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Miguel Cervera

Universidad Politecnica de Catalunya, Civil and Environmental Engineering, Faculty Member

CIMNE, Structural analysis, Faculty Member

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Prof. Miguel Cervera is Ingeniero de Caminos, Canales y Puertos (UPC, 1983), Doctor of Philosophy in Civil Engineering (University of Wales, 1986), Dr. Ingeniero de Caminos, Canales y Puertos (UPC, 1988), IACM Fellow (2016). His research interests are related to computational mechanics, structural analysis, constitutive modeling and finite element methods, with focus on applications of thermo-mechanical processes with phase change and chemical interactions. http://cervera.rmee.upc.edu/

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Papers by Miguel Cervera

Experimental, Computational, and Dimensional Analysis of the Mechanical Performance of Fused Filament Fabrication Parts

Polymers, 2021

Process parameters in Additive Manufacturing (AM) are key factors in the mechanical performance o... more Process parameters in Additive Manufacturing (AM) are key factors in the mechanical performance of 3D-printed parts. In order to study their effect, a three-zone model based on the printing pattern was developed. This modelization distinguished three different zones of the 3D-printed part, namely cover, contour, and inner; each zone was treated as a different material. The cover and contour zones were characterized via uniaxial tensile tests and the inner zones via computational homogenization. The model was then validated by means of bending tests and their corresponding computational simulations. To reduce the number of required characterization experiments, a relationship between the raw and 3D-printed material was established by dimensional analysis. This allowed describing the mechanical properties of the printed part with a reduced set of the most influential non-dimensional relationships. The influence on the performance of the parts of inter-layer adhesion was also addressed...

Fully coupled thermo-mechanical analysis of multi-pass welding and metal deposition processes

ABSTRACT The main idea of this paper is to introduce the appropriate formulation for numerical an... more ABSTRACT The main idea of this paper is to introduce the appropriate formulation for numerical analysis of multi-pass welding process as well as the strategy for the metal deposition (MD) involving the coupling of thermal, mechanical and metallurgical phenomena. MD process is widely used in industrial applications for constructing different features such as bosses or flanges directly on fabricated components. A fully coupled thermo-mechanical analysis is performed simulating buildup of different layers of filler material on a substrate including the phase-change phenomena in terms of latent heat release and thermal shrinkage effects. On one hand, the material used in the present study is a nickel alloy (INCONEL-718) with temperature dependent material properties. A thermo-elasto-visco-plastic model including isotropic hardening and thermal softening has been chosen. A smooth transition from the liquid-like to the solid phase behaviour and the incompressibility constraint has been considered to solve the mechanical model. On the other hand, enthalpy has been chosen to be the primary variable for the solution of the thermal problem including solidification evolution. The thermal boundary conditions combine heat convection and radiation to dissipate heat through the boundaries. The simulation of the MD process is based on an ad-hoc Finite Element activation methodology which it is able to reproduce the deposition of the melted material along the predefined welding path by new elements generation. Such amount of elements are switched on at each time step based on the born-dead elements strategy. At each pass a new layer is deposited on the previous substrate allowing the fabrication of the desired geometry. In the proposed simulation stabilized mixed U/P linear tetrahedral elements and Q1/P0 hexahedral elements have been utilized. The presented model can accurately predict the temperature evolution, distortions and residual stresses induced by the successive welding layers deposited during the process. The simulation based on the proposed model in terms of the temperature evolution at different locations and the distortion of the welded part shows good agreement between numerical results and experimental data.

Coupled thermo-mechanical analysis for Frictional Stir Welding processes

The aim of this work is to describe the formulation adopted for the numerical simulation of the F... more The aim of this work is to describe the formulation adopted for the numerical simulation of the Frictional Stir Welding (FSW) process. FSW is a solid-state joining process (the metal is not melted during the process) and is used for applications where the original metallurgical characteristics must be retained. This process is primarily used on aluminum alloys, and most often on large pieces which cannot be easily heat treated to recover temper characteristics. On one hand, heat is induced by the friction between the tool shoulder and the material of the work pieces. On the other hand, the heat is generated by the mechanical mixing (stirring) process without reaching the melting point (solid-state process).

A novel stress-accurate FE technology for the numerical simulation of the FSW process

In this work a novel finite element technology based on a three-field mixed formulation is presen... more In this work a novel finite element technology based on a three-field mixed formulation is presented. The Variational Multi Scale (VMS) method is used to circumvent the LBB stability condition allowing the use of linear piece-wise interpolations for displacement, stress and pressure fields, respectively. The result is an enhanced stress field approximation which enables for stress-accurate results in non-linear computational mechanics [4-5]. The use of an independent nodal variable for the pressure field allows for an ad-hoc treatment of the incompressibility constraint. This is a mandatory requirement due to the isochoric nature of the plastic strain in metal forming processes. The highly non-linear stress field typically encountered in the Friction Stir Welding (FSW) process is used as an example to show the performance of this new FE technology. The numerical simulation of the FSW process is tackled by means of an Arbitrary-Lagrangian-Eulerian (ALE) formulation [1-3]. The computational domain is split into three different zones: the work-piece (defined by a rigid visco-plastic behaviour in the Eulerian framework), the pin (within the Lagrangian framework) and finally the stir-zone (ALE formulation). A fully coupled thermo-mechanical analysis is introduced showing the heat fluxes generated by the plastic dissipation in the stir-zone (Sheppard rigid-visco-plastic constitutive model) as well as the frictional dissipation at the contact interface (Norton frictional contact model). Finally, tracers have been implemented to show the material flow around the pin allowing a better understanding of the welding mechanism. Numerical results are compared with experimental evidence.

Visualization of material flow for friction stir welding processes simulation

Computational Modeling and Sub-Grid Scale Stabilization of Incompressibility and Convection in the Numerical Simulation of Friction Stir Welding Processes

ABSTRACT This paper deals with the computational modeling and sub-grid scale stabilization of inc... more ABSTRACT This paper deals with the computational modeling and sub-grid scale stabilization of incompressibility and convection in the numerical simulation of the material flow around the probe tool in a friction stir welding (FSW) process. Within the paradigmatic framework of the multiscale stabilization methods, suitable pressure and convective derivative of the temperature sub-grid scale stabilized coupled thermomechanical formulations have been developed using an Eulerian description. Norton-Hoff and Sheppard-Wright thermo-rigid-viscoplastic constitutive material models have been considered. Constitutive equations for the sub-grid scale models have been proposed and an approximation of the sub-grid scale variables has been given. In particular, algebraic sub-grid scale (ASGS) and orthogonal sub-grid scale (OSGS) methods for mixed velocity, pressure and temperature P1/P1/P1 linear elements have been considered. Furthermore, it has been shown that well known classical stabilized formulations, such as the Galerkin least-squares (GLS) for incompressible (or quasi-incompressible) problems or the Streamline Upwind/Petrov-Galerkin (SUPG) method for convection dominant problems, can be recovered as particular cases of the multiscale stabilization framework considered. Using a product formula algorithm for the solution of the coupled thermomechanical problem, the resulting algebraic system of equations has been solved using a staggered procedure in which a mechanical problem, defined by the linear momentum balance equation, under quasi-static conditions, and the incompressibility equation, is solved first at constant temperature. Then a thermal problem, defined by the energy balance equation, is solved keeping constant the mechanical variables, i.e. velocity and pressure. The computational model has been implemented in an enhanced version of the finite element software COMET, developed by the authors at the International Center for Numerical Methods in Engineering (CIMNE). Two numerical examples have been considered. The first one deals with the numerical simulation of a coupled thermomechanical flow in a 2D rectangular domain. Steady-state and transient conditions have been considered. The goal of this numerical example has been the comparison between different sub-grid scale stabilization methods for the velocity and temperature equations. In particular, using a GLS stabilization method for the pressure equation, a comparison between SUPG and OSGS convective stabilization methods has been performed. Additionally, using a SUPG stabilization method for the temperature equation, a comparison between GLS and OSGS pressure stabilization methods has been done. The second example deals with the 3D numerical simulation of a representative FSW process. Numerical results obtained have been compared with experimental results available in the literature. A good agreement on the temperature distribution has been obtained and predicted peak temperatures compare well, both in value and position, with the experimental results available.

