مجله مهندسی مکانیک امیرکبیر
سال پنجاه و دوم شماره 8 (آبان 1399)

The highest amount of reported failures in railway tracks are due to turnout problems. The main causes of these breakdowns are: high wheel-rail contact forces, creep in the switch blade due to changes in the rail profile, and inconsistency in the rail profile during wheel passage over wing rail and crossing nose causing collision forces. In this paper, a new method for crossing nose fatigue life prediction is presented using FE approach. Firstly, a dynamic model containing a complete turnout (switch and crossing panels) is simulated. For a closer look at crossing, the results of the forces generated by the dynamic model are transmitted to a more detailed static model at specific sections, because of the criticality of this point in track. Then the stress and strain results are extracted to perform the fatigue analysis on the crossing nose in order to calculate fatigue crack initiation life and critical planes. Regarding the importance of fatigue and the necessity of investigating the effect of different variables on fatigue life, a parametric study is conducted considering variables such as velocity, wagon weight, and turnout type. The results indicate that the predicted fatigue life in UIC60 crossings are less than U33. Also, by increasing the wagon weight and speed or the curve radius fatigue crack initiation life have increased.

In this paper, the behavior of inhomogeneous functionally graded rubber with large deformations and under bending loading is modeled by assuming an incompressible hyper-elastic material. In hyper-elastic inhomogeneous functionally graded materials, mechanical properties continuously changes from one point to another in the specified direction. In the other words, they gradually become material from material to another. For modeling the nonlinear behavior of material, hyperelasticity theory and strain energy density functions, which are a function of the invariants of the left deformation Cauchy-Green tensor, are used. In order to be able to apply the existing energy functions to inhomogeneous functionally graded materials, they must be changed, therefore, the changes in the constant of the energy functions are assumed power shape and in the direction of the curvature radius after bending, due to the inhomogeneous functionally graded of the material. Since many materials are inhomogeneous, using the assumption of inhomogeneous functionally graded of the material is one of the most practical methods. In this paper, the modeling of the hyperelastic behavior of inhomogeneous functionally graded rubber is done under bending loading and extraction of the Cauchy stress relations governing the cross-section caused by this loading. For modeling, the generalized Mooney-Rivlin energy function is used and the properties change in radial direction are considered and heterogeneity variations are also investigated.

Ring rolling is a metal forming process used to forge precise, seamless, circular, shaped parts. Due to the unique properties of the produced rings, the application in the advanced industries is high. The most important advantage of the process is the uniform flow of material in the ring after the process. Usually, the working temperature of the device is high during the rolling of super alloys. The high temperature could damage the work rolls. In spite of the cooling practice, the work roll temperature is raised. In general, this reduces the work roll strength. In this research, the effect of various temperature and cooling practice on thermo-mechanical stresses in work roll of ring rolling mill have been investigated. The results show that the amount of produced thermo-mechanical stresses on the work rolls is completely different. In the main roll, the mechanical stress has a greater effect on thermo-mechanical stresses. However, in the mandrel, thermal stress determines the amount of thermo-mechanical stresses. Unlike the mandrel, the effect of cooling practice on thermo-mechanical stresses of the main roll is negligible. The results show that cooling practice increases the amplitude of equivalent stress and reduces the mean stress in the work rolls.

In this paper, we try to use a multiple linear regression method to explicitly describe the explicit relation between the stress concentration coefficient of plates containing elliptic openings in terms of mechanical properties and the angle of rotation of openness. First, the stress concentration coefficient for a lot of composites was calculated by analytical method based on the mixed variable method and using different values of the mechanical properties of the composites. Using the obtained data, using the regression method Multiple Linear The explicit relation was used to estimate the stress concentration coefficient in unbounded composite plates containing elliptic openings under tensile load. It is important to note that several factors affect the concentration of tension around the openness. Therefore, by correct selection of these parameters, it can be significantly reduced the stress concentration and increased the structural strength. One of these factors is the angle of rotation of openness, which is discussed in this article. In addition to the ease of use, the proposed relationship, by eliminating complex and complex analytical solutions, saves time and allows the designer to obtain the desired parameters to achieve the desired stress. The results showed that the regression model was able to estimate the coefficient of stress concentration with an error of less than 1 percent.

