Deformational Behavior of Fiber-Reinforced Concrete Beams in Bending

An experimental study aimed at understanding the deformational behavior of conventionally reinforced steel fiber concrete beams in pure bending is reported in this paper. One group of beams has steel fibers dispersed in the entire volume of the beam and the second has fibers dispersed over half the depth of the beam on the tension side. A comparative study of the deformational characteristics of these beams has been made. Half-depth fiber inclusion, requiring only half the quantity of fibers of full-depth inclusion, is found to be equally effective in improving the deformational behavior of beams. Thus, by such modes of inclusion of fibers, an economical and efficient use of expensive steel fibers can be realized.

Consistency limits behavior of bentonites exposed to sea water

This article examines the changes in interparticle forces brought about on prolonged contact (1 year period) of a bentonite clay with artificial seawater. The study is undertaken with the purpose of identifying the physico-chemical factors that impart a nonswelling character to smectite clays deposited in marine environments. Results show that equilibration of the bentonite clay with artificial seawater (total pore salinity approximately 42 gL-1) for a 1 year period does not lead to any mineralogical changes in the clay specimens; however, their exchangeable cation positions become prominently dominated by magnesium ions. The consistency limits of the seawater-equilibrated bentonite was determined on stepwise leaching to lower salinities. The predominance of diffuse double-layer repulsion forces in the pore salt concentration range of 42 gL-1 to 1.1 gL-1 caused an increase in the liquid limits of the seawater-equilibrated bentonite specimens on reducing the salinity in the corresponding range (42 gL-1 to 1.1 gL-1). The attraction forces, however, prevail over the repulsion forces at salt concentrations <1.1 gL-1 and cause a decrease in liquid limit of the clay specimens with reduction in pore salinity, which is typical of nonswelling clays. The attraction forces cause aggregation of the clay unit layers into domains that break down on sodium saturation of the clay specimens. It is inferred that the physico-chemical factors responsible for the nonswelling character of the seawater-equilibrated bentonite specimens at pore salt concentrations below 1.1 gL-1 are inadequate to explain the nonswelling character of smectite-rich Ariake marine clays. The lower consistency limits of the Ariake marine clays in comparison to the nonswelling character, seawater-equilibrated bentonite specimens is attributed to a relative deficiency of interparticle forces in the Ariake marine clay.

Evidence for decoupled two-dimensional vortex behavior of YBa2Cu3O7-delta in La0.7Sr0.3MnO3/YBa2Cu3O7-delta/La0.7Sr0.3MnO3 trilayer

We investigate the vortex behavior of YBa2Cu3O7-delta thin films sandwiched between two ferromagnetic layers (La0.7Sr0.3MnO3/YBa2Cu3O7-delta/La0.7Sr0.3MnO3). The magnetization study on La0.7Sr0.3MnO3/YBa2Cu3O7-delta/La0.7Sr0.3MnO3 trilayers conspicuously shows the presence of both ferromagnetic and diamagnetic phases. The magnetotransport study on the trilayers reveals a significant reduction in the activation energy (U) for the vortex motion in YBa2Cu3O7-delta. Besides, the ``U'' exhibits a logarithmic dependence on the applied magnetic field which directly indicates the existence of decoupled two-dimensional (2D) pancake vortices present in the CuO2 layers. The evidence of 2D decoupled vortex behavior in La0.7Sr0.3MnO3/YBa2Cu3O7-delta/La0.7Sr0.3MnO3 is believed to arise from (a) the weakening of superconducting coherence length along the c-axis and (b) enhanced intraplane vortex-vortex interaction due to the presence of ferromagnetic layers. (C) 2010 American Institute of Physics. doi: 10.1063/1.3524545]

Temperature dependent electrical transport behavior of InN/GaN heterostructure based Schottky diodes

InN/GaN heterostructure based Schottky diodes were fabricated by plasma-assisted molecular beam epitaxy. The temperature dependent electrical transport properties were carried out for InN/GaN heterostructure. The barrier height and the ideality factor of the Schottky diodes were found to be temperature dependent. The temperature dependence of the barrier height indicates that the Schottky barrier height is inhomogeneous in nature at the heterostructure interface. The higher value of the ideality factor and its temperature dependence suggest that the current transport is primarily dominated by thermionic field emission (TFE) other than thermionic emission (TE). The room temperature barrier height obtained by using TE and TFE models were 1.08 and 1.43 eV, respectively. (C) 2011 American Institute of Physics. doi: 10.1063/1.3549685]

