Thank you very much Axel, your answers help me a lot; Is the fix deform command appropriate for obtainning the strain-stress curve, if yes, is it necessary to wait the deformed box balance for a while and continue stretching the box instead of stretching the box incessantly? It seems that the former somewhat simulate the real stretching better.
this is not my area of research and thus i cannot give you an authoritative answer on this. however, this is a very common type of calculation, so there should be text book material, review papers, residual knowledge from coworkers and supervisors and comments and discussions in just regular publications that do such calculations where the authors discuss their choices (at least in good papers there should be or at least a reference to a publication that has).
axel.