مجله مکانیک سازه ها و شاره ها
سال دهم شماره 3 (پاییز 1399)

One of the important sections of cold roll forming lines is the shearing of profiles, which is usually performed as the guillotine shearing for open profiles. Despite the importance of shearing quality on the assemblability of the open profiles, few researches have been done on their shearing operation. In this paper, the guillotine shearing of a complex open profile, door frame, was investigated. For this purpose, the guillotine shearing of this profile was simulated in Abaqus software and the effects of the important process parameters on the shearing quality were determined. In the experiments, the profiles were initially annealed to remove residual stresses produced during deformation and then were sheared. The experimental and finite element–predicted shearing cross-section were compared and the shearing defects were analyzed. The comparison revealed a good agreement and confirmed the finite element model. The results showed the increase of the shearing angle and the clearance between dies and profile reduces the shearing quality. In addition, the increase of the clearance between dies and profile and the clearance between dies reduces the shearing force. Finally, the best shearing conditions were proposed based on the results.

Up to now, several analytical and numerical methods are proposed for free vibration analysis of fluid-structure interaction systems. Concrete gravity or arch dam-reservoir systems and the fluid containers in various shapes, such as rectangular, cylindrical or spherical shapes are the most well-known instances of these “coupled” systems. It should be emphasized that the governing equations of both fluid and structural parts of these coupled systems should be solved simultaneously. In the present article, two classical fluid-structure interaction (FSI) problems, including one-dimensional compressible fluid domain and one or two single degree of freedom (SDOF) system(s) as structural part, are solved in free vibration situation by using differential transform method (DTM). To this end, the solution process is thoroughly described and the numerical results, involving natural frequencies and mode shapes of both systems are obtained in detail. Moreover, to verify the DTM results, the closed-form solution is comprehensively derived for both systems.

One of the important issues is to reduce the depth of projectile penetration in the target. In this paper, the analytical, experimental and numerical investigation of the ballistic performance of ceramic composite targets is discussed. The targets are perforated in the first layer. In the analytical section, presents a new analytical model for these targets. In the experimental section, the depth of penetration was measured by conducting experiments on these targets. In the numerical section, Abaqus software is used. The validation of the numerical results with the experimental results are performed to develop the numerical simulation and to investigate the results of the new analytical model. The results of numerical simulation are in good agreement with the experimental results. The results of the new analytical model are also in good agreement with the numerical simulation results. At low and high velocities, the amount of ceramics in contact with the projectile is important, but at moderate velocities, the angle of obliquely is more important. Each of these factors has reduced the depth of projectile penetration in the target for those velocities. At all velocities, the hole has caused the projectile to move obliquely at the target.

Cylinder head bolts in high-speed diesel engines must meet classical standard requirements. Moreover, they should seal cylinder head gasket due to uniform distribution of clamping forces with high durability under thermal and mechanical cyclic loads. In present study, to evaluate the performance of newly designed bolts, at first, the clamping force in cold assembly was calculated using theoretical and experimental results. Then in real engine condition, the amount of clamping force variation was derived with combustion load under hot condition. The most critical force was used to test the bolt durability in a uniaxial high cycle fatigue test. The experimental test confirmed infinite life of the bolt. Also, results show preload of used bolt could be increased about 8% that is important for maintenance and repair services. In addition, torque-angle assembly procedure precisely tightened the bolt near yield stress condition and improved sealing of the cylinder head gasket. Fuji paper test confirmed this analytical uniform clamping force results too.

