A Novel Specimen Design for Multiaxial Loading Experiments at High Strain Rates

A specimen geometry that has four flat dog bones circumferentially arranged around the axis of the sample is proposed for combined tensile-torsional loading experiments. Finite-element modelling was implemented to optimise the design and achieve appropriate deformation and failure in both tensile and torsional loading conditions. The capability of the proposed specimen configuration is demonstrated via an experimental campaign on commercially pure titanium at various strain rates. The quasi-static tests were conducted using a universal screw-driven testing machine, whereas the high-rate experiments were carried out on an in-house designed combined tension-torsion Hopkinson bar system. A wide range of stress states were obtained using the ligament specimen, covering uniaxial tension, shear, and different combinations of tension and shear. Three distinct failure modes of the ligament specimens subjected to monotonic tension, monotonic torsion, and combined tension-torsion loading at high strain rates are presented and discussed. The quasi-static and high-rate ultimate stress loci will be constructed using direct experimental measurements to assess the strain rate sensitivity of the material.