A stress relaxation model for the viscoelastic solids based on the steady-state creep equation

Creep and stress relaxation are inherent mechanical behaviors of viscoelastic materials. It is considered that both are different performances of one identical physical phenomenon. The relationship between the decay stress and time during stress relaxation has been derived from the power law equation of the steady-state creep. The model was used to analyse the stress relaxation curves of various different viscoelastic materials (such as pure polycrystalline ice, polymers, foods, bones, metal, animal tissues, etc.). The calculated results using the theoretical model agree with the experimental data very well. Here we show that the new mathematical formula is not only simple but its parameters have the clear physical meanings. It is suitable to materials with a very broad scope and has a strong predictive ability.

Structure and magnetic properties of mono- and bi-dispersed ferrofluids as revealed by simulations

Using a Langevin molecular dynamics simulation, we show that the magnetic properties of a mono- and bi-dispersed ferrofluid system depend on the volume fraction and the dipolar coupling parameter. For the bi-dispersed system, most of the chains are formed by the large particles, but the aggregation behavior of the large particles is hindered by the presence of the small particles, which are predominantly attached to the end of the particle chain. To further elucidate the microscopic fluid structure, anisotropic structure factors are calculated.

Microstructural, structural and electrical properties of La(3+)-modified Bi(4)Ti(3)O(12) ferroelectric ceramics

Bi(4-x)La(x)Ti(3)O(12) (BLT) ceramics were prepared and studied in this work in terms of La(3+)-modified microstructure and phase development as well as electrical response. According to the results processed from X-ray diffraction and electrical measurements, the solubility limit (XL) of La(3+) into the Bi(4)Ti(3)O(12) (BIT) matrix was here found to locate slightly above x = 1.5. Further, La(3+) had the effect of reducing the material grain size, while changing its morphology from the plate-like form, typical of BIT ceramics, to a spherical-like one. The electrical results presented and discussed here also include the behavior of the temperature of the ferroelectric-paraelectric phase transition as well as the normal or diffuse and/or relaxor nature of this transition depending on the La(3+) content. (c) 2008 Elsevier Ltd. All fights reserved.

Thermo-optics parameters measurements in photorefractive sillenite Bi(12)SiO(20) crystals by thermal lens technique

The present work reports on the thermo-optical properties of photorefractive sillenite Bi(12)SiO(20) (BSO) crystals obtained by applying the Thermal Lens Spectrometry technique (TLS). This crystals presents one high photorefractive sensitivity in the region blue-green spectra, since the measurements were carried out at two pump beam wavelengths (514.5 nm and 750 nm) to study of the light-induced effects in this material (thermal and/or photorefractive). We determine thermo-optical parameters like thermal diffusivity (D), thermal conductivity (K) and temperature coefficient of the optical path length change (ds/dT) in sillenite crystals. These aspects, for what we know, not was studied in details up to now using the lens spectrometry technique and are very important against of the promising potentiality of applications these crystals in non linear optics, real time holography and optical processing data.

Processing and hygrothermal effects on viscoelastic behavior of glass fiber/epoxy composites

Fiber reinforced epoxy composites are used in a wide variety of applications in the aerospace field. These materials have high specific moduli, high specific strength and their properties can be tailored to application requirements. In order to screening optimum materials behavior, the effects of external environments on the mechanical properties during usage must be clearly understood. The environmental action, such as high moisture concentration, high temperatures, corrosive fluids or ultraviolet radiation (UV), can affect the performance of advanced composites during service. These factors can limit the applications of composites by deteriorating the mechanical properties over a period of time. Properties determination is attributed to the chemical and/or physical damages caused in the polymer matrix, loss of adhesion of fiber/resin interface, and/or reduction of fiber strength and stiffness. The dynamic elastic properties are important characteristics of glass fiber reinforced composites (GRFC). They control the damping behavior of composite structures and are also an ideal tool for monitoring the development of GFRC's mechanical properties during their processing or service. One of the most used tests is the vibration damping. In this work, the measurement consisted of recording the vibration decay of a rectangular plate excited by a controlled mechanism to identify the elastic and damping properties of the material under test. The frequency amplitude were measured by accelerometers and calculated by using a digital method. The present studies have been performed to explore relations between the dynamic mechanical properties, damping test and the influence of high moisture concentration of glass fiber reinforced composites (plain weave). The results show that the E' decreased with the increase in the exposed time for glass fiber/epoxy composites specimens exposed at 80 degrees C and 90% RH. The E' values found were: 26.7, 26.7, 25.4, 24.7 and 24.7 GPa for 0, 15, 30, 45 and 60 days of exposure, respectively. (c) 2005 Springer Science + Business Media, Inc.

