Sharif Digital Repository

To days the heterogeneous material are used extensively in the engineering materials. Optimization ability is a key feature of these materials to reach desired properties. Heterogeneous materials are the materials that make up from the constituents of multiphase materials in lower length scale such as mesoscopic, microscopic or/and Nano scales. So the properties of these materials at each scale are depending on to several characteristics of heterogeneities such as geometry, material and packing. In these materials the effects of heterogeneities at the lower scales are very significant and the constitutive equations are different for each range of scale. The proper selection of this range... 

The aim of this study is numerical simulation of progerasive failure in conical structures that manufactured by tape-winding proceses in various temperatures. For this purpose, flat laminates by tape-winding proceses firstly manufactured using carbon/phenolic prepreg tape. Then tensile, compression, shear and thermal expansion coefficient specimens are cut from these laminates. Tensile and compression test are done on these specemens in fiber and matrix directions in temperature range 23-200℃ and stress-strain and stress-time curves are achavied. Using these curves tensile elastic modulus in fiber and matrix directions and Poisson's ratio are obtained in room temperature. Also tensile and... 

The material presented in thesis uses concepts of finite element and doublet panel method to develop a structural aerodynamic coupled mathematical model for the analysis of morphing wing tip composed of smart materials. Much research is currently being performed within many facets of engineering on the use of smart of intelligent material. Examples of the beneficial characteristics of smart materials might include altering a structure's mechanical properties, controlling its dynamic response and sensing flaw that might progressively become detrimental to the structure. This thesis describes a bio inspired adaptive structure that will be used in morphing an aircraft's wing tip. The actuation... 

In the present work, the beginning and progressive growth of delamination in laminated composites plates has been studied. Interlaminar stresses at free edge of laminate may give rise to edge delamination and result in failure of laminate. So we need to determine the exact interlaminar stresses to predict the failure of laminated composites. A computer code based on layerwise theory that can obtain the exact interlaminar stresses been developed. Comparison between the present study and finite element software result showed the good agreement.A fracture mechanics approach is used to analysis delamination propagation between layers of composites laminate. In this approach damage propagation... 

The two-dimensional composites are benefited from different advantageous; however, they may not be good enough in terms of strength along the thickness, manufacturing speed, cost management and resistance to delamination. To overcome these unpleasant challenges, the three-dimensional composites are defined as having a 3D pattern of yarns which not only includes yarns along the length and width, but also in thickness or vertical direction. One of these common types of composites is 3D-orthogonal woven composites. In this comprehensive research study, the 3D orthogonal woven composite modelling is conducted through a specialized and advanced software, TexGen software. Then Voxel mesh were... 

In this study, through explaining a micromechanical model of progressive damage, failure in a composite conical structure under mechanical and thermal loads with 36 layers of carbon/phenolic material have been studied. To this end, first, mechanical behaviors of composite layers in micro and macro scale and their properties via fiber and matrix characterizations are discussed. In the following, selection of a proper RVE along with different types of boundary conditions on RVE are discussed. Then stress reinforcement coefficients in order to convert the mechanical and thermal stresses from macro to micro scale for carbon / phenolic material are extracted through defining appropriate RVE for... 

Adaptive vibration control of a structure under different condition of exciting forces or structural response is the main scope of this research.Using a combination of the pole placement and online Empirical Mode Decomposition (EMD) methods, a new algorithm is proposed for adaptive active control of structural vibration. For this purpose, by structural response which is evaluated from Hilbert-Huang Transform (HHT) and using prior knowledge for corresponding conditions, proper and optimum control forces are applied to structure. Hence, error sources of EMD method in the HHT such as end effects error, mode mixing problem and decomposition resolution are being studied. A modified method based... 

The substructuring method provides a suitable tool to conduct modal analysis of large and complicated structures. In this method the original structure is decomposed into simpler and smaller substructures, and the dynamic properties of the resulting substructures are first evaluated independently. The original structure’s dynamic properties are subsequently estimated by coupling the substructures’ dynamic results. In this procedure, the boundary conditions need to be imposed in such a way that the overall behaviour of the original structure is properly represented. Substructuring method can be used in different types of dynamic analysis. The focus of this paper is on the modal type of... 

In this research, molecular structural mechanics method is employed to calculate the Young’s modules of a two-layered carbon graphene sheet. For this purpose, covalent bonds are modeled using non-linear beam elements and van der Waals interactions are replaced by nonlinear truss elements. Morse potential and Lennard-Jones potential equations are used to simulate the covalent bonds and van der Waals interactions, respectively. For each atom, van der Waals forces are considered from all other atoms located in its cut-off radius. Young’s modulus, bending modules and Poisson’s ratio of single and two-layered graphene sheets were calculated and the results revealed that Young’s modulus decreases... 

In this study, the thermal buckling behavior of thin metal liner reinforced by composite shell in present of the initial imperfection is investigated. For this purpose, the Ryzener – Myndlyn shear-deformation theory & the virtual work method are used to extract equilibrium equations. In this work, the conical shell with c-c, s-s, c-f boundary conditions has been studied. The outer layer of reinforced composite is exposed to constant ambient temperature and iner layer is exposed to heat. Metal liner and reinforced composite shell are merged with together. In other words, the degrees of freedom at each node for both shells are assumed to be equal. Solution method is finite element method using... 

