مجله مکانیک سازه ها و شاره ها
سال پنجم شماره 2 (تابستان 1394)

In this paper, a new method for extracting sliding surface has been presented about a linear system with parametric uncertainties with known bound. In the proposed method, common signal of first order sliding mode control has been applied to the system and then the sliding surface is extracted as a virtual controller with the aim of minimizing a cost function. The main advantage of the proposed method is the possibility of setting the distance of the state trajectory from the sliding surface setting one of the design parameters. Thus, It is possible to near the ultimate goal of sliding mode control, i.e. staying the state trajectory on the sliding surface, without increasing the degree of sliding mode control, while the amount of used energy will be controlled, too. In order to show the efficiency of the method, a quarter car active suspension system has been chosen and the proposed method in the selection of position subsystem sliding surface is used.

A mobile manipulator robot is known as a complex system due to some properties such as coupling between the manipulator and mobile chassis, holonomic and nonholonomic constraints, multivariable and nonlinear dynamics. The control of robot faces the external disturbance, parametric uncertainty and unmodeled dynamics. Therefore, the use of an adaptive fuzzy system is suggested for its capability in overcoming uncertainties and approximating of nonlinear functions based on the universal approximation theorem. However, the tracking error does not converge asymptotically to zero due to the approximation error of the fuzzy system. This paper presents a novel adaptive fuzzy control for a mobile manipulator robot. The novelty of paper is compensating the approximation error of fuzzy system for asymptotic convergence in tracking the desired trajectory in the presence of uncertainties. For this purpose, the closed loop system in the error space converges to a linear system with poles having negative real parts. The control design consists of two parts; the kinematic control and dynamic control in which the novelty is for the dynamic control. The dynamic modeling and motion control of the nonholonomic wheeled mobile manipulator robot is considered in this paper. Advantages of the proposed design are the simplicity and very good performance in tracking of the desired trajectory in the presence of uncertainties. The stability of control system and convergence to the desired trajectory are proven by the Lyapunov method. The simulation results show the superiority of the proposed control over a robust adaptive control.

This article is devoted to the derivation of formulation and isogeometric solution of nonlinear incompressible elastic problems, known as incompressible hyperelasticity. After problem definition, the governing equations are linearized for employing the Newton-Raphson iteration method. Then, the problem is discretized by using concepts of isogeometric analysis method and its solution algorithm is devised. To demonstrate the performance of the proposed approach, the obtained results are compared with finite elements. Due to large deformations in this kind of problems, the finite element method requires a relatively large number of elements, as well as the need for remeshings in some problems, that results in a large system of equations with a high computational cost. In the isogeometric analysis method, using B-Spline and NURBS (Non-Uniform Rational B-Spline) basis functions provides us with a good flexibility in modeling of geometry without any need for further remeshings. The examples studied in this article indicate that by using the isogeometric approach good quality results are obtained with a smaller system of equations and less computational cost. Also, influence of Gauss integration points for the incompressible materials are investigated.

In this study, an experimental investigation of repeated low velocity impact is performed on GLARE using drop weight testing machine. After the first impact on the plate, the second impact energy decreases and stays constant until the last impact. Damage due to repeated impact is investigated using visual inspection and C-scan. Three categories namely, no damage, small damage and serious damage are happened as a result of the first impact on the plate. The number of impacts required for the penetration is increased with decrease of the second impact energy. The threshold limit energy is defined as the maximum impact energy in repeated impact after the first impact which causes no damage on the specimen. For the successive impact energy larger than the threshold limit energy, the number of impacts becomes important, while for the successive impacts energies smaller than threshold limit energy, the number of impact has inconsiderable effect on damage occurrence.

In this article, a spectral finite element (SFE) formulation and its solution are described for free and force vibrations of cracked Euler-Bernoulli beam. The formulation based on SFE algorithm includes deriving partial differential equations of motion, spectral displacement field, dynamic shape functions, and dynamic stiffness matrix. Frequency-domain dynamic shape functions are derived from an exact solution of governing wave equations. The cracked beam with an open crack is modeled as two segments connected by a massless rotational spring at the crack position and frequency-domain dynamic stiffness matrix for cracked Euler-Bernoulli beam is extracted. By considering free vibration of the cracked beam, its natural frequencies are derived for different boundary conditions. In the SFE model, It is possible to represent the whole length of beam only by two spectral elements, while it may not be possible to do that in finite element (FE) model, for reaching the same order of accuracy. The accuracy of results obtained from SFE formulation is compared with that of either FE method or analytical formulations. The SFE results display remarkable superiority with respect to those of FE, for reducing the number of elements as well as increasing numerical accuracy.

