Computational evaluation of flexural toughness of FRC and fracture properties of plain concrete

In this paper, a computational tool concerning the computation of flexural and fracture toughness of cement based composites is presented. Firstly, RILEM`s (Reunion Internationale des Laboratoires d`Essais de Materiaux) recommendations related to the analysis of FRC in three-point bend tests are discussed in their relevant aspects regarding the computational implementations. The determination of other mechanical properties such as the Young modulus has been added to the program. Taking this into account, a new formulation based on displacements is used. In the second part of the paper, the determination of fracture properties of concrete, such as the fracture energy, G(F) , and the fracture toughness, K-IC(S), is discussed regarding the computational strategies used in the implementations. Several features whereby anterior data can be reanalyzed, obtained from other standards and recommendations, have been incorporated into the program, therefore allowing comparative studies and back analysis activities.

A critical reassessment of elastic unloading in sharp instrumented indentation experiments and the extraction of mechanical properties

This work examines the extraction of mechanical properties from instrumented indentation P-h(s) curves via extensive three-dimensional finite element analyses for pyramidal tips in a wide range of solids under frictional and frictionless contact conditions. Since the topography of the imprint changes with the level of pile-up or sink-in, a relationship is identified between correction factor beta in the elastic equation for the unloading indentation stage and the amount of surface deformation effects. It is shown that the presumption of a constant beta significantly affects mechanical property extractions. Consequently, a new best-fit function is found for the correlation between penetration depth ratios h(e)/h(max), h(r)/h(max) and n, circumventing the need for the assumption of a constant value for beta, made in our prior investigation [Acta Mater. 53 (2005) pp. 3545-3561]. Simulations under frictional contact conditions provide sensible boundaries for the influence of friction on both h(e)/h(max) and h(r)/h(max). Friction is essentially found to induce an overestimation in the inferred n. Instrumented indentation experiments are also performed in three archetypal metallic materials exhibiting distinctly different contact responses. Mechanical property extractions are finally demonstrated in each of these materials.

Finite element and dimensional analysis algorithm for the prediction of mechanical properties of bulk materials and thin films

In this work, the applicability of a new algorithm for the estimation of mechanical properties from instrumented indentation data was studied for thin films. The applicability was analyzed with the aid of both three-dimensional finite element simulations and experimental indentation tests. The numerical approach allowed studying the effect of the substrate on the estimation of mechanical properties of the film, which was conducted based on the ratio h(max)/l between maximum indentation depth and film thickness. For the experimental analysis, indentation tests were conducted on AISI H13 tool steel specimens, plasma nitrated and coated with TiN thin films. Results have indicated that, for the conditions analyzed in this work, the elastic deformation of the substrate limited the extraction of mechanical properties of the film/substrate system. This limitation occurred even at low h(max)/l ratios and especially for the estimation of the values of yield strength and strain hardening exponent. At indentation depths lower than 4% of the film thickness, the proposed algorithm estimated the mechanical properties of the film with accuracy. Particularly for hardness, precise values were estimated at h(max)/l lower than 0.1, i.e. 10% of film thickness. (C) 2010 Published by Elsevier B.V.

Structure and properties of low temperature plasma carburized austenitic stainless steels

Austenitic stainless steels cannot be conventionally surface treated at temperatures close to 550 degrees C due to intense precipitation of nitrides or carbides. Plasma carburizing allows introducing carbon in the steel at temperatures below 500 degrees C without carbide precipitation. Plasma carburizing of AISI 316L was carried out at 480 degrees C and 400 degrees C, during 20 h, using CH(4) as carbon carrier gas. The results show that carbon expanded austenite (gamma(c)), 20 mu m in depth, was formed on the surface after the 480 degrees C treatment. Carbon expanded austenite (gamma(c)), 8 mu m in depth, was formed on the surface after the 400 degrees C treatment. DRX results showed that the austenitic FCC lattice parameter increases from 0.358 nm to 0.363 nm for the 400 degrees C treatment and to 0.369 nm for the 480 degrees C treatment, giving an estimation of circa 10 at.% carbon content for the latter. Lattice distortion, resulting from the expansion and the associated compressive residual stresses increases the surface hardness to 1040 HV(0.025). Micro-scale tensile tests were conducted on specimens prepared with the conditions selected above, which has indicated that the damage imposed to the expanded austenite layer was more easily related to each separated grain than to the overall macro-scale stresses imposed by the tensile test. (C) 2009 Elsevier B.V. All rights reserved.