Metallic pipelines are mainly corroded and damaged in operation that replacing damaged lines or their local welding is sometimes not feasible or economical, so for resolving the corrosion and functional damage of metallic pipelines, localized repair using composites as one of the most effective and cost effective solutions is used. The connection of composite repair to the metal substructure in this method is one of the most important design parameters. Therefore, deriving the important parameters at the junction will help engineers in designing and predicting the interlaminar crack initiation and propagation at the joint interface of composite repair to the metal substrate. Therefore, to investigate the second mode of failure in this method, the strain energy release rate is calculated experimentally and numerically in a steel/GFRP bond. According to an experimental standard for calculating the second mode strain energy release rate, ASTM-D7905, experimental tests are accomplished for three symmetric and asymettric unlike end-notched flexure specimens and a relationship for the thickness of each material to have symmetric specimens is proposed. To validate the thickness calculation relationship, the finite elemental modeling of unlike samples using virtual crack closure technique is used which indicates the good agreement of experimental and numerical results. Compareing the experimental results obtained in force-displacement diagrams of symmetric and asymmetric samples in crack propagation showed that the second mode strain energy release rate of symmetric samples are more than asymmetric and about 1.6 times as large.

Optimization in the design and maintenance of many engineering systems, economic and even social has been used to minimize the cost or maximize profits. In the buckling analysis the hybrid composite plates with cutout, the effective parameters on buckling are the cutout geometry, fiber angle, cutout size to plate size ratio, bluntness of cutout corners, and rotation angle of cutout. Therefore, in this study, using genetic algorithm method an attempt has been made to introduce the optimum parameters to achieve the Maximum amount of critical buckling temperature of hybrid composite plate with polygonal cutouts in different boundary conditions and stacking sequences, which are subject to uniform temperature rise. The cutout in this study are circular, pentagonal, and hexagonal. The solving method used to analyze this study is the finite element based on the energy method. Also, the theory used in this paper is the first-order shear deformation theory. The results presented in this case show that by choosing the appropriate shape of cutout and the Optimal selection of parameters affecting buckling, the plate's resistance to thermal buckling can be increased. It was also found that stacking sequences and boundary conditions have a significant effect on the critical buckling temperature of a hybrid composite plate with cutout.

Interference fit joints are widely used in the industry to produce tight joints. These type of connections have produced more beautiful and smaller joints by eliminating other joints such as keys which are defected with stress concentration. Interference fit joints could be applied under dynamics and statics design loads, successfully. Interference fit joints are always imperfect, such as other industrial products and also affected by some parameters which directly affect the operation of these joints. For instance, contact surface of every manufactured joint parts are not a perfect cylinder and always there are some form defects. The variation of these parameters could affect the performance and strength of interference joints, so these parameters should be considered. Some effective parameters on the strength of interference fit joints are friction coefficient, roughness, materials properties, dimensions and geometric irregularities of contact surfaces. In this study the effect of diameter of shaft and interference value variation on the friction coefficient in contact surface and strength of joints are investigated. Finite element results are interacted with experimental ones to estimate friction coefficients. Also, factorial method, which is a statistical method for design of experiments, are used to analysis the effects of pressure, which is vary by variation of interference, and radius of contact surface curvature on friction coefficient and strength of joints. Results indicate that the friction coefficient changes with diameter of interference surface, inversely and increase of interference and consequently, pressure leads to growth of friction coefficient.

Porous materials especially metallic foams are novel materials with high energy absorption and strength to weight ratio capability. In the present paper we investigate quasi-static uniaxial compression and crushing behavior of closed-cell graded aluminum foams and foam-filled tubes, both numerically and experimentally. To model the mentioned specimens, we place cubes with several densities and strengths to generate functionally graded specimens. Specimens are considered as to be graded with two and three layers and non-graded single layer, with and without tubes. Various standard uniaxial compression experiences are conducted for numerical model calibration and validation and also for non-linear mechanical properties and hardening characterization. To enhance strength and energy absorption capability and also tailoring purpose, we layout the cubic foams in tubes with square profile. The 3D Voronoi diagrams approach is manipulated to model stochastic foam microstructure. Also Novel unit cell is proposed based upon Kelvin cell. We implement the hybrid finite element analysis and Voronoi diagram using Python script and Abaqus 2017 commercial FEM-based code for more convenient modeling and efficient analysis. Finally, and after numerical model calibrations, numerical and experimental results showed good agreement.

