Representación de materiales viscoelásticos con aplicación a estructuras de hormigón

Se presenta una formulación para las relaciones constitutivas viscoelásticas de sólidos estructurales basada en el concepto de variables de estado, enfatizando algunas características particulares del hormigón. Combinando estas ecuaciones con un método variacional adecuado para representar procesos incrementales en deformaciones finitas se obtiene la correspondiente formulación de elementos finitos que es aplicada a problemas de estabilidad, de estructuras no homogéneas y de relajación térmica.A unified foriilulation for the viscoelastic constltutive relations of structural solids based in the state space concept with emphasis in concrete-like materials is presented. The equations are combined with a variational principle adequate for finite deformations in order to obtain a Finite Eleinents formulation, which is applied to stability problexqs, to non-homogeneous structures and to the analysis of thermal relaxation.Peer Reviewe

Tratamento do problema de contato-impacto entre corpos flexíveis em grandes deformações pelo método dos elementos finitos

O tratamento do problema de contato entre corpos flexíveis foi implementado no módulo METAFOR4, que permite o tratamento de problemas de conformacáo mecanica. O presente algoritmo emprega o método da penalidade para o cálculo de forcas de contato. Os elementos de contato sáo percorridos duas vezes em cada passo de tempo para que seja assegurada a deteccáo de todas as formas de interpenetracao. O algoritmo apresentou-se extremamente robusto. Um exemplo de impacto entre um tubo e uma placa é mostrado.The treatment of the contact problem between flexible bodies has been implemented in the METAFOR modulus4, which allows the treatment of metal forming problems. The described algorithm uses the penalty method for the calculation of the contact forces. The contact elements are checked twice at each time step to assure the detection of al1 forms of penetration. The algorithm demonstrated to be extremely robust. An example of impact between a tube and a plate is given.Peer Reviewe

Tratamento do problema de contato-impacto entre corpos flexíveis em grandes deformações pelo método dos elementos finitos

O tratamento do problema de contato entre corpos flexíveis foi implementado no módulo METAFOR4, que permite o tratamento de problemas de conformacáo mecanica. O presente algoritmo emprega o método da penalidade para o cálculo de forcas de contato. Os elementos de contato sáo percorridos duas vezes em cada passo de tempo para que seja assegurada a deteccáo de todas as formas de interpenetracao. O algoritmo apresentou-se extremamente robusto. Um exemplo de impacto entre um tubo e uma placa é mostrado.The treatment of the contact problem between flexible bodies has been implemented in the METAFOR modulus4, which allows the treatment of metal forming problems. The described algorithm uses the penalty method for the calculation of the contact forces. The contact elements are checked twice at each time step to assure the detection of al1 forms of penetration. The algorithm demonstrated to be extremely robust. An example of impact between a tube and a plate is given.Peer Reviewe

Análise viscoelástica de estruturas laminadas mediante emprego da inversao numérica da transformada de Laplace

Este trabalho focaliza a análise viscoelástica de estruturas laminadas em material compósito de matriz polimérica, propondo o emprego de métodos de inversa0 numérica da transformada de Laplace numa formulacao adaptada ao método dos elementos finitos. Nesta abordagem alternativa,as equacóes constitutivas viscoelásticas, formuladas no plano complexo da variável de transformacáo S , reduzem o problema a urna análise elástica equivalente. As solu~óesc omplexas sáo entiio retornadas ao plano físico para o valor de tempo desejado, dispensando processos incrementais. Exemplos de aplicaqóes siio executados comparando-se os resultados do Método da Transformada de Laplace com o Método das Variáveis de Estado que trata o problema viscoelástico de forma incremental.The viscoelastic analysis of laminated structures of polymeric matrix composite materials using Laplace transform is the objective of this work. The inverse of the Laplace transform is carried out using numerical methods adapted to a finite element method formulation. In this alternative aproach, as the constitutive equations are formulated on the complex plane in the transformation variable 'S', the viscoelasti~problemre duces to an equivalent elastic one. The complex solutions are then sent back to the physical plane for the desired time value, dispensing incremental processes. Examples of aplications compairing results obtained with the Laplace Transformation Method and the State Variables Method (that treats the viscoelastic problem in an incremental form) are presented.Peer Reviewe

Tratamento do problema de contato-impacto entre corpos flexíveis em grandes deformações pelo método dos elementos finitos

O tratamento do problema de contato entre corpos flexíveis foi implementado no módulo METAFOR4, que permite o tratamento de problemas de conformacáo mecanica. O presente algoritmo emprega o método da penalidade para o cálculo de forcas de contato. Os elementos de contato sáo percorridos duas vezes em cada passo de tempo para que seja assegurada a deteccáo de todas as formas de interpenetracao. O algoritmo apresentou-se extremamente robusto. Um exemplo de impacto entre um tubo e uma placa é mostrado.The treatment of the contact problem between flexible bodies has been implemented in the METAFOR modulus4, which allows the treatment of metal forming problems. The described algorithm uses the penalty method for the calculation of the contact forces. The contact elements are checked twice at each time step to assure the detection of al1 forms of penetration. The algorithm demonstrated to be extremely robust. An example of impact between a tube and a plate is given.Peer Reviewe

Computational Viscoelasticity

This text is a guide how to solve problems in which viscoelasticity is present using existing commercial computational codes. The book gives information on codes’ structure and use, data preparation  and output interpretation and verification. The first part of the book introduces the reader to the subject, and to provide the models, equations and notation to be used in the computational applications. The second part shows the most important Computational techniques: Finite elements formulation, Boundary elements formulation, and presents the solutions of Viscoelastic problems with Abaqus

Computational Viscoelasticity

XI, 124p. 23 illus.online resource

New Findings for the Composition and Structure of Ni Nanoparticles Protected with Organomercaptan Molecules

Here we explore the synthesis of alkanethiol-coated Ni NPs following the one-phase reaction method by Brust et al. The reduction of NiCl₂ with NaBH₄ in the presence of dodecanethiol (C₁₂SH) yields a complex product that is difficult to identify as illustrated in the figure of merit. We synthesized Ni­(II) dodecanethiolate (C₁₂S) (without the addition of NaBH₄) for comparison and performed an exhaustive characterization with TEM, HR-TEM, AFM, MFM, XPS, XRD, UV–vis, magnetism, and FT-IR. It is found that the organic coating is not quite a well-organized self-assembled monolayer (SAM) surrounding the Ni cluster as previously reported., XPS and XRD data show slight differences between both syntheses; however, Ni­(II) thiolate appears to be more stable than reduced Ni when exposed to ambient air, indicating the propensity of metallic Ni to oxidize. It has been shown that irradiating with TEM electrons over various metal thiolates leads to nanoparticle formation. We irradiated over Ni­(II) thiolate and observed no evidence of NP formation whereas irradiating a reduced Ni sample exhibited an ∼3.0 nm nanoparticle diameter. Magnetism studies showed a difference between both samples, indicating ferromagnetic character for the reduced Ni sample. According to our results, the product of the synthesis is comprised of ultrasmall metallic clusters embedded in some form of Ni­(II) C₁₂S. In this work, we open a discussion of the chemical nature of the core and the shell in the synthesis of Ni NPs protected with organomercaptan molecules