The dual fuel diesel engine (DDF) is a one of the various IC engine that can used alternative fuel for power generation. Emission and fuel consumption reduction are some of the properties that use a combination of fuel mixture in dual fuel diesel engine (gas-diesel). We focused in this research to study the effect of start of injection (SOI) and blend of fuel variation in OM355 EU2 dual fuel diesel engine at two various speeds with the help of the computational fluids dynamic. The modeling of the artificial neural network (ANN) has been used to study the interaction effects of SOI and blend of fuel on the operational parameters. The non-sorted genetic algorithm (NSGA II) has been used for determining the optimized levels of the variables. The results of this study show that the RBFNN has acceptable predictions about the outputs’ variables (R^2=0.99 ,RMSE=0.01), and the optimized range of the engine function in the specific speeds has been attained with the help of the responding surface of the neural network. Besides, the optimizing capabilities of the NSGA II have provided the optimized levels of the input and output variables in the various speeds.

The degrees of freedom of the workpiece can be constrained during its loading in the fixture, resulting in the jamming occurrence. It may cause improper workpiece positioning, damage to the workpiece, and even the fixturing elements. In the present study, theoretical and experimental analyses are performed to investigate the jamming phenomenon in fixturing workpiece using the peg-in-hole case study. The theoretical model is established based on the minimum norm principle which resulted in an optimization problem. The jamming occurrence is then predicted by solving this model in different configurations of the peg-in-hole problem. A theoretical model that was developed in the previous study is used to validate the predictions of the suggested theoretical model. The main validation of the theoretical predictions is conducted through the experimental analysis. For this purpose, an experimental setup is designed and fabricated for the peg-in-hole mechanism with the clearance fit. The friction coefficient is first measured using the fabricated setup. Then, the jamming-in and jamming-out angles of the peg into the prepared hole are measured using a rotary encoder. By comparing the experimental results to the predictions of the suggested theoretical model, the worst-case error values are determined as 8.2% and 14.4% for the jamming-in and jamming-out angles, respectively. These values represent the accuracy of the suggested mathematical model in the prediction of the jamming occurrence conditions and reliability of its results.

In this study, the crashworthiness of cylindrical tubes with variable thickness, under axial load, is investigated. The loading is applied quasi static. Wall-Thickness changes are considered to follow the nonlinear power distribution equatiion. Firstly, by considering the elastic-perfectly plastic behavior for material, an analytical relition is extracted based on Alexander's theory. Then, in order to increase the accuracy of the results, the effects of work hardening is applied to material behavior and the formulas are rewritten. Finally, to validate the analytical results, using simulation in ABAQUS FE software, several models have been studied in different modes and the results have been compared. The results show an acceptable agreement between the analytical results and the simulation. The final result of this research is presenting two analutical relition beteen absorbed energy and mean force with mechanical properties and geometic parameters of anergy absorber. Moreover, by simplifying the drived analytical equation, we can obtain the basic equations in later researches.

One of the major faults accrued in the aircraft is corresponding to its actuators. In order to fault diagnosis and isolation in the aircraft actuator, this paper presents a new robust method based on the incremental nonlinear dynamic inversion, which is robust to the disturbance and uncertainties. The produced residual is robust to the system uncertainties, and the adaptive threshold is designed to evaluate this residual with fuzzy logic, which is altered in the presence of disturbance or variation in the input command to prevent faulty detection. Since this method requires the online knowledge of the upper bound of undesirable factors (disturbance and uncertainty) and the disturbance occurrence on the system, new equations is developed to calculate this bound and an innovative structure is suggested to detect the disturbance event. In order to evaluate the proposed method, it is simulated on the nonlinear dynamics of the Boeing-747 considering the coupling between the longitudinal and lateral dynamics, whereas the disturbance is applied on the three axes at different times and the rudder actuator fault is locked to the aircraft. Simulations verify that despite the uncertainty and disturbance, the following of the input commands is well performed and the timing of the operator's fault and the location of the operator is determined according to the adaptive threshold.

Machining is a complex process that many variables determine the result. Among them, tool vibration is a critical phenomenon because it causes unacceptable dimensional errors on the work pieces and a sharp reduction in tools life. Whereas the dynamic of the tool and the work piece depends on many parameters, it's very difficult to prediction tool vibration by using relationship and formulas. In this research, internal tuning experiments done with various parameters. Output of experiments shows occurrence or non- occurrence of chatter phenomenon by finishing surface shape, Fast Furrier transform and Spectral density of acceleration signal. Then, by writing a program with Matlab software, using different artificial neural networks, various processes are performed to make the program with highest efficiency between them and using it to predict chatter phenomenon this specific turning machine. Finally, for the validation of results, tests have been designed and planned. The results show that the predication of chatter phenomenon with an acceptable percentage is done correctly.

