  The evolution of metals micro/nano-structure upon severe plastic deformation
(SPD) is still far to be theoretically explained, while experimental datasets
are persistently growing. Major problem associated with understanding of SPD is
a synergetic effect of several competing processes which alter material
structure. In this paper we propose a model to reveal, as free-fitting
parameters, strain breakpoints where predominantly one mechanism determines the
micro/nano-structure in SPD materials from material hardness vs true strain
experimental data. The model is applied to analyse SPD data for pure
polycrystalline iron and two distinctive strain breakpoints have been revealed.
