Showing papers by "École Polytechnique de Montréal published in 1996"

Continuous approximation models in freight distribution: An overview

Abstract: We present an overview of continuous approximation models that have been developed for freight distribution problems. A taxonomy of six classes is used to differentiate the problems and a brief review of each paper is provided. This paper stresses important principles and key results from the continuous approximation models. This paper also seeks to highlight gaps in the literature and areas for future work.

Concurrent methods for permeability measurement in resin transfer molding

Abstract: The numerical simulation of the resin transfer molding process (RTM) requires knowledge of the physical properties of the fibrous material. In particular, the resistance to the resin flow is measured by the permeability of the preform in the mathematical model of Darcy's law. A concurrent method for low-cost permeability estimation is proposed. The method uses a rectangular mold for the numerical determination of the principal permeabilities. The experimental data include a built-in correlation with Darcy's law and allow an estimation of both experimental and numerical errors. Since the experimental procedure can introduce a significant uncertainty on the estimated permeability, practical considerations are pointed out and some relevant parameters such as the minimum injected length and maximum injection pressure are introduced to increase the reliability of permeability measurements.

Fundamental limitations on the contrast-transfer efficiency in elastography: an analytic study.

Abstract: Elastography is a new ultrasonic imaging technique introduced to produce images of the Young's modulus distribution of compliant tissue. This Young's modulus distribution is derived from the ultrasonically estimated longitudinal internal strains induced by an external compression of the tissue. The displayed two-dimensional images are called elastograms. Recently, contrast-transfer efficiency, defined as the ratio of elasticity contrast as measured from elastogram to the true contrast, was used to illustrate by simulation the fundamental limitation of elastography in displaying the elastic modulus contrast of soft inclusion in a hard background and vice versa. In this paper, using a classical analytic solution of the elasticity equations derived for an infinite medium subjected to a uniaxial compression, we confirm such earlier simulations results. For this purpose we derive an analytic expression predicting the observed contrast in elastograms.

Function points analysis: an empirical study of its measurement processes

Abstract: Function point analysis (FPA) was initially designed on the basis of expert judgments, without explicit reference to any theoretical foundation. From the point of view of the measurement scales used in its measurement process, FPA constitutes a potpourri of scales not admissible without the transformations imbedded in the implicit models of expert judgments. The results of this empirical study demonstrate that in a homogeneous environment not burdened with major differences in productivity factors there is a clear relationship between FPA's primary components and work-effort. This empirical study also indicates that there is such a relationship for each step of the FPA measurement process prior to the mixing of scales and the assignments of weights. Comparisons with FPA productivity models based on weights confirm, on the one hand, that the weights do not add information and, on the other, that the weights are fairly robust and can be used when little historical data is available. The full data set is provided for future studies.

Biocompatibility testing of NiTi screws using immunohistochemistry on sections containing metallic implants.

Abstract: NiTi is one of the most innovative concepts to have appeared in the field of metallic biomaterials in recent years but its biocompatibility remains controversial. We evaluated the biocompatibility of Nitinol screws using immunohistochemistry to observe the distribution of bone proteins during bone remodeling process around NiTi implant. Results were compared with screws made of Vitallium, c.p. titanium, Duplex austenitic-ferritic stainless steel (SAF), and Stainless Steel 316L. Screws were implanted in rabbit tibia for 3, 6, and 12 weeks. Embedding was performed in the hard resin Technovit, and for the immunohistochemical procedure undecalcified sections with bone-anchored implants could thus be used. The immunostaining method developed seemed to be a reliable technique to stain proteins in undecalcified sections. Biocompatibility results of the NiTi screws compared with the other screws showed a slower osteogenesis process characterized by no close contact between implant and bone, disorganized migration of osteoblasts around the implant, and a lower activity of osteonectin synthesis.

Early development of fatigue cracking at manual fillet welds

Abstract: — An experimental study within the Canadian Offshore Corrosion Fatigue Research Programme was performed on the early development of fatigue cracking along the wavy toe of manual fillet welds between structural steel plates. Stress relieved and as-welded cruciform joints were tested under R =−1 and R= 0 loading at different stress amplitudes. The depth and the opening level of cracks as small as 10–20 μm were monitored using miniature strain gauges installed along the toe apex, in combination with beach marking. Most of the “initiation life” (25% to 50% of total life), conventionally defined by a crack depth of 0.5 mm, is consumed in short crack propagation. Three types of short crack development for different combinations of local mean stress and stress range are identified and analyzed. Growth rates in as-welded specimens are faster than in stress relieved specimens, which results in shorter “initiation lives”. This is associated with a higher effective stress range, particularly under R = - 1 loading where cracks are open over nearly the full stress range. The V-notch stress intensity factor is a promising parameter to rationalize the crack “initiation life”. It takes into account the thickness effect experimentally observed. Under R = - 1 loading of as-welded joints, using R = 0 data and taking the whole stress range gives a reasonably conservative approximation of the crack “initiation life”.

