CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
DYNAMIC CHARACTERIZATION OF COMPLIANT/BRITTLE
MATERIALS USING SPLIT HOPKINSON BAR
N. S. Brar* and Vasant S. Joshi**
*University of Dayton Research Institute, Dayton, OH 45469-0182
Naval Surface Warfare Center, 101 Strauss Avenue, Indian Head, Maryland 20640
Abstract. High strain rate compression response of thermoplasticoleflns (TPO) and explosives (Cast
TNT) is determined using the split Hopkinson pressure bar (SHPB). The conventional SHPB technique has been routinely used for measuring high strain rate properties of high strength materials. Attempts are underway in a number of research laboratories to use this technique to determine the high
strain rate behavior of compliant materials, such as plastics, rubbers, and foams. A split Hopkinson bar
consisting of 15.9-mm 7075 aluminum incident, transmitter, and striker bars was used to determine the
compressive response of TNT and other explosives. Initial tests were performed on aluminum 1100-O
to compare the stress-strain data with the published data. Stress-strain data on two types of TPOs and
TNT at strain rates in the regime 200-2000/s are presented.
INTRODUCTION
and transmitter bars. In both cases,useful data can
be obtained only by adapting the basic SHPB system
for these specific applications.
The objective of this paper is to present compression stress-strain data on TPOs and TNT at strain
rates in the range of 200-2000/s.
Mechanical property determination of extremely
brittle as well as soft polymeric material-based explosives has been a topic of considerable interest for
investigators in industrial and government research
laboratories. Stress-strain data at quasi-static and
high strain rates of theses materials are very crucial
to generate improved material models required to
determine the vulnerability and reactivity of munitions. These two classes of materials are in sharp
contrast to the normal metallic materials in their deformation characteristics. During dynamic testing,
deformation in soft polymeric materials may be nonlinear and strain may be more than 100% [1-2]. In
case of brittle materials, deformation is almost linear
and the failure strain is relatively small (<5%). Mechanical characterization of both types of materials is
difficult due to the problems associated with data
acquisition and resolution issues. In the case of
polymeric materials, transmitted signals through
specimen are extremely small. Brittle materials often
fail even before the stress equilibrium is reached at
the interfaces between the specimen and the incident
EXPERIMENTAL CONFIGURATION
The SHPB system used in this work was originally configured for 10-mm diameter steel pressure
bars to test metallic materials. Steel bars were replaced with 15.8-mm diameter magnesium bars to
test soft polymeric materials. Mechanical impedance
of magnesium matches more closely with that of
polymeric materials compared to that of steel or titanium. Although magnesium bar has a better impedance match with polymeric materials, its low yield
strength makes it unsuitable at higher strain rates.
This led to the choice of high strength aluminum
alloy (7075) as pressure bar material. A SHPB system using 15.8-mm diameter pressure bars was used
in the final configuration, as shown schematically in
Figure 1. The lengths of the incident, trans-mitter,
1323
and striker bars were 1.22-m, 1.22-m, and 0.30-m,
respectively. The bars are aligned in specially config ured bar supports to achieve impact planarity.
Pairs of 350-Q strain gages were mounted on the
incident and transmitter pressure bars 0.61-m away
from the specimen/bar interfaces. The striker bar is
launched at velocities of 5 to 15 m/s using a compressed helium gas gun.
Striker bar impact on the incident bar produces a
compressive stress pulse that propagates along the
length of the incident bar. The magnitude of the
stress pulse a is given by
a = pC0Vs
FIGURE 1. Schematic of the SHPB configuration.
RESULTS AND DISCUSSION
Compression tests were performed on three materials: (1) Relatively soft ductile alloy, aluminum
1100-0, (2) Thermoplasticolefm (TPO), and, (3) cast
TNT. Aluminum 1100-0 was chosen because of its
relatively low flow stress of about 100 MPa. Measured flow stress on 6.35-mm thick and 6.35-mm diameter AM 100-O specimens of, as shown in Figure
2, at strains of 5% and 10% agree within 2% of the
values reported earlier [2]. Such good agreement
convinced us that our SHE configuration and stressstrain data analysis are reliable.
where p ( 2800 kg/m3) is the density, C0 (4995 m/s)
is the sound speed in bar material (7075 aluminum),
and Vs (m/s) is the striker bar velocity. The stress
pulse width for a 0.51-m long aluminum striker bar is
200 jis. This compressive pulse in the incident bar
subjects the specimen to a compressive load. A portion of the incident compressive pulse, ei? is transmitted through the specimen £t and the remainder is reflected back in the incident bar £r. The amplitude of
the incident, reflected, and transmitted pulses are
recorded by the strain gages mounted on the pressure
bars. Using the recorded strains, the stress (a), strain
(e) and strain rate (8 ) in the specimen are determined using the following equations:
0
(2)
0.02
0.04
0.06
0.08
True Strain
FIGURE 2. Stress-strain curve for AM 100-0
(3)
Test specimens of both types of TPOs (R-2 and R3) measured 12.7 -mm in diameter and 3.8 -mm in
thickness. Experiments were performed at three
strain rates ranging from 800/s to 1,500/s. Stressstrain data on R-2 and R-3 TPOs at a strain rate of
1,400/s are shown in Figure 3 and 4 respectively.
