Stability and mobility of sand-bed channels affected by seepage

Seepage effects on the stability, mobility, and incipient motion of sand-bed particles are experimentally investigated. Seepage through a sand bed in a downward direction (suction) reduces the stability of particles, and it can even initiate their movement. The bed erosion is increased with the increased rates of suction. Whereas the seepage in an upward direction (injection) increases the stability of bed particles, it does not aid initiating their movement. The rate of bed erosion is reduced or even stopped by the increased infection rates. Hydrodynamic conditions leading to the so-called "pseudoincipient motion'' with suction (for the initiation of particles movement that are otherwise at rest under no-seepage conditions), and with injection (for only arresting the particles movement that are otherwise moving initially) are evaluated. The conventional Shields curve cannot be used to predict such pseudoincipient motion conditions with seepage. The concepts thus developed are useful for a better understanding of the sediment transport mechanics and in the design of stable alluvial channels affected by seepage.

Tensile flow behaviour of 2.25Cr-1Mo ferritic steel base metal and simulated heat affected zone structures of 2.25Cr-1Mo weld joint

Tensile tests in the temperature range 298 to 873 K have been performed on 2.25Cr-1Mo base metal and simulated heat affected zone (HAZ) structures of its weld joint, namely coarse grain bainite, fine grain bainite and intercritical structure. Tensile flow behaviour of all the microstructural conditions could be adequately described by the Hollomon equation (sigma = K-1 epsilon(n1)) at higher (> 623 K) temperatures. Deviation from the Hollomon equation was observed at low strains and lower (< 623 K) temperatures. The Ludwigson modification of Hollomon's equation, sigma = K-1 epsilon(n1) + exp (K-2 + n(2) epsilon), was found to describe the flow curve. In general, the flow parameters n(1), K-1, n(2) and K-2 were found to decrease with increase in temperature except in the intermediate temperature range (423 to 623 K). Peaks/plateaus were observed in their variation with temperature in the intermediate temperature range coinciding with the occurrence of serrated flow in the load-elongation curve. The n(1) Value increased and the K-1 value decreased with the type of microstructure in the order: coarse grain bainite, fine grain bainite, base metal and intercritical structure. The variation of nl with microstructure has been rationalized on the basis of mean free path (MFP) of dislocations which is directly related to the inter-particle spacing. Larger MFP of dislocations lead to higher strain hardening exponents n(1).

Converging swirling liquid jets from pressure swirl atomizers: Effect of inner air pressure

Converging swirling liquid jets from pressure swirl atomizers injected into atmospheric air are studied experimentally using still and cine photographic techniques in the context of liquid-liquid coaxial swirl atomizers used in liquid rocket engines. The jet exhibits several interesting flow features in contrast to the nonswirling liquid jets (annular liquid jets) studied in the literature. The swirl motion creates multiple converging sections in the jet, which gradually collapse one after the other due to the liquid sheet breakup with increasing Weber number (We). This is clearly related to the air inside the converging jet which exhibits a peculiar variation of the pressure difference across the liquid sheet, DeltaP, with We. The variation shows a decreasing trend of DeltaP with We in an overall sense, but exhibits local maxima and minima at specific flow conditions. The number of maxima or minima observed in the curve depends on the number of converging sections seen in the jet at the lowest We. An interesting feature of this variation is that it delineates the regions of prominent jet flow features like the oscillating jet region, nonoscillating jet region, number of converging sections, and so on. Numerical predictions of the jet characteristics are obtained by modifying an existing nonswirling liquid jet model by including the swirling motion. The comparison between the experimental and numerical measurements shows that the pressure difference across the liquid sheet is important for the jet behavior and cannot be neglected in any theoretical analysis. (C) 2002 American Institute of Physics.

