Hrynenko M. Modelling of the stress-strain state and the experimental-calculation method for evaluation of the fracture resistance of heat power engineering steel

The work includes an analytical review of modern theoretical and experimental approaches to the mechanics of deformable solids, which are used to establish the strain and fracture patterns of structural materials under complex loading. The main strength criteria are given and an analysis of the characteristics of the material's resistance to fracture is made. Taking into account the analysis carried out, the scientific and technical task of assessing the strength and resistance to fracture of low-carbon steels, which are used for the manufacture of structural elements of thermal power equipment and pipelines, taking into account their possibility to be hydrogen-charged, is set and solved in the work. The paper proposes a theoretical-experimental approach to predict the resistance to fracture of structural elements based on the damage accumulation model using the energy approach. One of the unique characteristics of the proposed approach is that it uses local parameters of the STS, which are determined by a highly accurate and easy-to-use non-contact method of ODIC, which can be used both in laboratory conditions and for structural elements under real operating conditions. The proposed method of plotting true stress-strain diagrams makes it possible to determine true stresses near concentrators in structural elements under complex loading. The use of these diagrams in engineering calculations and computer modelling using the FEM significantly increases their accuracy. Modelling of the damage accumulation was carried out using the energy criterion. The damage parameter is introduced as the ratio of the elastoplastic strain energy of the local volume to its critical value. The elastoplastic strain energy of the local volume of the structural element is calculated using FEM. The critical value of the elastoplastic strain (fracture) energy is taken as a characteristic of the resistance to fracture of the material under the given conditions of load, temperature, and working environment. It is determined experimentally as the area under the "true stress - true strain" curve, where the true strains in the local volume are calculated on the basis of displacement data in two directions on the basis of 0.5 mm using the ODIC methid. The value of the true stresses in the area of maximum strain is determined by taking into account the change in the cross-sectional area due to the change in the diameter of the sample during its deformation by uniaxial tension. The fracture energy and damage parameter for the modified Bridgman sample made of steel 22K under different load regimes were determined theoretically and experimentally. The research methodology and the mathematical apparatus used in the study are substantiated. To solve the formulated scientific problems, the methods of the mechanics of a deformable solid were used, primarily the theory of elastoplasticity and continuum damage mechanics. Numerical studies were carried out using the finite element method in the environment of the ANSYS 2022R1 software complex, in which three-dimensional computer models were created. All the theoretical developments of the thesis are brought to a specific engineering methodology for predicting the safe operation of elements of thermal power equipment, taking into account the action of the operating environment.