This type behaviour may be observed for staircase structures such as the double stranded DNA such as alpha polypeptide where the sudden drop in the stress indicates that one of those strands is ruptured but the rest of the structure is still in tacked. Between two successive strand rupture the main frame helical structure shows elastic deformation........Note: dsDNA (alpha peptide) has four strands.

In the case of 3D periodic copper ???? nanostructure I don't wish to speculate before I haven't had any micro structural information at hand!!

Dear Tarık Ömer Oğurtanı

Thank you for your response. As you said, this type of behavior indicate standard DNA, but my system is copper under 300K temperature (40X40X40 angstrom) using EAM potential which the plot of stress strain is strange for me and when I want to see its visualization in ovito the sample won't ruptured and it can not be separated.

It looks like there are a number of data sets at low stress on this plot. Are they monotonic? What are they? Does a modulus of 60GPa make sense?

Dear Mr. Farrington

Thanks, I could solve the problem by changing the fix command in my lammps code.

yes, they are monotonic and my data sets related to the EAM potential of copper which is correct but as I said my lammps code wasn't accurate.

Thanks again.

Hi. I guess your "LAMMPS code" was correct, but your "LAMMPS input" was incorrect. What change did you make in the "fix" command that solved the problem? How does the new stress-strain curve look like?

Thank you.

Hi Mr. Warrier

Actually, I used fix npt in deformation. I change it to nvt then my curve convert to standard form.

Just mention that I have to work on it to improve my curve.

Please find the attachment at below.

Dear Mr. Khademabbasi,

I think I faced same problem earlier. What I have observed carefully that under periodic condition the stress-strain curve shows weird cyclic behavior (positive and negative) if you run the simulation till its visual failure. But what I have observed under periodic condition the nanostructures rupture from edge portion which leave the boundary from one side and enter from another side. If you observe carefully you can notice that thing. So I think your first curve is correct and you should take the data upto 0.1 strain for your case and ignore the rest. It will show enhanced mechanical properties like in nanowires with constrained length. But if you use non-periodic condition you will get some another result. Periodic boundary mainly shows high strength with brittle behavior whereas non-periodic boundary show less strength but ductile behavior.

This is my experience on many nanowires I have studied and also upto certain size. Above that size it behaves normally under periodic condition. I have communicated a paper based on this study.

Dear Mr. Sarkar

Thank you so much for your information. It was very helpful for me.

May I have your paper about this study.

Dear Mr. Khademabbasi,

I am glad that my information was helpful for you. Currently the paper is under review in the journal Material Research Express. Kindly wait for few days, I will surely provide you the paper as soon as it is accepted/published.

This is because you deformed under uniaxial stress conditions (allowing changes in other dimensions (e.g. Y and Z) while applying tension in one direction (e.g. X). You will see failure if you deform under uniaxial strain conditions (no change in other dimensions is allowed while applying deformed in one direction).

Exactly, in the first plot, my sample failure failed but in second one the phenomena of failure happened in visualization.

Thanks

I largely endorse the observations of Mr. Sarkar. You may find some useful direction of realising your problem. In this context my suggestion is that you should try to understand the mechanical behaviour of your material in the light of its large grain boundary area and distribution of energy at it which may dynamically influence and facilitate the nucleation of micro crack to final fracture under monotonic condition of uniaxial tensile loading.