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 STRENGTH BEHAVIOUR OF KEL-F- 800™
AND ESTANE POLYMERS
N.K. Bourne, J.C.F. Millett, G.T. Gray III*, P. Mort
Royal Military College of Science, Cranfield University, Shrivenham, Swindon, SN6 8LA, UK.
*Los Alamos National Laboratory, MS-G755, Los Alamos, NM 87545, USA.
Abstract. One motivation to understand the shock response of polymers has arisen from the need to
understand and model the inert polymeric composites that are used in the aerospace and automotive
industries, and the reactive response of the polymeric binder constituents used in polymer bonded
explosives (PBXs) when subjected to impact loading. The latter materials need to be understood firstly
in relation to their long-term, high-temperature cycling under storage, and secondly to ensure that the
accidental loading of such materials remains below the levels where DDT processes may be initiated.
The effect of plate impact on the shock-induced damage evolution in estane and Kel-F-800™ is
presented. Lateral stress measurements were used to measure the strength of each polymer as a
function of impact stress. An increase of shear strength, determined in this manner, was noted for both
polymers and comparisons were drawn with others investigated using the same technique.
INTRODUCTION
both from the work of Marsh (3), and from Johnson
et al. (4). It is interesting to note that in this latter
work, the authors observed a change in slope in the
shock velocity (Us) - particle velocity (wp) curves.
They have suggested that this may be a reflection of
the non-linear behaviour of this material, noting that
computational results were insensitive to whether
linear or quadratic fits were used. In a recent
investigation of polychloroprene (5), a change in
shock velocity behaviour with particle velocity was
noted when comparing low stress data to the higher
values presented by Marsh (3). It was suggested
that this behaviour may be due to a shock-induced
phase change. Dynamic transformations at much
higher stresses (ca. 20 GPa) have been suggested
for virtually all polymers studied by Carter and
Marsh (6). The data for Kel-F-800™ is not as
extensive, but much of the recent work can be
found in the work of Anderson (7).
Apart from basic equation-of-state data, there
remains little information on other high-rate
mechanical properties for either polymer. The spall
strengths of both polymers have been previously
investigated by Bourne and Gray (8). Here they
As polymeric materials are used as the binder
phases of energetic composite materials such as
polymer-bonded explosives, it is becoming
increasingly important to understand their response
to high-strain-rate loading stimuli. Knowledge of
their explosive properties (1), and their behaviour to
mechanical events during production, transportation
and handling allows accidental reaction to be
avoided. It is thus perhaps surprising that there is a
paucity of data concerning the shock behaviour of
polymeric systems, the exception being polymethylmethacrylate (PMMA). Since this is transparent it
has found application as a backing window for rear
surface velocity measurements during shock
loading experiments (2).
In this investigation, the deviatoric response of
two polymers used as explosive binders has been
characterized. These materials are estane, a
polyester, polyurethane elastomer, and Kel-F-800™
(poly-chloro-trifluroethylene) which is a further
elastomer with low modulus. Shock Hugoniot data
for both materials is available, in the case of estane
653
GPa were induced by firing 5 and 10 mm plates of
PMMA, dural (aluminium alloy 82-T6) and copper,
in the velocity range 350 to 750 m s"1. Shear
strengths (T) were calculated from the measured
lateral stresses (cry), and known longitudinal stresses
(<7X) deduced from the known impact conditions via
impedance matching techniques, using the wellknown relation,
noted that Kel-F-800™ underwent a reduction in
spall strength with increasing impact stress whilst
estane displayed a finite value, although at a much
higher impact stress level. The authors also
observed that both materials had anomalous pulse
shapes, and whilst they offered no explanation as to
this behaviour, they noted that the shock response
of polymers is a largely unexplored area at low
impact stresses and worthy of additional research.
Thus it is the intention of this paper to investigate
the effect of shock loading on the shear strength of
these two polymers.
(1)
Kel-F-800™ has a longitudinal sound speed (CL)
of 1.74 mm j^s"1, shear wave speed (cs) of 0.77 mm
ps'1 and a density of 2.12 g cm"3. The properties of
estane are, cL=1.75 mm jus"1, cs=0.75 mm ps"1 and a
density of 1.19 g cm"3. Specimen configurations and
gauge placements are presented in Fig. 1.