Damping behavior of continuous fiber/metal composite materials by the free vibration method

Fiber metal laminates (FML) offer significant improvements over current available materials for aircraft structures due to their excellent mechanical characteristics and relatively low density. Non-destructive testing techniques are being used in the characterization of composite materials. Among these, vibration testing is one of the most used tools because it allows the determination of the mechanical properties. In this work, the viscoelastic properties such as elastic (E') and viscous (E) responses were obtained for aluminum 2024 alloy; carbon fiber/epoxy; glass fiber/epoxy and their hybrids aluminum 2024 alloy/carbon fiber/epoxy and aluminum 2024 alloy/glass fiber/epoxy composites. The experimental results were compared to calculated E modulus values by using the composite micromechanics approach. For all specimens studied, the experimental values showed good agreement with the theoretical values. The damping behavior, i.e. The storage modulus and the loss factor, from the aluminum 2024 alloy and fiber epoxy composites can be used to estimate the viscoelastic response of the hybrid FML. (c) 2005 Elsevier Ltd. All rights reserved.

Effect of microwave irradiation and water storage on the viscoelastic properties of denture base and reline acrylic resins

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Exothermic Behavior, Degree of Conversion, and Viscoelastic Properties of Experimental and Commercially Available Hard Chairside Reline Resins

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Simultaneous and Direct Determination of As, Bi, Pb, Sb, and Se and Co, Cr, Cu, Fe, and Mn in Milk by Electrothermal Atomic Absorption Spectrometry

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Evaluation of Ir, Ru, and W as permanent modifiers for the simultaneous determination of As, Bi, Pb, Sb, and Se in natural water by electrothermal atomic absorption spectrometry

Iridium-, Ru-, and W-coated platforms were prepared by thermal treatment of the transversely heated graphite atomizer and investigated for the simultaneous determination of As, Bi, Pb, Sb, and Se in tap water by electrothermal atomic absorption spectrometry. The maximum pyrolysis temperature for As and Bi increased in a modifier sequence W < Ru < Ir. For Pb, Sb, and Se, this sequence was W < Ru, It. Calculated characteristic masses in the presence of It, Ru, and W were 35, 33, and 35 pg for As; 63, 51, and 52 pg for Bi; 50, 32, and 34 pg for Pb; 40, 35, and 31 pg for Sb; and 39, 39, and 93 pg for Se, respectively. Ruthenium was elected as the optimum modifier.Repeatability of the measurements was typically < 6%. Recoveries of As, Bi, Pb, Sb, and Se added to tap water samples varied from 79 to 109%. Accuracy was also checked by analysis of five certified reference materials (CRMs) from the National Institute of Standards and Technology (NIST1640 - Trace Elements in Natural Water; NIST 1643d Trace Elements in Water) and High Purity Standards (Trace Metals in Drinking Water Standards, lots #812708, #591107, and #710710). A paired t-test showed that the results for the CRMs were in agreement at the 95% confidence level with the certified values. The graphite tube lifetime was about 650 firings. multi-element determination is particularly challenging due to the necessity of carefully optimizing compromise conditions.Based on the considerations listed above, the aim of this paper was to evaluate the behavior of Ir, Ru, and W as permanent modifiers for the simultaneous determination of As, Bi, Pb, Sb, and Se. The performance of the proposed procedure was also verified after the ETAAS analysis of tap waters and reference materials.

Evolution of rheological properties and local structure during gelation of siloxane-polymethylmethacrylate hybrid materials

Hybrid siloxane-polymethylmethacrylate (PMMA) nanocomposites with covalent bonds between the inorganic (siloxane) and organic (polymer) phases were prepared by the sot gel process through hydrolysis and polycondensation of 3-(trimethoxysilyl)propylmethacrylate (TMSM) and polymerization of methylmethacrylate (MMA) using benzoyl peroxide (BPO) as initiator. The effect of MMA, BPO and water contents on the viscoelastic behaviour of these materials was analysed during gelation by dynamic rheological measurements. The changes in storage (G') and loss moduli (G), complex viscosity (eta*) and phase angle (6) were measured as a function of the reaction time showing the viscous character of the sot in the initial step of gelation and its progressive transformation to an elastic gel. This study was complemented by Si-29 and C-13 solid-state nuclear magnetic resonance (NMR/MAS) measurements of dried gel. The analysis of the experimental results shows that linear chains are formed in the initial step of the gelation followed by a growth of branched structures and formation of a three-dimensional network. Near the gel point this hybrid material demonstrates the typical scaling behaviour expected from percolation theory.

