Metodologias de investigação para metais utilizados em conformação plástica

Doutoramento em Engenharia MecânicaNeste trabalho apresentam-se novas metodologias experimentais de análise dos mecanismos físicos que controlam a formabilidade das ligas de Al-Mg e o encruamento transitório em metais unifásicos. Para as ligas de Al-Mg a metodologia baseia-se na combinação entre ensaios de tracção com variações de temperatura, mapeamento do domínio de sensibilidade negativa à velocidade de deformação e tratamentos térmicos de recozimento e arrefecimento. Para temperaturas superiores a 120 ºC, o coeficiente de sensibilidade a velocidade de deformação (m) depende da gama de velocidades de deformação e do valor de deformação plástica para a qual o valor é determinado. No domínio de temperaturas negativas -80ºC - -120 ºC, o valor de m mantém-se constante e praticamente igual a zero. As diferentes energias de activação fora do domínio de PLC originam uma transição gradual de m de um valor positivo para um valor negativo para -100 ºC e abrupta para 100 ºC. O valor negativo de m resulta da presença de solutos na liga. A dissolução dos aglomerados de solutos através de tratamento térmico, conduz, independentemente do valor de deformação plástica, a um valor de m próximo de zero. Para o estudo de encruamento transitório a metodologia baseia-se na comparação entre os comportamentos mecânico e microestrutural de um aço ferrítico e de uma liga de alumínio. Á temperatura ambiente, os materiais apresentam um comportamento semelhante. A temperaturas negativas o comportamento do alumínio não sofre grandes alterações, enquanto que no aço ocorre um aumento significativo da tensão de atrito da rede cristalina e a formação de uma estrutura de deslocações homogénea. Testes mecânicos com inversão de carga e alteração brusca de temperatura mostraram que o mecanismo físico responsável pela estagnação da taxa de encruamento está relacionado com a natureza das deslocações criadas durante a predeformação e sua evolução após a recarga. Assim, o comportamento mecânico é controlado pelas interacções individuais entre as deslocações móveis e os obstáculos situados nos seus planos de escorregamento. A evolução da microestrutura desempenha um papel secundário. A extrusão em canal permite a obtenção de metais com grão altamente refinado e com elevada resistência mecânica e formabilidade. No entanto, este processo é de difícil implementação industrial. A laminagem assimétrica é bastante promissora para aplicação industrial mas carece de estudos complementares que permitam analisar os efeitos produzidos pelas subestruturas.In this work new experimental methodologies for studying the physical mechanisms that control formability of aluminium-magnesium alloys and strain hardening transient in single phase metals are presented. The behaviour of Al-Mg alloys is strongly dependent on temperature since this parameter controls solute mobility and clustering rate. For analysing the behaviour of this alloy, we propose an experimental methodology based on combination of the uniaxial tension tests with temperature, mapping the domain of negative strain rate sensitivity, and special combinations of annealing and cooling. It was shown that at temperatures higher than 120oC, m depends on the strain rate differential and the plastic strain at which the readings are made. In the negative temperature range, -80ºC to –120ºC, the variation of m is very slow stay near to zero value in the positive sense of the axis. The variation of m with temperature suggests that the activation energy for PLC at the low and the high temperature ends of the PLC range is different. The transition from positive to negative m is very gradual at T ~ -100oC, while at T ~ 100oC it is quite abrupt. The presence of solute structures was shown to be the reason for negative strain rate sensitivity of Al-Mg alloys. Dissolving all clusters that may exist in the material by annealing, gives an m value closer to zero and not depend on strain. The analysis of transient strain hardening in single phase metals was carried out with a new experimental methodology based on the comparison between the mechanical and microstructural behaviors of a ferritic steel and a commercial purity aluminium. These two materials involve identical plastic processes at room temperature, but not at low temperature were the related dislocation substructures are quite different. A temperature drop does not modify substantially the f.c.c. aluminum substructure but drastically increases the friction stress and delays patterning of the b.c.c. steel. Strain reversal tests coupled to temperature changes revealed that the physical process responsible for the transient stagnation of strain hardening is related to the nature of the dislocations generated during the prestrain and their evolutionary law during reloading. In other words, the mechanical behavior is controlled by individual interactions between the moving dislocations and the obstacles lying on their slip planes rather than to some cell structure evolution. Equal channel angular extrusion leads to the improvement of strength and formability of metals by grain refinement. However, the process is not adequate for mass production. On the contrary, asymmetric rolling appears to be a promising process for industrial applications. However, further studies are necessary to carried out in order to distinguish between the effects produced by the sub-grains and actual grains on the crystallographic texture

