Issue 24

M.P. Tretiakov et alii, Frattura ed Integrità Strutturale, 24 (2013) 96-101 ; DOI: 10.3221/IGF-ESIS.24.10 96 Special Issue: Russian Fracture Mechanics School Tests in tension-torsion conditions with descending sections of strain curve construction M.P. Tretiakov, V.E. Vildeman Center of Experimental Mechanics, Perm National Research Polytechnic University, Komsomolskiy Pr. 29, 614990, Perm, Russia cem_tretyakov@mail.ru A BSTRACT . The paper is devoted to theoretical and experimental investigation of materials behavior on postcritical deformation stage (strain softening). Tests results, which confirm theoretical justification of specimen configuration impact on the possibility of creating descending sections of strain curve, are given. It is shown that, with sufficient stiffness of loading system, equilibrium postcritical deformation of materials is possible. It is confirmed in uniaxial tension tests with unloading on postcritical deformation stage. Test results on tension with torsion of thin-walled tubular specimens in strain softening conditions are adduced. K EYWORDS . Experimental mechanics; Postcritical deformation; Proportional tension with torsion. I NTRODUCTION issipative processes of inelastic deformation, including the processes of structural failure and fracturing, reflect on the deformation curve as nonlinearity. At the final stage this leads to softening of materials and the appearance of a descending section on strain curve. Completeness of implementation of the load-carrying capacity of strength constructions is determined by the degree of postcritical deformation. When postcritical deformation stage is taken into account in adjusted computation, it allows a reserve of constructions load-carrying ability to be revealed. It makes possible to forecast the destruction moment more accurately [1, 2]. Questions of theoretical, experimental and numerical study of postcritical deformation mechanisms of materials attract attention of researches in connection with the questions of use of materials deformation reserves, the rise of load-carrying ability and survivability of materials, and the analysis of possibilities of failure processes management [1-9]. On the postcritical deformation stage the formation of macro-destruction conditions takes place. These conditions, unlike the traditional view that defines the use of force or deformation criteria are not definitely related to the stress-strain state at the point of deformed body. During the transition from the equilibrium stage of damage accumulation to non- equilibrium stage of destruction, interaction of the deformed body with the loading system plays the key role. As a result, based on loading conditions, each point on the descending section of stress-strain curve can correspond to the time of the loss of load-carrying ability as a result of transitioning 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 process of destruction due to local dissipation of elastic energy [1, 2]. However the force or deformation criteria can be considered with a view to transition of material to postcritical strain stage (similarly the condition of transition to plastic state). It is necessary to experimentation verify of criteria, particularly by test results in plane stress state under combined tension-torsion loading [11- 13]. D