Magnitudes of compressive stresses at strains of
10% and 20% compare well to those reported by
Brar and Simha [5].
where Ab and As are the cross-sectional area of the
pressure bar and the specimen in the gauge section,
respectively. L represents the gauge length of the
specimen. Stress, strain, and strain rate are average
values, and are determined by assuming a uniform
uniaxial stress-state condition [3-4].
1324
stress pulse of 140 jis corresponding to extremely
low failure strain value of (1-1.5%).
A high-speed analog camera (Imacon) was used to
observe the failure of TNT specimens during the
loading phase of the specimen. It was observed that
specimens fail prematurely before reaching the dynamic stress equilibrium even when the loading pulse
has a long rise time of ~3 0-40 |as[6]. As the testing of
TNT progressed, steps were taken to improve the
pulse shaping process in order to avoid premature
failure of the specimens.
FIGURE 3(a). Stress-strain curves for R-2.
TNTrOOt, strain rate 275/s
TPOr3-1, strain rate 1,400/s
0.000
0.005
0.010
0.015
Engg. Strain
FIGURE 4(a). Stress-strain curves for TNT at a strain rate
of 275/s without pulse shaping
FIGURE 3(b). Stress-strain curves for R-3.
Stress-strain curves for TPOs (Figure 3 a and 3b)
in the present work are obtained using 15.8-mm diameter aluminum bars and, therefore, are relatively
smooth (with higher resolution and strain rates)
compared to those determined using 25.4-mm diameter aluminum bars [5].
TNT specimens were fabricated from cast samples. Specimens measured 9.5 -mm in diameter and
were 4 -to 9 -mm thick. A Lexan test chamber (300mm x 250-mm) enclosed the specimen/bar interface
portions of the SHE. In the preliminary tests
PMMA/plastic disks of appropriate thickness (total
of 2 -mm) were placed at the striker bar/incident bar
interface to produce long rise time of -30-40 (is input stress pulses in the incident bar (pulse shaping).
Measured stress-strain curves for TNT specimens are
shown in Figure 4. The data clearly suggests that
specimens fail during the first 30-40 JLIS of the entire
TNTr002, strain rate 275/s
i. 15
0.010
0.005
Engg. Strain
FIGURE 4(b). Stress-strain curves for TNT at a strain rate
of 275/s with pulse shaping
Frew [7], based on his experience in characterization of brittle materials (concrete and ceramics), recommended using a linear ramp stress pulse of shorter
duration. A simple wave analysis model developed
1325
by him suggested placing a 3-mm diameter annealed
copper disk of appropriate thickness at the incident/striker bar interface to produce the linear ramploading wave. This approach of optimizing the shape
of loading pulse is currently being followed to perform further tests on TNT specimens.
photography to augment split Hopkinson pressure
bar measurements of energetic materials," in this
Proceedings.
7. Frew, D., Private communication, 2000.
CONCLUSIONS
Compression stress-strain curves at strain rates
of 200-2000/s for two types of TPOs are generated
using SHPB technique. These curves are smoother
than those reported earlier [3]. Preliminary high
strain rate data on TNT are presented. A proper loading stress pulse shaping procedure needed to avoid
premature failure of the TNT specimens during the
initial compressive loading phase is currently being
pursued.
ACKNOWLEDGEMENTS
This research was supported by NSWC (Indian
Head, MD) under the ASEE senior faculty summer
fellowship awarded to one of the authors (NSB) during summer 2000. The research was conducted under grant from Navy EOD Tech Center, Indian Head,
MD.
REFERENCES
1. Sawas. O.," High Strain Rate Characterization of
Low-Density Low-Strength Materials," Ph. D.
Thesis, The University of Dayton, Dayton, Ohio
(1997).
2. Sawas, O., Brockman, R. A., and Brar N. S., "Dynamic characterization of compliant materials using an All-Polymeric Split Hopkinson Bar," Exp.
Mech., 38, 204-10 (1998).
3. Nicholas, T., "Tensile testing of materials at high
rates of strain," Exp. Mech. 21, 117-185 (1980).
4. Lindholm, U. S., " Some experiments with split
Hopkinson pressure bar", J. of Mech. and Phys.
of Solids, 12,317-22(1964).
5. Brar, N. S. and H. Simha, "Strain-rate sensitivity
of TPO," Proceedings of 6th International Conference on TPO's in Automotive'99, Novi, MI,
120-126, October 1999.
6. Lee, R. J., and Joshi, V. S., "Use of high-speed
1326