Sharif Digital Repository

The vibration of a simply-supported beam with rotary springs at either ends is studied. The governing equations of motion are investigated considering the nonlinear effect of stretching. These equations are made non-dimensional and solved to the first-order approximation using the two known methods, namely, the multiple scales and the mode summation. The first five natural frequencies of the beam for different pairs of the boundary condition parameters are evaluated. A multilayer feed-forward back-propagation artificial neural network is trained using these natural frequencies. The artificial neural network used in this study shows high degree of accuracy for the natural frequency of the... 

Vibration characteristics of laminated composite beams with magnetorheological (MR) layer are investigated using layerwise theory. In most studies, shear strain across the thickness of MR layer has been considered as a constant value, which does not precisely describe the shear strain. In this study, layerwise theory is employed to develop a finite element formulation to investigate MR-laminated beams. Experimental tests under different magnetic fields are carried out to verify the numerical results. Layerwise numerical results are compared with the experimental results and other theories. An empirical expression for complex shear modulus is presented. The effects of MR layer thickness on... 

Vibration behavior of adaptive laminated composite beams integrated with magnetorheological (MR) fluid layer has been investigated using layerwise displacement theory. In most of the existing studies on the adaptive laminated beams with MR fluids, shear strain across the thickness of magnetorheological (MR) layer has been assumed a constant value, resulting in a constant shear stress in MR layer. However, due to the high shear deformation pattern inside MR layer, this assumption is not adequate to accurately describe the shear strain and stress in MR fluid layer. In this work a modified layerwise theory is employed to develop a Finite Element Model (FEM) formulation to simulate the laminated... 

Vibration behavior of adaptive laminated composite beams integrated with magnetorheological (MR) fluid layer has been investigated using layerwise displacement theory. In most of the existing studies on the adaptive laminated beams with MR fluids, shear strain across the thickness of magnetorheological (MR) layer has been assumed a constant value, resulting in a constant shear stress in MR layer. However, due to the high shear deformation pattern inside MR layer, this assumption is not adequate to accurately describe the shear strain and stress in MR fluid layer. In this work a modified layerwise theory is employed to develop a Finite Element Model (FEM) formulation to simulate the laminated... 

Laminated composite beams with non-uniform thickness are being used as primary structural elements in a wide range of advanced engineering applications. Tapered composite structures, formed by terminating some of the plies, create geometry and material discontinuities that act as sources for delamination initiation and propagation. Any small damage or delamination in these structures can progress rapidly without any visible external signs. Due to this reason early detection of damage in these systems during their service life is receiving increasing attention. The presence of a crack in a component or structure leads to changes in its global dynamic characteristics results in decreases in... 

In this paper the vibration of a spinning cylindrical shell made of functional graded material (FGM) made is investigated. After a brief introduction of FG materials, by employing higher order theory for shell deformation, constitutive relationships are derived. In the next step by utilizing energy method and Hamilton's principle governing deferential equation of spinning cylindrical shell is obtained. By making use of the principle of minimum potential energy, the characteristic equation of natural frequencies is derived. After verification of the results, the effect of changing different parameters such as material grade, L/R, h/R, and spinning velocity on the natural frequency are... 

A composite beam with single delamination traveled by a constant amplitude moving force is modeled accounting for the Poisson's effect, shear deformation and rotary inertia. The mechanical behavior between the delaminated surfaces is modeled using a piecewise-linear spring foundation. The governing differential equations of motion for such system are derived. Primarily, eigen-solution technique is used to obtain the natural frequencies and their corresponding mode shapes of such beam. Then, the Ritz method is employed to derive the dynamic response of the beam due to the moving force. The obtained results for the free and forced vibrations of beams are verified against reported similar... 

This paper investigates the free vibration analysis of a doubly tapered magnetorheological rotating sandwich beam based on the Euler-Bernoulli theory. The beam is made of a magnetorheological elastomer core sandwiched between two elastic layers. Through energy approach the kinetic and potential energies of the system are written and using the Lagrange equation the discretized form of the governing equation is derived based on the Ritz method. The free vibration analysis is carried out to obtain the natural frequency and the corresponding loss factor of the beam. Finally, after validating the formulation in order to provide a deep insight the effects of different parameters on the free... 

Functionally graded materials (FGMs) are inhomogeneous composites which are usually made of a mixture of metals and ceramics. Properties of these kinds of materials vary continuously and smoothly from a ceramic surface to a metallic surface in a specified direction of the structure. The gradient compositional variation of the constituents from one surface to the other provides an elegant solution to the problem of high transverse shear stresses that are induced when two dissimilar materials with large differences in material properties are bonded. FGMs have extracted much attention as advanced structural materials in recent years. In this paper, free vibration of a rotating FGM cantilever... 

In this paper, a thorough investigation of response of a composite Timoshenko simply-supported beam with actuating layers, under the motion of a partially distributed mass is studied and a control system based on the feedback of beam's deflection velocity is applied to alleviate and suppress the vibration of the beam in either case when the mass is still traveling on the beam or departed the beam. The actuating layers are made up of Terfenol-D smart material which are sensitive to magnetic field (magnetostrictive materials) and this trait makes them very suitable to be used for vibration control. They introduce damping to the system through which the energy of system dissipates. The response... 

