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.

[1]  Guruswami Ravichandran,et al.  Critical Appraisal of Limiting Strain Rates for Compression Testing of Ceramics in a Split Hopkinson Pressure Bar , 1994 .

[2]  Jia-Lin Tsai,et al.  Use of split Hopkinson pressure bar for testing off-axis composites , 2001 .

[3]  M. J. Forrestal,et al.  A split Hopkinson pressure bar technique to determine compressive stress-strain data for rock materials , 2001 .

[4]  A. Lomunov,et al.  Methodological aspects of studying dynamic material properties using the Kolsky method , 1995 .

[5]  D J Parry,et al.  Materials testing at high constant strain rates , 1982 .

[6]  W. Baker,et al.  A Split Hopkinson Bar Technique to Evaluate the Performance of Accelerometers , 1996 .

[7]  Fangyun Lu,et al.  High-strain-rate compressive behavior of a rigid polyurethane foam with various densities , 2002 .

[8]  Howard Kuhn,et al.  Mechanical testing and evaluation , 2000 .

[9]  Sia Nemat-Nasser,et al.  Hopkinson techniques for dynamic recovery experiments , 1991, Proceedings of the Royal Society of London. Series A: Mathematical and Physical Sciences.

[10]  M. J. Forrestal,et al.  A split Hopkinson bar technique for low-impedance materials , 1999 .

[11]  Kenneth S. Vecchio,et al.  Response of NiTi shape memory alloy at high strain rate: A systematic investigation of temperature effects on tension–compression asymmetry , 2006 .

[12]  M. J. Forrestal,et al.  Dynamic small strain measurements of a metal specimen with a split Hopkinson pressure bar , 2003 .

[13]  D. J. Parry,et al.  Hopkinson bar pulse smoothing , 1995 .

[14]  Nancy A. Winfree,et al.  Compressive superelastic behavior of a NiTi shape memory alloy at strain rates of 0.001–750 s−1 , 2001 .

[15]  Sia Nemat-Nasser,et al.  High Strain-Rate, Small Strain Response of a NiTi Shape-Memory Alloy , 2005 .

[16]  J. Duffy,et al.  On the Use of a Torsional Split Hopkinson Bar to Study Rate Effects in 1100-0 Aluminum , 1971 .

[17]  Weinong W Chen,et al.  Pulse shaping techniques for testing brittle materials with a split hopkinson pressure bar , 2002 .