Andreas Kuppert

Time Dependent FLC Determination Comparison of Different Algorithms to Detect the Onset of Unstable Necking before Fracture

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 [2]), gliding correlation coefficient, linear best fit (modified method based on [3]) 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.

Enhancement of formability of aluminum alloys in multi-stage forming operations by a local intermediate heat treatment

Within this paper a new approach to enhance the formability of aluminum alloys in multi-stage forming processes will be presented. The technology’s key idea is the local adaption of the mechanical properties after a first forming step and their optimization for the subsequent forming operation. The partial change of the mechanical properties is obtained by a short term heat treatment between two forming steps. Based on the new property distribution the material flow during the second forming is improved and the formability of the material can be enhanced. The presented work covers all necessary steps for a successful application of the technology. After a material characterization in dependency of the pre-straining and the heat treatment, the heat affected zone, which is a result of the high heat conductivity of aluminum alloys was analyzed. In the end appropriate heat treatment layouts were designed via numerical simulation and the enhancement of formability was demonstrated with a real multi-stage forming process.

Optimization of the Heat Treatment Layout and Blank Outline of THTB

Tailored heat treated blanks (THTB) is the generic term for an innovative approach to enhance the formability of blanks made out of high strength steel or aluminum alloys. Key idea of the technology is the adaption of the mechanical properties by a local heat treatment. Based on the new property distribution, the material flow during the forming operation can be improved and the forming limit can be enhanced. In comparison to conventional temperature assisted approaches the forming is performed at room temperature and therefore all advantages of a cold forming process can be used. Most challenging within the application is the definition of the heat treatment layout. Up to now the layout is dimensioned in a time-consuming trial and error procedure. In this paper a new approach for the automatic optimization of the heat treatment layout and the blank outline is presented.

Einfluss der Auswertestrategie auf die Grenzformänderungskurve

Kurzfassung Die Grenzformänderungskurve stellt bis heute das bewährte und etablierte Verfahren zur Bestimmung der Versagensgrenze des Werkstoffes dar und kann für nahezu alle modernen Simulationsprogramme der Blechumformung als Eingangsparameter verwendet werden. Die Qualität der FLC ist entscheidend von der Definition des Beginns der Einschnürung während der Umformung abhängig und geht auf Arbeiten von Keeler [1] und Goodwin [2] zurück. In diesem Beitrag wird dargestellt, dass besonders bei Leichtbauwerkstoffen das standardisierte Verfahren nach ISO 12004-2 [3] Schwächen aufweisen kann, wodurch Abweichungen zwischen Simulationsergebnissen und realen Umformversuchen entstehen können. Die unterschiedlichen Ansätze zur Ermittlung der FLC werden gegenübergestellt. Im besonderen Fokus stehen dabei die zeitlichen Auswertestrategien, welche aktuell auch im Rahmen der Arbeitsgruppe „Erweiterung FLC ISO 12004“ diskutiert werden.

Comprehensive Material Characterization for an Intermediate Heat Treatment

Encouraged by increasing climate regulations there is a trend to lightweight constructions in the transportation sector, particularly in the automotive industry. An auspicious possibility to reduce the weight of the vehicle is the substitution of conventional steel by aluminum alloys. However, aluminum has a low formability and therefore new technologies have to be found in order to enhance the materials spectrum of application. A new strategy to extend the process limit is the short-term aluminum intermediate heat treatment between two forming operations. Key idea is the partial adaption of the mechanical properties with a short term heat treatment. By the interaction of soft and hard areas the material flow during the forming operation can be improved and the formability can be enhanced. Prerequisite for a successful application of the technology and the numerical simulation is a comprehensive knowledge about the interaction of pre-straining and a subsequent short term heat treatment. Within this paper the mechanical properties in dependency of the two parameters will be presented and a process window for the heat treatment after first forming operations will be derived. Moreover, the influence of batch fluctuations and the impact of ageing will be demonstrated.

Investigation of the Beginning of Plastic Yielding and the Hardening Behaviour under Biaxial Tension

The main focus of the experimental observation deals with the investigation of the plasticyielding of DC06 and DP600 under biaxial tension in comparison to an identified material modelwith an isotropic hardening behaviour. The isotropic hardening law describes the hardeningbehaviour of the material by application and approximation of the flow curve. Prevalent hardeninglaws are proposed by Swift (1952) or Hockett and Sherby (1975) and lead to an expansion of theyield surface in the stress space. By reasons of good accordance in an earlier survey the givenmaterials are modeled by the yield criterion Yld2000-2d and the isotropic hardening law of Swift.In this case the yield loci at different experimental states of plastification are compared with theyield loci given from the isotropic expansion of the Yld2000-2d yield surface. Furthermore apossible approach for modelling the change of the yield criterion’s shape in the stress space duringplastification is shown. With respect to further investigations additional research work is neededincluding extended and complex hardening laws to describe the real material behaviour sufficiently.