By developing composites, difficulties of modeling of degradation of properties as stiffness is most considered. The recent advances in the modeling of degradations in stratified composites have led to improved models on all scales. Macro scales methods are good to estimate fracture but not to estimate the failure in micro scale (as microcrack) and moreover, they need many experimental data. Therefore by considering a stratified structure which contains fiber, matrix and intermediate area, mesoscale modeling method is most suitable. In this research Micromechanical Progressive Algorithm (MPA) is used. The method employed is Micromechanic Method of Fracture (MMF). In this work by developing a... 

The J-integral, as a powerful tool in fracture mechanics, is used to analysis of fracture behavior of materials. In order to, evaluate of three dimensional J-integral, an integral evaluation of line and surface is required. However, because surface integral evaluation requires the calculation of the second derivative of displacement field, an commercial finite element codes cannot calculate it.In this thesis, a method for computing 3D J-integral is presented using finite element analysis. In the analysis, the second derivative evaluation of displacement field is employed. In this method, error-minimal points for stress computation are not suitable for second derivative displacement... 

Some types of composite material used in aerospace industry make with braiding technology and Some type of fabric textilled with braiding machine, one of them is 2-D Triaxial braid fabrics.In this thesis, generated new an analytical model for predicted mechanical properties of 2-D Triaxial braid composites. This can prediceted equal elastic modulus and equal shear modulus and equal passion ratio for this composites and this model can predicetd volume fraction of fibers and resin  

The objective of this research is to model the wing structure of a flapping wing MAV and investigate structural dynamic behavior of its wing. in this research, to investigate and identify dynamic instabilities, modeling of the structure based on a nonlinear four-node plane element and aerodynamic modeling based on the strip theory in unsteady flow has been done, and by coupling these two models, the final model is formed. In the model of the wing structure, the displacement and changes angle inside the plane of the wing sections have been ignored and only outside the plane have been assumed for the degrees of freedom. In addition, the veins in the flapping wing have been omitted and the... 

Today, composites are widely used in various industries, thus the study and analysis of properties and their behaviors are very important. In addition, the use of multi-layered composites in the aerospace industry is important. The composite-elastomer structures, specially their spherical shape, are widely used as flexible joint in aerospace industries, such as the thrust vector control system in solid-propellant rockets and the helicopter industry. In this study, an analytical approach is proposed for calculating the interlaminar stresses of laminated plates and spherical shells with arbitrary laminations and boundary conditions based on a threedimensional multi-term extended Kantorovich... 

When vibration of the structure is unrecognized we call it random vibrations which their analysis is very important in space structures because these structures are exposed to acoustic forces or forces produced by turbulence of the atmosphere when a spacecraft is launched. Also in the aviation field, the random vibration analysis use to examine the response of the aircraft and missile structures to atmospheric turbulence and gust. In this research dominated equations of a truncated conical axisymmetric shell structure with linearized approximation of Love’s equations under uniform pressure is utilized. Considering displacements in the form of Frobenius series expansion the frequencies and... 

The purpose of this study, is to investigate and analyze the thermal resistance of phenolic composite insulation reinforced with high silica fibers. The properties of composites depend on the properties of their components .Therefore, by changing the type and percentage of its components, the performance of the insulation changes. Today, these types of composites are used as sacrificial thermal insulation in nozzles, motors, warheads or rocket launchers and rocket wings. In this project, phenolic composite bed flat samples have been made to test the thermal resistance and ablation. In this study, IL800 / 3 phenolic resin, high silica fabrics and 600 g / m2 carbon were used. Highsilica... 

Composite structures are subjected to different kinds of loadings in different workspaces, and sometimes these load conditions can damage them. Delamination is a common damage in composites. In this research, the delamination effect on modal behavior of a laminated composite plate is studied. To investigate this effect, the frequency response method is used and piezoelectric patches are used as a sensor and an actuator for extracting the frequency response. The aim of this project is to investigte presence and severity of the delamination. To survey the frequency response method, an intact and some damaged plates with one sensor and actuator are modeled with abaqus software and frequency... 

This study presents delamination in fiber-reinforced composite laminates by using multiscale modeling. The meso modeling is used to derive the relationship between microcrack density and damage parameters. Next the selected failure model is applied to analyze the macroscale modeling. The progress of failure terms and the reduction of fiber and matrix properties implemented into ABAQUS/Standard, which enables an individual to create a new material behavior through the user subroutine UMAT. In the following, the contours associated with each of the damage parameters are obtained in each of the damage mode. Then for a specific material, the relationship between microcrack density and damage... 

This thesis focuses on how the dimensions and location of piezoelectric patches affect thedynamic behavior of multi layered sandwich panels by the use of finite element method (FEM). To accomplish this objective the finite element formulation of the problem, including coupling of electrical and mechanical characteristic equations for piezoelectric patches is derived. Finite element modeling of sandwich panels is presented on the basis of classic theory and also first order shear displacement hypothesis for multi-layered plates. To perform finite element analysis, ABAQUS is utilized and the development procedure of a finite element model by this software is also introduced. After verifying...