Showing papers by "Iran University of Science and Technology published in 2001"

The role of T-stress in brittle fracture for linear elastic materials under mixed-mode loading

Abstract: The purpose of this paper is to revisit the maximum tensile stress (MTS) criterion to predict brittle fracture for mixed mode conditions. Earlier experimental results for brittle fracture of polymethylmethacrylate (PMMA) using angled cracked plates are also re-examined. The role of the T-stress in brittle fracture for linear elastic materials is emphasized. The generalized MTS criterion is described in terms of mode I and II stress intensity factors, K I and K II and the T-stress (the stress parallel to the crack), and a fracture process zone, r c . The generalized MTS criterion is then compared with the earlier experimental results for PMMA subjected to mixed mode conditions. It is shown that brittle fracture can be controlled by a combination of singular stresses (characterized by K) or non-singular stress (T-stress). The T-stress is also shown to have an influence on brittle fracture when the singular stress field is a result of mode II loading.

Finite–element model updating using experimental test data: parametrization and regularization

Abstract: Two critical issues in model updating are deciding how a finite-element model should be parametrized and estimating the unknown parameters from the resulting ill- conditioned equations. A lack of understanding of these issues will lead to updated models without physical meaning. This paper outlines the authors' approach to parametrization, using physical, geometric and generic element parameters. It also applies useful methods of regularization, namely parameter constraints, the singular- value decomposition, L-curves and cross-validation to model updating. Finite-element model updating has become a viable approach to increase the cor- relation between the dynamic response of a structure and the predictions from a model. In model updating, parameters of the model are adjusted to reduce a penalty function based on residuals between a measurement set and the corresponding model predictions. Typical measurements include the modal model (natural frequencies and mode shapes) and the frequency response functions. The choice of penalty function, and also the optimization approach, has been the subject of much research and is well covered by the authors' survey paper (Mottershead & Friswell 1993), book (Friswell & Mottershead 1995) and special issue of Mechanical Systems and Signal Processing (Mottershead & Friswell 1998). This paper considers the issues of how to parametrize a finite-element model and how to regularize the resulting estimation equations to obtain a well-conditioned solution. These are critical issues in model updating. 2. Parametrization of the finite-element model Parametrization is a key issue in finite-element model updating. It is important that the chosen parameters should be able to clarify the ambiguity of the model, and in that case it is necessary for the model output to be sensitive to the parameters. Usually elements in the mass and stiffness matrices perform very poorly as candidate

A fast adaptive digital distance protection

Abstract: This paper presents a new adaptive technique which can improve the performance of digital distance protection under high resistance earth fault conditions. In this technique, the relay operating characteristic is divided into certain and uncertain fault regions. The certain region is a preset fixed region which is obtained by the system studies under different operating conditions. The faults in the certain region are tripped quite instantly. However, for the other faults an adaptive routine is instigated which uses prefault system data available from a low speed communication channel such as SCADA or RTU. The distance relay with necessary data can provide a reliable trip decision for the whole line length in zone-1 setting. The results of computer simulation show the improvement of sensitivity and selectivity of the relay.

Electrodeposition of graphite-brass composite coatings and characterization of the tribological properties

Abstract: Co-deposition of Cu(Zn) and graphite powders with size less than 10 μm was studied. Electrocomposite coatings were prepared by means of the conventional electrodeposition (CECD) and the sediment co-deposition (SCD) techniques. Brass electroplating cyanide bath has been selected out of electrodeposition brass. The effect of parameters such as rate of agitation and current density on the composition of electrodeposition brass and also the effect of the concentration of graphite particles in brass bath, air agitation rate on graphite incorporation within brass matrix and deposition efficiency were studied. The wear and friction properties of G–Cu(Zn) composite coatings containing different percents of graphite particles were examined. The presence of graphite particles improved antifriction properties of composite coatings. It was shown that the composite coating containing approximately 3.7 (vol.%) graphite has the best tribological properties.

