The Development of Split Hopkinson Tension-Torsion Bar for the Understanding of Complex Stress States at High Rate

Abstract

A novel Tension-Torsion Split Hopkinson Bar apparatus conceived to achieve an arbitrary combination of tension and shear at high strain rates in a single loading event is proposed. The apparatus allows for the population of the failure and yield surfaces with experimental data and, in turn, for a better understanding of materials during deformation and failure. The proposed loading system enables combined direct-shear stress pulses of duration exceeding one millisecond to be applied to the sample under consideration; hence, allowing for the achievement of a wider range of strain rates than what currently exists in the open literature. Force, torque, and velocity histories at the interface between bars and specimen are measured via a data analysis procedure based on the method of characteristics and on D’Alambert’s solution of wave equations. Additional velocity measurements are conducted by means of photon Doppler velocimetry. The capability of the apparatus is validated by means of bar-attached tests and will be further demonstrated via experiments on commercially available pure titanium. The rate-dependent behavior of the material subjected to complex stress states will be analyzed and reported. The failure surface of specimens subjected to a prescribed combination of tension and torsion will be assessed by means of optical profilometry and scanning electron microscope.