Sugarcane bagasse is one of the most abundant types of natural fibers which can be utilized to fabricate natural composite materials. Since bagasse is one of the wastes of Khuzestan province sugarcane factories, recycling might be an opportunity to enjoy its economic and environmental benefits. In the present study, the mechanical and microstructural properties of Bagasse/Polypropylene natural composite fabricated by the injection molding method were investigated. Bagasse fibers after drying process were mixed up to polypropylene with 10, 30, 40 and 50% weight fraction of bagasse. In order to investigate the mechanical properties, experimental tests consist of tensile test and Charpy impact tests were carried out. The results showed that the maximum material strength was obtained from the sample made of 40% weight fraction of bagasse. The strength of 40% bagasse was found about 10%more than 50% bagasse the microstructural analysis was indicated that the failure mechanism of 40% bagasse was mainly affected by fiber breakage. However, the main failure mechanism of 50% bagasse was changed to fiber pull out. Additionally, impact absorbed energy was significantly decreased by increasing the bagasse weight fraction.

Dual axis torsion micro-actuators find variety of applications such as optical switches, biomedical imaging and adaptive optics. Squeezed film air damping is one of the most important energy loss mechanisms in these devices. This damping, is a key factor in determination of the dynamic performance of the micro-electro-mechanical systems. So they have been widely considered by the researchers. The objective of this paper is to suggest a model for the squeezed film damping of dual axis torsional micro-actuators by considering the bending of the supporting torsion beams. To achieve so, the Reynolds equation governing the pressure of the trapped gas between the actuator plate and the underneath electrodes is considered and simplified for the case in which the inertial effects are negligible compared to the viscous effects. The dimensionless form of this equation is then solved using the extended Kantorovich method and the pressure distribution under the actuator is obtained. This pressure distribution is then employed to calculate the resulting force and force of the squeezed film air damping. The effects of the design parameters of the actuator on the dimensionless damping torques, dimensional damping torques and dimensional damping force are studied. The presented results in this paper can be effectively utilized for an accurate dynamic modeling and control of such structures.

In the present research, vibration behavior is presented for a thermally postbuckled double layered graphene sheet (DLGS). The DLGS is modeled as a nonlocal orthotropic plate which contains small scale effects. The formulations are based on the Kirchhoff's plate theory, and von Karman-type nonlinearity is considered in strain-displacement relations. The thermal effects, van der Waals forces between layers and chirality are also included and the material properties are assumed to be temperature-dependent. A semi analytical solution is obtained using multiple time scales method. The effects of variation of small scale parameter to the natural frequencies, deflections and response curve of DLGS are analyzed and the numerical results are obtained from the nonlocal plate model. Numerical results are compared with those of similar researches. Effects of various parameters on the postbuckled vibration of DLGS in thermal environments such as scale parameter, length and thermal load are presented. Stability of vibration modes around a buckled configuration is investigated. Results show that the scale parameter and thermal changes have important roles on nonlinear vibrational behavior of nano scale buckled structures.

In this research, the free vibration analysis of doubly curved composite sandwich panels with variable thickness is studied using higher order sandwich panel theory. For the first time, considering different radii of curvatures of the face sheets in this paper, the thickness of the core is a function of plane coordinates (x,y). In addition, in the current model, the continuity conditions of the transverse shear stress, transverse normal stress and transverse normal stress gradient at the layer interfaces, as well as the conditions of zero transverse shear stresses on the upper and lower surfaces of the sandwich panel are satisfied, which is unique. The formulation is based on an enhanced higher order sandwich panel theory and the vertical displacement component of the face sheets is assumed as a quadratic one, while a cubic pattern is used for the in-plane displacement components of the face sheets and the all displacement components of the core. The equations of motion and boundary conditions are derived using the Hamilton principle. The effects of some important parameters including composite layup sequences, length to width ratio, varying properties of the face sheets materials, Face sheet thicknesses ratio and varying materials of the face sheets were investigated. The results are validated by the latest results published in the literature.

