Improved Pulse Shaping to Achieve Constant Strain Rate and Stress Equilibrium in Split-Hopkinson Pressure Bar Testing

Assuring a constant strain rate during dynamic testing is highly desirable to support the development of physically based predictive, constitutive material models. Many dynamic tests conducted on high-work-hardening materials, or materials that do not display a classic power-law-type hardening behavior, such as materials exhibiting complex sigmoidal concave-upward hardening (shape-memory alloys or a number of textured hexagonal metals due to deformation twinning), often result in continuously decreasing strain rates as a function of strain throughout the test. Incident pulse shaping has not been fully developed or successfully demonstrated over a large range of strain in high work hardening or complex-hardening materials. To shape an incident pulse for a constant strain rate in a split-Hopkinson pressure bar (SHPB) test, a high-strength, high-work-hardening rate (HSHWHR) material was selected to fabricate the pulse shaper. Several test sample materials, namely, 50-50 NiTi superelastic alloy, higher strength 60NiTi alloy, tungsten single crystals, interstitial-free (IF) steel, and MACOR (a glassy ceramic), which display a range of strength levels, work-hardening rates, and superelastic hardening behavior in the case of 50-50 NiTi, were tested in the SHPB with and without a pulse shaper at different temperatures and strain rates. The current experiments demonstrate that HSHWHR pulse-shaper materials are ideally suited to shape the incident pulse to achieve constant strain rates and achieve stress state equilibrium, while inherently dampening high frequency oscillations in the incident pulse.