A direct elimination algorithm for quasi-static and dynamic contact problems

This paper deals with the computational modeling and numerical simulation of contact problems at Unite deformations using the Finite element method. Quasi-static and dynamic problems are considered and two particular frictional conditions, full stick friction and frictionless cases, are addressed. Lagrange multipliers and regularized formulations of the contact problem, such as penalty or augmented Lagrangian methods, are avoided and a new direct elimination method is proposed. Conserving algorithms are also introduced for the proposed formulation for dynamic contact problems. An assessment of he performance of the resulting formulation is shown in a number of selected benchmark tests and numerical examples, including both quasi-static and dynamic contact problems under full stick friction and frictionless contact conditions. Conservation of key discrete properties exhibited by the time stepping algorithm used for dynamic contact problems is also shown in an example. (C) 2014 Elsevier B.V. All rights reserved.Peer ReviewedPostprint (author’s final draft

A mixed stabilized finite element formulation for strain localization analysis

This work exploits the concept of stabilization techniques to improve the behaviour of either mixed or pu/mixed u/εlinear/linear triangular elements when strain localization analysis is required. Different stabilization methods suitable for either Mode-I or Mode-II structural failure are proposed and compared to check the global stability of the corresponding discrete finite element formulation. Both elasto-J2-plastic and isotropic damage models have been used for the constitutive behaviour of the material. In both cases, exponential softening has been introduced in the post-peak regime. The results obtained do not suffer from spurious mesh-size or mesh-bias dependence, comparing very favorably with those obtained with standard approach.Postprint (published version

Mecànica de medis continus per a enginyers

Descripció del recurs: 18 de novembre de 2016Reimpressions: 2011Mecànica de medis continus per a enginyers vol ser una eina per a la formació dels enginyers en la mecànica dels medis continus, que manté un equilibri adequat entre el rigor del seu plantejament i la claredat dels principis físics que aborda. El contingut es divideix en dues parts ben diferenciades, que es presenten seqüencialment. A la primera part -capítols de 1 al 5-, s’introdueixen els aspectes fonamentals i descriptius comuns de tots els medis continus (moviment, deformació, tensió i equacions de conservació-balanç). A la segona part -capítols del 6 a 11-, s’estudien famílies específiques de medis continus, com ara els sòlids i els fluids, en un plantejament que s’inicia amb l’equació constitutiva corresponent i conclou amb les formulacions clàssiques de la mecànica de sòlids (elàstics lineals i elastoplàstics) i de la mecànica de fluids (règim laminar). Finalment, es fa una incursió breu en els principis variacionals (principis dels treballs virtuals i de minimització de l’energia potencial). Aquesta estructura permet utilitzar el text amb finalitats docents, tant en un sol curs d’unes 100 hores lectives, com en dos cursos diferenciats: el primer, basat en els cinc primers capítols, on s’introdueixin els fonaments de la mecànica de medis continus, i el segon, dedicat específicament a la mecànica de sòlids i a la mecànica de fluids

Mecánica de medios continuos para ingenieros

Mecánica de medios continuos para ingenieros pretende ser una herramienta para la formación de los ingenieros en la mecánica de medios continuos, que mantiene un equilibrio adecuado entre la rigurosidad de su planteamiento y la claridad de los principios físicos tratados. El contenido del texto está claramente dividido en dos partes, que se presentan secuencialmente. En la primera (capítulos 1 a 5), se introducen los aspectos fundamentales y descriptivos comunes a todos los medios continuos (movimiento, deformación, tensión y ecuaciones de conservación-balance). En la segunda (capítulos 6 a 11), se estudian familias específicas de medios continuos, como son los sólidos y los fluidos, en un planteamiento que se inicia con la correspondiente ecuación constitutiva y concluye con las formulaciones clásicas de la mecánica de sólidos (elásticos-lineales y elastoplásticos) y de la mecánica de fluidos (régimen laminar). Finalmente, se realiza una breve incursión en los principios variacionales (principios de los trabajos virtuales y de minimización de la energía potencial). Esta estructura permite la utilización del texto con propósitos docentes, tanto en un único curso, de unas 100 horas lectivas, como en dos cursos diferenciados: el primero, basado en los primeros cinco capítulos y dedicado a la introducción de los fundamentos de la mecánica de medios continuos, y el segundo, dedicado específicamente a la mecánica de sólidos y la mecánica de fluidos.2a ed

