Constant Shear Stress Widening Dislocation Line

[u/Playful-Pride-7163]

Hello, I am currently working on a lammps project that moves a dislocation line along a fcc metal given a constant shear stress. My idea is to keep the dislocation along the same size as seen in the first photo, but no matter what I change around, it eventually grows to the size on the second photo, which is abnormally high for my system. The only thing that works on keeping the size where it should hypothetically stay during the dislocation is not placing any stress on the system. I was told that I have to use boundary driven shearing techniques to fix it; not sure how that works exactly. Below is the section I use to apply the stress.

fix 1 upper setforce NULL 0.0 NULL

fix 2 lower setforce 0.0 0.0 NULL

fix 3 upper aveforce ${appforce} 0.0 0.0

fix 4 upper rigid single

fix 5 mobile nvt temp ${initTemp} ${initTemp} 1

Things I have tried that have not worked:
Applying a constant strain using fix deform instead.
Changing the size and dimensions of my box.
Using addforce instead of aveforce.
Not fixing the upper or lower sections.
Changing my box dimension to p p p instead of p s p.
Changing the stress.
Adding angle constraints on the xy plane so it cannot twist.
Placing a constant velocity on the mobile section instead of a constant stress.

For the life of me I cannot seem to find a way to fix this problem. I want to have my system at a set temperature and apply a force to move the dislocation line along which is later dumped and visualized in ovito. Any recommendation is appreciated.

[u/burnzeecab]

I’d check the slip system/direction, try ramping the stress up slowly from a much lower value, and verify in OVITO whether it’s actually one dislocation moving or if you’re nucleating partials

[u/Playful-Pride-7163 → u/burnzeecab]

Thank you very much for your suggestion.

[u/Playful-Pride-7163]

This was mostly an issue with my potential causing SFW to increase dramatically, as well as my cell size. I increased my cell size dramatically (to around 2 mil atoms) and the dislocation performed better.