Sharif Digital Repository

Plane strain compression (PSC) test was used to study the flow stress of ultrafine grained commercially pure aluminum at large strains. AA 1050 sheets were processed by various Accumulative Roll-Bonding (ARB) cycles up to 10 cycles as the initial specimens for the test. An approach was developed to measure the coefficient of friction and to suppress its effect on the results. It is shown that as a result of having an anisotropy parameter (R-value) of less than one, Von-Mises tensile strengths are significantly higher than PSC strengths. Comparing these strengths, the R-value as an average anisotropy parameter of rolling and transverse directions is estimated for the ARBed sheets, where it is... 

Abstract Flow behavior of ultrafine grained (UFG) AA 1050 sheets processed by Accumulative Roll-Bonding (ARB) and its viscous nature are investigated by plane strain compression test (PSC) along with stress relaxation. Occurrence of dynamic recovery is validated by TEM observations as the microstructural explanation of the flow softening at the start of deformation of the 8-cycles specimen. Significant recovery of the UFG specimens during the stress relaxation test is also disclosed. It is discussed that neither the internal stress (σi) nor the density of mobile dislocations are constant during the test. The possible effects of these two factors as well as contribution of the... 

An experiment was conducted to determine whether allowing an aluminum melt to solidify in the crucible and later remelting it would improve the quality of the melt and the casting. To investigate this, data were collected through reduced pressure test, density measurement, metallography, X-ray radiography and tensile testing and analyzed statistically. Results indicated that remelting improved the quality of the melt by reduction of the number of defects in the casting and their area and volume fractions. The improvement in elongation of remelted specimens was found to be statistically significant. Analysis of deformation characteristics showed that the improvement in elongation is due to... 

In current studies, equal channel angular pressing process (ECAP) and modified ECAP processes are simulated under ideal conditions to compare the deformation characteristics. The deformation behaviour is more complicated and the strain induced during the processes is highly non-uniform in the modified ECAP processes except in the equal channel multi-angular pressing (ECMAP) process with Route C. The strain homogeneity is more of a possibility with ECAP and ECMAP with Route C processes. The deformation stress state is widely distributed in nature in modified ECAP processes than in conventional ECAP. In addition, the load requirements are also higher in modified ECAP processes with that of the... 

Nanostructured Cu and Cu-2 vol% SiC nanocomposite were produced by high energy mechanical milling and hot pressing technique. Microstructure development during fabrication process was investigated by X-ray diffraction, scanning electron microscope, scanning transmission electron microscope, and electron backscatter diffraction techniques. The results showed that the microstructure of copper and copper-based nanocomposite composed of a mixture of equiaxed nanograins with bimodal and non-random misorientation distribution. The presence of SiC nanoparticles refined the grain structure of the copper matrix while the fraction of low angle grain boundaries was increased. Evaluation of mechanical... 

Using Hamilton's principle, exact equations of motion for non-linear planar and spatial Euler-Bernoulli beams are derived. In the existing non-linear Euler-Bernoulli beam formulations, some elastic terms are dropped by differentiation from the incomplete Green-Lagrange strain tensor followed by negligible elastic deformations of cross-sectional frame. On the other hand, in this article, the exact strain field concerning considerable elastic deformations of cross-sectional frame is used as a source in differentiations. As a result, the achieved closed-form equations are exact and more accurate than formerly reported equations in the literature. Moreover, the applicable dynamic model of... 

Natural flyers have flexible wings, which deform significantly under the combined inertial and aerodynamic forces. In this study, we focus on the role of chord wise flexibility in 2D pitch and plunge motions. We derive the exact nonlinear 2D equations of motion for a flexible flapping wing with flying support. In achieving the closed-form equations, we use the exact strain field concerning considerable elastic deformations. After numerically solving the novel equations, we validate them in simulations with highly deformable wings. By coupling the derived equations of motion with fluid flow, we study the aerodynamic performance of the geometrically nonlinear flexible flapping wing. Through... 

Double oxide films (hereafter: oxides), inclusions and Fe-rich phases are known to be the most detrimental defects in cast Al-Si-Mg alloys. The effects of these defects on reproducibility of tensile properties in Al-7Si-0.35Mg alloy have been investigated in this study. Four different casting conditions (low oxide-low Fe, high oxide-low Fe, low oxide-high Fe and high oxide-high Fe) were studied. In each case, 30 tensile test samples were prepared by casting in a metallic mold and machining (total of 120 tensile test samples). Results of tensile test were analyzed by Weibull three-parameter and mixture analyses. The microstructure and fracture surface of samples were studied by optical and... 

Nanostructured Cu and Cu-2. vol% SiC nanocomposite were produced by high energy mechanical milling and hot pressing technique. Microstructure development during fabrication process was investigated by X-ray diffraction, scanning electron microscope, scanning transmission electron microscope, and electron backscatter diffraction techniques. The results showed that the microstructure of copper and copper-based nanoco mposite composed of a mixture of equiaxed nanograins with bimodal and non-random misorientation distribution. The presence of SiC nanoparticles refined the grain structure of the copper matrix while the fraction of low angle grain boundaries was increased. Evaluation of mechanical... 

The microstructure and phase transformational behavior of NiTi-based composites reinforced with 6 wt.% of α-alumina nanoparticles have been investigated. Two kinds of starting materials, elemental Ni–Ti and prealloyed austenitic NiTi, were used to prepare the composites. The samples were consolidated using a hot isostatic pressing method. The X-ray diffraction results showed that while unreinforced NiTi mainly contained B2 phase at room temperature, martensitic B19′ phase appeared in the microstructure after addition of the α-alumina nanoparticles. The differential scanning calorimetry measurements indicated that the martensitic transformation temperatures were elevated in the composite... 

