SaiD
Ansys Employee

If you do not include a damage or failure model, then the ultimate strength is again used only to calculate the factor of safety during postprocessing. 

Unless you include a plasticity model or a damage/failure mode, then material will continue to behave elastically at every load, even though in real-life the material might be failing or yielding. So it is necessary to model the material correctly in order to get an accurate response from the lattice structure.