نشریه کارافن
سال نوزدهم شماره 57 (بهار 1401)

Construction waste is one of the most important environmental concerns in the world. Iran is not an exception to this concern and more than 20 million tons of construction waste is produced annually in Tehran alone. To overcome this, researchers suggest recycling concrete and reusing it for construction and non-construction purposes. The aim of this paper was to predict the compressive strength of 28-day concrete with recycled aggregate using support vector machines (SVM) and multiple linear regression (MLR). Training and experimental data were developed for the development of the SVM model using 124 existing datasets from 11 published references. In the modeling process, an optimal network is considered to have the lowest mean square error and the highest correlation coefficient. Therefore, to evaluate the efficiency of the proposed model, the method of backup vector machines was compared with the method of multiple linear regression using the k-fold cross validation method. The results of comparing two 28-day compressive strength prediction tools including support vector machines and multiple linear regression using k-fold cross validation technique showed that support vector machines performed better compared to multiple linear regression method. Therefore, the support vector machine method can be mentioned as an effective method for predicting the 28-day compressive strength of recycled concrete.

In this paper, the increased bearing capacity of the circular and ring footings in sandy soil was investigated. Therefore, vertical skirts were used to increase the bearing capacity of the footing. In this regard, axisymmetric numerical modeling was performed using the lower and the upper bounds of the Finite Element Limit Analysis (FELA). More than 1,000 simulations were performed using OptumG2 software to ensure accurate results. The impact of different parameters on the bearing capacity of the ring footing was studied by considering structural and geotechnical parameters. The factors included the soil's internal friction angle, interaction coefficient between the footing and the soil, radius ratio of the ring footing, and vertical skirt length. Dimensionless design charts were used to present the research results. Additionally, statistical analysis was employed to derive a nonlinear regression equation for calculating the bearing capacity of the ring footings.

12 groups of soil samples were prepared with various percentage of PP-fiber content (i.e. 0.0%, 0.4%, 1%, 1.7% and 2.5% by weight of soil), cement (i.e. 0% and 4% by weight of soil) as well as lime (0% and 3% by weight of soil) to study the effect of PP-fiber on mechanical behaviour and strength of non-stabilized and stabilized clayey soil with cement and lime. The unconfined compression tests were carried out on samples that were cured at 30°C and kept for 14 and 28 days in oven. The test results showed that inclusion of fiber reinforcement in stabilized and non-stabilized clayey soil mixtures caused an increase in the unconfined compressive strength (UCS) and failure strain. However, such effects were much more evident in stabilized samples when compared with those of non-stabilized samples. Furthermore, maximum unconfined compressive strength of samples was obtained at an optimum percentage of fiber content of 0.4%. The results indicated that the existence of fiber in specimens decreased the stiffness of samples, caused more ductile behaviour and loss of post-peak strength. The interactions at the interface between fiber and soil matrix were studied by using Scanning Electron Microscopy (SEM). The results demonstrated that the friction and bound strength between fiber surface and soil were significantly effective in changing the mechanical behaviour of soil.

Green concrete can be produced as environmentally friendly concrete using recycled materials. In this study, the compressive and endurance behavior of green concrete containing waste aggregates as a replacement to coarse (gravel) and cement-replaced marble powders was evaluated experimentally. In the present research, 10 mix designs in addition to the control sample mix design were constructed and compared. The main variables included the volume percentage of waste aggregates as replacement sand with percentages of 0, 25, 50 and 100, and the volume percentage of marble powder as replacement cement with percentages of 0, 10 and 20. Compressive strength at 28 days and water absorption percentage and density tests were performed to evaluate the endurance of concrete. According to the results, gravel substitute waste stone had a negative effect and cement substitute marble powder had a positive effect on the compressive strength of concrete. By replacing gravel with 100% waste stone, approximately 49% reduction in strength was observed compared to the control sample, and by replacing cement with 10% marble powder, approximately 13.4% increase in resistance was observed compared to the control sample, demonstrating the minimum and maximum changes in compressive strength of concrete compared to the control test, respectively.