An apropos kinematic framework for the numerical modelling of friction stir welding

ABSTRACT This paper describes features of a fully coupled thermo-mechanical model for Friction St... more ABSTRACT This paper describes features of a fully coupled thermo-mechanical model for Friction Stir Welding (FSW) simulation. An apropos kinematic setting for different zones of the computational domain is introduced and an efficient coupling strategy is proposed. Heat generation via viscous dissipation as well as frictional heating is considered.The results of the simulation using the proposed model are compared with the experimental evidence. The effect of slip and stick condition on non-circular pin shapes is analyzed. Simulation of material stirring is also carried out via particle tracing, providing insight of the material flow pattern in the vicinity of the pin.

A novel stress-accurate FE technology for highly non-linear analysis with incompressibility constraint. Application to the numerical simulation of the FSW process

In this work a novel finite element technology based on a three-field mixed formulation is presen... more In this work a novel finite element technology based on a three-field mixed formulation is presented. The Variational Multi Scale (VMS) method is used to circumvent the LBB stability condition allowing the use of linear piece-wise interpolations for displacement, stress and pressure fields, respectively. The result is an enhanced stress field approximation which enables for stress-accurate results in nonlinear computational mechanics. The use of an independent nodal variable for the pressure field allows for an adhoc treatment of the incompressibility constraint. This is a mandatory requirement due to the isochoric nature of the plastic strain in metal forming processes.The highly non-linear stress field typically encountered in the Friction Stir Welding (FSW) process is used as an example to show the performance of this new FE technology.The numerical simulation of the FSW process is tackled by means of an Arbitrary-Lagrangian-Eulerian (ALE) formulation. The computational domain is split into three different zones: the work.piece (defined by a rigid visco-plastic behaviour in the Eulerian framework), the pin (within the Lagrangian framework) and finally the stirzone (ALE formulation).A fully coupled thermo-mechanical analysis is introduced showing the heat fluxes generated by the plastic dissipation in the stir-zone (Sheppard rigid-viscoplastic constitutive model) as well as the frictional dissipation at the contact interface (Norton frictional contact model).Finally, tracers have been implemented to show the material flow around the pin allowing a better understanding of the welding mechanism. Numerical results are compared with experimental evidence.

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

Finite element modeling of multi-pass welding and shaped metal deposition processes

This paper describes the formulation adopted for the numerical simulation of the shaped metal dep... more This paper describes the formulation adopted for the numerical simulation of the shaped metal deposition process (SMD) and the experimental work carried out at ITP Industry to calibrate and validate the proposed model. The SMD process is a novel manufacturing technology, similar to the multi-pass welding used for building features such as lugs and flanges on fabricated components (see Fig. 1a and b). A fully coupled thermomechanical solution is adopted including phase-change phenomena defined in terms of both latent heat release and shrinkage effects. Temperature evolution as well as residual stresses and distortions, due to the successive welding layers deposited, are accurately simulated coupling the heat transfer and the mechanical analysis. The material behavior is characterized by a thermo-elasto-viscoplastic constitutive model coupled with a metallurgical model. Nickel super-alloy 718 is the target material of this work. Both heat convection and heat radiation models are introduced to dissipate heat through the boundaries of the component. An in-house coupled FE software is used to deal with the numerical simulation and an ad-hoc activation methodology is formulated to simulate the deposition of the different layers of filler material. Difficulties and simplifying hypotheses are discussed. Thermo-mechanical results are presented in terms of both temperature evolution and distortions, and compared with the experimental data obtained at the SMD laboratory of ITP.

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

by Miguel Cervera and Carlos Agelet de Saracibar

Pin geometry is a fundamental consideration in friction stir welding (FSW). It influences the the... more 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.

Numerical modeling of the electron beam welding and its experimental validation

by Carlos Agelet de Saracibar, Miguel Cervera, and Narges Dialami

Fully coupled thermo-mechanical analysis of the EB welding process.Accurate description of the mo... more Fully coupled thermo-mechanical analysis of the EB welding process.Accurate description of the moving heat source used for the analysis.FE technology suitable for isochoric behavior of both the liquid-like and plastic deformation of the solid phases.Continuous transition model from room temperature to (and above) the melting point.Experimental validation of the model carried out the authors of the paper by using the facilities at BAMTRI labs.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.

2015-CMAME-u-epsilon-plast

by Michele Chiumenti, Lorenzo Benedetti, and Miguel Cervera

ENCOMPASSING INCOMPRESSIBILITY AND STRAIN LOCALIZATION IN PLASTICITY WITH MIXED FE

by Michele Chiumenti, Lorenzo Benedetti, and Miguel Cervera

Analytical solutions of laminar perturbed flows to validate a finite elements model applied to the numerical simulation of friction stir welding processes

by Narges Dialami and Miguel Cervera

Una formulación mixta estabilizada explícita para plasticidad con localización de deformaciones

by Miguel Cervera and Nelson M Lafontaine

Este artículo presenta la aplicación de la formulación mixta estabilizada explícita en desplazami... more Este artículo presenta la aplicación de la formulación mixta estabilizada explícita en desplazamientos y deformaciones (MEX-FEM)[23, 24] para la solución de problemas no lineales de la mecánica de sólidos con localización de deformaciones. A fin de emplear el mismo orden lineal de interpolación para el campo de los desplazamientos y deformaciones, nuestra formulación emplea el método de las sub-escalas variacionales. Comparada con la formulación estándar en desplazamientos, la formulación propuesta proporciona mejores campos de deformaciones y tensiones, y es capaz de abordar situaciones quasi-incompresibles. En este trabajo se investigan los efectos que tienen las deformaciones y tensiones mejoradas en los modelos de plasticidad de Mohr-Coulomb y Drucker Prager, incluyendo el fenómeno de la localización de las deformaciones. Los ejemplos numéricos validan la capacidad de la formulación propuesta para predecir correctamente los mecanismos de fallo, cargas últimas y la dirección de la banda de localización, virtualmente independientes de la malla utilizada y sin necesidad de emplear un algoritmo de rastreo.