Discrete linear bilevel programming problem

Abstract: In this paper, we analyze some properties of the discrete linear bilevel program for different discretizations of the set of variables We study the geometry of the feasible set and discuss the existence of an optimal solution We also establish equivalences between different classes of discrete linear bilevel programs and particular linear multilevel programming problems These equivalences are based on concave penalty functions and can be used to design penalty function methods for the solution of discrete linear bilevel programs

Diblock copolymers as emulsifying agents in polymer blends: Influence of molecular weight, architecture, and chemical composition

Abstract: Interfacial agents used in the compatibilization of immiscible polymer blends often consist of block copolymers containing at least one segment compatible with each of the two phases of the blend. This work examines the influence of the molecular weight, architecture, and chemical composition of the interfacial agent on its ability to emulsify a polymer blend. The system chosen is a blend containing 80% polystyrene and 20% ethylene-propylene rubber, compatibilized by diblock copolymers of poly(styrene-hydrogenated butadiene). The emulsification curve, which relates the dispersed phase particle size to the concentration of interfacial agent added to the system, was used as a tool to characterize the efficacy of the different interfacial agents. The observed behavior is similar to that of classical emulsions: a rapid drop in phase size at low concentrations of interfacial modifier, followed by a levelling off to an equilibrium diameter value once a “critical” concentration has been reached. For systems compatibilized by symmetrical diblocks (i.e., containing approximately 50% styrene by weight), the volume average particle diameter decreased from 2.7 μm for the unmodified system to about 0.4 μm once interfacial saturation is reached. The critical concentration for emulsification decreased with increasing interfacial agent molecular weight, due to the higher interfacial area occupied by longer molecules; however, this parameter did not affect the equilibrium particle diameter. The asymmetrical diblock copolymer (30% styrene) was found to be less effective than the symmetrical ones over the entire range of concentrations studied (5 to 35% modifier, based on the volume of the minor phase). Asymmetrical diblock copolymers would tend to form micelles, whereas symmetrical copolymers are less constrained at the interface. No significant difference was observed between the emulsifying capability of tapered and pure diblocks of similar composition and molecular weight. © 1996 John Wiley & Sons, Inc.

A comparative study of four open-ended coaxial probe models for permittivity measurements of lossy dielectric/biological materials at microwave frequencies

Abstract: A comparative study of four open-ended coaxial probe models which relate the coaxial line end impedance to the complex permittivity of the material under test is presented. The accuracy of the models in measuring lossy dielectric/biological material and their robustness as a function of the calibration materials are investigated. The four open-ended coaxial probe models studied are: capacitive model, antenna model, virtual line model, and rational function model. Experimental results taken on saline solutions as lossy materials are obtained for the four models.

Molecular Wires: Extended Coupling and Disorder Effects

Abstract: Molecular wires are studied with respect to disorder and to increased electronic connectivity. We find that, with increased connectivity, the tendency to exhibit sharp changes of the conductance with modifications of the bridge structure is reduced. We find that the conductance falls off exponentially with distance for injection energies below the bridge energy band, and shows quantum behavior (oscillatory with wire length) for injection in the band. With disorder, we find that the distribution of conductances is lognormal, the parameters of which depend on disorder and on connectivity.

Characterization of in vivo wear debris from ceramic-ceramic total hip arthroplasties.

Abstract: In contrast to the much-studied mechanism of aseptic loosening of the metal-polyethylene joint couple, the mechanism responsible for failure of ceramic-ceramic (CC) total hip arthroplasties (THAs) has not been evaluated. The aim of this study was to conduct a systematic characterization of the in vivo wear debris from 15 cases of CC THAs revised for aseptic loosening. Two methods were used to evaluate the wear debris; a semiquantitative histological analysis of HE and an evaluation of isolated debris particles using SEM, energy-dispersive X-ray analysis, and image analysis. The three main types of particulate debris identified were titanium alloy (TiAlV) and alumina ceramic (Al2O3) of prosthetic origin, and zirconium dioxide (ZrO2) from the contrast agent used in the cement for prosthetic fixation. Alumina debris was present in the smallest proportion (12%) and was consistent with the low wear rate of the CC joint couple. Zirconium dioxide debris was present in the greatest proportion (76%) and was an unexpected finding. The ZrO2 debris represented microstructural grains of the original ZrO2 particles added as contrast agent to the cement. The presence of a histiocytic foreign body reaction to ZrO2 debris on histologic sections leads us to believe that these particles play an important role in aseptic loosening of the CC THAs evaluated in this study.

