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
RESOLVING MECHANICAL RESPONSE OF PLASTIC BONDED
EXPLOSIVES AT HIGH STRAIN-RATE USING SPLIT HOPKINSON
PRESSURE BAR
Vasant S. Joshi and Richard J. Lee
*Naval Surface Warfare Center, 101 Strauss Avenue, Indian Head, Maryland 20640
The mechanical properties of two explosives (PBXN-110 and PBXW-128) were determined using a
split-Hopkinson pressure bar at strain rates between 103 /s and 104 /s. The stress-strain data for 1, 2
and 3-wave analysis were compared to determine when stress equalization was achieved in the test
samples. PBXN-110 behaved similar to conventional Hopkinson bar samples, i.e., stress equalization
was maintained for most of the loading cycle. Stress equalization was not achieved until late in the
loading cycle for PBXW-128. This behavior eventually terminates during the compression process
yielding a uniform response.
INTRODUCTION
data obtained from these investigations are used in
modeling of two-part materials.
High strain rate testing and characterization of
materials using conventional techniques have been
extensively investigated and reviewed [1, 2]. New
composites containing a low strength matrix and high
strength particulate fillers are being used in structural
and automotive applications, These composites have
emphasized the need to develop special techniques
and methods to obtain accurate results during mechanical testing [3, 4]. Although a variety of composites have been tested using a split Hopkinson
pressure bar, the range of strain rates is invariably
tailored for specific application. Due to this consideration, prediction of high strain rate behavior of a
composite with analogous filler material (same size
but different strength) has been difficult. In order to
understand the mechanical behavior of such a composite, it is necessary to understand the effect on
elastic and plastic deformation characteristics of
changing the proportion of filler material to polymeric binder.
Polymeric binder -based explosives with various
solids loading are currently of interest to us. The
response of these materials to mechanical stimuli is
important in developing a fundamental understanding
of sensitivity and ignition. The mechanical properties
BACKGROUND
Explosive-loaded composite materials are currently in use in military applications. One of these
explosives, PBXW-128 was previously studied by
John et al. [5] at low strain rates (up to 1.2 x 103) and
by Tasker et al. [6] at higher strain rates (up to 3 x
104). The other composite explosive examined here,
viz. PBXN-110 has a binder matrix similar to that in
PBXW-128, but higher solid loading. Preliminary
results on high strain-rate behavior of PBXN-110
had large scatter [8], and therefore, new data is now
being reported.
Previous results from Tasker et al. [6] indicate a
two-slope behavior of PBXW-128. Their results
show scatter in the slopes at different strain rates.
Anomalous peaks observed by them could not be
attributed to any specific experimental condition.
Some of the samples were tested using aluminum
6061 bars and others were tested using PMMA bars.
The minimum strain rate reported was 2,500/sec.
Samples tested by John et al. [5] used 4150 steel
bars. The maximum strain rate reported was
1,100/sec. The data from these two sets of reports
701
cannot be compared since there was no range of
overlapping strain rate. They also differed in sample
sizes, 1/d ratio, which made comparison even more
difficult. During the current investigation, experiments were conducted in the intermediate range of
strain rates, with one experiment overlapping each of
the previous series for comparison.
tions were implemented before final data reduction,
resulting in better match between 1- and 2-wave
plots. These methods are described elsewhere in the
proceedings of this meeting [9].
Cylindrical test samples were fabricated out of
bulk PBXW-128 sheet. The samples were 9.5-mm in
diameter and 4.75 mm long, giving 1/d -0.5. All
samples were loaded for 200-|is, obtained by using
508-mm striker. The data was recorded on a Nicolet
Integra 40 (20 MS/s, 12 bit) digital oscilloscope.
EXPERIMENTAL
The Hopkinson bar system available at our facility allows variation in bar diameter (0.25" to 1.00")
and bar lengths (4'-6'). The original system consisted
of Ti6-4Al-V bars, 15.8-mm diameter, with 304-mm
long striker and 1.2 m long incident as well as transmitted bars. The data reduction scheme, consisting of
a dispersion correction as well as 1-, 2- and 3-wave
analyses [2] was employed to obtain stress-strain
plots. Initial results showed an apparent increase in
strain rates beyond a certain strain, and some scatter
in the stress-strain curves. The data set also showed
oscillations in the plots of both the 1-wave and 2wave analyses. These oscillations raised doubts
about the constant volume assumption during the
compression cycle.
Several high-speed extensometer and imaging
techniques were explored to resolve this issue [7].
Replacing Ti-6Al-4V bars with Al 7075-T6 bars for
a better impedance match only made the oscillations
more pronounced. Recently, comparison of reduced
strain gage data with the radial expansion measurements from high-speed digital photographic records
presented elsewhere [8] validated the constant volume assumption for PBXW-128 experiments.
