The main goal of this study is to evaluate the influence of work-hardening modeling in springback... more The main goal of this study is to evaluate the influence of work-hardening modeling in springback prediction in the first phase of the Numisheet'05 ''Benchmark 3'': the U-shape ''Channel Draw''. Several work-hardening constitutive models are used in order to allow the different materials' mechanical behavior to be better described: the Swift law (a power law) or a Voce type saturation law to describe the classical isotropic work-hardening; a Lemaître and Chaboche type law to model the non-linear kinematic hardening, which can be combined with the previous two; and Teodosiu's microstructural work-hardening model. This analysis was carried out using two steels currently used in the automotive industry: mild (DC06) and dual phase (DP600).
This paper seeks to present a new approach to reverse analysis in depth-sensing indentation which... more This paper seeks to present a new approach to reverse analysis in depth-sensing indentation which makes use of numerical simulation. This methodology allows the results of experimental hardness tests acquired with single indenter geometry to be used to determine the plastic properties of materials. Forward and reverse analyses of high deformation three-dimensional numerical simulations of Vickers indentation tests are used to determine different mechanical properties of materials: Young's modulus, yield stress and strain-hardening exponent. The Vickers indenter used in the numerical simulations is formulated as a rigid body and takes into account the presence of the most common imperfection of the tip, so-called offset. The contact friction between the Vickers indenter and the deformable body is also considered. The forward analysis uses materials with Young's modulus values from 50 to 600 GPa, yield stress values from 0.3 to 10 GPa and strain-hardening exponents from 0 to 0.6; the Poisson ratio did not vary from 0.3. The representative plastic strain e r and the correspondent stress r r , as previously defined by other authors [Dao M, Chollacoop N, Vliet KJ, Venkatesh TA, Suresh S. Acta Mater 2001;49:3899], were identified by an independent numerical method. The values of the representative plastic strain e r obtained for the Vickers indenter confirm those of the above-mentioned authors, despite showing a slight influence from the Young's modulus values. The forward study enables the production of a unique plot of the hardness H IT vs. representative stress r r , where both are normalized by the Young's modulus E. The proposed reverse analysis provides a unique solution to the representative stress r r and the strain-hardening exponent, n, given that the Young's modulus is predetermined from the experimental hardness test. Depending on the material properties, the value of n can be more or less sensitive to the scatter of the experimental results obtained using the depth-sensing equipment, particularly the stiffness of the unloading curve. The validity of the proposed reverse analysis method is checked using three real materials: stamping quality steel (DC 06), stainless AISI 304 steel and BK7 glass.
In this paper we present a methodology for trimming 3D solid finite element meshes using a non-un... more In this paper we present a methodology for trimming 3D solid finite element meshes using a non-uniform rational B-spline (NURBS) surfaces representation. The algorithms applied in the identification of the spatial position of the finite element (FE) nodes and elements relative to the trimming surface are described, as well as, the explanation of the correction methods adopted to geometrically rearrange the trimmed elements. Three different strategies are proposed to adjust, with greater or lesser accuracy, the trimmed FE mesh to the trimming surface. The first consists only of an element elimination strategy, while the other two are based on a nodal stretching strategy, aiming for more accurate adjustment of the finite elements boundary to the trimming surface. Finally, to highlight the capabilities of the developed program, a mesh generation example is given by trimming a mesh with the shape of a blank automotive panel.
This work aims to analyse the influence of temperature, strain-rate and aging time in the mechani... more This work aims to analyse the influence of temperature, strain-rate and aging time in the mechanical behaviour of an EN AW 6016-T4 alloy, naturally aged. This Aluminium Alloy was select since it is commonly used in the automotive industry. Uniaxial tensile tests were performed within a temperature range between room temperature and 300oC, for different strain rates and at different times after its production. The results collected constitute an experimental database for evaluating the potential application of warm forming processes to this alloy Mots clefs: Al-Mg-Si Alloys; Mechanical Characterization; Temperature; Strain rate; Aging
Computer Methods in Applied Mechanics and Engineering, 2016
Highlights • A new 3D contact surface smoothing approach for large deformation contact problems b... more Highlights • A new 3D contact surface smoothing approach for large deformation contact problems between deformable bodies is proposed. • The local Nagata patch interpolation is used to smooth arbitrary surface meshes. • The original curvature of the master surface is recovered using a relatively coarse mesh. • The non-physical contact force oscillations usual in the faceted surface representation are eliminated. • The accuracy, robustness and performance of the numerical simulations is improved adopting the surface smoothing method.