Tweel is a new type of non-pneumatic tire that is going to be replaced instead of tire in machines. The purpose of this research is to simulate the wheel from beginning of the movement to reaching a speed of 60 kilometers per hour and study the vibrations and the effect of curvature and speed on its value. Spokes curvature has effect on the vibrational behavior of the wheel. For this reason, in addition to the wheel with spokes curvature of 5 millimeters, two other models with spokes curvature of 4 and 6 millimeters are also modeled. These studies have shown that with changes in curvature of spokes, domains of spokes vibrations and vibrational frequencies have changed dramatically. Two important disadvantages of this wheel are vibrations and making loud noises at speeds more than 80 kilometers per hour. For this purpose, the speed of the wheel in the simulation is increased to 100 kilometers per hour. It is observed that at speeds of higher than 70 km/h, the spokes of the wheel are highly vibrated. At this moment spokes vibrational frequencies are as same as their natural frequencies and the resonance occurs. The final result is that the tire designed in this project is just operational in cars with low speed.

The present work investigates the nonlinear vibration of a cantilever cylindrical micro-beam subjected to voltage and fluid flow as a micro vortex generator. As the microbeam is subjected to the fluid with given velocity, in addition to the load due to fluid added mass, the lift and drag forces as the two basic flow-induced factors affecting the dynamics of the micro-beam are modelled using the Van der pol equation. The Euler-Bernoulli beam theory is used to model the cross fluid motion of the beam under nonlinear electrostatic force as result of applied voltage. The Galerkin method is used to convert the partial differential equation to regular differential equations as well as to solve the coupled nonlinear equations governing the micro-beam motion and the wake oscillation to evaluate the response of the coupled structure to a combined applied voltage and fluid flow. The effect of fluid flow on Reynolds number and fluid vortex frequency as two main parameters in creation of Lock-in phenomenon is studied. In addition to effect of different fluid velocities, response of the micro-beam to different input voltages in the presence of fluid flow is investigated and it is shown that for a given flowing fluid, applied voltage can be used to control the lock-in regime.

The contrast of electrical and mechanical forces of microbeam causes pull-in instability in microelectromechanical systems and introduces them as vulnerable systems sustainability. One of the proposed solutions for controlling this instability and increasing the stable range of system are curved microbeams. This curvature causes snap-through in which the microbeam moves to its second stable position with a large-scale vibration. Despite of the advantages of this phenomenon, sometimes its beginning ends in the systems instability. In order to use the advantages of the curvature of structure and to avoid the disadvantages of Snap-trough, in present study, the performance of a structure with curved electrode has been investigated. In this regard, by assuming the Euler-Bernoulli beam theory, the governing differential equation is obtained by using Hamilton’s principle and based on the modified couple stress theory. This equation is converted to a nonlinear ordinary differential equation by using a reduced order model based on the Galerkin procedure and its numerical solution is obtained by MATLAB software. In order to compare the performance of the two curved systems with each other as well as with a straight structure, in different ratios of maximum curvature to gap, the variation of maximum deflection versus voltage is plotted for each of the systems and their instabilities characteristics are calculated. The results show that in the case of snap-through causes instability in curved microbeam, the curved electrode with the same dimensions leads to structural stability until higher position and voltage

Vibration in wind turbine blades is one of the important factors that result in reducing performance of wind turbines. Therefore, control or reduction of the mentioned vibrations can be of great help in increasing efficiency of wind turbines. In this paper, an optimal tuned mass damper (TMD) is proposed to reduce edgewise vibration of an small-scale horizontal axis wind turbine blade (5 KW) with considering the coupling between edgewise and flapwise vibrations. For this purpose, partial differential equations governing dynamics of the system are derived using the Lagrange method. These equations are completely nonlinear and linearization is not performed to avoid possible errors in the analysis and also, the blade is considered as a flexible member. In deriving governing equations, coupling effect between in-plane and out-of-plane vibrations of the blade, effect of pre-twist angle in the coupling, and effect of centrifugal forces and gravity are considered. In order to reduce vibration of the blade, TMD is used and its parameters are optimized using one of the genetic algorithm methods for a real blade sample. Finally, with applying wind force as a sweep sine excitation, effectiveness of the optimized TMD in vibration reduction of the blade in four different wind speeds is investigated and the related results are presented. Results show that the wind turbine blade vibration reduction is achieved properly using the optimal TMD.

