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).

Molecular self-assembly of cadmium-triazolate complexes via hydrogen bonding: Synthesis, structures and photoluminescent properties

Two new cadmium coordination polymers namely Cd(HAmTrz-COO)(4)(NH4+)(2)] 1; and Cd(HAmTrz)(2)I-2](n) 2; (HAmTrz-COOH = 3-amino-1,2,4-triazole-5-carboxylic acid), have been prepared based on HAmTrz-COOH as ligand. The crystal structures of 1 and 2 have been determined by single-crystal X-ray diffraction technique. In coordination-complex 1 four triazole ligands coordinate via N1 nitrogen leading to a tetrahedral geometry around cadmium ion, while in 2 the ligand prefers to coordinate to the metal in a bidentate bridging mode. The structures of both the coordination polymers can be envisaged as 3D hydrogen bonded networks. Thermogravimetric analysis shows that 2 is more stable than 1 owing to different coordination numbers of cadmium atoms. Photoluminescence properties of both the compounds have been investigated in the solid state. (C) 2011 Elsevier B.V. All rights reserved.

Autoignition in a non-premixed medium: DNS studies on the effects of three-dimensional turbulence

Direct numerical simulation (DNS) results of autoignition in anon-premixed medium under an isotropic, homogeneous, and decaying turbulence are presented. The initial mixture consists of segregated fuel parcels randomly distributed within warm air, and the entire medium is subjected to a three-dimensional turbulence. Chemical kinetics is modeled by a four-step reduced reaction mechanism for autoignition of n-heptane/air mixture. Thus, this work overcomes the principal limitations of a previous contribution of the authors on two-dimensional DNS of autoignition with a one-step reaction model. Specific attention is focused on the differences in the effects of two- and three-dimensional turbulence on autoignition characteristics. The three-dimensional results show that ignition spots are most likely to originate at locations jointly corresponding to the most reactive mixture fraction and low scalar dissipation rate. Further, these ignition spots are found to originate at locations corresponding to the core of local vortical structures, and after ignition, the burning gases move toward the vortex periphery Such a movement is explained as caused by the cyclostrophic imbalance developed when the local gas density is variable. These results lead to the conclusion that the local ignition-zone structure does not conform to the classical stretched flamelet description. Parametric studies show that the ignition delay time decreases with an increase in turbulence intensity. Hence, these three-dimensional simulation results resolve the discrepancy between trends in experimental data and predictions from DNSs of two-dimensional turbulence. This qualitative difference between DNS results from three- and two-dimensional simulations is discussed and attributed to the effect of vortex stretching that is present in the former, but not in the latter.

Helical structures: The geometry of protein helices and nanotubes

In nature, helical structures arise when identical structural subunits combine sequentially, the orientational and translational relation between each unit and its predecessor remaining constant. A helical structure is thus generated by the repeated action of a screw transformation acting on a subunit. A plane hexagonal lattice wrapped round a cylinder provides a useful starting point for describing the helical conformations of protein molecules, for investigating the geometrical properties of carbon nanotubes, and for certain types of dense packings of equal spheres.

Identification of delamination in composite beams using spectral estimation and a genetic algorithm

An efficient strategy for identification of delamination in composite beams and connected structures is presented. A spectral finite-element model consisting of a damaged spectral element is used for model-based prediction of the damaged structural response in the frequency domain. A genetic algorithm (GA) specially tailored for damage identification is derived and is integrated with finite-element code for automation. For best application of the GA, sensitivities of various objective functions with respect to delamination parameters are studied and important conclusions are presented. Model-based simulations of increasing complexity illustrate some of the attractive features of the strategy in terms of accuracy as well as computational cost. This shows the possibility of using such strategies for the development of smart structural health monitoring softwares and systems.