In the present research, the modulus of a double lap bonded joint in the strain rate ranges of 0 to 1055 S-1 is predicted by a theoretical model. In this approach, using geometrical parameters of bonded joint, elastic young modulus of adherends and shear modulus of the adhesive layer at two different strain rates, strain rate sharing of bonded joint’ constituents can be calculated. Also, using these data, shear modulus of the adhesive layer at its share of strain rate is calculated; Then, using these data, dynamic modulus of double lap bonded joint can be computed. Calculating stiffness of the double lap bonded joint using geometrical parameters of bonded joint and mechanical parameters of constituents without need to do the test is the advantage of this model. Also, in this manuscript, a finite element model (using Abaqus Explicit) is presented in order to predict dynamic modulus of double lap bonded joint using elastic mechanical properties of bonded joint constituents, their densities and bonded joint geometrical parameters. The two presented models show a similar trend in the prediction of double lap bonded joint’ modulus; in a way that, at the two models, bonded joint’ modulus is increased by increasing strain rate. The maximum difference between the two models is about nine percent.

In this research, dissimilar joint between AISI 430 ferritic stainless steel and AISI 2304 duplex stainless steel was made by Nd:YAG laser welding process. At first, microstructural and phase evolutions and also changes in mechanical properties due to the welding process were investigated. Hence, mechanical properties of weld joints were optimized using statistical methods. For this purpose, response surface methodology based on central composite design was used in order to find the optimum values of laser power, welding speed and defocusing distance and to obtain the maximum value of fracture force of joints. Uniaxial tensile test and optical microscopy were used to study the mechanical properties and structural features of weld joints, respectively. It was found that the fracture force of weld joints increases by decrease in welding speed and laser power and the maximum fracture force reached to 3587 N. Regarding microstructural evolutions studies, grain growth and presence of dispersed carbides in the fusion zone were observed. Finally, the effect of heat input on the hardness profile was evaluated and discussed. According to the result, the maximum hardness was obtained in the weld metal of the sample with low heat input and it was measured at about 332 vickers.

The vibrational behavior of an axially graded micro-beam with both axial and rotational motion under axial load have been studied in the magnetic field. A detailed parametric study was performed to explain the effect of various factors such as range of axial graded of materials, type of material distribution, viscosity coefficient, and magnetic field strength, length scale parameter of material, rotation and coupling crossing motion on the dynamical characteristics of the system. It is assumed that the material properties of the system change linearly or exponentially in the longitudinal direction. The critical axial and rotational speeds of the system are obtained by using Galerkin discretization technique and eigenvalue analysis. An analytical method has also been used to identify system instability thresholds. System stability maps were tested, and it has been shown that by increasing the magnetic field strength and the length scale parameter, the system stability can be improved. The results also show that the simultaneous determination of density gradient and elastic modulus in the longitudinal direction can reduce the destructive effects of compressive axial load.

In order to improve the performance or manufacture of new micro-components, research on the vibrations of the micro-technologies is one of the most pressing needs in the industry. In this paper, the nonlinear vibration analysis of rotating Euler-Bernoulli microbeames subjected to loading is investigated using the strain gradient theory. Firstly, the non-dimensional equations of motion are obtained by insertion of the strain energy, the kinetic energy, and the work of the external force in Hamilton's Principle. Then, by applying Galerkin method, the natural frequencies are extracted. Then the validity and accuracy of the presented analysis is confirmed through comparison of the the present analysis result with the result of another reference. Next, after extracting the mode shapes, the effects of some important mechanical and geometrical parameters such as Poisson's ratio, thickness to material length scale parameter ratio, material, rotation speed and size effect parameters on the vibration response of the microbeam are investigated. The results reveal that based on the strain gradient theory, increase of the Poisson's ratio reduces the natural frequencies, while the modified couple stress theory can't do this prediction. The results of the present research can be valuable for development of micro-technologies.

