Measurement of sheet-metal planar anisotropy at medium strain-rates by digital image correlation

The knowledge of deformation behavior at medium strain rates is very important because they are typically reached in many industrial forming processes. A novel test methodology to investigate the strain rate influence on planar and normal anisotropic behavior in thin metals is presented. Dynamic tests at medium strain rate were carried out using an electromechanical material testing machine equipped with a specialized clamping system that contains a set of preloaded springs. When released, the springs create a tensile pulse on the specimen that may reach a deformation speed of 100 s-1. Spring release is caused by the rupture of a sacrificial specimen, clamped together with the tested one, but differently shaped, so that it fails when the other one is still almost undeformed. The surface of the tested specimen is prepared with a random speckle pattern by spray painting. Deformation is measured by digital image correlation of adjacent frames of a digital high-speed camera with up to 7500 frames per second at 256 x 384 pixels. To cope with the huge amount of data the triggering of image acquisition and efficient image processing algorithms are vital. Results of simulation and experimental validation are presented and compared to demonstrate the functional capability of this new testing methodology.