Fit joints are widely used in industry to connect bush to shaft and its manufacturing process is always associated with geometric deviations similar to other processes. In a cylindrical fit connection, the geometrical deviation of the shaft and the bush is in order of one hundredth of millimeter, which is a common precision in manufacturing. This range of error seems to be significant compared to the required dimensional interference to make the connections and affects the strength of the interference fit joints. Usually, certain geometrical deviations will occur in the production process considering the selected manufacturing method. This research attempted to investigate the effect of common geometric deviations at the surface of interference of cylindrical interference fit on the experimental extraction strength. In this regard, 8 specimens of interference fit with same characteristic of dimensions, surface roughness and interference were produced. Furthermore, shafts with certain geometrical deviation (standard sample, two-lobes, three-lobes and conical) were made and fitted. Then, the extraction force of each part was extracted experimentally. Taking into consideration precise measurements of the parts dimension, the effect of shaft geometrical deviations on the strength of the joint was investigated. The results showed that the geometrical deviation affects the extraction strength. However, in the quality control process of these parts, by measuring the dimensions of the parts, the effects of these usual geometrical deviations on the joint strength could be predicted and the quality control process can be performed more effectively.

In this paper, the microstructural and mechanical properties as well as the defects in the joining of aluminum sheets using the friction welding method are investigated. First, the microstructural properties of the friction stir welding zone are investigated, and then the effect of process parameters on the microstructural properties is discussed. The grain size in the stir zone was greatly improved compared to the base metal, which resulted in the improvement of the mechanical properties in this zone. Then, the effect of process parameters on the grain size was investigated. Welding defects, which is one of the most important reasons for joint failure, were investigated and the effect of tool pin profile and process parameters on the formation of defects in the welding area were discussed. For instance, by using the rotational speed of 500 rpm, a defect-free joint can be produced by the threaded pin even in the traverse speed of 500 mm/min, while the amount for the square and triangular pin profiles were 400 and 315 mm/min, respectively. Finally, the hardness of the welded area was studied in different samples. The hardness of the weld metal was lower than that of the base material due to the annealing of hardness which was generated as a result of primary rolling process. The hardness decreased by increasing the ratio of rotational speed to tool traverse speed.

Tailor welded blanks (TWBs) are increasingly used in the automotive and aerospace industries with the aim of reducing weight. Studying the forming processes of these sheets, examining their formability, as well as controlling factors such as the displacement of the weld line during forming can help to develop their application. The rubber pad forming process is a flexible forming process in which the sheet is deformed due to the pressure of a rigid punch with the help of a rubber cushion. In this paper, the rubber pad forming of tailor welded blank consisting of two welded sections with the same material but different thicknesses were simulated based on the finite element method and using ABAQUS software. The effects of various factors such as friction, thicknesses of the sheets that compose the TWB and the force applied to each section on the maximum displacement of the weld line were investigated through the use of a design of an experimental method. The results illustrated that increasing the thicknesses of the sheets that compose TWB, increasing the blank holding forces applied to each section, and reducing the friction between the TWB and the punch reduce the maximum amount of weld line displacement. In addition, increasing the thickness ratio of tailor welded blank led to an increase in the maximum weld line displacement. The simultaneous increase in the thickness of each section and the blank holding force applied to that section or the opposite section reduced the maximum amount of weld line displacement.

The present study investigates the effect of point defects in the structure of monolithic tungsten disulfide (WS2) using basic principles. This study was performed on six vacancies to investigate their effects on the electronic and magnetic properties of WS2 monolayer. The structure under study was a supercell with 36 atoms and the atomic positions were optimized. In the present study, density functional theory calculations were performed within the framework of local density approximation. Structural analysis of this material showed that the WS2 monolayer had a direct band gap of 1.89 eV. The simulation results illustrated that depending on the type of defects in the structure and their position, the behavior of the structure can change from semiconductor to quasi-metal and non-magnetic to magnetic. For instance, the removal of one tungsten atom leads to the metallization and magnetization of the structure. Moreover, the bandgap energy of the WS2 monolayer decreased in the absence of one sulfur atom.  In addition, there was a transition from direct to indirect semiconductors and a reduction in the energy of the band gap in comparison with its pristine. These cases indicate that the presence of defects in semiconductor nanostructures paves the way for the application of such nanostructures in tunable electronics, optical electronics, and spintronics.

