A modified criterion for shear band formation in bulk metallic glass under complex stress states

A new criterion for shear band formation in metallic glasses is proposed based on the shear plane criterion proposed by Packard and Schuh [1]. This modified shear plane (MSP) criterion suggests that a shear band is not initiated randomly throughout the entire material under stress but is initiated at the physical boundaries or defects and at locations where the highest normal stress modified maximum shear stress occurs. Moreover, the same as in the shear plan criterion, the shear stress all over the shear band should exceed the shear yield strength of the material. For a complete shear band to form, both requirements need to be fulfilled. The shear yield strength of the material is represented by the shear stress of the point at which the shear band stops. The new criterion agrees very well with experimental results in both the determination of the shear yield strength and the shear band path. (C) 2010 Elsevier B.V. All rights reserved.

Strain rate-dependent shear band behavior in bulk metallic glasses

In this article, we review our recent advances in understanding the deformation behavior of a typical tough Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vit 1) bulk metallic glass (BMG), as a model material, under various loading modes and strain rates, focusing particularly on the rate-dependence and formation mechanism of shear-banding. Dynamic and quasi-static mechanical experiments, including plate shear, shear punch and spherical indentation, and continuum as well as atomistic modeling on shear-banding are discussed. The results demonstrate that higher strain rate slows down the annihilation process of free volume, but promotes the free-volume coalescence, which is responsible for the rate-dependent shear banding. The physical origin of shear bands, that is the free volume softening underpinned by irreversible rearrangements of atoms, is unveiled. Finally, some concluding remarks are given.

A multi-scale numerical modelling of crack propagation in a 2D metallic plate

A new multi-scale model of brittle fracture growth in an Ag plate with macroscopic dimensions is proposed in which the crack propagation is identified with the stochastic drift-diffusion motion of the crack-tip atom through the material. The model couples molecular dynamics simulations, based on many-body interatomic potentials, with the continuum-based theories of fracture mechanics. The Ito stochastic differential equation is used to advance the tip position on a macroscopic scale before each nano-scale simulation is performed. Well-known crack characteristics, such as the roughening transitions of the crack surfaces, as well as the macroscopic crack trajectories are obtained.

An application of bi-directional evolutionary structural optimisation for optimising energy absorbing structures using a material damage model

The Bi-directional Evolutionary Structural Optimisation (BESO) method is a numerical topology optimisation method developed for use in finite element analysis. This paper presents a particular application of the BESO method to optimise the energy absorbing capability of metallic structures. The optimisation objective is to evolve a structural geometry of minimum mass while ensuring that the kinetic energy of an impacting projectile is reduced to a level which prevents perforation. Individual elements in a finite element mesh are deleted when a prescribed damage criterion is exceeded. An energy absorbing structure subjected to projectile impact will fail once the level of damage results in a critical perforation size. It is therefore necessary to constrain an optimisation algorithm from producing such candidate solutions. An algorithm to detect perforation was implemented within a BESO framework which incorporated a ductile material damage model.

Development of Free Space Method for Material Characterisation and GPR Study & A Case Study on Biological Samples

