No 4 (2019)

While in operation, the parts with the applied coatings often work under variable loads, which affects the service life of the products. Many papers deal with the study of fatigue properties of such products, however, the study of the influence of variable loads on the structure and properties of the coatings has a fragmented nature. As the determination of fatigue characteristics of samples with the applied coatings is still important, the goal of this paper is to develop the design of the plant for fatigue tests and the technique of testing the samples with coatings under the variable loads. To develop the technique of fatigue tests, the authors created a unique design plant. When developing the plant, the authors considered the fact that the loading of a sample should be regular that the base and the substrate materials will experience stresses constant along the full length of a sample. Using the analytical solution of the strength task, the authors developed the technique of determining the stresses in a coating and a substrate. To prove the appropriateness of using the suggested technique, the numerical methods were used. For this purpose, using the Comsol Multiphysics software in a 2-D setup, the authors were solving the task of determining stresses in a sample consisted of two layers of the materials. In the result of the study, the authors developed the design of an experimental plant for testing samples with coatings with the maximum tensile force of 5000 N and the possibility of smooth change of vibrational frequency. The authors developed and tested the technique of testing samples with coatings without separation of a coating from a substrate. The study showed that the application of the described technique is reasonable if the thickness of a coating is not more than 7 % of the thickness of a substrate.

The paper considers a three-link crawling snake-like robot, the links of which are connected by two-axis hinges. The robot is equipped with four bearing supports with the controlled coefficient of friction between them and the surface. The device is designed to move inside buildings after the emergencies to search for people under the debris and to transport the essentials to them. The robot is controlled by the operator in two modes: the sequential execution of commands (forward, backward, turn, etc.) and moving from the starting point to the final one. Working in each of the control modes requires the use of the gaits bank. The authors developed the classification of gaits for a crawling robot based on several criteria: the admissible periodic separation of links from a surface, the ability to control by friction in the supports, the combinations of controllable bearing supports, and the implemented type of motion. The authors studied in detail the controllable planar gaits of a robot when moving it forward and distinguished four types of gaits: longitudinal, transverse, transverse s -shaped, and longitudinal-transverse. For each gait the sequence of stages is developed, the conditions of their beginning and end imposed on the movements of the links are formulated and the vector of generalized coordinates is defined. As a result of numerical simulation, the authors built the graphs of trajectories of the centers of mass of links and the center of mass of the entire device for each gait, as well as the graphs of time dependences of the angles of rotation of links. The study identified the influence of angles of links relative positions on the distance traveled by the robot. It is revealed that for the same time of movement, the robot will pass the greatest distance with a longitudinal gait irrespective of angles of links relative position. The device will be able to march the same distance at two types of transverse gaits at the maximum possible angle of the links relative position.

The traditional approach to ranking structures and fractures as the comparison with standards (pictures) does not allow to objectively describe the existing diversity of their geometry, to provide the direct comparison of the morphology of structures and fractures to identify critical parameters of structures determining the difference in their resistance to fracture. Formalization of approaches to the description of digital images of structures and fractures is complicated, in particular, due to the differences in the mechanisms of destruction of nominally similar structures that differ in the geometry of the structure of its individual elements and their configuration as a whole; the resulting differences in the metrological support of image processing procedures. It is usually understood that this is provided by default within the framework of the specialized software products used, but in practice, the necessary attention to comparing the alternative options for image processing procedures to choose the optimal one is not always paid. In this regard, the paper considers some aspects of obtaining digital images of structures and fractures, their processing, providing reproducible and comparable results that carry meaningful information about their morphology. In particular, the authors evaluated the role of the etching duration, the choice of the optimal magnification of the microscope (in the range of values comparable in their capabilities to solve a specific problem), and the noise removal procedure. The paper discusses the approaches to the selection of effective image processing algorithms, for example, when changing over from the point estimates of structures to the measuring of their geometry (taking into account the ideas about the statistical nature of structures and fractures, measurement scales). The authors estimated the efficiency of using classical and nonparametric statistics when comparing the results of measurements of the structures and fractures geometry, and the possibility of classifying “blurry” images of microstructures based on the Fourier transform. Based on the results obtained, the authors reviewed some procedures for processing the images of structures and fractures and showed that the use of statistical characteristics of images of structures and fractures makes it possible to rank more objectively the structures according to their geometry.

This paper gives the information analysis on the ratio of the melting rate of electrode wire on welding arcs of direct and reverse polarity in СО₂. At equal currents, the melting rate on direct polarity arc is about two times higher than the melting rate on reverse polarity arc. When welding in shielding gases, the reason to refuse the use of direct polarity arc is the low melting rate stability of the electrode wire. It is caused by the intense moving of arc cathode spot affected by the emissivity change of the electrode surface. Within the scope of this paper, the authors propose a calculation method for arc power transmitted to a consumable aluminum electrode on various polarities. The calculated specific power (per 1 А of the current) is significantly higher for electrode-cathode, and when step-up the current, the power increases more intensively than for anode. The experiment determined the melting rate of aluminum electrode wire of 1.2 mm diameter for direct polarity arc in argon. It is as well about two times higher than for reverse polarity. Within the limits of 80-180 А currents on the arc direct polarity, there was not defined any significant dependence between the melting ratio of aluminum wire and arc current. The calculation method ensures good convergence of designed and experimental data on the ratio of electrode melting rates on different polarities. The formulas obtained allowed evaluating the effective arc power in argon for aluminum products. Further researches are to be aimed at defining the stability conditions of the melting rate of electrode wire on direct polarity arc in shielding gases. It is especially necessary for welding of heavy thickness parts to reach more efficient filling of edge preparation.

In the present work, the authors studied the development of the oxidation process in some binary copper alloys (Cu - 40 % Ni, Cu - 30 % Ni, Cu - 1.6 % Fe, Cu - 0.4 % Cr). The authors determined the principal locations for the formation of corrosion centers on the surface of textured tape substrates of Cu-Me alloys (where Me=Ni, Cr, Fe) after annealing in an oxidizing atmosphere for 5, 30, and 250 min at the temperature of 700 °С. The study established that the oxidation of the surface of thin tapes of Cu - 0.4 % Cr and Cu - 1.6 % Fe alloys is not homogeneous, in contrast to the Cu - 40 % Ni and Cu - 30 % Ni alloys. The corrosion centers formed more intensively on the segregated particles of the alloying element - pure chromium or iron with a bcc lattice. The study discovered that the oxide film formed as a result of prolonged annealing, in Cu-Cr and Cu-Fe alloys, has a greater thickness in the zone of grain boundaries. According to the X-ray spectrum analysis, in the spectra taken from the boundaries, the higher oxygen content is registered than in the central zone of a grain. The study shows that in the textured tapes of Cu-Cr and Cu-Fe alloys, both the surface oxidation and internal oxidation of tapes occur in the process of short-term annealing (700 °С, 5 and 30 min). As a result of electron-diffraction analysis, the authors identified that, in the course of oxidation, a layer of complex spinel-type CuMe₂O₄ (Me=Cr, Fe) oxide appears on the alloying element particles during the annealing, and the dispersed copper oxides, mainly Cu₂O with a small quantity of CuO, are produced in the copper matrix.