Behavior of Ca(OH)2/CaO pellet under dehydration and hydration

With construction of a thermochemical energy conversion prototype system to store solar heat, thermal dissociation of pellets of Ca(OH)2 and hydration of CaO have been investigated in some detail for its application to the system. The inorganic substance is very attractive as a material for long term heat storage, but molar density changes associated with the reaction are fairly large. Therefore, this factor has been taken into account in the kinetic equation. The importance of additives and pellet size has been discussed considering reactivity and strength of pellets. An analysis has been attempted when chemical reaction is important. The deformation of pellets was observed during hydration.

High-temperature low-cycle fatigue behavior of a NIMONIC PE-16 superalloy—Correlation with deformation and fracture

Low-cycle fatigue (LCF) responses of NIMONIC PE-16 for various prior microstructures and strain amplitudes have been evaluated and the fatigue behavior has been explained in terms of the operative deformation mechanisms. Total strain-controlled LCF tests were performed at 923 K on samples possessing three different prior microstructures: alloy A in solution-annealed condition (free of γ′ and carbides), alloy B with double aging treatment (spherical γ′ of 18-nm diameter and M23C6), and alloy C with another double aging treatment (γ′ of size 35 nm, MC and M23C6). All three microstructures exhibited an intial cyclic hardening followed by a period of gradual softening at 923 K. Coffin-Manson plots describing the plastic strain amplitudevs number of reversals to failure showed that alloy A had maximum fatigue life while C showed the least. Alloy B exhibited a two-slope behavior in the Coffin-Manson plot over the strain amplitudes investigated. This has been ascribed to the change in the degree of homogeneity of deformation at high and low strain amplitudes. Transmission electron microscopic studies were carried out to characterize the various deformation mechanisms and precipitation reactions occurring during fatigue testign. Fresh precipitation of fine γ′ was confirmed by the development of “mottled contrast” in alloy C. Evidence for the shearing of the ordered γ′ precipitates was revealed by the presence of superdislocations in alloy C. Repeated shearing during cyclic loading led to the reduction in the size of the γ′ and consequent softening. Coarser γ′ precipitates were associated with Orowan loops. The observed fatigue behavior has been rationalized based on the micromechanisms stated above and on the degree of homogenization of slip assessed by slipband spacing measurements on tested samples.

Probing potential energy surfaces in confined systems: behavior of mean-square displacement in zeolites

Time evolution of mean-squared displacement based on molecular dynamics for a variety of adsorbate-zeolite systems is reported. Transition from ballistic to diffusive behavior is observed for all the systems. The transition times are found to be system dependent and show different types of dependence on temperature. Model calculations on a one-dimensional system are carried out which show that the characteristic length and transition times are dependent on the distance between the barriers, their heights, and temperature. In light of these findings, it is shown that it is possible to obtain valuable information about the average potential energy surface sampled under specific external conditions.

Structural Behavior of Story-High Cement-Stabilized Rammed-Earth Walls under Compression

A rammed-earth wall is a monolithic construction made by compacting processed soil in progressive layers in a rigid formwork. There is a growing interest in using this low-embodied-carbon building material in buildings. The paper investigates the strength and structural behavior of story-high cement-stabilized rammed-earth (CSRE) walls, reviews literature on the strength of CSRE, and discusses results of the compressive strength of CSRE prisms, wallettes, and story-high walls. The strength of the story-high wall was compared with the strength of wallettes and prisms. There is a nearly 30% reduction in strength as the height-to-thickness ratio increases from about 5 to 20. The ultimate compressive strength of CSRE walls predicted using the tangent modulus theory is in close agreement with the experimental values. The shear failures noticed in the story-high walls resemble the shear failures of short-height prism and wallette specimens. The paper ends with a discussion of structural design and characteristic compressive strength of CSRE walls. DOI: 10.1061/(ASCE)MT.1943-5533.0000155. (C) 2011 American Society of Civil Engineers.

