The paper presents assumptions of the Gurson-Tvergaard-Needleman (GTN) model for a porous materials, which is now one of the basic damage material models applied in the analysis of the load-bearing capacity of metal engineering structures. The practical information about the selection of GTN material parameters and performing numerical calculations is given. An example of the numerical simulation of the failure of tensile element made of S235JR steel is presented. The calculation procedure, the determined GTN microstructure parameters and the obtained results are described. The simulations included the strength analysis and the study of the micro-defects growth occurring during the plasticity process is presented. As a result the critical volume void fraction was determined which can be used as a microvoid criterion corresponding to the expected S235JR steel failure.