نشریه مهندسی مکانیک مدرس
سال بیست و چهارم شماره 1 (دی 1402)

In this paper, by performing heat treatment on IN718 superalloy specimens that are manufactured by additive manufacturing method, the purpose is to investigate the experimental and analytical behavior of stress relaxation. The 3D printed specimens were made by the selective laser melting (SLM) method and after homogenization and solution heat treatment; they were subjected to stress relaxation at the temperature of 650 °C with an initial strain of 1.1% and 2.1% for 8 hours. Due to investigate the effect of strain changes on the stress relaxation, the stress relaxation limit diagram, which is 119.55 and 514.35 MPa for strains of 1.1% and 2.1%, respectively, shows that the stress relaxation limit increases with the increase of the initial strain. Furthermore, by examining the stress relaxation behavior in the experimental specimens, it was found that the amount and slope of the relaxation curve is higher in the specimen with a strain of 2.1%. In the analytical study, the creep constitute equations were also used to investigate the stress relaxation behavior, which by checking the presented comparative curves, by recording the error amount of 2.17% and 3.85% for the strains of 1.1% and 2.1%, respectively, the result of the comparison indicates a good agreement between the analytical results and the experimental curves.

In this article, the determination of the mechanical properties of a micro-cellular auxetic structure is investigated. This lattice structure consists of hexagonal cells, whose cell-wall dimensions are on a micro-scale. First, using modified strain gradient theory (MSGT) and energy method, the mechanical properties of the micro-cellular auxetic structure are analytically obtained. Then for validation, Young's modulus of a micro-cellular auxetic structure is derived by tensile test and compared with theoretical results. The comparison of analytical and experimental results shows good agreement. A nanosecond laser cutting machine is used to fabricate the microcellular auxetic structure, and the ISO 6892-1 standard is used to perform tensile tests. The results show that the modified strain gradient theory plays an important role in determining the mechanical properties of micro-cellular auxetic structures. In some cases, the results of this theory are more than 100% different from the classical theory. In addition, it can be seen that by changing the dimensional parameters of the micro-cells, the mechanical properties of the auxetic structure can be tunable. For example, by reducing the magnitude of the angle of the cell wall, Young's modulus in the X 1  direction increases, and Young's modulus in the X 2  direction and the shear modulus of the structure decrease

In this article, the effect of using phase change materials to improve heat transfer in power distribution transformers has been investigated experimentally. To enhance the cooling of the transformer, a new method has been proposed, which involves adding paraffin inside aluminium containers that are sealed to the transformer oil. The test setup includes an electric transformer filled with transformer oil, two electrical heaters, a power regulator, a thermal camera, oil insulation measuring device, and temperature sensors placed at various locations. The experimental results demonstrated that the addition of phase change materials to the electronic transformer oil led to a decrease in the temperature of the transformer, particularly in summer weather conditions. Additionally, the mean temperature of the transformer oil was reduced from 46.7 to 42.5 degrees Celsius by adding 8 kg of paraffin. However, it was observed that when the temperature increases suddenly and rapidly within an hour, these materials are ineffective in dissipating the heat and reducing the temperature of the transformers. Additionally, the research examines the impact of continuous and high-temperature increases on the oil electrical insulation. The results revealed that using phase change materials increased the voltage that the oil could withstand as an electrical insulator from 56.8 kV to 61 kV.

The effect of texturing the tool rake surface on the surface quality in hard turning of 1191/1 steel with a surface hardness of 45 HRC was studied in this research. The pattern parameters including, cavity diameter, pitch, and depth, as well as the pattern distance from the main cutting edge were changed in 3 levels, assuming the cutting tool with regular cavity texture. Nine tests were designed using the Taguchi DOE and conducted in dry and lubricated conditions with 2 repeats. Machining forces during the tests and surface roughness of the machined workpieces were measured in machining under lubricated and dry conditions. The results showed that in turning with a textured tool under lubrication, changing the parameters of the texture pitch and the distance from the cutting edge increased the surface roughness of the workpiece by 57.6% and 39.2%, respectively. This is while the increase in the diameter of the tissue cavity, due to the reduction of the contact area in the tool-chips interface and better lubrication near the cutting region, improved the surface roughness up to 40.7%. The cavities depth of also did not have a significant effect on improving lubrication and reducing the roughness of the final surface. In dry turning, increasing the cavities diameter in texture and decreasing the pattern pitch, reduced the surface roughness by 10.6% and 29%, respectively. Examining the SEM images also indicated the production of the workpiece surface with smoothed texture when turning using optimized textured tool.

Hydrogen is considered as a new source of energy. One of the most used methods of hydrogen gas production is water electrolysis. Electrolysis of water by an electrolyzer and by passing an electric current through water molecules, using an intercellular membrane causes the breakdown of water molecules into hydrogen and oxygen. If the intercellular membrane is removed, hydrogen gas and oxygen are mixed and hydroxy gas is produced. This gas, which includes hydrogen and oxygen molecules, has been considered as an auxiliary fuel. For this purpose, in this research, the design and construction of a multi-cell electrolyzer producing hydroxy gas without a membrane using 316 steel sheet has been discussed. To increase the conductivity of water, potassium hydroxide electrolyte was used, and solar panels with different powers were used to supply the required energy consumption of the electrolyzer. Then, by conducting various experiments, the application of this technology using solar energy has been investigated. The results of the tests show that each electrolyzer plate has a voltage of 2 to 3 volts and there must be a proper proportion between the number of electrolyzer plates and the voltage of the panels. The production per solar panel surface for the three studied conditions was obtained as 1919, 4542.5 and 6919 liters per square meter, respectively, which indicates that there are optimal values corresponding to the panel surface and the area of the electrolyzer plates, which leads to an optimal current passing. It becomes more appropriate from the device and its performance.

An improved method for noise reduction from a time series obtained from a chaotic system is presented. This improved method is based on a noise reduction technique presented by Schreiber and Grassberger that has good performance and less complexity compared to other noise reduction methods from chaotic data. Here a global model created using a neural network has been used as a prediction model for chaotic time series. This global prediction model performs better compared to the local prediction model used in the original method. The improved method also takes advantage of the singular spectrum analysis reconstruction technique. Both of these improvements led to a more accurate noise reduction method while preserving the unique properties of the original. The improved method is applied to a time series obtained from the chaotic state of Lorenz equations that is polluted with Gaussian noise. The final results show a 33 percent reduction in mean absolute error values compared to the original method. Also, the error of calculating the correlation dimension from the data has been reduced to 2 percent after applying the improved method.