Please post to the mail list, not to me personally.
Stress/strain curves imply the volume changes, else you
don’t have strain. There is nothing in your script that
is changing the volume of the system. Just fix move on
the atoms, won’t likely change the volume. Since you
have non-periodic boundariies, the atoms will just
relax back to where they want to be. I suggest you
use a periodic box and look at fix deform. That is the
usual way of measuring stress/strain effects.