Applicability of Butler's equation in interpreting the thermodynamic behavior of surfaces and adsorption in Fe-S-O melts

Expressions for various second-order derivatives of surface tension with respect to composition at infinite dilution in terms of the interaction parameters of the surface and those of the bulk phases of dilute ternary melts have been presented. A method of deducing the parameters, which consists of repeated differentiation of Butler's equations with subsequent application of the appropriate boundary conditions, has been developed. The present investigation calculates the surface tension and adsorption functions of the Fe-S-O melts at 1873 and 1923 K using the modified form of Butler's equations and the derived values for the surface interaction parameters of the system. The calculated values are found to be in good agreement with those of the experimental data of the system. The present analysis indicates that the energetics of the surface phase are considerably different from those of the bulk phase. The present research investigates a critical compositional range beyond which the surface tension increases with temperature. The observed increase in adsorption of sulfur with consequent desorption of oxygen as a function of temperature above the critical compositional range has been ascribed to the increase of activity ratios of oxygen to sulfur in the surface relative to those in the bulk phase of the system.

Consolidation behavior of clayey soils under radial drainage

For Barren's degree of consolidation, U-r, versus the time factor, T-r, relationship for soils undergoing consolidation with radial drainage for the equal vertical strain condition, a simple method has been developed to determine the value of the coefficient of consolidation with radial drainage c(r). Theoretical log(10)(d(e)(2)/t) versus U-r curves where d(e) is the diameter of influence and r is the real time for the different known value of c(r) have been generated. A method has been developed wherein both the theoretical and experimental behaviors of soils undergoing consolidation with radial drainage can be simultaneously compared and studied on the same plot. The experimental log(10)(d(e)(2)/t) versus U-r curves have been compared with the theoretical curves. Effects of initial compression, secondary compression, and duration of load increment are studied. Simple procedures are presented for calculating the values of c(r) using the experimental log(10)(d(e)(2)/t) versus U-r curves. A comparative study of the coefficient of consolidation and the coefficient of permeability between the cases of vertical and radial drainage has been done.

Investigations of the reduction behavior of Iron-impregnated alumina gels (Fe/AlOOH) and the formation of Fe0-Al2O3 metal-ceramic composites

Fe/AlOOH gels calcined and reduced at different temperatures have been investigated by a combined use of Mossbauer spectroscopy, x-ray diffraction, and electron microscopy in order to obtain information on the nature of the iron species formed as well as the various reduction processes. Calcination at or below 1070 K mainly gives reducible Fe3+ while calcination at higher temperatures gives substitutional Fe3+ in the form of Al2-xFexO3. The Fe3+ species in the calcined samples are, by and large, present in the form of small superparamagnetic particles. Crystallization of Al2O3 from the gels is catalyzed by Fe2O3 as well as FeAl2O4. Fe (20 wt. %)/AlOOH gels calcined at or below 870 K give FeAl2O4 when reduced in hydrogen at 1070 K or lower and a ferromagnetic Fe0-Al2O3 composite (with the metallic Fe particles >100 angstrom) when reduced at 1270 K. Samples calcined at 1220 K or higher give the Fe0-Al2O3 composite when reduced in the 870-12,70 K range, but a substantial proportion of Fe3+ remains unreduced in the form of Al2-xFexO3, showing thereby the extraordinary stability of substitutional Fe3+ to reduction even at high temperatures. Besides the ferromagnetic Fe0-Al2O3 composite, high-temperature reduction of Al2-xFexO3 yields a small proportion of superparamagnetic Fe0-Al2O3 wherein small metallic particles (<100 angstrom) are embedded in the ceramic matrix. In order to preferentially obtain the Fe0-Al2O3 composite on reduction, Fe/AlOOH gels should be calcined at low temperatures (less-than-or-equal-to 1100 K); high-temperature calcination results in Al2-xFexO3. Several modes of formation of FeAl2O4 are found possible during reduction of the gels, but a novel one is that involving the reaction, 2Fe3+ + Fe0 --> 3Fe2+.

A Comparative Study of Electrical Switching Behavior of Certain Tellurium based Chalcogenide Thin Films for Phase Change Memory (PCM) Applications

This work describes the electrical switching behavior of three telluride based amorphous chalcogenide thin film samples, Al-Te, Ge-Se-Te and Ge-Te-Si. These amorphous thin films are made using bulk glassy ingots, prepared by conventional melt quenching technique, using flash evaporation technique; while Al-Te sample has been coated in coplanar electrode geometry, Ge-Se-Te and Ge-Te-Si samples have been deposited with sandwich electrodes. It is observed that all the three samples studied, exhibit memory switching behavior in thin film form, with Ge-Te-Si sample exhibiting a faster switching characteristic. The difference seen in the switching voltages of the three samples studied has been understood on the basis of difference in device geometry and thickness. Scanning electron microscopic image of switched region of a representative Ge15Te81Si4 sample shows a structural change and formation of crystallites in the electrode region, which is responsible for making a conducting channel between the two electrodes during switching.

