Sheet-Metal Assessment at Medium Strain-Rates by Digital Image Correlation

The aim of the paper is to present an experimental test methodology able to investigate the strain-rate influence on the mechanical behaviour of thin sheet metals at large strains. Dynamic tensile tests at medium strain rate were carried out using an electromechanical material testing machine and a specifically developed clamping device, which is able to create a traction pulse on the specimen by releasing the energy stored in a set of preloaded viscoelastic springs by means of the rupture of a sacrificial element. A numerical model of the testing system has been created to optimise the shape of the strain rate pulse. Due to sheet metal anisotropic behaviour, it is advisable to measure both axial and transverse strains and to assure that the stress field is purely axial. To fulfil both requirements, digital image correlation technique has been used for strain measurement on specimen surface. In fact, this method is capable to return full-field maps of all in-plane strain components. In addition, strain map analysis allows to detect when transverse stress components arise. To make use of image processing for strain and strain rate measurement, the specimen surface has been speckle-prepared and, because of the dynamic nature of the phenomenon in exam, a high speed camera, able to grab and store 1024 x 1024 pixel images at 1000 fps, was used. To verify the functionality of the testing system, tests on DP 600 and AISI 304 steels have been run in which strain rates up to 150 s-1 have been reached.