A micromechanical constitutive model for ductile fracture: numerical treatment and calibration strategy

This contribution describes the numerical treatment and calibration strategy for a new micromechanical damage model, which employs two internal damage variables. The new micromechanical model is based on Gurson's theory incorporating the void volume fraction as one damage parameter and a shear mechanism, which was formulated considering geometrical and phenomenological aspects, as the second internal damage variable. The first and the second damage variables are coupled in the constitutive formulation in order to affect the hydrostatic stress and deviatoric stress contributions, respectively. Both internal damage variables are independent and, as a consequence, they also require independent nucleation mechanisms for each one in order to trigger the growth contribution. These mechanisms require the determination of material parameters that are obtained through two calibration points: one for high and the other for low stress triaxiality. This is in contrast to other damage models that typically require one calibration point. In the first part of this paper, theoretical aspects of the constitutive formulation are presented and discussed. Then, an implicit numerical integration algorithm is derived, based on the operator split methodology, together with a methodology to perform the calibration of all material parameters. In order to assess the performance of the new model, the “butterfly” specimen was used and the 1045 steel was employed under a wide range of stress triaxiality. The results obtained from the numerical simulations are presented such as: the evolution of both damage parameters, the evolution of the equivalent plastic strain, the reaction versus displacement curve and the contour of the effective damage parameter. From the comparison of the numerical results with experimental evidence, it will be highlighted that the present formulation is able to predict accurately the location of fracture onset and the level of the associated equivalent plastic strain at fracture

On Measuring Non-Recursive Trade-Offs

We investigate the phenomenon of non-recursive trade-offs between descriptional systems in an abstract fashion. We aim at categorizing non-recursive trade-offs by bounds on their growth rate, and show how to deduce such bounds in general. We also identify criteria which, in the spirit of abstract language theory, allow us to deduce non-recursive tradeoffs from effective closure properties of language families on the one hand, and differences in the decidability status of basic decision problems on the other. We develop a qualitative classification of non-recursive trade-offs in order to obtain a better understanding of this very fundamental behaviour of descriptional systems

Counter Machines and Distributed Automata: A Story about Exchanging Space and Time

We prove the equivalence of two classes of counter machines and one class of distributed automata. Our counter machines operate on finite words, which they read from left to right while incrementing or decrementing a fixed number of counters. The two classes differ in the extra features they offer: one allows to copy counter values, whereas the other allows to compute copyless sums of counters. Our distributed automata, on the other hand, operate on directed path graphs that represent words. All nodes of a path synchronously execute the same finite-state machine, whose state diagram must be acyclic except for self-loops, and each node receives as input the state of its direct predecessor. These devices form a subclass of linear-time one-way cellular automata.Comment: 15 pages (+ 13 pages of appendices), 5 figures; To appear in the proceedings of AUTOMATA 2018

MODELAGEM NUMÉRICA DE UM DISPOSITIVO DE SEGURANÇA UTILIZADO EM SISTEMAS OFFSHORE

  Abstract. As válvulas Blowout Preventer (BOP) são projetadas para serem acionadas em casos de emergência impondo uma força de compressão no tubo, que irá causar deformações plásticas, até provocar a ruptura do tubo. Segundo o relatório apresentado pela Det Norske Veritas em 2011, os registros fotográficos da tubulação mostram que a BOP funcionou, porém não atuou de forma adequada, ao invés de selar o tubo, apenas realizou um esmagamento do material. Desta forma, há indícios de falha de funcionamento do dispositivo de segurança, este fato pode ter direta relação com a forma que são projetados esses dispositivos, iniciando pelo critérios de falha considerando os modelos matemáticos utilizados. Com base nas informações anteriores se faz necessário realizar investigações para propor critérios de projeto que atendam às exigências desse tipo de dispositivos, levando em consideração que as tubulações aonde as válvulas atuam apresentam propriedades elasto-plásticas. Este trabalho tem por objetivo realizar a modelagem computacional deste dispositivo considerando um estágio de funcionamento, simulando o acionamento e, assim, analisar os resultados numéricos. Serão incluídos no modelo a geometria do ríser e dois aríetes que constituem o mecanismo da válvula, com suas propriedades correspondentes. Inicialmente, foi implementado o modelo elastoplástico considerando endurecimento isotrópico. A partir dos resultados preliminares será possível avaliar o modelo utilizado e, posteriormente, realizar as modificações necessárias para aprimorar o estudo.Palavras Chaves: Modelagem, BOP, Critérios de Falha, Elasto-plástic

Geração de tecnologias para a produção sustentável e processamento de frutos de açaí no estuário amazônico.