Influence of parameters of the HVOF thermal spray process on the properties of multicomponent white cast iron coatings

High velocity oxi-fuel (HVOF) thermal spray process has been used in order to deposit a new alloy known as multicomponent white cast iron. The coatings were characterized in terms of macrostructure, phase composition, porosity and hardness. Coating characteristics and properties were found to be dependent on the particles size range, spray distance, gases flow rate and oxygen to propane ratio. For set of parameters utilized in this job a narrow particle size range between 20 and 45 gm with a spray distance of 200 mm and oxygen to propane ratio of 4.6 are the preferred coating parameters. Coating porosity of 0.9% and hardness of 766 HV were obtained under these conditions. (c) 2007 Elsevier B.V. All rights reserved.

MEASUREMENT OF ELASTIC PROPERTIES OF MATERIALS BY THE ULTRASONIC THROUGH-TRANSMISSION TECHNIQUE

The elastic mechanical behavior of elastic materials is modeled by a pair of independent constants (Young`s modulus and Poisson`s coefficient). A precise measurement for both constants is necessary in some applications, such as the quality control of mechanical elements and standard materials used for the calibration of some equipment. Ultrasonic techniques have been used because wave velocity depends on the elastic properties of the propagation medium. The ultrasonic test shows better repeatability and accuracy than the tensile and indentation test. In this work, the theoretical and experimental aspects related to the ultrasonic through-transmission technique for the characterization of elastic solids is presented. Furthermore, an amorphous material and some polycrystalline materials were tested. Results have shown an excellent repeatability and numerical errors that are less than 3% in high-purity samples.

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.

Photodegradation of Thermodegraded Polypropylene/High-Impact Polystyrene Blends: Mechanical Properties

The influence of the addition of high-impact polystyrene (HIPS) on polypropylene (PP) photodegradation was studied with blends obtained by extrusion with and without styrene-butadiene-styrene (SBS) copolymer (10 wt % with respect to the dispersed phase). The concentrations of HIPS ranged from 10 to 30 wt %. The blends and pure materials were exposed for periods of up to 15 weeks of UV irradiation; their mechanical properties (tensile and impact), fracture surface, and melt flow indices were monitored. After 3 weeks of UV exposure, all of the materials presented mechanical properties of the same order of magnitude. However, for times of exposure greater than 3 weeks, an increasing concentration of HIPS resulted in a better photostability of PP. These results were explained in light of morphological observations. This increase of photostability was even greater when SBS was added to the blends. It was more difficult to measure the melt flow index of the binary PP/HIPS blends than that of PP for low concentrations of HIPS; this was most likely due to energy transfer between the blend domains during photodegradation. This phenomenon was not observed for the ternary blends. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 770-779, 2011

Influence of the Type of Quaternary Ammonium Salt Used in the Organic Treatment of Montmorillonite on the Properties of Poly(Styrene-co-Butyl Acrylate)/Layered Silicate Nanocomposites Prepared by In Situ Miniemulsion Polymerization

Hybrid latices of poly(styrene-co-butyl acrylate) were synthesized via in situ miniemulsion polymerization in the presence of 3 and 6 wt % organically modified montmorillonite (OMMT). Three different ammonium salts: cetyl trimethyl ammonium chloride (CTAC), alkyl dimethyl benzyl ammonium chloride (Dodigen), and distearyl dimethyl ammonium chloride (Praepagen), were investigated as organic modifiers. Increased affinity for organic liquids was observed after organic modification of the MMT. Stable hybrid latices were obtained even though miniemulsion stability was disturbed to some extent by the presence of the OMMTs during the synthesis. Highly intercalated and exfoliated polymer-MMT nanocomposites films were produced with good MMT dispersion throughout the polymeric matrix. Materials containing MMT modified with the 16 carbons alkyl chain salt (CTAC) resulted in the largest increments of storage modulus, indicating that single chain quaternary salts provide higher increments on mechanical properties. Films presenting exfoliated structure resulted in the largest increments in the onset temperature of decomposition. For the range of OMMT loading studied, the nanocomposite structure influenced more significantly the thermal stability properties of the hybrid material than did the OMMT loading. The film containing 3 wt % MMT modified with the two 18 carbons alkyl chains salt (Praepagen) provided the highest increment of onset temperature of decomposition. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 3658-3669, 2011

