Sharif Digital Repository

The modeling of mechanical behavior of Reinforced concrete Structures has been an important issue in civil engineering. In this thesis, an approach to the numerical modeling of different parts of these structures such as concrete and steel bars was presented. The model is based on damage and plasticity in porous materials. The behavior of steel materials was modeled through Von-Misses plasticity model while the coupling of the Gurson plasticity and a rate dependent damage model based on elastic strains were used for modeling the behavior of concrete materials. The effect of interaction between concrete and steel bar in RC members was considered through an experimental relation in which the... 

In this thesis, we aim to study the elastic-plastic behavior of short fiber reinforced composites experimentally and analytically. The experimental investigation is applied to glass fiber-polypropylene composites (PP/GF). The samples are produced by injection process with maximum temperature of 190C. Fibers are randomly oriented in this procedure of fabrication. The samples are tested in tension, and the mechanical strength is measured by changes in fiber content. The Young's modulus is also investigated in these experiments.Also in this thesis, an analytical model for calculation of Young’s modulus of fiber reinforced composite materials is proposed. In this analysis a displacement field is... 

This project presents an outline of the crack tip stress fields in the presence of a plastic enclave around a growing fatigue crack using RKPM and Drucker-Prager criterion for pressure sensitive materials such as concrete and rock. Two distinguished features of RKPM are the arbitrarily high order smoothness and the interpolation property of the shape function which has kronecker delta property at the associated nodes. This method has proved to handle extreme material deformation and moving discontinuities. For the frictional materials such as rock in geotechnical engineering, a non-associated or associated Drucker- Prager plasticity model is appropriate for modeling its constitutive... 

Recent research studies on ductile fracture of metals have shown that the ductile fracture initiation is significantly affected by the stress state. In this study, the effects of the stress invariants as well as the effect of the reverse loading on ductile fracture are considered. To estimate the reduction of load carrying capacity and ductile fracture initiation, a scalar damage expression is proposed. This scalar damage is a function of the accumulated plastic strain, the first stress invariant and the Lode angle. To incorporate the effect of the reverse loading, the accumulated plastic strain is divided into the tension and compression components and each component has a different weight... 

Most engineering components must be manufactured so they are safe to use and are ‘‘fit for purpose’’. However despite all the efforts, cracking of the engineering components is inevitable. Residual stresses play an important role in either increasing or decreasing the possibility of failure. For instance, enhancement in apparent toughness following pre-loading arises principally because of the creation of local crack tip compressive residual stresses. Introducing compressive residual stresses can also slow the fatigue crack growth. Yet, the dilemma is choosing an efficient method growth to introduce such stresses for retardation of fatigue crack growth. There are many methods available in... 

In this thesis an analytical model is applied to the dynamic response of fiber-metal laminate beams subjected to low-velocity impact. By using quasi-static consider-tions, a static model is developed using first order shear deformation beam theory and minimum total potential energy. In the next step, this static model is applied to the one degree of freedom non-linear spring-mass system for developing a model to predict dynamic response of low-velocity impact with the inclusion of material plasticity. By new explicit and simple expression, load-deflection curve, velocity-deflection curve, force-time history and deflection-time history can be derived. By using this analytical approach... 

The use of metal matrix composite materials (MMC) in aerospace structures and engine parts has become more frequent recently. These materials are composed of metal matrices reinforced by hard discontinuous ceramic fibers, usually oriented longitudinally within the matrix, and due to the stability of the mechanical properties, including the hardness and the strength at high working temperatures, have found various applications.Stress transfer from the matrix to the fiber is known as the most important mechanism governing the deformation and the fracture response of such MMCs. The stress transfer characteristic of fiber reinforced composite materials under various mechanical and thermal... 

A finite element model for seismic analysis of concrete arch dams is proposed. Material inelasticity as well as joints nonlinearity is considered. In order to provide a precise description of concrete response, a novel plastic-damage constitutive model for plain concrete is developed. For this purpose, the microplane theory is proposed for overcoming the deficiency of available anisotropic continuum plastic-damage models in reproducing the true anisotropic nature of damage in multidimensional loadings.Based on the microplane theory, the degeneration process in concrete is considered along with plastic deformations. Using the principle of strain energy equivalence, a transformation between... 

In present research forming process of nanopowders, which is a part of powder metallurgy was investigated by molecular dynamics method. Powder metallurgy is a relatively new method for production of industrial parts by pouring powder into die and compaction to desired density. One can reach parts with higher quality and strength by decreasing size of powder’s particles and entering the nano scale. Particle with smaller size have higher specific surface and due more intensity to react. Classic methods for investigation of this process don’t cover the atomic scale effects, so using newer procedures such as molecular dynamics is highly recommended. In present research, at first compaction of... 

Concrete has long been considered by engineers as one of the most used constructional materials. Therefore, recognition of mechanical behavior of concrete materials is of particular importance. So, extensive studies have been undertaken to understand and analyze its behavior under different loading conditions. Based on these studies, many models for describing the behavior of concrete are presented. Of these, many models are based on the combination of plastic theory and damage mechanics, and by using them, significant success has been achieved in the correct modeling of concrete materials. On the other hand, one of the important loading conditions encountered by concrete structures is in... 

The present research focuses on the simulation of the metallic powder compaction process with the Peridynamics theory. Various methods are exploited to simulate this process in literature. Studying the nonlinear behavior of powders includes different phenomena such as dislocation and grain boundary, making it complicated. However, numerous research has been shaped to consider these phenomena on the micro-scale. There is also another batch of nano-scale studies underway. In this class of simulations, considering atoms as rigid particles, interatomic potentials, and molecular dynamics methods are used. Because of atomic-scale precision, this approach has very high accuracy. The massive... 

The primary purpose of this study is to perform nonlinear behavior simulations in the compaction and forming of metal powders and their alloys and investigate their behavior in the plastic state. Powder compression is one of the methods of producing metal parts with high precision and improved mechanical properties, which is why it has been considered in addition to its need in the industry.There are various methods for performing these simulations, including multi-scale methods with optimal performance. Given that only the use of high-scale methods or only low-scale methods will have problems and disadvantages in such simulations, multi-scale methods will be a good alternative in which both... 

Currently, one of the most important problems in the field of drilling is the issue of drilling fluid loss in different formations. The problem of mud loss has been evident since the beginning of oil and gas well drilling operations, so that oil companies spend millions of dollars annually to solve the problem of mud loss. Problems such as pipe jamming, loss of rig production time, well blowout, loss of a large volume of drilling mud and damage to reservoir formations due to loss of drilling fluid are caused. Regarding the drilling variables, many factors cause the decrease or increase of the loss, the most important of which are the weight on the drill bit, drilling speed, the rate of... 

The proposition of the critical nature of brain activities has been discussed for approximately two decades. This notion has been empirically observed through measurements of neuronal ensembles on brain slices of mice. Subsequently, numerous models have been introduced to rationalize and interpret this brain property. Additionally, it has been demonstrated that the neural system exhibits optimal performance at the critical point, both in computational and memory aspects. Therefore, the hypothesis of the critical brain is supported both empirically and theoretically. Typically, critical behavior occurs at phase transition points that require adjustment of external parameters such as...