International Journal of Mechanical Sciences, 2017
The high-strength steel sheets currently used in the automotive industry are prone to non-traditi... more The high-strength steel sheets currently used in the automotive industry are prone to non-traditional behaviour during forming, being wrinkling and springback two of the most challenging geometrical predictions for numerical simulation. Thus, the finite element method requires accurate and reliable numerical models. This study presents the experimental and numerical analysis of a rail component with high tendency to develop wrinkling and 2D springback. Two different materials are used for the sheet blank, namely a mild steel (DC06) and a dual phase steel (DP600). The frictional behaviour between each metallic sheet and the forming tools is evaluated through the flat-die test, allowing the determination of a friction coefficient as a function of the normal pressure. The influence of the applied boundary conditions on the numerical results is evaluated by means of two distinct numerical models (full blank geometry and 1/4 of the blank with symmetry conditions). The results show that the wrinkling behaviour is strongly affected by the blank's material, as well as by the symmetry conditions defined in the numerical model. In fact, considering the full model of the blank, the numerical results are in better agreement with the experimental ones. However, the computational cost of the numerical simulation considering the full blank is substantially higher than using 1/4 of the blank.
Journal of Physics: Conference Series, Jul 1, 2018
Both the plastic deformation and the friction forces generate heat in sheet metal forming process... more Both the plastic deformation and the friction forces generate heat in sheet metal forming processes. This yields a temperature rise on the blank and tools, which is mainly dictated by the blank properties and the press speed. Although modest values of temperature rise yield negligible changes in the mechanical behavior of metals, the lubricant properties are rather influenced by the temperature. Thus, the deep drawing of an aluminum cylindrical cup was selected to evaluate, experimentally and numerically, the temperature variation of the forming tools and blank at room temperature. Different values of punch speed are compared to evaluate the impact of the heat losses to the environment. The experimental results show that the temperature rise on the die surface is higher than 9ºC when the punch speed is 10 mm/s. The temperature is underestimated by the finite element model due to the isothermal conditions imposed on the forming tools.
International Journal of Mechanical Sciences, 2004
In order to study the influence of the heat-affected zone softening on the fracture behaviour of ... more In order to study the influence of the heat-affected zone softening on the fracture behaviour of welds with cracks in the weld metal centre line, a large variety of weld geometries and undermatch conditions of the heat affected zone mechanical properties, relative to the weld metal and base material, were addressed in this study. With this aim, the opening stress distribution in notched welded specimens was analysed using the numerical simulation of the three-point bending test. The numerical results show a reduction in the stress levels ahead of the crack tip for welded specimens with severe heat-affected zone undermatch. The stress distribution is strongly influenced by the crack position relative to the weld material/heat-affected zone interface, independently of heat-affected zone width.
International Journal of Solids and Structures, 2016
The forming tools are commonly assumed as rigid in the finite element simulation of sheet metal f... more The forming tools are commonly assumed as rigid in the finite element simulation of sheet metal forming processes. This assumption allows to simplify the numerical model and, subsequently, reduce the required computational cost. Nevertheless, the elastic deformation of the tools can influence considerably the material flow, specifically the distribution of the blank-holder pressure over the flange area. This study presents the finite element analysis of the reverse deep drawing of a cylindrical cup, where the forming tools are modelled either as rigid or as deformable bodies. Additionally, the numerical results are compared with the experimental ones, in order to assess the accuracy of the proposed finite element model. Considering the elastic deformation of the tools, the numerical results are in better agreement with the experimental measurements, namely the cup wall thickness distribution. On the other hand, the computational time of the simulation increases significantly in comparison with the classical approach (rigid tools).
