The ISO standard 12004-2:2008E for the determination of forming limit curves based on the section method was approved in 2008. About 4 years of measuring experience in different laboratories has shown advantages and weaknesses of the standard and is leading to some minor changes in the specification. In the years from the development of this standard until today a further technical development of the optical measuring devices occurred, so that it is now possible to determine forming limit curves using the time history of the test. This procedure of determination is referred to a time dependent technique and could be the basis of the ISO 12004 part 2 proposal worked out by the work group Erweiterung FLC ISO 12004 of the German group of the IDDRG.
This publication recapitulates existing work which was carried out from the IDDRG work group regarding the determination of forming limit curves for sheet metal materials. On one hand known issues with the current section based approach are discussed and on the other hand it deals with a comparison of different algorithms to determine the FLC from the time history of the Nakajima test using strategies to identify the instant of onset of instable necking. The different time dependent algorithms [ utilised are automatically selecting the area where necking is leading to fracture and then analyze the time history of such points using the first or the second time derivative of the true major strain, or of the true thinning strain using methods like: correlation coefficient (modified method based on ), gliding correlation coefficient, linear best fit (modified method based on ) and gliding difference of mean to median. The resulting experimental FLC points are compared with the results from the section technique described in ISO 12004 part 2 and with the maximum strain values measured in each test. Further a large number of forming limit curves were determined and used for a comparison of these different methods to define the most promising time dependent algorithm, which was selected as a suggestion for the working group defining the new proposed ISO standard 12004 part 2.
Tailored Heat Treated Blanks (THTB) are blanks, which exhibit locally different material properties optimized for the succeeding sheet metal forming process. The distribution of the material properties is obtained by a local, short-term heat treatment. As a result of the optimized property gradients in the sheet metal's plain, THTB allow enhancing the forming limits significantly. The research work covered in this paper focuses on THTB made out of aluminum alloys. The article presents the microstructural mechanisms due to a short-term heat treatment as well as design principles for the heat treatment layout for THTB made out of AlMgSi alloys and ultra-fine grained (UFG) aluminum alloys produced by the Accumulative Roll Bonding process.
The current lightweight design is realized by the multi-material mix in body-in-white production of automotive industry. In car bodies, the proportion of high-strength steels and aluminum alloys increases. Therefore, it is necessary to join these materials. Mechanical joining processes are suitable for a multi-material mix. But in combination with high-strength steels, only processes with additional elements, like rivets, are useful. The development of a new clinching technology shows a solution for an economical method for joining these different materials. An innovative single-stage process without any additional joining elements is created to extend the current limits of mechanical joining technologies. This makes it possible to join aluminium alloys and high-strength steel by punching and clinching in one step. Using the special material behavior of the parts to be joined is most important for this mechanical joining process. A clinching sequence, a scheme and a gradual analysis support the understanding of this joining method. Some examples of connected material combinations complete this description. Further steps to improve this joining process are shown. Especially the finite element analysis will provide more process details for improving this innovative joining technology.
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