Abel D. Santos

The use of finite element simulation for optimization of metal forming and tool design

Abstract When producing a new component there is usually a trial and error stage to tune the process in order to obtain a part without defects, using the right quantity of raw material. At this stage, the previous experience of designers and manufacturers should give an important aid to reduce trials. However, the use of new materials associated with new shape designs creates the possibility of new behaviours. For this reason, while producing a component it is very important that we get some directions in order to avoid possible defects in it. For such an objective, the finite element simulation has been proving that, at the design stage, it can give important answers in analysing the process and predicting the defects that may occur. Therefore, modifications can be made easily, before tool manufacturing and part production, reducing the trial and error stage and its associated costs. This paper presents the modelling of components with complex shape to be manufactured by closed die forging and sheet metal forming and one tries to show how numerical simulation may help in defining the shape and size of initial material or blank and predict the forces needed to define the press to be used in the process, as well as the possible defects.

Inverse methods in design of industrial forging processes

Abstract An approach to optimal shape design in forging is presented. The design problem is formulated as an inverse problem incorporating a finite element 3D analysis model and an optimisation technique conducted on the basis of design sensitivities. The mechanical analysis provides information to predict deformation, stresses and strains necessary to the shape optimisation problem. The objective is to minimise a function describing die underfill and excessive material waste. Analytical sensitivities of the objective function are required and the calculation of discrete derivatives based on the differentiation of the discrete problem equations is considered. Due to the time-dependent deformation process the direct differentiation method has been implemented. The capability of the proposed inverse approach to deal with optimal forging of industrial parts is demonstrated.

On the Determination of Flow Stress Using Bulge Test and Mechanical Measurement

The standard uniaxial tensile test is a widely accepted method to obtain relevant properties of sheet metal materials. These fundamental parameters can be used in numerical modeling of sheet forming operations to predict and assess formability and failure analysis. However the range of strain obtained from tensile test is limited and therefore if one will need further information on material behavior, extrapolation of tensile data is performed. The bulge test is an alternative to obtain ranges of deformation higher than tensile test, thus being possible to obtain non-extrapolated data for material behavior. Several methods may be used to obtain stress-strain data from bulge test, but a common concept is behind them, which needs the measurement of bulge pressure, curvature of bulge specimen, its thickness at the pole and the application of membrane theory. Concerning such measurements, optical methods are being used recently but classical mechanical methods are still an alternative with its own strengths. This paper presents the use and development of a mechanical measuring system to be incorporated in a hydraulic bulge test for flow curve determination, which permits real-time data acquisition under controlled strain rates up to high levels of plastic deformation. Numerical simulations of bulge test using FEM are performed and a sensitivity analysis is done for some influencing variables used in measurements, thus giving some directions in the design and use of the experimental mechanical system. Also, first experimental results are presented, showing an efficient testing procedure method for real time data acquisition with a stable evaluation of the flow curve.

Study of Tool Trajectory in Incremental Forming

Single point incremental forming (SPIF) is an innovative flexible sheet metal forming process which can be used to produce complex shapes from various materials. Due to its flexibility, it attracts a more and more attention in the recent decades. Several studies show that besides the major operating parameters, namely feed rate, tool radius, and forming speed etc., tool path is also an important processing parameter to affect the final forming component. In view of that, the present paper studies the influence of tool paths on the work piece quality by the finite element method coupled with the Continuum Damage Mechanics (CDM) model. The formability of incremental forming in different tool paths is also analyzed.

Towards standard benchmarks and reference data for validation and improvement of numerical simulation in sheet metal forming

Abstract The last decade has witnessed many advances and a lot of improvement in FE codes for simulation of sheet metal forming processes. Such advances could be followed mainly by benchmarks proposed in Numisheet conferences. It was possible to notice that the scatter of results among numerical codes has decreased so significantly that recently scattering of experimental results among different corporations was evident. However in order to pursue further developments and validate numerical results it is fundamental to have reliable reference experimental data. This is one of the objectives of a current IMS project called 3DS-Digital Die Design System. In this paper such objectives are presented as well as some of the proposed benchmarks. It is intended to show part of the developed work concerning tool design and manufacturing methodology. Also an experimental case study about the use of piercing holes in parts and the use of counter-punch is presented. Finally some simulation results are also shown concerning one of the proposed benchmarks.

Determination of Flow Curve Using Bulge Test and Calibration of Damage for Ito-Goya Models

The standard uniaxial tensile test is the widely accepted method to obtain relevant properties of mechanical characterization of sheet metal materials. However the range of strain obtained from tensile test is limited. The bulge test is an alternative to obtain ranges of deformation, higher than tensile test, thus permitting a better characterization for material behaviour. This paper presents a sensitivity analysis for some influencing variables used in bulge measurements, thus giving some guidelines for the evaluation of the stress-strain curve from experimental results using a developed experimental mechanical system. Additionally, using bulge test up to fracture shall give material information regarding damage, which in turn may be used to evaluate and calibrate damage models. A methodology is presented to be used for evaluation and calibration of Ito-Goya damage model of damage prediction.