The thesis mainly focuses on material characterization in different environments: freely available samples taken in planar fonn, biological samples available in small quantities and buried objects.Free space method, finds many applications in the fields of industry, medicine and communication. As it is a non-contact method, it can be employed for monitoring the electrical properties of materials moving through a conveyor belt in real time. Also, measurement on such systems at high temperature is possible. NID theory can be applied to the characterization of thin films. Dielectric properties of thin films deposited on any dielectric substrate can be determined. ln chemical industry, the stages of a chemical reaction can be monitored online. Online monitoring will be more efficient as it saves time and avoids risk of sample collection.Dielectric contrast is one of the main factors, which decides the detectability of a system. lt could be noted that the two dielectric objects of same dielectric constant 3.2 (s, of plastic mine) placed in a medium of dielectric constant 2.56 (er of sand) could even be detected employing the time domain analysis of the reflected signal. This type of detection finds strategic importance as it provides solution to the problem of clearance of non-metallic mines. The demining of these mines using the conventional techniques had been proved futile. The studies on the detection of voids and leakage in pipes find many applications.The determined electrical properties of tissues can be used for numerical modeling of cells, microwave imaging, SAR test etc. All these techniques need the accurate determination of dielectric constant. ln the modem world, the use of cellular and other wireless communication systems is booming up. At the same time people are concemed about the hazardous effects of microwaves on living cells. The effect is usually studied on human phantom models. The construction of the models requires the knowledge of the dielectric parameters of the various body tissues. lt is in this context that the present study gains significance. The case study on biological samples shows that the properties of normal and infected body tissues are different. Even though the change in the dielectric properties of infected samples from that of normal one may not be a clear evidence of an ailment, it is an indication of some disorder.ln medical field, the free space method may be adapted for imaging the biological samples. This method can also be used in wireless technology. Evaluation of electrical properties and attenuation of obstacles in the path of RF waves can be done using free waves. An intelligent system for controlling the power output or frequency depending on the feed back values of the attenuation may be developed.The simulation employed in GPR can be extended for the exploration of the effects due to the factors such as the different proportion of water content in the soil, the level and roughness of the soil etc on the reflected signal. This may find applications in geological explorations. ln the detection of mines, a state-of-the art technique for scanning and imaging an active mine field can be developed using GPR. The probing antenna can be attached to a robotic arm capable of three degrees of rotation and the whole detecting system can be housed in a military vehicle. In industry, a system based on the GPR principle can be developed for monitoring liquid or gas through a pipe, as pipe with and without the sample gives different reflection responses. lt may also be implemented for the online monitoring of different stages of extraction and purification of crude petroleum in a plant.Since biological samples show fluctuation in the dielectric nature with time and other physiological conditions, more investigation in this direction should be done. The infected cells at various stages of advancement and the normal cells should be analysed. The results from these comparative studies can be utilized for the detection of the onset of such diseases. Studying the properties of infected tissues at different stages, the threshold of detectability of infected cells can be determined.

Nonlinear and magneto-optical transmission studies on magnetic nanofluids of non-interacting metallic nickel nanoparticles

Oxide free stable metallic nanofluids have the potential for various applications such as in thermal management and inkjet printing apart from being a candidate system for fundamental studies. A stable suspension of nickel nanoparticles of ∼5 nm size has been realized by a modified two-step synthesis route. Structural characterization by x-ray diffraction and transmission electron microscopy shows that the nanoparticles are metallic and are phase pure. The nanoparticles exhibited superparamagnetic properties. The magneto-optical transmission properties of the nickel nanofluid (Ni-F) were investigated by linear optical dichroism measurements. The magnetic field dependent light transmission studies exhibited a polarization dependent optical absorption, known as optical dichroism, indicating that the nanoparticles suspended in the fluid are non-interacting and superparamagnetic in nature. The nonlinear optical limiting properties of Ni-F under high input optical fluence were then analyzed by an open aperture z-scan technique. The Ni-F exhibits a saturable absorption at moderate laser intensities while effective two-photon absorption is evident at higher intensities. The Ni-F appears to be a unique material for various optical devices such as field modulated gratings and optical switches which can be controlled by an external magnetic field

NOVEL CELLULOSE/NbOPO4 center dot nH(2)O HYBRID MATERIAL FROM SUGARCANE BAGASSE

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

Influence of Crown Ferrule Heights and Dowel Material Selection on the Mechanical Behavior of Root-Filled Teeth: A Finite Element Analysis

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

Effect of variations in annealing temperature and metallic cations on nanostructured molybdate thin films

Crystalline molybdate thin films were prepared by the complex polymerization method. The AMoO(4) (A = Ca, Sr, Ba) films were deposited onto Si wafers by the spinning technique. The Mo-O bond in the AMoO(4) structure was confirmed by FTIR spectra. X-ray diffraction revealed the presence of crystalline scheelite-type phase. The mass, size, and basicity of A(2+) cations was found to be dependent on the intrinsic characteristics of the materials. The grain size increased in the following order: CaMoO4 < SrMoO4 < BaMoO4. The emission band wavelength was detected at around 576 nm. Our findings suggest that the material's morphology and photoluminescence were both affected by the variations in cations (Ca, Sr, or Ba) and in the thermal treatment.

