Convex optimization and limit analysis: Application to Gurson and porous Drucker-Prager materials

First, we summarize our convex optimization method to solve the static approach of limit analysis. Then, we present the main features of a quadratic extension of a recently proposed mixed finite element method of the kinematic approach. Both methods are applied to obtain precise solutions to a forming problem with Gurson and Drucker-Prager materials. Finally, in order to analyze the criterion of “Porous Drucker-Prager” materials, the Gurson micro-macro model involving a Drucker-Prager matrix containing cylindrical cavities is investigated. Comparing previous results shows, among other things, a similarity in the compression case not always observed for the “Porous von Mises” material between cylindrical and spherical cases.     

Process analysis and optimization for failure energy of spot welded titanium alloy

Titanium and its alloys have been applied in many industrial fields because of their high specific strength, good corrosion resistance and high thermal stability. Whereas, there is limited valuable references for recommendations of welding parameter selection and specific standards of the small scale resistance spot welding (SSRSW) of titanium alloy though it has been applied in many industrial production fields. Seventeen tests were designed according to the three-level three-factor Box–Behnken experimental design and the mathematical model correlating the process parameters and the failure energy was established on the basis of response surface methodology (RSM) technique. And then this model was used to analyze the effects/interactions of the welding parameters on the failure energy. The verification test results which were conducted with completely new welding parameters verified that the model presented in this paper was effective and robust. Sensitivity analysis was also carried out to explore the impact of each process parameter on the quality of welding joint. The optimal combination of process parameters for maximum failure energy of the welded joint was obtained using the model based on artificial fish swarm algorithm (AFSA).     

Mirror-based double diffraction grating system for coherence generation: additional study of periodicity error

The Lau effect consists of generating spatial coherence by using an extended light source that illuminates a double diffraction grating system. The price of this coherence generation is a huge energy loss caused by the successive amplitude gratings, which are blocking elements. To significantly reduce the energy loss, our approach consists in using a longitudinal mirror system to collect the light rays initially blocked by the gratings of the Lau set-up. This technique offers high power efficiency and can be straightforwardly extended to provide Talbot array illuminators. The impact of periodicity error is studied and illustrations are given at the end.     

Normalized parameter relating the spherical aberration of a thin lens to the diffraction limit and fiber coupling efficiency

In this paper, we calculate the transverse spherical aberration TA of a thin lens and defines a normalized aberration Y equal to TA divided by the theoretical resolution limit. As a rule of thumb, (a) a thin lens that suffers only from spherical aberration may be considered effectively diffraction-limited as long as Y < 1.6. Similarly, (b) the coupling efficiency of a Gaussian beam to a single-mode fiber may be high even when Y > 1.6, and, specifically, (c) the lens need be diffraction-limited only over a radius approximately equal to the radius (to the 1/e-point) of the Gaussian beam.     

An improved ant colony optimization algorithm and its applications to limit analysis of frame structures

This article presents a modified ant colony systems approach, which allows reduction of the computational effort needed to converge to the optimal solution of a given engineering problem. The proposed algorithm is applied to the evaluation of the plastic load and failure modes of planar frames. The approach is based on the generation of elementary collapse mechanisms and on their linear combination, aimed at minimizing the collapse load factor. Many applications have been performed on frames of different sizes subjected to seismic load scenarios, i.e. permanent vertically distributed and variable concentrated forces at each floor. The values of the collapse load obtained with the proposed algorithm have been compared with those obtained by means of other optimization algorithms and nonlinear pushover analysis. An extended performance comparison between the proposed procedure and standard ant colony system algorithms shows the expected improvements in terms of a great reduction in the computational effort.     

Brazing of titanium to steel with different filler metals: analysis and comparison

Evaluations of vacuum brazed commercially pure titanium and low-carbon steel joints using one copper-based alloy (Cu–12Mn–2Ni) and two silver-based braze alloys (Ag–34Cu–2Ti, Ag–27.25Cu–12.5In–1.25Ti) have been studied. Both the interfacial microstructures and mechanical properties of brazed joints were investigated to evaluate the joint quality. The optical and scanning electron microscopic results showed that all the filler metals interact metallurgically with steel and titanium, forming different kinds of intermetallic compounds (IMC) such as CuTi, Cu2Ti, Cu4Ti3, and FeTi. The presence of IMC (interfacial reaction layers) at the interfacial regions strongly affects the shear strength of the joints. Furthermore, it was found that the shear strength of brazed joints and the fracture path strongly depend on the thickness of the IMC. The maximum shear strength of the joints was 113 MPa for the specimen brazed at 750 °C using an Ag–27.25Cu–12.5In–1.25Ti filler alloy.     

A comparison of localized finite element formulations for two-dimensional wave diffraction and radiation problems

Two of the most promising localized finite element methods are compared: the boundary series element method, in which a series of eigenfunctions is used to represent the far field solution; and the boundary integral element method, in which an integral equation is satisfied at the boundary between localized finite element and outer regions. The methods are applied to water of arbitrary depth. The theory of the two methods is summarized, and typical numerical results are discussed. Consideration is given to the well-known hydrodynamical reciprocal relations, and to the phenomenon of ‘irregular’ frequencies. The relative merits of the two methods are established.     

