Evaluation of the extended finite element method for the analysis of bonded joints

The extended finite element method is an innovative approach based on finite element method. Where finite element method needs to change the mesh at every step of a crack propagation, extended finite element method does not because the mesh does not need to follow the crack anymore, that is why this method is very well suited for fracture problems. This master’s thesis is the development of the 2D part of this method. The XFEM tool in Abaqus is evaluated for two dimensional stationary cracks with a variety of parameters and features such as meshing technique, element size, symmetry and sub modeling. The purpose is to find a robust and flexible strategy to model cracks. The strategy is verified through handbook cases modeled in Abaqus, where the accuracy has been evaluated. The study was conducted through a numerical analysis by the eXtended Finite Element Method (XFEM) based on stress and strain criteria, which was validated with previously obtained experimental results. The numerical analysis allowed the study of the stress distributions, the effect of the damage initiation criterion, the effect of the propagation law and also the joints strength. The XFEM analysis revealed that this method is fully accurate when using the QUADS initiation criterion, growth criteria and the triangular propagation law. It was shown that the SLJ and DLJ adherend’s geometry and the adhesive type are the most important parameters affecting the joints strength.O Método de Elementos Finitos eXtendido é uma abordagem inovadora baseada no Método de Elementos Finitos. Enquando o Método de Elementos Finitos precisa mudar a malha em cada etapa da propagação da fenda, o Método de Elementos Finitos eXtendido não faz mais porque a malha não precisa de seguir a fenda, e é por isso que esse método é muito adequado para problemas de fratura. Esta dissertação de mestrado é o desenvolvimento da parte 2D deste método. A ferramenta XFEM no Abaqus é avaliada para fendas estacionárias bidimensionais com uma variedade de parâmetros e características como técnica de malha, tamanho do elemento, simetria e sub modelação. O objetivo é encontrar uma estratégia robusta e flexível para modelar fendas. A estratégia é verificada por meio de casos estudo modelados no Abaqus, cuja precisão foi avaliada. O estudo foi conduzido através de uma análise numérica pelo Método dos Elementos Finitos eXtended (XFEM) baseado nos critérios de tensão e deformação, o qual foi validado com resultados experimentais previamente obtidos. A análise numérica permitiu o estudo das distribuições de tensão, o efeito do critério de início de dano, o efeito da lei de propagação e também a resistência das juntas. A análise XFEM revelou que este método é totalmente preciso ao usar o critério de iniciação QUADS, critérios de crescimento e a lei de propagação triangular. Foi demonstrado que a geometria do aderente SLJ e DLJ e o tipo de adesivo são os parâmetros mais importantes que afetam a resistência das juntas

Evaluation of the extended finite element method for the analysis of bonded joints

The extended finite element method is an innovative approach based on finite element method. Where finite element method needs to change the mesh at every step of a crack propagation, extended finite element method does not because the mesh does not need to follow the crack anymore, that is why this method is very well suited for fracture problems. This master’s thesis is the development of the 2D part of this method. The XFEM tool in Abaqus is evaluated for two dimensional stationary cracks with a variety of parameters and features such as meshing technique, element size, symmetry and sub modeling. The purpose is to find a robust and flexible strategy to model cracks. The strategy is verified through handbook cases modeled in Abaqus, where the accuracy has been evaluated. The study was conducted through a numerical analysis by the eXtended Finite Element Method (XFEM) based on stress and strain criteria, which was validated with previously obtained experimental results. The numerical analysis allowed the study of the stress distributions, the effect of the damage initiation criterion, the effect of the propagation law and also the joints strength. The XFEM analysis revealed that this method is fully accurate when using the QUADS initiation criterion, growth criteria and the triangular propagation law. It was shown that the SLJ and DLJ adherend’s geometry and the adhesive type are the most important parameters affecting the joints strength.O Método de Elementos Finitos eXtendido é uma abordagem inovadora baseada no Método de Elementos Finitos. Enquando o Método de Elementos Finitos precisa mudar a malha em cada etapa da propagação da fenda, o Método de Elementos Finitos eXtendido não faz mais porque a malha não precisa de seguir a fenda, e é por isso que esse método é muito adequado para problemas de fratura. Esta dissertação de mestrado é o desenvolvimento da parte 2D deste método. A ferramenta XFEM no Abaqus é avaliada para fendas estacionárias bidimensionais com uma variedade de parâmetros e características como técnica de malha, tamanho do elemento, simetria e sub modelação. O objetivo é encontrar uma estratégia robusta e flexível para modelar fendas. A estratégia é verificada por meio de casos estudo modelados no Abaqus, cuja precisão foi avaliada. O estudo foi conduzido através de uma análise numérica pelo Método dos Elementos Finitos eXtended (XFEM) baseado nos critérios de tensão e deformação, o qual foi validado com resultados experimentais previamente obtidos. A análise numérica permitiu o estudo das distribuições de tensão, o efeito do critério de início de dano, o efeito da lei de propagação e também a resistência das juntas. A análise XFEM revelou que este método é totalmente preciso ao usar o critério de iniciação QUADS, critérios de crescimento e a lei de propagação triangular. Foi demonstrado que a geometria do aderente SLJ e DLJ e o tipo de adesivo são os parâmetros mais importantes que afetam a resistência das juntas

Twinning and the mechanical behavior of magnesium alloys at very high strain rates

