Abstract
Sheet metal forming boundaries are established using the forming limit diagram (FLD). The Nakajima and Marciniak tests, which are based on stretching a material using a punch, are the most commonly used methods for determining the FLD or fracture forming limit diagram (FFLD). The results are usually evaluated by calculating local strain, strain rates, specimen thickness reduction or fracture strain. When the amount of experimental material is insufficient, miniaturization of the testing specimens may be a solution. However, the interchangeability of the results for standard and miniaturized specimens has not been proven yet. In this study, the Nakajima tests were performed using standard and sub-sized specimens made of manganese–boron steel 22MnB5, commonly used in the automotive industry. Afterwards, four FLD/FFLD evaluation methods were applied and compared. The miniaturized specimens yielded higher strain values, which was explained by the varied ratio of material thickness/punch diameter and different bending conditions. The highest compliance of the results was recorded for the standard and miniaturized FFLD.
Highlights
For almost all kinds of forming operations in which sheet metal is used as the input material, it is essential to know the conditions under which the material exhibits necking or direct fracture
This study aims to investigate the effect of the irregularity between the forming limit diagram (FLD) and the final fracture of the specimens, namely the difference between the conditions created by the testing and forming tools, and the size of the formed material
In the case of miniaturized FLD specimens, the results showed that the principle of using miniaturized samples is possible
Summary
For almost all kinds of forming operations in which sheet metal is used as the input material, it is essential to know the conditions under which the material exhibits necking (material instability) or direct fracture. A typical example can be found in the automotive industry, in the forming of critical body elements such as A, B and C pillars. The tendency on the global market is to reduce the material consumption for body in white (BIW) design by using innovative, lightweight materials and simultaneously maintaining the material strength characteristics required for operational safety. Knowledge of the borders of material processing is crucial. Such limits can be presented as a forming limit curve/diagram (FLC/FLD), where the curve of the coordinates of major and minor strains is plotted
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