Polypropylene/ ethylene–propylene-diene monomer/ Nanoclay (PP/EPDM/ Nanoclay) nanocomposite with different loadings of clay content is used in many industries because of balance on the tensile and impact strengths. Laser welding, as a fabrication method, is applied to the butt-welding of these nanocomposites. The input parameters (clay content, laser power, scan velocity, and stand-off-distance) were varied to achieve the best responses (tensile strength of welds). Response surface methodology (RSM) was utilized to relate the input parameters and the response. The effects of all input parameters on the responses were investigated. Results demonstrated that increasing the clay content from 0 to 6 wt% and increasing stand-off-distance from 5 to 8 mm decreased the tensile strength from 9.6 MPa to 6.1 MPa and 9.19 MPa to 8.36 MPa, respectively. The models showed that laser power of 100 W, traverse speed of 400–750 mm min21, and stand-off-distance of 5 – 6.5 mm, led to weld strength from 8 to 9.9 MPa.

The purpose of this research was to simulate the investment casting process of the supercharger blade and its practical implementation based on the simulation results. For this purpose, the desired specimen after modeling was first analyzed in Pro-Cast software. The results of this simulation showed that the solidification of the piece starts from the edges and moves towards the center of the piece, and therefore the volumetric and surface shrinkage defects are not concentrated due to unbalanced thermal gradients in the center. Heat transfer from the edges to the center is much faster due to the thermal conductivity of the edges as well as the thinness. For practical implementation, the desired specimen was designed and used by a polymer 3D printer. Then, the silicone mold of the piece was made of RTV2 and a wax model produced by injecting wax. This was followed by the ceramic mold of the piece being made of SiO2 for melting. The results of the practical execution of the specimen showed that due to the defects identified in the simulation, the vacuum system should be used after the end of melting and at high temperature; otherwise, the piece produced would be completely defective. The use of vacuum caused the piece to be produced in a completely desirable and flawless manner.

In the present study, the feasibility of processing of MMC tubes via friction stir backward extrusion (FSBE) technique was investigated. In this regard, an aluminum alloy 1100 bar was utilized as initial raw material. In the FSBE method, the initial material is placed inside a chamber and a rotating punch is fed downward penetrating into the material softened due to the generated frictional heat. Consequently, the material is back extruded around the punch and a cylindrical tube is formed. In order to compare the results, first, a tubular specimen was produced by the FSBE method. Then, several through holes with 1.5 mm diameter were drilled by a super drill machine. These holes were filled with SS 316 powder having micrometric dimensions. The prepared specimens were processed via FSBE technique and MMC tubes were successfully produced. The distribution of powder particles was investigated by metallographic observations. Furthermore, the mechanical properties of the processed tubes were documented by Vickers hardness measurements and tensile tests. The comparison of the results demonstrates that the MMC tubes have higher strength and lower ductility. The material deformation behavior during FSBE was simulated by a coupled structural-thermal model via DEFORM finite element software.

Roller bearings are one of the most widely used mechanical components. The load on the bearing is supported by the pressure that is developed in fluid-film and thus the lubricant is less able to escape and a longer-lasting lubricant film is generated called elastohydrodynamics lubrication (EHL). In general, to understand the lubrication regimes, it is important to consider the thickness of the lubrication layer. Lubricant film thickness is one of the most important parameters affecting the performance of bearings. In the present study, using theoretical equations and using MATLAB programing software, the load distribution along the rotary kiln was investigated and calculated, and then the film thickness of the lubricant (base oil and grease) in the roller bearings of the rotary kiln of Butia Iranian Steel Pelletizing Complex was calculated. Finally, the effect of various parameters such as rotary kiln speed, lubricant operating temperature, fill ratio, base oil viscosity and viscosity index of base oil were deliberated. The results showed that by increasing the base oil viscosity from 150 cst to 1000 cst, the film thickness increased by 239%. Furthermore, increasing the viscosity index caused small changes in the base oil viscosity in the operating temperature and reduced the starting torque at the beginning of application.

