Effect of input variables uncertainty in free tube hydroforming process

Tube hydroforming (THF) is a plastic forming process that uses tubes with an initial circular cross section, in which pressurized fluid and axial feeds are applied for producing parts with various cross-sectional shapes. Despite of the complexity of THF process, a great progress in the automotive and aerospace industry has been made due to its advantages, such as, consolidation and weight reduction over conventional stamped and welded parts. The analysis of THF process is typically based on deterministic approaches, excluding scattering effects that influence the process reliability. Thus, robust design of tube hydroforming aims to vanish noise factors effects on process responses by considering the influence of process parameters variability. If this fluctuation is not monitored, then the fluctuation of the hydroformed parts quality may contribute to high scrap rates. In this work, the influence of variability in the THF material and process parameters (e.g. yield stress, strength coefficient, strain hardening exponent, plastic anisotropy, initial tube thickness and bulged length) on the bursting pressure is analyzed resorting to a response surface model. The statistically significant variables, which mostly influence the free bulge hydroforming process, are identified through an analysis of variance. Assuming that the input parameters variability follows the normal distribution, the probability distribution of the bursting pressure is evaluated by involving random process variables into the built response surface model. It was shown that the initial tube thickness is the most statistically significant variable, whereas the strain hardening exponent is the least statistically significant variable.publishe

Characterization of Ni-CNTs Nanocomposites Produced by Ball-Milling

This research focuses on the characterization of a metal matrix nanocomposite (MMNC) comprised of a nickel matrix reinforced by carbon nanotubes (CNTs). The aim of this study was to characterize Ni-CNTs nanocomposites produced by powder metallurgy using ball-milling. CNTs were initially untangled using ultrasonication followed by mixture/dispersion with Ni powder by ball-milling for 60, 180, or 300 min. The mixtures were cold-pressed and then pressureless sintered at 950 degrees C for 120 min under vacuum. Their microstructural characterization was mainly performed by optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). The mechanical properties were evaluated by Vickers microhardness. The results indicate that combining ultrasonication and ball-milling can successfully produce Ni-CNTs nanocomposites. The ball-milling time has a significant effect on both the CNT dispersion and the final nanocomposite microstructure

A ética do silêncio racial no contexto urbano: políticas públicas e desigualdade social no Recife, 1900-1940

Mais de meio século após o preconceito racial ter se tornado o principal alvo dos movimentos urbanos pelos direitos civis nos Estados Unidos e na África do Sul, e décadas depois do surgimento dos movimentos negros contemporâneos no Brasil, o conjunto de ferramentas legislativas criado no Brasil para promover o direito à cidade ainda adere à longa tradição brasileira de silêncio acerca da questão racial. Este artigo propõe iniciar uma exploração das raízes históricas desse fenômeno, remontando ao surgimento do silêncio sobre a questão racial na política urbana do Recife, Brasil, durante a primeira metade do século XX. O Recife foi eé um exemplo paradigmático do processo pelo qual uma cidade amplamente marcada por traços negros e africanos chegou a ser definida política e legalmente como um espaço pobre, subdesenvolvido e racialmente neutro, onde as desigualdades sociais originaram na exclusão capitalista, e não na escravidão e nas ideologias do racismo científico. Neste sentido, Recife lança luzes sobre a política urbana que se gerou sob a sombra do silêncio racial.More than half a century after racial prejudice became central to urban civil rights movements in the United States and South Africa, and decades after the emergence of Brazil’s contemporary Black movements, Brazil's internationally recognized body of rights-to-the-city legislation still adheres to the country's long historical tradition of racial silence. This article explores the historical roots of this phenomenon by focusing on the emergence of racial silence in Recife, Brazil during the first half of the 20th Century. Recife was and remains a paradigmatic example of the process through which a city marked by its Black and African roots came to be legally and politically defined as a poor, underdeveloped and racially neutral space, where social inequalities derived from capitalist exclusion rather than from slavery and scientific racism. As such, Recife'sexperience sheds light on the urban policies that were generated in the shadow of racial silence

