Sharif Digital Repository

The morphology of a quaternary blend containing a diglycidyl ether of bisphenol-A (DGEBA), a thermoplastic modifier (PMMA), a phase-separating curing agent (diaminodiphenylmethane, DDM), and a non-phase-separating curing agent (methylenebis(3-chloro-2,6-diethylaniline, MCDEA) was studied as a function of volume fraction of the thermoplastic modifier and fractional concentration of the curing agents in their mixture. It was found that using mixtures of curing agents a co-continuous morphology could be obtained at PMMA concentrations as low as 2.5 volume percent. Using FTIR spectroscopy it was proved that specific interactions are present between PMMA and individual amine curing agents. On the... 

We present in this paper a new method of modifying hard-core potentials to correctly predict the equilibrium and non-equilibrium properties of real fluids at low densities (specifically when ρ ≤ B/C where ρ is density, and B and C are the second and third virial coefficients) only by one set of potential parameters. Because the molecular diameter becomes smaller when temperature increases, we introduced a new expression for the variation of molecular diameter with temperature that incorporates this effect. The temperature dependence of the diameter was used in both Sutherland (ST) and square-well (SW) potential models to modify the second virial coefficient. Then we have shown that the... 

We consider a class of optimization problems called movement minimization on euclidean plane. Given a set of m nodes on the plane, the aim is to achieve some specific property by minimum movement of the nodes. We consider two specific properties, namely the connectivity (Con) and realization of a given topology (Topol). By minimum movement, we mean either the sum of all movements (Sum) or the maximum movement (Max). We obtain several approximation algorithms and some hardness results for these four problems. We obtain an O(m) -factor approximation for ConMax and ConSum and extend some known result on graphical grounds and obtain inapproximability results on the geometrical grounds. For the... 

Abstract: Shot peening is well-known process for mechanical properties integrity in metallic materials. In present study influences of different shot peening treatments on the surface hardness of different carbon steels were investigated experimentally and then alternative approach by using artificial neural network is presented for hardness prediction of the shot peened surface. After modeling a comprehensive parametric investigations and sensitivity analysis were applied according to the influence of the related effective parameters on surface hardness improvements. Graphic Abstract: [Figure not available: see fulltext.] © 2020, The Korean Institute of Metals and Materials  

Al-15wt.% Mg2Si composites were prepared by in situ casting and characterized in wear tests. Previous to the extrusion of specimens at 470°C - varying extrusion ratio (7.4, 14.1 and 25), the as-cast composites were homogenized at 500°C for 5h, followed by slow furnace cooling. The microstructure, hardness and sliding wear behavior were characterized for both, the as-cast and hot extruded composites. Results show that increasing the extrusion ratio causes a significant improvement in hardness and wear resistance. This is ascribed to the observed decrease in average size and better distribution of Mg2Si particles, in tandem with a remarkable decrease in porosity percentages, which goes from... 

In this study, electronic structure, stability, and tendency to exchange electron of neutral, anionic, and cationic RhxCu4-x (x = 0-4) small clusters were investigated by density functional theory calculations. For neutral small clusters, it was found that the most stable structures of Rh4, Rh3Cu and Rh2Cu2 have distorted tetrahedral shape while the most stable structures of RhCu3 and Cu4 have quasi-planer shape. Adding charges to the clusters, caused shapes of the most stable structures undergo variations. Stabilities of the neutral, anionic, and cationic clusters decrease linearly with increasing the copper content. In addition, calculated chemical harnesses indicated that the small... 

We present in this paper a simple method of obtaining various equations of state for hard sphere fluid in a simple unifying way. Using the first several virial coefficients of hard sphere fluid, we will guess equations of state by using the asymptotic expansion method. Among the equations of state obtained in this way are Percus-Yevick, Scaled Particle Theory, and Carnahan-Starling equations of state. Also by combining the Monte Carlo results on hard sphere fluid with the asymptotic expansion method many other equations of state for hard sphere fluid can be found where all of them give essentially similar results in the region of isotropic hard sphere liquid, i.e., up to η < 0.5, in which η... 

Circular chromatic number, χc is a natural generalization of chromatic number. It is known that it is NP-hard to determine whether or not an arbitrary graph G satisfies χ(G)= χc(G). In this paper we prove that this problem is NP-hard even if the chromatic number of the graph is known. This answers a question of Xuding Zhu. Also we prove that for all positive integers k ≥ 2 and n ≥ 3, for a given graph G with χ(G)= n, it is NP-complete to verify if χc(G)≤ n -1/k. © 2004 Wiley Periodicals, Inc  

Abstract: Shot peening is well-known process for mechanical properties integrity in metallic materials. In present study influences of different shot peening treatments on the surface hardness of different carbon steels were investigated experimentally and then alternative approach by using artificial neural network is presented for hardness prediction of the shot peened surface. After modeling a comprehensive parametric investigations and sensitivity analysis were applied according to the influence of the related effective parameters on surface hardness improvements. Graphic Abstract: [Figure not available: see fulltext.] © 2020, The Korean Institute of Metals and Materials  

