Deformation Analysis of Autobody Panels
Abstract
This paper describes experimental results for the forming limits for steel sheet DC 04 (KOHAL 200) and the deformation states in large-sized autobody panels of the Skoda, with examples of the application of these results to the analysis of an actual press forming operation. The forming-limit diagram (FLD) was determined using the in-plane stretching method. The deformation states in the large-sized autobody panels were marked on the FLD and compared with the limit strains of the sheet metal tested. Since successful forming requires the right combination of material, lubrication, sheet-blank configuration and die design, the trial-and-error method was used to determine the proper forming parameters. By analysing the strain patterns, it was possible to obtain the changes needed to convert an unfavourable stamping into a favourable one
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Introduction
Autobody stampings can be divided generally into two main categories [1]: (i) skin panels, i.e. those that are visible on the exterior and the interior of the vehicle after all the functional and decorative trim has been added; and (ii) interior panels. These different panels are generally widely different in character, the skin panels involving large areas of metal with very low strain, the strain being largely biaxial stretching, whilst the interior panels are much more complicated in form, involving a higher level of strain in the finished part, and more complicated strain histories.
With the large flat panels dictated by modern styling requirements, uniform stretching is particularly difficult to achieve, as the direction of strain over the larger part of the panel is normally perpendicular to the direction of punch travel. This means that large areas of the panel are inevitably subjected to small strains, whereas the periphery, i.e. those areas strained over the punch and die radii, are heavily strained. When considering interior panels, the same criterion applies in some cases.
It is generally necessary with both types of stampings to produce them in large quantities at high production speed without any failure in the part. It is not sufficient to define a failed stampings as one which has a split, this term applying equally to stampings exhibiting any severe localised thinning in the metal and, for that matter, panels with excessive buckling, wrinkling and of general bad shape must also be considered failures.
Whether or not a particular sheet of metal can be formed without failure depends on the material properties, the surface condition, the blank size and shape, the lubrication, the press speed, the blank-holder pressure, the punch and die design, and many other known and unknown factors.
Conclusion
The forming limits of steel sheet and the deformation state of the most critical area were investigated to avoid fracture in the press forming of autobody panels. By comparing the deformation state with the forming limit, it was possible to provide some technically effective means for avoiding fracture. As a consequence of these investigations, is was shown that small changes in the technological parameters, i.e. blank sheet configuration, the blank-holder pressure, the lubrication and the tool geometry, enabled the achieving of a satisfactory reduction of breakage of the stampings, without change of the material quality.