A localized mapped damage model for orthotropic materials

by Miguel Cervera and Sergio Oller

This paper presents an implicit orthotropic model based on the Continuum Damage Mechanics isotrop... more This paper presents an implicit orthotropic model based on the Continuum Damage Mechanics isotropic models. A mapping relationship is established between the behaviour of the anisotropic material and that of an isotropic one. The proposed model is used to simulate the failure loci of common orthotropic materials, such as masonry, fibre-reinforced composites and wood. The damage model is combined with a crack-tracking technique to reproduce the propagation of localized cracks in the discrete FE problem. The proposed numerical model is used to simulate the mixed mode fracture in masonry members with different orientations of the brick layers.

A stabilized formulation for incompressible plasticity using linear triangles and tetrahedra

In this paper, a stabilized finite element method to deal with incompressibility in solid mechani... more In this paper, a stabilized finite element method to deal with incompressibility in solid mechanics is presented. Both elastic and J2-plastic constitutive behavior have been considered. A mixed formulation involving pressure and displacement fields is used and a continuous linear interpolation is considered for both fields. To circumvent the Babu ska–Brezzi condition a stabilization technique based on the orthogonal sub-scale method is introduced. The main advantage of the method is the possibility of using linear triangular or tetrahedral finite elements , which are easy to generate for real industrial applications. Results are compared with standard Galerkin and Q1P0 mixed formulations in either elastic or elasto-plastic incom-pressible problems.

CURRENT DEVELOPMENTS ON THE COUPLED THERMOMECHANICAL COMPUTATIONAL MODELING OF METAL CASTING PROCESSES

In this paper, current developments on the coupled thermomechanical computational simulation of m... more In this paper, current developments on the coupled thermomechanical computational simulation of metal casting processes are presented A thermodynamically consistent constitutive material model is derived from a thermoviscoplastic free energy function. A continuous transition between the initial fluid-like and the final solid-like is modeled by considering a J2 thermoviscoplastic model. Thus, an thermoelastoviscoplastic model, suitable for the solid-like phase, degenerates into a pure thermoviscous model, suitable for the liquid-like phase, according to the evolution of the solid fraction function. A thermomechanical contact model, taking into account the insulated effects of the air-gap due to thermal shrinkage of the part during solidification and cooling, is introduced. A fractional step method, arising from an operator split of the governing differential equations, is considered to solve the coupled problem using a staggered scheme. Within a finite element setting, using low-order interpolation elements, a multiscale stabilization technique is introduced as a convenient framework to overcome the Babuska-Brezzi condition and avoid volumetric locking and pressure instabilities arising in incompressible or quasi-incompressible problems. Computational simulation of industrial castings show the good performance of the model.

Local-global strategy for the prediction of residual stresses in FSW processes

by Miguel Cervera and Narges Dialami

This work describes the local-global strategy proposed for the computation of residual stresses i... more This work describes the local-global strategy proposed for the computation of residual stresses in Friction Stir Welding (FSW) processes. 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. As a first step, the local problem solves the material stirring as well as the heat generation induced by the pin and shoulder rotation at the heat affected zone. The Arbitrary Lagrangian Eulerian (ALE) formulation is adopted to deal with the rotation of complex pin shapes. A thermo-rigid-viscoplastic constitutive law is employed to characterize the viscous flow of the material, driven by the high strain rates induced by the FSW process. A mixed temperature-velocity-pressure finite element technology is used to deal with the isochoric nature of the the strains. The output of this local analysis is the heat generated either by plastic dissipation or by friction and it is used as the power input for the welding analysis at structural (global) level. The global problem is tackled within the Lagrangian framework together with a thermo-elasto-viscoplastic constitutive model. Also in this case the mixed temperature-displacement

Experimental, Computational, and Dimensional Analysis of the Mechanical Performance of Fused Filament Fabrication Parts

Polymers, 2021

Process parameters in Additive Manufacturing (AM) are key factors in the mechanical performance o... more Process parameters in Additive Manufacturing (AM) are key factors in the mechanical performance of 3D-printed parts. In order to study their effect, a three-zone model based on the printing pattern was developed. This modelization distinguished three different zones of the 3D-printed part, namely cover, contour, and inner; each zone was treated as a different material. The cover and contour zones were characterized via uniaxial tensile tests and the inner zones via computational homogenization. The model was then validated by means of bending tests and their corresponding computational simulations. To reduce the number of required characterization experiments, a relationship between the raw and 3D-printed material was established by dimensional analysis. This allowed describing the mechanical properties of the printed part with a reduced set of the most influential non-dimensional relationships. The influence on the performance of the parts of inter-layer adhesion was also addressed...

Fully coupled thermo-mechanical analysis of multi-pass welding and metal deposition processes

ABSTRACT The main idea of this paper is to introduce the appropriate formulation for numerical an... more ABSTRACT The main idea of this paper is to introduce the appropriate formulation for numerical analysis of multi-pass welding process as well as the strategy for the metal deposition (MD) involving the coupling of thermal, mechanical and metallurgical phenomena. MD process is widely used in industrial applications for constructing different features such as bosses or flanges directly on fabricated components. A fully coupled thermo-mechanical analysis is performed simulating buildup of different layers of filler material on a substrate including the phase-change phenomena in terms of latent heat release and thermal shrinkage effects. On one hand, the material used in the present study is a nickel alloy (INCONEL-718) with temperature dependent material properties. A thermo-elasto-visco-plastic model including isotropic hardening and thermal softening has been chosen. A smooth transition from the liquid-like to the solid phase behaviour and the incompressibility constraint has been considered to solve the mechanical model. On the other hand, enthalpy has been chosen to be the primary variable for the solution of the thermal problem including solidification evolution. The thermal boundary conditions combine heat convection and radiation to dissipate heat through the boundaries. The simulation of the MD process is based on an ad-hoc Finite Element activation methodology which it is able to reproduce the deposition of the melted material along the predefined welding path by new elements generation. Such amount of elements are switched on at each time step based on the born-dead elements strategy. At each pass a new layer is deposited on the previous substrate allowing the fabrication of the desired geometry. In the proposed simulation stabilized mixed U/P linear tetrahedral elements and Q1/P0 hexahedral elements have been utilized. The presented model can accurately predict the temperature evolution, distortions and residual stresses induced by the successive welding layers deposited during the process. The simulation based on the proposed model in terms of the temperature evolution at different locations and the distortion of the welded part shows good agreement between numerical results and experimental data.

Coupled thermo-mechanical analysis for Frictional Stir Welding processes

The aim of this work is to describe the formulation adopted for the numerical simulation of the F... more The aim of this work is to describe the formulation adopted for the numerical simulation of the Frictional Stir Welding (FSW) process. FSW is a solid-state joining process (the metal is not melted during the process) and is used for applications where the original metallurgical characteristics must be retained. This process is primarily used on aluminum alloys, and most often on large pieces which cannot be easily heat treated to recover temper characteristics. On one hand, heat is induced by the friction between the tool shoulder and the material of the work pieces. On the other hand, the heat is generated by the mechanical mixing (stirring) process without reaching the melting point (solid-state process).