The Swift test was originally proposed as a formability test to reproduce the conditions observed... more The Swift test was originally proposed as a formability test to reproduce the conditions observed in deep drawing operations. This test consists on forming a cylindrical cup from a circular blank, using a flat bottom cylindrical punch and has been extensively studied using both analytical and numerical methods. This test can also be combined with the Demeri test, which consists in cutting a ring from the wall of a cylindrical cup, in order to open it afterwards to measure the springback. This combination allows their use as benchmark test, in order to improve the knowledge concerning the numerical simulation models, through the comparison between experimental and numerical results. The focus of this study is the experimental and numerical analyses of the Swift cup test, followed by the Demeri test, performed with an AA5754-O alloy at room temperature. In this context, a detailed analysis of the punch force evolution, the thickness evolution along the cup wall, the earing profile, th...
Influence of the characteristics of the 3D FE mesh on the evolution of variables used to characte... more Influence of the characteristics of the 3D FE mesh on the evolution of variables used to characterize the stress state
This study presents a new staggered coupled strategy to deal with thermomechanical problems. The ... more This study presents a new staggered coupled strategy to deal with thermomechanical problems. The proposed strategy is based on the isothermal split methodology, i.e. the mechanical problem is solved at constant temperature and the thermal problem is solved for a fixed configuration. Nevertheless, the procedure for this strategy is divided into two phases within each increment: the prediction and the correction phases, while the interchange of information is performed on both. This allows taking advantage of automatic time-step control techniques, previously implemented for the mechanical problem, which is the main feature that distinguishes it from the classical strategies. The aim of the proposed strategy is to reduce the computational cost without compromising the accuracy of the results. The new coupling strategy is validated using three numerical examples, comparing its accuracy and performance with the ones obtained with the classical (commonly employed) strategies for solving thermomechanical problems. Moreover, the influence of the time-step size on the accuracy is analysed. The results indicate that the proposed strategy presents accuracy close to the one obtained with the implicit coupling algorithm, while the computational cost is only slightly higher than the one required by the explicit strategy.
The Swift test was originally proposed as a formability test to reproduce the conditions observed... more The Swift test was originally proposed as a formability test to reproduce the conditions observed in deep drawing operations. This test consists on forming a cylindrical cup from a circular blank, using a flat bottom cylindrical punch and has been extensively studied using both analytical and numerical methods. This test can also be combined with the Demeri test, which consists in cutting a ring from the wall of a cylindrical cup, in order to open it afterwards to measure the springback. This combination allows their use as benchmark test, in order to improve the knowledge concerning the numerical simulation models, through the comparison between experimental and numerical results. The focus of this study is the experimental and numerical analyses of the Swift cup test, followed by the Demeri test, performed with an AA5754-O alloy at room temperature. In this context, a detailed analysis of the punch force evolution, the thickness evolution along the cup wall, the earing profile, the strain paths and their evolution and the ring opening is performed. The numerical simulation is performed using the finite element code ABAQUS, with solid and solid-shell elements, in order to compare the computational efficiency of these type of elements. The results show that the solid-shell
Nowaday s the use of the finite element approach is indispensable to save time and effort in the ... more Nowaday s the use of the finite element approach is indispensable to save time and effort in the production and process design of deep drawn parts. The advances of commercial software developers and universities research teams have been focused in topics that can be divided into numerical and technological. The numerical simulation results are influenced by several numerical parameters and
The International Journal of Advanced Manufacturing Technology
Heat flow across the interface of solid bodies in contact is an important aspect in several engin... more Heat flow across the interface of solid bodies in contact is an important aspect in several engineering applications. This work presents a finite element model for the analysis of thermal contact, which takes into account the effect of contact pressure and gap dimension in the heat flow across the interface between two bodies. Additionally, the frictional heat generation is also addressed, which is dictated by the contact forces predicted by the mechanical problem. The frictional contact problem and thermal problem are formulated in the frame of the finite element method. A new law is proposed to define the interfacial heat transfer coefficient (IHTC) as a function of the contact pressure and gap distance, enabling a smooth transition between two contact status (gap and contact). The staggered scheme used as coupling strategy to solve the thermomechanical problem is briefly presented. Four numerical examples are D. M. Neto
