applying tensile load in periodic dimension

Dear all,

I have a question about applying tensile load in periodic dimension. I know the “deform” command is able to apply tensile load but I have not found it useful for the tension test. For example when I use “fix 1 all deform 1 z erate 0.01 units box remap x”, it changes its sizes in both +Z and –Z directions. It seems force is applied in the both directions of Z. I need a command which applies load in only one direction of Z ( one of +Z or –Z ). I mean it increases its size only in one direction. Is it possible to do this? Because in non-periodic cases, I do tension test easily by fixing one end and applying the “velocity” command at the other end. But what about periodic cases? I have done tension and compression tests by deform command but the results are not valid.

I really need your help.

It's a periodic box. If it starts as 0 to L,
then it doesn't matter if it ends up as -delta to L+delta
or as 0 to L + 2*delta. The dynamics will be the same.
LAMMPS is simply applying the strain symmetrically.