M. R. Rahimi Pour and M. Moayeri, ,
Volume 25, Issue 1 (7-2006)
Abstract
The effective parameters that influence in situ cast ferroTic composites were investigated. Centrifugal casting of specimens was carried out using ceramic & metallic molds. OM, SEM and XRD techniques were used to examine the existence of flows in the specimens. Results show that the control of chemical composition, processing, cooling rate and heat treatment has
a promising effect on the quality of specimens. Also remelting process leads to the hemogeneity of matrix by uniform distribution of secondary phase.
M. R. Toroghinejad, M. Sayadi and M. M. Moshksar,
Volume 25, Issue 1 (7-2006)
Abstract
Aluminum matrix composites reinforced with Al2O3 and SiC particles (5 Vol%) were produced using the hot powder extrusion method. Extrusion temperature and extrusion reduction in area were chosen in the range of 500 to 600°C and 90 to 95%, respectively. The physical and mechanical properties of the extruded composites such as density, tensile strength, elongation and microhardness were evaluated and discussed as a function of extrusion parameters. The microstructure and fracture surface of the products were examined using SEM. The results showed that the composites were fully densified and reinforcement particles were distributed uniformly in the matrix. Presence of Al2O3 and SiC particles increased both strength and microhardness, but decreased the ductility of the composites. Experimental results for hot extrusion of the compacted powder billets also showed that the extrusion pressure was dependent on the ram speed or deformation strain rate.
S. M. Miresmaeili and S. Shabestari,
Volume 25, Issue 1 (7-2006)
Abstract
The formation of microporosity in modified Al-Si alloys has been reviewed in the present study. A major concern in modification is the increased tendency to form microporosity in the macro-shrinkage free Al-Si alloy castings. It has also been demonstrated that at low hydrogen contents (0.1cc/ 100g, Al), where only shrinkage porosity should occur, the effect of Sr-modification on porosity content is not considerable, indicating that the increase in porosity is due to an increase in gas porosity. Modification treatment, however, does not add hydrogen to the melt, nor does it increase the rate of regassing of the liquid, revealing that it can not enhance pore formation by increasing the melt hydrogen content. Modification treatment raises the freezing range (4-10 oC), but this increased freezing range exerts only a very small effect on microporosity formation, which cannot, by itself, explain the increased tendency to microporosity formation observed in modified alloys. The presence of modifiers slightly decreases the surface tension of the melt (5%), although this decrease in surface tension is not sufficiently high to considerably enhance pore formation in modified alloys. Many researchers have reported that modification treatment might favour the formation of porosity due to its effect on oxide use in the heterogeneous pore formation although the systematic observation of pores has shown that SrO does not take part in pore fomation in Sr-modified alloys. Strontium and other modifiers which increase pore formation (Na and Ca) in Al-Si alloys have a high chemical affinity to form complex intermetallic compounds with Si and Al. Systematic observation of pores have shown that Sr-rich intermetallics take part in pore formation. Thus, Sr-modification may increase the porosity content through the formation of Sr-rich compounds during solidification.
N. Setoudeh, A. Saidi, A. Shafyei and N. J. Welham,
Volume 25, Issue 1 (7-2006)
Abstract
Anatase-to-rutile phase transformation was studied in milled and unmilled samples. Ball milling was carried out in two types of ball mills, planetary and tumbler, with a ball-to-powder ratio of 40:1 over 2-48 hours. First, the unmilled samples
were heated in the furnace at various temperatures for different periods of time. The results revealed that the anatase-to-rutile transformation completed at 980 after 48 hours. The rate of transformation in milled samples was greatly higher than that of unmilled ones. Activation energy in unmilled samples was about 440 kj/mol. The rate of transformation in the planetary ball mill was higher than that in tumbler mill. In the former, transformation almost finished after 16 hours of milling while in the lattar, it did not finish even after 48 hours. XRD results revealed that the transformation proceeds through an intermediate srilankite phase in all milled samples. However, srilankite was not observed in the unmilled samples.
