Sharif Digital Repository

In this study, a novel multi-scale hierarchical method has been employed to explore the role of edge dislocation on Nano-plates with hexagonal atomic structure in large deformation. multiscale hierarchical atomistic/molecular dynamics (MD) finite element (FE) coupling methods are proposed to demonstrate the impact of dislocation on mechanical properties of Magnesium in large deformation. The atomic nonlinear elastic parameters are attained via computing first-order derivation of stress with respect to strain of Representative Volume Element (RVE). To associate between atomistic and continuum level, the mechanical characteristics are captured in the atomistic scale and transferred to the... 

In this study, hot extrusion of AA6061-%5 SiCp composite has been predicted and flow behavior of material during deformation process has been investigated. At the first step, hot compression tests have been carried out to obtain the flow behavior of the material at different temperatures and strain rates. In the next step, an FE-based temperature-displacement coupled model by employing ABAQUS software has been proposed in order to solve the energy and heat transfer equations and to determine the required energy and force for the extrusion process. Experimental investigations have also been performed to verify the proposed model. Comparison between the results from the model and the... 

Processing an alloy can enhance its mechanical properties. Based on repeated radiation of high-power pulsed laser beam, laser shock peening (LSP) is one way to impart favorable residual stress in an alloy and to enhance its damage tolerance property. Study of overlapping LSP is possible through the simulation of limited laser shots. Still, its computational cost is higher than that of a single shot. This research, primarily, is aimed at improving a methodology for reduction of computations in the study of an overlapping LSP and its impacts on fatigue crack growth. Addressing research gaps by utilizing the methodology is the secondary goal. The methodology consists of plastic strain... 

In recent decades, the study of Nano and Micro Electromechanical Systems (NEMS and MEMS) as in micro sensors and actuators or in Atomic Force Microscopes (AFM) attracted considerable attention. Since an indispensable part of such systems is a nano or micro flexible beam, the inevitable undesirable vibrations of the flexible beam necessitate the implementation of vibration control strategies. Recently, some cutting edge experiments on statics and dynamics behavior of micro beams have been conducted that reveal some deviations from the prediction of classical theories, and hence this leads to development of strain gradient elasticity theory. In addition, the often-adopted finite dimensional... 

The purpose of this study is to apply the governing equations of the theory of porous media with the concept of volume fractions to calculate the isotropic half-space responses subjected to a surface and asymmetric loading and to compare the responses with Biot’s theory. For this purpose, each of the components is first considered continuous and the equilibrium equations of mass and momentum for the solid and fluid phases based on the mixture theory are expressed. The concept of volume fractions and the effect of interaction between the two phases, distinguish these equations from the equations of the continuum theory. Thus, the governing coupled partial differential equations of the theory... 

The present study aims at determining the elastic fields of ultra-small flaws and defects. These defects are often introduced undesirably in elastic solids during fabrication and their sizes are normally in the order of couple of nano-meters. In this work, the elastic fields around a circular nano-void subjected to a uniform farfield uniaxial tension, also the elastic fields of a nano-sized mode I crack under remote uniform loading are studied. In this paper the strain gradient theory developed by Mindlin and co-workers in 1960s is employed. According to this theory, the strain energy density assumes the form of a positive-definite function of the strain components and their first gradient.... 

The present study aims to evaluate the effect of the shear-lag on non-rectangular RC shear walls and develop equations to determine the axial stress and strain distributions and calculate the effective flange width. Research has shown that a non-rectangular shear wall under a lateral load experiences the largest axial stress and strain in the flange-web cojunction. This phenomenon is referred to as the shear-lag effect and reduces the bending capacity of the shear wall. As a result, the effective flange width is typically defined to consider the shear lag effect. The present work first reviewed the literature on the effects of shear lag on non-rectangular RC shear walls. Then, flanged shear... 

Shear modulus and damping ratio are the most important parameters for evaluating and designing the performance of geotechnical systems under dynamic loads. According to the definition, the secant shear modulus is the slope of the hysteresis shear stress-shear strain loop. Also, the damping ratio indicates the dissipation of wave energy during propagation in the soil. This research presents an experimental laboratory study on the effect of non-plastic fines content and hydraulic hysteresis on the Gmax of unsaturated sand-silt mixtures. A significant part of the previous research has been done to determine Shear modulus and damping ratio parameters. It has been done in completely saturated or... 

Austenite in the 300 stainless steels is not a table phase in deformation processing during which martensite phase forms. This causes significant changes in the microstructures as well as the mechanical properties of deformed steel. In this work, the effects of rolling speed, reduction, temperature and strain pass on the strain-induced martensite during strip rolling operation of stainless steel 304L are studied. Cold rolling experiments under different speeds, reduction and temperature are conducted and then, the microstructural events and mechanical properties of the rolled steel are evaluated. For doing so, different testing techniques including tensile test, hardness measurement, optical... 

The purpose of this research is to study the behavior of conventional and retrofitted Khorjini connections. Thus the software ABAQUS 6.11 was used to fabricate the FEM models of the connections with experimental prototypes. In analytical studies, 21 specimens with different size of the connection components were analyzed. True stress–strain curves for steel and weld materials were used and damage initiation and revolution were considered in the weld material. Also cracking of the welds were considered by a micromechanical method. Behavioral characteristics of the conventional connections are specified and appropriate relations are suggested, using Stability equations of the connection... 