Numerical Modelling and Experimental Study of Sandwich Shells with Metal Foam Cores

The trends in automobile industry always include the use of new materials such as those needed for the passive safety of vehicles and they are one of the most important strategies to reduce injury of passengers during traffic accidents. Associated with the development of security systems, there is the possibility of improving efficiency by the introduction of materials that lead to weight reduction, having a direct impact on fuel consumption and lower carbon emissions. The present work aims to study the behaviour of sandwich structures, composed by a foam core with two outer layers of metal sheet (all structure being aluminium). The study of the composite structure behaviour, its mechanical characterization and numerical modelling is essential to analyse the mechanical performance of structures based on this type of materials. This step is fundamental in preliminary design, since the different materials of the composite structure show different mechanical responses. The differences in mechanical behaviour are demonstrated by the axisymmetric compressive stress states tests and also by the influence of hydrostatic pressure in the yield of the aluminium foam porous material, while the yield of the homogeneous solid material (aluminium sheet) can be considered as pressure insensitive. In order to correctly characterize separately these two materials of the composite (outer layers and core), a set of tests were performed. The characterization of the aluminium sheet was performed in a series of tensile tests, using three different rolling directions. For the metal foam core characterization a series of uniaxial compression tests were performed. The experimentally obtained results were applied in the development of numerical models for this kind of sandwich structure. The models include elastoplastic constitutive relation, where a distinct plastic domain for different materials is accounted for, as well as, the influence of hydrostatic pressure in the yield of the porous material. Also, the validation of the elastoplastic models is performed by comparing results obtained by numerical simulations with those obtained experimentally.

Analysis of Sandwich Shells with Metallic Foam Cores based on the Uniaxial Tensile Test

On this work, the authors present the development and evaluation of an innovative system able to perform reliable panels of sandwich sheets with metallic foam cores for industrial applications, especially in automotive and aeronautical industries. This work is divided into two parts; in the first part the mathematical model used to describe the behavior of sandwich shells with metal cores form is presented and some numerical examples are presented. In the second part of this work, the numerical results are validated using the experimental results obtained from the mechanical experiments. Using the isotropic hardening crushable foam constitutive model, available on ABAQUS, a set of different mechanical tests were simulated. The isotropic hardening model available uses a yield surface that is an ellipse centered at the origin in the p‐q stress plane. Using this constitutive model, the uniaxial tensile test for this material was simulated, and a comparison with the experimental results was made.

Aluminum foam sandwich with adhesive bonding: Computational modeling

ABSTRACT A numerical approach to simulate the delamination effect occurring in metal foam composites is being presented in this work. It is shown that in order to create reliable numerical models to simulate general components produced with aluminum metal foam sandwiches, the delamination effect of the aluminum skins from the metal foam must be considered. Delamination occurs within the polyurethane adhesive layer, causing the loss of the structural integrity of the structure. Foam is not a continuum medium, nevertheless, when simulating foam structures, foam is commonly assumed as a continuum, with homogeneous properties. This approach requires the calibration of the mechanical properties of the polyurethane adhesive layer, in order to compensate the effect of the foam’s discontinuous structure. The finite element method was used to numerically simulate a three-points bending test and an unconstrained bending test. The cohesive behavior was modelled by using a traction separation law. For the damage initiation criteria, a maximum-stress-based criterion was used, whereas for the damage evolution, a displacement-based damage evolution law was adopted. The experimental data were obtained from the group’s previous work, including a compression test, a tension test, a three-points bending test, and an unconstrained bending test.

Prediction of Forming Limit Diagrams for Materials with HCP Structure

The forming limit diagram (FLD) is an important tool to be used when characterizing the formability of metallic sheets used in metal forming processes. Experimental measurement and determination of the FLD is time-consuming and therefore the analytical prediction based on theory of plasticity and instability criteria allows a direct and efficient methodology to obtain critical values at different loading paths, thus carrying significant practical importance. However, the accuracy of the plastic instability prediction is strongly dependent on the choice of the material constitutive model [1–3]. Particularly for materials with hexagonal close packed (HCP) crystallographic structure, they have a very limited number of active slip systems at room temperature and demonstrate a strong asymmetry between yielding in tension and compression [4, 5]. Not only the magnitude of the yield locus changes, but also the shape of the yield surface is evolving during the plastic deformation [4]. Conventional phenomenological constitutive models of plasticity fail to capture this unconventional mechanical behavior [4, 6]. Cazacu and Plunkett [6] have proposed generic yield criteria, by using the transformed principal stress, to account for the initial plastic anisotropy and strength differential (SD) effect simultaneously. In this contribution, a generic FLD MATLAB script was developed based on Marciniak–Kuczynski analytical theory and applied to predict the localized necking. The influence of asymmetrical effect on the FLD was evaluated. Several yield functions such as von Mises, Hill, Barlat89, and Cazacu06 were incorporated into analysis. The paper also presents and discusses the influence of different hardening laws on the formability of materials with HCP crystal structures. The findings indicate that the plastic instability theory coupled with Cazacu model can adequately predict the onset of localized necking for HCP materials under different strain paths.