Hassan Haddadpour,
Volume 25, Issue 1 (7-2006)
Abstract
A method is presented for the stress analysis of flight vehicles under different flight conditions including gust and control surface deflection (or maneuver) using the governing equations of rigid-body motions and elastic deformations. The Lagrangian approach is used to derive the governing equations of motions. For this purpose, the basic equations of motions are derived in terms of potential energy, kinetic energy and generalized forces, which are, in turn, computed in terms of rigid-body motion variables, elastic mode shapes and distribution for aerodynamic forces. By replacing them into the relations obtained, the governing equations for aeroelastic behavior of the vehicle are derived. The system of aeroelastic equations of
motions is solved in time domain using numerical methods. The stress distribution is determined using the relation between modal variables and strain at each point. Finally, the prepared code is verified through comparison of the results obtained from the proposed method for the stability of a rocket and the same results reported by other studies. Also additional information such as maximum stress in the body is presented for various flight conditions.
A. R. Safari, M. Ghayour, and A. Kabiri,
Volume 25, Issue 1 (7-2006)
Abstract
It is empirically established that, due to a number of factors involved, a classical (linear) analysis of buckling pressure is impossible. Nonlinear theories of buckling are, therefore, required that involve effective factors such as imperfections and welding effects. In this study, models are developed which are as close to allowable standard deviations as possible. In the next stage, their buckling behavior is investigated both experimentally and numerically using finite element packages ADINA, ANSYS, COSMOS, and MARC based on specific capabilities of each. Results show that reasonable estimates of real buckling pressure will become possible when material and geometrical nonlinearities and initial imperfections are introduced into the analytical system. Finally, in the light of the results obtained, a submarine pressure hull is analyzed.
G. Mirjalily, H. Hossieni, and A. Sheikhi,
Volume 25, Issue 2 (1-2007)
Abstract
The theory of distributed detection is receiving a lot of attention. A common assumption used in previous studies is the conditional independence of the observations. In this paper, the optimization of local decision rules for distributed detection networks with correlated observations is considered. We focus on presenting the detection theory for parallel distributed detection networks with fixed fusion rules to develop a numeric algorithm based on Neyman-Pearson criterion. Simulation results are
presented to demonstrate the efficiency and convergence properties of the algorithm.
Gh. Moslehi and H. Ghahar,
Volume 25, Issue 2 (1-2007)
Abstract
This paper deals with resource unconstrained project scheduling problems with the objective of maximizing the net present value (NPV) of project cash flows. Here we present a heuristic algorithm named as differential procedure (Dif_AOA).
In order to evaluate the efficiency of this algorithm, networks with node numbers between 10-1000 and network complexity coefficients between 1.3-6.6 have been generated. We have compared both the total time for solving the problem and NPV of the Dif_AOA with those of the recursive search procedure. Computational results show that the Dif_AOA performs very effectively.
Extensive analysis have been performed to evaluate the node number, complexity network coefficients(CNC), and deadline.
D. Mostofinejad and M. Hoseinian,
Volume 25, Issue 2 (1-2007)
Abstract
It is well known that the characteristics of concrete components greatly affect the durability of high strength/high performance (HS/HP) concrete against frost action. Undoubtedly, precise recognition of this relationship leads
to appropriate selection of the type and proportions of concrete components in any particular project. In the current study, the aim is to investigate the possibility of developing some mathematical-experimental models to explain the frost resistance of high-performance concrete, regarding the role of some of its main components. To do so, the effects of four key elements, i.e. water, silica fume, coarse aggregate, and number of freeze-thawing cycles, were studied on the frost resistance of HS/HP concrete were studied. 24 concrete mix designs including 3 ratios of water to cementitious materials, i. e. 0.4, 0.3, and 0.25 4 ratios of silica fume to cementitious materials, i.e. 0, 5, 10, and 15 percent and 2 types of coarse aggregates, i. e. Limestone and Quartzite were utilized for HS/HP concrete. Overall, about 432 concrete cubes were cast, cured and tested under freeeze-thaw cycles. Finally, some models were proposed for describing the frost resistance of high strength concrete.