III European Conference on Computational Mechanics, 2006
In the simulation of the deep drawing process one of the challenges is the correct prediction of ... more In the simulation of the deep drawing process one of the challenges is the correct prediction of the actual contact surface and kind of contact established between the tools and the blank sheet, since they determine the boundary conditions. In the numerical simulation of the deep drawing process the contact conditions change continuously, increasing the importance of a correct evaluation of these parameters in each load step. In the finite element implicit code DD3IMP , devoted to the simulation of the deep drawing process, the tools are modeled with Bézier parametric surfaces, for which a frictional contact algorithm has been developed and continuously improved . The description of the tools by parametric surfaces allows the direct use of the information provided by CAD software. However, in order to guarantee the contact algorithm convergence it is necessary to impose some continuity conditions between the surfaces that define the forming tools. In many situations the tools are defined by a set of plane surfaces connected by fillings with surfaces of constant or variable radius. To guarantee the correct tools' modeling it is necessary to correctly model the curve that defines the radius. Previous works had shown that, to assure a correct correlation between tools' geometry and Bézier description at least cubic segments should be used to define the radius . The Bézier tools description must accurately represent the tools' geometry, but also satisfy the continuity conditions that guarantee the contact algorithm convergence. In this work the influence of the degree of the polynomial functions used in the Bézier surfaces on the contact algorithm is evaluated. Also the error in the 0 C and 1 C continuity of the Bézier surfaces obtained with the CAD system is estimated, and its influence on the convergence of the contact algorithm is studied.
This study deals with the description of the anisotropic behaviour of the mild steel sheet used i... more This study deals with the description of the anisotropic behaviour of the mild steel sheet used in the reverse deep drawing process of a cylindrical cup, which was proposed as benchmark at the Numisheet'99 conference. The effect of the yield criterion on the numerical results is analysed using three yield functions, von Mises, Hill'48 and Barlat Yld'91, combined with the Swift hardening law. The anisotropy parameters of the Hill'48 model are identified using either the yield stresses or r-values, obtained from the uniaxial tensile test at three different directions. On the other hand, the anisotropy parameters of the Yld'91 are determined taking into account both the yield stresses and r-values, minimizing an objective function. The comparison between experimental and numerical results is presented, being the punch force evolution and the thickness distribution along the cup wall the principal variables under study. In both forming stages, the predicted punch force evolution is close to the experimental one, whatever the yield criterion adopted. Nevertheless, the cup wall thickness distribution is strongly influenced by the yield criteria, being clearly overestimated by the von Mises yield criterion. On the other hand, the Yld'91 yield criterion provides a thickness distribution closer to the experimental one, for both forming stages. The strain paths during both forming stages ranges from uniaxial compression, when the material flows between the die and blank-holder, to plane strain in the cup wall, whereas the important strain path changes occurs in the die radius.
It is well known that residual stresses strongly influence the behaviour of most materials and, i... more It is well known that residual stresses strongly influence the behaviour of most materials and, in particular, of composite materials. This chapter presents one approach to the numerical determination of thermal residual stresses in metal matrix composites (MMC). The subject of residual stresses is introduced and the corresponding mathematical and constitutive models are described in detail. It is considered that the reinforcement material is elastic and that the metallic matrix may exhibit thermoelastic-viscoplastic behaviour. A progressive gradient based time-integration algorithm is described that leads to the implementation of the proposed constitutive models in a finite element analysis code. The corresponding variational formulation and discretisation into finite elements is also described. In order to guarantee stabilised convergence and to increase the precision of results, the authors also propose a time-step optimisation algorithm. All the formalisms are tested measuring the influence of the reinforcement volume fraction and cooling rate on the resulting residual stresses.