A fast and accurate two-stage strategy to evaluate the effect of the pin tool profile on metal flow, torque and forces in friction stir welding

Pin geometry is a fundamental consideration in friction stir welding (FSW). It influences the thermal behaviour, material flow and forces during the weld and reflects on the joint quality. This work studies four pin tools with circular, triflute, trivex, and triangular profiles adopting a validated model of FSW process developed by the authors. The effect of the rotating tool geometry on the flow behaviour and process outcomes is analysed. Additionally, longitudinal and transversal forces and torque are numerically calculated and compared for the different pin shapes. The study is carried out for slip and stick limiting friction cases between pin and workpiece. The main novelties of the paper are a “speed-up” two-stage simulation methodology and a piecewise linear version of the constitutive model, both of them conceived for the use in real case industrial applications, where the achievement of accuracy with affordable simulation times is of importance. The Norton-Hoff constitutive model is adopted to characterize the material behaviour during the weld. The piecewise linear version of the model developed by the authors greatly facilitates the convergence of the numerical solution ensuring both computational efficiency and accuracy. A two-stage computational procedure is applied. In the first stage, a forced transient is carried out; in the second one, the magnitudes of interest are computed. The study shows that the proposed modelling approach can be used to predict and interpret the FSW behaviour for a specific pin geometry. Moreover, the reduction of the simulation time using the two-stage strategy can be up to 90%, compared to a standard single stage strategy.Peer ReviewedPostprint (author's final draft

Numerical modeling of the electron beam welding and its experimental validation

Electron Beam Welding (EBW) is a highly efficient and precise welding method increasingly used within the manufacturing chain and of growing importance in different industrial environments such as the aeronautical and aerospace sectors. This is because, compared to other welding processes, EBW induces lower distortions and residual stresses due to the lower and more focused heat input along the welding line. This work describes the formulation adopted for the numerical simulation of the EBW process as well as the experimental work carried out to calibrate and validate it. The numerical simulation of EBW involves the interaction of thermal, mechanical and metallurgical phenomena. For this reason, in this work the numerical framework couples the heat transfer process to the stress analysis to maximize accuracy. An in-house multi-physics FE software is used to deal with the numerical simulation. The definition of an ad hoc moving heat source is proposed to simulate the EB power surface distribution and the corresponding absorption within the work-piece thickness. Both heat conduction and heat radiation models are considered to dissipate the heat through the boundaries of the component. The material behavior is characterized by an apropos thermo-elasto-viscoplastic constitutive model. Titanium-alloy Ti6A14V is the target material of this work. From the experimental side, the EB welding machine, the vacuum chamber characteristics and the corresponding operative setting are detailed. Finally, the available facilities to record the temperature evolution at different thermo-couple locations as well as to measure both distortions and residual stresses are described. Numerical results are compared with the experimental evidence.Peer ReviewedPostprint (author's final draft

Formulación estabilizada de elementos finitos triangulares y tetraédricos para problemas de incompresibilidad en grandes deformaciones

Se presenta una formulación en multiescalas del método de elementos finitos capaz de estabilizar el comportamiento de elementos mixtos en problemas de elasticidad y de plasticidad incompresibles en grandes deformaciones. Esta formulación se fundamenta en el concepto de las sub-escalas ortogonales (OSGS) y se aplica a elementos triangulares y tetraédricos mixtos, con interpolaciones de desplazamientos y presión continuas. La formulación permite eludir la condición de estabilidad de Babu˘ska-Brezzi, y ofrece como principal ventaja la posibilidad de utilizar interpolaciones lineales en elementos mixtos triangulares y tetraédricos, muy convenientes en aplicaciones de interés práctico debido a su versatilidad para la generación de mallas sobre configuraciones geométricas complejas. Se explican tanto las consideraciones empleadas en el planteamiento, como los principales aspectos de implementación. Una de las contribuciones más relevantes de esta formulación es la eficacia y originalidad de la aproximación propuesta para el parámetro de estabilizaci ón. Finalmente, mediante ejemplos de simulación se muestra el buen comportamiento de los elementos obtenidos en comparación con elementos estándar y Q1P0.Peer Reviewe