The formability of twinning: induced plasticity steels predicted on the base of Marciniak-Kuczynski theory

The purpose of this work is to predict and analyze the formability of twinning – induced plasticity steels through the Marciniak-Kuczynski (MK) theory with emphasis on the solutions for improving the prediction results. The selected constitutive equations involve the Yld00-2d of Barlat et al. (2003) plane stress yield function, the Swift strain–hardening power law and the dislocation density based constitutive model proposed by Kim et al. (2013), taking into account the dislocation glide, twinning and dynamic strain aging. Three types of high manganese content TWIP steels sheet were selected. To understand the formability of the TWIP steel and the factors influencing it, a sensitive study on the effect of the mechanical properties of the TWIP steel on the MK theory concept and the predicted forming limits is performed. Using the dislocation density based microstructural model, the deformation twinning effect and the contribution of dynamic strain aging to the FLDs of TWIP steel is analyzed. The relevant influence of the sharpness of the yield surface in the biaxial stretching region in the prediction of FLDs of TWIP steels is highlighted. The extended MK model can be adapted to predict the forming limits of the TWIP steels by using an unusual high initial geometrical defect imposed by their high strain hardening. In this way it was showed that the MK theory cannot be applied for predicting the forming limits of TWIP steels unless by applying imperfection factors that are not physically reasonable. Therefore, new failure models are required for TWIP steel.publishe

Twist Springback of Asymmetric Thin-walled Tube in Mandrel Rotary Draw Bending Process

AbstractThis paper aims to develop an effective numerical model and analyse the twist springback behaviour of asymmetric thin-walled tube in mandrel rotary draw bending. Yld2000-2d anisotropic yield criterion integrated with mixed isotropic and kinematic hardening model was used to describe the material properties including anisotropy and Bauschinger effect. The corresponding mechanical experiments such as uniaxial tension, monotonic and forward-reverse shear tests were performed to obtain the material parameters. A three-dimensional elastic-plastic finite element model was developed, and its validity was assessed by comparing the predicted twist springback with experiment one. Based on the present FE model, the tangential stress distribution during different bending steps were analysed to explore the source of twist springback. The results indicate that the torsion moment of cross sections caused by the non-homogenous stress states play a considerable role in twist springback prediction

Development of a device compatible with universal testing machine to perform hole expansion and Erichsen cupping tests

The material characterization regarding sheet metal formability is usually assessed by the forming limit curve (FLC). The FLC requires specialized and expensive equipment, several samples with different geometries, and can be a very time-consuming procedure for data treatment. Alternatively, the hole expansion test (HET) and the Erichsen Cupping Test (ECT) can be used for routine evaluations of sheet metal mechanical behavior. These formability tests require fewer quantities of material and easy specimen preparation with a fast analysis of results. The HET and ECT procedures provide a proper evaluation of sheet material stretch-flangeability, formability, strength, and ductility. In this work, we developed a low-cost mechanical device capable of performing the HET and ECT tests using a universal testing machine. The equipment is designed to meet the test parameters set by ISO 16630 (HET) and ISO 20482 (ECT) standards. In order to verify its functionality, tests were carried out with the dual-phase steels DP600 and DP780. The corresponding values determined for the hole expansion ratio and the Erichsen index provided reliable results in terms of the accuracy and repeatability of the proposed testing device.publishe

