Experimental evaluation of strains in the tension-compression using a new tool geometry, X-Die

Sheet-metal forming involves a complex distribution of strains throughout the part. The strains occur due to tension, compression and a mix of both. A geometry has been developed, the X-Die, in order to gain insight into the strain behavior of different materials. The X-Die enables strain paths far into the tension-compression region, thus creating the possibility to extend the experimental base both for definition and for further extrapolation of the forming limit curve (FLC) in the tension-compression region, as well as to evaluate FE-simulation results for the same region. The experimental results show that the strain signature is impacted by material quality. In qualities such as extra high strength steel (EHSS) and aluminum the strains do not reach as far into the tension-compression region as the strains do in e.g. mild steel. This is due to failure in plane strain tension. Strain paths in materials such as mild steel and high strength steel (HSS) reach far into the tension-compression region before failure. Use of the X-Die provides possibilities to reach farther into the tension-compression region compared with traditional test methods for creating a forming limit diagram (FLD). Use of the X-Die yields well-defined strain signatures. These clearly defined strain signatures are favorable for comparison with numerical simulations, especially for strain signatures in the tension-compression region. Furthermore, the experiments using the X-Die indicate that a possible additional forming limit curve, which intersects the original forming limit curve (shear failure), exists so far into the tension-compression region that it is not applicable. Even though the experiments indicate compression strains > 100% (material DX56D), the experiments show potential for an experimentally determined extrapolation of the FLC up to similar to 75% compression strain. The results of the experiments indicate that the X-Die geometry is suitable as a supplementary tool in identifying the strain behavior of different materials far into the tension-compression region and is also a good tool for verification of numerical results in the tension-compression region. (c) 2007 Elsevier B.V. All rights reserved.