Finite element computational modeling of externally bonded CFRP composites flexural behavior in RC beams

This paper focuses on the flexural behavior of RC beams externally strengthened with Carbon Fiber Reinforced Polymers (CFRP) fabric. A non-linear finite element (FE) analysis strategy is proposed to support the beam flexural behavior experimental analysis. A development system (QUEBRA2D/FEMOOP programs) has been used to accomplish the numerical simulation. Appropriate constitutive models for concrete, rebars, CFRP and bond-slip interfaces have been implemented and adjusted to represent the composite system behavior. Interface and truss finite elements have been implemented (discrete and embedded approaches) for the numerical representation of rebars, interfaces and composites.

Optimal design of periodic functionally graded composites with prescribed properties

The computational design of a composite where the properties of its constituents change gradually within a unit cell can be successfully achieved by means of a material design method that combines topology optimization with homogenization. This is an iterative numerical method, which leads to changes in the composite material unit cell until desired properties (or performance) are obtained. Such method has been applied to several types of materials in the last few years. In this work, the objective is to extend the material design method to obtain functionally graded material architectures, i.e. materials that are graded at the local level (e.g. microstructural level). Consistent with this goal, a continuum distribution of the design variable inside the finite element domain is considered to represent a fully continuous material variation during the design process. Thus the topology optimization naturally leads to a smoothly graded material system. To illustrate the theoretical and numerical approaches, numerical examples are provided. The homogenization method is verified by considering one-dimensional material gradation profiles for which analytical solutions for the effective elastic properties are available. The verification of the homogenization method is extended to two dimensions considering a trigonometric material gradation, and a material variation with discontinuous derivatives. These are also used as benchmark examples to verify the optimization method for functionally graded material cell design. Finally the influence of material gradation on extreme materials is investigated, which includes materials with near-zero shear modulus, and materials with negative Poisson`s ratio.

Synthesis, characterization, and bactericidal properties of composites based on crosslinked resins containing silver

Two different commercial crosslinked resins (Amberlite GT73 and Amberlite IRC748) were employed for anchoring silver. The -SH and -N(CH2COOH)2 groups, respectively, present on these resins were used for Ag+ chelation from an aqueous solution. The Ag+ ions were reduced with three different reductants: hydrazine, hydroxylamine, and formaldehyde (under an alkaline pH). The produced composites were characterized with thermogravimetry/differential thermogravimetry and scanning electron microscopy combined with a backscattered scanning electron detector. Energy-dispersive X-ray spectroscopy coupled to scanning electron microscopy allowed the observation of submicrometer particles of silver, and chemical microanalysis of emitted X-rays revealed the presence of metal on the internal and external surfaces of the composite microspheres. The amount of incorporated silver was determined by titration. The antibacterial activity of the silver/resin composites was determined toward 10(3)-10(7) cells/mL dilutions of the auxotrophic AB1157 Escherichia coli strain; the networks containing anchored submicrometer silver particles were completely bactericidal within a few minutes because of the combined action of silver and functional groups of the resins. (c) 2007 Wiley Periodicals, Inc.

Influence of the rubbery phase on the crystallinity and thermomechanical properties of poly(3-hydroxybutyrate)/elastomer blends

Poly(3-hydroxybutyrate) (PHB) is a very promising biopolymer. In order to improve its processability and decrease its brittleness, PHB/elastomer blends can be prepared. In the work reported, the effect of the addition of a rubbery phase, i.e. ethylene - propylene-diene terpolymer (EPDM) or poly(vinyl butyral) (PVB), on the properties of PHB was studied. The effects of rubber type and of changing the PHB/elastomer blend processing method on the crystallinity and physical properties of the blends were also investigated. For blends based on PHB, the main role of EPDM is its nucleating effect evidenced by a decrease of crystallization temperature and an increase of crystallinity with increasing EPDM content regardless of the processing route. While EPDM has a weak effect on PHB glass transition temperature, PVB induces a marked decrease of this temperature thank to its plasticizer that swells the PHB amorphous phase. A promising solution to improve the mechanical properties of PHB seems to be the melt-processing of PHB with both plasticizer and EPDM. In fact, the plasticizer is more efficient than the elastomer in decreasing the PHB glass transition temperature and, because of the nucleating effect of EPDM, the decrease of the PHB modulus due to the plasticizer can be counterbalanced. (C) 2010 Society of Chemical Industry

Thermal and mechanical characteristics of polyurethane/curaua fiber composites

Polyurethane composites reinforced with curaua fiber at 5, 10 and 20% mass/mass proportions were prepared by using the conventional melt-mixing method. The influence of curaua fibers on the thermal behavior and polymer cohesiveness in polyurethane matrix was evaluated by dynamic mechanical thermal analysis (DMTA) and by differential scanning calorimetry (DSC). This specific interaction between the fibers and the hard segment domain was influenced by the behavior of the storage modulus E` and the loss modulus EaEuro(3) curves. The polyurethane PU80 is much stiffer and resistant than the other composites at low temperatures up to 70A degrees C. All samples were thermoplastic and presented a rubbery plateau over a wide temperature range above the glass transition temperature and a thermoplastic flow around 170A degrees C.

