominant mode of loading; eg. elastic, plastic, folding, rigid body motion etc.

Indentation using a sphere head has already a too complex mechanics which make interpretation of its results difficult, choosing such a wide range of stroke only makes it more complex. The plateau might represent a steady force needed to keep the balance of forces during the final stages (e.g. rigid body motion)

Hope this helps.

Dear all,

I have found the reason for this plateau happening. Before sharing about it I would like to thank Shahin Khoddam, Ross James Friel and Soud Choudhury for taking time out to answer my question. I am sure that you are really busy and I appreciate you taking the time to respond. Thank you again.

After few experimental trials conducted on the metal sheet, it was found that the grip itself moving upward upon reaching 3kN of load, the displacement up to 7 mm. The indenter was gripped by using Pneumatic Wedge Grips vee jaw face. After reaching 3 kN of load, the cylinder starts to move upwards according to quasi-static speed which is 1.25 mm/minute (0.02083333 mm/s).  As the pneumatic pressure insufficient and could not provide more force than 3 kN to penetrate the sheet metal, and it starts to move in another way around (in upwards) until the cylinder reached its maximum level(till stopper). Then the load starts to increase till reach the ultimate and fail. Due to very slow movement, through the multi-stage test, this was encountered. Since the 0.5 mm aluminium load is less than 3kN thus it has a trend without the constant region as 1 mm and 2 mm. in conclusion, the fixture caused the problem here and has to be changed in order to conduct the test.

Again thanks to everyone and my sincere apologies for any inconvenience caused.

If it is a tensile test and the sample is fixed well, this effect could be due to exeeding a force required for moving some element of the clamping system. This effect can occur when movable traverse is loose. Its weight should be exceeded to displace it.

It turns out that thickening the material leads to new mechanisms of resistance/plasticity. A reasonable question arises here, but what is the minimum thickness for such a situation? It is clear that 0.5 < x < 1, but I would like to be more precise. Perhaps this is not one mechanism, but several.