To meet the energy demand, the expansion of transmission networks has caused low frequency oscillations in the power system. These fluctuations, if not attenuated, reduce the capability of the transmission lines and sometimes cause the whole system to be unstable. Proper selection of stabilizing parameters have great importance and increase the optimum performance during unwanted disturbances. In this article, a new algorithm based on teaching and learning is used to increase the stability of the multi-machine power system and thus increase the performance of the power system. In fact, since the optimal tuning of the control parameters has a direct effect on the low frequency stability, it becomes an optimization problem and is accomplished with a new algorithm based on the optimized training and learning algorithm of the power system stabilizer. In addition, the results are compared with the colonial competition algorithm. A four-Machin test system is used to evaluate the effectiveness of the proposed method. By examining the results in different modes, it is shown that the proposed method using the learning-based optimization algorithm is much more efficient than the proposed method using the colonial competition-based algorithm.

The electrochemical machining process is an anodic dissolution process. Due to the inherent nature of the process and the complex physical, chemical, and hydrodynamic phenomena that occur during the process, it is difficult to model the process parameters. Therefore, a highly repeatable and appropriate method in terms of time and cost is important for modeling. In this paper, the simulation of the process is firstly performed by the numerical method and the Comsol software. The voltage, tool feed rate and electrolyte concentration are considered as input parameters and machining overcut and the material removal rate are considered as the response variables. 15 experiments were selected using the Box-Behnken design method to implement the response surface methodology. Then, the simulation was performed according to the experimental design in the Comsol software. Nickel-based single crystal superalloy workpiece is selected due to its improved mechanical properties. As a result, two quadratic mathematical models showing the relationship between two response variables with the input parameters are obtained. The adequacy and accuracy of these two mathematical models have been examined by the analysis of variance, correlation coefficient, and related graphs. Therefore, the combination of numerical simulation and design of experiments for modeling and investigating the effect of input parameters of electrochemical machining of the nickel-based single-crystal superalloy has been implemented in a favorable manner.

Freezing water in the surface cracks of wind turbine blades can remarkably deteriorate its mechanical properties. In order to investigate this phenomena, a finite element model is developed to simulate the icing process in a small scale fiber reinforced composite beam with a sharp notch. The load exerted from the ice onto the beam is calculated by using the model taking into account the expansion of water at freezing temperature. Based on this technique, a new parameter, called the ice expansion effect, is introduced and employed to study the influence of notch opening angle and the beam thickness on the distribution of stress and strain along the notch faces. Compromising between the actual problem, in-service damaged blades, and the existing standard which is commonly applied for characterization of fracture behavior of composite materials, an optimum geometry is designed for experimental investigations. The experimental results acquired from small scale composite beams based on the designed geometry and simultaneously imposed to humidity and freezing temperature demonstrate the significance of ice expansion factor on the monotonic response of the materials and structures conditioned at different number of freeze-thaw cycles.

The aim of this study is investigating the center of mass deflection to thickness ratio of triangular plates using the GMDH-type neural networks and comparing it with results of laboratory tests performed on a narrow triangular plate using water-hammer apparatus. Also, the study focuses on the overall deformation, strain and impact transmission. Dimensionless input variables are used to investigate the center of mass deflection of triangular plate with changing variables. A simpler polynomial expression is derived using GMDH-type neural network and dimensionless number. The vector of coefficients of quadratic sub-expressions involved in GMDH-type networks is obtained by Singular Value Decomposition (SVD) method. SVD can improve the proficiency of GMDH-type networks to model the intricate process of deformation of triangular plates. Obtaining results by applying a GMDH model and comparing them with actual data indicates good agreement between model output and experimental data. The advantages of this approach are in the simplification of computation and convenient application to parametric study for impact behavior.