Estampagem de chapas de aço macio : estudo da evolução da microestrutura de deslocações durante a deformação plástica

Tese de doutoramento em Ciências de Engenharia (Engenharia Mecânica - Tecnologia de Produção) apresentada à Fac. de Ciências da Univ. de CoimbraDe maneira a simular as trajectórias de deformação que se observam em chapas metálicas durante as operações de estampagem, submeteram-se provetes retirados de uma chapa de aço macio a deformações segundo trajectórias simples (tracção, deformação plana e expensão biaxial simétrica) e complexas (tracção seguida de expansão biaxial simétrica e sequência inversa). Constatou-se que durante a deformação as deslocações tendem a dispor-se em paredes delimitando células dentro das quais a densidade de deslocações é baixa; a configuração da microestrutura em células de deslocações e a sua evolução em função da deformação dependem da trajectória imposta. Caracterizou-se a microestrutura de deslocações após diferentes etapas de deformação, tendo-se analisado a forma das células e medido alguns dos seus parâmetros geométricos (tamanho das células, espessura e orientação das paredes). Verificou-se que o tamanho das células e a espessura das paredes de deslocações diminuem com o aumento da deformação, qualquer que seja a trajectória. Além disso, a orientação dos planos das paredes de deslocações, em cada grão, parece estar relacionada com a orientação dos planos dos sistemas de escorregamento activos. Nas trajectórias complexas de deformação, a forma das células evolui, durante a segunda trajectória, da forma típica da trajectória inicial para a forma típica da trajectória em curso

Modelling and Simulation of Sheet Metal Forming Processes

The numerical simulation of sheet metal forming processes has become an indispensable tool for the design of components and their forming processes. This role was attained due to the huge impact in reducing time to market and the cost of developing new components in industries ranging from automotive to packing, as well as enabling an improved understanding of the deformation mechanisms and their interaction with process parameters. Despite being a consolidated tool, its potential for application continues to be discovered with the continuous need to simulate more complex processes, including the integration of the various processes involved in the production of a sheet metal component and the analysis of in-service behavior. The quest for more robust and sustainable processes has also changed its deterministic character into stochastic to be able to consider the scatter in mechanical properties induced by previous manufacturing processes. Faced with these challenges, this Special Issue presents scientific advances in the development of numerical tools that improve the prediction results for conventional forming process, enable the development of new forming processes, or contribute to the integration of several manufacturing processes, highlighting the growing multidisciplinary characteristic of this field

Investigation on the Strengthening Mechanisms of Nickel Matrix Nanocomposites

The strengthening effect of carbon nanotubes (CNTs) in metal matrix nanocomposites occurs due to several mechanisms that act simultaneously. The possible strengthening mechanisms for metal matrix nanocomposites reinforced with CNTs consist of: (1) load transfer, (2) grain refinement and texture strengthening, (3) second phase strengthening, and (4) strain hardening. The main focus of this work is to identify the strengthening mechanisms that play a role in the case of the Ni-CNT nanocomposite produced by powder metallurgy. For the dispersion and mixing of the metallic powders with CNTs, two different routes were performed by ultrasonication and ball milling. The results indicated that four different strengthening mechanisms are present in the nanocomposites and had a different contribution to the final mechanical properties. The load transfer and the increase in dislocation density seem to strongly affect the properties and microstructure of the nanocomposite. The grain refinement and the presence of second phase particles have a small contribution in the strengthening of this nanocomposite, since the introduction of CNTs in the Ni matrix slightly affects the size and orientation of the grains in the matrix and a few nanometric particles of Ni3C were identified

Strengthening Mechanisms of Aluminum Matrix Nanocomposites Reinforced with CNTs Produced by Powder Metallurgy