A novel stress-accurate FE technology for the numerical simulation of the FSW process

In this work a novel finite element technology based on a three-field mixed formulation is presen... more In this work a novel finite element technology based on a three-field mixed formulation is presented. The Variational Multi Scale (VMS) method is used to circumvent the LBB stability condition allowing the use of linear piece-wise interpolations for displacement, stress and pressure fields, respectively. The result is an enhanced stress field approximation which enables for stress-accurate results in non-linear computational mechanics [4-5]. The use of an independent nodal variable for the pressure field allows for an ad-hoc treatment of the incompressibility constraint. This is a mandatory requirement due to the isochoric nature of the plastic strain in metal forming processes. The highly non-linear stress field typically encountered in the Friction Stir Welding (FSW) process is used as an example to show the performance of this new FE technology. The numerical simulation of the FSW process is tackled by means of an Arbitrary-Lagrangian-Eulerian (ALE) formulation [1-3]. The computational domain is split into three different zones: the work-piece (defined by a rigid visco-plastic behaviour in the Eulerian framework), the pin (within the Lagrangian framework) and finally the stir-zone (ALE formulation). A fully coupled thermo-mechanical analysis is introduced showing the heat fluxes generated by the plastic dissipation in the stir-zone (Sheppard rigid-visco-plastic constitutive model) as well as the frictional dissipation at the contact interface (Norton frictional contact model). Finally, tracers have been implemented to show the material flow around the pin allowing a better understanding of the welding mechanism. Numerical results are compared with experimental evidence.

Visualization of material flow for friction stir welding processes simulation

Computational Modeling and Sub-Grid Scale Stabilization of Incompressibility and Convection in the Numerical Simulation of Friction Stir Welding Processes

ABSTRACT This paper deals with the computational modeling and sub-grid scale stabilization of inc... more ABSTRACT This paper deals with the computational modeling and sub-grid scale stabilization of incompressibility and convection in the numerical simulation of the material flow around the probe tool in a friction stir welding (FSW) process. Within the paradigmatic framework of the multiscale stabilization methods, suitable pressure and convective derivative of the temperature sub-grid scale stabilized coupled thermomechanical formulations have been developed using an Eulerian description. Norton-Hoff and Sheppard-Wright thermo-rigid-viscoplastic constitutive material models have been considered. Constitutive equations for the sub-grid scale models have been proposed and an approximation of the sub-grid scale variables has been given. In particular, algebraic sub-grid scale (ASGS) and orthogonal sub-grid scale (OSGS) methods for mixed velocity, pressure and temperature P1/P1/P1 linear elements have been considered. Furthermore, it has been shown that well known classical stabilized formulations, such as the Galerkin least-squares (GLS) for incompressible (or quasi-incompressible) problems or the Streamline Upwind/Petrov-Galerkin (SUPG) method for convection dominant problems, can be recovered as particular cases of the multiscale stabilization framework considered. Using a product formula algorithm for the solution of the coupled thermomechanical problem, the resulting algebraic system of equations has been solved using a staggered procedure in which a mechanical problem, defined by the linear momentum balance equation, under quasi-static conditions, and the incompressibility equation, is solved first at constant temperature. Then a thermal problem, defined by the energy balance equation, is solved keeping constant the mechanical variables, i.e. velocity and pressure. The computational model has been implemented in an enhanced version of the finite element software COMET, developed by the authors at the International Center for Numerical Methods in Engineering (CIMNE). Two numerical examples have been considered. The first one deals with the numerical simulation of a coupled thermomechanical flow in a 2D rectangular domain. Steady-state and transient conditions have been considered. The goal of this numerical example has been the comparison between different sub-grid scale stabilization methods for the velocity and temperature equations. In particular, using a GLS stabilization method for the pressure equation, a comparison between SUPG and OSGS convective stabilization methods has been performed. Additionally, using a SUPG stabilization method for the temperature equation, a comparison between GLS and OSGS pressure stabilization methods has been done. The second example deals with the 3D numerical simulation of a representative FSW process. Numerical results obtained have been compared with experimental results available in the literature. A good agreement on the temperature distribution has been obtained and predicted peak temperatures compare well, both in value and position, with the experimental results available.

An apropos kinematic framework for the numerical modelling of friction stir welding

ABSTRACT This paper describes features of a fully coupled thermo-mechanical model for Friction St... more ABSTRACT This paper describes features of a fully coupled thermo-mechanical model for Friction Stir Welding (FSW) simulation. An apropos kinematic setting for different zones of the computational domain is introduced and an efficient coupling strategy is proposed. Heat generation via viscous dissipation as well as frictional heating is considered.The results of the simulation using the proposed model are compared with the experimental evidence. The effect of slip and stick condition on non-circular pin shapes is analyzed. Simulation of material stirring is also carried out via particle tracing, providing insight of the material flow pattern in the vicinity of the pin.

A novel stress-accurate FE technology for highly non-linear analysis with incompressibility constraint. Application to the numerical simulation of the FSW process

In this work a novel finite element technology based on a three-field mixed formulation is presen... more In this work a novel finite element technology based on a three-field mixed formulation is presented. The Variational Multi Scale (VMS) method is used to circumvent the LBB stability condition allowing the use of linear piece-wise interpolations for displacement, stress and pressure fields, respectively. The result is an enhanced stress field approximation which enables for stress-accurate results in nonlinear computational mechanics. The use of an independent nodal variable for the pressure field allows for an adhoc treatment of the incompressibility constraint. This is a mandatory requirement due to the isochoric nature of the plastic strain in metal forming processes.The highly non-linear stress field typically encountered in the Friction Stir Welding (FSW) process is used as an example to show the performance of this new FE technology.The numerical simulation of the FSW process is tackled by means of an Arbitrary-Lagrangian-Eulerian (ALE) formulation. The computational domain is split into three different zones: the work.piece (defined by a rigid visco-plastic behaviour in the Eulerian framework), the pin (within the Lagrangian framework) and finally the stirzone (ALE formulation).A fully coupled thermo-mechanical analysis is introduced showing the heat fluxes generated by the plastic dissipation in the stir-zone (Sheppard rigid-viscoplastic constitutive model) as well as the frictional dissipation at the contact interface (Norton frictional contact model).Finally, tracers have been implemented to show the material flow around the pin allowing a better understanding of the welding mechanism. Numerical results are compared with experimental evidence.

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

Finite element modeling of multi-pass welding and shaped metal deposition processes

This paper describes the formulation adopted for the numerical simulation of the shaped metal dep... more This paper describes the formulation adopted for the numerical simulation of the shaped metal deposition process (SMD) and the experimental work carried out at ITP Industry to calibrate and validate the proposed model. The SMD process is a novel manufacturing technology, similar to the multi-pass welding used for building features such as lugs and flanges on fabricated components (see Fig. 1a and b). A fully coupled thermomechanical solution is adopted including phase-change phenomena defined in terms of both latent heat release and shrinkage effects. Temperature evolution as well as residual stresses and distortions, due to the successive welding layers deposited, are accurately simulated coupling the heat transfer and the mechanical analysis. The material behavior is characterized by a thermo-elasto-viscoplastic constitutive model coupled with a metallurgical model. Nickel super-alloy 718 is the target material of this work. Both heat convection and heat radiation models are introduced to dissipate heat through the boundaries of the component. An in-house coupled FE software is used to deal with the numerical simulation and an ad-hoc activation methodology is formulated to simulate the deposition of the different layers of filler material. Difficulties and simplifying hypotheses are discussed. Thermo-mechanical results are presented in terms of both temperature evolution and distortions, and compared with the experimental data obtained at the SMD laboratory of ITP.