This paper focused on the transient heat transfer inside a convergent-divergent nozzle which has applications in propulsion systems. Time-averaged Navier Stokes equations in the compressible form were solved using the finite volume method. The flow was assumed to be axisymmetric and the results of simulations were compared with available experimental data. The flow and heat transfer parameters were investigated in different nozzle geometries. The results revealed that the SST k-ω turbulence model gives better predictions compared to other applied turbulence models. Also, for a constant length of the nozzle, increasing the divergence angle caused higher exit Mach numbers and lower exit pressure and temperature. Bell nozzles had more exit Mach number, and less exit temperature and pressures compared to the conical nozzles. Decreasing in the exit angle of the bell nozzle led to an increase in the Mach number and thrust and causes lower exit temperature and pressure. For various nozzle shapes, the values of the heat flux and temperatures were nearly constant at the sections which have the same area ratios. The solid surface temperature at the outlet was greater for the bell shape than that for conical shape. The maximum value of the convection heat transfer coefficient occurred at the nozzle throat. The maximum thrust was obtained by bell shape nozzle. Higher outlet angles gave lower thrusts.

In the present work a three-dimensional simulation of the slip flow regime for a small-sized channel of a fuel cell with lower porous wall. This study is used to determine the variations of average velocity, pressure difference, and friction coefficient considering the penetrated mass from porous wall, the wall and channel cross-sectional area, and aspect ratio of channel. Furtermore, the results are comprised with Continuous flow regime. In this solution, the equations of mass and momentum for continuous and slip regimes evaluated with developed Fortran code, and Validated with papers.The results with different channel area for two fluid regimes are obtained. Considering the aspect ratio equal to one, the friction coefficient average velocity, and the pressure difference are increased with decreasing at Knudsen number. Considering the slip regime into the channel, the friction coefficient, pressure difference and average velocity values are calculated more accurate (at best 5%), than non-slip regimes. Furthermore, when Knudsen number and aspect ratio are respectively equal to 0.001 and 5, changing the flow assumption from non-slip regime to slip one is occurred to increasing the expected pressure difference and average velocity.

This study presents an analytical investigation of the natural convection hydromagnetic flow of a particulate suspension fluid through an infinitely long vertical channel in the presence of heat sink or source effects. The channel walls are maintained at isoflux–isothermal condition. That is, the thermal boundary conditions are such that one of the channel walls is maintained at constant heat flux, while the other is maintained at a constant temperature. In addition, the magnetic field is exerted on channel from the iso-thermal wall. Various closed-form and exact solutions of the governing (Navier Stokes) differential equations for different special cases are obtained. A parametric study of the physical variables involved in the problem is done to illustrate the influence of these non dimensional parameters on the velocity and temperature profiles of both phases. Results show that replacing the physical boundary conditions, will have a different behavior of velocity and temperature distributions.

In this research, an effective formulation based on the method of fundamental solutions is presented for analyzing the two- and three-dimensional transient heat conduction problems including moving point heat sources. This is a new formulation and has not been presented yet. The path of motion and the intensity of the moving point heat source are arbitrary functions of time and the number of moving point heat sources is unlimited. Solution of the problem is considered as a linear combination of time-dependent fundamental solutions and a particular solution involving the effect of the moving point heat source. The particular solution is presented as a time and space-dependent integral and is obtained without using any internal cells or points and without any time transformation. Numerical examples show the efficiency and accuracy of the proposed method in comparison with the finite element method. In the finite element modeling of the moving point heat source, it is necessary to use a fine mesh in the path of the moving point heat source that increases the cost of the preprocessing step. Compared to the finite element method, the proposed method is very simple and gives very good results even with a small number of source points. For example, in the second solved example, the average relative difference percentage between the results of the proposed method with 218 source points and the finite element method with 7268 nodes is 0.65%.