MAFVRO is one of the out-put only modal analysis methods which is able to determine the modal parameters of a vibration system through the free vibration responses. It is worth to note that most of the vibration systems are subjected to random excitations. Therefore, in such cases, the traditional MAFVRO cannot extract the modal parameters. In this study, the mentioned method is developed to randomly excited vibration systems. Then, the modified MAFVRO is applied to a randomly excited vibration system. The random excitation has been considered as a set of white noises which are applied to all masses of a discrete system. Then by employing the developed method, the estimated results have been compared with those obtained through the structural eigenvalue problem. Comparing the results reveals that the developed method can give the natural frequencies and mode shapes of a randomly excited system with a good accuracy, but it estimates the damping ratios lower than their exact values. In addition, the effect of the noise on the accuracy of the estimated modal parameters have been investigated.

Nowadays, damage diagnosis of the engineering structures, applying vibration signal for early damage identifying during normal operation of the structure or after some disasters such as earthquake are very important from academic and technology viewpoints. Among different methods, proposed for damage diagnosis of the structures, the methods, based on FE model updating using modal analysis features are very important because of its practicability such that there are many researches in this field. Also because of node measurement limitations in real structures, model reduction is accompanied with damage detection methods. In addition for on-line applications of the mentioned methods for structural health monitoring with complex and intensive FE modeling, speed and accuracy increasing has attracted many researchers’ attentions. In this paper one of new model updating methods, which is lately proposed and proved to be accurate and fast, are applied for designing a damage diagnosis method. A beam, modeled with FE, has been considered with artificial damage on it is simulated in MATLAB for studying the proposed methods’ abilities and the noise effects on the method was studied exactly. Additionally, for studying the capability of the method in practice, the proposed method was applied on laboratory beam and the results were studied.

In this paper, thermo mechanical buckling of functionally graded plates (FG Plates) with circular cutout and subjected to combined thermal and mechanical loads are investigated by Finite Element Method (FEM). Unlike other studies in which the plate is subjected to only one type of loading at once, in present study it was assumed that mechanical and thermal loads are applied simultaneously. The material properties are assumed to vary across plate thickness according to power law distribution of the volume fraction of constituents. The plate formulation is based on first order shear deformation theory (FSDT) and element stiffness matrices are derived based on principle of minimum potential energy. A flexible mesh generation algorithm is prepared in which the mesh density around the hole can be controlled easily. After validating the results of developed finite element code with those available in the literature the effect of boundary conditions in edges of plate and cut out, plate aspect ratio and cut out size on thermo mechanical buckling behavior of FG plates are studied thoroughly and stability boundary graphs are presented. Finally useful conclusions are presented.

In the current paper a new method is introduced to analyze and measure the cracks dimensions in solid materials such as mechanical tools and bricks. Since the cracks do not have a regular or predictable shape, in order to achieve the exact dimensions of such cracks, the conventional mathematical formulas are by no means applicable. Hence, while studying different crack analyzing methods, we argue on their faults and limits and propose our method which aims to measure the crack dimensions in a solid object by utilizing machine vision, image capturing and image processing techniques. We define new algorithms and perform picture scaling in real dimensions and analyze the acquired data to obtain the most precise results. This optimal machine vision technique is performed in two different ways. The first method is based on the measurement of the length of the image skeleton, while the second technique is based on measuring the half of the perimeter of the crack’s image. After obtaining the measurements, we optimize the results with the help of some pre-defined algorithms. It is shown that our proposed algorithms provide a reliable method which can be used to measure any crack dimensions. Also, we apply these techniques on a sample brick with some random cracks on it. After gaining the binary images, filters are applied to gain the best results among all images.

In the present study, a mixed mode fracture specimen was simulated by the extended finite element method. For this, a specimen called "Diagonally loaded square plate" was selected. For this specimen, experimental test results are available. The loading process including the fracture and crack growth is simulated. Displacement control loading is considered. The crack growth increment and direction are determined using the fracture criteria. Various test specimens are simulated using a model with the fixed mesh. Therefore, computation costs has decreased dramatically. Furthermore, numerical integration in the enriched elements is studied and the optimum number of gauss points in these elements are determinrd. Comparing the results of the extended finite element method with the experimental data show that the critical fracture load and the stress intensity factors at the fracture moment differ less than 10%. Furthermore, there is a convincing agreement between the crack growth path in the experimental test results and numerical analysis by the extended finite element method.

In present research, the numerical and experimental investigation of progressive ductile damage has been conducted in free bulging of 304 stainless steel seamed tube using oil pressure. In the numerical section, using the Lagrangian finite element method with ABAQUS/Explicit solver, the seam weld simulated as a thin strip, contains non-homogeneity factors of strength and formability. Forming Limit Diagram (FLD) criterion was used as a measure of damage initiation, as well as effective plastic displacement factor with linear approach in order to model damage evolution. In the experimental section, tensile test of tube has been conducted to attain mechanical properties and also tube free bulging has been perform. Maximum bulging diameter of tube and critical pressure were recorded at the moment of bursting. In numerical analysis, the effect of material non-homogeneity factor on critical pressure, also the outcome of effective plastic displacement on the maximum bulging diameter investigated and compared with experiments. Finally, the optimum values of non-homogeneity factor and effective plastic displacement obtained respectively equal to 0.9 and 0.05. Using these factors, the accuracy of numerical prediction for tube diameter and oil pressure at bursing moment were more than 99%.

