Open hole tensile tests for the determination of the edge-crack sensitivity of sheared holes dependent on specimen geometry, cutting parameters, and the notch factor

摘要

Taking into account current trends in the automotive industry, one can see that components having a high functional integration in combination with a high material strength are integrated into the body shell in order to reduce the total weight. Regardless of the type of sheet metal component to be produced, a shear cutting process is involved in almost every case. Separation with unsuitable parameters leads to the risk that sheared component edges will fail during the forming process due to edge-cracks. Steel and automotive manufacturers as well as scientific institutes have developed a variety of non-standardized test methods to identify the edge-crack sensitivity of high-strength steels. The only standardized test set-up for identifying the edge-crack sensitivity of sheared edges, ISO16630, proved to be unsuitable due to the scatter of results and induced friction. An alternative method could be the Open Hole Tensile Test, which was developed for fiber composites testing. This method makes it possible to classify steels with regard to their edge-crack sensitivity depending on the machining method and the notch factor of the initial holes. The investigations described in this paper show that the specimen geometry and the process parameters used in the shearing process significantly influence the edge-crack sensitivity. For the identification of the respective influences the logarithmic major strain at the beginning of local necking is used. The characteristics are related to the cut-surface characteristics of the shear-cut starting holes.