Stress of the loading- unloading uni-axial tension

Dear Everyone,

I did a set of loading-unloading uni-axial tension tests on polymers, i.e. first use “fix deform” to deform the box to 100% strain along one direction, then unload the box by setting the pressure along all three directions equal to zero.

Please see the figure attached for the stress vs time curve, time = 1ns is the point of unloading.

My question is: the stresses along all directions seem reasonable during the loading part,
but the stresses don’t go to zero during the unloading part although I set them all equal to zero at this part.

Does anyone have experience on this problem?
Thanks for your comments and suggestions.

Lili Zhang

Loading-unloading stress.gif

The system might take a while to relax. Are you using
fix npt aniso, so the xyz dims can relax independently?