We present a description of the current status of research on deformation properties of elastoplastic materials under quasi-static loading. Based both on experimental and theoretical approach and on the developed method, true stress-strain curves were produced in the case of stretching of a cylindrical shell and a solid rod until fracture. The difference between produced curves for these types of specimen is shown for the first time for heterogeneous and non-uniaxial stress-strain state after beginning of the localization process of deformation. We show that the difference between the curves is inversely proportional to the volume rate of stress-strain state at the neck of a specimen.