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
CYLINDER FRAGMENTATION USING GAS GUN TECHNIQUES
T. F. Thornhill1, W. D. Reinhart2, L. C. Chhabildas2, D. E. Grady3, L. T. Wilson4
l
Ktech Corporation, Suite 400, 2201 Buena Vista SE, Albuquerque, NM 87106-4265
Sandia National Laboratories, Department 1610, PO Box 5800, Albuquerque, NM 87185
3
Applied Research Associates, 4300 San Mateo Blvd. NE, Albuquerque, NM 87110
4
Naval Surface Warfare Center, Dahlgren Division, Dahlgren, VA 22448-5000
2
Abstract. In this study an experimental technique for study of cylinder fracture fragmentation
characteristics has been developed on a two-stage light gas gun. This test method allows the study of
cylinder fracture fragmentation in a laboratory environment under well-controlled loading conditions.
Application of this technique allows measure of failure strain, strain rates, expansion velocity, and
fragmentation toughness. Results of several experiments on Aermet steel are presented*.
INTRODUCTION
development. AerMet® 100 steel was chosen
because it is a well-characterized material [2,3].
From these experiments observed strain, strain rate,
and fragmentation toughness measurements are
compared with the Aermet® 100 sphere on plate
and explosively driven cylinder experiments.
Past studies of properties critical to dynamic
fracture and fragmentation to support computational
model development and simulation have included
sphere on plate impacts and explosively driven
cylinder expansion.
In the present study, a
technique to expand a cylinder to failure on the
two-stage light gas gun has been developed. This
technique is an adaptation of tests performed by R.
E. Winter in 1979 [1]. A stationery plug is
impacted inside of a bored out cylinder, the plug is
trapped inside the cylinder by an anvil located on
the backside of the plug and cylinder. Late in time
the impact longitudinal momentum is translated
into a radial momentum in the plug, expanding the
cylinder and producing fracture and fragmentation.
This technique allows the precision application of
diagnostics including flash radiography, high-speed
photography, VISAR, and soft catch fragment
recovery.
Aermet® 100 steel fabricated to two hardness
levels are being investigated to validate the gas gun
technique, and further the computational model
TEST DESCRIPTION
The test technique uses a 50.80 mm long
AerMet® 100 cylinder precision machined to match
the gun launch tube muzzle bore of 13.08 mm
diameter. The cylinder wall thickness is 3.18 mm.
Two conditions of the Aermet® 100 material are
used, an AR (As-Received) treatment, and HT
(Heat-Treated). The material is heat treated by
performing a solution treatment, followed by
quenching, refrigeration, and then aging. The
major alloying elements are nickel, chromium,
molybdenum, and cobalt [4]. The average density
of the AerMet® 100 material used in this study is
7.99 ± .01 g/cm3. The cylinder is mounted directly
on the end of the gun muzzle using a precision
This work was supported by the U. S. Department of Energy under contract DE-AC04-94AL85000. Sandia is a multiprogram
laboratory operated by Sandia Corporation a Lockheed Martin Company, for the United States Department of Energy.
515
nine shot series to accommodate a variety of
different diagnostics and test purpose. Most
promising of test configurations is the free
suspended cylinder shown in figure 2. The cylinder
is glued directly to the muzzle with the anvil
assembly resting on a v-block against the back of
the cylinder and plug. This configuration provides
unobstructed radial access to the cylinder for x-ray
radiography, high-speed photography, and VISAR.
Fragments are still contained inside the first target
chamber of the gun range providing good recovery
statistics.
aligned freestanding mount, or gluing the cylinder
directly on the gun muzzle. A 25.4 mm long,
13.07mm diameter Lexan plug is inserted into the
rear half of the cylinder and the cylinder/plug
combination is against a Copper/Foam/Steel anvil.
The projectile is fabricated from Lexan to match the
cylinder plug to provide a symmetric impact
condition.
TEST DATA
Three shots were done using the free suspended
cylinder configuration, this data is presented below.
A detailed study of fragment distribution is
performed by weighing and measuring recovered
fragments individually for each shot. Table 1
summarizes the recovery results. The mean
fragment recovery by mass is 99.6% for any given
test. Radial failure strain is estimated from the
recovered fragments by measuring the fragment
wall thickness. The failure strain (sf) is defined in
equation 1, where w0 is the original cylinder wall
thickness, and Wi is the posttest fragment thickness.
In this study measurements were made on four to
eight large fragments per test. The mean failure
strain is reported in table 1.
Test
Cylinder
FIGURE 1. Initial test configuration
The initial test configuration is illustrated in
figure 1. The test configuration evolved during the
X-ray Head
£f = '
Acrylic
Fragment
Arrest Ring
(1)
Wo
Table 1. Experimental Summary
Test
No.
Test Cylinder
CF-6
CF-7
CF-10
Anvil
Assembly
Film Cassette
WQ — Wi
Projectile
Velocity
(km/s)
1.83
1.91
1.72
Material
Condition
No. of
Frag.
(8f)
HT
AR
HT
51
19
29
0.104
0.295
0.101
The radial diameter expansion velocity (Vex)
measured from the orthogonal radiographs of CF-6
is 0.48 km/s and exhibits symmetric expansion. An
estimate of strain rate (s*f) at failure made from
radiographs of CF-6 is 2.49E+4 s"1. This strain rate
is defined in equation 2, where Vex is the diameter
Camera ^^^i^
Lens
FIGURE 2. Suspended cylinder test setup
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A two beam VISAR, was located horizontally on
the side of the cylinder to measure the radial
expansion velocity. One beam was 12.7 mm from
the anvil, and the second beam was located 16.5
mm from the anvil. Figure 5 plots the cylinder
radial velocity and strain rate from the VISAR, time
is referenced to shock arrival at the cylinder
plug/anvil interface.
