Abstract
Blanked edge surfaces are rough and hardened. They therefore lead to inhomogeneous deformation on the edge, which can trigger localization within the shear affected zone (up to few mm from the edge). The size and extent of these phenomena are primarily a function of the shearing process and are only marginally coupled to the global/homogeneous deformation behavior of the blank. A direct numerical simulation of such local deformation effects would require a prohibitively high resolution to capture the microgeometry of the edge and thus remains unfeasible in the current industrial practice. A predictive model can therefore only be achieved by determining limit strains on the edge, which are compatible with the homogeneous numerical framework used. The present contribution aims discussing the basic mechanics of edge cracking based on tensile tests with edges blanked with different die clearances. The local and global strain evolutions in the vicinity of the edge are analysed and a new evaluation procedure is proposed for the reliable determination of limit strains. The application of this method in industrial context is also discussed.
Highlights
The most common failure mode in stamping operations is splitting due to localized necking
It is reasonable to set the failure limit at the onset of localized necking (FLC), which in turn is compatible with the state of the art numerical framework based on shell elements and continuum mechanics
It is more challenging to find a deformation limit which adequately represents the fracture phenomenon but at the same time remains compatible with state of the art numerical computation results
Summary
The most common failure mode in stamping operations is splitting due to localized necking. It is seen that the DH800 provides superior edge formability especially in the critical low clearance case Using these values quantitatively, as limiting strains for stamping simulations, would clearly be too conservative. The remedy commonly resorted to is the definition of the significant quantities using a non-local formulation In this approach the quantities are defined not directly in terms of the local values but as a function of the neighbouring values as well, within a length scale independent of the mesh size. A robust evaluation of the strains in a DIC results needs to be defined in terms of a length scale which is independent of the resolution The latter should be chosen at a similar scale to the FEM element size in order to ensure compatibility to numerical results. For the material under consideration, the thickness is 1.5mm which is in the same ballpark as the ideal mesh size for adaptively refined regions in numerical simulations for most automotive applications
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