Today, there is a growing utilization of fractional order systems in the modeling of various phenomena. Furthermore, the capabilities and performance of fractional order controllers in the analysis and design of fractional order systems are of interest to many researchers. In the present research, the most important toolboxes developed for analyzing and designing fractional-order systems were introduced and their capabilities and performance in the analysis and design of fractional order controllers compared. In this regard, the basic features and capabilities of the four different toolboxes were examined. The advantages and disadvantages of each were listed. Then, with the help of three toolboxes, the performance of two fractional-order proportional, integral, and derivative (FOPID) controllers optimized with a meta-heuristic algorithm to stabilize the synchronous generator voltage regulation system and brushless DC motor were simulated. The simulation results showed that the performances of these toolboxes depend to a large extent on their employed algorithms. Thus, the results of Fractional-order Transfer Function (FOTF) and Fractional-order Modelling and Control (FOMCON) toolboxes were very similar and non-integer (NINTEGER) toolbox results were significantly different.

Due to the low efficiency of internal combustion engines, a large part of the chemical energy of the fuel they consume is dissipated to the environment as heat through a cooling system. Unfortunately, most engines still use the classic cooling system, which is practically unable to cool the engine optimally unnder different operating conditions, and this category increases fuel consumption and increases emissions from engines. Therefore, in this paper, first, a classic cooling system was tested and its performance recorded. Then, in the cooling system of the same engine, a mechanical water pump and mechanical thermostat were respectively replaced with electric water pump and an electronic thermostat. After this upgrade, the engine was tested under the same conditions as the classic mode, and its results were compared with the results obtained from the classic system. The results illustrated that if a good and complete program is used to control the electric water pump and electronic thermostat, fuel consumption was reduced by at least 1.19% and at most 4.33% compared to the classic system in addition to increasing the lifespan of engine parts. In addition, the engine warm-up time was reduced by a minimum of 6.25% and a maximum of 18.84%. Moreover, HC and NO pollution in most tests showed a minimum of 15.32% and a maximum of 70% and a minimum of 9.28% and a maximum of 46.58%, respectively.

In the present study, the performance of nanofluid flow in a Linear Parabolic Trough Solar Collector was simulated and studied. The flow is considered three-dimensional, turbulent and incompressible. Flow modeling is a combination of Monte Carlo Ray Tracing (MCRT) and numerical simulation which was implemented by Soltrace and Ansys Fluent software, respectively. The radiative flux reflected from reflector on the absorber tube was calculated by Soltrace software and introduced to Ansys Fluent by UDF code. Thermophysical properties including density, specific heat, thermal conductivity and viscosity were considered as a function of temperature. It was observed that in Reynolds 1000 number and in two volume fractions of 2 and 5 percent, the enhancement of heat transfer coefficients were respectively 3 and 11 percent for Al2O3, 7 and 15 percent for copper powder (Cu), and 7 and 42 percent for silver powder (Ag). In Reynolds number 15,000, increases of 8 and 33 for Al2O3, 9 and 29% for copper powder (Cu), and 8 and 25% for silver powder (Ag) were respectively observed. The results showed that at a constant Reynolds number, the heat transfer coefficient increased with increased nanofluid volume. It was also observed that the performance of silver powder (Ag) nanoparticles at low Reynolds numbers was better than other nanoparticles, but it decreased with increasing Reynolds number. Unlike silver powder (Ag), the performance of Al2O3 at low Reynolds number was insufficient, but improved with increasing Reynolds number.

In this study, energy and exergy for different parts of a 5 MWe direct steam generation (DSG) solar power plant was analyzed for Yazd city climate. It was observed that the most energy in the condenser and the most exergy losses occurred in the parabolic solar concentrator. To improve the efficiency of the concentrator, this study recommends to preheat the water entering the concentrator with one or more open preheaters in which the extractions of steam from turbine should be optimized by their pressures and mass flow rates. For different cases in which the cycle has up to 4 preheaters, the efficiency of the first and second laws of thermodynamics was investigated. In the case of 4 preheaters, based on obtained results, it was revealed that the efficiency of the first and second laws were 17.2% and 16%, respectively which was a significant increase compared to the case where the cycle had one, two or three preheaters. Therefore, according to the obtained results from the efficiency of the first and second laws as well as the irreversibility of power plant components, the cycle of the present article can be recommended for the city of Yazd.