(2)
expansion velocity from the radiographs, and D0 is
the original cylinder diameter.
A Cordin rotating mirror camera was also used to
photograph the cylinder expansion process. The is
set to monitor cylinder fracture formation. The
camera magnification frames 49 mm of the cylinder
length providing high resolution pictures (figure 3)
of the expansion and fragmentation process. Frame
by frame measurement of the cylinder deformation
has been conducted at six locations and at the
maximum diameter that moves longitudinally with
time. Figure 4 plots strain vs. time as obtained by
the photographic deformation measurements. The
strain rate is measured from the slope of the linear
fit to strain vs. time data at each location. The
observed failure strain is defined as the maximum
strain at the time of the first visible crack formation.
0.30
Module 1,12.7 mm From Anvil
Module 4,16.5 mm From Anvil
-0.05
-1.0E-05
-5.0E-06
O.OE+00
5.0E-06
Q
QJT+QO
-5.1E+03
1.0E-05
Time (s)
FIGURE 5. Rate of expansion and strain rate of the cylinder as
determined by VISAR
Strain rate in fig. 5 is calculated by applying eq. 2
using the radial velocity and original cylinder
radius.
ANALYSIS
Grady-Kipp [5] model is used to calculate
fragmentation toughness. This relationship is shown
in equation 3, and is developed from an energybased theory of dynamic fragmentation .
FIGURE 3. High speed photography sample frame
0.5 T
24
The strain rate was measured on shots using
radiographs, photography, and VISAR respectively.
The mean fragment mass |i is determined from the
fragment mass distribution fit with a bilinear
exponential distribution of the form in equation 4.
Based on a Poisson statistical approach [5], N is the
total number of fragments, the coefficients A,
o.o
4.0E-06
6.0E-06
8.0E-08
1.0E-05
1.2E-05
1.4E-05
(3)
1.6E-05
Time (s)
FIGURE 4. Photographically observed strain, strain rate is 5-24
xlO 3 s"1 over a 17.5 mm interval starting 1.5 mm from the anvil.
Top line is maximum observed deformation irrespective of
longitudinal position.
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Table 2. Fragmentation Toughness and Strain Summary
Test Type
Gas Gun Cylinder
Gas Gun Cylinder
Material Condition
Failure Threshold
Strain
HT
AR
0.10-0.14
0.26-0.30
Maximum
Strain Rate xlO3
(s"1)
25
5-24
Fragmentation Toughness
Kf (MPa*m'/2)
72-95
105
of materials. This technique can provide fragment
distribution
statistics,
fracture
formation
characteristics, strain, strain rates, expansion
velocities, and fracture formation characteristics
supplementing computational studies and model
development to improve understanding of candidate
material fragmentation phenomena.
(4)
and £ represent the reciprocal of the mean fragment
size for the large and small fragments respectively,
and the coefficients N0l and N0S represent the
number of large and small fragments in eq. 4. For
these tests fragment distribution is dominated by the
larger fragments which account for the vast
majority of the fragmented mass, therefore the A,
term is used as the mean fragment mass (|i) for
calculation of the fragmentation toughness (Kf).
Table 2 summarizes the fragmentation toughness
and failure threshold strain for the gas gun cylinder
expansion.
REFERENCES
1. Winter, R. E., "Measurement of Fracture Strain at
High Strain Rates", in Inst. Phys. Conf-1979, Ser.
No. 47, Chapter 1, pp. 81-89
2. Chhabildas, L. C, Thornhill, T. F., Reinhart, W. D.,
Kipp, M. E., Reedal, D. R., Wilson, L. T., Grady, D.
E. "Fracture Resistant Properties of Aermet 100
Steel" in Int. J. Impact Engng., V26, (2001)
3. Chhabildas, L. C., Reinhart, W. D., Wilson, L. T.,
Reedal, D. R., Kuhns, L. D., Grady, D. E., "Dynamic
Properties of Aermet Steels to 25 GPa", Proceedings
ofEXPLOMET., 2000
4. Carpenter Technology Corporation, Alloy Data Sheet,
AerMet® 100
5. Grady, D. E., Kipp, M. E. International Journal of
Impact Engineering "Fragmentation Properties of
Metals", Vol. 20, pp. 293-308. (1997)
CONCLUSIONS
This study of AerMet® 100 in the initial
development of the fragmenting cylinder technique
has shown to be quite promising. The failure strain
for the as-received material is about half that of the
heat-treated material (.12, and .28 respectively) and
produces approximately half the number of
fragments. The expansion velocities of the two
material treatments are equivalent for the same
loading conditions although the heat-treated
material responds and fragments earlier in time
relative to the as-received. The as-received material
has a mean longitudinal fracture propagation
velocity of 1.3 km/s, and initial fracture formation
does not occur simultaneously around the
circumference. Additionally posttest inspection of
the fragments indicates some late time fracture
formation not evident in the first 25 jus of the
fragmentation process. Hoop strain producing shear
failure is the predominant failure mechanism for
these tests at maximum strain rates of 2.5 x 104 s"1.
Based on this series of cylinder fragmentation
scoping shots the gas gun technique is a useful tool
for studying fracture fragmentation characteristics
518