Fracture energy and softening behavior of high-strength concrete

An experimental investigation on the fracture properties of high-strength concrete (HSC) is reported. Three-point bend beam specimens of size 100 x 100 x 500 mm were used as per RILEM-FMC 50 recommendations. The influence of maximum size of coarse aggregate on fracture energy, fracture toughness, and characteristic length of concrete has been studied. The compressive strength of concrete ranged between 40 and 75 MPa. Relatively brittle fracture behavior was observed with the increase in compressive strength. The load-CMOD relationship is linear in the ascending portion and gradually drops off after the peak value in the descending portion. The length of the tail end portion of the softening curve increases as the size of coarse aggregate increases. The fracture energy increases as the maximum size of coarse aggregate and compressive strength of concrete increase. The characteristic length of concrete increases with the maximum size of coarse aggregate and decreases as the compressive strength increases, (C) 2002 Elsevier Science Ltd. All rights reserved.

Investigation of reversible and irreversible polarizations in thin films of SrBi2(Ta-0.5,Nb-0.5)(2)O-9

The reversible and irreversible components of the total polarization in a thin film of SrBi2(Ta-0.5,Nb-0.5)(2)O-9 were calculated. The C-V loop was integrated to obtain the reversible part of the total polarization. The reversible polarization was only 20% of the total polarization and showed almost no hysteresis. However, the dielectric constant due to the total polarization was almost the same as that for the reversible polarization in the saturation region of the large signal P-E hysteresis loop. The reversible part was subtracted from the total polarization to calculate the irreversible counterpart of it. The irreversible polarization showed a near-square shaped hysteresis loop, while the reversible polarization was obeying the Rayleigh law. The small signal hysteresis was simulated from the parameters obtained from the Rayleigh-curve fit with the experimental curve and then it was compared with the result obtained from direct measurement with small amplitude. (C) 2002 Elsevier Science B.V. All rights reserved.

Nonlinear Finite-Element Modeling of Batter Piles under Lateral Load

In this paper, a finite-element model is developed in which the nonlinear soil behavior is represented by a hyperbolic relation for static load condition and modified hyperbolic relation, which includes both degradation and gap for a cyclic load condition. Although batter piles are subjected to lateral load, the soil resistance is also governed by axial load, which is incorporated by considering the P-Δ moment and geometric stiffness matrix. By adopting the developed numerical model, static and cyclic load analyses are performed adopting an incremental-iterative procedure where the pile is idealized as beam elements and the soil as elastoplastic spring elements. The proposed numerical model is validated with published laboratory and field pile test results under both static and cyclic load conditions. This paper highlights the importance of the degradation factor and its influence on the soil resistance-displacement (p-y) curve, number of cycles of loading, and cyclic load response.

Studies on compressive failure features in syntactic foam material

Syntactic foam made by mechanical mixing of glass hollow spheres in epoxy resin matrix is characterized for compressive properties in the present study. Volume fraction of hollow spheres in the syntactic foam under investigation is kept at 67.8%. Effect of specimen aspect ratio on failure behavior and stress-strain curve of the material is highlighted. Considerable differences are noted in the macroscopic fracture features of the specimen and the stress-strain curve with the variation in specimen aspect ratio, although compressive yield strength values were within a narrow range. Post compression test scanning electron microscopic observations coupled with the macroscopic observations taken during the test helped in explaining the deviation in specimen behavior and in gathering support for the proposed arguments.

Water content - void ratio swell-shrink paths of compacted expansive soils

This paper addresses the behaviour of compacted expansive soils under swell-shrink cycles. Laboratory cyclic swell-shrink tests were conducted on compacted specimens of two expansive soils at surcharge pressures of 6.25, 50.00, and 100.00 kPa. The void ratio and water content of the specimens at several intermediate stages during swelling until the end of swelling and during shrinkage until the end of shrinkage were determined to trace the water content versus void ratio paths with an increasing number of swell-shrink cycles. The test results showed that the swell-shrink path was reversible once the soil reached an equilibrium stage where the vertical deformations during swelling and shrinkage were the same. This usually occurred after about four swell-shrink cycles. The swelling and shrinkage path of each specimen subjected to full swelling - full shrinkage cycles showed an S-shaped curve (two curvilinear portions and a linear portion). However, the swelling and shrinkage path occurred as a part of the S-shaped curve, when the specimen was subjected to full swelling - partial shrinkage cycles. More than 80% of the total volumetric change and more than 50% of the total vertical deformation occurred in the central linear portion of the S-shaped curve. The volumetric change was essentially parallel to the saturation line within a degree of saturation range of 50-80% for the equilibrium cycle. The primary value of the swell-shrink path is to provide information regarding the void ratio change that would occur for a given change in water content for any possible swell-shrink pattern. It is suggested that these swell-shrink paths can be established with a limited number of tests in the laboratory.