Flow evolution of a metallic slurry during thixoextrusion of an automotive valve

The aim of this work is to characterize a metallic slurry (Al-4.5%Cu) flow during thixoforming of an automotive valve. The necessary globular structure was obtained by first inoculating the alloy with TIBAL (5%Ti, 1%B, Al - rest) at 750.0°C, and then reheating to a state between liquidus and solidus prior to thixoforming. Two metallic slurries, with a solid phase of approximately 86.1 and 78.2 percent, were used to generate different experimental flow patterns during the thixoforming process. The flow of the material into the die was observed for total, and partial displacement (2.7, 5.4, 7.5mm) of the punch. The first displacement shows formation of the valve rod. The patterns at each step of displacement of the punch were preserved by quenching in water, thus revealing the profile of the die fill and microstructural evolution. Degeneration of the globular phase was observed along the piece thixoextruded. Thixoextrusion forces versus time curves were generated for partial and full displacement of the punch. Porosity was visible along the billet prior to thixoforming. However, some areas show that the porosity gradually decreased to zero as the thixoextrusion pressure increased. Turbulent, transient and laminar flow are analyzed in this work.

Novel cellulose/NbOPO 4.nH 2O hybrid material from sugarcane bagasse

In recent years studies concerning the applications of lignocellulosic/ inorganic couples have resulted in the development of an interesting class of functional materials. In this work a cellulose/NbOPO 4.nH 2O hybrid using cellulose from surgacane bagasse was prepared and characterized in order to test for adsorption applications. The preparation process was conducted by carrying out metallic niobium dilution in hydrofluoric acid in the presence of nitric acid, then adding boric acid to form the complex and, finally, the cellulose sugar cane bagasse was added. Concentrated phosphoric acid was also inserted to precipitate hydrous niobium phosphate particles in the cellulose fiber. This material was characterized by X-ray diffractometry (XRD), thermogravimetry (TG/DTG), and scanning electronic microscopy (SEM) connected to an energy dispersive spectrophotometer (EDS). Results by SEM/EDS show that NbOPO 4.nH 2O was present in structure of the cellulose. During the preparation of the material, using boric acid it was observed that the formation of precipitate occurred in a shorter time than the material prepared without boric acid.

Supplementation of soft drinks with metallic ions reduces dissolution of bovine enamel

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

Comparação da desadaptação marginal de infra-estruturas metálicas obtidas com variações na técnica de fundição, revestimentos e material para obtenção dos padrões

Pós-graduação em Odontologia - FOA

Avaliação de risco ambiental por contaminação metálica e material orgânico em sedimentos da bacia do Rio Aurá, Região Metropolitana de Belém - PA

A bacia do Rio Aurá está situada na região metropolitana de Belém, entre os municípios de Belém e Ananindeua, onde a taxa populacional tem aumentado sem qualquer medida de controle social ou ambiental. A região é intensamente explorada, sendo que os principais problemas ambientais são o desmatamento, erosão, inundação, poluição e contaminação das águas, especialmente por metais pesados e compostos orgânicos. O comportamento geoquímico dos elementos Al, Fe, Mn, Cr, Ni e Cu e os teores de compostos orgânicos foram avaliados em 30 pontos de amostragem no período entre 2008 e 2010 nos sedimentos fluviais. O aterro sanitário não controlado localizado nas proximidades da bacia do Rio Aurá é responsável, em parte, pela contaminação dos sedimentos. O estresse ambiental é resultado das atividades antrópicas locais, que contribuem no transporte de material clástico contendo metais para o rio. As variáveis estudadas foram classificadas segundo mecanismos de transporte e fonte (autóctone ou alóctone). Os resultados demonstraram que a principal contribuição de íons Al e Fe foi o aterro sanitário; Mn e Ni vieram principalmente dos solos adjacentes; Cr foi modificado (III/VI) por processo alobioquímico e Cu por processo bioinduzido.

The role of superstructure material on the stress distribution in mandibular full-arch implant-supported fixed dentures. A CT-based 3D-FEA

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