S. M. Zahrai and B. Rad,
Volume 25, Issue 2 (1-2007)
Abstract
In recent years, destructive earthquakes have shown the deficiencies of the existing buildings. One of the most effective mechanisms for dissipating the earthquake energy is inelastic deformation of the steel components. The objective of this research is to study the application of metallic dampers for dissipation of the earthquake energy and to investigate the behavior of concrete structures incorporating these dampers. Therefore, the metallic dampers and the behavior of concrete structures having these dampers are studied first. Afterwards, a typical metallic damper is used in four different types of concrete structure. The required dampers are designed and nonlinear earthquake analysis is applied to investigate the behavior of the structures. Finally, the buildings are subjected to various earthquakes to generalize the results. The results show that the incorporation of the metallic dampers significantly decreases the relative and absolute drift, the structure and the stories damage indices and, finally, the number of plastic hinges. Furthermore, the hysteretic energy dissipation demand also decreases in structural components. Despite the reduction in the inner forces of structural components, story shear forces slightly increase due to increase of lateral stiffness, but much of these forces will concentrate in dampers. Moreover, the combination of moment resisting frame, shear wall, and metallic dampers are studied. The results show a similar trend in the stated parameters- especially the drift and the hysteresis energy dissipation demand.
M. Sheikhi and H. Haji-Kazemi,
Volume 25, Issue 2 (1-2007)
Abstract
Jacketing of reinforced concrete columns is a common and useful strengthening method. This method substantially improves mechanical properties of the column, such as flexural strength as well as shear and ductility. In this paper, the behavior of confined reinforced concrete columns are investigated. The results indicate that the method is more effective for slender columns in the region of their failure zone.
M. R. Tabeshpour, A. A. Golafshani, A. Bakhshi, and A. Moayed Alaei,
Volume 25, Issue 2 (1-2007)
Abstract
The assessment of seismic performance of existing structures is becoming an important problem in earthquake engineering. Some important ructures are considerably old and, therefore, their strengths and ductilities are less than strength and ductility demands because of changes in codes and design methodologies. Such structures must be strengthened to resist future earthquakes. First, a structural model must be developed and then, based on seismic hazard and seismic risk analysis or code quantities, the design (or control) parameters can be determined. The next step is defining the damage indices in order to quantify the structural damage. Then the nonlinear dynamic analysis is carried out and damage indices are calculated. In the present paper, a power plant stack (located in Mashhad) is investigated for some levels of peak ground acceleration (PGA) considering return periods of 75, 500, 1000, and 2500 year. The height of stack is 100 m and the external diameter of the structure is 10 m. Several records are used for nonlinear dynamic analysis. It has been used from Park-Ang damage index that is a suitable model for concrete structures and has been considered in IDARC. It is clear that nonlinear dynamic analysis is necessary for the seismic vulnerability of existing structures. Special structures such stacks can be modeled with some 2-D or 3-D elements. However, the beam-column element is a proper model for special structures such as chimneys, considering calculation cost.