Solid propellant binders

Solid, propellants are widely used in modern rockets and missiles. Although the history of solid rockets could be traced to the discovery of gunpowder over a thousand years ago, the technology could be perfected only by the later half of the 20(th) century. The failure of gunpowder rockets was largely due to the unknown consolidating technique of the powder composition. The emergence of large solid propellant motors had, to await the dawn of polymer. science and technology(S&T). Specific syntheses of functionally terminated polymers having cross-linking capability led to the emergence of casting technology of solid composite propellants. This review describes the various polymeric fuel/binder systems used or considered for use in solid,propellants. It includes a brief background, advantages, and shortcomings of the various systems, an account of the currently used binders and a critical survey of the advanced polymers envisaged for future usage. Special emphasis has been laid on recently synthesized polymers having N-N bonds in their structures, and-on the feasibility of developing smokeless propellants based on ammonium nitrate.

Sulfates of organic diamines: hydrogen-bonded structures and properties

In order to investigate the supramolecular hydrogen-bonded networks and other structural features exhibited by compounds containing an organic cation and an inorganic anion, sulfates of the organic diamines, ethylenediamine (I), 1,3-diaminopropane (II), piperazine (III), and 1,4-diazabicyclo[2.2.2]octane (DABCO) (IV) have been prepared investigated by X-ray crystallography. While II, III, and IV crystallize in the centrosymmetric space group, Pbca, P2(1)/n, Pbcn, respectively, I crystallizes in the non-centrosymmetric space group, P4(1) exhibiting chirality and weak NLO properties. I-IV exhibit different types of supramolecular H-bonded networks involving the organic cation and the SO42- anion. The nature and strength of the H-bonding network vary from one compound to another, with the strongest network found in piperazinium sulfate, III, and the weakest in II. While in III, water molecules form part of the H-bonded network, they are present as guest molecules in the channels of IV. Thermal stability of the compounds as well as the infrared spectra reflect the stabilities of these H-bonded solids. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

Supramolecular hydrogen-bonded structures in organic amine squarates

Structures of monohydrogen squarates of methylamine, ethylenediamine, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, N,N'-diemethylpiperazine and N,N,N,N-tetramethylguanidine have been studied in detail. The supramolecular hydrogen-bonded molecular networks are formed by the monoanion of squaric acid by itself or in association with the parent acid. Three types of hydrogen-bonded motifs are observed in these compounds, namely a liner chain, a cyclic dimer and a cyclic tetramer. These hydrogen-bonded motifs formed by the squaric acid species interact with the amine through N-H...O hydrogen-bonding and give rise to predominantly layered structures, while some of them also exhibit three-dimensional structures. Two of the monohydrogen squarate structures also exhibit pi-pi interactions between two squarate rings. The various hydrogen-bonding parameters in the amine squarates are discussed at length. (C) 2002 Elsevier Science B.V. All rights reserved.

Transformations of two-dimensional layered zinc phosphates to three-dimensional and one-dimensional structures

Transformations of the layered zinc phosphates of the compositions [C6N4H22](0.5) [Zn-2 (HPO4)(3)], I, [C3N2H12][Zn-2 (HPO4)(3)], II and [C3N2OH12][Zn-2 (HPO4)(3)], III, containing triethylenetetramine, 1,3-diaminopropane, and 1,3-diamino-2-hydroxypropane, respectively, have been investigated under different conditions. On heating in water, I transforms to a one-dimensional (1-D) ladder and a three-dimensional (3-D) structure, while II gives rise to only a two-dimensional (2-D) layered structure. In the transformation reaction of I with zinc acetate, the same ladder and 3-D structures are obtained along with a tubular layer. Under similar conditions II gives a layered structure formed by the joining of two ladder motifs. III, on the other hand, is essentially unreactive when heated with water and zinc acetate, probably because the presence of the hydroxy group in the amine which hydrogen bonds to the framework. In the presence of piperazine, I, II and III give rise to a four-membered, corner-shared linear chain which is likely to be formed via the ladder structure. In addition, 2-D and 3-D structures derived from the 1-D linear chain or ladder structures are also formed. The primary result from the study is that the layers produce 1-D ladders, which then undergo other transformations. It is noteworthy that in the various transformations carried out, most of the products are single-crystalline.