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

by Miguel Cervera and Carlos Agelet de Saracibar

Pin geometry is a fundamental consideration in friction stir welding (FSW). It influences the the... more 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.

Numerical modeling of the electron beam welding and its experimental validation

by Carlos Agelet de Saracibar, Miguel Cervera, and Narges Dialami

Fully coupled thermo-mechanical analysis of the EB welding process.Accurate description of the mo... more Fully coupled thermo-mechanical analysis of the EB welding process.Accurate description of the moving heat source used for the analysis.FE technology suitable for isochoric behavior of both the liquid-like and plastic deformation of the solid phases.Continuous transition model from room temperature to (and above) the melting point.Experimental validation of the model carried out the authors of the paper by using the facilities at BAMTRI labs.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.

2015-CMAME-u-epsilon-plast

by Michele Chiumenti, Lorenzo Benedetti, and Miguel Cervera

ENCOMPASSING INCOMPRESSIBILITY AND STRAIN LOCALIZATION IN PLASTICITY WITH MIXED FE

by Michele Chiumenti, Lorenzo Benedetti, and Miguel Cervera

Analytical solutions of laminar perturbed flows to validate a finite elements model applied to the numerical simulation of friction stir welding processes

by Narges Dialami and Miguel Cervera

Una formulación mixta estabilizada explícita para plasticidad con localización de deformaciones

by Miguel Cervera and Nelson M Lafontaine

Este artículo presenta la aplicación de la formulación mixta estabilizada explícita en desplazami... more Este artículo presenta la aplicación de la formulación mixta estabilizada explícita en desplazamientos y deformaciones (MEX-FEM)[23, 24] para la solución de problemas no lineales de la mecánica de sólidos con localización de deformaciones. A fin de emplear el mismo orden lineal de interpolación para el campo de los desplazamientos y deformaciones, nuestra formulación emplea el método de las sub-escalas variacionales. Comparada con la formulación estándar en desplazamientos, la formulación propuesta proporciona mejores campos de deformaciones y tensiones, y es capaz de abordar situaciones quasi-incompresibles. En este trabajo se investigan los efectos que tienen las deformaciones y tensiones mejoradas en los modelos de plasticidad de Mohr-Coulomb y Drucker Prager, incluyendo el fenómeno de la localización de las deformaciones. Los ejemplos numéricos validan la capacidad de la formulación propuesta para predecir correctamente los mecanismos de fallo, cargas últimas y la dirección de la banda de localización, virtualmente independientes de la malla utilizada y sin necesidad de emplear un algoritmo de rastreo.

A localized mapped damage model for orthotropic materials

by Miguel Cervera and Sergio Oller

This paper presents an implicit orthotropic model based on the Continuum Damage Mechanics isotrop... more This paper presents an implicit orthotropic model based on the Continuum Damage Mechanics isotropic models. A mapping relationship is established between the behaviour of the anisotropic material and that of an isotropic one. The proposed model is used to simulate the failure loci of common orthotropic materials, such as masonry, fibre-reinforced composites and wood. The damage model is combined with a crack-tracking technique to reproduce the propagation of localized cracks in the discrete FE problem. The proposed numerical model is used to simulate the mixed mode fracture in masonry members with different orientations of the brick layers.

A stabilized formulation for incompressible plasticity using linear triangles and tetrahedra

In this paper, a stabilized finite element method to deal with incompressibility in solid mechani... more In this paper, a stabilized finite element method to deal with incompressibility in solid mechanics is presented. Both elastic and J2-plastic constitutive behavior have been considered. A mixed formulation involving pressure and displacement fields is used and a continuous linear interpolation is considered for both fields. To circumvent the Babu ska–Brezzi condition a stabilization technique based on the orthogonal sub-scale method is introduced. The main advantage of the method is the possibility of using linear triangular or tetrahedral finite elements , which are easy to generate for real industrial applications. Results are compared with standard Galerkin and Q1P0 mixed formulations in either elastic or elasto-plastic incom-pressible problems.

CURRENT DEVELOPMENTS ON THE COUPLED THERMOMECHANICAL COMPUTATIONAL MODELING OF METAL CASTING PROCESSES

In this paper, current developments on the coupled thermomechanical computational simulation of m... more In this paper, current developments on the coupled thermomechanical computational simulation of metal casting processes are presented A thermodynamically consistent constitutive material model is derived from a thermoviscoplastic free energy function. A continuous transition between the initial fluid-like and the final solid-like is modeled by considering a J2 thermoviscoplastic model. Thus, an thermoelastoviscoplastic model, suitable for the solid-like phase, degenerates into a pure thermoviscous model, suitable for the liquid-like phase, according to the evolution of the solid fraction function. A thermomechanical contact model, taking into account the insulated effects of the air-gap due to thermal shrinkage of the part during solidification and cooling, is introduced. A fractional step method, arising from an operator split of the governing differential equations, is considered to solve the coupled problem using a staggered scheme. Within a finite element setting, using low-order interpolation elements, a multiscale stabilization technique is introduced as a convenient framework to overcome the Babuska-Brezzi condition and avoid volumetric locking and pressure instabilities arising in incompressible or quasi-incompressible problems. Computational simulation of industrial castings show the good performance of the model.

Local-global strategy for the prediction of residual stresses in FSW processes

by Miguel Cervera and Narges Dialami

This work describes the local-global strategy proposed for the computation of residual stresses i... more This work describes the local-global strategy proposed for the computation of residual stresses in Friction Stir Welding (FSW) processes. 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. As a first step, the local problem solves the material stirring as well as the heat generation induced by the pin and shoulder rotation at the heat affected zone. The Arbitrary Lagrangian Eulerian (ALE) formulation is adopted to deal with the rotation of complex pin shapes. A thermo-rigid-viscoplastic constitutive law is employed to characterize the viscous flow of the material, driven by the high strain rates induced by the FSW process. A mixed temperature-velocity-pressure finite element technology is used to deal with the isochoric nature of the the strains. The output of this local analysis is the heat generated either by plastic dissipation or by friction and it is used as the power input for the welding analysis at structural (global) level. The global problem is tackled within the Lagrangian framework together with a thermo-elasto-viscoplastic constitutive model. Also in this case the mixed temperature-displacement

Prediction of residual stresses and distortions induced by the Additive Manufacturing process

by Miguel Cervera, Carlos Agelet de Saracibar, Narges Dialami, and L. Wei

On the Numerical Simulation of AM Processes

by Miguel Cervera, Narges Dialami, and Carlos Agelet de Saracibar

Elementos Finitos Mixtos Estabilizados para Flujos Viscoplásticos. Formulación y Aplicaciones

by Elvira Moreno and Miguel Cervera

Palabras claves: Método de los elementos finitos estabilizados, incompresibilidad, método del lev... more

Análisis estructural de edificios históricos mediante modelos localizados de fisuración

Nonlinear analysis of reinforced concrete structures using three dimensional and shell finite element models

On the Orthogonal Subgrid Scale Pressure Stabilization of Small and Finite Deformation J2 Plasticity

by Carlos Agelet de Saracibar, Miguel Cervera, and Quino Valverde

The use of stabilization methods is becoming an increasingly well-accepted technique due to their... more

A mixed finite element formulation for incompressibility using linear displacement and pressure interpolations

by Miguel Cervera and Carlos Agelet de Saracibar

In this work shall be presented a stabilized finite element method to deal with incompressibility... more In this work shall be presented a stabilized finite element method to deal with incompressibility in solid mechanics. A mixed formulation involving pressure and displacement fields is used and a continuous linear interpolation is considered for both fields. To overcome the Ladyzhenskaya-Babuška-Brezzi condition, a stabilization technique based on the orthogonal sub-grid scale method is introduced. The main advantage of the method is the possibility of using linear triangular finite elements, which are easy to generate for real industrial applications. Results are compared with several improved formulations, as the enhanced assumed strain method (EAS) and the Q1P0-formulation, in nearly incompressible problems and in the context of linear elasticity and J2-plasticity.