EXPERIMENTAL
Ffy&r
RESULTS AND DISCUSSION
FIGURE 1. Experimental arrangement for lateral stress
experiments.
All shots were performed on a 50 mm bore, 5 m
long single stage gas gun. 12 mm thick targets of
Kel-F-800™ and estane were sectioned and
manganin stress gauges (Micro Measurements J2MSS-380SF-025) were introduced 4 mm from the
impact face in each case. Samples were
reassembled using a low viscosity epoxy adhesive
and held in a special jig for approximately 12 hours.
Afterwards, the impact faces were lapped flat and
parallel to 5 optical fringes (approximately 25 ^im
over 50 mm). The gauges were thus placed in an
impedance-matched material to the target. The
specimens were aligned using an adjustable
specimen mount, and impact velocities measured
via the shorting of sequentially mounted pairs of
graphite pins. Gauge data was converted from
voltage to stress using the methods of Rosenberg
and Partom (9), using a modified analysis that
requires no prior knowledge of the longitudinal
stress (10). Impact stresses in the range 0.6 to 3.5
Particle Velocity (mm /us~)
FIGURE 2. The shock Hugoniots of Kel-F-800™ and Estane.
The Hugoniots of both materials are shown in
Fig. 2. In addition to the points added from this
work, data for Kel-F-800™ was taken from that of
Sheffield and Alcon (11), whilst that for estane was
found in Marsh (3). The Hugoniot for Kel-F-800™
lies above that for estane reflecting its stiffer nature
which leads to greater impedance. Indeed its elastic
impedance is nearly twice that of estane.
In figures 3 and 4, lateral stress gauge traces are
presented for Kel-F-800™ and estane, respectively.
In the case of Kel-F-800™, it can be seen that the
lateral stress rises rapidly to a constant or near
654
The lateral stresses measured in figures 3 and 4,
in combination with the Hugoniot stresses from Fig.
2, have been used to calculate the shear strengths of
Kel-F-800™ and estane (using equation 1). The
results are presented in figure 5.
constant level before releases enter the gauge
location. It has been observed that in a polymer
such as PMMA (which is also below its glass
transition temperature) the rise of stress pulses is
ramped reflecting non-linearity in the shock
velocity-particle velocity plots (2). This is not
apparent in the traces of Fig. 3 with only a slight
rounding of the traces visible before a plateau is
reached. These traces also contrast with those of
PMMA in that the peak stress does not relax with
time (12).
1.5
I
1
a X= 3.50 GPa
0.5
aX - 2.56 GPa
I
CO
a. X= 0.59 GPa
0
0.5
1
1.5
Time (jus)
2
2.5
FIGURE 4. Lateral stress gauge traces in estane.
In both materials, there is an overall increase in
shear strength with increasing shock stress.
However, at higher stress levels, clear differences
can be seen. In estane, the shear strength increases,
whilst in Kel-F-800™, it reaches a peak value
before decreasing again. A possible reason for this
may be that Kel-F-800™ damages causing a
decrease in shear strength consistent with its more
brittle macroscopic response at ambient temperature
quasi-statically.
a=
1
a X= 0.95 GPa
0
o = 1.40 GPa
0
a-0.172
GPa
X
2
Time (jus)
FIGURE 3. Lateral stress gauge traces in Kel-F-800™.
Since Kel-F-800™ is below its ductile to brittle
transition at room temperature, this may be
reasonable and relaxation may not be favoured
since reorientation or rearrangement of the polymer
chains is not thermodynamically favoured.
In contrast, the two higher stress traces in estane
display a slight, but consistent decrease in the
lateral stress as a function of time. If it is assumed
that the longitudinal stress remains constant behind
the shock wave, then this behaviour suggests (from
equation 1) that there is a corresponding increase in
shear strength. Such behaviour has been noticed in
other polymeric systems such as PMMA (12) and
an epoxy resin (13), were it was suggested that this
was a manifestation of viscoelastic/viscoplastic
behaviour of these materials. Thus it is possible that
estane is behaving in a similar manner.
Clearly, the contrasting response of these two
elastomers begs several questions as to the
interpretation of the measurements made. Both
materials are open polymers but estane shows a
strong pressure dependency of its properties whilst
Kel-F-800™ does less so.
1
B-.
;-
^0.4
rsj
:
0.8
5^0.6
:
r
'.