The present work aims to investigate the strengthening mechanisms in aluminum matrix nanocomposites reinforced by carbon nanotubes (CNTs). A classical powder metallurgy route produced Al-CNT nanocomposites using ultrasonication and ball milling as dispersion/mixture techniques. The microstructural characterization is crucial for this study to reach the objective, being performed mainly by electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM). Uniform dispersion without damaging the CNTs structure is the key for the nanocomposite by powder metallurgy production process. The reinforcement effect occurs due to several strengthening mechanisms that act simultaneously. For the Al-CNT nanocomposites produced by ultrasonication as a dispersion/mixture technique, the observed improvement in the mechanical properties of nanocomposites can be attributed to the load transfer from the matrix to the CNTs. The strain hardening and the second-phase hardening can also have a small contribution to the strengthening of the nanocomposites

NUMERICAL STUDY ON HOLE EXPANSION TESTS OF METAL SHEETS

In this work, a numerical analysis of the hole expansion test is presented. This analysis focuses on the impact that certain geometric and material features have on the strain path and maximum values achieved for the principal strains, during the test. The features analysed are the die diameter, the sheet thickness and the material hardening behaviour. In order to study the impact of each feature individually, a reference study case was defined, which strictly follows the ISO 16630 standard, to serve as a basis of comparison. Then, other study cases were created from the reference one, by changing each feature in study. The results show that a uniaxial tension strain path is observed at the outer edge of the blank hole, which is not significantly changed with the variation of the features under study

Non-uniform deformation after prestrain

The deformation behaviour of prestrained metal sheets is analysed in this work. The non-uniform deformation observed during reloading in tension was studied, by following deformation in different regions of the samples. It takes into account the presence of geometrical defects in the samples and explains the importance of mechanical behaviour. A simplified analysis was used, to model the behaviour in tension of a metallic specimen with geometrical imperfection. The flow behaviour is described using a Swift law equation, which includes strain-rate sensitivity. A modified law was used for prestrained materials and this incorporates the plastic prestrain value, adjusted to the path change. The model predicts imperfection growth kinetics with strain, and strain saturation in the homogeneous region, due to the onset of necking.http://www.sciencedirect.com/science/article/B6VKW-3YVKWPR-2/1/654804ecaa421fb49a96f69023e4b5a

New mandrel design for ring hoop tensile testing

The determination of mechanical and fracture properties of anisotropic tubular materials along hoop direction needs the use of the ring hoop tensile test, for which, the obtained results are deceived by the effect of friction between the ring sample and the Dshaped block mandrel. Commonly, lubricants are applied to reduce the friction, which are inefficient in some specific cases; despite of that, it was noticed that scarce works have focused on the development of new mechanical mandrel designs or trying to improve the current ones to resolve the friction concern. The aim of this research is to correctly address the friction issue between the ring sample and the fixture mandrel to significantly reduce its effect on the ring hoop tensile test results without using any kind of lubricants. New mechanical design of D-shaped block mandrels are developed to carry out ring hoop tensile tests to simply characterize the mechanical behaviour of tubular materials. New mechanical D-shaped block mandrels were designed, manufactured and used to carry out experimental ring hoop tensile tests. An inverse identification method based on an artificial neural network trained by finite element simulation responses, was developed to efficiently segregate the flow stress curve from the influence of the friction, inherent in the global force-displacement curve for the classical ring hoop tensile test. The experimental force – displacement curves using five mandrel-types are established and quantitatively compared on the base of their ability to reduce the friction issue. The analysis of the finite element simulations, related to the investigation of the influence of the friction on the ring hoop tensile test results, shows that one of the new developed mandrels reduces the friction coefficient by about 10 times compared to that identified using the classical D-shaped block mandrel. It has been found that, the finite element simulation of ring hoop tensile test using the identified material parameters matches the experimental results. This investigation provides a useful fixture mandrel, which is able to drastically reduce the friction without resort to any lubricants to just determine the material flow stress curve using ring hoop tensile test, regardless the friction level between the sample and mandrel.info:eu-repo/semantics/publishedVersio