Thermodynamics and kinetics of solids fragmentation at severe plastic deformation

The approach of nonequilibrium evolution thermodynamics earlier offered is developed. It helps to describe the processes of defect formation within the adiabatic approximation. The basic equations system depends on the initial defects distribution (dislocations and grain boundaries). The phase diagram is determined with the domains of the realization of different limiting structure types. The interaction effect of several defect types on the formation of a limiting structure is investigated in terms of the internal energy. The conditions of the formation of two limiting structures are found. The kinetics of the steady-state values establishment of the defects density is investigated within the scope of the adiabatic approximation. The dislocations density change follows the evolution of the grain boundaries density in this approach. It is shown that grain sizes, in limiting structures, decrease with an increase of the elastic strains.Comment: 14 pages, 6 figure

Nonlocalized and localized fatigue damage in metals and alloys

Проаналізовано основні закономірності розсіяного і локалізованого втомного пошкодження металів і сплавів. Запропоновано та обґрунтовано методи визначення умов переходу від розсіяного до локалізованого втомного пошкодження, засновані на аналізі закономірностей розповсюдження «коротких» і «довгих» тріщин. Із використанням розроблених методів визначено розміри тріщин і кількість циклів навантаження, що відповідають переходу від розсіяного до локалізованого втомного пошкодження вуглецевих, легованих і аустенітних сталей та алюмінієвого сплаву з урахуванням рівня напружень і властивостей досліджених матеріалів.The main regularities of nonlocalized and localized damage in metals and alloys are analyzed. Based on the analysis of the regularities of «short» and «long» crack propagation, the methods for determining the conditions for the transition from nonlocalized to localized fatigue damage are proposed and justified. By the «conditions for the transition» are mainly meant the crack sizes corresponding to the main crack initiation and the number of load cycles to this crack initiation. The first of the methods is based on the analysis of the kinetics of the growth of short fatigue cracks evaluated from either the crack-size or the crack growth rate dependence on the number of load cycles. In the transition from nonlocalized to localized fatigue damage at stresses above the fatigue limit, the form and parameters of the equations describing the crack size and the crack growth rate dependences on the number of load cycles are changed. The second method is based on the analysis of the kinetics of variation in the short crack propagation rate depending on the stress intensity factor. The stress intensity factor range and its corresponding crack size at which the SIF begins to change at an increasing rate is taken as the condition for the transition from nonlocalized to localized fatigue damage. Using the developed methods, the crack sizes and the numbers of load cycles corresponding to the transition from nonlocalized to localized fatigue damage in carbon, alloyed and austenitic steels and in an aluminum alloy are determined by considering the stress level and the properties of the materials under investigation. It is shown that the fatigue crack sizes corresponding to the transition from nonlocalized to localized fatigue damage at stresses above the fatigue limit and found with both the first and the second methods, decrease with increasing stresses, always remaining lower than the crack size at the fatigue limit. For the metals and alloys under investigation, the fatigue crack values corresponding to the transition from nonlocalized to localized fatigue damage in the region of high-cycle fatigue (105–107 cycles) vary in the range from 0,01 to 0,74 mm. In this case, the fatigue crack value corresponding to the transition from nonlocalized to localized fatigue damage is 0,07 to 0,02 mm for carbon and alloy steels, 0,2 to 0,74 mm for ductile austenitic steels, and 0,155 mm for an aluminum alloy. The ratio of the number of cycles to main crack initiation to the number of cycles to fracture increases with increasing number of cycles to fracture. The value of this ratio for the materials under investigation in the region of high-cycle fatigue varies within the range of 0,3 to 0,85

Fatigue and inelasticity of metals

A method for the investigation of high-cycle fatigue and inelasticity of metals under conditions of symmetric cyclic loading in tension-compression, bending and torsion is described. A procedure for the calculation of true stresses and inelastic cyclic strains in surface layers under nonuniform stress conditions (bending, torsion) is proposed. A large group of steels has been studied and it is shown that the use of inelastic cyclic strains at the stage of stabilization of the deformation process as a measure of the fatigue damage accumulation intensity makes it possible to explain some general laws of high-cycle fatigue, including the influence of stress gradient, load biaxiality, befrequency character of loading, loading frequency on the fatigue behaviour

Multiparametric Optimization of Complex System Management Scenarios Based on Simulation Models

This work is devoted to the development of a multiparametric optimization module for a digital management decision support tool based on simulation models. It is noted that the optimization of simulation models of complex socioeconomic and sociotechnical systems involves the generation of multiple scenarios of system development, their calculation, and further comparison, which imposes additional requirements on the optimization algorithms used. Moreover, complex socioeconomic and sociotechnical systems are characterized by a multiplicity of goals, which leads to multiparametric optimization.  The result of the work is the algorithm for solving the problem of optimization of multiparametric scenario calculations using the example of a two-parameter optimization problem. The scope of the calculation optimization problem is to form the optimal set of scenarios that will ensure satisfactory computing time and, at the same time, give a representative scenario calculation result. Thus, the contribution of the current research is to formalize the processes of optimizing the parameters of simulation models of complex systems. In the course of the study, existing approaches to process optimization are considered. Based on the analysis of existing approaches to the formation of an optimal set of scenarios, ways to improve the algorithm type using approaches to scenario reduction or the introduction of genetic algorithms for the formation of an optimal set of scenarios are proposed. This work is carried out within a project to develop a digital tool to support managerial decision-making in sociotechnical and socioeconomic systems