Hi Ahmed,

it looks like some slipping happens. But I am not familiar with your experimental set up. You may say something about your set-up?

But you said nothing about yor sample.  There may  by be internal cracks popping up during the experiments.

What is your sample? Only a rough descrpition if you are not allowed to tell us details. Is it your sample a fibre composite structure or something else?

Does your experimental set up has got vibrations isolation to the ground floor?

Shock transients  from your laboratory surrounding may enter your system ( e.g. from a workshop nearby or from a street having a lot of traffic etc.). You cannot feel the very most of those vibrations.

It is better to isolate your machine to the surrounding area, especially with big machine,which can introduce the vibrations. 

Besides, though it is likely the slip during the test. But it will be easier to find the reason to detain decribe your test and material. 

Dear Kun Liu , Gerhard Martens , 

My Sample is PLA polymer ,  according to tensile sample ASTM D638-03 type I , rate 5mm/min . 

but lets speak generally , is this obtained curve accepted ? regardless of solving and checking the reason of the problem .

Hi Ahmed,

the overall shape of your stress-strain curve looks quite ok.

By gooling for 'stress strain curve' you will find a lot of such typical curves.

Do you have repeated your  measurements taking another sample of your material ?

If the overall shape is the same (including the coarse structures) you will then have probably  done the right experiment.

 The coarse structures  are probably caused by some structural changes in your material when it is exposed to the excessive stress.

It think that it is unlikely that the sharp spikes showing up in your curves are also due structural changes because these should not show up in a plastic by a sudden event (looking like cracks). For crystalls or fibre based composite structures I can imagine such a behaviour because of suddenly apperaring cracks.

I may be wrong by excluding upcoming cracks in PLA; there may be other colleagues commenting on that.

May I ask about the test whether it is carried out in pure tensile or in pure compression? Slippage during tensile looks logical? I don,t think so. Slippage may result from shearing effect of stress means compression.

regards

Dear Ijaz , 

YES PURE tensile