Haluk Darendeliler

Finite element analysis of springback in bending of aluminium sheets

Abstract Bending is one of the processes frequently applied during manufacture of aluminium components. The bending operation involves springback, which is defined as elastic recovery of the part during unloading. In manufacturing industry, it is still a practical problem to predict the final geometry of the part after springback and to design appropriate tooling in order to compensate for springback. In this paper, commercially available finite-element analysis (FEA) software is used to analyse bending and springback of different aluminium materials of different thickness. The amount of springback, the total equivalent plastic strains and the equivalent von Mises stresses are presented. The FEA results are compared with empirical data.

Analysis of tapered preforms in cold upsetting

Abstract Upset forging is usually carried out with a sequence of stages and the tapered preforms are commonly used. The preforms should be free from flash formation and buckling injuries. In this paper, the results that depend on elastic–plastic finite element analysis of taper upset forging are given. The buckling analysis was realized by using the Modified Riks method. For a given upset ratio which is the ratio of unsupported length to diameter of the initial billet, reduction in the height of the billet and the diameter ratio are the important design parameters in taper upsetting. For several values of the design parameters, the analysis has been carried out and the results have been compared with the results of the well-known Meyers experiment and Goklers suggestions. In the analysis, the perfectly square end (ideal billet case) and the end with face inclination angle were considered as two different end conditions of the billet. The results for the ideal billet case are in good-agreement with Meyers results. The end-face inclination angle, which is a significant imperfection on the billet, reduces the limits of allowable upset ratio. It has been observed that although the tapered header dies designed at the limits suggested by Meyer produce flashless preforms, in some of the cases, injuries due to buckling are not eliminated. However, Goklers suggested limits provide elimination of both flash formation and injuries due to buckling.

Deformation Analysis of Deep-Drawing by a Finite Element Method

A finite element method is developed to study the elastic-plastic deformation of sheet materials in the presence of large strains and large displacements. It is based on updated Lagrangian type formulation and membrane shell theory. The sheet is assumed to be isotropic and rate insensitive which obeys J2 flow theory. The work-hardening characterstics of material and Coulomb friction between the sheet metal and forming tools are incorporated. The method is used for modelling partial deep-drawing with the appropriate boundary, conditions. Numerical solutions are compared with the experimental results.

Effect of variable friction coefficient on sheet metal drawing

A variable friction model that relates the parameters of sheet metal drawing to the local lubrication conditions taking place during the deformation, has been integrated to a finite element program. Variable friction coefficients for the contacting surfaces are determined from the friction model, which uses the related parameters obtained at each time step of the finite element program as inputs. A number of numerical runs have been performed and the strains are compared with the experimental results for circular blanks. A good agreement is obtained between the numerical and experimental results for the variable friction model used.

The optimum die profile for the cylindrical bending of plates

Abstract This paper presents a methodology for the design of plate-forming dies in cylindrical bending using optimization techniques to reduce the cost of die production by reducing the trial-and-error procedure considerably in determining the final die geometry. The plate thickness is discretized by plane-strain finite-elements. The die is taken to be rigid and its profile is approximated by Bezier curves the control-point coordinates of which are the design variables. The die profile is varied to minimize the difference between the required shape and the shape of the bent plate, considering springback action. The unconstrained optimization problem is solved by the BFGS (Broyden-Fletcher-Goldfarb-Shanno) method. A numerical example is presented where the optimum die profile is obtained for a plate bent into a quarter circle.

Analysis of axisymmetric cup drawing in relation to friction

Abstract The application of computer-aided simulation to manufacturing by using sheet-metal forming processes in industry, leads to the elimination of the repetitive production of expensive dies, and the loss of time, labor and material. The finite-element method has been widely used in the analysis of sheet-metal forming processes, in this way making it possible to determine the deformed shape and the variation in the thickness of the material. Hence, the drawability and the shape of the blank can be determined in advance. In this paper, an analysis of axisymmetric cup drawing is made using a three-dimensional finite-element method considering the effect of friction, which is one of the important factors that affects the drawing, and is difficult to predict and simulate.

A pseudo-layered, elastic-plastic, flat-shell finite element

Abstract A three-node, C0-type, layered flat-shell finite element is developed for the analysis of large elastic-plastic deformations in plate and shell structures. The system equations are derived by using virtual work principle and the updated Lagrangian formulation. Material is assumed to be isotropic and rate insensitive obeying J2-flow rule. The displacement field assumption of the MIN3 plate bending element is employed. A layered structure is used to model through-the-thickness distribution of elastic and plastic zones. The finite element results for three nonlinear plate bending problems are compared with experimental results to verify the accuracy of the formulation.

Analysis of roller hemming process for a vehicle tailgate closure

Hemming is a sheet metal joining process which is widely used for vehicle closures. As the latest hemming process type, the roller hemming process uses industrial robots therefore; main advantage of the process is achieved as flexibility with improved product quality. Trial and error method is the general approach to design the process in the industry due to limited know-how in the roller hemming. However, due to advantages of the process, the recent studies have also been focused on numerical simulations. In this study, the roller hemming process of the tailgate of a vehicle has been investigated by using the finite element method. The points of interest are selected as cycle time reduction and reducing the undesired wrinkling formation in the process. In the current roller hemming process of the tailgate, three stages including two pre-hemming and one final hemming stages are being applied. For the cycle time reduction, simulations have been performed to complete the hemming process in two stages. Effec...

Comparison of Different Constitutive Models in Sheet Metal Forming

The effects of different constitutive models in sheet metal forming are investigated by considering the cylindrical and square cup drawing and V-bending processes. Numerical analyses are performed by employing eight different constitutive models. These are elastic plastic constitutive model with isotropic hardening, elastic plastic constitutive model with kinematic hardening, elastic plastic constitutive model with combined hardening, power law isotropic plasticity, piecewise linear isotropic plasticity, three-parameter Barlat, anisotropic plasticity and transversely anisotropic elastic plastic models. The simulations are performed for three different materials, St12 steel, Al-5182 aluminum and stainless steel 409 Ni, by using a commercial finite element code. A number of experiments are carried out and the experimental and analytical results are utilized to evaluate the results of simulations.

A Study on the Constitutive Equation Effects in the Fracture Initiation of AA5450 Sheets

Determination of the fracture initiation in the sheet metal forming applications can be achieved successfully using ductile fracture criteria (DFCs) and finite element codes together. In this study three different uncoupled, energy based ductile fracture criteria have been employed to predict the onset of the fracture in the AA5450 aluminum alloy sheets. Also, two different constitutive models namely, isotropic von-Mises and anisotropic Hill, have been implemented to the finite element code ABAQUS through VUMAT subroutine to investigate the effect of constitutive equations on the applicability of the utilized DFCs. It was shown that the constitutive model has significant influence on the estimation of the time and place of the fracture initiation in the sheets.