ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation
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113. F. Gascon, A. Bayon, and A. Varade, Measurement of Longitudinal Vibrations in a Slender Rod
by Optical Heterodyne Interferometry, Eur. J. Mech.-A/Solids, Vol 17, 1998, p 167–177
114. S. Nemat-Nasser, J.B. Isaacs, and J.E. Starrett, Hopkinson Techniques for Dynamic Recovery
Experiments, Proc. R. Soc. (London) A, Vol 435, 1991, p 371–391
Classic Split-Hopkinson Pressure Bar Testing
George T. (Rusty) Gray III, Los Alamos National Laboratory
Acknowledgments
The author would like to thank Thomas A. Mason and George C. Kaschner for their assistance in this work.
Helpful discussions with Stephen Walley, John E. Field, Neil K. Bourne, Paul F. Follansbee, Robert K. Garrett,
D. Goto, and Kenneth S. Vecchio are gratefully acknowledged. This work was supported in part by the U.S.
Department of Energy.
Classic Split-Hopkinson Pressure Bar Testing
George T. (Rusty) Gray III, Los Alamos National Laboratory
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Experiments, Proc. R. Soc. (London) A, Vol 435, 1991, p 371–391
Recovery Hopkinson Bar Techniques
Sia Nemat-Nasser, University of California, San Diego
Introduction
THE CLASSICAL split-Hopkinson bar technique is described in separate articles in this Section for tension,
compression, and torsion tests. For all three cases, the stress pulse travels along the incident bar and is partly
transmitted to the transmission bar by the specimen sandwiched between the two bars. The remaining part of
the stress pulse is reflected back in the incident bar as tension in the compression bars, as compression in the
tension bars, and as reverse torsion in the torsion bars. Both the transmitted and reflected pulses travel along the
corresponding bar and are then reflected back and eventually reload the sample. If the sample does not fail in
the course of the first loading, it will then be subjected to repeated loading by these elastic waves traveling back
and forth along the bars.
There are many applications where it is necessary to subject a sample to a single loading and then recover it,
without the sample having been subjected to any additional loads. Such a capability allows, for example, to
relate the microstructural changes to the loading history or, as is shown later, to produce isothermal stress-strain
relations for certain metals, at high strain rates (Ref 1, 2, 3).
For compression tests, special fixtures, such as “stopper rings,” can be used to limit the total axial strain of the
sample and to transmit the remaining compression pulse through the stopper ring once the sample length equals
that of the ring (Ref 4, 5, 6). A similar technique, however, does not apply to the tension or torsion tests.
Moreover, even in compression, such a technique works only if large axial permanent straining of the sample is
involved, and even then, the sample with the stopper ring is repeatedly loaded elastically. For hard, brittle
materials, such as ceramics and their composites, the total axial strain to failure is very small, and, therefore, a
“stopper-ring approach” is difficult, if not impossible, to implement. For this class of materials, once the initial
compressive pulse has produced microcracks in the brittle sample, the subsequent reflected compression pulses
may shatter the specimen, making recovery essentially impossible by that technique. For tension tests, and in
the absence of wave trapping, both the transmitted and reflected pulses load the sample repeatedly.
The stopper-ring approach fails also when recovery tests are to be performed at high temperatures. For high
strain-rate tests at elevated temperatures, it is necessary to heat the sample to the required temperature, while
keeping the incident and transmission bars at suitably low temperatures. If the bars are in contact with the
specimen within the furnace, their temperature will increase, having a variable distribution along the bars. This
affects the elastic properties of the bar and, hence, the stress pulses. Moreover, the bars being good heat
conductors (usually of maraging steel), the considerable heat loss that occurs makes controlling the experiment
difficult.
To remedy these major problems, techniques have been developed where, for compression and tension tests
(Ref 7), both the reflected and transmitted pulses are trapped at the far end of the bars, once the sample is
subjected to the initial loading. Moreover, this can be done at elevated temperatures, where the sample is heated
within an attached furnace, with the Hopkinson bars held outside the furnace. These bars are then brought into
contact with the specimen microseconds before the stress pulse reaches the end of the incident bar. The method
ensures that the sample is subjected to a single stress pulse with a time variation that is also fully controlled.
In addition to the compression and tension recovery Hopkinson bar techniques, a technique has been worked
out that allows recovery of the sample after it has been subjected to a compression followed by a tension pulse
(Ref 7). This technique permits the study of the Bauschinger effect and the strain-rate history effects on the
mechanical properties and microstructural evolution of materials under various dynamic loading conditions
(Ref 8, 9).
The dynamic tests of hard materials (e.g., ceramics) at elevated temperatures present a new challenge. At
suitably low temperatures, the sample strain can be measured directly by placing a strain gage on the sample
(Ref 10, 11, 12, 13). This, however, may not be possible at elevated temperatures. Even at low temperatures,
indentation of the bars by the sample must be prevented in order to obtain reliable results and preclude
damaging of the bars. This latter problem is solved by using properly confined tungsten carbide (WC) inserts,
placed between the sample and the bars (Ref 12). The impedance of the inserts is carefully matched with that of
the bars. For strain measurements at high temperatures, a new technique is differential strain measurement
(DSM), which can be used to obtain accurate and reliable values of the strain in the dynamic test of very hard
ceramics at elevated temperatures. This method is briefly discussed in the section “High-Temperature, High
Strain Rate Testing of Hard, Brittle Materials” in this article.
