23,637 research outputs found

    Stretch and shrink flanging of AA2024-T3 sheet by single point incremental forming

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    Flanging is a forming process widely used to increase the sti ness of sheet parts in industrial applications such as the production of aircraft and automobile components. Flanges are usually formed by rubber forming or using a punch and a die, as these processes are fast and economical for producing a large number of parts. However, there is increasing interest in the manufacturing of anges using nonconventional processes, such as single point incremental forming (SPIF), that allow cost savings due to their exibility and low tooling. This thesis studies the manufacturing of open stretch and shrink anges of AA2024-T3 sheets by SPIF for a wide range of process parameters, including the ange principal radius, spindle speed and tool diameter. In this context, an experimental campaign was performed for each type of ange, classifying the principal modes of failure in the sheet and assessing its formability within the forming limit diagram (FLD). For stretch anging, FE modelling in combination with a theoretical framework based on Barlat's 89 anisotropic yield criterion was used to evaluate the formability in the stress triaxiality space. The fracture forming limit (FFL) obtained for proportional loading and the evolution of the anges were compared in the average stress triaxiality versus equivalent strain space, showing that this space might be the most appropriate for making failure predictions in SPIF. A geometry analysis of shrink anges obtained by SPIF was carried out to evaluate their formability in terms of their tendency to develop wrinkling. The elastic recovery in successful and failed is also studied. Finally, a new approach based on the analysis of compression stresses was proposed to predict the onset of failure in this type of ange. In this regard, it was shown that there exists a stress limit at which the anges analysed fail by wrinkling.El rebordeado de ancos es un proceso de conformado de chapa ampliamente usado en industrias como la aeron autica o la automobilistica para la fabricaci on de componentes estructurales de chapa delgada. Este tipo de proceso se suele realizar sobre metal, normalmente aplicando presion a trav es de una almohadilla de goma o directamente usando una matriz y un punz on met alicos. Estos dos m etodos tienen la ventaja de ser r apidos y rentables en la fabricaci on de grandes lotes. Sin embargo, son cada vez m as los trabajos que estudian el uso de procesos no convencionales, como el conformado incremental monopunto o SPIF (por sus siglas en ingl es), en aplicaciones tales como el rebordeado de agujeros, aprovechando as la exibilidad y el ahorro de utillaje que proporciona esta t ecnica. En este contexto, esta tesis presenta un estudio sobre el conformado incremental monopunto aplicado al rebordeado de ancos abiertos de AA2024-T3 para geometr as c oncavas y convexas. Este trabajo se centra en la evaluaci on de la conformabilidad en el diagrama l mite de conformado (FLD), teniendo en cuenta di erentes par ametros tales como el radio principal del anco, la velocidad de giro de la herramienta o el tama~no de la misma. As mismo, se han obtenido las ventanas de proceso para las dos geometr as propuestas, analizando las caracter sticas de los diferentes modos de fallo. Adem as, se ha usado un modelo num erico junto con un marco te orico basados en el criterio de plasticidad de Barlat 89 para analizar la conformabilidad de los ancos c oncavos en el espacio de la triaxialidad, mostrando que este enfoque puede ser m as adecuado para la predicci on del fallo que el basado en el analysis de deformaciones en el FLD. Por otro lado, se ha realizado un an alisis geom etrico de los ancos convexos, evaluando los efectos de la recuperaci on el astica en ancos exitosos y fallidos. Finalmente, se ha propuesto para este tipo de ancos un nuevo enfoque basado en la predicci on del fallo mediante el an alisis de las tensiones de compresi on. En este sentido, los resultados muestran que existe un l mite de compresi on en t erminos de tensiones a partir del cual este tipo de ancos falla por arrugamiento

    On the Use of Strain Path Independent Metrics and Critical Distance Rule for Predicting Failure of AA7075-O Stretch-Bend Sheets

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    The strain-based forming limit curve is the traditional tool to assess the formability of metal sheets. However, its application should be restricted to proportional loading processes under uniform strain conditions. Several works have focused on overcoming this limitation to characterize the safe process windows in industrial stretch-bend forming processes. In this paper, the use of critical distance rule and two path-independent stress-based metrics are explored to numerically predict failure of AA7075-O stretch-bend sheets with 1.6 mm thickness. Formability limits of the material were experimentally obtained by means of a series of Nakazima and stretch-bending tests at di erent thickness-over-radius ratios for inducing controlled non-uniform strain distributions across the sheet thickness. By using a 3D calibrated finite element model, the strain-based forming limit curve was numerically transformed into the path-independent stress and equivalent plastic strain polar spaces. The numerical predictions of necking strains in the stretch-bending simulations using the above approaches were successfully compared and critically discussed with the experimental results for di erent values of the critical distance. It was found that failure was triggered by a critical material volume of around the half thickness, measured from the inner surface, for the both path-independent metrics analyzed.Gobierno de España PGC2018-095508-B-I0

