High strain rate tensile testing of sheet materials using three Hopkinson pressure bars

¹ Solid Mechanics Laboratory (CNRS UMR 7649), Department of Mechanics, École Polytechnique, Palaiseau, France
² Impact and Crashworthiness Laboratory, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge MA, USA

Abstract

In an attempt to circumvent the inherent problems associated with Split Hopkinson Tensile Bar (SHTB) systems, a new experimental technique is proposed for the high strain rate tensile testing of sheet specimens in Split Hopkinson Pressure Bar (SHPB) systems. Existing solutions employ a multiple gage section hat-shaped specimen to transform the incoming pulse from compression into tension at the specimen level. However, multiple gage section specimens are not suitable for studying the post-necking behavior of materials as the required symmetry of the mechanical system is no longer guaranteed in the post-necking range. Here, we present a new load-inversion device that is used in conjunction with three Hopkinson pressure bars for the tensile loading of single gage section sheet specimen. The device is designed to transform the compression load applied at its boundaries into a tensile loading of the specimen. Two output bars are used to measure the total force applied on the specimen boundaries, while Digital Image Correlation (DIC) is used to determine the strain histories on the specimen surface based on photographs acquired at a frequency of about 100 kHz. In addition to uniaxial tensile experiments on TRIP780 steel sheet specimens at strain rates ranging from 200/s to 1000/s, results are presented on the dynamic fracture testing of notched tensile specimens.