Modeling and prediction of bulk properties of open-cell carbon foam

The emerging ultralightweight material, carbon foam, was modeled with three-dimensional microstructures to develop a basic understanding in correlating microstructural configuration with bulk performance of open-cell foam materials. Because of the randomness and complexity of the microstructure of the carbon foam, representative cell ligaments were first characterized in detail at the microstructural level. The salient microstructural characteristics (or properties) were then correlated with the bulk properties through the present model. In order to implement the varying anisotropic nature of material properties in the foam ligaments, we made an attempt to use a finite element method to implement such variation along the ligaments as well as at a nodal point where the ligaments meet. The model was expected to provide a basis for establishing a process-property relationship and optimizing foam properties.The present model yielded a fairly reasonable prediction of the effective bulk properties of the foams. We observed that the effective elastic properties of the foams were dominated by the bending mode associated with shear deformation. The effective Young's modulus of the foam was strongly influenced by the ligament moduli, but was not influenced by the ligament Poisson's ratio. The effective Poisson's ratio of the foam was practically independent of the ligament Young's modulus, but dependent on the ligament Poisson's ratio. The effective Young's modulus of the carbon foam was dependent more on the transverse Young's modulus and the shear moduli of the foam ligaments, but less significantly on the ligament longitudinal Young's modulus. A parametric study indicated that the effective Young's modulus was significantly improved by increasing the solid modulus in the middle of the foam ligaments, but nearly invariant with that at the nodal point where the ligaments meet. Therefore, appropriate processing schemes toward improving the transverse and shear properties of the foam ligaments in the middle section of the ligaments rather than at the nodal points are highly desirable for enhancing the bulk moduli of the carbon foam.     

Turbulent flow structure at concordant and discordant open-channel confluences

Models of flow at river-channel confluences that consist of two concordant confluent channels with avalanche faces dipping into a scour zone are limited because this morphology may be the exception rather than the rule in nature. In this paper the mean and turbulent flow structure in the streamwise and vertical directions at both concordant and discordant laboratory confluences were examined in order to determine the effect of bed discordance on the flow field, and to assess its influence on sediment transport. Instantaneous velocities were measured with a laser Doppler anemometer using a dense spatial sampling grid. The spatial distribution of normal stress varies with bed geometry as bed discordance generates a distortion of the mixing layer between the confluent streams. Turbulent shear stress is larger in the discordant bed case and its peak is associated with the position of the mixing layer whereas for concordant beds the zone of mixing is characterised by a decrease in the Reynolds shear stress. Quadrant analysis also revealed differential dominating quadrants between the two bed geometries which will influence sediment transport routing and, consequently, the resulting bed morphology. These results highlight the need for significant modifications to current models of confluence flow dynamics in order to account for the bed configuration.We would like to thank Phil Fields of the Earth Sciences workshop at Leeds for his skillful construction of the confluence model and continued help during the course of this research. The LDA facility has been funded through grants from the UFC, NERC and University of Leeds. PB thanks NSERC for financial support and the Fonds FCAR for funding an eight month study-visit held at the Department of Earth Sciences, University of Leeds. AGR acknowledges the financial support of NSERC whilst JLB is grateful for the award of a Nuffield Science Foundation Fellowship which facilitated the preparation of this paper. We thank two anonymous reviewers for their constructive criticisms that helped clarifying the paper.     

Fluides viscoélastiques non linéaires satisfaisant à un principe de superposition étude théorique et experimentale

Summary This study is devoted to incompressible viscoelastic fluids in which a superposition principle for stress is assumed. In order to respect the material objectivity, this summation of incremental stress, with corresponding memory factor, is operated in a rheological (corotational) frame. Theoretical study for these fluids in viscometric flows was conducted up to explicit solutions. This study shows some impossibility ranges for such flows. The determination of the memory functions from this type of flow involvesFourier transform and so, would need the knowledge of all the kinematic range, and this is impossible.The study in relaxation tests permits the determination of the memory functions. So, we performed these experiments, to get memory functions which were independent of the kinematics.Using the function so obtained, we can compute and predict some experimental results in steady flows; and the comparison between these two results was quite good.

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Zusammenfassung Wir untersuchen inkompressible viskoelastische Fluide, die einem Superpositionsprinzip für die Spannungen genügen. Um dem Prinzip der Bezugsindifferenz zu entsprechen, wird die Summation der Spannungs-inkremente in einem korotationalen Bezugssystem vorgenommen. Die theoretische Untersuchung wird für viskosimetrische Strömungen mit kreisförmigen oder geradlinigen Stromlinien für einfache Randwertprobleme bis zu expliziten Lösungen durchgeführt. Dabei findet man, daß es Bereiche gibt, für die solche Strömungen nicht existieren. Die Bestimmung der Gedächtnisfunktion aus diesen Strömungsformen würde eineFourier-Transformation erfordern und darum die Kenntnis des gesamten kinematischen Bereichs voraussetzen, was nach vorstehendem Ergebnis unmöglich ist.Mit Hilfe von Relaxationsversuchen kann man die Gedächtnisfunktion unmittelbar erschließen. Durch solche Experimente erhalten wir Gedächtnisfunktionen, die von der Kinematik unabhängig sind. Unter Verwendung dieser Funktionen lassen sich einige theoretische Aussagen über stationäre Strömungen erhalten, die mit dem Experiment verglichen werden können. Es ergibt sich eine recht gute Übereinstimmung.