Effect of combined bending strain and thermal cycling on the voltage-current characteristic curves of Bi-2223 tapes

High critical temperature superconductors are evolving from a scientific research subject into large-scale application devices. In order to meet this development demand they must withstand high current capacity under mechanical loads arising from thermal contraction during cooling from room temperature down to operating temperature (usually 77 K) and due to the electromagnetic forces generated by the current and the induced magnetic field. Among the HTS materials, the Bi2Sr2Ca2Cu3Ox, compound imbedded in an Ag/AgMg sheath has shown the best results in terms of critical current at 77 K and tolerance against mechanical strain. Aiming to evaluate the influence of thermal stress induced by a number of thermal shock cycles we have evaluated the V-I characteristic curves of samples mounted onto semicircular holders with different curvature radius (9.75 to 44.5 mm). The most deformed sample (epsilon = 1.08%) showed the largest reduction of critical current (40%) compared to the undeformed sample and the highest sensitivity to thermal stress (I-c/I-c0 = 0.5). The V-I characteristic curves were also fitted by a potential curve displaying n-exponents varying from 20 down to 10 between the initial and last thermal shock cycle.

Evaluation of Bi as internal standard to minimize matrix effects on the direct determination of Pb in vinegar by graphite furnace atomic absorption spectrometry using Ru permanent modifier with co-injection of Pd/Mg(NO3)(2)

Bismuth was evaluated as an internal standard for the direct determination of Pb in vinegar by graphite furnace atomic absorption spectrometry using Ru as a permanent modifier with co-injection of Pd/Mg(NO3)(2). The correlation coefficient of the graph plotted from the non-nalized absorbance signals of Bi versus Pb was r=0.989. Matrix effects were evaluated by analyzing the slope ratios between the analytical curve, and analytical curves obtained from Pb additions in red and white wine vinegar obtained from reference solutions prepared in 0.2% (v/v) HNO3, samples. The calculated ratios were around 1.04 and 1.02 for analytical curves established applying an internal standard and 1.3 and 1.5 for analvtical curves without. Analytical curves in the 2.5-15 pg L-1 Pb concentration interval were established using the ratio Pb absorbance to Bi absorbance versus analvte concentration, and typical linear correlations of r=0.999 were obtained. The proposed method was applied for direct determination of Pb in 18 commercial vinegar samples and the Pb concentration varied from 2.6 to 31 pg L-1. Results were in agreement at a 95% confidence level (paired t-test) with those obtained for digested samples. Recoveries of Pb added to vinegars varied from 96 to 108% with and from 72 to 86% without an internal standard. Two water standard reference materials diluted in vinegar sample were also analyzed and results were in agreement with certified values at a 95% confidence level. The characteristic mass was 40 pg Pb and the useful lifetime of the tube was around 1600 firings. The limit of detection was 0.3 mu g L-1 and the relative standard deviation was <= 3.8% and <= 8.3% (n = 12) for a sample containing, 10 mu L-1 Pb with and without internal standard, respectively. (C) 2007 Elsevier B.V. All rights reserved.

Evaluation of sample treatment for the simultaneous flow injection hydride generation and determination of As, Bi, Sb and Se by GRAS

A new method was developed for the simultaneous determination of As, Bi, Sb, and Se by flow injection hydride generation graphite furnace atomic absorption spectrometry. An alternative two-step sample treatment procedure was used. The sample was heated (80degreesC) for 10 min in 6 M HCl to reduce Se(VI) to Se(IV), followed by the addition of 1% (m/v) thiourea solution to reduce arsenic and antimony from the pentavalent to the trivalent states.With this procedure, all analytes were converted to their most favorable and sensitive oxidation states to generate the corresponding hydrides. The pre-treated sample solution was then processed in the flow system for in situ trapping and atomization in a graphite tube coated with iridium. The impermanent modifier remained stable up to 300 firings and new coating out significant were possible wit changes in the analytical performance.The accuracy was checked for As, Bi, Sb, and Se determination in water standard reference materials NIST 1640 and 1643d and the results were in agreement with the certified values at a 95% confidence level. Good recoveries (94-104%.) of spiked mineral waters and synthetic As(V), Sb(Ill), mixtures of As(Ill), Sb(V), Se(VI), and Se(IV) were also found. Calculated characteristic masses were 32 mug As, 79 mug Bi, 35 mug Sb, and 130 pg Se, and the corresponding limits of detection were 0.06, 0.16, 0.19, and 0.59 mug L-1, respectively. The repeatability for a typical solution containing 5 mug L-1 As, Bi, Sb, and Se was in the 1-3% range.