In spite of the abundant number of experimental friction tests that have been reported, the conta... more In spite of the abundant number of experimental friction tests that have been reported, the contact with friction modeling persists to be one of the factors that determine the effectiveness of sheet metal forming simulation. This difficulty can be understood due to the nature of the friction phenomena, which comprises the interaction of different factors connected to both sheet and
The main goal of this study is to evaluate the influence of work-hardening modeling in springback... more The main goal of this study is to evaluate the influence of work-hardening modeling in springback prediction in the first phase of the Numisheet'05 ''Benchmark 3'': the U-shape ''Channel Draw''. Several work-hardening constitutive models are used in order to allow the different materials' mechanical behavior to be better described: the Swift law (a power law) or a Voce type saturation law to describe the classical isotropic work-hardening; a Lemaître and Chaboche type law to model the non-linear kinematic hardening, which can be combined with the previous two; and Teodosiu's microstructural work-hardening model. This analysis was carried out using two steels currently used in the automotive industry: mild (DC06) and dual phase (DP600).
This paper seeks to present a new approach to reverse analysis in depth-sensing indentation which... more This paper seeks to present a new approach to reverse analysis in depth-sensing indentation which makes use of numerical simulation. This methodology allows the results of experimental hardness tests acquired with single indenter geometry to be used to determine the plastic properties of materials. Forward and reverse analyses of high deformation three-dimensional numerical simulations of Vickers indentation tests are used to determine different mechanical properties of materials: Young's modulus, yield stress and strain-hardening exponent. The Vickers indenter used in the numerical simulations is formulated as a rigid body and takes into account the presence of the most common imperfection of the tip, so-called offset. The contact friction between the Vickers indenter and the deformable body is also considered. The forward analysis uses materials with Young's modulus values from 50 to 600 GPa, yield stress values from 0.3 to 10 GPa and strain-hardening exponents from 0 to 0.6; the Poisson ratio did not vary from 0.3. The representative plastic strain e r and the correspondent stress r r , as previously defined by other authors [Dao M, Chollacoop N, Vliet KJ, Venkatesh TA, Suresh S. Acta Mater 2001;49:3899], were identified by an independent numerical method. The values of the representative plastic strain e r obtained for the Vickers indenter confirm those of the above-mentioned authors, despite showing a slight influence from the Young's modulus values. The forward study enables the production of a unique plot of the hardness H IT vs. representative stress r r , where both are normalized by the Young's modulus E. The proposed reverse analysis provides a unique solution to the representative stress r r and the strain-hardening exponent, n, given that the Young's modulus is predetermined from the experimental hardness test. Depending on the material properties, the value of n can be more or less sensitive to the scatter of the experimental results obtained using the depth-sensing equipment, particularly the stiffness of the unloading curve. The validity of the proposed reverse analysis method is checked using three real materials: stamping quality steel (DC 06), stainless AISI 304 steel and BK7 glass.
In this paper we present a methodology for trimming 3D solid finite element meshes using a non-un... more In this paper we present a methodology for trimming 3D solid finite element meshes using a non-uniform rational B-spline (NURBS) surfaces representation. The algorithms applied in the identification of the spatial position of the finite element (FE) nodes and elements relative to the trimming surface are described, as well as, the explanation of the correction methods adopted to geometrically rearrange the trimmed elements. Three different strategies are proposed to adjust, with greater or lesser accuracy, the trimmed FE mesh to the trimming surface. The first consists only of an element elimination strategy, while the other two are based on a nodal stretching strategy, aiming for more accurate adjustment of the finite elements boundary to the trimming surface. Finally, to highlight the capabilities of the developed program, a mesh generation example is given by trimming a mesh with the shape of a blank automotive panel.
This work aims to analyse the influence of temperature, strain-rate and aging time in the mechani... more This work aims to analyse the influence of temperature, strain-rate and aging time in the mechanical behaviour of an EN AW 6016-T4 alloy, naturally aged. This Aluminium Alloy was select since it is commonly used in the automotive industry. Uniaxial tensile tests were performed within a temperature range between room temperature and 300oC, for different strain rates and at different times after its production. The results collected constitute an experimental database for evaluating the potential application of warm forming processes to this alloy Mots clefs: Al-Mg-Si Alloys; Mechanical Characterization; Temperature; Strain rate; Aging
Computer Methods in Applied Mechanics and Engineering, 2016
Highlights • A new 3D contact surface smoothing approach for large deformation contact problems b... more Highlights • A new 3D contact surface smoothing approach for large deformation contact problems between deformable bodies is proposed. • The local Nagata patch interpolation is used to smooth arbitrary surface meshes. • The original curvature of the master surface is recovered using a relatively coarse mesh. • The non-physical contact force oscillations usual in the faceted surface representation are eliminated. • The accuracy, robustness and performance of the numerical simulations is improved adopting the surface smoothing method.