Anatomy of turbulence in thermally stratified lakes

Abstract: Turbulence within the thermocline of two thermally stratified lakes—Lake Biwa, Japan, and Lake Kinneret, Israel—was investigated using a portable flux profiler. This instrument provided high-resolution profiles of temperature, conductivity, and two components of velocity within a measuring volume of approximately 8 mm 3 . Each data profile was segmented into statistically stationary segments. A range of properties of the turbulence, including direct estimates of the vertical mass flux, were then calculated for each segment. It was found that turbulence in the thermocline was generally patchy, but within a patch, dissipation levels were relatively high. The turbulent motions were found to be fine grained with small Thorpe scales and a large skewness of the distribution of the displacement scale. This is distinct to that found in the ocean, where shear instability produces large overturns. The measurement showed that the net vertical mass flux in the thermocline was negligible and less than that predicted by Osborn (1980). This is explained by noting that the net buoyancy flux consisted of two opposing parts—a down-gradient irreversible flux due to turbulent mixing and an up-gradient reversible flux due to restratification—that often cancel each other within a segment. The low measured net buoyancy flux within the interior of the lakes suggested that other processes, such as gravitational adjustments and benthic boundary layer processes, should be responsible for the basin-averaged vertical transport in these lakes.

A pipelined ROM-less architecture for sine-output direct digital frequency synthesizers using the second-order parabolic approximation

Abstract: A mathematical approximation for the sine function is proposed which is so close to the sine function that it satisfies the accuracy requirements for sine computation in a typical sine-output direct digital frequency synthesizers (DDS). Then, it is shown that the proposed approximation is realizable by standard digital circuitry, so, a novel ROM-less architecture for sine-output DDS is developed, optimized, and implemented. Experimental results for the implemented ROM-less sine-output DDS are in complete agreement with simulation results in showing that the approximation error will cause harmonic levels below the spurious levels associated with the output quantization error. Another important advantage of the proposed architecture is that it dispenses with ROM, so it can be easily pipelined in order to be as fast as is required.

Effect of Ti and Nb on the Formation of Carbides and the Mechanical Properties in As-cast AISI-M7 High-speed Steel

Abstract: In this research, the carbides of as-cast AISI M7 high-speed steel, in which different amounts of V and W were replaced by equivalent Ti and Nb, were investigated using light and electron microscope. EDX and WDX analysers were used in order to recognize the type and formulae of carbides. The volume fractions of each type of carbides are determined using especial etchant, so that the effects of Nb and Ti on the carbides types, shapes and volume percentages could be understand. The results show that Nb and Ti increase the volume fraction of MC instead of M 2 C or M 6 C and decrease the volume fractions of total carbides. It is also found that, the volume fractions of M 2 C decrease and M 6 C increase with increasing Nb percentage.

Gain equation for a free-electron laser with a helical wiggler and ion-channel guiding.

Abstract: The theory of ion-channel guiding in a helical wiggler is presented. Electron motion in the combined ion electrostatic and wiggler magnetostatic fields is analyzed in the absence of the radiation field. The Phi function that determines the rate of change of axial velocity with energy is derived and studied numerically. A detailed analysis of the pendulum equation and the gain equation in the low-gain-per-pass limit are presented. It is shown that the gain for stable group I orbits is positive, while for group II orbits the gain is negative in the negative mass regime and positive in the positive mass regime.

About Some Problems Raised by the Relativistic Form of De-Broglie-Bohm Theory of Pilot Wave

Abstract: The standard relativistic de-Broglie–Bohm theory has the problems of tacyonic solutions and the incorrect non-relativistic limit. In this paper we obtain a relativistic theory, not decomposing the relativistic wave equations but looking for a generalization of non-relativistic Bohmian theory in such a way that the correct non-relativistic limit emerges. In this way we are able to construct a relativistic de-Broglie–Bohm theory both for a single particle and for a many-particle system. At the end, the theory is extended to the curved space-time and the connection with quantum gravity is discussed.

Oxidation behaviour of titanium coated copper substrate

Abstract: In order to enhance the oxidation resistance of a copper substrate, titanium was added by inward diffusion into the surface of oxygen free high conductivity (OFHC) copper by a pack cementation process. Microstructural examination revealed that the specimen surface zone consists of three layers, the outermost layer is the transformation product of β phase which was formed at the diffusion temperature and contains α-Ti and Ti2Cu, the intermediate layer is composed of intermetallic compounds including the TiCu and Ti2Cu phases, and the innermost layer contains a solid solution of titanium in copper. Static oxidation tests of coated and uncoated samples at 700 and 800°C indicated a decrease in the oxidation rate by a factor of more than two orders of magnitude. While uncoated copper samples formed a rather thick oxide scale at these temperatures, the presence of intermetallic compound layers effectively increased the oxidation resistance and protected the copper substrate.

Three-dimensional forced Burgers turbulence supplemented with a continuity equation.