Numerical analysis of the manufacturing processes of a mock-up of the ITER NHF First Wall Panel

The objective of ITER is to build a new Tokamak, with the goal of demonstrating the scientific and technical feasibility of fusion power. The First Wall Panels are the inner component of the reactor, built with different materials that must support high heat flux levels inside the vacuum vessel. The manufacturing processes of the First Wall are a complex procedure including bending, hipping and cutting procedures which, in general, lead to residual stresses and distortions of the fabricated component. In this work, the analysis of the thermo-mechanical response of a simplified prototype of the ITER NHF First Wall Panel is presented from the numerical point of view. The experimental procedure within each phase of the whole manufacturing process is described. Residual stresses and distortions have been measured and analyzed. The numerical simulation of the manufacturing process includes the description of the main hypothesis, the applied loads and the boundary conditions assumed at every stage of the process. Special attention is paid to the simulation of machining and cutting by means of an ad-hoc element deactivation strategy. The numerical results are compared with the experimental evidence to show the prediction capability and the limitations of the proposed numerical model.Peer ReviewedPostprint (author's final draft

Material flow visualization in Friction Stir Welding via particle tracing

This work deals with the modeling of the material flow in Friction Stir Welding (FSW) processes using particle tracing method. For the computation of particle trajectories, three accurate and computationally efficient integration methods are implemented within a FE model for FSW process: the Backward Euler with Sub-stepping (BES), the 4-th order Runge-Kutta (RK4) and the Back and Forth Error Compensation and Correction (BFECC) methods. Firstly, their performance is compared by solving the Zalesak's disk benchmark. Later, the developed methodology is applied to some FSW problems providing a quantitative 2D and 3D view of the material transport in the process area. The material flow pattern is compared to the experimental evidence.Peer ReviewedPostprint (author’s final draft

Local and global approaches to Friction Stir Welding

This paper deals with the numerical simulation of Friction Stir Welding (FSW) processes. FSW techniques are used in many industrial applications and particularly in the aeronautic and aerospace industries, where the quality of the joining is of essential importance. The analysis is focused either at global level, considering the full component to be jointed, or locally, studying more in detail the heat a§ected zone (HAZ). The analysis at global (structural component) level is performed deÖning the problem in the Lagrangian setting while, at local level, an apropos kinematic framework which makes use of an e¢ cient combination of Lagrangian (pin), Eulerian (metal sheet) and ALE (stirring zone) descriptions for the di§erent computational sub-domains is introduced for the numerical modeling. As a result, the analysis can deal with complex (non-cylindrical) pin-shapes and the extremely large deformation of the material at the HAZ without requiring any remeshing or remapping tools. A fully coupled thermo-mechanical framework is proposed for the computational modeling of the FSW processes proposed both at local and global level. A staggered algorithm based on an isothermal fractional step method is introduced. To account for the isochoric behavior of the material when the temperature range is close to the melting point or due to the predominant deviatoric deformations induced by the visco-plastic response, a mixed Önite element technology is introduced. The Variational Multi Scale (VMS) method is used to circumvent the LBB stability condition allowing the use of linear/linear P1/P1 interpolations for displacement (or velocity, ALE/Eulerian formulation) and pressure Öelds, respectively. The same stabilization strategy is adopted to tackle the instabilities of the temperature Öeld, inherent characteristic of convective dominated problems (thermal analysis in ALE/Eulerian kinematic framework). At global level, the material behavior is characterized by a thermo-elasto- viscoplastic constitutive model. The analysis at local level is characterized by a rigid thermo-visco-plastic constitutive model. Di§erent thermally coupled (non-Newtonian) áuid-like models as Norton-Ho§, Carreau or Sheppard-Wright, among others are tested. To better understand the material áow pattern in the stirring zone, a (Lagrangian based) particle tracing is carried out while post-processing FSW results. A coupling strategy between the analysis of the process zone nearby the pin-tool (local level analysis) and the simulation carried out for the entire structure to be welded (global level analysis) is implemented to accurately predict the temperature histories and, thereby, the residual stresses in FSW.Postprint (published version