Development and optimization of dark Fenton oxidation for the treatment of textile wastewaters with high organic load

The examination of the effectiveness of the chemical oxidation using Fenton's reagent (H(2)O(2)/Fe(2+)) for the reduction of the organic content of wastewater generated from a textile industry has been studied. The experimental results indicate that the oxidation process leads to a reduction in the chemical oxygen demand (COD) concentration up to 45%. Moreover, the reduction is reasonably fast at the first stages of the process, since the COD concentration is decreased up to 45% within four hours and further treatment time does not add up to the overall decrease in the COD concentration (48% reduction within six hours). The maximum color removal achieved was 71.5%. In addition, the alterations observed in the organic matter during the development of the process, as indicated by the ratios of COD/TOC and BOD/COD and the oxidation state, show that a great part of the organic substances, which are not completely mineralized, are subjected to structural changes to intermediate organic by-products.     

Verification and comparison of a fast fourier transform-based full diffraction method for tilted and offset planes

The recent introduction of a fast Fourier transform- (FFT-) based method for calculating the Rayleigh-Sommerfeld full diffraction integral for tilted and offset planes permits high-speed evaluation of integrated optical systems. An important part of introducing a new calculational tool is its validation and an assessment of its limitations. The validity of the new FFT-based method was determined by comparison of that method with direct integration (DI) of the Rayleigh-Sommerfeld integral, a well-established method. Points of comparison were accuracy, computational speed, memory requirements of the host computer, and applicability to various optical modeling situations. The new FFT-based method is 228 times faster, yet requires 14 times more memory, than the DI method for a 500 mum by 500 mum real computational window.     

Indices for assessment of hemodialysis adequacy: a comparison of different formulae

Of the various indices used in the assessment of dialysis adequacy, fractional urea clearance controlled for volume of distribution "Kt/V" remains the most widely used. Its determination is best performed by formal urea kinetic modeling (UKM), which is laborious and cumbersome, and the computational softwares are largely unavailable, particularly in developing countries. Consequently, different equations have been developed that approximate the formal UKM determination. Of the available formulae, that from second-generation logarithmic equation have been found to approximate values derived from formal UKM closely. We set out to determine the clinical utility of percent reduction of urea and Kt/V formulae derived from it, using the logarithmic equation as the standard.     

Application and comparison of metaheuristic techniques to generation expansion planning in the partially deregulated environment

The deregulation of the electric utility industry is an on-going process in many developing countries. A novel generation expansion planning (GEP) model is proposed that is suitable for developing countries such as India in a partially deregulated environment. In a partially deregulated/restructured environment, both utilities and independent power producers (IPPs) participate in the generation market. In this model, the utility purchases electric power from the IPPs and sells it to the consumer. The utility maximises its profit and ensures profits for all the participating IPPs. In addition, the utility checks under/over investment and considers system security, national security (fuel-mix ratio), social welfare and reliability simultaneously. The budget constraints of the utility are to be taken into consideration during the expansion plan. Metaheuristic techniques, such as genetic algorithms, differential evolution, evolutionary programming, evolutionary strategy, particle swarm optimisation, tabu search, simulated annealing, and the hybrid approach are used to solve the restructured GEP problem, and their performances are evaluated and validated against the dynamic programming (DP) method for a synthetic test system having five types of candidate plant for the utility and three types of candidate plant for IPP, with a 6 year planning horizon. The effectiveness of the proposed modifications and techniques is also addressed.     

Estimating surface area of copper powder: A comparison between electrochemical,microscopy and laser diffraction methods

For heterogeneous catalytic/electrocatalytic reaction systems, estimation of the surface area of the catalyst is of great importance. As far as systems involving copper as catalyst are concerned, the literature reveals lack of a robust method to determine the surface area. In this work, we compare the surface area determined from electrochemical measurements, optical and electron microscopy and from laser diffraction experiments. Specifically, cyclic voltammetry in two different solutions involving two separate surface reactions has been used to measure the surface area. First involves the formation of a monolayer of Cu₂O and the second involves formation of a monolayer of Pb. Additionally, the surface area was also estimated from the double layer capacitance of the copper electrode. The surface areas determined from these electrochemical measurements are in good agreement with those obtained from microscopy and particle size analyzers (for average copper particle diameter?=?10?µm).     

Parametric hypoplasticity as continuum model for granular media: from Stokesium to Mohr-Coulombium and beyond

Fluid-particle systems, in which internal forces arise only from viscosity or intergranular friction, represent an important special case of strictly dissipative materials defined by a history-dependent 4th-rank viscosity tensor. In a recently proposed simplification, this history dependence is represented by a symmetric 2nd-rank fabric tensor with evolution determined by a given homogeneous deformation. That work suggests an essential physical link between idealized suspensions (“Stokesium”) and granular media (“Mohr-Coulombium”) along with possible models for the visco-plasticity of fluid-saturated and dry granular media. The present paper deals with the elastoplasticity of dilatant non-cohesive granular media composed of nearly rigid, frictional particles. Based on the underlying physics and past modeling by others, a continuum model based on parametric hypoplasticity is proposed, which involves a set of rate-independent ODEs in the state-space of stress, void ratio and fabric. As with the standard theory of hypoplasticity, the present model does not rely on plastic potentials but, in contrast to that theory, it is based explicitly on positive-definite elastic and plastic moduli. The present model allows for elastic loading or unloading within a dissipative yield surface and also provides a systematic treatment of Reynolds dilatancy as a kinematic constraint. Some explicit forms are proposed and comparisons are made to previous hypoplastic models of granular media.