Some numerical issues on the use of XFEM for ductile fracture

The XFEM is a powerful method to handle strong discontinuities in a finite element environment, especially in the study of the final stages of material failure, modelling the propagation of cracks, suppressing the need of remeshing. Nevertheless, for some materials undergoing large strain processes without noticeable volume changes, the discretization technique employed must not only describe the material behaviour but also correctly address the incompressibility constraints. In order to develop a robust formulation for this type of problems, an approach based on the analyses of the underlying sub-space of incompressible deformations embedded in the XFEM approximation is used, in the context of both infinitesimal and finite strains. This study motivated the extension of the conventional formulations of B-bar and F-bar to include the XFEM enrichment functions, whose performance is evaluated through some numerical examples and compared with competing methods such as the enhanced strain formulation.     

Water vapor absorption and permeability of films based on chitosan and sodium caseinate

The objective of this work was to characterize the moisture sorption and water vapor permeation behavior of edible films made from sodium caseinate and chitosan for future applications as protective layers on foods. Glycerol was used as a plasticizer, and the films were obtained by a casting/solvent‐evaporation method. The moisture sorption kinetics and water vapor permeability (WVP) were investigated. The effect of the addition of glycerol on the WVP characteristics of the films was determined at 25°C with a relative humidity (RH) gradient of 0–64.5% (internal to external). Experimental data were fitted with an exponential function with two fitting parameters. WVP increased with increasing glycerol content in both films, chitosan samples being much more permeable than caseinate ones at any glycerol content. WVPs of sodium caseinate, chitosan, and chitosan/caseinate films with 28 wt % glycerol were also determined for two RH gradients, 0 to 64.5% and 100 to 64.5%, higher WVPs being measured at higher RHs. The moisture sorption kinetics of caseinate films prepared with various glycerol contents were determined by the placement of the films in environments conditioned at 20°C and 75% RH. Peleg's equation and Fick's second law were used to predict the moisture sorption behavior over the entire time period. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Influence of surface features on the adhesion of Staphylococcus epidermidis to Ag–TiCN thin films

Abstract

Staphylococcus epidermidis has emerged as one of the major nosocomial pathogens associated with infections of implanted medical devices. The initial adhesion of these organisms to the surface of biomaterials is assumed to be an important stage in their colonization. The main objective of this work is to assess the influence of surface features on the adhesion of S. epidermidis to Ag–TiCN coatings deposited by dc reactive magnetron sputtering. The structural results obtained by x-ray diffraction show that the coatings crystallize in a B1-NaCl crystal structure typical of TiC_0.3N_0.7. The increase of Ag content promoted the formation of Ag crystalline phases. According to the results obtained with atomic force microscopy, a decrease on the surface roughness of the films from 39 to 7 nm is observed as the Ag content increases from 0 to 15 at.%. Surface energy results show that the increase of Ag promotes an increase in hydrophobicity. Bacterial adhesion and biofilm formation on coatings were assessed by the enumeration of the number of viable cells. The results showed that the surface with lower roughness and higher hydrophobicity leads to greater bacterial adhesion and biofilm formation, highlighting that surface morphology and hydrophobicity rule the colonization of materials.     

Steady state analysis of snowball effects for reaction–separation–recycle systems with thermally coupled distillation sequences

It is known that the steady and dynamic characteristics of processes with recycles can be significantly different from those without recycle streams. Particular attention has been given to the reactor–separator–recycle (RSR) system; because of the existence of snowball effects, a small load change can translate into a large change in the recycle flowrate, which propagates throughout the whole system. Previous works have analyzed the influence of separators on snowball effects using flash units and conventional distillation columns. This work explores the existence of snowball effects in steady state conditions under the implementation of thermally coupled distillation sequences in RSR systems. In particular, the implementation of coupled systems with side columns (sequences with side rectifiers or side strippers) is considered. A simplified process for ethylbenzene production is taken as a basis, and the effects of implementing thermally coupled systems on snowball effects are compared to those generated by conventional distillation systems.     

Azo peroxide initiators

Summary Azo diacyl peroxide compounds have been successfully used as sequential bifunctional initiators, in reactions of radicalic polymerization of vinyl monomers, for block copolymer obtaining. Resuming previous investigations, the 0-[4-(t-butylazo)-4-cyanovaleryl]-0'-actyl-peroxide has been synthesized; this is a new azo diacyl peroxide compound, obtained either by the direct condensation of the 4-(t-butylazo)-4-cyanovaleric acid(the BACV acid) with peracetic acid — under the dehydrating action of the dicyclohexylcarbodiimide — or from the chloride of the respective acid by treatment with peracetic acid in the presence of pyridine. The initiator structure is confirmed by the characteristics evidenced by IR and H-NMR, as well as by the ¹³C-NMR spectra. The synthesized product may be kept, in cold conditions, for a long time; nevertheless, by contact with a base it manifests the tendency of changing itself into a symmetrical azo diacyl peroxide.     

Minmax topology optimization

We describe a systematic approach for the robust optimal design of linear elastic structures subjected to unknown loading using minmax and topology optimization methods. Assuming only the loading region and norm, we distribute a given amount of material in the design domain to minimize the principal compliance, i.e. the maximum compliance that is produced by the worst-case loading scenario. We evaluate the principal compliance directly by satisfying the optimality conditions which take the form of a Steklov eigenvalue problem and thus we eliminate the need of an iterative nested optimization. To generate a well-posed topology optimization problem we use relaxation which requires homogenization theory. Examples are provided to demonstrate our algorithm.     

Methane oxidation – effect of support, precursor and pretreatment conditions – in situ reaction XPS and DRIFT

The methane combustion was studied on Pd supported catalysts searching for explanation of the effects of support, pretreatment and induction periods on the activity of the Pd/alumina, Pd/zirconia, Pd/niobia and Pd/niobia/alumina catalysts. Characterization of methane oxidation in XPS and DRIFTS chambers indicated that an induction period of at least 6 h is necessary to form Pd in a higher oxidation state Pd^2+δ coexisting with PdO as the surface active phases. Similarly, DRIFTS measurements showed distinct species at the surface during the oxidation of methane after 6 h, indicating formate species on Pd/Alumina and CH_x-species and O₂⁻ adsorption on the Pd/zirconia catalyst, suggesting different reaction mechanisms. The reverse pathway depends on the surface modification due to the temperature and induction time.

The activity of the Pd/niobia/alumina was better than of the Pd/niobia. The effect of the precursor was observed, indicating that palladium nitrate precursor allows to lower dispersion but better performance on methane oxidation. The stability was studied and shows that the Pd/zirconia catalyst was stable and is regenerated, while the Pd/alumina catalyst deactivates very fast and is not regenerated. The effect of pretreatment of the catalysts on the methane oxidation showed different behavior depending of the support.     

Component and system reliability-based topology optimization using a single-loop method

We perform reliability-based topology optimization by combining reliability analysis and material distribution topology design methods to design linear elastic structures subject to random inputs, such as random loadings. Both component reliability and system reliability are considered. In component reliability, we satisfy numerous probabilistic constraints which quantify the failure of different events. In system reliability, we satisfy a single probabilistic constraint which encompasses the component events. We adopt the first-order reliability method to approximate the component reliabilities and the inclusion-exclusion rule to approximate the system reliability. To solve the probabilistic optimization problem, we use a variant of the single loop method, which eliminates the need for an inner reliability analysis loop. The proposed method is amenable to implementation with existing deterministic topology optimization software, and hence suitable for practical applications. Designs obtained from component and system reliability-based topology optimization are compared to those obtained from traditional deterministic topology optimization and validated via Monte Carlo simulation.