Abstract: We investigate turbulent limit of the forced Burgers equation supplemented with a continuity equation in three dimensions. The scaling exponent of the conditional two-point correlation function of density, i.e., approximately /x1-x2/(-alpha3), is calculated self-consistently in the nonuniversal region from which we obtain alpha3=3. Also we derive an equation governing the evolution of the probability density function (PDF) of longitudinal velocity increments in length scale, from which a possible mechanism for the dependence of the inertial PDF to one-point u(rms) is developed.

Syntheses and X-Ray Crystal Structures of New Copper(I) and Silver(I) Complexes containing Thione Ligands

Abstract: The reaction of [(Ph3P)2CuCl]2 with 4-amino-6-methyl-1,2,4-triazine-thione-5-one (AMTTO, 1) in methanol and further recrystallization from methanol/acetone solution gives [(C4H4N3SON(=CMe2)Cu(PPh3)2Cl] (2) as a neutral complex. [(C4H4N3SON(=CMe2)Ag(PPh3)2]NO3 (4) can be obtained in excellent yield by the reaction of [(AMTTO)2Ag]NO3 (3) with triphenylphosphane in methanol/acetone. Both complexes were characterized by infrared spectroscopy, elemental analyses as well as by X-ray diffraction studies. Crystal data for 2 at –80 °C: space group P1 with a = 1233.8(1), b = 1389.7(1), c = 1417.1(1) pm, α = 89.36(1)°; β = 65.10(1)°, γ = 65.95(1)°, Z = 2, R1 = 0.0582 and for 4 at –80 °C: space group P1, with a = 1193.3(1), b = 1308.5(1), c = 1385.3(1) pm, α = 94.69(1)°, β = 109.14(1)°, γ = 93.42(1)°, Z = 2, R1 = 0.0716. Synthesen und Kristallstrukturen neuer Kupfer(I)- und Silber(I)-Komplexe mit Thion-Liganden Die Reaktion von [(Ph3P)2CuCl]2 mit 4-Amino-6-methyl-1,2,4-triazin-thion-5-on (AMTTO, 1) in Methanol sowie Umkristallisation aus Methanol/Aceton ergibt den Neutralkomplex [(C4H4N3SON(=CMe2)Cu(PPh3)2Cl] (2). [(C4H4N3SON(=CMe2)Ag(PPh3)2]NO3 (4) kann in sehr guter Ausbeute erhalten werden, wenn [(AMTTO)2Ag]NO3 (3) mit Triphenylphosphan in Methanol/Aceton umgesetzt wird. Die Komplexe wurden mittels IR-Spektroskopie, Elementaranalysen und Rontgenstrukturanalysen charakterisiert. Kristalldaten fur 2 bei –80 °C: Raumgruppe P1 mit a = 1233,8(1), b = 1389,7(1), c = 1417,1(1) pm, α = 89,36(1)°; β = 65,10(1)°, γ = 65,95(1)°, Z = 2, R1 = 0,0582 und fur 4 bei –80 °C: Raumgruppe P1, mit a = 1193,3(1), b = 1308,5(1), c = 1385,3(1) pm, α = 94,69(1)°, β = 109,14(1)°, γ = 93,42(1)°, Z = 2, R1 = 0,0716.

FORMATION OF Ti–Cu INTERMETALLIC COATINGS ON COPPER SUBSTRATE

Abstract: In an attempt to modify the surface properties of copper substrate, Ti–Cu intermetallic compounds were produced on oxygen-free high-conductivity (OFHC) copper by physical vapor deposition followed subsequently with diffusion annealing. The structure and phase identification were evaluated by means of X-ray diffraction, electron probe microanalysis (EPMA), metallographic examination (optical and scanning electron microscopy [SEM]), energy-dispersive X-ray analysis (EDS), and microhardness measurements. Microstructural examination revealed that the compound coating consisted of three successive layers: the outermost layer (including the transformation product of β phase formed at diffusion temperature) contains α-Ti and Ti2Cu, the intermediate layer consists of TiCu4, and the innermost layer is a solid solution of copper in titanium. Ti–Cu intermetallic coatings significantly improved the hardness of the coated surfaces. The microhardness in the intermetallic layer was about seven times and the Ti–Cu solid ...

Spectral trisection of finite element models

Abstract: In this paper a new method is proposed for finite element domain decomposition. A weighted incidence graph is first constructed for the finite element model. A spectral partitioning heuristic is then applied to the graph using the second and the third eigenvalues of the Laplacian matrix of the graph, to partition it into three subgraphs and correspondingly trisect the finite element model.

