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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
ON THE SHOCK RESPONSE OF POLYCHLOROPRENE
J.C.F. Millett, N.K. Bourne, G.T. Gray III* and G. Cooper
Royal Military College of Science, Cranfield University, Shrivenham, Swindon, SN6 8LA, UK.
*Los Alamos National Laboratory, MS-G755, Los Alamos, NM 87545, USA.
Abstract. It is increasingly import to understand the high-strain rate response during impact of
polymeric materials due to their widespread use in automotive and aerospace applications. There are
three broad classes of polymer materials; elastomers, thermoplastics and thermosets. It is one of the
first of these three classes that is investigated in this work. Research has been conducted to determine
the Hugoniot of polychloroprene which was investigated using plate-impact experiments measuring the
stress at varying positions whilst simultaneously probing the shock velocity in the material and the
stress-particle velocity state. The shock-particle velocity data allows deduction of the constants c0 and S
for the material whilst the other measurements made allow plotting of the measured stresses with the
hydrodynamic curve to view the offset of the Hugoniot in a single series of experiments. For perfectly
elastic/plastic materials, the shear strength of the material is given by the offset of the Hugoniot and
hydrostat. In this material it is found to be either constant or to reduce slightly with increasing shock
stress. The shear strength of polychloroprene is explored further using embedded lateral stress gauges.
From these measurements, the shear strength has been determined as a direct experimental variable, and
it's variation with shock stress deduced.
INTRODUCTION
investigated for use as a transparent material used
as a window in interferometric experiments (1),
there is relatively few experimental investigations
on polymers. Knowledge of the response of
polychloroprene to mechanical stimuli is also
scarce, with high-rate data even more so. Some data
for polychloroprene does exist for specific
applications such as seismic events (2) and for use
as underwater rubbers (3). Interest has also been
expressed in the effect upon the mechanical
properties of polychloroprene by chemical attack
from hydrocarbons (4, 5). Basic shock properties of
polychloroprene above 7 GPa have been assembled
(6) and recently, polychloroprene has been
proposed as a backing material for manganin stress
gauges (7), where it was suggested that its close
match in impedance with the gauge backingmaterial would result in less imposed noise on the
gauge signal. In response to this lack of data, the
Hugoniot of polychloroprene has recently been
reported at low stress levels (8). It was observed in
that work that whilst the Hugoniot measured in
The response of polymeric materials to shock
loading has excited increased interest in recent
years. In particular, they are of significance as
components of composite materials, both in inert
systems such as automotive, aerospace, and armour
applications, and in energetic compositions, where
they are used as the binder phase in polymerbonded explosives. In both, a fundamental
requirement to understand the shock behaviour of
the individual polymeric constituents is necessary if
the response of the whole system is to be physically
modelled using a micromechanical approach.
Polymers fall into three broad categories;
thermoplastics such as polymethymethacrylate
(PMMA), thermosets such as epoxy resins and
elastomers such as polychloroprene. It is the latter
material, which is the subject of this paper.
However, the understanding of the response of
polymers to shock loading is not extensive. With
the exception of PMMA, which has been
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behaviour at high pressures for these materials may
give an indication of the mechanisms operating.
stress-particle velocity space lay on an extension of
the data of Marsh (6), the shock velocity-particle
velocity data was very different, indicating a
change in behaviour at ca. 6 GPa. Whilst no
definitive explanation was given, it was noted that
similar observations were made by Champion (9)
who investigated the shock response of
polytetrafluroethylene (PTFE). Here it was
suggested that this was due to changes in the
rotation of the CF2 units, from 180° every 13 units
to 180° every 9. It was thus suggested that similar
processes might explain the differences between the
data collected (8) and that of Marsh (6). Finally,
when the shock velocity data (which was used to
determine the hydrodynamic curve) was plotted
alongside the Hugoniot, the points lay within
experimental error of the measured stresses.
However, it was also observed that the
hydrodynamic curve calculated from measured
parameters, was slightly above the measured
stresses. If genuine, this would suggest that the
shear strength of polychloroprene (that is the offset
of the Hugoniot from the hydrostat) decreases with
increasing shock stress.
The observation of the complex response of
polymeric materials drives the experimentalist to
determine techniques that complement and check
upon measurements made using specific tools. The
aims of this work are thus two fold. Firstly, to
develop a novel arrangement by which the shock
velocity and the induced stress are measured using
a pair of Lagrangian measurement stations within
the target. Secondly, having deduced the
parameters necessary for the evaluation of the
hydrodynamic curve, to check the deduced values
of shear strength with those determined by the
direct measurement of the lateral and longitudinal
stresses during impact.
