Quantitative Beschreibung der Kaltumformbarkeit von Stahlfeinblech mittels Schädigungskurve
Final Report Abstract
The general idea of the project is to establish a universal ductile failure criterion in the framework of damage mechanics for arbitrary sheet metal forming processes. Compared to conventional forming limit diagram (FLD), damage mechanics are able to consider the nonlinear stress and strain loading and the occurrence of failure prior to necking or minimal necking observed in advanced high stress steels (AHSS) forming. AHSS are mostly multi-phase steels in which microstructure plays important roles in the behavior and performance of steels and needs to be taken into account for formability characterization. In the study, a general empirical model postulated by Bai and Wierzbicki (BW model) is employed to characterize the formability of a dual-phase steel, DP600. The ductile failure locus of this model is constructed on a wide range of stress state and dependent on the hydrostatic pressure and lode angle. To validate the applicability of the model in sheet metal forming process, it is calibrated and applied to predict FLD and afterwards compared with the conventional one obtained from Nakajima test. The advantage of the BW modelling approach is that it is applicable in a wide range of stress states regardless the underlying failure mechanisms, shear fracture or void nucleation controlled ductile fracture. However, the shortcoming of this model is it is lack of link between the failure prediction and the material microstructure. Like this, the model is not suitable for the computeraided design of sheet metals with substantially improved material performance during cold forming. For this reason different micromechanical models shall be used to predict the BW ductile failure locus curve for the stress states where the respective model is applicable. Initially, the micromechanical Gurson-Tvergaard-Needleman (GTN) model is used for a microstructure-based computation of the equivalent strain to ductile crack initiation in a range of high stress triaxialities. From this the following conclusions can be drawn: (1) The general empirical BW model is calibrated in the specific application of sheet metal by a series of loading cases. To achieve the various stress state, pure shear, notched dog-bone, central-hole and flat-grooved plane strain specimens are loaded on uniaxial tensile test machine up to failure. Furthermore, hydraulic bulge tests are performed to actualize the equi-biaxial stress state. Direct current potential drop (DCPD) method and metallography are used to identify the onset of the micro cracks. From these tests, the influence of stress state on the ductile failure locus as well as on metal plasticity is observed and quantitatively calibrated into the model. (2) The predicted FLD by BW model gives a reasonable and reliable result. The predicted forming limit curve (FLC) lies under the experimental FLC. To author’s understanding, this is a more critical limit for sheet forming. The ductile failure criterion in this study is based on the onset of micro cracks due to the method of crack identification, whereas the conventional FLC is based on the visual observation of macro necking on the surface of sheet. In a forming process that is evaluated with FLC criterion, the formed components could already contain micro cracks inside, which significantly decrease the crashworthiness in the ultimate application. (3) GTN model was assumed to be applicable at the high stress triaxiality range. In this study it shows that GTN is able to predict the ductile failure locus at the lode angle parameter = 1, but for low lode angle parameter = 0, it fails to predict the failure locus even at high stress triaxiality range. The lode angle therefore is a limitation for the applicability and reliability of the micromechanical models other than stress triaxiality. On a microscale level, it should be also correlated to the fracture mechanisms. Stress triaxiality was believed as the main factor to provoke the different fracture modes. When it is above 0.33, the void growth dominated mode prevails. When it is near and less than 0, the shear band mode dominates. When it is in between, these two modes coexist and compete with each other. After the introduction of the lode angle, this needs to be reexamined.
Publications
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4th Baosteel Biennial Academic Conference: Design of microstructure for damage tolerance in high strength steels, November 16-18, 2010, Shanghai, China
W. Bleck, J. Lian, S. Münstermann
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6th German-Japan Seminar “Materials, Processes and Components”: A ductile failure criterion in cold formability prediction of AHSS sheets, July 8-10, 2010, Fraunhofer Institute for Mechanics of Materials, Freiburg, Germany
J. Lian, S. Münstermann, W. Bleck
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International Conference on Advanced Steels 2010: Formability characterization of DP steel with damage mechanics approaches, November 9-11, 2010, Guilin City, Guangxi, China
J. Lian, S. Münstermann, W. Bleck