International Journal of Mechanical Sciences, 2017
The high-strength steel sheets currently used in the automotive industry are prone to non-traditi... more The high-strength steel sheets currently used in the automotive industry are prone to non-traditional behaviour during forming, being wrinkling and springback two of the most challenging geometrical predictions for numerical simulation. Thus, the finite element method requires accurate and reliable numerical models. This study presents the experimental and numerical analysis of a rail component with high tendency to develop wrinkling and 2D springback. Two different materials are used for the sheet blank, namely a mild steel (DC06) and a dual phase steel (DP600). The frictional behaviour between each metallic sheet and the forming tools is evaluated through the flat-die test, allowing the determination of a friction coefficient as a function of the normal pressure. The influence of the applied boundary conditions on the numerical results is evaluated by means of two distinct numerical models (full blank geometry and 1/4 of the blank with symmetry conditions). The results show that the wrinkling behaviour is strongly affected by the blank's material, as well as by the symmetry conditions defined in the numerical model. In fact, considering the full model of the blank, the numerical results are in better agreement with the experimental ones. However, the computational cost of the numerical simulation considering the full blank is substantially higher than using 1/4 of the blank.
Journal of Physics: Conference Series, Jul 1, 2018
Both the plastic deformation and the friction forces generate heat in sheet metal forming process... more Both the plastic deformation and the friction forces generate heat in sheet metal forming processes. This yields a temperature rise on the blank and tools, which is mainly dictated by the blank properties and the press speed. Although modest values of temperature rise yield negligible changes in the mechanical behavior of metals, the lubricant properties are rather influenced by the temperature. Thus, the deep drawing of an aluminum cylindrical cup was selected to evaluate, experimentally and numerically, the temperature variation of the forming tools and blank at room temperature. Different values of punch speed are compared to evaluate the impact of the heat losses to the environment. The experimental results show that the temperature rise on the die surface is higher than 9ºC when the punch speed is 10 mm/s. The temperature is underestimated by the finite element model due to the isothermal conditions imposed on the forming tools.
International Journal of Mechanical Sciences, 2004
In order to study the influence of the heat-affected zone softening on the fracture behaviour of ... more In order to study the influence of the heat-affected zone softening on the fracture behaviour of welds with cracks in the weld metal centre line, a large variety of weld geometries and undermatch conditions of the heat affected zone mechanical properties, relative to the weld metal and base material, were addressed in this study. With this aim, the opening stress distribution in notched welded specimens was analysed using the numerical simulation of the three-point bending test. The numerical results show a reduction in the stress levels ahead of the crack tip for welded specimens with severe heat-affected zone undermatch. The stress distribution is strongly influenced by the crack position relative to the weld material/heat-affected zone interface, independently of heat-affected zone width.
International Journal of Solids and Structures, 2016
The forming tools are commonly assumed as rigid in the finite element simulation of sheet metal f... more The forming tools are commonly assumed as rigid in the finite element simulation of sheet metal forming processes. This assumption allows to simplify the numerical model and, subsequently, reduce the required computational cost. Nevertheless, the elastic deformation of the tools can influence considerably the material flow, specifically the distribution of the blank-holder pressure over the flange area. This study presents the finite element analysis of the reverse deep drawing of a cylindrical cup, where the forming tools are modelled either as rigid or as deformable bodies. Additionally, the numerical results are compared with the experimental ones, in order to assess the accuracy of the proposed finite element model. Considering the elastic deformation of the tools, the numerical results are in better agreement with the experimental measurements, namely the cup wall thickness distribution. On the other hand, the computational time of the simulation increases significantly in comparison with the classical approach (rigid tools).
