The research is devoted to study of dependence of loading system stiffness on ultimate states of material at failure. Questions associated with theoretical and experimental study and analysis of mechanisms and control methods of damage accumulation processes, problems of implementation of the postcritical deformation stage of materials during testing where considered. On the postcritical strain stage is formation macro-destruction conditions. During the transition from the stage of equilibrium damage accumulation to non-equilibrium destruction stage the key role play the interaction of a deformed body with the loading system. As a result, depending of loading conditions, each point on the descending branch of stress-strain diagram can correspond to the time of the loss of bearing capacity resulting from a transition from stable to non-equilibrium stage of the damage accumulation process. Thus, the rigid loading system may contribute to the "adaptation" of the object in the destruction process due to local dissipation of elastic energy [1, 2].
The experimental researches relating to finding a probably effects of resource plastic deformation increase for description of the materials behavior in real operational influence environment. Shown that additional vibrational impact (cyclic torsion with small amplitude) could lead to equilibrium deformation of material on the postcritical deformation stage during tension of solid cylindrical specimen. It is shown that obtaining of strain curves with advanced softening stage and complete deformation curves of materials with sufficient high stiffness of loading system are possible. Test results of uniaxial tension of steel specimens with different loading system stiffness with using of special test tooling and tension-torsion of thin-walled tubular specimens were carried out. Accounting of the postcritical deformation stage in the more exact calculations is reveals reserve load-carrying capacity of structures. Completeness of the implementation of the load carrying capacity of critical constructions and buildings is determined by the degree of postcritical deformation. Realization of postcritical deformation stage allows using the reserves of load-carrying ability, increase vitality and safety of deformable systems.
The uniaxial tension, torsion and proportional tension-torsion tests carried out by universal biaxial servohydraulic test system Instron 8850 (100 kN/1000 Nm).
The work was carried out in the Perm National Research Polytechnic University with support of the Government of Russian Federation (The decree № 220 on April 9, 2010) under the Contract № 14.В25.310006, on June 24, 2013.