On the Computational Modeling and Numerical Simulation of FSW Processes

by Carlos Agelet de Saracibar, Miguel Cervera, Narges Dialami, and Guilermo Lombera

Shear Band Localization via Local J2 Continuum Damage Mechanics

by Miguel Cervera and Carlos Agelet de Saracibar

Computational modelling of mould manufacture for lost foam processes

by Carlos Agelet de Saracibar and Miguel Cervera

This paper presents a numerical model of mould manufacture for the lost foam casting process. The... more This paper presents a numerical model of mould manufacture for the lost foam casting process. The process of mould filling with sand and sand compaction by vibration are modelled using spherical (in 3D) or cylindrical (in 2D) discrete elements. The motion of discrete elements is described by means of equations of rigid body dynamics. Rigid particles interact among one another with contact forces, both in normal and tangential directions. Numerical simulation predicts defects of the mould due to insufficient sand compaction around the pattern. Combining the discrete element model of sand with the finite element model of the pattern allows us to detect possible distortion of the pattern during mould filling and compaction. Results of numerical simulation are validated by comparison with experimental data.

Stabilized finite element formulation for incompressible large strain plasticity using orthogonal subscales

by Carlos Agelet de Saracibar and Miguel Cervera

This paper proposes a possible solution to the problem of incompressibility observed in solid mec... more This paper proposes a possible solution to the problem of incompressibility observed in solid mechanics in case of non-linear kinematics for both incompressible elasticity and J2-plasticity. The objective is to avoid the so called volumetric locking, an undesirable effect exhibited by all low order elements based on the standard Galerkin formulation. The main advantage of the method is the possibility of using linear triangular or tetrahedral finite elements, which are easy to generate for real industrial applications such as hot-forging and metal casting among others. A mixed formulation involving pressure and displacement fields is used and a continuous linear interpolation is considered for both fields. To circumvent the Babuska-Brezzi condition a stabilization technique based on the orthogonal sub-scale method is introduced. This technique has been recently proposed by the authors to solve the problem of incompressible elasticity in the framework of small strain kinematics. Effectiveness and robustness of the method have encouraged the authors to extend the approach to geometrically non-linear problems. Results are compared with standard Galerkin and Q1P0 mixed large strain formulations in either elastic or elasto-plastic incompressible problems.

Numerical simulation of sand mould manufacture for Lost Foam Casting process

by Carlos Agelet de Saracibar and Miguel Cervera

This paper presents a numerical model of mould manufacture for the lost foam casting process. The... more This paper presents a numerical model of mould manufacture for the lost foam casting process. The process of mould filling with sand and sand compaction by vibration are modeled using spherical (in 3D) or cylindrical (in 2D) discrete elements. The motion of discrete elements is described by means of equations of rigid body dynamics. Rigid particles interact among one another with contact forces, both in normal and tangential directions. Numerical simulation predicts defects of the mould due to insufficient sand compaction around the pattern. Combining the discrete element model of sand with the finite element model of the pattern allows us to detect possible distortion of the pattern during mould filling and compaction. Results of numerical simulation are validated by comparison with experimental data.

Computational modeling of aluminium casting processes

by Carlos Agelet de Saracibar and Miguel Cervera

The numerical formulation of coupled thermo-mechanical solidification processes has been one of t... more The numerical formulation of coupled thermo-mechanical solidification processes has been one of the research topics of great interest over the last years. Also, during the last decade, a growing interest on this and related topics has been shown by many industrial companies, such as automotive and aeronautical, motivated by the need to get high-quality final products and to reduce manufacturing costs. However, and despite the enormous progress achieved in computational mechanics, the large-scale numerical simulation of these problems continues to be nowadays a very complex task. This is mainly due to the highly non-linear nature of the problem, involving non-linear constitutive behavior, liquid-solid phase-change, non-linear thermal and mechanical boundary conditions and thermo-mechanical contact interaction, among others. This paper presents an up-to-date finite element numerical tool for the simulation of aluminium foundry processes. A fully coupled thermo-mechanical formulation including phase change phenomena is considered. The mathematical framework to account for both thermal and mechanical constitutive and boundary assumptions is introduced. The proposed constitutive model is consistently derived from a thermo-elasto-viscoplastic free energy function. Mechanical and thermal material properties are assumed to be temperature-dependent. A continuum transition from the initial fluid-like to the finial solid-like behavior of the part is modeled considering a temperature dependent viscoplastic-surface evolution. Phase-change contribution is taken into account assuming both latent-heat release and shrinkage effects. Moreover, an accurate definition of the interfacial heat transfer between the solidifying casting and the mold is essential in producing a reliable casting model. In fact, both the solidification process and the temperature evolution strongly depend on the heat exchange at the contact interface. This exchange is affected by the insulating effects of the air-gap due to the thermal shrinkage of the casting part during the solidification and cooling phases. The need for a so closely coupled formulation is the reason why the finite element code VULCAN, developed by the authors, is presented as an accurate, efficient and robust numerical tool, allowing the numerical simulation of solidification and cooling processes for the aluminium casting industry.

Thermomechanical simulation for aluminium casting

by Miguel Cervera and Carlos Agelet de Saracibar

Numerical simulation of aluminium foundry processes

by Miguel Cervera and Carlos Agelet de Saracibar

This paper presents an up-to-date finite element numerical tool for the simulation of aluminium f... more This paper presents an up-to-date finite element numerical tool for the simulation of aluminium foundry processes. A fully coupled thermo-mechanical formulation including phase change phenomena is considered. The mathematical framework to account for both thermal and mechanical constitutive and boundary assumptions is introduced. The proposed constitutive model is consistently derived from a thermo-elasto-viscoplastic free energy function. Mechanical and thermal material properties are assumed to be temperature-dependent. A continuum transition from the initial fluid-like to the finial solid-like behavior of the part is modeled considering a temperature dependent viscoplastic-surface evolution. Phase-change contribution is taken into account assuming both latent-heat release and shrinkage effects. Moreover, an accurate definition of the interfacial heat transfer between the solidifying casting and the mold is essential in producing a reliable casting model. In fact, both the solidification process and the temperature evolution strongly depend on the heat exchange at the contact interface. This exchange is affected by the insulating effects of the air-gap due to the thermal shrinkage of the casting part during the solidification and cooling phases. The need for a so closely coupled formulation is the reason why the finite element code VULCAN, developed by the authors, is presented as an accurate, efficient and robust numerical tool, allowing the numerical simulation of solidification and cooling processes for the aluminium casting industry. Introduction: need for a coupled thermo-mechanical formulation