    Identification of inelastic parameters of the 304 stainless steel using multi-objective techniques

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    This work addresses identification of inelastic parameters based on an optimization method using a multi-objective technique. The problem consists in determining the best set of parameters which approximate three different tensile tests. The tensile tests use cylindrical specimens of different dimensions manufactured according to the American ASTM E 8M and Brazilian ABNT NBR ISO 6892 technical standards. A tensile load is applied up to macroscopic failure. The objective functions for each tensile test/specimen is computed and a global error measure is determined within the optimization scheme. The Nelder-Mead simplex algorithm is used as the optimization tool. The proposed identification strategy was able to determine the best set of material parameters which approximate all tensile tests up to macroscopic failure

    Ductile damage prediction in sheet metal forming and experimental validation

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    Tese de doutoramento. Engenharia Mecânica. Universidade do Porto. Faculdade de Engenharia. 201

    On the Use of Maximum Force Criteria to Predict Localised Necking in Metal Sheets under Stretch-Bending

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    The maximum force criteria and their derivatives, the Swift and Hill criteria, have been extensively used in the past to study sheet formability. Many extensions or modifications of these criteria have been proposed to improve necking predictions under only stretching conditions. This work analyses the maximum force principle under stretch-bending conditions and develops two different approaches to predict necking. The first is a generalisation of classical maximum force criteria to stretch-bending processes. The second approach is an extension of a previous work of the authors based on critical distance concepts, suggesting that necking of the sheet is controlled by the damage of a critical material volume located at the inner side of the sheet. An analytical deformation model is proposed to characterise the stretch-bending process under plane-strain conditions. Different parameters are considered, such as the thickness reduction, the gradient of variables through the sheet thickness, the thickness stress and the anisotropy of the material. The proposed necking models have been successfully applied to predict the failure in different materials, such as steel, brass and aluminiumGobierno español DPI2015-64047-

    A Study of Ductile Damage and Failure of Pure Copper – Part I: Constitutive Equations and Experiments

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    This paper presents the results of an experimental study of ductile damage and failure of pure copper. Uniaxial tension tests were performed for specimens with different arrangements of pre-drilled micro-holes representing the simulation models of cylindrical voids. This experimental method has already been applied by a number of researchers in order to investigate the damage of metals under plastic deformation and proved to be useful for studying an evolution of damage in ductile materials in terms of local strains of both representative volume elements (RVE) and meso-elements (i.e., material unit cells with a single void). Two measures are used for the assessment of damage in the deformed material. The first one relates damage to an increase in the void volume. The second measure accounts for the damage associated with a change in the void shape. Both measures were introduced as part of a tensorial theory of damage in Zapara et al. (2008). They are based on experimental studies of damage kinetics in metallic materials under plasticity conditions. In combination with similar data from the literature the obtained results are important for the modeling of metal forming processes with dominating tensile deformation (e.g., deep-drawing, ironing, wire drawing)

    Strain-Based Design Methodology of Large Diameter Grade X80 Linepipe

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    Continuous growth in energy demand is driving oil and natural gas production to areas that are often located far from major markets where the terrain is prone to earthquakes, landslides, and other types of ground motion. Transmission pipelines that cross this type of terrain can experience large longitudinal strains and plastic circumferential elongation as the pipeline experiences alignment changes resulting from differential ground movement. Such displacements can potentially impact pipeline safety by adversely affecting structural capacity and leak tight integrity of the linepipe steel. Planning for new long-distance transmission pipelines usually involves consideration of higher strength linepipe steels because their use allows pipeline operators to reduce the overall cost of pipeline construction and increase pipeline throughput by increasing the operating pressure. The design trend for new pipelines in areas prone to ground movement has evolved over the last 10 years from a stress-based design approach to a strain-based design (SBD) approach to further realize the cost benefits from using higher strength linepipe steels. This report presents an overview of SBD for pipelines subjected to large longitudinal strain and high internal pressure with emphasis on the tensile strain capacity of high-strength microalloyed linepipe steel. The technical basis for this report involved engineering analysis and examination of the mechanical behavior of Grade X80 linepipe steel in both the longitudinal and circumferential directions. Testing was conducted to assess effects on material processing including as-rolled, expanded, and heat‑treatment processing intended to simulate coating application. Elastic-plastic and low-cycle fatigue analyses were also performed with varying internal pressures. Proposed SBD models discussed in this report are based on classical plasticity theory and account for material anisotropy, triaxial strain, and microstructural damage effects developed from test data. The study results are intended to enhance SBD and analysis methods for producing safe and cost effective pipelines capable of accommodating large plastic strains in seismically active arctic areas

