In-plane biaxial loading of sheet metal until fracture

The last decades have seen a tremendous effort to analyze and understand the strain path dependency of sheet metals until fracture for incorporation in advanced forming simulations. Compared to Nakazima tests, which are traditionally used for experimental verification of these simulations, in-plane biaxial loading (IPBL) has the advantage of well-known boundary conditions, full control of the followed strain path, and possibility for online microscopic inspection. The use of IPBL setups, however, is limited by the geometry of the cruciform specimens which are reported to fail away from the central region when tested until fracture. This work solves this issue by addressing the cruciform shape. Elasto-plastic FE simulations demonstrated that a significant thickness reduction in the cruciform center is necessary to reach fracture in the biaxial strain path. Specimens of this geometry are produced using electro-discharge machining (EDM) and extensive characterization using scanning electron microscopy revealed no significant microstructure distortion during the EDM process. A biaxial testing apparatus for IPBL until fracture was developed and digital image correlation was applied for local strain determination. The obtained local strains revealed good correlation with the simulations. © 2008 Society for Experimantal Mechanics Inc