Abstract
This study addresses the analysis of edge crack sensitivity of DP800 steel and AA5182 aluminum alloy in dependency of punching and machining operation as well as load case of subsequent forming. The inserting of a round hole by punching with defined punch-to- die-clearance, milling and drilling is compared. Subsequent forming is performed by standardized hole expansion test and by Nakajima-tests with three different specimen geometries. Local strain distribution at the surface for Nakajima-tests is measured by optical strain measurement technique and investigated in order to evaluate local deformation before failure. Additionally, resulting hole expansion ratio λ is determined. Significant higher X as well as local strain values εmax are achieved by machined holes. This is directly coupled to higher local formability and stretchability for both materials. Furthermore, the load condition has a strong impact on the edge crack sensitivity of the material. Prior failure is observed with changing stress conditions using different specimen geometries also influencing the reachable maximum failure strain. Higher edge crack sensitivity is observed for DP800, which is in good accordance to the material properties in terms of ductility and strength. These data in dependency of the process parameter can be used for the design of automotive components.
Highlights
Within the fabrication of automotive components multi-stage process sequences with punching and subsequent forming operations are commonly used
The highest hole expansion ratios are reached in the hole expansion test, because the load condition is different to the Nakajima test
As within the hole expansion test a first flanging of the hole leads to further tangential stresses and strains, the hole is expanded in plane within the Nakjima test, which leads to more critical stresses directly at the edge of the hole
Summary
Within the fabrication of automotive components multi-stage process sequences with punching and subsequent forming operations are commonly used. In this context holes in blanks are required for assembly in case of safety relevant structural components or as relief holes for deep drawing operations of body parts, for instance. According to economic reasons the holes are commonly punched in industry This leads to a significant reduction of materials formability. Pre-punched areas have a negative effect on forming operations, which primarily occur during component production, concerning crack initiation. The impact of shearing edge quality on subsequent forming is mainly assessed by the standardized hole expansion test [1]. The achievable hole expansions ratio λ, defined as the ratio of the initial and the resulting diameter of the hole, is generally higher with lower material strength [4] and higher
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