An appropriate joining technology is the laser beam welding process, because of its low localized energy input leading to low distortion, high strength of the joint and high processing speeds. The growing of aircraft industry in reducing the weight of aerospace structure has led the introduction of laser beam welding into the fabrication of aerospace structure with stiffeners, instead of riveted joints. In the present paper an analytical model for calculation of the residual stresses induced by laser welding is proposed and numerically validated. The aim of this work is to study the transverse residual stress distribution of plates made of an aluminum alloy 6061-T6, which is used for fabrication of fuselage panels. The results show that, if the maximum stress amount induced in the workpiece does not exceed the material yield stress, no plastic penetration will occur and there will be no residual stress. It was found that high stresses are developing close to the plate surface while these stresses decrease rapidly to almost zero values at the lower surface. An ideal elastic-plastic material curve was assumed. Good accordance is found between the calculated and numerical results.

In this paper, designing optimal PID controller using modified particle swarm optimization is presented. The advantage of this new method compared to conventional methods of controller design is that it is not limited to a certain class of systems. In designing phase, sum of rising time, settling time, overshoot and integral of squared-error are minimized. There kind of particle swarm optimization algorithms such as ePSO, mPSO and sPSO are compared with other methods of optimization including Ant Colony. The results clearly show how superior the new proposed method is to the other methods. The proposed method differs from the other optimization algorithms in such a way that, the proposed algorithm does not need a velocity equation. The position of the particle is updated directly by extrapolating the current particle position with the global best particle position obtained so far.

In the present investigation, in spite ofintroducing the effective parameters in the Bladeless fan performance,the profile of fan cross section was studied precisely because it is the most important sectionof designing this kind of fan. In order to modify the fan cross section and by considering the similarity of bladeless fans to airfoils, five profiles were chosen among the standard airfoil profiles by considering the important geometric parameters such asradius of leading edge, maximum thickness of airfoil compatible with the original airfoil. In addition, five profile were designed to create uniform airflow in front of fan and to prevent the separation of flow as well as the manufacturing criteria. By solving the momentum and continuity equations for incompressible fluid, the flow was analyzed numerically in 3D form. The aerodynamic characteristics of the designed airfoils and the original airfoil of the Bladeless fan wereindicated and compared to eachother.The fan was located in the centerof a 4×2×2m room and Eppler473 airfoil profile was used as the cross section of this fan.Accoring tothe obtainednumerical results, the flow increase curve of the fan versus different inlet flowrate was depicted. The flow increase curve shows that the outlet flow rate increasedlinearely by increasing the inlet flow rate.

On hot days, due to the high power consumption of the compressor, using the refrigeration cycles cause to increase in energy consumption. According to lack of fossil fuels, refrigeration consumption reduction is very valuable. In this paper, simulation and design of a refrigeration cycle are presented that ejector and separator added to a conventional compression cycle. These changes, can reduce the power consumption of refrigerator. Comparison results show with the addition of ejector to conventional compression cycle, depending on the type of fluid and the temperature of the condenser and evaporator, lead to energy saving up to 35%. Also, whatever temperature difference between evaporator and condenser increases, or at the same temperature difference between the condenser and the evaporator, as lower evaporator temperature, coefficient performance of ejection-compression cooler is better than conventional refrigerant. In comparision between working fluids, Ammonia has the minimum coefficient of performance improvement and one reason for increased compression ratio is that this fluid has the lowest increasing compression ratio. The result show that R290 and R134a have the highest improvement coefficient of performance because the operation of these fluids associated with the highest increasing compression ratio.

Low swirl burner provides an effective and low cost solution for stable lean premixed combustion. Several studies have been conducted on the performance of these burners in different pressures, temperatures, capacities, mixture velocities and equivalence ratios. The main design parameters of low swirl burners are the swirl number and the recess length. The objective of this paper is the investigation of recess length effects on the burner performance. A rig test has been established and utilized to study the effects of different equivalence ratios and recess lengths on the temperature distribution and flame regime of low swirl burners. Results revealed that increasing the recess length causes the increase in axial bulk velocity and hence the lifted flame would be stable in wider range of equivalence ratios. Observations also show the significant effect of the specific divergent flame regime of low swirl burners on result in uniform temperature distribution inside the combustion chamber and lower NOx production.