A Grobner basis for certain affine monomial curves

Let K be a field and let m(0),...,m(e-1) be a sequence of positive integers. Let W be a monomial curve in the affine e-space A(K)(e), defined parametrically by X-0 = T-m0,...,Xe-1 = Tme-1 and let p be the defining ideal of W. In this article, we assume that some e-1 terms of m(0), m(e-1) form an arithmetic sequence and produce a Grobner basis for p.

A method of tracking the peak power points for a variable speed wind energy conversion system

In this paper, a method of tracking the peak power in a wind energy conversion system (WECS) is proposed, which is independent of the turbine parameters and air density. The algorithm searches for the peak power by varying the speed in the desired direction. The generator is operated in the speed control mode with the speed reference being dynamically modified in accordance with the magnitude and direction of change of active power. The peak power points in the P-omega curve correspond to dP/domega = 0. This fact is made use of in the optimum point search algorithm. The generator considered is a wound rotor induction machine whose stator is connected directly to the grid and the rotor is fed through back-to-back pulse-width-modulation (PWM) converters. Stator flux-oriented vector control is applied to control the active and reactive current loops independently. The turbine characteristics are generated by a dc motor fed from a commercial dc drive. All of the control loops are executed by a single-chip digital signal processor (DSP) controller TMS320F240. Experimental results show that the performance of the control algorithm compares well with the conventional torque control method.

On the Magnetic Ground State of La(0.85)Sr(0.15)CoO(3) Single Crystals

We present an extensive study on magnetic and transport properties of La(0.85)Sr(0.15)CoO(3) single crystals grown by a float zone method to address the issue of phase separation versus spin-glass (SG) behavior. The dc magnetization study reveals a kink in field-cooled magnetization, and the peak in the zero-field-cooling curve shifts to lower temperature at modest dc fields, indicating the SG magnetic phase. The ac susceptibility study exhibits a considerable frequency-dependent peak shift (similar to 4 K) and a time-dependent memory effect below the freezing temperature. In addition, the characteristic time scale tau(0) estimated from the frequency-dependent ac susceptibility measurement is found to be similar to 10(-13) s, which matches well with typical values observed in canonical SG systems. The transport relaxation study evidently demonstrates the time-dependent glassy phenomena. In essence, all our experimental results corroborate the existence of SG behavior in La(0.85)Sr(0.15)CoO(3) single crystals.

Effect of Post-Metallization Hydrogen Annealing on C-V Characteristic of Zirconia Grown Using Atomic Layer Deposition

Substantial amount of fixed charge present in most of the alternative gate dielectrics gives rise to large shifts in the flat-band voltage (VFB) and charge trapping and de-trapping causes hysterectic changes on voltage cycling. Both phenomena affect stable and reliable transistor operation. In this paper we have studied for the first time the effect of post-metallization hydrogen annealing on the C-V curve of MOS capacitors employing zirconia, one of the most promising gate dielectric. Samples were annealed in hydrogen ambient for up to 30 minutes at different temperatures ranging from room temperature to 400°C. C-V measurements were done after annealing at each temperature and the hysteresis width was calculated from the C-V curves. A minimum hysteresis width of ∼35 mV was observed on annealing the sample at 200°C confirming the excellent suitability of this dielectric

Interface states of laser ablated BaTiO3 and Ba0.9Ca0.1TiO3 thin films in MFS structure determined by DLTS and C – V technique