The evaluation of laser weldability of the third-generation advanced high strength steel

To meet the demands of vehicular safety and greenhouse gas emission reduction, the automotive industry is increasingly using advanced high strength steels (AHSS) in the production of the components. With the development of the new generation of AHSS, it is essential to study their behavior towards manufacturing processes used in the automotive industry. For this purpose, the welding capability of newly developed third-generation Gen3 980T steel was investigated using the Nd:YAG (Neodymium:Yittrium Aluminum Garnet) laser-welding with different parameter conditions. The analysis was made by uniaxial tensile tests, micro-hardness, Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). The criteria used to evaluate the quality of the weld were the distance between the fracture and the weld bead and the surface finish. A relationship between the quality of the weld and the energy density was observed, expressed by a partial penetration for values below the optimal, and by irregularities in the weld bead and a high number of spatters for the values above the optimal.publishe

Strain-path dependent hardening models with rigorously identical predictions under monotonic loading

Accurate sheet metal simulation often requires advanced strain-path dependent material models, in order to predict the material response under complex loading conditions, including monotonic, reverse and orthogonal paths. More and more flexible models imply higher and higher costs in terms of parameter identification, computer implementation and simulation time, and robust comparison is often compromised by the inconsistent predictions of advanced models under monotonic loading. In this paper, a simple and general approach is proposed for the alteration of advanced hardening models in order to make them rigorously identical to each other under monotonic loading. This objective was reached without any drawback other than the addition of the corresponding equations. On the contrary, the flexibility and accuracy of the selected models was improved, and the parameter identification procedure became simpler, more accurate and more robust. Three material models of increasing complexity were selected to demonstrate the interest of this approach with respect to a complete set of characterisation experiments for a DP600 sheet steel

COMPARISON of DATA AVAILABILITY and QUALITY for PHARMACOECONOMIC ANALYSIS in BRAZIL VERSUS the UNITED STATES and EUROPEAN UNION: the CASES of DIABETES & HYPERTENSION

Axia Bio Consulting, São Paulo, BrazilI3 Innovus, Medford, MA USAUniversidade Federal de São Paulo, São Paulo, BrazilNovartis Biociencias SA, São Paulo, BrazilNovartis Pharma AG, Basel, SwitzerlandUniversidade Federal de São Paulo, São Paulo, BrazilWeb of Scienc

The dopamine D2 receptor gene and depressive and anxious symptoms in childhood: Associations and evidence for gene-environment correlation and gene-environment interaction

ObjectiveS: Research implicates the A1 allele of the dopamine D2 receptor gene (DRD2) Taq1A polymorphism in the development of depression and anxiety. Furthermore, recent papers suggest that children with A1 allele of this gene may receive less positive parenting, and that the effects of this gene on child symptoms may be moderated by parenting. We sought to replicate and extend these findings using behavioral measures in a nonclinical sample of young children. Methods: In a sample of 473 preschool-aged children and their mothers, structured clinical interview measures and maternal reports of child symptoms were collected, and standardized observations of parent-child interactions were conducted. Results: An association was detected between the DRD2 A1 allele and symptoms of depression and anxiety indexed using interview and parent report methods. As found in previous reports, children with the DRD2 A1 allele received less supportive parenting and displayed higher levels of negative emotionality during parent-child interactions. Tests of mediation and moderation were conducted. Conclusion: We found associations between the DRD2 A1 allele and early-emerging anxious and depressive symptoms in a community sample of preschool-aged children, and evidence of a gene-environment correlation and moderation of the main effect of child genotype on child symptoms by parenting. © 2010 Wolters Kluwer Health | Lippincott Williams & Wilkins

Fire hazard modulation by long-term dynamics in land cover and dominant forest type in eastern and central Europe

