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This article describes a fast design oriented tool to provide predictions of springback in automotive stampings. The new program, FAST_SPRINGBACK, utilizes the results of another membrane finite element code, FAST_3D, which performs a one step simulation of the forming process. The significant feature of the new springback tool is its ability to quickly predict the different modes of springback deformations as well as residual stresses that will affect the final shape and performance of the formed part. An important aspect of the program is its capability to integrate with existing CAD tools, thus enabling the designer to perform checks on the candidate part geometry in the early design stages.

The paper introduces the concept of process signatures, the main intention being to change the grid analysis from a merely data processing activity to a knowledge based methodology. The process signature is the shape of the strain envelope, which depicts the strain state of contiguous elements on the surface of a stamping. The signature is strongly influenced by the tooling geometry, process induced modes of deformation, and to a lesser extent by the lubricating conditions, material characteristics, etc. Essentially the deformation of the whole part can be analysed by subdividing it into the fundamental modes. It is shown that the total response of a stamping depends on certain combination of these basic fundamental signatures. Distinct areas on the signature correspond to the punch and die actions. The work demonstrates that the process signatures respond in a predictable fashion to changes in tooling, material and set up.

The paper describes a system currently under development, intended to be the spine for organizing an open-ended knowledge-based system for stampings within the sheet metal industry. The system is designed for those not necessarily expert in plasticity and strain theory, as well as applications where lead time is of prime importance. The objectives of the system are to develop: i a common vocabulary for BASIC SHAPES and DEFORMATION MODES ii an open-ended filing system for the organization of the knowledge i.e. the documentation of solutions and procedures used, as well as assessing historical data iii a procedure for deriving the FIRST FORM SHAPE and the number of forming stages and iv a system for the assessment of forming severity, force and energy calculations, material selection, tribological requirements and process planning.

A simultaneous Design/Process/Material engineering approach for the analysis of automotive structural cross member designs is presented. It is shown that it is possible at the product design stages to evaluate i. forming severity, ii. blank shape and nestability, iii. spotting requirements, and iv. structural analysis based on yield stress and thickness distributions after forming. The approach allows the reduction of material content in the part (by nesting and possible weight reduction), a better estimate of part performance during stamping, the evaluation of the effect of the material properties, the construction of the blanking die at the same time as production form tooling, and the reduction of a significant portion of the product design/manufacturing cycle. A production cross member is used to validate the approach. The production cross member information also served as a basis for a new prototype cross member.

Surface defects occur during forming, material unloading and during further material processing. During forming operations the compressive forming stresses are the cause of defects. During material unloading and further material processing, springback and residual stresses are the major cause of such defects. The conditions for the occurrence of surface defects at these three stages are examined and discussed. A computer aided methodology is presented for estimation of the residual stresses, application of the approach is given for some practical examples.

The paper describes some advances in the use of grid analysis and forming limit diagram techniques. It is shown that a limited amount of process or tooling related information can be derived from the measurements of the grid distortion at localized areas. It is illustrated that a full three dimensional grid analysis of the part is required for a comprehensive understanding of the interrelation between the state of deformation at different regions and the possible improvement in process.

The theme of this work is the interaction of the part geometry, material properties, forming severity and tooling design for box-shaped stampings. It is shown that the forming severity decreases with the increase of material normal anisotropy and corner radius. The lower the material normal anisotropy, the larger are the required tooling clearances and the thickening of the material under the blankholder. Understanding these relationships is of primary importance in applications requiring a major change in material properties, such as cold-rolled to hot-rolled conversion of products, or product shape redesign. The work also introduces a new type of idealization to metal forming problems. Beside the traditional continuum mechanics axisymmetric and plane strain idealizations, the work introduces the “corner analysis” approximation.

The work describes an approach for a closer interface between product design and manufacturing. Starting from the part print a finite element model of the final part is constructed. The model is used for a one step unfolding of the part to determine the blank shape. In addition, by tracking the shape of the material “finite” elements before and under unfolding, the approximate forming severity of the stamping can be obtained. The one step unfolding solution also provides the thickness distribution together with the yield stress distribution in the stamped part. This information is of great value to product design for the performance of a more realistic fatigue analysis. Since during product design many design alternatives are to be evaluated, the finite element formulation used at this stage is based on a one step approach, with the emphasis on solution speed as opposed to exactness.

The effect of binder shape on the formability of metal stampings is addressed in this work. The paper starts with a discussion of a binder development approach for developable or near developable binders. An FEM based test for closeness to developable surfaces is also presented. In this case, the binder surface is meshed using standard FEM discretization techniques. The test is used as an evaluator for the proneness of wrinkle formation at the binder closing stage under the binder surface. The formability of a door outer panel is evaluated and the forming severity related to binder shape. It is shown that addendum and binder designs are key process variables that have to be designed up front and related to product characteristics.

This paper presents several guidelines useful when applying axisymmetric solution techniques to the problem of estimating the forming severity of complex three dimensional stampings. Although the axisymmetric solutions are seemingly restricted in their number of applications, these two dimensional techniques can be used to analyze a wide variety of stamping configurations. However, great care must be taken to ensure that the underlying requirements of Axisymmetry are not “seriously” violated. Presented in this paper are several guidelines, including a discussion of the rationale behind them, suggesting how to apply AS solutions to stretch and shrink flanging stamping operations.