A Novel High-Speed Bulge Test to Identify the Large Deformation Behavior of Sheet Metals

Background Despite the widespread use of quasi-static bulge tests to investigate the plastic deformation of sheet metals, a dynamic counterpart able to provide reliable measurements of the bulge pressure, displacement and strain fields at the sample surface is still missing.Objective Aiming at an in-depth identification of the mechanical response of sheet metals at high strain rates under nearly equibiaxial stresses, a novel high-speed bulge (HSB) test was developed.Method The working principle of the HSB setup combines the strengths of conventional split Hopkinson pressure bar (SHPB) and static hydraulic bulge facilities. The main innovation of the HSB test facility, compared to existing setups, is the possibility to implement high-speed stereo digital image correlation (DIC) measurements of the 3D-displacement and in-plane strain fields at the sample surface. Moreover, from strain measurements on the output Hopkinson bar, the time history of the pressure imposed to the sample is obtained.Results The potential of the novel technique is demonstrated by experiments on AA2024-T3 sheets. The measurements reveal a nearly oscillation-free pressure signal which indicates a stable sample loading. The material is deformed up to large levels of plastic strain at strain rates of about 300 to 350 s(-1). The strain rate at the sample apex has a stable value during most of the experiment. From the measurements, the material flow curves are calculated using the methodology presented in the ISO-16808:2014 standard for bulge experiments.Conclusion The ability of the proposed HSB test facility to capture bulge pressure, displacement and deformation fields of the entire sample surface, provides unique opportunities to investigate sheet metals behaviour under a nearly biaxial stress state at high strain rates. Furthermore, the available measurement data can be used to calibrate and validate complex, strain rate dependent plasticity models.