The Swift test was originally proposed as a formability test to reproduce the conditions observed... more The Swift test was originally proposed as a formability test to reproduce the conditions observed in deep drawing operations. This test consists on forming a cylindrical cup from a circular blank, using a flat bottom cylindrical punch and has been extensively studied using both analytical and numerical methods. This test can also be combined with the Demeri test, which consists in cutting a ring from the wall of a cylindrical cup, in order to open it afterwards to measure the springback. This combination allows their use as benchmark test, in order to improve the knowledge concerning the numerical simulation models, through the comparison between experimental and numerical results. The focus of this study is the experimental and numerical analyses of the Swift cup test, followed by the Demeri test, performed with an AA5754-O alloy at room temperature. In this context, a detailed analysis of the punch force evolution, the thickness evolution along the cup wall, the earing profile, th...
Influence of the characteristics of the 3D FE mesh on the evolution of variables used to characte... more Influence of the characteristics of the 3D FE mesh on the evolution of variables used to characterize the stress state
This study presents a new staggered coupled strategy to deal with thermomechanical problems. The ... more This study presents a new staggered coupled strategy to deal with thermomechanical problems. The proposed strategy is based on the isothermal split methodology, i.e. the mechanical problem is solved at constant temperature and the thermal problem is solved for a fixed configuration. Nevertheless, the procedure for this strategy is divided into two phases within each increment: the prediction and the correction phases, while the interchange of information is performed on both. This allows taking advantage of automatic time-step control techniques, previously implemented for the mechanical problem, which is the main feature that distinguishes it from the classical strategies. The aim of the proposed strategy is to reduce the computational cost without compromising the accuracy of the results. The new coupling strategy is validated using three numerical examples, comparing its accuracy and performance with the ones obtained with the classical (commonly employed) strategies for solving thermomechanical problems. Moreover, the influence of the time-step size on the accuracy is analysed. The results indicate that the proposed strategy presents accuracy close to the one obtained with the implicit coupling algorithm, while the computational cost is only slightly higher than the one required by the explicit strategy.
The Swift test was originally proposed as a formability test to reproduce the conditions observed... more The Swift test was originally proposed as a formability test to reproduce the conditions observed in deep drawing operations. This test consists on forming a cylindrical cup from a circular blank, using a flat bottom cylindrical punch and has been extensively studied using both analytical and numerical methods. This test can also be combined with the Demeri test, which consists in cutting a ring from the wall of a cylindrical cup, in order to open it afterwards to measure the springback. This combination allows their use as benchmark test, in order to improve the knowledge concerning the numerical simulation models, through the comparison between experimental and numerical results. The focus of this study is the experimental and numerical analyses of the Swift cup test, followed by the Demeri test, performed with an AA5754-O alloy at room temperature. In this context, a detailed analysis of the punch force evolution, the thickness evolution along the cup wall, the earing profile, the strain paths and their evolution and the ring opening is performed. The numerical simulation is performed using the finite element code ABAQUS, with solid and solid-shell elements, in order to compare the computational efficiency of these type of elements. The results show that the solid-shell
Nowaday s the use of the finite element approach is indispensable to save time and effort in the ... more Nowaday s the use of the finite element approach is indispensable to save time and effort in the production and process design of deep drawn parts. The advances of commercial software developers and universities research teams have been focused in topics that can be divided into numerical and technological. The numerical simulation results are influenced by several numerical parameters and
The International Journal of Advanced Manufacturing Technology
Heat flow across the interface of solid bodies in contact is an important aspect in several engin... more Heat flow across the interface of solid bodies in contact is an important aspect in several engineering applications. This work presents a finite element model for the analysis of thermal contact, which takes into account the effect of contact pressure and gap dimension in the heat flow across the interface between two bodies. Additionally, the frictional heat generation is also addressed, which is dictated by the contact forces predicted by the mechanical problem. The frictional contact problem and thermal problem are formulated in the frame of the finite element method. A new law is proposed to define the interfacial heat transfer coefficient (IHTC) as a function of the contact pressure and gap distance, enabling a smooth transition between two contact status (gap and contact). The staggered scheme used as coupling strategy to solve the thermomechanical problem is briefly presented. Four numerical examples are D. M. Neto