Structural and magnetic properties of pure and nickel doped SnO(2) nanoparticles

Ni-doped SnO(2) nanoparticles prepared by a polymer precursor method have been characterized structurally and magnetically. Ni doping (up to 10 mol%) does not significantly affect the crystalline structure of SnO(2), but stabilizes smaller particles as the Ni content is increased. A notable solid solution regime up to similar to 3 mol% of Ni, and a Ni surface enrichment for the higher Ni contents are found. The room temperature ferromagnetism with saturation magnetization (MS) similar to 1.2 x 10(-3) emu g(-1) and coercive field (H(C)) similar to 40 Oe is determined for the undoped sample, which is associated with the exchange coupling of the spins of electrons trapped in oxygen vacancies, mainly located on the surface of the particles. This ferromagnetism is enhanced as the Ni content increases up to similar to 3 mol%, where the Ni ions are distributed in a solid solution. Above this Ni content, the ferromagnetism rapidly decays and a paramagnetic behavior is observed. This finding is assigned to the increasing segregation of Ni ions (likely formed by interstitials Ni ions and nearby substitutional sites) on the particle surface, which modifies the magnetic behavior by reducing the available oxygen vacancies and/or the free electrons and favoring paramagnetic behavior.

Experimental study of the structural, microscopy and magnetic properties of Ni-doped SnO(2) nanoparticles

A polymer precursor method has been used to synthesize Ni-doped SnO(2) nanoparticles. X-ray diffraction (XRD) data analyses indicate the exclusive formation of nanosized particles with rutile-type phase (tetragonal SnO(2)) for Ni contents below 10 mol%. In this concentration range, the particle sizes decrease with increasing Ni content and a bulk solid solution limit was determined at similar to 1 mol%. Ni surface enrichment is present at concentrations higher than the solution limit. Only above 10 mol% Ni. the formation of a second NiO-related phase has been determined. Magnetization measurements suggest the occurrence of ferromagnetism for samples in the solid solution regime (below similar to 1 mol%). This ferromagnetism is associated with the exchange interaction between electron spins trapped on oxygen vacancies, and is enhanced as the amount of Ni(2+) substituting at Sn(4+) sites increases. Above the solid solution limit, ferromagnetism is destroyed by the Ni surface enrichment and the system behaves as a paramagnet. (C) 2010 Elsevier B.V. All rights reserved.

Thermal properties of amphiphilic biodegradable triblock copolymer of l,l-lactide and ethylene glycol

Samples of poly(l,l-lactide)-block-poly(ethylene glycol)-block-poly(l,l-lactide) (PLLA-PEG-PLLA) were synthesized from l,l-lactide polymerization using stannous 2-ethylhexanoate, Sn(Oct)(2) as initiator and di-hydroxy-terminated poly(ethylene glycol) (PEG) (M (n) = 4000 g mol(-1)) as co-initiator. The chemical linkage between the PEG segment and the PLA segments was characterized by Fourier transform infrared spectroscopy (FTIR). Thermogravimetry analysis (TG) revealed the copolymers composition and was capable to show the deleterious effect of an excess of Sn(Oct)(2) in the polymer thermal stability, while Differential Scanning Calorimetry (DSC) allowed the observation of the miscibility between the PLLA and PEG segments in the different copolymers.

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

Controlling the luminescence properties of poly(p-phenylene vinylene) entrapped in Langmuir and Langmuir-Blodgett films of stearic acid

Properties of hybrid films can be enhanced if their molecular architecture is controlled. In this paper, poly (p-phenylene vinylene) was mixed with stearic acid in order to form stable hybrid Langmuir monolayers. Surface properties of these films were investigated with measurements of surface pressure, and also with polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS). The films were transferred from the air-water interface to solid supports through the Langmuir-Blodgett technique, and the viability of the film as optical device was investigated with fluorescence spectroscopy. Comparing the fluorescent spectra for the polymer in solution, as a casting film, and as an LB film, the emission bands for LB films were narrower and appeared at lower wavelengths. The interactions between the film components and the design for the LB film may take advantage of the method to immobilize luminescent polymers in mixed ultrathin films adsorbed in solid matrices. (C) 2011 Elsevier B.V. All rights reserved.