Utilizing the differential scanning calorimetry (DSC) and Vickers hardness tests, the relationship between the stored energy and indentation hardness of copper after compression test is achieved experimentally. Three dislocation models are utilized to develop the relationships between the stored energy and hardness for justifying the experimental relationship. The relationships show that the stored energy is increased by increasing the hardness, non-linearly. By comparing the models' results with the experimental data, the validity of each model at different ranges of hardness is determined. © 2008 Springer Science+Business Media, LLC  

Given a random variable (Formula presented.) and a set of experts (Formula presented.), we describe a method for finding a subset of experts (Formula presented.) whose aggregated opinion best predicts the outcome of (Formula presented.). Therefore, the problem can be regarded as a team formation for performing a prediction task. We show that in case of aggregating experts' opinions by simple averaging, finding the best team (the team with the lowest total error during past (Formula presented.) rounds) can be modeled with an integer quadratic programming and we prove its NP-hardness whereas its relaxation is solvable in polynomial time. At the end, we do an experimental comparison between... 

It is commonly known that the properties of materials are strongly related to their microstructure. This paper introduces a method that is a combination of rapid solidification (melt spinning) and semisolid forming (thixoforming) processes, which are used to refine the microstructure of “as-cast” Al–30Si–5Fe alloy and improve its mechanical properties. Produced Al–30Si–5Fe ribbons by melt-spinning process, were pulverized and subsequently consolidated at 300 °C in argon atmosphere. Prepared specimens were thixoformed at temperatures ranging from 580 to 700 °C in various holding times. Some samples were also directly thixoformed from as-cast condition. Microstructure of the as-cast, meltspun,... 

A low-carbon steel sheet containing 0.05 C, 0.203 Mn, and 0.0229 Si (all in wt%) was rapidly annealed in a temperature range of 300 °C to 600 °C after severe plastic deformation by using constrained groove pressing (CGP) technique. Microstructure evolution was investigated by scanning electron and optical microscopes. Mechanical properties were evaluated by hardness measurements and shear punch test. The results showed a thermal stability up to 400 °C where recrystallization did not occur in the specimens even after 7200 s. This thermal stability is in agreement with previously reported results of conventional annealing of the same steel after CGP. However, annealing at 500 °C and 600 °C led... 

The cold drawing process is one of the precise metal forming techniques which have attracted attention by many researchers with respect to the given properties such as potential for improvement of mechanical properties of the material, achieving of accurate dimensional tolerances and rise of quality at final surface so that different studies are conducted on properties of various materials in the course of analysis of its effect. Using cold drawing and also exertion of special heat treatment cycle in this investigation, initially the seamless steel tube Ck60 has been produced and then effect of cold drawing has been assessed on the mechanical properties and structure of steel. The production... 

We present sliding experiments of a sledge on wetted sand and describe that the frictional response is controlled by the penetration hardness of the granular medium. Adding a small amount of water to sand increases the hardness which results in a decrease of the sliding friction. Pouring even more water to sand results in a decrease of the hardness and a subsequent increase of the friction. This inverse correlation between hardness of a wetted granular material and its frictional response to sliding is found to be due to ploughing of the sledge. When the load of the sledge exceeds the penetration hardness of the water-sand mixture the granular material is irreversibly deformed, which is... 

The Ghotbi-Vera hard sphere equation of state as the reference system, coupled with the mean spherical approximation, (GV-MSA), has been used to accurately correlate the mean ionic activity coefficients of aqueous solutions of single 1:1 electrolytes. The new equation of state considers the ions to be hard spheres in Kelvin tetrakaidekahedron cells. The mean ionic activity coefficients of the electrolytes can be correlated with constant diameter of anions, equal to the crystallographic Pauling diameter and with a concentration dependent relation for the cations. In comparison to their crystallographic Pauling diameters, the new model for the mean ionic activity coefficient produces realistic... 

The lazy bureaucrat scheduling is a new class of scheduling problems that was introduced in [1]. In these problems, there is one employee (or more) who should perform the assigned jobs. The objective of the employee is to minimize the amount of work he performs and to be as inefficient as possible. He is subject to a constraint, however, that he should be busy when there is some work to do. In this paper, we focus on the cases of this problem where all jobs have the same common deadline. We show that with this constraint, the problem is still NP-hard, and prove some hardness results. We then present a tight 2-approximation algorithm for this problem under one of the defined objective... 

The predictions of the pair radial distribution function (RDF) at contact value, compressibility factor, and chemical potentials for binary and ternary hard-sphere mixtures obtained with the Barrio-Solana, the Santos et al., and the generalized Lebowitz mixing rules are compared using the Carnahan-Starling, Kolafa, and Khoshkbarchi-Vera one-component hard-sphere EOS. An expression for the pair RDF at contact value for Barrio-Solana mixing rule and the expressions for the chemical potentials in multicomponent hard-sphere fluids are derived. As a general rule, the equations of state obtained based on the generalized Lebowitz mixing rule predict better the pair RDF at contact value simulated... 

Assume that we are given a set of points some of which are black and the rest are white. The goal is to find a set of convex polygons with maximum total area that cover all white points and exclude all black points. We study the problem on three different settings (based on overlapping between different convex polygons): (1) In case convex polygons are permitted to have common area, we present a polynomial algorithm. (2) In case convex polygons are not allowed to have common area but are allowed to have common vertices, we prove the NP-hardness of the problem and propose an algorithm whose output is at least (Formula presented.). (3) Finally, in case convex polygons are not allowed to have...