Sharif Digital Repository

In this research, a new method for producing bimetallic rods is introduced. The method is a new application of equal channel angular extrusion (ECAE) process. Advantages of using this method can be expressed as; good dimensional control on both the core and clad layer, possibility of performing the process in more than one pass which can result in larger bonding strength, and the possibility of performing the process in different temperatures which is critical for producing most of the bimetallic joints as well as aluminum-copper joint. The shear strength obtained using this method is compared with the same final product produced by general extrusion process. While the shear strength after... 

The present study is concerned with introducing a new configuration of equal channel angular extrusion (ECAE) process nominated as multi-stage ECAE. This die configuration is not a real processing die and is just used to calculate strain in ECAE dies with outer curved corner. A solution is presented for evaluating total strain in those dies consisting of two or more ECAE sub-dies. The solution is extended for calculating strain in frictionless ECAE dies with outer curved corner. A complete agreement is found between calculated and previously published results by other authors. © 2006 Elsevier B.V. All rights reserved  

In this research, deformation of material in equal channel angular extrusion (ECAE) process with outer curved corner is analyzed using an upper bound solution. The effects of die angle, friction between the sample and the die walls, and the angle of the outer curved corner, on the extrusion pressure are all considered in the analysis. It is found that the extrusion pressure decreases with increasing both the die angle and the outer curved corner angle and increases with increasing the friction coefficient. Moreover, a good agreement is found between the predicted and experimental results of extrusion pressure relating to two dies of different outer curved corner angles used in ECAE tests of... 

In this research, deformation of material during equal channel angular extrusion (ECAE) is analyzed using an upper bound model. The model considers the effect of die angle and friction between the sample and the die walls on the geometry of dead metal zone and total strain. The relationship between the friction coefficient, the extent of dead metal zone and total strain is derived. It is found that strain decreases with increasing die angle and friction coefficient. Moreover, a critical value for friction coefficient is determined that when the friction coefficient is more than the critical, deformation zone and therefore, the dead metal zone forms which consequently total strain decreases.... 

In this paper, an Al-Mg-Si-Cu aluminum alloy was exposed to various heat treatment processes and the effect of precipitation hardening on hardness was investigated as a function of time and temperature. It was concluded that the variation in time and temperature can improve the hardness of the alloy. The work revealed that time and temperature play an important role in the precipitation hardening process of the alloy. To achieve the maximum hardness, an optimum aging cycle should be used which is heating at 180-190 °C for 15-18 h. Kinetics studies show an apparent activation energy of 58 kJ/mol. A relationship was derived which can be used to estimate the hardness at a given time and... 

In this paper, with introducing and based on a new configuration of equal channel angular extrusion (ECAE) process, nominated as multi-stage equal channel angular extrusion, two relationships are derived to calculate the effective strain based on shear and principal strains. First, a solution is presented for evaluating the effective strain in multi-stage ECAE dies consisting of two or more ECAE sub-dies. Then, the solution is extended to calculate the effective strains in ECAE dies with outer curved corner. The effective strain based on shear strains shows an exact agreement with previously published results. The derived relationships were compared with each other, with the relationships... 

The most applicable configuration of the equal channel angular extrusion (ECAE) dies is the axi-symmetric one. However, most of the previous analytical solutions are focused on the plane strain conditions. In this research, an upper bound model is used to investigate the deformation of the material during axi-symmetric ECAE. The analysis considers the effect of die angle, friction between the sample and the die walls, and the angle of the outer curved corner of the die, on the extrusion pressure. It is found that increasing the die angle and outer curved corner angle and decreasing the friction coefficient results in decreasing extrusion pressure. The proposed model is verified using two... 

As computer simulation increasingly supports engineering design, the requirement for a computer software environment providing an integration platform for computational engineering software increases. A key component of an integrated environment is the use of computational engineering to assist and support solutions for complex design. In the present paper, an integrated software environment is demonstrated for multi-disciplinary computational modeling of structural and geotechnical problems. The SUT-DAM is designed in both popularity and functionality with the development of user-friendly pre- and post-processing software. Pre-processing software is used to create the model, generate an... 

In this paper, the influence of powder-tool friction on the mechanical properties of the final product is investigated in pressing metal powders. A computational algorithm is presented for simulation of frictional contact in the compaction process of powder. The large deformation finite element (FE) formulation is characterized by the use of penalty approach in which a plasticity theory of friction is incorporated to simulate sliding resistance at the powder-tool interface. The constitutive relations for friction are derived from a Coulomb friction law. A double-surface cap plasticity model is employed together with the nonlinear contact friction behavior in numerical simulation of powder... 

In this paper, an atomistic–continuum homogenization multiscale method is developed to study the nonlinear behavior of heterogeneous nanomaterials. The atomistic representative volume element (RVE) with vacancy and/or void defects are analyzed by employing the fully atomistic method, in which the nucleation, migration, and elimination of dislocation, as well as the dislocation-vacancy interaction, are captured. The coarse-scale material domain is modeled within the framework of the nonlinear finite element method, and the impression of nanoscale material defects is investigated by upscaling the stress tensor and tangent modulus from the atomistic RVE based on the Hill-Mandel principle. The... 