Review of failure criteria of fibrous composite materials

Abstract: Composite materials exhibit various and complex failure behavior. Different formalisms have been used to predict failure. Improvement of old theories and new ones continue to be published. In this paper, the most recent and widely used models are presented. Failure criteria such as Tsai-Wu, parametric formulations, maximal stress and strain, Hashin criterion, Hart-Smith criterion, and the method based on kriging are presented. These failure theories may be classified in two categories, depending whether they integrate failure modes or not. The formalism of each theory is briefly described and their application to model failure of composite laminates is discussed by comparing the advantages and limitations of each method. The diversity of experimental failure envelopes, as reported in the literature on composites, is outlined and it is shown that most criteria permit modeling only particular failure properties of composite laminates.

A closed-form approximation for ground return impedance of underground cables

Abstract: Calculation of the ground return impedance generally requires the evaluation of a complex infinite integral. Converging infinite series that usually converge to the exact value have been known for many years, and most handbook formulas were derived from these series, with only the first one or two terms retained. Several approximations with closed-form solutions have been proposed for overhead lines, but nothing seems to have been done in the case of underground cables. This paper describes a complete, simplified closed-form approximation for the self and mutual impedances of underground cables. The simplified equations are first presented and then related to Pollaczek's and Wedephol's equations. Finally, the errors, which in most cases are very satisfactory, are numerically evaluated.

The effect of oxygen concentration on the oxidation of low-carbon steel in the temperature range 1000 to 1250°C

Abstract: This paper describes the oxidation behavior of low-carbon steel samples in binary gas mixtures of oxygen and nitrogen, at oxygen concentrations ranging between 1% and 15% and temperatures ranging between 1000 and 1250°C. Sample weight gains versus time were analyzed, along with measurements and calculations of sample heating rates due to exothermic heat of reaction at the sample surface. It was found that initial rates of oxidation depended on oxygen content in the gasmixture and that these reaction rates were linear up to oxide thicknesses of 0.4 to 0.5 mm. Calculations of linear oxidation rate constants based on equations for mass transport of oxygen in the gas mixture to the sample surface showed good agreement with those measured experimentally, indicating that the initial period of oxidation is controlled by the mass transport of oxygen to the reaction interface. The linear rate constants showed little dependency on temperature, an activation energy of approximately 17kJ/mole being obtained. Measurements of sample surface temperatures have shown that within this linear-oxidation regime, interfacial temperatures of the samples increase with increasing oxygen contents in the gas mixture, owing to exothermic heats of oxidation. Subsequent oxidation kinetics were found to be parabolic. Measured parabolic rates constants were in good agreement with previous investigations, with activation energy values of approximately 127kJ/mole.

Reduced Dynamics with Initial Correlations, and Time-Dependent Environment and Hamiltonians

Abstract: We consider a small system {ital s} in a bath {ital b}, in the case that the state of {ital b} and all Hamiltonians are possibly time dependent. We obtain for the reduced density matrix of {ital s} and exact evolution equation {rho}{sub {ital s}}({ital t})={Lambda}({ital t},{tau}){rho}{sub {ital s}}({tau}), with ({partial_derivative}/{partial_derivative}{ital t}){Lambda}({ital t},{tau})={Gamma}({ital t},{tau}){Lambda}({ital t},{tau}), where {Gamma}({ital t},{tau}) depends on the system-bath correlations at time {tau}. The open evolutor {Lambda}({ital t},{tau}) can (but need not) be chosen completely positive. It is argued that as {ital t}{minus}{tau} increases, {Gamma}({ital t},{tau}){r_arrow}{Gamma}{sub 0}({ital t}) forgets the initial correlations and tends to Lindblad form in time-coarse-grained weak coupling limits. {copyright} {ital 1996 The American Physical Society.}

Dispatching, routing, and scheduling of two automated guided vehicles in a flexible manufacturing system

Abstract: This article presents a new approach for planning the dispatching, conflict-free routing, and scheduling of automated guided vehicles in a flexible manufacturing system. The problem is solved optimally in an integrated manner, contrary to the traditional approach in which the problem is decomposed in three steps that are solved sequentially. The algorithm is based on dynamic programming and is solved on a rolling time horizon. Three dominance criteria are used to limit the size of the state space. The method finds the transportation plan minimizing the makespan (the completion time for all the tasks). Various results are discussed. A heuristic version of the algorithm is also proposed for an extension of the method to many vehicles.