Polyethylene/(organo-montmorillonite) composites modified with ethylene/ methacrylic acid copolymer: Morphology and mechanical properties

Several composites based on high-density polyethylene (PE), organically modified montmorillonite (OMMT) and ethylene/methacrylic acid copolymer (EMAA) were prepared by melt compounding. Three Na(+)-montmorillonites (MMT) of different precedence were modified with hexadecyl trimethyl ammonium chloride in order to change their nature from hydrophilic to organophilic. The composites morphology was examined by XRD, SEM and TEM. Mechanical properties were evaluated under static conditions. A slight reinforcement was achieved only when OMMT was added to PE. When EMAA was added to the composites, it negatively interacted with OMMT, diminishing the interlayer distance of OMMT, changing the composite morphology, as if OMMT was not present in composites, and deteriorating their mechanical properties. (C) 2008 Elsevier Ltd. All rights reserved.

Toluene and naphthalene sorption by iron oxide/clay composites

Commercial bentonite (BFN) and organoclay (WS35), as well as iron oxide/clay composite (Mag_BFN) and iron/oxide organoclay composite (Mag_S35) were prepared for toluene and naphthalene sorption. Mag_BFN and Mag_S35 were obtained, respectively, by the precipitation of iron oxide hydrates onto sodium BFN and S35 clay particles. The materials were characterized by powder X-ray diffraction (XRD), X-ray Fluorescence (XRF), and TG and DTA. From XRF results and TG data on calcined mass basis, a quantitative method was developed to estimate the iron compound contents of the composites, as well as the organic matter content present in WS35 and Mag_S35.

Adsorbent new materials and composites produced in a single step

The aim of this work is the production and preliminary characterization of adsorbent new materials useful for sensor development. A new plasma chamber was simulated and designed in order to obtain multiple layers and/or composites in a single step. Plasma deposited organic fluorocompound and hexamethyldisilazane (HMDS) thin films were produced and tested as adsorbent layers. Chemical characterization used ellipsometry, Raman. infrared and X-ray photoelectron spectroscopy. Hydrophobic and oleophobic character were determined by contact angle measurements. Adsorption characteristics were evaluated using quartz crystal microbalance. Not only HMDS but also the fluorocompound can polymerize but intermixing and a double layer are only obtained in very narrow conditions. The films are adsorbent and mildly hydrophobic. Films deposited on a microchromatographic column can be used on sample pretreatment to remove and/or preconcentrate volatile organic Compounds. Therefore, with this approach it is possible to obtain films with different monomers on double layer or composites, with organic/inorganic materials or particles and use them on sample pretreatment for chemical analysis. (C) 2008 Elsevier B.V. All rights reserved.

Long-term quantification of the release of monomers from dental resin composites and a resin-modified glass ionomer cement

This study quantified the release of monomers from polymerized specimens of four commercially available resin composites and one glass ionomer cement immersed in water:ethanol solutions. Individual standard curves were prepared from five monomers: (1) triethylene glycol dimethacrylate (TEGDMA), (2) 2-hydroxy-ethyl methacrylate (HEMA), (3) urethane dimethacrylate (UDMA), (4) bisphenol A glycidyl dimethacrylate (BISGMA), and (5) bisphenol A. The concentration of the monomers was determined at Days 1, 7, 30, and 90 with the use of electrospray ionization/mass spectrometry. Data were expressed in mean mumol per mm(2) surface area of specimen and analyzed with Scheffe's test (P < 0.05). The following monomers were found in water: monomers (1) and (2) from Delton sealant, monomer (5) from ScotchBond Multipurpose Adhesive and Delton sealant, monomer (3) from Definite and monomer (4) from Fuji II LC, ScotchBond Multipurpose Adhesive, Synergy and Definite. All these monomers increased in concentration over time, with the exception of monomer (1) from Delton sealant. Monomers (3) and (5) were found in extracts of materials despite their absence from the manufacturer's published composition. All monomers were released in significantly higher concentrations in water:ethanol solutions than in water. The greatest release of monomers occurred in the first day. The effect of the measured concentrations of monomers (1-5) on human genes, cells, or tissues needs to be considered with the use of a biological model. (C) 2002 Wiley Periodicals, Inc.

A new approach to the synthesis of conjugated polymer-nanocrystal composites for heterojunction optoelectronics

We report a simple one pot process for the preparation of lead sulfide (PbS) nanocrystals in the conjugated polymer poly (2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV), and we demonstrate electronic coupling between the two components.

