The inability to reliably predict the mechanical behavior of new automotive alloys during forming has generated strong demand for more advanced constitutive relationships and property data necessary to calibrate them. There is a particular need for models that incorporate combined kinematic and isotropic hardening in the sheet. The primary objective for these models is to increase the understanding of the complex loading around draw-beads, where calibration tests must include full-reversal of the loading (i.e., to probe both sides of the yield surface during plastic deformation). The test developed to generate the necessary property data subjects a tensile sheet specimen to a fully reversed uniaxial load cycle. Uniaxial loading produces a homogeneous stress/strain distribution in the gauge area of the specimen thereby enabling a better assessment of the springback that will occur during forming. While many designs for this test have been developed, proper assessment of the lateral forces required to prevent buckling during compression and the ensuing friction remains a considerable measurement challenge.
Our approach addresses this problem by using two opposing piezoelectric actuators in closed-loop control to simultaneously apply and instrument the forces on the anti-buckling guides. The principal advantage of this approach is, during the tensile segments of the load cycle, the actuators can fully retract the anti-buckling guides and eliminate the contact between the specimen and the anti-buckling guides. During compression, the lateral forces on the anti-buckling guides will be sampled in real-time, enabling direct assessment of the local friction conditions. In addition, this approach allows for variability in the specimen shape, the amount of applied strain, and the strain rate.