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Avec 6 figures     

Dispersive bending waves in uniform bars

The traveling bending waves in a long beam of rectangular cross section were measured and calculated. The bending waves were induced by impacting with a steel sphere and measured with strain gages at several distances from the point of impact. The impact force was calculated as a function of time by integrating the dynamic equations of the sphere and the beam. The force spectrum was then found using a fast-Fourier-transform (FFT) calculation and multiplied by the moment-frequency response of the beam to determine the moment spectrum. The moment-time function was calculated by an inverse FFT. The traveling wave is dispersive; its spectrum was found from that at the point of impact by phase shifting each component by an angle proportional to the distance and to the square root of the frequency. Again the time curve was determined by an inverse FFT. The indentation stiffness of the beam was found to be very much less than that of the elastic half space because of transverse bending. After the impact force was recalculated with this correction, the calculated moment-time traces agreed very well with the measured ones.     

Seismic Evaluation of Multi-Storey RC Frame Using Modal Pushover Analysis

The recently developed pushover analysis procedure has led a new dimension to performance-based design in structural engineering practices. With the increase in the magnitude of monotonic loading, weak links and failure modes in the multi-storey RC frames are usually formed. The force distribution and storey displacements are evaluated using static pushover analysis based on the assumption that the response is controlled by fundamental mode and no mode shift takes place. Himalayan-Nagalushai region, Indo-Gangetic plain, Western India, Kutch and Kathiawar regions are geologically unstable parts of India and some devastating earthquakes of remarkable intensity have occurred here. In view of the intensive construction activity in India, where even a medium intensity tremor can cause a calamity, the authors feel that a completely up-to-date, versatile method of aseismic analysis and design of structures are essential. A detailed dynamic analysis of a 10-storey RC frame building is therefore performed using response spectrum method based on Indian Standard Codal Provisions and base shear, storey shear and storey drifts are computed. A modal pushover analysis (MPA) is also carried out to determine the structural response of the same model for the same acceleration spectra used in the earlier case. The major focus of study is to bring out the superiority of pushover analysis method over the conventional dynamic analysis method recommended by the code. The results obtained from the numerical studies show that the response spectrum method underestimates the response of the model in comparison with modal pushover analysis. It is also seen that modal participation of higher modes contributes to better results of the response distribution along the height of the building. Also pushover curves are plotted to illustrate the displacement as a function of base shear.     

Synthesis, electrical and thermal properties of Bi4V2−xMexO11 (x = 0.0 and 0.02) ceramics

Polycrystalline ceramic samples of Bi₄V_2?xMe_xO₁₁ (Me = Nb, Zr, Y and Cu and x = 0.0 and 0.02) have been synthesized by standard solid state reaction method using high purity oxides. The formation of the compounds have been analysed by X-ray diffraction method. The dielectric constant, dielectric loss and AC conductivity as a function of frequency and temperature have been measured. The dielectric studies indicate that the material is highly lossy and hence its AC conductivity increases with the increase of temperature. The DC conductivity of material has been measured as a function of temperature from room temperature to 380 °C and its activation energy was calculated using the relation σ = σ ₀exp (?E _a/kT). The modulated differential scanning calorimetry has been used to investigate the effect of substitution on the heat capacity and heat flow of the compounds. The results are discussed in detail.     

Molecular Composites Comprising Rodlike Polyamides and Vinyl Polymers

Abstract

At present we have strong evidence that several members of a series of wholly-aromatic, para-linked, rodlike polyamides, polyesters, and polyesteramides form molecular composites with certain flexible-chain, thermoplastic polymers over a wide range of compositions. This paper reports on the initial results of an investigation of intermolecular interactions using spectroscopy and various scattering techniques as well as characterization of some of the mechanical and optical properties of these materials. The composites are made by two techniques: 1) photo-polymerization of a homogeneous solution of a rodlike polymer in a monomer containing a photoinitiation; 2) solvent evaporation from homogeneous solutions of very limited combinations of solvent, rodlike polymers and flexible polymers. While both of these techniques produce optically clear, nonscattering films of various thicknesses over the entire compositional range, e.g., 1–99 wt% of rodlike polymer, the latter is generally more convenient and has been used extensively in this study. Optical and electron microscopy, wide angle light scattering, and spectroscopic and thermal analysis support the view that these polymer combinations are truly molecularly dispersed.     

First Bis(σ)-borane Complexes of Group 6 Transition Metals: Experimental and Theoretical Studies

A series of bis(σ)-borane complexes of Group 6 transition metals were prepared by direct dihydroborane coordination to the metal center. Reaction of [M(CO)₃(PCy₃)₂] and two dihydroboranes [DurBH₂] and [(Me₃Si)₂NBH₂] (Dur=2,3,5,6-Me₄C₆H) yielded bis(σ)-borane complexes fac-[M(CO)₃(PCy₃){η²-(H₂BR)}] (R=Dur; 1 : M=Cr, 2 : M=W; R=N(SiMe₃)₂; 3 : M=Cr, 4 : M=W). In the case of molybdenum, we have isolated an arene complex ( 5 ) with [DurBH₂] in which the Dur group acts as a η⁶-bound ligand, and with [(Me₃Si)₂NBH₂] a similar bis(σ)-borane complex was isolated, cis,trans-[Mo(CO)₂(PCy₃)₂{η²-(H₂BN(SiMe₃)₂}] ( 6 ), with a different pattern of auxiliary ligands. The complexes were investigated by multinuclear NMR spectroscopy, mass spectrometry, X-ray diffraction analysis, and computational methods. Quantum theory of atoms in molecules (QTAIM) calculations demonstrated that the borane complexes may be described as pure bis(σ)-borane complexes rather than elongated or stretched examples given that the calculations do not show the presence of a ring-critical point (RCP) at the ring formed by the interactions of the B−H with metal center.