Nevertheless, oscillatory behavior in the stressstrain curve could not be explained readily. In an
effort to refine the data reduction scheme, dispersion
correction routines were revisited. A single bar, 1.8m
long, with two sets of strain gages mounted at 0.6 m
apart replaced the incident bar. Data was recorded at
gage 1 and at gage 2 for various impact velocities
and dispersion corrections were applied. Source of
errors were identified by measuring wave velocities
at various impact pressures to obtaining appropriate
values. The correct wave velocity obtained was used
in dispersion correction routines. Wave velocities
were also corrected for the variations due to travel
through compressed and uncompressed regions of
the bar. These accurate inputs for dispersion correc-
RESULTS
Stress-strain curves from 1- and 2-wave analyses
for PBXW-128 at a strain-rate of 2,300/s is shown in
Figure 1. The strain rate gradually increases after
initial ring-up and stabilization, and the values indicated in Table 1 and Figures 2 and 3 are average
values beyond the initial oscillations.
0.00
0.40
0.50
FIGURE 1 Stress-strain and strain rates PBXW-128 at
2,300/s.
The difference between the one and two-wave
plots shown here would normally indicate a loss of
stress equalization in the sample resulting from some
internal damage, e.g., de-wetting of solids from the
binder. However, high-speed photographic data [8]
suggested that the samples were uniformly deformed.
The oscillations recorded from the transmitted gage
signal, as well as in 1-wave stress-strain curve appear
to be real, occurring only at intermediate strain levels
(0.05-0.25) and disappearing beyond a certain strain
(0.3).
Figure 2 and 3 show the stress-strain curves at
four different strain rates for PBXW-128 and PBXN702
110 respectively. Although four plots seem to overlap at lower strains, measured slopes on individual
plots clearly indicate that increasing strain rate leads
to an increase in the slope of the stress-strain curve,
as shown in table 1.
with their transition points for four different strain
rates. MI and M2 are the two slopes: first, slopel,
and the second, slope2, respectively, and e^ans is the
transition strain where material changes slope.
TABLE 1. Slopes and transition strains for samples.
Iope2 (M2)
3,500 /sec
40
35
3,100/sec
30
25
20
15
10
5
0
0.1
0,2
0.3
0.4
0.5
0.6
0.7
M!
1,700
2,400
3,100
3,500
1,700
2,400
3,100
3,500
20
20
22
25
33
25
30
33
M2
29
70
100
_
70
130
160
^Trans
-
.27
.44
.44
_
.35
.38
.38
In the current investigation, we have observed
that the slope changes gradually as a function of
strain rate, although the transition point remains essentially unchanged at about 45% strain for PBXW128. The corresponding transition point for PBXN110 occurs at a lower strain of 38%, which is consistent with the higher solid loading in that material.
The initial slope of about 20 MPa agrees with data
from John et al. [5], for the highest strain rates reported by them, as well as with the data of Tasker et
al. [6], at the lowest strain rate reported by them.
Since all the samples tested by us had identical
dimensions, we are able to distinguish minute differences in mechanical response. We prefer to obtain
higher strain rates by increasing striker velocity, but
there are limitations on the highest velocity that bar
material can withstand. For obtaining higher strain
rates than those reported, 1/d needs to be changed.
However, calibrations are needed for new 1/d ratios
at similar strain rates.
50
PBXN-110
Strain rate
128-001
128-002
128-003
128-004
110-001
110-002
110-003
110-004
DISCUSSIONS
FIGURE 2 Stress-strain plot for PBXW-128 at four different strain rates.
45
Sample #
0.8
Strain
FIGURE 3 Stress-strain plot for PBXW-110 at four different strain rates.
The oscillations and changing slope as seen in
these plots can be described as a process of squeezing of binder until adjacent crystals in the matrix
touch one another, at which point they continue to
slide against each other until there is no more space
to slide. Beyond this point, the material behaves as a
single solid. As a result of this compaction, the material appears to have stiffened, as indicated by an
increased slope of the stress-strain curve.
Table 1 shows the two slopes for each material
CONCLUSIONS
Two composite explosives tested under identical
conditions have revealed lower strain level lock-up
conditions for higher solid loading. The data agree
with previously published data for PBXN-128. Fur703
ther studies are in progress to obtain data at Mgher
strain rates than those reported here.
ACKNOWLEDGEMENTS
The Office of Naval Research (ONR) under the
Explosive Response Modeling Program supported
this work. Authors wish to thank Dr. Chak-Pan
Wong and Dennis Budd for their assistance in performing these experiments.
REFERENCES
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