Encapsulated and hollow closed-cage structures of WS2 and MoS2 prepared by laser ablation at 450–1050°C

Encapsulated and hollow closed-cage onion-like structures of WS2 and MoS2 were prepared by laser ablation of the corresponding layered structures in argon atmosphere at four varied temperatures. A detailed study for WS2 indicates that only metal-filled onion-like structures are produced at temperatures Tless-than-or-equals, slant650°C, whereas a mixture of metal-filled and hollow structures are produced at Tgreater-or-equal, slanted850°C. The encapsulated metal is identified to be predominantly the metastable β phase of tungsten. Very short tube-like or elongated polyhedral structures are also obtained at high temperatures.

Reliability Based Design Optimization for Internal Stability of Reinforced Soil Structures Using Pseudo‐Static Method

Seismic design of reinforced soil structures involves many uncertainties that arise from the backfill soil properties and tensile strength of the reinforcement which is not addressed in current design guidelines. This paper highlights the significance of variability in the internal stability assessment of reinforced soil structures. Reliability analysis is applied to estimate probability of failure and pseudo‐static approach has been used for the calculation of the tensile strength and length of the reinforcement needed to maintain the internal stability against tension and pullout failures. Logarithmic spiral failure surface has been considered in conjunction with the limit equilibrium method. Two modes of failure namely, tension failure and pullout failure have been considered. The influence of variations of the backfill soil friction angle, the tensile strength of reinforcement, horizontal seismic acceleration on the reliability index against tension failure and pullout failure of reinforced earth structure have been discussed.

A new beam finite element for the analysis of functionally graded materials

A new beam element is developed to study the thermoelastic behavior of functionally graded beam structures. The element is based on the first-order shear deformation theory and it accounts for varying elastic and thermal properties along its thickness. The exact solution of static part of the governing differential equations is used to construct interpolating polynomials for the element formulation. Consequently, the stiffness matrix has super-convergent property and the element is free of shear locking. Both exponential and power-law variations of material property distribution are used to examine different stress variations. Static, free vibration and wave propagation problems are considered to highlight the behavioral difference of functionally graded material beam with pure metal or pure ceramic beams. (C) 2003 Elsevier Science Ltd. All rights reserved.

A new parallel overlapped domain decomposition method for nonlinear dynamic finite element analysis

In this paper a new parallel algorithm for nonlinear transient dynamic analysis of large structures has been presented. An unconditionally stable Newmark-beta method (constant average acceleration technique) has been employed for time integration. The proposed parallel algorithm has been devised within the broad framework of domain decomposition techniques. However, unlike most of the existing parallel algorithms (devised for structural dynamic applications) which are basically derived using nonoverlapped domains, the proposed algorithm uses overlapped domains. The parallel overlapped domain decomposition algorithm proposed in this paper has been formulated by splitting the mass, damping and stiffness matrices arises out of finite element discretisation of a given structure. A predictor-corrector scheme has been formulated for iteratively improving the solution in each step. A computer program based on the proposed algorithm has been developed and implemented with message passing interface as software development environment. PARAM-10000 MIMD parallel computer has been used to evaluate the performances. Numerical experiments have been conducted to validate as well as to evaluate the performance of the proposed parallel algorithm. Comparisons have been made with the conventional nonoverlapped domain decomposition algorithms. Numerical studies indicate that the proposed algorithm is superior in performance to the conventional domain decomposition algorithms. (C) 2003 Elsevier Ltd. All rights reserved.

Transmission electron microscopy study of domain structures in ferroelectric SrBi2Nb2O9 ceramics

A transmission electron microscopy study has been carried out on the domain structures of SrBi2Nb2O9 (SBN) ferroelectric ceramics which belong to the Aurivillius family of bismuth layered perovskite oxides. SBN is a potential candidate for Ferroelectric Random access memory (FeRAM) applications. The 90° ferroelectric domains and antiphase boundaries (APBs) were identified with dark field imaging techniques using different superlattice reflections which arise as a consequence of octahedral rotations and cationic shifts. The 90° domain walls are irregular in shape without any faceting. The antiphase boundaries are less dense compared to that of SrBi2Ta2O9(SBT). The electron microscopy observations are correlated with the polarization fatigue nature of the ceramic where the domain structures possibly play a key role in the fatigue- free behavior of the Aurivillius family of ferroelectric oxides.