    Study of HFQ forming process on lightweight alloy components

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    In order to reduce CO2 emissions and improve fuel efficiency for the aerospace industry, a leading edge sheet metal forming technology, namely solution heat treatment, forming and in-die quenching (HFQ) was utilised to form lightweight, complex-shaped components, efficiently and cost-effectively. The work performed in this research project contains two major achievements. The first achievement is successfully forming a complex AA2060 (Al-Li alloy) wing stiffener demonstrator part, and an L-shape AA7075 demonstrator part, without necking or fracture, using HFQ forming technology. The feasibility of forming the aluminium alloys was based on a series of fundamental experimental tests including uniaxial tensile test, isothermal forming limit test and artificial aging test. The second achievement is the development of a novel forming limit prediction model, namely the viscoplastic-Hosford-MK model. This model enables the forming limit prediction of AA2060 and AA7075 alloys under hot stamping conditions, featuring non-isothermal and complex loading conditions. This prediction model fills a significant need in industry for accurately predicting the forming limit of aluminium alloys under such complex forming conditions. The effectiveness of the developed model was analytically verified for AA2060, demonstrating accurate material responses to cold die quenching, strain rate and loading path changes. By applying the developed model to the hot stamping of an AA2060 component, its accuracy was successfully validated. Furthermore, the viscoplastic-Hosford-MK model was also demonstrated for use in industry by determining the optimum initial blank shape of an L-shape AA7075 component. An iterative simulation procedure implementing the forming limit prediction model was used to arrive at an optimum blank shape by the minimisation of the failure criterion. The optimised initial blank shape design was applied in the experimental hot stamping of a demonstrator AA7075 component. The accuracy of the developed model was validated by the successful forming of the component, without necking or fracture.Open Acces

    Modeling the mechanics of amorphous solids at different length and time scales

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    We review the recent literature on the simulation of the structure and deformation of amorphous glasses, including oxide and metallic glasses. We consider simulations at different length and time scales. At the nanometer scale, we review studies based on atomistic simulations, with a particular emphasis on the role of the potential energy landscape and of the temperature. At the micrometer scale, we present the different mesoscopic models of amorphous plasticity and show the relation between shear banding and the type of disorder and correlations (e.g. elastic) included in the models. At the macroscopic range, we review the different constitutive laws used in finite element simulations. We end the review by a critical discussion on the opportunities and challenges offered by multiscale modeling and transfer of information between scales to study amorphous plasticity.Comment: 58 pages, 14 figure

    Study on Ductile Fracture with Anisotropic and Strain Rate Effects in Manufacturing Processes

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    Ductile fracture is a topic of great importance in automotive and aerospace industries. Prediction of ductile fracture in engineering structures relies on developing robust material models under complex loading conditions. This dissertation addresses the anisotropic and strain rate effects in constitutive and ductile fracture models of lightweight metals. In the present modeling framework, the anisotropic plasticity behavior is modeled by combination of an initial anisotropic yield function and an isotropic hardening correction by Lode dependence. A new all-strain based anisotropic fracture model is proposed based on the approach of linear transformation on plastic strain rate tensor. The strain rate effects in ductile fracture is considered as an extension of the modified Mohr-Coulomb (MMC) fracture model by coupling strain rate with stress state in terms of Lode angle parameter. The rate-dependent MMC model provides a well-bound solution up to the intermediate strain rate range ( \u3c 1000/s) for metal forming and crashworthiness applications. The present modeling framework is calibrated from coupon tests of aluminum alloy and advanced high strength steel (AHSS) sheets using digital image correlation (DIC) technique and validated through correlations by finite element (FE) simulations. This study also demonstrates the applications of ductile fracture modeling in manufacturing processes. The thermo-mechanical FE simulations of orthogonal cutting processes using the Johnson-Cook constitutive and damage models show that the highly damaged regions in zones of material separation form a thin boundary layer at the tool tip. The numerical simulation results explain the success of analytical model with uncoupled component works of plasticity, friction and separation. The FE modeling results of formability and component-level testing suggest that part behavior and material failure is well predicted using calibrated ductile fracture models under different loading conditions
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