Connecting wind turbines to power networks without adversely affecting system stability is one of the most important future challenges to expand wind farms. In this paper, to improve the power network stability, damping the oscillations caused by torsional modes and increasing the energy conversion efficiency of wind turbines, a new control strategy was proposed for the active power control loop of wind turbines as well as HVDC transmission systems. First, the nonlinear speed-power curve of the wind turbine was shown to be effective in damping coefficients of torsional modes, energy conversion efficiency and the stability of the wind turbine. Accordingly, the use of wind turbine stabilizer in the wind turbine power control loop was proposed. Since the wind power plant is effective in the flow of the active-reactive power components, the use of supplementary damping controllers in the rectifier of VSC HVDC system was proposed to improve the voltage profile and the dynamic stability of the power network. The fractional order PID controllers were used in the proposed control strategy whose coefficients were adjusted using the proposed genetic-bat algorithm. In this algorithm, in order to avoid the rapid convergence of bats to local extremes and the optimal development of the search space, two operators of genetic algorithms, the dynamic mutation based on probabilities and the crossover were used. The simulation results showed that under the proposed control strategy, the stability of the system and the voltage profile in the network were significantly improved.

This paper introduces a versatile single-phase seven-level inverter for integrating lower voltage renewable energy sources to the higher voltage amplitude power grids. The input voltage was increased by combining the voltage of the capacitors in the predetermined current paths. The output staircase waveform was formed by using a DC voltage source, two capacitors, and the aid of the phase disposition pulse width modulation (PD-PWM) strategy. The self-voltage balancing ability of the capacitors without the need for any external controllers nor sensors, high efficiency, and ease of operation were the most important features of the proposed converter. Requiring fewer semiconductor devices compared to other inverters mentioned in recently published papers ensures the superiority of the proposed boosting multilevel inverter. The ability to increase the input voltage by three times at the output in addition to the ability to self-balance the voltage of the capacitors without the use of a side control circuit or sensor were other advantages of the proposed structure. Furthermore, by calculating the power losses and circuit capacitances, 95.8% efficiency was obtained. Finally, the performance of the proposed seven-level inverter under different load conditions was simulated in MATLAB SIMULINK environment and the results were confirmed by the results obtained from the laboratory prototype.

Thus far, controllers have often used the equations governing direct kinematics or reverse kinematics to control the position of the final implement of the robotic arm. Difficulties in unravelling direct kinematic equations and reverse kinematics, errors in solving equations, lack of user-friendly graphical environment, lack of flexibility in controller decision making and large volumes of calculations are problems of control systems in controlling robotic arms. In this paper, a skeletal lift robot with two classic PID and Fuzzy control methods with 4 degrees of freedom, in addition to a simulation in which four parts of the arm were examined by the controller and Matlab / Simulink as a tool for feature testing were used. Robotic arm movements were used. Implementation outputs show the satisfactory performance of the fuzzy controller which was able to control the arms of this robot with a percentage of uplift and a desired sitting time of 1.23 seconds. Furthermore, in order to test the performance of the scaffolding robot, the movement nodes of the arm were placed optimally and the controlled system showed the high accuracy of the fuzzy controller so that after 1.23 seconds at the critical point, the wrist area, it was able to continue with the desired sitting with a permanent error of zero and without any distortion.

In this study, the flow patterns, heat transfer rate and entropy generation rate of Al2O3/Water nanofluid was simulated in a closed cavity containing 4 circular cylinders with 2 different arrangements. The renowned single-phase nonhomogeneous technique (Buongiorno Model) was chosen to assess the nanofluid flow and heat transfer rate numerically. However, the diffusion term in the concentration equation of the technique usually possesses a relatively low value, which might cause instability issues during the iteration process. Hence, in order to deal with this problem, an in-house Fortran 90 code was developed using a novel hybrid FD-LBM method with TVD characteristics. The mentioned algorithm has been proved to be a great asset when facing flows with lower rate of diffusion compared to convection. The active parameters in this study were Ra, Le, and Al2O3 nanoparticle volume fractions, and their influence on the Nu and Sh numbers as well as the entropy generation rate of the system were investigated in the cavity for both cylinder arrangements. According to the results of this simulation, the stream function and consequently the flow velocity in the diamond arrangement were much higher compared to the square arrangement. Moreover, it was observed that not only the addition of nanoparticles could not raise the heat transfer rate from the wall, but in φ = 9 %, the Nusselt number experienced an approximate 15 % decrement.