The experimental and theoretical study of large deformations of cylindrical samples from steel 09G2S with stress concentrators under tension-torsion loading to failure

The paper presents the experimental and numerical results of elastoplastic deformation of specimens with a cylindrical working part and specimens with a stress concentrator made of 09G2C steel under monotonic kinematic tension-torsion loadings prior to failure, taking into account large deformations and inhomogeneity of the stress-strain state (SSS). A complete system of equations describing generalized axisymmetric torsion problems is written in a cylindrical coordinate system. Kinematic relations are formulated at speeds in the metric of the current state, which makes it possible to describe large deformations. The equation of motion of a continuous medium follows from the equation of the balance of virtual capacities. Since we consider active loading processes close to proportional ones, we describe the elastoplastic properties of materials by the flow theory with nonlinear isotropic hardening. The system of equations, supplemented by kinematic boundary and initial conditions, is solved by the finite element method in a combination with an explicit integration scheme of the "cross" type. On the basis of the experimental-calculation method, true deformation diagrams for tension and torsion were constructed to supplement the plasticity model with material functions. These diagrams differ significantly with deformations of more than 15 %, which is caused by the sensitivity of the plastic properties of the test material to the form of the stressed state. To describe the tension-torsion, the dependence of the deformation diagram on the form of the stressed state is introduced. The refined deformation diagram is a linear combination of tension and torsion diagrams, which coefficients depend on the parameter of the form of the stressed state, the parameter of the stress triaxiality or the Nadai-Lode stress parameter. There is a good correspondence of the numerical results with the experiment on the integral characteristics (axial force from axial displacement and torque from the twist angle). The analysis of the SSS parameters in the neck and circular concentrator is performed. The estimated the peculiarities of the mutual influence of tension and torsion on the localization of deformations and destruction of cylindrical samples and samples with a stress concentrator.