nanowire rotation issue for small size nanowires during tensile deformation

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1

Dear LAMMP users and developers,

I am simulating the tensile testing of Cu nanowires under strain rate controlled testing. Equilibrated nanowires rotate during deformation for small size nanowires (square cross-section nanowire width < 4 nm and length < 8nm), whereas i didn’t observe this rotation problem for larger size nanowires. nanowire has been maintained periodic boundary conditions along loading direction, while the remaining directions were left free. What is the reason behind this rotation behaviour of nanowires during deformation for small sized nanowires and how to solve this issue.

Thanks in advance,

P. Rohith

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Well this really isn’t a LAMMPS question, more of a material science question. Here a few thoughts you can chase down. The ‘rotation’ I am going to assume is the formation of slip bands or motion of twin boundaries, which is very much a real physical effect of loading(see https://www.nature.com/articles/nnano.2012.116 ). The observation of slip is an effect of resolved shear stress, change your loading axis to something else in the crystal to see this effect. In MD your strain rates are very high, and if these samples are defect free (lack of stress concentrators), you will be at the mercy of thermal fluctuations to nucleate the slip band. Also, a square nanowire really isn’t physical, surface energy is going to dominate and will prefer a rounded surface. Hope that helps guide your next set of experiments.

-Mitch

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I am sorry for not mentioning my issue clearly. I had observed the rotation phenomenon during elastic deformation itself (not during plastic deformation). I am performing simulations at a strain rate of 10^8 s^-1. I had observed this issue only for small size nanowires as mentioned previously.

Thanks in advance,

With regards,
P. Rohith

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Ok, I still don’t know what you mean by rotation then. Are you using the “rot yes” portion of the velocity command (http://lammps.sandia.gov/doc/velocity.html)? It really isn’t productive for me to keep guessing as to what the problem is though.

-Mitch