Based on the code specifications, in moment-resisting frames the span-to-depth ratio has to be larger than a minimum value to make sure of adequate plastic hinging at beam-ends. This provision poses a restriction on the use of deep beams with short spans, especially for the framed-tube system in tall buildings. In this paper, a new energy dissipation method is introduced for moment-resisting frames by modifying the mid-span of the beam by two different methods, resulting in: (i) Perforated Shear Links (PSLs) and (ii) Slit Shear Links (SSLs). The links dissipate energy through shear yielding of the web, weakened by perforation or slitting, respectively. A thorough study is conducted on the... 

In this paper, a 2D boundary element approach able to model viscoelastic functionally graded materials (FGM) is presented. A numerical implementation of the Somigliana identity for displacements is developed to solve 2D problems of exponentially graded elasticity. An FGM is an advanced material in which its composition changes gradually resulting in a corresponding change in properties of the material. The FGM concept can be applied to various materials for structural and functional uses. Our model needs only the Green's function of nonhomogeneous elastostatic problems with material properties that vary continuously along a given dimension. We consider the material properties to be an... 

In this study, the torsional vibration analysis of a rotating tapered sandwich beam with a magnetorheological elastomer core has been investigated. The magnetorheological elastomer material is used as a constrained damping layer embedded between two elastic constraining skins in order to improve the vibrational behavior of the sandwich beam. The three layers of the sandwich beam have rectangular cross-sections with symmetric arrangement. The problem formulation is set up based on the torsional theory of rectangular laminated plates. The assumed modes method and the Lagrange equations are used to derive the governing equations of motion of the system. The validity of the presented formulation... 

In this research, the mechanical properties of glass and carbon fiber reinforced polymer (FRP) bars with epoxy resin matrices embedded in concrete were investigated under an extensive range of elevated temperatures (i.e., 25–800 °C). Embedded FRP bars with various bar diameters were studied in order to determine bar diameter influence on the results. In addition, analysis of variance (ANOVA) was performed on the experimental results to investigate the contribution of exposure temperature and bar diameter to the tensile behavior of embedded in concrete FRP bars at elevated temperatures. The results show that the tensile strength of embedded FRP bars generally decreases with increasing... 

In this study, the thermal buckling analysis of hybrid laminated plates made of two-layered functionally graded materials (FGMs) that are integrated with surface-bonded piezoelectric actuators referred to as (P/FGM)s are investigated. Material properties for both substrate FGM layers and piezoelectric layers are temperature-dependent. Uniform temperature rise as a thermal load and constant applied actuator voltage are considered for this analysis. By definition of four new analytic functions, the five coupled governing stability equations, which are derived based on the first-order shear deformation plate theory, are converted into fourth-order and second-order decoupled partial differential... 

This research investigates a rotary disk with variable cross-section and incompressible hyperelastic material with functionally graded properties in large hyperelastic deformations. For this purpose, a power relation has been used to express the changes in cross-section and properties of hyperelastic material. So that (m) represents the changes in cross-section and (n) represents the manner of changes in material properties. The constants used for hyperelastic material have been obtained from experimental data. The obtained equations have been solved for different m, n, and (angular velocity) values, and the values of radial stresses, tangential stresses, and elongation have been compared.... 

A modified dynamic-based pushover (MDP) analysis is proposed to properly consider the effects of higher modes and the nonlinear behavior of the structural systems. For this purpose, first, a dynamic-based story force distribution (DSFD) load pattern is constructed using a linear dynamic analysis, either time history (THA) or response spectrum (RSA). Performing an initial pushover analysis with the DSFD load pattern, a nonlinearity modification factor (NMF) is calculated to modify the DSFD load pattern. The envelope of the peak responses of the structure obtained from 2 pushover analyses with the modified DSFD load pattern as well as the code suggested first mode load pattern are considered... 

In this paper, a size-dependent formulation is presented for Timoshenko beams made of a functionally graded material (FGM). The formulation is developed on the basis of the modified couple stress theory. The modified couple stress theory is a non-classic continuum theory capable to capture the small-scale size effects in the mechanical behavior of structures. The beam properties are assumed to vary through the thickness of the beam. The governing differential equations of motion are derived for the proposed modified couple-stress FG Timoshenko beam. The generally valid closed-form analytic expressions are obtained for the static response parameters. As case studies, the static and free... 

To perform a long lasting, crack-free repair welding on ultrahigh strength steels, the filler metal must be chosen and applied properly. Avoiding several short-term repairs or replacements, the repaired weldment should reveal comparative characteristics such as wear resistance, toughness and hardness to base metal. In the present study, a novel functionally graded material have been introduced to obtain enhanced wear resistance and hardness at surface as well as improved fracture toughness at fusion line of repaired weldments. A comparative study of wear resistance of repaired weld metals has been carried out by pin-on-disk apparatus at 5N normal load and 0.14ms-1 sliding speed. Fracture... 

Usage of concrete-filled pultruded glass fiber-reinforced polymer (GFRP) tubes (CFPGT) as columns can increase the service life of structures. However, marine structures such as oil platforms are always prone to fire because of the low resistance to the elevated temperatures. The purpose of this investigation is to evaluate the effects of concrete core strength (30 and 60 MPa), and exposure temperature (25, 100, 200, 300, and 400 °C) and time (60 and 120 min) on the compressive and bond behavior of CFPGTs. The properties of unexposed and exposed concrete core, pultruded GFRP hollow tubes, and CFPGTs were determined via compressive and disk-split tests. Also, the push-out test was used to...