EEG signals can be used to detect the voluntary hand movements by using an enhanced resource-allocating neural network

Abstract: This article explores the use of single trial EEG signals to predict the voluntary movements of single hand and two hands. During single-hand movements, three kinds of task, grasping, releasing, and holding were considered. The tasks considered during two-hand movements are left and right grasping, left and right releasing, and holding. The subject performs the tasks spontaneously without waiting for and responding to any external cues. In addition, a neural adaptive noise canceller is developed that accomplishes eye blinks suppression. The neural adaptive filter is here implemented by means of a three-layer feed-forward neural network. The feature vectors are formed from the three channels (Fz, C3, and F3). We employ the multilayer perceptron (MLP) with back-propagation learning algorithm and Radial Basis Function (RBF) network with stochastic gradient learning rule for discriminating different patterns of the EEG signals. In the classical approach to RBF and MLP network implementation, the number of hidden units is predetermined. It, usually, results in too many hidden units. To overcome this drawback, we develop an enhanced resource-allocating network (RAN) for discriminating the EEG patterns. These networks start with no hidden units and grow by allocating new hidden units based on the novelty in the EEG signals, which arrive sequentially. The results of this analysis show that the neural networks would be able to detect the movements of a single hand and two hands with an average classification accuracy of 98.82% and 96.40%, respectively. Moreover, the RAN provides a reduction in the training epochs as compared to the MLP and RBF networks. This work represents a promising approach to control prosthesis device.

Topological aspects of meshless methods and nodal ordering for meshless discretizations

Abstract: The meshless element-free Galerkin method (EFGM) is considered and compared to the finite-element method (FEM). In particular, topological aspects of meshless methods as the nodal connectivity and invertibility of matrices are studied and compared to those of the FE method. We define four associated graphs for meshless discretizations of EFGM and investigate their connectivity. The ways that the associated graphs for coupled FE-EFG models might be defined are recommended. The associated graphs are used for nodal ordering of meshless models in order to reduce the bandwidth, profile, maximum frontwidth, and root-mean-square wavefront of the corresponding matrices. Finally, the associated graphs are numerically compared. Copyright © 2001 John Wiley & Sons, Ltd.

Effect of the addition of Y2O3 on the structure, microstructure and piezoelectric properties of PZT(53/47)

Abstract: In this work, an attempt is made to study \( Pb_{1 - x} Y_x \left( {Zr_{0.53} Ti_{0.47} } \right)O_3 \) system with x=0.0125, 0.025, 0.050, 0.075 and 0.1. It was shown that there is limited solid solubility (0.625 mol %) of \( Y_2 O_3 \) or 1.25 mol % of \( Y^{3 + } \) in PZT(53/47). For higher levels of dopant, mainly two other extra second phases were detected. The first was a Zr-rich phase in which some \( Y_{^2 } O_3 \) and small amounts of \( TiO_2 \) was dissolved. The second one was a Pb solid solution \(\left( {PbO_{SS} }\right)\) composed of mainly PbO, \(Y_2 O_3 \) and \(ZrO_2 \) which was initially also seen in calcined samples. The formation of Zr-rich phase is thought possibly to originate due to the sublimation of Pb from \(PbO_{SS}\) source during the sintering process. For higher x values, a structural shift towards Ti-rich region of PZT's phase diagram is seen. All piezoelectric parameters of the doped samples such as \(d_{33}\), \(g_{33}\) are seen to decline sharply compared to that of undoped samples. Increasing the level of dopant gave rise to the increase of conductivity and dielectric loss of sintered samples. The formation of non-ferroelectric extra phases, and the Zr/Ti change of the main formed phase is believed to be responsible for this behavior.

Analysis of a ridged circular disc microstrip antenna element using the cavity model

Abstract: A theoretical analysis of a ridged circular patch microstrip antenna element is presented by modelling it as a cylindrical cavity with a magnetic wall along its edge. The approach is based on conserving the reaction at the ridged position. This antenna, excited in the dominant mode, resonates at higher frequencies and has a wider bandwidth than a simple conventional circular patch antenna with the same size. Design graphs show the variation of the resonant frequency, quality factor, radiation efficiency, feeding position at resonance and bandwidth (VSWR ≤ 2), for the dominant mode as a function of the ratio of inner to outer-disc radius for different ridge thicknesses.