Current issues in the numerical simulation of casting processes

by Carlos Agelet de Saracibar and Miguel Cervera

Current issues in the numerical simulation of casting processes

by Carlos Agelet de Saracibar and Miguel Cervera

VULCAN 2000: A finite element system for the simulation of casting processes

by Carlos Agelet de Saracibar and Miguel Cervera

A coupled thermo-mechanical multi-phase constitutive model

by Carlos Agelet de Saracibar and Miguel Cervera

This paper presents an up-to-date finite element numerical model for fully coupled thermo-mechani... more This paper presents an up-to-date finite element numerical model for fully coupled thermo-mechanical problems, focusing in the simulation of industrial solidification processes. The proposed constitutive equations have been identified to model the thermo-mechanical multi-phase behavior of cast iron from liquidus to room temperature. A fractional step method arising from an operator split of the governing equations is used to solve the non-linear coupled system of equations, leading to a staggered product formula solution algorithm suitable for large scale computations. INTRODUCTION: The numerical simulation of foundry processes is one of the most interesting R+D topics for many industrial companies such as automotive and aeronautical, motivated by the need to spend less effort on experimental prototyping and to get high quality final products reducing manufacturing costs. The objective of this paper is to show the relevance of a model that takes into account the phase changes transformations in spheroidal graphite ferritic cast iron during its solidification.

A coupled thermo-mechanical multi-phase constitutive model

by Carlos Agelet de Saracibar and Miguel Cervera

This paper presents an up-to-date finite element numerical model for fully coupled thermo-mechani... more This paper presents an up-to-date finite element numerical model for fully coupled thermo-mechanical problems, focusing in the simulation of industrial solidification processes. The proposed constitutive equations have been identified to model the thermo-mechanical multi-phase behavior of cast iron from liquidus to room temperature. A fractional step method arising from an operator split of the governing equations is used to solve the non-linear coupled system of equations, leading to a staggered product formula solution algorithm suitable for large scale computations. INTRODUCTION: The numerical simulation of foundry processes is one of the most interesting R+D topics for many industrial companies such as automotive and aeronautical, motivated by the need to spend less effort on experimental prototyping and to get high quality final products reducing manufacturing costs. The objective of this paper is to show the relevance of a model that takes into account the phase changes transformations in spheroidal graphite ferritic cast iron during its solidification.

Stabilization of incompressibility through orthogonal subgrid scales in finite strain plasticity

by Carlos Agelet de Saracibar, Miguel Cervera, and Quino Valverde

In this work a stabilized finite element method to deal with incompressibility in finite strain J... more In this work a stabilized finite element method to deal with incompressibility in finite strain J2 plasticity is presented. A mixed formulation involving pressure and displacement fields is considered, using a continuous linear interpolation for both fields. To overcome the Babuska-Brezzi condition a stabilization technique based on the orthogonal subgrid scale method is introduced.

On the orthogonal sub-grid scale pressure stabilization in finite deformation J2 plasticity

by Carlos Agelet de Saracibar, Miguel Cervera, and Quino Valverde

Use of stabilization methods is becoming an increasingly well-accepted technique due to their suc... more Use of stabilization methods is becoming an increasingly well-accepted technique due to their success in dealing with numerous numerical pathologies that arise in a variety of applications in computational mechanics. In this paper, a multi-scale finite element method technique to deal with pressure stabilization of incompressibility and nearly incompressibility problems in nonlinear solid mechanics, using low order finite elements, is presented. An Orthogonal Sub-Grid Scales (OSGS) method for both incompressible elasticity and J2-plasticity at finite deformations is proposed. Standard mixed finite element formulations, particularly those using low order interpolations, perform poorly or totally fail to perform for nearly incompressibility or incompressibility problems, producing results throughly polluted by spurious oscillations of the pressure. The goal here is to avoid this undesirable effect while retaining the use of low order elements. To achieve this goal we consistently derive, within the framework of the OSGS method, a modified variational mixed formulation of the original problem with enhanced stability properties. An assessment of the behavior of the formulation is presented. Results are compared with standard Galerkin and Q1P0 mixed large strain formulations.

Shear Band Localization and Isotropic Continuum J2 Damage Models

by Miguel Cervera and Carlos Agelet de Saracibar

This paper describes a novel formulation for the solution of problems involving shear band locali... more This paper describes a novel formulation for the solution of problems involving shear band localization using a local isotropic J2 continuum damage model and mixed linear simplex (triangles and tetrahedra). Stabilization methods are used to ensure existence and uniqueness of the solution, attaining global and local stability of the corresponding discrete finite element formulation. Consistent residual viscosity is used to enhance robustness and convergence of the formulation. Implementation and computational aspects are also discussed. A simple isotropic local J2 damage constitutive model is considered, either with linear or exponential softening. The softening modulus is regularized according to the material mode II fracture energy and the element size. Numerical examples show that the formulation derived is fully stable and remarkably robust, totally free of volumetric locking and spurious oscillations of the pressure. As a consequence, the results obtained do not suffer from spurious mesh-size or mesh-bias dependence, comparing very favourably with those obtained with the ill-posed standard approaches.

On the localization of shear bands using J2 damage models and multiscale stabilization techniques

by Carlos Agelet de Saracibar and Miguel Cervera

The aim of the paper is to present a robust and accurate formulation allowing the simulation of s... more The aim of the paper is to present a robust and accurate formulation allowing the simulation of shear bands localization using a local isotropic one-parameter J2 damage model and low-order mixed simplex (triangles and tetrahedral). Multi-Scale Stabilization techniques, Orthogonal Sub Grid Scales and Galerkin Least Square, are considered in order to ensure the existence and uniqueness of the solution, bypassing the Babuska-Brezzi condition and attaining global and local stability of the corresponding discrete finite element formulation. A simple isotropic one-parameter J2 damage model, either with linear or exponential softening, is considered. The softening modulus is regularized according to the material mode II fracture energy and the element size. Consistent residual viscosity is used to enhance robustness and convergence of the model. Computational and implementation aspects to obtain an efficient formulation are also discussed. Numerical examples show that the proposed formulation is fully stable, remarkably robust, totally free of undesirable effects such as volumetric locking and spurious oscillations of the pressure. Results obtained do not suffer from spurious mesh-size or mesh-bias dependence, comparing very favourably with those obtained using ill-posed standard approaches. INTRODUCTION: Finite element simulation of shear bands have been modeled using both weak and strong discontinuities approaches. Modeling of weak discontinuities using standard local approaches lead to unphysically and unacceptable solutions, fully determined by the fineness and orientation of the discretization. Up to now, this fact has been attributed to the strain softening behavior of the constitutive equation and different solutions, effective to some extend but introducing new difficulties not fully mastered, such as non-local, gradient-enhanced, micropolar and regularized constitutive models, have been proposed. Strong discontinuities approach is appealing but also poses new computational challenges, such as the need to use complex tracking algorithms (Oliver et al. [1999]). The aim of the paper is to show that difficulties found in localization problems are related to the fulfillment of the Babuska-Brezzi stability condition and are not related to the local definition of the constitutive equations. Multiscale finite element stabilization techniques, such as OSGS and GLS, are proposed within the framework of a local isotropic J2 damage model (Cervera et al. [2004a]).