The dynamic mechanical behavior of magnesium and its alloys is a subject of interest primarily because of its high specific strength. This makes it attractive for structural components and vehicles. The hexagonal close packed crystal structure of magnesium makes it highly anisotropic in terms of its mechanical behavior. Extension twinning is a dominant deformation mechanism in these materials. This is often reflected in a characteristic sigmoidal profile of the stress–strain curve when crystals are compressed along directions perpendicular to the crystallographic c-axis. Past experiments have been limited to strain rates of 103 s–1 . This study focuses on microstructural twinning effects on the mechanical behavior of AZ31 magnesium alloy at higher strain rates. We perform very high-strain rate experiments on AZ31 magnesium alloy, using a miniature Kolsky compression bar apparatus coupled with a high speed camera for whole field imaging. This experiment is capable of achieving strain rates on the order of 105 s–1 . Experiments at these strain rates have shown substantial plastic deformation without failure when compared with the lower rates of loading. This is evidence of deformation mechanisms that tend to delay failure in the material. We also observe a change in the hardening rates between these experiments and experiments done at 103 s–1. Examination of the microstructure of deformed samples gives us information about the relative activation and growth of deformation mechanisms that cause plastic deformation at these rates

Cloning and semi-quantitative expression of endochitinase (ech42) gene from Trichoderma spp.

Species of Trichoderma such as T. harzianum, T. viride and T. atroviride are some of the most potent antagonistic fungi, and have been used as plant growth promoters in developed countries. Endochitinase (ech42) gene which is involved in mycoparasitism, was isolated from Trichoderma spp. taken from hot-arid soils of Rajasthan, cloned, sequenced and its expression profiling was carried by reverse transcription-polymerase chain reaction (RT-PCR) technique. The cloned DNA sequence was 1,476 base pairs. Gene encoding endochitinase was ligated in pGEMT cloning vector. The plasmids were transformed in DH5α Escherichia coli competent cells and clones were confirmed through sequencing and restriction analysis. Endochitinase gene expression was then observed for different Trichoderma isolates viz., T. harzianum (T14 and T12) and T. atroviride (T5). Among the three, higher expression of endochitinase was observed in T14 and T12, whereas T5 showed lesser expression with respect to T14 and T12 strain. The Trichoderma chitinase enzyme activity was monitored for all isolates under study. The highest chitinase activity was observed in T14 and T11 viz., 17.21 (1 enzyme μg/ml) and 13.11 enzyme μg/ml, respectively.Keywords: Endochitinase, cloning, expression, Trichoderma atroviride, Trichoderma harzianum, Trichoderma virid

Modeling the Effect of Environmental Geometries on Grid Cell Representations

Grid cells are a special class of spatial cells found in the medial entorhinal cortex (MEC) characterized by their strikingly regular hexagonal firing fields. This spatially periodic firing pattern is originally considered to be independent of the geometric properties of the environment. However, this notion was contested by examining the grid cell periodicity in environments with different polarity (Krupic et al., 2015) and in connected environments (Carpenter et al., 2015). Aforementioned experimental results demonstrated the dependence of grid cell activity on environmental geometry. Analysis of grid cell periodicity on practically infinite variations of environmental geometry imposes a limitation on the experimental study. Hence we analyze the dependence of grid cell periodicity on the environmental geometry purely from a computational point of view. We use a hierarchical oscillatory network model where velocity inputs are presented to a layer of Head Direction cells, outputs of which are projected to a Path Integration layer. The Lateral Anti-Hebbian Network (LAHN) is used to perform feature extraction from the Path Integration neurons thereby producing a spectrum of spatial cell responses. We simulated the model in five types of environmental geometries such as: (1) connected environments, (2) convex shapes, (3) concave shapes, (4) regular polygons with varying number of sides, and (5) transforming environment. Simulation results point to a greater function for grid cells than what was believed hitherto. Grid cells in the model encode not just the local position but also more global information like the shape of the environment. Furthermore, the model is able to capture the invariant attributes of the physical space ingrained in its LAHN layer, thereby revealing its ability to classify an environment using this information. The proposed model is interesting not only because it is able to capture the experimental results but, more importantly, it is able to make many important predictions on the effect of the environmental geometry on the grid cell periodicity and suggesting the possibility of grid cells encoding the invariant properties of an environment

Managing pregnancy in women with Sturge-Weber syndrome: case report and review of the literature

Sturge-Weber syndrome (SWS) is a sporadic congenital neuro-cutaneous anomaly with capillary-venous malformation involving the brain, eye, and the ophthalmic division of the trigeminal nerve. In these cases, physiological changes in pregnancy, including hormonal and hemodynamic changes, may predispose to increased seizure frequency and even a life-threatening intracranial haemorrhage. There are only few case reports available about the management of women with pregnancy and SWS. We report two patients with SWS diagnosed in childhood and managed during pregnancy and reviewed the outcomes and complications during pregnancy in women with this disorder

A dynamic assertion-based verification platform for UML statecharts over rhapsody

For quite some time, the Unified Modeling Language (UML) has been adopted by designers of safety critical control systems such as automotive and aviation control. This has led to an increased emphasis on setting up a validation flow over UML that can be used to guarantee the correctness of UML models. In this paper, we propose a dynamic Assertion-based verification (ABV) framework for validation of UML Statecharts over the Rhapsody platform of I-logix. We present an extension of Linear Temporal Logic (LTL), named Action-LTL that allows assertions to be specified over data attributes and events of UML models. We present a methodology for automatic generation of Rhapsody Statecharts from Action-LTL specifications. These generated Statecharts are added as simulation observers to an existing UML model to detect specification violations during simulation. In view of the capacity limitations of existing formal assertion-based verification tools, we believe that our methods are of immediate practical value to the UML-based design community