Finite element analysis of mechanical properties in discontinuously reinforced metal matrix composites with ultrafine microstructure

This investigation focused on the finite element analyses of elastic and plastic properties of aluminium/alumina composite materials with ultrafine microstructure. The commonly used unit cell model was used to predict the elastic properties. By combining the unit cell model with an indentation model, coupled with experimental indentation measurements, the plastic properties of the composites and the associated strengthening mechanism within the metal matrix material were investigated. The grain size of the matrix material was found to be an important factor influencing the mechanical properties of the composites studied. (C) 1997 Elsevier Science S.A.

Conductive hearing loss in children with bronchopulmonary dysplasia: A longitudinal follow-up study in children aged between 6 and 24 months

Aims: To determine the occurrence of isolated and recurrent episodes of conductive hearing loss (CHL) during the first two years of life in very low birth weight (VLBW) infants with and without bronchopulmonary dysplasia (BPD). Study design, subjects and outcome measures: In a longitudinal clinical study. 187 children were evaluated at 6, 9, 12,15 18 and 24 months of age by visual reinforcement audiometry, tympanometry and auditory brain response system. Results: Of the children with BPD, 54.5% presented with episodes of CHL, as opposed to 34.7% of the children without BPD. This difference was found to be statistically significant. The recurrent or persistent episodes were more frequent among children with BPD (25.7%) than among those without BPD (8.3%). The independent variables that contributed to this finding were small for gestational age and a 5 min Apgar score. Conclusions: Recurrent CHL episodes are more frequent among VLBW infants with BPD than among VLBW infants without BPD. (C) 2010 Elsevier Ireland Ltd. All rights reserved.

Do Low-shrink Composites Reduce Polymerization Shrinkage Effects?

Progress in polymer science has led to continuous reduction of polymerization shrinkage, exemplified by a new generation of ""low-shrink composites"". The common inference that shrinkage stress effects will be reduced in teeth restored with such restoratives with lower shrinkage was tested in extracted human premolars. Mesio-occluso-distal slot-shaped cavities were cut and restored with a conventional (SupremePlus) or low-shrink (RefleXions, Premise, Kalore, and LS) composite (N = 5). We digitized the coronal surfaces before and 10 min after restoration to determine cuspal deflection from the buccal and lingual volume change/area. We also determined the main properties involved (total shrinkage, post-gel shrinkage, degree of conversion, and elastic modulus), as well as microleakage, to verify adequate bonding. It was shown that, due to shrinkage stresses, buccal and lingual surfaces pulled inward after restoration (9-14 microns). Only Kalore and LS resulted in significantly lower tooth deformation (ANOVA/Student-Newman-Keuls post hoc, p = 0.05). The other two low-shrink composites, despite having the lowest and highest total shrinkage values, did not cause significant differences in cuspal deflection. Deflection seemed most related to the combination of post-gel shrinkage and elastic modulus. Therefore, even for significantly lower total shrinkage values, shrinkage stress is not necessarily reduced.

Characterization of dimethacrylate polymeric networks: A study of the crosslinked structure formed by monomers used in dental composites

The resin phase of dental composites is mainly composed of combinations of dimethacrylate comonomers, with final polymeric network structure defined by monomer type/reactivity and degree of conversion. This fundamental study evaluates how increasing concentrations of the flexible triethylene glycol dimethacrylate (TEGDMA) influences void formation in bisphenol A diglycidyl dimethacrylate (BisGMA) co-polymerizations and correlates this aspect of network structure with reaction kinetic parameters and macroscopic volumetric shrinkage. Photopolymerization kinetics was followed in real-time by a near-infrared (NIR) spectroscopic technique, viscosity was assessed with a viscometer, volumetric shrinkage was followed with a linometer, free volume formation was determined by positron annihilation lifetime spectroscopy (PALS) and the sol-gel composition was determined by extraction with dichloromethane followed by (1)H NMR analysis. Results show that, as expected, volumetric shrinkage increases with TEGDMA concentration and monomer conversion. Extraction/(1)H NMR studies show increasing participation of the more flexible TEGDMA towards the limiting stages of conversion/crosslinking development. As the conversion progresses, either based on longer irradiation times or greater TEGDMA concentrations, the network becomes more dense, which is evidenced by the decrease in free volume and weight loss after extraction in these situations. For the same composition (BisGMA/TEGDMA 60-40 mol%) light-cured for increasing periods of time (from 10 to 600 s), free volume decreased and volumetric shrinkage increased, in a linear relationship with conversion. However, the correlation between free volume and macroscopic volumetric shrinkage was shown to be rather complex for variable compositions exposed for the same time (600 s). The addition of TEGDMA decreases free-volume up to 40 mol% (due to increased conversion), but above that concentration, in spite of the increase in conversion/crosslinking, free volume pore size increases due to the high concentration of the more flexible monomer. In those cases, the increase in volumetric shrinkage was due to higher functional group concentration, in spite of the greater free volume. Therefore, through the application of the PALS model, this study elucidates the network formation in dimethacrylates commonly used in dental materials. (C) 2010 Elsevier Ltd. All rights reserved.