The purpose of this research is to study the behavior of some kind of conventional and retrofitted Khorjini connections, such as stress and strain concentration and eventually strain-jumping phenomenon. Thus the software ABAQUS 2019 was used to fabricate the FEM models of the connections with experimental prototypes. In analytical studies, 21 specimens with different sizes of the connection components were analyzed. True stress-strain curves for steel and weld materials were used and damage initiation and revolution were considered in the weld material. Also cracking of the welds was considered by a micromechanical method. The main defects of the connection are severe initial stiffness... 

In this research, non-isothermal annealing process at temperatures ranging between 400°C to 830 °C was investigated in warm and cold-rolled austenitic stainless steel 304L. First, the wide variety of austenitic to martensitic structure was produced by the multi-pass rolling process at room temperature -8 °C, 200 °C, and 280°C. The n exponent in Olson-Cohen equation was governed 2.83 after non-linear curve fitting to assess the fraction of deformation-induced martensite as a function of strain. Afterward, Abaqus/Explicit method was utilized to achieve the stored energy and plastic strains distribution through the thickness. Then, Cellular automata and Galerkin-finite element models were... 

Abstract- Due to vulnerability of metallic structures to fire, to have an advance estimate of the behaviour and performance of the structure during fire, can greatly assist the designers to make preparations and take precautionary measures to prevent excessive damage to the building. The study of the behaviour of a structure at high temperatures can be divided into two parts, that of its members, and that of its joints. The survey of the literature reveals that most of the previous work concentrated on the former, and it seems that there has not been much work on the latter. The objective of this project was to study the behaviour of structural joints at elevated temperatures, in general,... 

In some of the austenitic stainless steels, the strain-induced martensite transformation occurs during the deformation processing owing to instability of austenite phase at low temperatures. In this study, formation of strain-induced martensite as well as the impact of strain path on the rate of martensite transformation in 316L austenitic stainless steel were investigated. At first, the multi-pass cold rolling experiments at room temperature and -8 ℃ were conducted in which both direct and reverse rolling layouts were taken into account. Then, as the third path, the uni-axial tensile loading with an appropriate strain rate was considered. After that, in order to examine the mechanical and... 

The stiffness rate of the 300 series austenitic stainless steel has long been known to depend on the stability of the austenite and its susceptibility to martensite formation. In this project, the cold deformation of austenitic stainless steel (304L) and the occurrence of martensite deformation during it are studied. Experimental cold rolling experiments under different strain velocities and initial temperatures in the range of -10° C to 25° C; Performed and microstructure, mechanical properties and percentage of martensite generated in rolled steel are determined and evaluated employing different testing techniques. mathematical modeling of cold rolling operation is also performed using... 

In many of the geotechnical engineering projects, such as construction of embankments and roads, soil is compacted with different water contents. Difference in compaction water content leads to different dynamic behaviors under cyclic loadings like earthquake and traffic. On the other hand, the magnitude and the direction of principal stresses on soil element, affect the dynamic response of the soil. The main objective of this research is to study the effects of compaction water content on dynamic properties of soil in anisotropic loading conditions. Therefore, the dynamic parameters of Babolsar sand, including shear strain amplitude, shear modulus and damping ratio under induced anisotropic... 

In this thesis the elastic energy of plates is computed. Primarily, some physical related concepts such as stress, strain and strain energy density are defined. Then based on rigidity estimate theorem, the gradient map on bounded Lipschitz domains is approximated by a constant rotation map R.Thanks to this approximation theorem,we prove the precompactness of sequences of finite bending energy, and the limiting energy of two dimensional thin plates is shown to be an elastic energy. This limit innovation is called Γ –convergence introduced in 1993. It will indeed be revealed that the induced elastic energy of a deformable body is the limit of the sequence of stored energy of body under... 

Shear band is more relevant in porous medias rather than continuum media. Shear band is a narrow region of solid experiencing intense shearing. After initiation of shear band, the sliding on two sides of the band occurs. The area inside the shear band undergoes extreme plastic deformation, while experiences elastic unloading outside of the band. In porous materials, the direction of slip and discontinuity in displacement field is not same, as a result of dilation. In fact dilation requires an increase in the volume of the shear band zone. The main difficulty in modeling the shear band is that the width of shear band in contrast to the dimension of the media, is too small. Consequently, the... 

Traumatic brain injury (TBI) has long been known as one of the most anonymous reasons for death around the world. A presentation of a model of what happens in the process has been under study for many years; and yet it remains a question due to physiological, geometrical and computational complications. Although the modeling facilities for soft tissue modeling have improved, the precise CT-imaging of human head has revealed novel details of brain, skull and the interface (the meninges), a comprehensive FEM model of which is still being studied. This study aims to present an optimized model of human head after a comprehensive study of the previous models; which includes the brain, skull, and... 

Magnetic Shape Memory Alloys (MSMAs) are a category of active materials which can be excited by magnetic field. These alloys have been used in sensor and actuator applications recently. MSMAs posses special properties such as large magnetic field-induced strains (up to %10) and high actuation frequency (about 1kHz), while ordinary Shape Memory Alloys can’t act in frequencies above 5Hz due to the time involved with heat transformation. In this thesis, MSMAs are modeled by means of incremental modelling which is a useful method in analysis of nonlinear behaviour of materials. Furthermore, stress-strain curve of MSMA is approximated using an analytical expression whose parameters can simply be...