620.172.25/625.143 and N. Ya. OpravkhataWe present the results of investigations into the influence of high-density electric current pulses on the mechanical characteristics of rail steels in the initial state and after a long in-service operating time. The effect of in-service loading conditions and modes of exposure to electric current pulses on the steel mechanical properties and their anisotropy is shown.Introduction. In terms of freight and passenger traffic volumes, railway transport occupies one of the first places in many countries of the world, including Ukraine. Recently, the strategy of railway transportation system development is aimed at increasing the traffic volume due to a decrease in the time interval between trains, an increase in the total weight of trains and their movement speed, the comfort and safety of passenger traffic. For this purpose, rapid transit railroads and new types of rolling stock are introduced.As a result of an increase in the weight of trains and their motion velocity, the levels of mechanical loads on track structure and the density of propulsion currents supplied by electric locomotives increase. The railroad bed operates under conditions of a complex stress state: it undergoes the action of vertical and horizontal forces exerted by the rolling stock, longitudinal traction forces exerted by driving wheels of an electric locomotive and braking forces, longitudinal tensile and compressive forces induced by the temperature variation relative to the rail length fixing temperature [1,2]. In many ways, the standard mode of operation of the rail transport depends on the ability of heavily-loaded structural elements of track structure to resist deformation and fracture, their continuous interaction with the rolling stock, the life and reliability of these elements during the whole period of operation. A structural feature of the electrified railway transport that separates it into a single category of electric energy suppliers is the method of supplying current to the rolling stock. The electric systems that provide power supply of the rolling stock, alarm signaling and traffic safety control are integrated with rail tracks and are terminated in the zone of interaction of the wheel-rail contact pair. Under the action of high-level operating mechanical loads and high densities of electric current, the railhead material in small areas operates under special extreme conditions [3].Numerous investigations on the assessment of the effect of the electromagnetic control on the mechanical properties of metallic materials of different grades [4,5] show that the action of electric current can change them considerably. The earlier [6] analysis of literature data on the mechanical behavior of the railway track, the level and modes of the electric influence on rails enables one to state that the conditions of their operation correspond to those under which the electroplastic deformation is possible [5]. The investigation of the rail material behavior under the action of the electric curren...
620.178.15/625.143 and N. Ya. OpravkhataThe analysis of the mechanical state of the materials of a new rail and a rail after long-term operation are performed on the basis of the obtained data on the distributions of hardness over the cross sections of rails and the spread of the characteristics of hardness.Keywords: residual and contact stresses, hardness, spread of values of the characteristics of hardness. Introduction.In the course of operation, the materials of heavy-duty structures undergo various types of changes. Thus, in new products, we observe the processes of adjustment of the components with intense wear of the working surfaces, as well as the processes of adaption of the upper layers of the materials to the modes of operation, temperature conditions, etc. The indicated adaptation leads to irreversible structural changes in the materials, which, as a rule, increase their serviceability [1]. The subsequent long-term operation promotes the initiation and development of defects in the materials of the workpieces. It is clear that all these processes are typical, in particular, of the materials of wheel-rail systems.The structural inhomogeneity of structural materials prior to operation manifests itself in the spread of values of its mechanical characteristics in local volumes. A noticeable structural inhomogeneity of the material of the rail in the intact state caused by the technological processes of its manufacturing (rolling, thermomechanical treatment of the entire rail or its head, etc.) increases in the course of operation. The inhomogeneity of the mechanical properties of structural materials over the cross section of any product is estimated according to the distribution of the values of hardness. In this case, the analysis of inhomogeneity of the properties of materials is performed both on the basis of direct values of hardness [2, 3] and by using statistical methods [4][5][6]. The approaches extensively used in practice enable one to monitor (without significant costs) both the state of load-carrying elements and the accumulation of defects in the local zones of structures without their destruction [4].If a wheel rests on a rail, then a three-dimensional stressed state is formed in the central part of its head. In the case of hunting of a train, the contact zone shifts toward the working fillet. In this case, one may observe the realization of two-point contact as a result of the interaction of the wheel flange with the rail. This leads to the formation of a stressed state close to pure shear [7]. The action of numerous factors of different nature affecting the stress-strain state of the railway bed results in the necessity of a large number of replacements of defective and highly defective rails in the railway tracks and frequent failures of rails, which is especially dangerous for the mainline railways with fast motion of passenger and heavy freight trains.The levels of operating loads and the fields of residual stresses play a significant role in the operation of rails. In passing all ...
The methods developed at the Pisarenko Institute of Problems of Strength of the National Academy of Sciences of Ukraine for determining the allowable stresses for structural elements operating at cryogenic temperatures are considered. The methods make it possible to take into account the effect of the low-temperature hardening and the softening electromagnetic action on the value of allowable stresses.Introduction. By a mechanical strength of any component or structural element is meant their ability to withstand the action of various factors while maintaining the integrity and serviceability. The retention of the integrity and serviceability of the structure is governed by its ultimate state and is dependent on the stress-strain state of its elements, level and nature of loads, environment, etc. Depending on the type of the structure ultimate state (in terms of the load-carrying capacity, the development of excessive strains, the formation or opening of cracks), one or another ultimate state criterion is chosen. The maximum loads resulting in fracture or occurrence of residual strains, the residual strains, crack initiation, time or number of cycles to fracture, etc., can be chosen as the criteria. The onset of the ultimate state testifies to the fact that further operation of the given structural element is impossible. As noted in [1], "... such approach does not coincide completely with the commonly accepted one when the strength is limited by the fracture of the component. If the ultimate state of the object under study is known to us, and the criterion is chosen that represents the quantity describing the attainment of the ultimate state, namely, the force, stress, temperature, time, size (variation in the length or thickness) of the worn out layer or the crack length, then this criterion should be taken as a limiting value, using which the allowable value of this quantity with a certain safety margin should be chosen that will be dependent on the degree of accuracy of the estimated parameter adopted as a criterion of the ultimate state." For a number of structures operating at low and cryogenic temperatures, the onset of the ultimate state can be defined by the occurrence of residual strains and crack nucleation. Accordingly, in these cases, the criteria for its onset will be the following: the equality of the calculated equivalent stresses in the dangerous zone of the structure to the stresses of the onset of the material plastic flow (σ σ
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