A new control strategy to achieve sinusoidal line current in a cascade buck-boost converter

Abstract: This work presents a detailed theoretical analysis and experimental results of a novel means of obtaining sinusoidal input current and unity power factor (UPF) via a cascade buck-boost power converter. Using the new configuration, sinusoidal line current in phase with the bus voltage is achieved, thanks to a new and simple to implement control strategy. Comparison between the input and output voltages is used to select the instantaneous operating mode of the converter. Offline references are calculated and stored in two EPROM circuits and then compared to measured currents to generate the gating signals of the appropriate switches. Complete theoretical analysis, simulation results and experimental data on a 500 W power converter are presented, to demonstrate the superiority of the new control strategy. Low order harmonics in the input current are eliminated and the input power factor is found to be over 0.99.

Critical evaluation and optimization of the thermodynamic properties and phase diagrams of the CrO-Cr2O3, CrO-Cr2O3-Al2O3, and CrO-Cr2O3-CaO systems

Abstract: Available thermodynamic and phase diagram data have been critically assessed for all phases in the CrO-Cr2O3, CrO-Cr2O2-Al2O3, and CrO-Cr2O2-CaO systems from 298 K to above the liquidus temperatures and for oxygen partial pressures ranging from equilibrium with metallic Cr to equilibrium with air in the case of the first two systems and toP O 2 = 10−3 atm for the CrO-Cr2O3-CaO system. All reliable data have been simultaneously optimized to obtain one set of model equations for the Gibbs energy of the liquid slag and all solid phases as functions of composition and temperature. The modified quasichemical model was used for the slag. The models permit phase equilibria to be calculated for regions of composition, temperature, and oxygen potential where data are not available.

Five Percent Platinum‐Tungsten Oxide‐Based Electrocatalysts for Phosphoric Acid Fuel Cell Cathodes

Abstract: A Pt-tungsten oxide-based electrocatalyst has been fabricated by an inexpensive chemical route for use as an oxygen cathode in 99% phosphoric acid at 180 C. The effect of %WO{sub 3} (wt/wt) on the Pt-tungsten oxide/C-based electrode performance was studied. The electrocatalytic properties for the oxygen reduction reaction (ORR), e.g., exchange current density and mass activity of a 5% Pt-40% WO{sub 3}-based electrode were found to be twice as high as those of 10% Pt, which contains double the amount of platinum. The Tafel slope and specific activity of the two electrodes are similar. It was shown that an increase in its electrochemically active surface area was the only reason for the performance of the 5% Pt-40% WO{sub 3}-based electrode. The electrocatalytic parameters of the 5% Pt-40% WO{sub 3}-based electrode for the ORR were compared to those of the 2% Pt-1% H{sub 2}WO{sub 4}-based electrode.

Single station magnetotelluric impedance estimation: Coherence weighting and the regression M-estimate

Abstract: The first step in the interpretation of magnetotelluric (MT) data involves estimating 101-102 frequency‐domain impedances, Z(w), from the raw electric and magnetic field time series e(t), h(t) (approx. 106 real numbers/site) [e.g., Swift, 1967, Sims et al., 1971]. Superficially, this initial data reduction step is almost trivial. Making the usual MT assumptions that the external sources are spatially uniform, and allowing for noise in the simplest way, e and h are related in the frequency domain via the linear statistical model e = Zh + e, (1) where e represents noise. The impedance Z can then be estimated quite simply by Fourier transforming the time series, and using linear least squares (LS] to minimize the misfit to equation (1) [Sims et al., 1971]. Unfortunately, this simple approach can fail catastrophically for noisy data, producing estimates that are heavily biased or wildly oscillatory [e.g., Gamble et al., 1979a; Jones et al., 1989; Figure 1]. As a consequence a number of refinements to the simp...

Optimization-Based Multifrequency Test Generation for Analog Circuits

Abstract: A robust test set for analog circuits has to detect faults under maximal masking effects due to variations of circuit parameters in their tolerance box. In this paper we propose an optimization based multifrequency test generation method for detecting parametric faults in linear analog circuits. Given a set of performances and a frequency range, our approach selects the test frequencies that maximize the observability on a circuit performance of a parameter deviation under the worst masking effects of normal variations of the other parameters. Experimental results are provided and validated by HSpice simulations to illustrate the proposed approach.