Publicado também: FRAZÃO, D. A. C.; HOMMA, A. K. O; VIÉGAS, I. de J. M. (Ed.). Contribuição ao desenvolvimento da fruticultura na Amazônia. Belém, PA: Embrapa Amazônia Oriental, 2006. p. 113-117

Manejo e cultivo de açaizais para produção de frutos.

Os açaizais, áreas de florestas de varzea dominadas pela palmeira Euterpe oleracea, são um recurso florestal não-madeireiro da região do Estuario Amazônico

Topological defects and Goldstone excitations in domain walls between ferromagnetic quantum Hall effect liquids

It is shown that the low-energy spectrum of a ferromagnetic quantum Hall effect liquid in a system with a multi-domain structure generated by an inhomogeneous bare Zeeman splitting $\epsilon_{Z}$ is formed by excitations localized at the walls between domains. For a step-like $\epsilon_Z(r)$, the domain wall spectrum includes a spin-wave with a linear dispersion and a small gap due to spin-orbit coupling, and a low-energy topological defects. The latter are charged and may dominate in the transport under conditions that the percolation through the network of domain walls is provided.Comment: 4 pages, 1 fi

In vitro protective effect and antioxidant mechanism of Resveratrol induced by Dapsone Hydroxylamine in human cells

Dapsone (DDS) hydroxylamine metabolites cause oxidative stress- linked adverse effects in patients, such as methemoglobin formation and DNA damage. This study evaluated the ameliorating effect of the antioxidant resveratrol (RSV) on DDS hydroxylamine (DDSNHOH) mediated toxicity in vitro using human erythrocytes and lymphocytes. The antioxidant mechanism was also studied using in-silico methods. In addition, RSV provided intracellular protection by inhibiting DNA damage in human lymphocytes induced by DDS-NHOH. However, whilst pretreatment with RSV (10-1000 μM significantly attenuated DDS-NHOH-induced methemoglobinemia, but it was not only significantly less effective than methylene blue (MET), but also post-treatment with RSV did not reverse methemoglobin formation, contrarily to that observed with MET. DDS-NHOH inhibited catalase (CAT) activity and reactive oxygen species (ROS) generation, but did not alter superoxide dismutase (SOD) activity in erythrocytes. Pretreatment with RSV did not alter these antioxidant enzymes activities in erythrocytes treated with DDS-NHOH. Theoretical calculations using density functional theory methods showed that DDS-NHOH has a pro-oxidant effect, whereas RSV and MET have antioxidant effect on ROS. The effect on methemoglobinemia reversion for MET was significantly higher than that of RSV. These data suggest that the pretreatment with resveratrol may decrease heme-iron oxidation and DNA damage through reduction of ROS generated in cells during DDS therapy

Spin Orientation and Spin Precession in Inversion-Asymmetric Quasi Two-Dimensional Electron Systems

Inversion asymmetry induced spin splitting of the electron states in quasi two-dimensional (2D) systems can be attributed to an effective magnetic field B which varies in magnitude and orientation as a function of the in-plane wave vector k||. Using a realistic 8x8 Kane model that fully takes into account spin splitting because of both bulk inversion asymmetry and structure inversion asymmetry we investigate the spin orientation and the effective field B for different configurations of a quasi 2D electron system. It is shown that these quantities depend sensitively on the crystallographic direction in which the quasi 2D system was grown as well as on the magnitude and orientation of the in-plane wave vector k||. These results are used to discuss how spin-polarized electrons can precess in the field B(k||). As a specific example we consider GaInAs-InP quantum wells.Comment: 10 pages, 6 figure