loading-unloading tensile test

Hello,

i would like to simulate a tensile test under stress-controlled loading and to compare numerical results to experiments.

Experimentally, i have imposed the form of this cycle (figure):

                                                      loading sequence: load up 4kN, 1mm/min

                                                      unloading sequence: unloading with a displacement -1mm/min

so my unloading sequence is not under load control (it is impossible with our tensile machine).

How can i simulate this test under stress-controlled loading?what is the form of my loading-unloading sequence?

Regards