BaTiO3 and Ba0.9Ca0.1TiO3 thin films were deposited on the p – type Si substrate by pulsed excimer laser ablation technique. The Capacitance – Voltage (C-V) measurement measured at 1 MHz exhibited a clockwise rotating hysteresis loop with a wide memory window for the Metal – Ferroelectric – Semiconductor (MFS) capacitor confirming the ferroelectric nature. The low frequency C – V measurements exhibited the response of the minority carriers in the inversion region while at 1 MHz the C – V is of a high frequency type with minimum capacitance in the inversion region. The interface states of both the MFS structures were calculated from the Castagne – Vaipaille method (High – low frequency C – V curve). Deep Level Transient Spectroscopy (DLTS) was used to analyze the interface traps and capture cross section present in the MFS capacitor. There were distinct peaks present in the DLTS spectrum and these peaks were attributed to the presence of the discrete interface states present at the semiconductor – ferroelectric interface. The distribution of calculated interface states were mapped with the silicon energy band gap for both the undoped and Ca doped BaTiO3 thin films using both the C – V and DLTS method. The interface states of the Ca doped BaTiO3 thin films were found to be higher than the pure BaTiO3 thin films.

MFIS and MFS structures using SrBi2Ta2O9 thin films for the FRAM applications

Recently there is an increasing demand and extensive research on high density memories, in particular to the ferroelectric random access memory composed of 1T/1C (1 transistor/1 capacitor) or 2T/2C. FRAM's exhibit fast random acess in read/write mode, non - volatility and low power for good performance. An integration of the ferroelectric on Si is the key importance and in this regard, there had been various models proposed like MFS, MFIS, MFMIS structure etc., Choosing the proper insulator is very essential for the better performance of the device and to exhibit excellent electrical characteristics. ZrTiO4 is a potential candidate because of its excellent thermal stability and lattice match on the Si substrate. SrBi2Ta2O9 and ZrTiO4 thin films were prepared on p - type Si substrate by pulsed excimer laser ablation technique. Optimization of both ZT and SBT thin films in MFS and MFIS structure had been done based on the annealing, oxygen partial pressures and substrate temperatures to have proper texture of the thin films. The dc leakage current, P - E hysteresis, capacitance - voltage and conductance - voltage measurement were carried out. The effect of the frequency dependence on MFIS structure was observed in the C – V curve. It displays a transition of C - V curve from high frequency to low frequency curve on subjection to varied frequencies. Density of interface states has been calculated using Terman and high - low frequency C - V curve. The effect of memory window in the C - V hysteresis were analysed in terms of film thickness and annealing temperatures. DC conduction mechanism were analysed in terms of poole - frenkel, Schottky and space charge limited conduction separately on MFS, MIS structure.

MFIS and MFS structures using SrBi2Ta2O9 thin films for the FRAM applications

Recently there is an increasing demand and extensive research on high density memories, in particular to the ferroelectric random access memory composed of 1T/1C (1 transistor/1 capacitor) or 2T/2C. FRAM's exhibit fast random acess in read/write mode, non - volatility and low power for good performance. An integration of the ferroelectric on Si is the key importance and in this regard, there had been various models proposed like MFS, MFIS, MFMIS structure etc., Choosing the proper insulator is very essential for the better performance of the device and to exhibit excellent electrical characteristics. ZrTiO4 is a potential candidate because of its excellent thermal stability and lattice match on the Si substrate. SrBi2Ta2O9 and ZrTiO4 thin films were prepared on p - type Si substrate by pulsed excimer laser ablation technique. Optimization of both ZT and SBT thin films in MFS and MFIS structure had been done based on the annealing, oxygen partial pressures and substrate temperatures to have proper texture of the thin films. The dc leakage current, P - E hysteresis, capacitance - voltage and conductance - voltage measurement were carried out. The effect of the frequency dependence on MFIS structure was observed in the C – V curve. It displays a transition of C - V curve from high frequency to low frequency curve on subjection to varied frequencies. Density of interface states has been calculated using Terman and high - low frequency C - V curve. The effect of memory window in the C - V hysteresis were analysed in terms of film thickness and annealing temperatures. DC conduction mechanism were analysed in terms of poole - frenkel, Schottky and space charge limited conduction separately on MFS, MIS structure.