NiTi alloys have exhibited significant potential as a matrix of wear resistant composites. In this study, in order to examine the effect of starting materials on the wear performance of NiTi-based composites, both elemental Ni/Ti and prealloyed NiTi powders were used to fabricate NiTi-6wt% nano-Al2O3 composites using hot isostatic pressing (HIP). Nanoindentation and microhardness test results indicate that the composite samples produced from the elemental Ni/Ti powders exhibited higher hardness and lower pseudoelasticity properties than those of the samples fabricated from the prealloyed NiTi powders; this is attributed to the higher amount of... 

Artificial Life can be used as an agent training approach in large state spaces. This paper presents an artificial life method to increase the training speed of some speech recognizer agents which where previously trained by genetic algorithms. Using this approach, vertical training (genetic mutations and selection) is combined with horizontal training (individual learning through reinforcement learning) and results in a much faster evolution than simple genetic algorithm. The approach is tested and a comparison with GA cases on a standard speech data base is presented. COPYRIGHT © ENFORMATIKA  

This paper reports on the NTIRE 2021 challenge on perceptual image quality assessment (IQA), held in conjunction with the New Trends in Image Restoration and Enhancement workshop (NTIRE) workshop at CVPR 2021. As a new type of image processing technology, perceptual image processing algorithms based on Generative Adversarial Networks (GAN) have produced images with more realistic textures. These output images have completely different characteristics from traditional distortions, thus pose a new challenge for IQA methods to evaluate their visual quality. In comparison with previous IQA challenges, the training and testing datasets in this challenge include the outputs of perceptual image... 

A new magnetic quaternary photocatalyst, K4Nb6O17/α-Fe2O3/Fe3N/g-C3N4, was synthesized via a simple one-step thermal pyrolysis process. The prepared photocatalyst was well characterized, and its photocatalytic activity was evaluated towards acetamiprid pesticide degradation, CO2 reduction reaction, and U87-MG cell eradication. The coexistence of four different crystalline structures in the prepared photocatalyst could simultaneously facilitate photoelectron transport and suppress charge recombination through their coupled heterogeneous interfaces. Because of the coexistence of various photosensitive compounds, the prepared photocatalyst showed the excellent capability to harvest the visible... 

We report a simple synthetic approach to grow uniform CH3NH3PbI3 perovskite (PSK) layers free of pinholes via varied portions of silver iodide (AgI) added to the precursor solution. XRD/EDS elemental mapping experiments demonstrated nearly uniform Ag distribution inside the perovskite film. When the 1% AgI-assisted perovskite films were fabricated into a p-i-n planar device, the photovoltaic performance was enhanced by ∼30% (PCE increased from 9.5% to 12.0%) relative to the standard cell without added AgI. Measurement of electronic properties using a hall setup indicated that perovskite films show p-type character after Ag doping, whereas the film is n-type without Ag. Transients of... 

The In-Situ Combustion (ISC) as a thermal EOR process has been studied deeply in heavy oil reservoirs and is a promising method for certain non-fractured sandstones. However, its feasibility in fractured carbonates remained questionable. The aim of the present work was to understand the recovery mechanisms of ISC in fractured models and to evaluate the effect of fractures geometrical properties such as orientation, density, location and networking on the ISC recovery performance. Combustion parameters of a fractured low permeable carbonate heavy oil reservoir in Middle East called KEM; applied to simulation study. Simulator has been validated with KEM combustion tube experimental data and... 

Switched reluctance machines (SRMs) suffer from ultra-low-frequency torque ripple in single pulse mode. This ripple is the result of an imbalance in phase currents, when multiphase excitation occurs. This paper investigates the origin of this imbalance and identifies the imbalanced phase. Excitation shifting strategy, which delays excitation of the imbalanced phase, is then proposed. The delay angle adjusts current amplitude in the imbalanced phase to attain a perfect balance between all phases. The excitation shifting strategy is a general solution, which is applicable to both three-phase and four-phase machines. In addition, the proposed method can be implemented by a much simpler drive... 

The evidence suggests that monetary policy post 1988 became more forward-looking, invalidating the identifying assumptions in conventional methods of measuring monetary policy's effects, leading to spurious and unlikely results for this period. We propose a new identification scheme that uses factors extracted from Fed Funds futures to measure exogenous changes in policy. Using this shock series in a VAR, we recover the contractionary effect of monetary tightening on output. Moreover, we find that as much as half of the variability in output was driven by monetary policy shocks, and that there is a mild price puzzle  

Organizational change has become complex and challenging, and employee attitudes and beliefs toward change are even more important. This article proposes a theoretical framework on how authentic leadership may influence followers’ change-oriented attitudes, beliefs, and behaviors through follower psychological resources including hope, trust, optimism, self-efficacy, and resilience, which influence readiness for change and change implementation. We maintain that an authentic leader’s behavior can influence his or her followers’ change-oriented behaviors in participation in decision-making processes and change initiatives, organization citizenship behavior, organizational learning processes,... 

This paper examines the effect of experiencing a research collaboration between African scientists and their international partners on the African scientists' research impact, measured by scores based on the number of citations and journal impact factor. Using a difference-in-difference regression analysis on publication data of African scientists, the results show that international research collaboration in a given year has a positive and significant effect on research impact, which lasts in the subsequent years. The presence of such permanent effect suggests a sort of 'permanent learning by international collaboration'. The positive effect of international collaboration on research...