Papers by Keyword: Nakajima Test

Abstract: The industrial necking prediction in sheet metal forming is still based on the Forming Limit Diagram (FLD) as initially proposed by Keeler. The FLD is commonly specified by the Nakajima tests and evaluated with the so called cross section method. Although widely used, the FLC concept has numerous serious limitations. In the paper the influences of bending on the FLC as well as postponed crack limits will be discussed. Both criteria will be combined to an extended FLC concept (X-FLC). The new concept demonstrates that the Nakajima tests are not only appropriate for the evaluation of the necking instability, but also for the detection of the real crack strains. For the evaluation of the crack strains, a new local thinning method is proposed and tested for special 6xxx Al-alloys.

Abstract: The paper deals with measurement of limit strains for tinplate TS 245 made by US Steel with nominal thickness 0.3 mm. The FLC curve has been measured by implementation of Nakajima test on testing device Erichsen 145-60. The Nakajima test has been measured according to EN ISO 12004-2. Limit strains have been measured using 3D photogrammetric system Argus by GOM. Forming limit curve was evaluated in the software Argus. During test, some problems have been solved concerning friction on hemispherical punch which affected fracture position on the dome. Reached FLC curve was implemented in material database of PAM-Stamp 2G simulation software.

Abstract: The ability to predict the forming severity with respect to crack and failure is essential to analysis of sheet metal forming process. The forming limit diagram (FLD) is commonly used to gauge the formability of sheet metal. In this article, forming limit diagrams of cold rolled carbon steel (JIS-SPCC), which widely used to produce the parts of automobile, are obtained by performing experiment and FE simulation with the Nakajima-test. By using the GTN (Gurson-Tvergaard -Needleman) damage mechanical model, a failure criterion based on void evolution was examined in this FE simulation. The parameters of GTN model are determined through comparison of experimental and numerical result with Nakajima-test. These parameters acceptably can be used in GTN model using given material. In application case, the reliability of the GTN model for failure criterion in simulation with automotive part was confirmed.

Abstract: To study the effects of temperature and thickness on forming limits of boron steel 22MnB5, Nakajima tests have been performed for the steels with thickness of 1.0mm and 1.4mm at 600°C and 700°C, respectively. The forming limit curve (FLC) of the steel at 700°C is higher than that at 600°C, and the FLC of the steel with 1.4mm is higher than that of the steel with 1.0mm. With increasing the forming temperature, the strain rate sensitive exponent m increases, and it results in a longer Swift’s diffuse instability phase and greater limit strains. The effect of thickness on yield path is different from the case at room temperature, due to the half angle of pointed Vertex, θ₀, which increases with increasing of the thickness, and then the limit strains increase.