Sharif Digital Repository

Control of two-arm flexible robot is studied in this research. The goal is to control the robot in order to carry a fluid tank installed in the top of the last arm, while the vibration of fluid must be damped. The equations governing the system are extracted by the energy method while Euler-Bernoulli beam is selected to model the elastic members. In order to develop the equations, all of the potential and kinematic energies due to rigid and elastic motions are calculated, then a proper controller is designed for the system to accomplish the considered missions. Since the system is classified as an under-actuated system, the exact feedback linearization method is not practical to control the... 

In this thesis, buckling of truncated conical shells made of composite laminates with general lamination sequence is investigated. The conical shell is considered under axial compression with simply supported or clamped boundary condition. First, Donnel type nonlinear equations and boundary conditions for doubly curved shells are obtained using Hamilton principle. Second, using adjacent equilibrium criterion, the nonlinear equations was linearized then with defining lame parameters and curvature radius, the linear equations of conical shell was extracted. The power series and Galerkin method was used to solve differential equations. The obtained results are in good agreement with available... 

In this study, static and dynamic buckling of laminated composite beams resting on an elastic foundation under thermal and mechanical load is studied. Beam is resting on an elastic foundation with hardening/softening term. Nonlinear governing equations are obtained based on the energy method and are solved via the multi-term Galerkin method and the Newton-Raphson numerical method. Critical dynamic load is estimated by the Hoff Simitses criterion. The results are validated with the results of available articles in this field. In the following, the effects of different parameters of the problem on the results are examined. Results reveal that for a sufficiently stiff softening elastic... 

In this thesis a nonlinear thermomechanical model based on Lagrangian-extended finite element method is proposed to simulate discontinuities under thermal and mechanical loads. At first, a geometrically nonlinear model is presented for large deformation problems and then the model is completed by considering thermomechanical aspects. In extended finite element method, different enrichment functions are stated and also using analytical methods, a new set of functions are introduced to enrich the temperature filed around biomaterial crack tips. For numerical simulations, an object oriented code is created in C++. The results of numerical simulations are verified by the use of ABAQUS  

In this investigation, the behavior of pin loaded composite plates is studied numerically. A failure prediction model is used to predict the ultimate strength considering three different mechanisms of failure: bearing, shearout and net tension. The model consists of two major parts: stress analysis and failure analysis. The main difference between this investigation and others is considering more realistic assumptions in modeling of the problem. As an example, we consider nonlinear behavior in the shear stress/ shear strain relationship of each unidirectional ply and use this model in optimization problem to reach maximum strength. The results are compared with experimental results available... 

The main purpose of this dissertation is to study thermoelastic behaviors in rotating disks subjected to thermal shock loads based on the generalized and classic theories of coupled thermoelasticity. To this end, this research has been carried out in two stages. In the first stage, thermoelasticity problems in an axisymmetric rotating disk with constant thickness made of a homogeneous isotropic material are analytically solved. In this stage, based on the classical and generalized coupled theories, and dynamic and quasi-static uncoupled theories, an analytical method based on the Fourier-Bessel transform is employed to obtain the thermoelastic solutions. Then, closed-form formulations are... 

The presence of spatial partial derivatives in the equations of classical continuum mechanics has led to the fact that methods based on this theory are not valid in displacement discontinuities (such as cracks). The peridynamic theory is a nonlocal formulation of solid mechanics which is most suited to model discontinuities and dynamic fractures in continuous or discrete media. By substituting integral expressions instead of partial differentials in the equations of motion, peridynamics provides an integrated model that is valid and the same in continuous, discontinuous, and discrete media.The capability of the peridynamic theory in modeling discontinuities and cracks has been demonstrated... 

The main purpose of this study is the analysis of low velocity impact on the light weight sandwich beams. Some parameters such as weight, failure mode, and impact energy absorption capability are very essential in the design of sandwich beams. The failure mode map is a technique to design sandwich structures, in which no single component is over-designed with respect to other components. In this study, the failure mode map of foam core sandwich beams composed of E-glass/Epoxy and PVC foam is drawn and investigated by an analytical method. For this purpose, displacement relations for bending of a sandwich beam are written using higher order theory. In addition, the sandwich beam is modeled... 

The object of this study is to determine the buckling load of reinforced composite conical shells under axial compression. . Shells are reinforced by stringers and rings and the boundary conditions are assumed to be simply supported. At first the equilibrium equations are obtained using the first order shear deformation theory (FSDT), smeared stiffener technique and principle of minimum potential energy. In the following, the resulting equations which are the system of five variable coefficient partial differential equations in terms of displacement components are investigated by generalized differential quadrature method (GDQM). Finally the standard eigenvalue equation is formed and the... 

In this dissertation, the buckling and postbuckling analysis of lattice conical shells are presented. The outer skins of the shell may be reinforced by cross-ply laminated composites. The shell is subjected to uniform axial compression and the boundary conditions are simply supported. This problem is solved using two different approaches. At first approach, the main priority is high speed calculations, which is investigated in static and dynamic types of loadings. In the case of static loads, the equilibrium equations are formulated based on the classical theory and von Karman type of nonlinearity. The equations are solved by Galerkin method and a closed-form relation is derived. In the case... 