Collapse Behavior of an Artificially Cemented Clayey Silt

The present study examines the role of interparticle cementation in the collapse behavior of two partly saturated (S-r = 4 to 12%) and very highly porous (initial void ratio = 1.5 to 2) laboratory-desiccated clayey silt specimens containing varying amounts (5 and 15% by dry weight of the respective specimens) of the cementitious iron oxides hematite and goethite, which are generally encountered in tropical residual soils. Kaolinite is the representative clay mineral of the soil matrix used for this research. Interparticle cementation by the crystalline iron oxides was generated in the laboratory by repeated (six times) wetting and drying of the iron-hydroxide-admixed clayey silt specimens under ambient conditions of temperature and humidity. Results showed that, for a given laboratory-desiccated clayey silt specimen (i.e., a specimen containing 5 or 15% of iron oxide on a dry weight basis), the amount of collapse (represented by Delta epsilon, the change in vertical strain upon wetting under constant pressure) increases with an increase in the experimental loading under which the specimen is inundated. The laboratory results also show that the desiccated specimen with a higher iron oxide content (containing 15% iron oxide by dry weight of the desiccated specimen) in spite of a lower dry unit weight (gamma(d) = 8.8 kN/m(3)) undergoes a lesser amount of collapse on soaking under a constant external stress (50 or 100 kPa) than the desiccated specimen with a lower iron oxide content (i.e., containing 5% iron oxide by dry weight of the desiccated specimen, gamma(d) = 10.4 KN/m(3)). Based on the X-ray diffraction results and the stress-strain relationships obtained from isotropically consolidated undrained triaxial tests, it is suggested that the laboratory-desiccated specimens are characterized by a metastable bonding provided by capillary suction and the crystalline iron oxides. On soaking under load owing to the loss of the metastable bonding, collapse of the laboratory-desiccated specimens occurs. Also, in the case of the laboratory-desiccated specimen with a higher iron oxide content, the presence of a stronger interparticle cementation (due to a greater abundance of crystalline iron oxides) and a higher initial moisture content are considered responsible for the specimen exhibiting a lower amount of collapse in comparison to that exhibited by the desiccated specimen with a lesser iron oxide content.

Permeability and Compressibility Behavior of Bentonite-Sand/Soil Mixes

As a seepage barrier slurry trench material should have a relatively low coefficient of permeability, in the range of 10(-7) cm/s, and at the same time should be compatible with surrounding material with regard to compressibility. Although bentonite-sand/soil mixes are used widely, there is no specific engineering approach to proportion these mixes that satisfies the above practical requirements. In this paper, a generalized approach is presented for predicting the permeability and compressibility characteristics of mixes with minimum input parameters. This approach will be helpful in proportioning mixes and predicting corresponding changes in engineering behavior. It is possible to proportion a mix to arrive at the required compressibility without affecting the permeability. This is explained using an illustrative example.

The effect of grain size on the high-strain, high-strain-rate behavior of copper

Copper with four widely differing grain sizes was subjected to high-strain-rate plastic deformation in a special experimental arrangement in which high shear strains of approximately 2 to 7 were generated. The adiabatic plastic deformation produced temperature rises in excess of 300 K, creating conditions favorable for dynamic recrystallization, with an attendant change in the mechanical response. Preshocking of the specimens to an amplitude of 50 GPa generated a high dislocation density; twinning was highly dependent on grain size, being profuse for the 117- and 315-mu m grain-size specimens and virtually absent for the 9.5-mu m grain-size specimens. This has a profound effect on the subsequent mechanical response of the specimens, with the smaller grain-size material undergoing considerably more hardening than the larger grain-size material. A rationale is proposed which leads to a prediction of the shock threshold stress for twinning as a function of grain size. The strain required for localization of plastic deformation was dependent on the combined grain size/shock-induced microstructure, with the large grain-size specimens localizing more readily. The experimental results obtained are rationalized in terms of dynamic recrystallization, and a constitutive equation is applied to the experimental results; it correctly predicts the earlier onset of localization for the large grain-size specimens. It is suggested that the grain-size dependence of shock response can significantly affect the performance of shaped charges.