M. H. Bagheripour, E. Shasavandi, and S. M. Marandi,
Volume 25, Issue 2 (1-2007)
Abstract
This paper introduces an accurate, fast, and applicable method for optimization of slip surfaces in earth slopes. Using Genetic Algorithm (GA), which is one of the modern and non-classic optimization methods, in conjunction with the
well -known Bishop applied method, the optimum slip surface in an earth slope is investigated and its corresponding lowest safety factor is determined. Investigations have shown that selection of appropriate variables to define and to solve the problem and determination of a good range for these variables have a profound effect on the speed of convergence in the problem. In the present study, appropriate variables have been defined for solving the problem in a way that the number of repetitions required to reach convergence are considerably reduced by up to 50% compared with other approaches. This has led to a drastic reduction in time and the memory required. The accuracy of the method is shown first by solving examples related to search for optimum failure surfaces of some homogenous, non-homogenous, and earth dam slopes and then by comparison of the results with those of other optimization techniques. In order to show the application of the present method in modern geotechnical engineering, a reinforced earth slope is studied and its failure surface is finally optimized
S. Mortazavi,
Volume 25, Issue 2 (1-2007)
Abstract
The cross-stream migration of a deformable drop in two-dimensional Poiseuille flow at finite Reynolds numbers is studied numerically. In the limit of a small Reynolds number (<1), the motion of the drop depends strongly on the ratio of the viscosity of the drop fluid to the viscosity of the suspending fluid. For a viscosity ratio 0.125, the drop moves toward the centre of the channe while for the ratio 1.0, it moves away from the centre until halted by wall repulsion. The rate of migration increases
with the deformability of the drop. At higher Reynolds numbers (5-50), the drop either moves to an equilibrium lateral position about halfway between the centerline and the wall according to the so-called Segre-Silberberg effect or undergoes oscillatory motions. The steady-state position depends only weakly on the various physical parameters of the flow but the length of the transient oscillations increases as Reynolds number is raised, the density of the drop is increased, or the viscosity of the drop is decreased. Once the Reynolds number is high enough, the oscillations appear to persist forever and no steady state is observed. The numerical results are in good agreement with experimental observations, especially for drops that reach steady-state lateral position.
M. Talebi, E. Shirani, and M. Ashrafizadeh,
Volume 25, Issue 2 (1-2007)
Abstract
In this study, turbulent flow around a tube bundle in non-orthogonal grid is simulated using the Large Eddy Simulation (LES) technique and parallelization of fully coupled Navier – Stokes (NS) equations. To model the small eddies, the Smagorinsky and a mixed model was used. This model represents the effect of dissipation and the grid-scale and subgrid-scale interactions. The fully coupled NS equations with the multiblock method was parallelized. Parallelization of the computer code was accomplished by splitting the calculation domain into several subdomains and using different processors in such a way that the computational work was equally distributed among processors. The discretized governing equations are second order in time and in space and the pressure is calculated by Momentum Interpolation Method (MIM) to prevent the checkerboard problem. The results are obtained for the turbulent flow over five parallel tube rows. The computational efficiency, flow patterns, and flow properties are also determined. The results showed high parallelization efficiency and high speed up for the computer code. The flow characteristics were determined and compared with experimental results which showed good agreement. Also, the results showed that the mixed model is better than the Smagorinsky model for evaluation of flow characteristics and lift and drag forces on tubes.
M. Mashayekhi, S. Ziaei-Rad, and J. Parvizian,
Volume 25, Issue 2 (1-2007)
Abstract
The continuum mechanic simulation of micro-structural damage process is important in the study of ductile fracture mechanics. In this paper, the continuum damage mechanics model formulation proposed by Lematire has been validated against ductile damage evolution experimentally measured in A533B-C1 steel under stress triaxiality conditions. First, a
procedure to identify the model parameters from test was defined. Then, the finite element model was used to simulate the experiment carried out on a notched flat rectangular bar. Good agreement was observed between the experimental results and finite element predictions. Next, the identified parameters on A533B-C1 steel were used to simulate the results from a conventional tensile test by finite element method. The specimen was prepared according to ASTM E08 standard. The stresses at necking stage and ultimate load calculated by the damage based method were compared with those obtained from the test. The comparisons indicate a good agreement between the simulated and the experimental results.
