Advanced Biaxial Cruciform Testing at the NIST Center for Automotive Lightweighting

Abstract

Modeling of sheet metal forming operations requires mechanical properties data at very large tensile strains and various biaxial strain paths. Typically these data are developed along strain ratio paths that are linear and monotonic, but actual forming strain paths are nonlinear and not necessarily monotonically increasing. A unique planar-biaxial testing facility at the National Institute of Standards and Technology (NIST) has been designed to address non-linear strain paths and other long standing measurement needs. The system uses a combination of four independently controlled hydraulic actuators, with either displacement, force, or strain feedback control, to deform the material, while measurements of the material response is accomplished through a unique combination of digital image correlation and X-ray diffraction. Results of commissioning tests are presented for displacement and force control along different axes. The system was able to deform the sample in the elastic and plastic regimes. The results show the difference between the displacement and strain paths followed, as well as some unexpected behavior (e.g. buckling). Other expanded system capabilities for future use are briefly described.