In this paper, a new criterion for passivity of haptic devices is obtained. This criterion creates a relationship between Coulomb friction coefficient, viscous friction coefficient, sampling rate, and the maximum simulated stiffness. The process of derivation of the passivity criterion is described in detail. This criterion is improved compared with other existing criteria and predicts passivity in haptic rendering more accurately. In particular, for speeds of less than 5 cm/s, the new passivity criterion should replace the previous criteria to avoid unwanted vibrations of stiff virtual walls. Analytical and numerical investigations are presented to validate the new criterion. A specific... 

The field emission properties of MoO2 nanostars grown on a silicon substrate and their emission performance in various vacuum gaps are reported in this article. A new structure of molybdenum oxides, named a nanostar, is grown by thermal vapor deposition with a length of ̃1 μm, a thickness of ̃50 nm, and its width in the range of 500-700 nm. The morphology, structure, composition, and chemical states of the prepared nanostars were characterized by scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). According to XRD analysis, the grown nanostructures are composed of both crystalline Mo4O11 and... 

Several exoskeletons with various configurations and degrees-of-freedoms (DOFs) have been introduced in the literature, but not many have addressed the optimal selection of the mechanism's DOFs. In the proposed system a semi-passive toe joint is introduced to achieve several advantages. The performance of the proposed novel passive toe joint is evaluated experimentally. In addition, the semi-passive abduction/adduction DOFs in the hip joint and eversion/inversion DOFs in ankle joint is proposed to facilitate turning while providing more comfort during normal walking. The effect of passive DoFs existence on the user comfortability is studied with the aid of three kinematic indices. In the... 

This paper models the dielectric corona pre-ionization, capacitor transfer type of flat-plane transmission line traveling wave transverse excited atmospheric pressure nitrogen laser by a non-linear lumped RLC electric circuit. The flat-plane transmission line and the pre-ionizer dielectric are modeled by a lumped linear RLC and time-dependent non-linear RC circuit, respectively. The main discharge region is considered as a time-dependent non-linear RLC circuit where its resistance value is also depends on the radiated pre-ionization ultra violet (UV) intensity. The UV radiation is radiated by the resistance due to the surface plasma on the pre-ionizer dielectric. The theoretical predictions... 

The reliability Studies are done in order to determine the probability of functioning or availability of different engineering systems. These studies can also provide a basis for comparing different structures of a system and can be used as a tool to improve the system design. Also, reliability is an important consideration in the planning, design and operation of transit system. Since the issue of the reliability of electric vehicles has not been sufficiently and seriously evaluated yet, so in this paper, the reliability of the various types of these kinds of vehicles will be discussed and then the reliability indices of them will be compared with each other and with the conventional cars  

Considerable interest in the indoor applications of wireless local-area networks (WLANs) demands some powerful simulation methods of these systems. Proper modeling of the physical propagation channel is crucial in this context. A novel simulation method for indoor wideband multi-input multi-output (MIMO) channel at the operating frequency band of IEEE 802.11a, HIPERLAN/2, and the emerging IEEE 802.11n standards is presented in this paper which is suitable for performance analysis of MIMO techniques in these systems. The presented model is developed on the basis of a previously proposed channel representation at the same frequency band. Simulation results for MIMO channel capacity are... 

To synthesize a novel biopolymer-based superabsorbent hydrogel, 2-acrylamido-2-methylpropanesulfonic acid (AMPS) was grafted onto kappa-carrageenan (KC) backbones. The graft copolymerization reaction was carried out in a homogeneous medium and in the presence of ammonium persulfate (APS) as an initiator, N,N,N',N'-tetramethyl ethylenediamine (TMEDA) as an accelerator, and N,N'-methylene bisacrylamide (MBA) as a crosslinker. A proposed mechanism for KC-g-AMPS formation was suggested and the hydrogel structure was confirmed using FTIR spectroscopy. The affecting variables on swelling capacity, i.e., the initiator, the crosslinker, and the monomer concentration, as well as reaction temperature,... 

In this work, a comparative study of hybrid solar cells based on P3HT and TiO2 nanofibers was accomplished. Electrospinning, a low cost production method for large area nanofibrous films, was employed to fabricate the organic-inorganic hybrid solar cells based on poly (3-hexylthiophene) and TiO2 nanofibers. The performance of the hybrid solar cells was analyzed for four density levels of TiO2 nanofibers which resulted in the average power conversion efficiency of about 0.0134% under AM 1.5 simulated illuminations (100 mWcm-2). It is found that the higher densities of TiO2 lead to more interface area and generating exciton, so the power conversion efficiency will be increased till the active... 

Forming limit diagram is often used as a criterion to predict necking initiation in sheet metal forming processes. In this study, the forming limit diagram was obtained through the inclusion of the Marciniak–Kaczynski model in the Nakazima out-of-plane test finite element model and also a flat model. The effect of bending on the forming limit diagram was investigated numerically and experimentally. Data required for this simulation were determined through a simple tension test in three directions. After comparing the results of the flat and Nakazima finite element models with the experimental results, the forming limit diagram computed by the Nakazima finite element model was more convenient... 

Radial forging and indentation forging processes are two processes used for sizing and/or internal profiling of tubes. In this paper, the two processes are simulated using finite element method and their results are compared with Experimental results. The purpose of this paper is to compare the pattern of residual stresses in both processes. Copyright © 2005 SAE International