M. Darbandi, V. Mokarizadeh, and E. Roohi,
Volume 25, Issue 2 (1-2007)
Abstract
The main objective of the present study is to utilize a novel linearization strategy to linearize the convection terms of the quasi-one-dimensional Euler governing equations on collocated grids and to examine its shock-capturing capabilities. To avoid a pressure checkerboard problem on the collocated grids, it is necessary to utilize two velocity definitions at each cell face. Similarly, we define two velocity expressions at cell faces known as convecting and convected velocities. We derive them from the proper combinations of continuity and momentum equations which, in turn, provide a strong coupling among the Euler discretized equations. To achieve this, we utilize an advanced linearization strategy known as Newton-Raphson to linearize the nonlinear convection terms. The key point in this linearization is to preserve the original physics behind the two velocities in the linearization procedure. The performance of the new formulation is then investigated in a converging-diverging nozzle flow. The results show great improvement in both the performance of the original formulation and in capturing shocks. The results also indicate that the new extended formulation is robust enough to be used as an all-speed flow solver.
M. H. Enayati and M. Salehi,
Volume 25, Issue 2 (1-2007)
Abstract
Fabrication and characterization of aluminum matrix composites containing different volume fractions of Ni3Al powder (5-40 Vol%) were investigated. Ni3Al powder was produced by mechanical alloying of elemental nickel and aluminum powder mixture. Al-Ni3Al composite parts were prepared using a powder metallurgy route involving two stages Al and Ni3Al powder mixtures were first compacted under 500MPa and then hot-pressed under 250MPa at 420 oC for 10min. The microstructure and hardness of consolidated parts were investigated by x-ray diffractometery, optical and scanning electron microscopy and
hardness measurements. Results showed that consolidated Al-Ni3Al samples included no significant porosity with a nearly uniform distribution of Ni3Al particles. Additionally, structural examinations showed that no significant reaction between Ni3Al and aluminum matrix occurred during sintering process. Al-Ni3Al composites exhibited a higher hardness value compared with pure aluminum sample prepared under identical conditions. The hardness value of Al-Ni3Al composites increased linearly as Ni3Al content increased.
R. Bagheri and M.a. Golozar,
Volume 25, Issue 2 (1-2007)
Abstract
Using Electrostatic Spray Coating Technique, Polypropylene Powder (EPD 60R) was applied on carbon steel substrates at room temperature. In order to obtain a uniform coating, steel substrates with powder coatings were heated in a vacuum oven at various temperatures up to 250° C for various periods of time up to 45 min and a pressure of 200 mb. The coatings produced had thicknesses of around 470 microns. In order to modify the chemical structure of this polymer, the powder coatings containing various weight percentages of maleic (anhydride (MA) and a peroxide (TBHP or DCP) were also applied onto the steel substrates under the above conditions. Adhesion strength, wear resistance, and ductility of polymer coatings produced were assessed using ASTM standard methods. Results obtained revealed that the polymer coating containing 5 wt%. MA and 0.1 wt% TBHP had the best mechanical properties. Adhesive strength and wear resistance of this coating were 14.3 kgf and 250.3 cm, at 6 kgf, respectively, under the applied load of 6kg. Results obtained from DSC thermographs and IR Spectroscopy also proved the chemical bond formation (grafting) between the polymer and MA. The mechanical properties of coatings on steel substrate stem from such graftings.
A. Zaery, and F. Sheikholeslam,
Volume 25, Issue 2 (1-2007)
Abstract
An important consideration in control issues is control of nonlinear systems. Sliding control is among those nonlinear controllers that can control the system desirably in the presence of unstructured uncertainties of carelessness in specifying parameters of the system. In sliding control, also called Variable Structure Control, the main objectives of the controller
are achieved by introducing a sliding surface. One of the fundamental problems which may occur in sliding control is the chattering phenomenon on unwanted oscillation around the sliding surface. Different solutions are introduced to eliminate chattering. One of the commonest solutions is using a constant boundary layer round the sliding surface. In this paper, efforts are made to reduce chattering and to increase stability of the system by varying the sliding controller with a constant boundary layer. Finally, the mathematical model of a pendulum/cart in the presence of uncertainty is developed and the result of the simulation of the introduced controllers are compared.