A coupled thermo-mechanical multi-phase constitutive model

by Carlos Agelet de Saracibar and Miguel Cervera

This paper presents an up-to-date finite element numerical model for fully coupled thermo-mechani... more This paper presents an up-to-date finite element numerical model for fully coupled thermo-mechanical problems, focusing in the simulation of industrial solidification processes. The proposed constitutive equations have been identified to model the thermo-mechanical multi-phase behavior of cast iron from liquidus to room temperature. A fractional step method arising from an operator split of the governing equations is used to solve the non-linear coupled system of equations, leading to a staggered product formula solution algorithm suitable for large scale computations. INTRODUCTION: The numerical simulation of foundry processes is one of the most interesting R+D topics for many industrial companies such as automotive and aeronautical, motivated by the need to spend less effort on experimental prototyping and to get high quality final products reducing manufacturing costs. The objective of this paper is to show the relevance of a model that takes into account the phase changes transformations in spheroidal graphite ferritic cast iron during its solidification.

Advanced numerical methods in drying preservation process of foods

by Miguel Cervera and Carlos Agelet de Saracibar

The objective of this work is the development of a new tool for the numerical simulation of dryin... more The objective of this work is the development of a new tool for the numerical simulation of drying preservation processes of foods. The finite elements method is used to solve the algebraic set of equations that define the phenomena of heat and mass transfer involved in such a process. Typical characteristics of the food materials have been considered as: anisotropy, irregular geometry, phase change and non-linear thermal properties. The characterization of the thermal properties has been approximated considering its proximal composition. As a result of the numerical simulation it is possible to have information about the evolution of relevant parameters which control of the drying preservation process such as: temperature field, moisture content and the water activity. The tool is orientated to the food companies as a valorous support in the process design optimization and to guarantee both safety and quality of their products.

Estimation of thermal properties of food by means of inverse optimization analysis

by Miguel Cervera and Carlos Agelet de Saracibar

In this work the parameters that define the thermal properties of foods as a function the tempera... more In this work the parameters that define the thermal properties of foods as a function the temperature field are optimised using an inverse optimisation algorithm. Food preservation processes such as sterilization or freezing has been considered. The method of inverse optimisation consists of an iterative algorithm in which the experimental measurements are taken as target functions for the numerical simulation to achieve the best set of parameters that characterize the food properties. In this work the Differential Evolution algorithm, DE, has been assumed. The numerical solution of the heat transfer equations that define the food preservation processes are solved considering food characteristics such as: anisotropy, irregular geometry, change of phase and thermal properties depending on the temperature field. The algebraic format of these equations necessary for its numerical solution has been obtained applying the finite elements method. As a result of this work a new software (ProFood) has been developed by the authors and it is now available for its industrial use as an useful tool for the design of new products and the optimisation of industrial preservation processes.

Numerical tools for the simulation and optimisation of food preservation processes

by Miguel Cervera and Carlos Agelet de Saracibar

One of the major concerns of food industry consists of offering high quality and safe products to... more One of the major concerns of food industry consists of offering high quality and safe products to the consumers. Food spoilage is a gradual process occurring because of enzymatic or chemical reactions, improper temperature control or microbial growth, which results in undesirable changes in the color, flavor, odor or texture. Food safety and quality control ensures that the desirable characteristics of food are retained throughout all the manufacturing chain that includes the production, handling, processing, packaging, distribution and preparation stages. It is well known that the use of adequate preservation technique results in a longer shelf life of the product. The different nature of food products results in several responses to the preservation treatments. Moreover numerous processing conditions result in different final product characteristics. As a consequence the food industry is interested in a better knowledge of the process to improve the design and the optimal operation policies. The modern computer-aided process engineering tools can be used for these purposes, allowing a considerable reduction of cost and time if compared with the traditional experimental or trial-and-error methods. Food preservation through processing is an extremely broad area in food science and there are several possible technologies, we will focus here on the thermal preservation processing of foods. This work presents a new user-friendly software environment based on the use of the finite element approach for the simulation of the most common preservation processes such as sterilization, pasteurization and freezing. This new tool offers the possibility of taking into account the effects of difficult tasks such as temperature-dependent thermo-physical properties, anisotropic and non homogeneous foods and complex geometries. In the following, the capabilities of this numerical tool will be presented and an experimental validation will be also considered.

Numerical Simulation Techniques Oriented to the Optimization of Food Preservation Processes

by Miguel Cervera and Carlos Agelet de Saracibar

The complexity of the mathematical models able to describe food preservation processes together w... more The complexity of the mathematical models able to describe food preservation processes together with the difficulties arising from the definition of geometrical data and boundary conditions make attractive the use of computational tools in food product, process and equipment design. This work describes the potential of Computer-Aided-Engineering (CAE) for the food industry as a valuable tool to better understand, predict and finally improve food preservation processes. To achieve these objectives it is important to understand that a good mathematical model is not the only necessary requirement to describe a physical process on the computer. Material property database available during all the processing conditions, geometry definition and its discretization together with an user friendly pre and post-processing environment must be considered as additional issues to let the food industry benefit significantly from this technology. The parameters, which define the thermal properties of foods, are optimized using an inverse optimisation algorithm taking as target functions the experimental data available. Optimization techniques have been also applied to optimize the preservation process variables such as the environmental temperature and humidity fields. A new software environment based on the Finite Element Method (FEM) has been developed for the simulation of the most common food preservation techniques such as sterilization, pasteurization, and freezing, among others. This new computer-aided tool allows users to have a better understanding of the physical process by observing the relationship between its input and output parameters leading to an optimization of the preservation process design in terms of safety and quality of the final food product.

Thermo-Mechanical Contact in Casting Analysis

by Carlos Agelet de Saracibar and Miguel Cervera

Shaped Metal Deposition Processes

by Carlos Agelet de Saracibar and Miguel Cervera

Shaped Metal Deposition Processes C. Agelet de Saracibar, A. Lundbäck, M. Chiumenti, M. Cervera i... more Shaped Metal Deposition Processes
C. Agelet de Saracibar, A. Lundbäck, M. Chiumenti, M. Cervera
in R.B. Hetnarski (Ed.), Encyclopedia of Thermal Stresses
Springer Dordrecht, Heidelberg, New York, London, 2014, pp. 4346-4355
Chapter DOI: 10.1007/978-94-007-2739-7_808
ISBN: 978-94-007-2738-0 (Print) 978-94-007-2739-7 (e-Book)

Numerical simulation and visualization of material flow in friction stir welding via particle tracing

by Narges Dialami, Miguel Cervera, Carlos Agelet de Saracibar, and J. Ponthot

This work deals with the numerical simulation and material flow visualization of Friction Stir We... more This work deals with the numerical simulation and material flow visualization of Friction Stir Welding (FSW) processes. The 4-th order Runge-Kutta (RK4) integration method is used for the computation of particle trajectories. The particle tracing method is used to study the effect of input process parameters and pin shapes on the weld quality. The results show that the proposed method is suitable for the optimization of the FSW process. 2

Thermo-Mechanical Analysis of Industrial Solidification Processes

by Miguel Cervera and Carlos Agelet de Saracibar