An Integration Algorithm for Constitutive Models with Two Intersecting Yield Surfaces

Abstract: In this paper, the constitutive models involving two active intersecting yield surfaces, such as Lade's model, are considered and a new integration algorithm is formulated. The formulation introduces a new class of the return mapping algorithm within the framework of the operator splitting methodology. The proposed algorithm is applicable to a general class of elastoplastic constitutive models exhibiting associative or nonassociative flow rule, arbitrary yield criteria and hardening laws. The algorithm can handle one or two active yield surfaces at a time. The development leads to a relatively simple, straightforward algorithm for updating the stresses at Gauss point.
Finally, the new algorithm is implemented in a generalized nonlinear finite element computer program incorporating Lade's model. The results of a number of boundary value problems are presented and compared with available experimental data and theoretical formulas. The comparisons indicate that incorporating the new class of return mapping algorithm gives reasonable solutions.     

A novel anti islanding detection method for grid connected fuel cell power generation systems

Renewable energy sources have been developed rapidly all around the world, and one of these green energy sources is hydrogen energy. The fuel cell systems have become prominent in renewable energy sources because of its minimal dimensions and energy conversion method. There have been developed, some applications, especially in domestic and automotive areas, and fuel cell systems are also have been started to use in grid connected systems. Fuel cell systems must have some electrical connection standards while they connected to an electrical grid. One of these electrical conditions and may be the most important one is unplanned islanding condition. Islanding is a very dangerous situation because it can damage to the fuel cell and related electrical systems and also working people have been at risk in islanding situation on the grid. In this study, a novel islanding detection method was introduced for grid connected fuel cell systems. 0.5 kW solid oxide fuel cell (SOFC) system used in developed experimental system and a novel anti islanding detection method was researched by using an effective method. The proposed method was also developed by using Matlab Simulink and its useful tools. The developed islanding detection method is robust, reliable and has a fast response time, according to present methods. The results confirm the suggested conditions, and it can be seen in this method, it can also be adapted easily to the grid connected fuel cell systems.     

Molecular methods for identification of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus using methionine biosynthesis and 16S rRNA genes

Yoghurt and starter culture producers are still searching strains of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus to produce healthier yogurt with longer shelf life, better texture, taste and quality. However, selective identification of Lb. delbrueckii subsp. bulgaricus and Strep. thermophilus from a mixed population using microbiological and biochemical methods is difficult, time consuming and may not be accurate. In this study, a quick, sensitive and accurate method is proposed to identify both Lb. delbrueckii subsp. bulgaricus and Strep. thermophilus using PCR. The method is comprised of two parts. In the first part, methionine biosynthesis genes, known to be present in both species were partially amplified by designed primers (cysmet2F and cysmet2R). Partial amplification of the methionine biosynthesis gene which gives 700 bp fragment resulted in selective identification of Lb. bulgaricus and Strep. thermophilus. All 16 Lb. bulgaricus and 6 Strep. thermophilus isolates assessed by this method gave the expected amplification. On the other hand, further analysis of other closely related species with the same primers have indicated that the same product was also amplified in two more lactobacilli namely, Lb. delbrueckii subsp. lactis and Lb. helveticus species. Thus, in the second part of the method, further differentiation of Lb. delbrueckii subsp. bulgaricus and Strep. thermophilus from each other and these species was achieved using restriction analysis of 16S rRNA gene with EcoRI.     

Accurate model for Schottky-contacted coplanar waveguide includingfinite epilayer resistance effects

A new quasistatic model for Schottky-contacted coplanar waveguide (CPW) on a semiconductor substrate is presented. Comparison between experimental measurements for a CPW on a GaAs epilayer and calculations show excellent agreement from 45 MHz to 40 GHz. The new model includes the effect of the finite resistance of the undepleted epilayers under the CPW electrodes     

Can primary failure of arteriovenous fistulas be anticipated?

Primary failure, early thrombosis, and inadequate maturation are the main complications encountered in arteriovenous fistulas. Doppler ultrasonographic assessment of flow‐mediated dilatation (FMD) is currently used for the early diagnosis of atherosclerosis. Clinical experience in the use of FMD for preoperative assessment of vasculature is rather limited; therefore, we sought to elucidate the relationship between preoperative FMD and primary failure of the fistula. Thirty‐three patients with end‐stage renal disease who were admitted to our hospital between January and July 2005 were included in our study. Medical histories were established and the internal diameter, wall thickness, peak systolic flow rate, and resistive index (RI) were measured in the cephalic vein and radial and brachial arteries. Flow‐mediated dilatation and nitrate‐mediated dilatation (NMD) of the brachial artery were assessed. Fistulas were evaluated 48 hours and 30 days postoperatively. Brachial arterial internal diameter was lower in all fistulas that developed primary failure in 48 hours (0.4 ± 0.07 cm vs. 0.35 ± 0.07 cm, P = 0.016). The radial artery RI was found to be significantly elevated in fistulas with both early (48‐hour) and late‐term (30‐day) failure (0.9 ± 0.08 vs. 0.68 ± 0.3, P = 0.01, and 0.86 ± 0.8 vs. 0.67 ± 0.3, P = 0.038, respectively). The brachial artery peak systolic flow rate was significantly reduced in patients in the radiocephalic fistula group that developed early and late‐term failure (42.9 ± 12 cm/sec vs. 68.4 ± 10 cm/sec, P = 0.01, and 44.1 ± 13 cm/sec vs. 57.7 ± 16 cm/sec, P = 0.038, respectively). Our study, constrained by a smaller, older patient group, was unable to show a statistically significant correlation between FMD, NMD, and fistula success. Any single parameter may not be sufficient to assess vascular health preoperatively. A multifactorial approach incorporating parameters evaluating arterial and venous function might be more effective in predicting fistula success. Further studies on larger patient groups may indeed demonstrate the value of these assessments.     

Styrenation of air-blown linseed oil by nitroxide-mediated radical polymerization

Styrenation of air-blown linseed oil by a nitroxide-mediated radical polymerization (NMRP) technique is described. In this technique, air-blown linseed oil bearing hydroperoxide groups was used as a macroinitiator in NMRP of styrene in the presence of 2,2′,6,6′-tetramethylpiperidinyl-1-oxy (TEMPO). The effects of various parameters, such as the amount of TEMPO and hydroperoxide groups, were investigated in terms of molecular weight and polydispersity. For comparison, a copolymer sample of air-blown linseed oil with styrene was also prepared in the absence of TEMPO. The film properties of all samples were determined according to the related standards and were compared with respect to surface protection. Samples prepared by the NMRP technique exhibited relatively narrow polydispersity and better film properties compared to those of the samples obtained by the conventional method.     

Synthesis and room temperature coating of nano ZrB2 on copper using mechanical roll-milling

In this study, ZrB₂ was prepared from ZrOCl₂.8H₂O, H₃BO₃ and citric acid by using the gel method. The gel was dried at 400 °C and then it was calcined at 1500 °C. The prepared 0.5–1 μm ZrB₂ powder was encapsulated into the Cu tube. For the purpose of coating ZrB₂ on Cu metal, the roll-milling of ZrB₂ particles inside the cylindrical Cu tube has been performed at room temperature. The prepared ZrB₂ coating on Cu was analyzed by using FE-SEM, EDS, XRD, electrical resistivity and CV measurements. The final ZrB₂ particle size on the surface of the coating layer was below 100 nm, and the coating thicknesses were about 15–20 μm. The long nanofiber like ZrB₂ structures was observed. The coating of nano ZrB₂ on Cu was achieved by using this novel roll-milling process which is an environmental friendly, low cost and practical technique.