Wildfire occurrence is influenced by climate, vegetation and human activities. A key challenge for understanding the risk of fires is quantifying the mediating effect of vegetation on fire regimes. Here, we explore the relative importance of Holocene land cover, land use, dominant functional forest type, and climate dynamics on biomass burning in temperate and boreo-nemoral regions of central and eastern Europe over the past 12 kyr. We used an extensive data set of Holocene pollen and sedimentary charcoal records, in combination with climate simulations and statistical modelling. Biomass burning was highest during the early Holocene and lowest during the mid-Holocene in all three ecoregions (Atlantic, continental and boreo-nemoral) but was more spatially variable over the past 3–4 kyr. Although climate explained a significant variance in biomass burning during the early Holocene, tree cover was consistently the highest predictor of past biomass burning over the past 8 kyr. In temperate forests, biomass burning was high at ~ 45% tree cover and decreased to a minimum at between 60% and 70% tree cover. In needleleaf-dominated forests, biomass burning was highest at ~60 %–65%tree cover and steeply declined at > 65% tree cover. Biomass burning also increased when arable lands and grasslands reached ~15 %–20 %, although this relationship was variable depending on land use practice via ignition sources, fuel type and quantities. Higher tree cover reduced the amount of solar radiation reaching the forest floor and could provide moister, more wind-protected microclimates underneath canopies, thereby decreasing fuel flammability. Tree cover at which biomass burning increased appears to be driven by warmer and drier summer conditions during the early Holocene and by increasing human influence on land cover during the late Holocene. We suggest that longterm fire hazard may be effectively reduced through land cover management, given that land cover has controlled fire regimes under the dynamic climates of the Holocene

Evaluating evidence-based content, features of exercise instruction, and expert involvement in physical activity apps for pregnant women: systematic search and content analysis

Background: Guidelines for physical activity and exercise during pregnancy recommend that all women without contraindications engage in regular physical activity to improve both their own health and the health of their baby. Many women are uncertain how to safely engage in physical activity and exercise during this life stage and are increasingly using mobile apps to access health-related information. However, the extent to which apps that provide physical activity and exercise advice align with current evidence-based pregnancy recommendations is unclear. Objective: This study aims to conduct a systematic search and content analysis of apps that promote physical activity and exercise in pregnancy to examine the alignment of the content with current evidence-based recommendations; delivery, format, and features of physical activity and exercise instruction; and credentials of the app developers. Methods: Systematic searches were conducted in the Australian App Store and Google Play Store in October 2020. Apps were identified using combinations of search terms relevant to pregnancy and exercise or physical activity and screened for inclusion (with a primary focus on physical activity and exercise during pregnancy, free to download or did not require immediate paid subscription, and an average user rating of ≥4 out of 5). Apps were then independently reviewed using an author-designed extraction tool. Results: Overall, 27 apps were included in this review (Google Play Store: 16/27, 59%, and App Store: 11/27, 41%). Two-thirds of the apps provided some information relating to the frequency, intensity, time, and type principles of exercise; only 11% (3/27) provided this information in line with current evidence-based guidelines. Approximately one-third of the apps provided information about contraindications to exercise during pregnancy and referenced the supporting evidence. None of the apps actively engaged in screening for potential contraindications. Only 15% (4/27) of the apps collected information about the user’s current exercise behaviors, 11% (3/27) allowed users to personalize features relating to their exercise preferences, and a little more than one-third provided information about developer credentials. Conclusions: Few exercise apps designed for pregnancy aligned with current evidence-based physical activity guidelines. None of the apps screened users for contraindications to physical activity and exercise during pregnancy, and most lacked appropriate personalization features to account for an individual’s characteristics. Few involved qualified experts during the development of the app. There is a need to improve the quality of apps that promote exercise in pregnancy to ensure that women are appropriately supported to engage in exercise and the potential risk of injury, complications, and adverse pregnancy outcomes for both mother and child is minimized. This could be done by providing expert guidance that aligns with current recommendations, introducing screening measures and features that enable personalization and tailoring to individual users, or by developing a recognized system for regulating apps