Two-phase fluids transportation is very important in industry. Rotating helical pump is a special form that can be used to transfer fluid-gas flows and also to generate pulsatile flows. The structure of this pump differs from conventional pumps and its geometry can be changed during operation. In this paper, while demonstrating a fabricated second version of the rotating helical pump, a dynamic analysis is performed for the first time and the governing equations are extracted based on the input control variables (rotational speed and tilt angle of the pump). In the dynamical analysis, a rotating control volume corresponding to a spiral tube is considered. In order to determine the values of the inputs corresponding to the desired outputs, we use the non-dimensional characteristic curves of the pump that was published in the previous study. Then the control is performed on the basis of two input variables to reach the desired pump head and flowrate. A sliding mode controller is implemented. The results are shown for the desired value of head and flowrate. The attractive features of this pump are that it can work with the low rotational speed, and it can change the point of operation (head-flowrate) without changing the speed (just with changing the tilt angle). The results include governing equations of the rotatical helical pump that can be used in future studies. Moreover, the results show the success of the sliding mode method in control of the pump.

This paper proposes a new hybrid controller based on combining the ANFIS method and PID controller, for vibration mitigation of structural system. The proposed controller although has the PID controller features, create a fuzzy inference system that has fewer bugs and errors than neural networks in calculations. The whale optimization algorithm is used for optimum tuning the PID parameters and also for identification of parameters related to the experimental structure. Considering four well-known earthquake real data (ground acceleration) namely, El Centro 1940, Northridge 1994, Athens 1999 and Mexico City 1999, the performance of the proposed controller is evaluated. Then the results are compared with two other controllers namely, fuzzy logic control and ANFIS, which are designed for a four-degree of freedom building which is equipped with active tuned mass damper (ATMD). The simulation results show that the proposed method which is a combination of two controllers namely, ANFIS and PID, performs better than other strategies with are developed. The results obtained from the simulation show the better performance of the proposed method than the other control methods in reducing the structural responses in terms of displacement and acceleration of all floors. The results show that the maximum acceleration related to the building’s floors while using proposed method has improvement of 36.3% for the El Centro, 35.4% for the Northridge, 27.7% for the Athens and 22.5% for the Mexico City earthquakes regarding two other common controllers namely, fuzzy control and ANFIS control.

Cable-driven manipulators are a generation of parallel cinematic chain robots which provide important features including wide workspace and cost effective high speed operations. However, due to wideness of the workspace and the flexibility of the cables, they are susceptible of unwanted vibrations which reduce their precision. Therefore, determination of velocity limits in the operation workspace is of high importance. In this study, stability analysis and critical velocities of a four cable plane robot is considered. Governing equations of the system are extracted by use of finite element method and employing variable length element. The characteristic coefficients of the extracted equations are nonlinear and velocity dependent ones. To provide a stability analysis, the equations are linearized assuming that the end-effector experiences quasi-static movements and the system is subjected to low amplitude vibrations. Afterwards, the corresponding eigenvalue problem is analyzed and critical speeds of the robot in whole workspace domain are calculate. Furthermore, vibration frequencies corresponding to the unstable eigenvalues are determined. It is observed that system critical speed reduces as the end-effector moves to the boundaries of the workspace. In contrast to this, the frequency of the corresponding unstable modes increase moving to the borders of the workspace.

Using social robots in autism diagnosis and treatment has been increasing in recent years. In this study, our objective (as one of the pioneers in Iran) is to explore the clinical application of humanoid robots as medical assistants in the treatment/education of children with autism in order to improve their social/cognitive skills. To reach this goal, we have designed and implemented a set of therapeutic games with the following topics: a)Investigation of social robots’ acceptability and effect on improving the fine/gross movement imitation of Iranian children with autism, b)Exploring the effect of a robot-assisted music-education program on children with ASD’s socio-cognitive skills improvement (as an individual clinical intervention program), and c)The impact of humanoid robots on improving the social and cognitive skills of high-functioning autistic children (as a group clinical intervention program). The results indicated that our robots were accepted by 70 percent of the participants as a communication tool from the first interaction. We also observed improvement in joint attention and fine movement imitation skills of both the high-functioning and low-functioning subjects. It was concluded that the high-functioning children’s social skills improved due to the robot-assisted group therapy sessions, while the stereotyped behaviors of the low-functioning subjects decreased during the course of this program. Moreover, the proposed FCM algorithm for the automatic assessment of facial expression imitation tasks shows a sufficient agreement on the automatic and manual scores for the participants. The observations indicate the potential use of robotic-platforms in behavioral analysis of children during the intervention sessions.