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This paper presents applications of a binder wrap calculation model to several experimental sheet steel forming dies. In one application of a doubly curved die, the calculated binder wrap agrees with physical tests and calculations reported elsewhere. In another door inner panel die, the designer's intent of having more material in certain part of the die cavity was verified. In the third case, the calculated binder wrap agrees quantitatively with measured data for a decklid stretch-draw die. The later result was used in guiding the modifications of the layout.

The accurate prediction of burst of hydroformed tubes is a research area of considerable importance in order to evaluate a design before prototyping. This report applies the presently available criteria (forming limit diagram, stress-based forming limit diagram, extended stress based forming limit curve and the plastic strain criterion) to some of the benchmark examples carried out by the Auto/Steel partnership. It was found that the formability predictions are lowest if the plastic strain criterion is used and highest if either the stress-based criteria are used. Predicted and measured results were also compared.

The bulge test in hydroforming is a simple fundamental experiment used to obtain basic knowledge in tube expansion. The results can be used to assist design and manufacturing of hydroformed automotive parts. It also can be used to develop a failure criterion for tubes in hydroforming. For these purposes, a section of a long unsupported tube with fixed ends was simulated numerically to obtain the mechanical states of the tube subjected to internal pressure. Steel and aluminum tubes are used. For the bulge tests, the internal pressure reaches a maximum and then decreases in value without failure while the stress, strain and volume of the tube keep increasing. A failure criterion for the bursting of a tube is proposed based on the stress-strain curve of the material.

Fundamental differences exist between sheet metal forming and hydroforming processes. Sheet metal forming is basically a one step metal fabrication process. Almost all plastic deformation of an originally flat blank is introduced when the punch is moved normal to a clamped sheet metal. Hydroforming, however, consists of multiple steps of tube making, pre-bending, crushing, pressurization, etc. Each of the above mentioned steps can introduce permanent plastic deformations. The forming limit diagram obtained for sheet metal forming may or may not be used in hydroforming evaluations. A failure criterion is proposed for predicting bursting failures in tube hydroforming. The tube material's stress-strain curve, obtainable from uniaxial tensile test and subjected to some postulations under large stress/strain states, is used in judging the failure.

Corner filling is a benchmark experiment in tube hydroforming. It was designed to gain knowledge pertinent of this new fabrication process. The corner filling benchmark has been widely used in the automotive and steel industries. Common sense as well as physical tests suggests that friction is an important parameter that affects the deformation of the tube and the bursting of the tubes. However, numerical simulations have yet to verify this fact. In this paper, the stress/strain states in the tube were computed using a finite element model. The dependence of bursting on friction for corner filling was estimated by using the forming limit diagram and a tensile-based failure criterion.

Binder wrap forming for automobile body panels can be evaluated by finite element analysis. Sometimes, however, numerical instabilities occur in the implicit nonlinear solution process. While this instability may be an artifact of the numerical model, it may also signal the onset of a real physical phenomenon, such as bifurcation. This paper presents a numerical experiment to examine bifurcation in binder wrap forming, using an experimental decklid die. Numerical instability occurred early in the deformation process. Two stable solutions for the binder wrap surface were obtained, demonstrating that the instability resulted from bifurcation. One of the deformation modes is saddle like, and the other is cylindrical. In practice, the blank will deform to one of the above modes under the clamping actions generated by the closing of the binders. If the deformation takes the other mode, substantially different strains will result in the subsequent punch forming.

Sheet metal forming of automobile body panel consists of two processes performed in series: binder forming and punch forming. Due to differences in deformation characteristics of the two forming processes, their analysis methods are different. The binder wrap surface shape and formed part shape are calculated using different mathematical models and different finite element codes, e.g., WRAPFORM and PANELFORM, respectively. The output of the binder forming analysis may not be directly applicable to the subsequent punch forming analysis. Interpolation, or approximation, of the calculated binder wrap surface geometry is needed. This surface representation requirement is carried out using computer aided geometric design tools. This paper discusses the use of such a tool, SURFPLAN, to link WRAPFORM and PANELFORM calculations.

A computational procedure is presented to evaluate the static dent resistance at the center of a steel door panel. Using the design parameters of geometric shape, thickness and the stress-strain relations of the steel, the static dent initiation load can be calculated. The method is based on the concept of plastic work which is the non-recoverable energy dissipated in the panel by the applied load. A threshold value of plastic work of 0.02 joule is used to signal the dent initiation. A comparison of the computed and measured dent initiation loads of ten experimental panels shows the maximum deviation is less than 20 newtons.

In the analysis of automotive structures using finite element models, areas of stress concentration are often uncovered. The calculated linear stress at these locations may exceed the yield strength of the material. In these situations, the analyst is confronted with the choice of making a nonlinear analysis of the structure or using an approximate technique to predict nonlinear strains. The former can be excessively expensive while the latter can lead to results of questionable accuracy. This paper will discuss a technique which substantially reduces the cost of the nonlinear analysis of complex structures with areas of localized plasticity. The basis of the technique is the substructuring of the localized plasticity areas. They are identified and disconnected from the linear elastic part of the model by boundary points.

This paper presents a numerical method for calculating the maximum force a steel automobile door panel can support at its upper character line without creasing failures. The method is based on the concept of plastic work, that is, the non-recoverable energy dissipated in the panel by the applied force and plastic deformation. By relating the calculated plastic works and the measured creasing forces of twelve experimental panels, a threshold value of plastic work of 0.03 joule has been obtained for signaling the onset of creasing. This critical value was used to calculate the creasing forces of these panels. The results so obtained are accurate. They have an average error of less than six percent from their corresponding measured data. The correlation coefficient between the calculated and measured creasing forces is 0.97.