In doing so, it must be realised that there are
several assumptions involved in this process. The
first is that the shock velocity is indeed constant
and that two stations are sufficient to determine this
speed and work is in hand to address this issue. The
second is that the assumptions of the simple theory
are correct for these materials. The strengthening of
polymers with increasing shock stress requires
some mechanism analogous to the storage of
dislocations in metals to explain the hardening or
direct affect of the shock on elastic response. The
determination of the shear modulus and its
EXPERIMENTAL
Plate impact experiments were performed on a 5
m long, 50 mm bore single stage gas gun.
Manganin stress gauges (MicroMeasurements type
J2M-SS-380SF-025) were introduced to 11 mm
thick samples of polychloroprene in such an
orientation that renders them sensitive to the lateral
component of stress. The gauge position was 4 mm
from the impact surface. Voltage-time data was
reduced to stress-time using the methods of
Rosenberg and Partom (10), using a modified
analysis that does not require prior knowledge (11)
of the impact stress. Specimens were aligned to
better than a milliradian using an adjustable stage,
and velocity measured to within 0.1 % by the
shorting of sequentially mounted pairs of pins.
Impact stresses in the range 1.2 to 3.7 GPa were
achieved by firing flat and parallel flyer plates of
PMMA, dural (aluminium alloy 6082-T6) and
copper at velocities between 347 to 778 ms'1.
Specimen configurations showing gauge placement
are presented in Fig. 1.
Flyer,
Plate
Stress Gauge
FIGURE 1. Specimen configuration and gauge placement.
MATERIALS DATA
The properties of the polychloroprene used in
this investigation were density (p0) 1.42 g cm"3,
longitudinal sound speed (CL) 1.23 mm us"1, tensile
strength 5.0 GPa and elongation 250 %.
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Additionally, the shock parameters of this material
(CQ and 5) were determined to be 1.4 mm jas"1 and
4.0 respectively.
4- /
(1)
Where P is the hydrostatic pressure.
In Fig. 3, a typical lateral stress gauge trace
from the experiments conducted is shown. The
impact conditions were a 10 mm dural flyer at 538
m s"1, inducing a longitudinal stress of 1.04 GPa.
The resultant lateral stress is ca. 0.7 GPa. The trace
rises to a plateau and shows a gentle rise and then a
decrease thereafter. This is in contrast to other
elastomers such as estane and Kel-F where the
pulse top is flatter. On the other hand it does not
show the falling pulse found for the thermoplastic
PMMA.
RESULTS AND DISCUSSION
In Fig. 2, the shock Hugoniot of polychloroprene is presented using the data from this
work and reference (8).
0.8
I
0.6
o<
0.4
0
0.2
0.4
0.6
0.8
1
Particle Velocity (mm jus' )
0.2
I
FIGURE 2. Shock Hugoniot of polychloroprene. The curve is a
hydrodynamic curve calculated from the CQ and S terms
measured in the experiments described thus o^=poU&up, with
C/s=l.4+4.0 iip.
0
0
1
Time (jus)
It will be noted that the calculated pressures
derived from the measured shock velocities are
slightly higher than their measured stress
counterparts at 2 and 4 GPa. Although this may be
in part related to experimental error, it is also
possible that it is a genuine reflection of the
materials response to shock-loading. Note that it
would have been usual to include a strength term,
(i.e. the HEL) in calculating the pressure from the
shock velocity which makes the fact that it is higher
more interesting. It is felt that the data points
should, however, lie close to the hydrodynamic
curve since the quoted quasi-static uniaxial stress
strength, at 5.0 MPa, is very low, and at the
uniaxial-strain state stress levels investigated here,
will have minimal effect upon the final result. If the
effect is real, it would indicate a reduction in shear
strength (T), as can be seen from the well-known
relation,
FIGURE 3. Typical lateral gauge trace in polychloroprene.
The lateral stresses deduced from this, and the
other shots were used, in combination with
knowledge of the longitudinal stresses, to calculate
the shear strengths, using the well-known formula
2r = ax-<7y.
(2)
The results are presented in Fig. 4. The calculated
points from these experiments are shown with
errors largely determined from the uncertainty in
the lateral stress noted. There is an initial increase
in the strength with stress up to ca. 1 GPa but from
this stress onwards the strength is effectively
constant with increasing impact stress, at an
average level of ca. 0.33 GPa. This would be the
case if the HEL of the material were 1 GPa but this
is high given the measured quasi-static strength.