III European Conference on Computational Mechanics, 2006
In the simulation of the deep drawing process one of the challenges is the correct prediction of ... more In the simulation of the deep drawing process one of the challenges is the correct prediction of the actual contact surface and kind of contact established between the tools and the blank sheet, since they determine the boundary conditions. In the numerical simulation of the deep drawing process the contact conditions change continuously, increasing the importance of a correct evaluation of these parameters in each load step. In the finite element implicit code DD3IMP , devoted to the simulation of the deep drawing process, the tools are modeled with Bézier parametric surfaces, for which a frictional contact algorithm has been developed and continuously improved . The description of the tools by parametric surfaces allows the direct use of the information provided by CAD software. However, in order to guarantee the contact algorithm convergence it is necessary to impose some continuity conditions between the surfaces that define the forming tools. In many situations the tools are defined by a set of plane surfaces connected by fillings with surfaces of constant or variable radius. To guarantee the correct tools' modeling it is necessary to correctly model the curve that defines the radius. Previous works had shown that, to assure a correct correlation between tools' geometry and Bézier description at least cubic segments should be used to define the radius . The Bézier tools description must accurately represent the tools' geometry, but also satisfy the continuity conditions that guarantee the contact algorithm convergence. In this work the influence of the degree of the polynomial functions used in the Bézier surfaces on the contact algorithm is evaluated. Also the error in the 0 C and 1 C continuity of the Bézier surfaces obtained with the CAD system is estimated, and its influence on the convergence of the contact algorithm is studied.
This study deals with the description of the anisotropic behaviour of the mild steel sheet used i... more This study deals with the description of the anisotropic behaviour of the mild steel sheet used in the reverse deep drawing process of a cylindrical cup, which was proposed as benchmark at the Numisheet'99 conference. The effect of the yield criterion on the numerical results is analysed using three yield functions, von Mises, Hill'48 and Barlat Yld'91, combined with the Swift hardening law. The anisotropy parameters of the Hill'48 model are identified using either the yield stresses or r-values, obtained from the uniaxial tensile test at three different directions. On the other hand, the anisotropy parameters of the Yld'91 are determined taking into account both the yield stresses and r-values, minimizing an objective function. The comparison between experimental and numerical results is presented, being the punch force evolution and the thickness distribution along the cup wall the principal variables under study. In both forming stages, the predicted punch force evolution is close to the experimental one, whatever the yield criterion adopted. Nevertheless, the cup wall thickness distribution is strongly influenced by the yield criteria, being clearly overestimated by the von Mises yield criterion. On the other hand, the Yld'91 yield criterion provides a thickness distribution closer to the experimental one, for both forming stages. The strain paths during both forming stages ranges from uniaxial compression, when the material flows between the die and blank-holder, to plane strain in the cup wall, whereas the important strain path changes occurs in the die radius.
It is well known that residual stresses strongly influence the behaviour of most materials and, i... more It is well known that residual stresses strongly influence the behaviour of most materials and, in particular, of composite materials. This chapter presents one approach to the numerical determination of thermal residual stresses in metal matrix composites (MMC). The subject of residual stresses is introduced and the corresponding mathematical and constitutive models are described in detail. It is considered that the reinforcement material is elastic and that the metallic matrix may exhibit thermoelastic-viscoplastic behaviour. A progressive gradient based time-integration algorithm is described that leads to the implementation of the proposed constitutive models in a finite element analysis code. The corresponding variational formulation and discretisation into finite elements is also described. In order to guarantee stabilised convergence and to increase the precision of results, the authors also propose a time-step optimisation algorithm. All the formalisms are tested measuring the influence of the reinforcement volume fraction and cooling rate on the resulting residual stresses.
In spite of the abundant number of experimental friction tests that have been reported, the conta... more In spite of the abundant number of experimental friction tests that have been reported, the contact with friction modeling persists to be one of the factors that determine the effectiveness of sheet metal forming simulation. This difficulty can be understood due to the nature of the friction phenomena, which comprises the interaction of different factors connected to both sheet and
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