The thesis addresses the buckling and post-buckling analysis of an elastic conical shell under non-uniform axial loading using finite element method. Non-uniform axial load is applied to one end of the truncated cone in the form of two equal-length loaded zones, diametrically opposite to each other. Material used in this thesis is isotropic with linear elasticity. The results are presented for different boundary condition. The results show that the buckling strength obtained by nonlinear analysis may be upto 40% less than the buckling strength obtained by linear analysis. The effects of boundary conditions and geometry on the results has been investigated  

The aim of this research is to analyze the failure and investigate crack growth in graphene using peridynamic theory. The presence of spatial partial derivatives in the equations of classical continuum mechanics has led to the fact that methods based on this theory are not valid in displacement discontinuities such as cracks. Peridynamic theory emerges as a nonlocal reformulation of mechanics, uniquely well-suited for modeling discontinuities and dynamic fractures in both continuous and discrete media. Its adaptability extends to various dimensions, encompassing phenomena at the nanoscale. In the present study, based on the ordinary state-based peridynamic theory, we investigated the... 

In this thesis, turbine disk thermo elastic and creep stresses has been extracted and the turbine disk cross-section is optimized. Gas turbine disks are used at high temperature and high rotational speed. Due to high temperatures gradients, the mechanical properties vary along the radius of the disk. Most of the stresses in the gas turbine are exerted by centrifugal force and thermal strains. At first, thermo elastic and creep equations for these disks are derived, assuming plane stress and axisymmetry. The resulting differential equations are nonlinear and can not be solved using analytical solutions. Two methods are used to solve the equations which finally are verified employing the... 

With the recent trend to use thin shell structures in severe operational conditions, it is not sufficient to employ the classical linear theory to analyze their dynamic behavior, especially with large amplitudes. When the transverse deflection of a shell raise to the order of its thickness, the nonlinear effects grow significantly, leading to a variety of complex responses, such as the variable frequencies depending on the amplitude and the jump phenomenon. This dynamic behavior should be analyzed by the nonlinear theory of shells. In this research, the nonlinear vibration of composite shallow circular cylindrical shells is considered. The geometric nonlinear strains are of the von Karman... 

The present study investigates the effect of MR materials on the vibrations of three-layer pretwisted rotating beams. For this purpose, by considering MR material as the core of the rotating beam and using Hamilton's principle, the equations governing the three-layer rotating beam are extracted. After solving the desired equation, we optimize the dynamic behavior of the three-layer beam at different rotational speeds. Using the Genetic Algorithm optimization tool, the desired beam weight will be minimized and maximum damping will be applied to the desired system. The goal is to find the optimum magnetic field strength for maximum damping in the system at any rotational speed, in addition to... 

Cylindrical shells are widely used in industries. For example, they are used as type 3 pressure vessel's liner. In this work, linear buckling of bead stiffened cylindrical shells under axial load or lateral pressure has been investigated by using numerical modeling. Bead form of cylindrical shell in longitudinal and circumferential direction and combination of both direction is considered. Different parameters such as number, dimension, direction (inside or outside) and cross-section's area of beaded form are considered. The obtained results show that creation of longitudinal beads decrease and increase the critical load and pressure compared to the simple cylindrical shell respectively. The... 

In the present study, buckling and free vibration of two joined conical shells made from isotropic and generally laminated composites are presented. The joined conical shells can be considered as the general case that can be used in analysis of single cylindrical and conical shells, joined cylindrical-conical shells, joined cylinder-plates or cone-plates, cylindrical and conical shells with stepped thicknesses, annular plates, laminates with ply drop-off or any case that the stiffness of the laminate changes in the shell. Governing equations are obtained using thin-walled shallow shell theory of Donnell type and Hamilton’s principle. The joining of shells is exerted using various methods and... 

In the present study, the buckling of conical sandwich panel with composite faces and flexible core is investigated. At first, the nonlinear differential equations based on Donnell and Novozhilov theories with relevant boundary conditions for conical shells are derived using energy method and Hamilton principle. In the following, by applying adjacent-equilibrium criterion, the equations are linearized and the equations of conical shell are obtained. Then, the results obtained from these theories are compared, and based on the benefits and application domain of the theories, Novozhilov theory is selected to model composite faces of the sandwich panel. High-order sandwich theory is used for... 

In this research, the dynamic behavior of a rotating stiffened cylindrical shell has been investigated using the refined finite element method. Refined FEM means decreasing degrees of freedom based on decoupling the structure to longitudinal and layerwise directions. The refined FEM has been used to solve the 3D rotor dynamics equations. The natural frequencies and the corresponding modes have been calculated and verified for some existed data and problems. In addition, the effects of the shell geometry dimensions on the natural frequencies and mode shapes have been studied. Also, the relationship between the length to radius ratio, radius to thickness ratio, rotational speed, Aspect ratio... 

In this thesis the dynamic behavior of the thick-walled shell with variable thickness is investigated. Such structures are used in a variety of industries including aerospace, oil, gas and petro-chemistry. In this study, the dynamic behavior of a rotating truncated conical thick-shell with various geometries is investigated. For this purpose, modal analysis is performed using the refined finite element method. Then,the natural frequencies and corresponding mode shapes are calculated for several different geometries. Also, the relationship between geometrical variables such as thickness, the ratio of length to radius, ratio of cross-sectional radius and so on, with the natural frequency of...