References cited in this section
1. S. Nemat-Nasser and J.B. Isaacs, Direct Measurement of Isothermal Flow Stress of Metals at Elevated
Temperatures and High Strain Rates with Application to Ta and Ta-W Alloys, Acta Metall., Vol 45,
1997, p 907–919
2. S. Nemat-Nasser and Y. Li, Flow Stress of FCC Polycrystals with Application to OFHC Cu, Acta
Mater., Vol 46, 1998, p 565–577
3. S. Nemat-Nasser, W. Guo, and M. Liu, Experimentally-Based Micromechanical Modeling of Dynamic
Response of Molybdenum, Scr. Mater., Vol 40, 1999, p 859–872
4. K.H. Hartman, H.D. Kunze, and L.W. Meyer, Metallurgical Effects in Impact Loaded Materials, Shock
Waves and High-Strain-Rate Phenomena in Metals, M.A. Meyers and L.E. Murr, Ed., Plenum, London,
1986, p 325
5. S. Nemat-Nasser, J.B. Isaacs, G. Ravichandran, and J.E. Starrett, High Strain Rate Testing in the U.S.,
Proc. TTCP TTP-1 Workshop on Techniques of Small Scale High Strain Rate Studies, 26 April 1988,
Materials Science Laboratory, Melbourne, Australia, 1988
6. S. Nemat-Nasser and S.N. Chang, Compression-Induced High Strain Rate Void Collapse, Tensile
Cracking, and Recrystallization in Ductile Single and Polycrystals, Mech. Mater., Vol 10, 1990, p 1–17
7. S. Nemat-Nasser, J.B. Isaacs, and J.E. Starrett, Hopkinson Techniques for Dynamic Recovery
Experiments, Proc. R. Soc. (London) A, Vol 435, 1991, p 371–391
8. A. Thakur, S. Nemat-Nasser, and K.S. Vecchio, Bauschinger Effect in Haynes 230 Alloy: Influences of
Strain Rate and Temperature, Metall. Mater. Trans., Vol 27A, 1996, p 1739–1748
9. A. Thakur, S. Nemat-Nasser, and K.S. Vecchio, Dynamic Bauschinger Effect, Acta Mater., Vol 44,
1996, p 2797–2807
10. W.P. Rogers, J.B. Isaacs, and S. Nemat-Nasser, Effect of Microstructural Damage on Ultrasonic
Velocity and Elastic Moduli of Partially Stabilized Zirconia, Review of Progress in Quantitative
Nondestructive Evaluation, D.B. Thomson and D.E. Chimenti, Ed., Plenum, New York, 1989, p 1827–
1832
11. W.P. Rogers and S. Nemat-Nasser, Transformation Plasticity at High Strain Rate in Magnesia-Partially-
Stabilized Zirconia, J. Am. Ceram. Soc., Vol 73, 1990, p 136–139
12. V. Sharma, S. Nemat-Nasser, and K.S. Vecchio, Dynamic-Compression Fatigue of Hot-Pressed Silicon-
Nitride, Exp. Mech., Dec 1994, p 315–323
13. G. Subhash and S. Nemat-Nasser, Dynamic Stress-Induced Transformation and Texture Formation in
Uniaxial Compression of Zirconia Ceramics, J. Am. Ceram. Soc., Vol 76, 1993, p 153–165
Recovery Hopkinson Bar Techniques
Sia Nemat-Nasser, University of California, San Diego
Momentum Trapping for Hopkinson Bars
In a tension recovery Hopkinson bar, both the reflected compression and the transmitted tension pulses must be
trapped once the loading of the sample is completed. In a compression bar, on the other hand, only the tension
pulse, which reflects off the sample, must be trapped since the transmitted compression cannot reload the
sample.
Figure 1(a) shows the momentum trapping scheme for a tension pulse. The free end of the bar terminates with a
transfer flange F in contact with a momentum-trap tube MT, which has the same impedance as the bar (e.g., the
tube has the same cross-sectional area and is made of the same material as the bar). The tensile pulse T in Fig.
1(a) reflects off the free end F of the transfer flange as compression. This compressive pulse is then fully
transferred into the momentum-trap tube MT in contact with the flange because of the matched impedances.
The compression then reflects off the free end of the tube as tension and is trapped in the tube, which begins to
move away from the transfer flange once the reflected tension reaches the tube's end in contact with the flange.
This process is similar to transferring compression pulses across contacting bars with fully matched
impedances. As shown in Fig. 1(b), the compression pulse C will be completely transmitted from bar A to bar B
across the contact surface S
c
when the bars have the same impedance. This transmitted compressive pulse then
reflects from the free end of the bar B as tension and is fully trapped in B.