The split Hopkinson bar bulge setup: a novel dynamic biaxial test method

Abstract

In sheet metal forming, very often, large plastic deformations are imposed to a thin plate. An accurate description of the material’s elastoplastic response is therefore of paramount importance to perform finite element (FE) simulations of an actual forming operation. Reliable stressstrain data till significantly larger strains compared to tensile tests can be identified by means of bulge test. In this work, a dynamic hydraulic bulge test is proposed. The novel split Hopkinson bar bulge setup, combines features of classical split Hopkinson pressure bar (SHPB) and hydraulic bulge tests. The special configuration of the Hopkinson bars leaves the sample surface fully accessible. As such, high-speed optical measurements can be performed on the sample surface allowing the application of, for instance, digital image correlation (DIC) for full-field displacement strain mapping. The potential of the facility is explored by performing experiments on 0.8mm thick Al2024-T3 sheet.

On the Anisotropic Mechanical Response of Ti6Al4V Sheet at High Strain Rates

“…Three nominal strain rates, i.e., 8 • 10 −5 , 0.5 and 1000s −1 , are considered. A novel high-speed bulge (HSB) test developed at Ghent University [10], designed to deform sheet metals in nearly equibiaxial loading conditions and high strain rates, is used for the dynamic bulge test. The tensile results reveal the influence of strain rate on the Lankford coefficient for the experiment along TD, namely a reduction of about 75% with respect to the quasi-static condition, while no variations on the normalized stress were observed.…”

“…For the HSB experiment, optical measurements at the sample surface are carried out using two highspeed Photron mini AX200 cameras positioned under an angle to allow stereo imaging with an acquisition rate of 36 • 10 3 fps. More information regarding the HSB setup can be found in [10].…”

“…The observed strain rate sensitivity and adiabatic heating effects are modeled using the Johnson-Cook constitutive law. Additionally, a dynamic bulge test is performed using the novel highspeed bulge (HSB) test developed at Ghent University [10]. The experimental results clearly show the need of including the effect of strain rate, and eventually temperature, on the formulation of the yield function in order to correctly model and predict deformation processes performed at high speed.…”

“…Indeed, during the HSB test, the sample is accelerated by a dynamic load, resulting in the vibration of the sample. The ringing of the sample is reflected in the radius of curvature, and thus in the bulge stress[10]. Nevertheless, Fig.9gives a valuable insight into the effect of a high strain rate on the mechanical behaviour of the tested material under nearly equibiaxial stress state.…”