0.2 0
* * * -'
o
*o
:
'.
o
+ Kel-F :
<> Estane ;
;
0 0.5
1 1.5 2 2.5 3 3.5 4
Longitudinal Stress (GPa)
FIGURE 5. Shear strength versus longitudinal stress in Kel-F800™ and Estane.
655
2. Barker, L.M. and Hollenbach, R.E., J, Appl Phys. 41,
4208-4226(1970).
3. Marsh, S.P., LASL Shock Hugoniot data, University
of California Press, Los Angeles, 1980.
4. Johnson, J.N., Dick, JJ. and Hixson, R.S., J. Appl,
Phys. 84, 2520-2529 (1998).
5. Millett, J.C.F. and Bourne, N.K., J. Appl. Phys. 89,
2576-2579(2001).
6. Carter, WJ. and Marsh, S.P., (1995), Hugoniot
equation of state of polymers, LA-12006-MS.
7. Anderson, M.U., in Shock Compression of Condensed
Matter -1991 (ed. S.C. Schmidt, R.D. Dick, J.W. Forbes,
and D.G. Tasker), Amsterdam: North-Holland, pp. 875878, (1992).
8. Bourne, N.K. and Gray III, G.T., in Plasticity 99:
Constitutive and Damage Modeling of Inelastic
Deformation and Phase Transformation, (ed. A.S. Khan),
Neat Press, Fulton, Maryland, pp. 619-622, (1998).
9. Rosenberg, Z. and Partom, Y., J. Appl. Phys. 58,
3072-3076 (1985).
10. Millett, J.C.F., Bourne, N.K. and Rosenberg, Z., J.
Phys. D: Appl. Phys. 29, 2466-2472 (1996).
11. Sheffield, S.A. and Alcon, R.R., in Shock
Compression of Condensed Matter 1991, (ed. S.C.
Schmidt, et al\ Amsterdam: North-Holland, pp. 909912, (1992).
12. Millett, J.C.F. and Bourne, N.K., Journal of Applied
Physics 88, 7037-7040 (2000).
13. Barnes, N., Bourne, N.K., Millett, J.C.F., and
Belcher, I., in Shock Compression of Condensed Matter
2001, (ed. M.D. Furnish, N. Thadani, and Y. Horie),
Melville, New York: American Institute of Physics, in
press, (2001).
14. Bourne, N.K., Millett, J.C.F., Rosenberg, Z. and
Murray, N.H., J. Mech. Phys. Solids 46, 1887-1908
(1998).
One might conjecture that brittle behaviour in
Kel-F-800™ might show features analogous to the
failure wave phenomenon seen in other materials
(14). However, rapid drops in strength indicative of
fracture are not observed and it is suggested that
this may be due to the larger strain-to-failure of the
polymer in comparison with brittle solids.
The spall strength in Kel-F-800™ has also been
found to decrease with increasing impact stress
levels (8), and thus these may be separate
manifestations of the same overall behaviour.
Further work is necessary to clarify these issues.
CONCLUSIONS
Lateral stresses have been measured in the
polymers Kel-F-800™ and estane, during plate
impact, using embedded manganin stress gauges. In
Kel-F-800™, lateral stresses are constant behind the
shock front whilst in estane, it appears that the
lateral stress decreases. Thus the shear strength
increases behind the main shock for an individual
shot. Where this has been observed in other
polymeric systems, it has been suggested that this is
due the viscoelastic/viscoplastic behaviour of the
material.
In both of the elastomers tested, shear strength
has been shown to increase with increasing impact
stress. However, in contrast to estane, where
strength increased continually in the impact stress
range investigated, Kel-F-800™ reached a peak
stress before decreasing again. It has been noted in
a previous paper that the spall strength of this
material showed similar trends (8).
These observations are consistent from shot to
shot and the materials are such that the gauges are
suited to the measurements required. The results
thus suggest that the simple assumptions in the use
of the gauge and the interpretations of polymer
behaviour under shock require careful consideration
to fully explain these observations.
REFERENCES
1. Tarver, C.M., Simpson, R.L. and Urtiew, P.A., in
Shock Compression of Condensed Matter 1995, (ed. S.C.
Schmidt and W.C. Tao), Woodbury, New York:
American Institute of Physics, pp. 891-896, (1996).
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