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Thus the interpretation of the rising portion of the
plot remains to be determined.
simple rationalisations of these phenomena cannot
explain these mechanical behaviours suggesting
that more complex consideration of the shock
response of polymers is necessary for a complete
understanding.
0.4
0.3
CONCLUSIONS
1.0.2
Lateral stress measurements have been made in
the elastomer, polychloroprene. From this, the
shear strength was calculated, and shown to be
effectively constant over the impact stress range
studied. The results here agree with previous
measurements of the Hugoniot, where differences
in the measured stress values and those calculated
from the shock velocity also indicated a near
constant shear strength.
0.1
0
#
1
2
3
Longitudinal Stress (GPa)
4
FIGURE 4. Shear strength versus impact stress in
polychloroprene. The straight line is a simple fit to indicate
trends.
REFERENCES
1. Barker, L.M. and Hollenbach, R.E. J. Appl Phys. 41 (1970)
4208-4226.
2. Buland, P., Dalbera, J. and Lafolie, R., in 10th European
conference on Earthquake Engineering, Sum, Editor. 1995,
Balkema: Rotterdam, p. 2029-2032.
3. Pillau, V.B. and Das, J.N. Plastics, Rubbers and Composites
Processing and Applications 18 (1992) 155-160.
4. Celina, M., Wise, J., Ottensen, D.K., Gillen, K.T. and
Clough, R.L. Polymer degradation and Stability 68 (2000)
171-184.
5. Unnikrishnan, G. and Thomas, S. Polymer 39 (1998) 19333938.
6. Marsh, S.P., LASL Shock Hugoniot data. 1980, Los
Angeles: University of California Press.
7. Marom, H., Sherman, D. and Rosenberg, Z., in Shock
Compression of Condensed Matter 1999, M.D. Furnish,
L.C. Chhabildas, and R.S. Hixson, Editors. 2000, American
Institute of Physics: Melville, New York. p. 597-600.
8. Millett, J.C.F. and Bourne, N.K. JAppL Phys. (2001) In
press.
9. Champion, A.R. /. Appl. Phys. 42 (1971) 5546-5550.
10. Rosenberg, Z. and Partom., Y. J. Appl. Phys. 58 (1985)
3072-3076.
11. Millett, J.C.F., Bourne, N.K. and Rosenberg, Z. J. Phys. D.
Applied Physics 29 (1996) 2466-2472.
12. Millett, J.C.F. and Bourne, N.K. J. Appl. Phys. 88 (2000)
7037-7040.
13. Batkov, Y.V., Novikov, S.A. and Fishman, N.D., in Shock
Compression of Condensed Matter 1995, S.C. Schmidt and
W.C. Tao, Editors. 1996, American Institute of Physics:
Woodbury, New York. p. 577-580.
14. Bourne, N.K., Millett, J.C.F., Barnes, N., and Belcher, I.
The deviatoric response of an epoxy resin to onedimensional shock loading, in These proceedings. (2001)
15. Bourne, N.K., Millett, J.C.F., Gray III, G.T. and Mort, P. On
the strength behaviour of Kel-F-800 and Estane polymers,
in These proceedings. (2001)
16. Barnes, N., Bourne, N.K., Millett, J.C.F. The shock
Hugoniot of an epoxy resin. These proceedings. (2001)
Similar measurements have been made in other
polymers such as PMMA, both by ourselves (12)
and Bat'kov et al. (13), an epoxy resin (14), Kel-F800™ and estane (15). However, in contrast to
polychloroprene, all those materials display a
rapidly increasing shear strength with increasing
impact stress. An increase in the shear modulus
(with pressure) could result in an increasing shear
strength with impact stress. The fact that such
behaviour does not occur in polychloroprene would
suggest that in this particular material, this may not
the case or higher longitudinal stresses are required
to see the effect. Other measurement of the
Hugoniot of an epoxy resin (16), where both stress
and shock velocity measurements were made,
showed that the calculated stresses from the shock
velocity were significantly lower than the directly
measured values. From equation 1, this suggests
that the shear strength is increasing with increasing
shock stress. This has been shown experimentally
in another paper (14) where shear stresses were
measured. Thus differences between the calculated
and measured Hugoniot in stress-particle velocity
space can be used to suggest the trends in shear
strength at different stress levels. However, the
trend seen here, where the hydrodynamic curve
plotted from measured values of shock velocity lies
above the stress points determined simultaneously,
lies at odds with the lateral stress measurements
showing a positive shear strength. It appears that
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