Development of new highly efficient equipment in power engineering, metallurgy, space, military and some other branches of industry is connected with the problem of exploitation of materials on the edge of their capabilities. In other words, under conditions of increased temperature and significant plastic deformations. Understanding of physical regularities and constructing authentic mathematical models of material deformation in such conditions is a quite difficult but accomplishable task for scientists who study properties of heat-resistant alloys.
Team of researchers from South Ural State University for two years has been participating in development of RUGK (space-based gas-cooled reactor) for a project of Roscosmos State Corporation in order to create a ‘transport and power module’. The work included research of deformation properties of materials and development of calculation methods.
Engineer of the SUSU Laboratory for Physical Modeling of Thermomechanical Processes, postgraduate student Sergey Samoilov, under supervision of Doctor of Engineering Sciences, Professor Aleksandr Chernyavskiy, studied an alloy on the basis of molybdenum within the frameworks of the research. Obtained results became a foundation for the project entitled “Development of mathematical model of cyclic unstable materials for the use in calculations of technological, operational and emergency modes of constructions’ deforming”.
“I analyzed bibliographical data and came to conclusion that the use of some comparatively common approaches allows for representing qualitatively different demonstrations of materials’ properties within a single model. My task is developing such a model,” says Sergey Samoilov.
Sergey Samoilov’s area of expertise is materials which are used in constructions simulated for comparatively small (up to 1 000) number of cycles before destruction. For example, power plants in space industry. The existing models of materials are poorly fit for simulating such constructions: it is necessary to describe deformation of material maximally accurately within the frameworks of the available resource.
“Usually, power plants are simulated for low-cycle fatigue. In this case we get in the range from tens to thousands of cycles. Comparatively recently, such concept as ‘ultra low-cycle fatigue’ appeared in English literature. Properties of materials in such conditions are poorly studies,” explains the young scientist. In domestic literature, ultra low-cycle fatigue is not singled out as a separate simulation case.
The goal of the SUSU postgraduate student’s work is to obtain a mathematical model which will allow describing material in sufficiently general conditions. At the stage of literature analysis it was discovered that the case of ultra low-cycle exposure is more complex compared with the case of one-time or low-cycle loads. First of all, it summarizes the effects characteristic for the latest ones, and secondly, it supplements these two cases with new effects which are not present in standard simulation cases.
“In particular, at big deformations specific for ultra low-cycle destruction, an abnormal curvature of deformation diagram in a cycle is observed for a series of materials (the dependency of strains on deformations under cycle exposure),” makes an example Sergey Samoilov. “Such effects can lead to unstable deformation of constructions: the appearance of zones of deformation’s localization in a form of thinning, folds, contractions, etc. At the present time there is no physically proved model which would reflect such effect. I assume that the model describing the ultra low-cycle properties should include this as well.”
Aside from the deformation anomaly, described above and called in foreign literature the ‘stagnation of strain hardening’, the case of ultra low-cycle fatigue is characterized by degradation of elastic properties: after preliminary deformation of material and its subsequent deformation backwards, a decrease of elasticity module’s value can be observed. This causes the reduction in rigidity in the most loaded zones of constructions. Nowadays there is no physical explanation to this effect. The work of Sergey Samoilov at the moment allows taking this effect into account.
“Physical models, as a rule, are complex and impossible to adapt completely to engineering calculations. Therefore in my work I’m trying to apply general prerequisites for physical models by combining them with the ‘phenomenological’ approach (describing a specific phenomenon) and therefore obtain a model which is considerably easy for perception by qualified specialists,” continues the young engineer.
Competitive advantage of Sergey Samoilov’s work is the presence at SUSU the necessary for research high-tech equipment, and the existence of serious scientific schools of metallurgists and specialists in the sphere of calculating structural strength. The SUSU Laboratory for Physical Modeling of Thermomechanical Processes is equipped with simulator Gleebe 3800. There are only 8 such devices all around Russia; they are located in Moscow, Saint Petersburg, and Magnitogorsk. Using the supervisor’s and his own experience in research, Sergey Samoilov intends to obtain considerable scientific results. Two articles of the young scientist, dedicated to the research, has already been published in an authoritative scientific journal entitled “Materials Science Forum”, indexed in Scopus international database.