Time: 10:00 a.m.
Place: IC 02/718
Erman Tekkaya, Technische Universität Dortmund, Dortmund, Germany
The presentation gives an overview of the activities of the Institute of Forming Technology and Lightweight Construction at the TU Dortmund. Three selected research topics are discussed in detail: Sheet metal characterization, the first topic, is one of the main activities of the Department for Sheet Metal Forming at IUL. The anisotropic plastic behavior of sheet metals is modeled with different yield conditions which necessitate the determination of different number of material parameters from various tests. Most of these tests exhibit an inhomogeneous deformation state in the gauge region. To overcome this problem, X-Ray diffraction method is applied to measure stress in-situ. A mobile X-ray diffractometer and an optical strain measurement system are mounted on a universal tensile testing machine. This enables the recording of the whole strain and stress history of a material point during testing. The method is applied to uniaxial tension tests, plane strain tension tests and shear tests to characterize initial yield locus of sheet materials. In order to assess the formability of sheet metals damage modeling is necessary. The identification of the material parameters for these models includes different tensile and shear tests. As a second topic material characterization using the in-plane torsion test is discussed. This test method for sheet materials can determine the flow curve in shear deformation for high strains (eq. strain up to 1.0). Evaluation methods using optical strain measurements are developed at the IUL. With one single measurement after deformation, a complete flow curve can be found due to the stress and strain gradient on the specimen. Furthermore, this test can be used to identify multiple cyclic shear flow curves simultaneously, allowing an effective investigation of the kinematic hardening of materials at different prestraining. By a modification of the specimen, the in-plane torsion test is suitable to determine shear fracture of sheet materials with perfect shear conditions without edge effects and inhomogeneous stress or strain distributions. The third topic is vaporizing foil actuator forming (VFA forming) and presents an innovative impulse forming process. During this process metallic foils or wires are rapidly vaporized by high pulsed electrical currents in order to form fast expanding plasma which generates an intensive mechanical pressure pulse. The major advantage of VFA forming compared to electromagnetic forming (EMF), which is the most common impulse forming process, is that the process is also suitable for non-conductive materials. Furthermore the lifetime of EMF actuators is limited, especially at high energies. Therefore, the process is only to a limited extent suitable for large numbers of operations. Low-cost, disposable foils or wires as actuators are a good approach to counteract this issue. Besides forming, vaporizing foil actuator forming is also suitable for joining by forming.