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
CONTROLLED FRAGMENTATION
Werner Arnold
European Aeronautic Defence and Space Company
TDW — Gesellschaftfur verteidigungstechnische Wirksysteme mbH
P.O. Box 1340, D-86523 Schrobenhausen, Germany
Abstract Contrary to natural fragmentation, controlled fragmentation offers the possibility to adapt
fragment parameters like size and mass to the performance requirements in a very flexible way. Known
mechanisms like grooves inside the casing, weaken the structure. This is, however, excluded for applications with high accelerations during launch or piercing requirements for example on a semi armor
piercing penetrator. Another method to achieve controlled fragmentation with an additional grid layer is
presented with which the required grooves are produced "just in time" inside the casing during detonation of the high explosive. The process of generating the grooves aided by the grid layer was studied
using the hydrocode HULL with respect to varying grid designs and material combinations. Subsequent
to this, a large range of these theoretically investigated combinations was contemplated in substantial
experimental tests. With an optimised grid design and a suitable material selection, the controlled
fragment admits a very flexible adaptation to the set requirements. Additional advantages like the increase of perforation performance or incendiary amplification can be realized with the grid layer.
mechanical stress raisers also weaken the casing
structure. To avoid this weakening effect, other
methods like roof shaped grooves in the high
explosive were proposed (3). Connected with this
technology, there arise other disadvantages of
having no longer a plain surface on the external side
of the high explosive charge.
INTRODUCTION
The detonation of a blast fragmentation warhead
delivers metal fragments to the target in near-miss
situations. Unfortunately, the natural fragmentation
process leads to a comminution of the shell case
into predominantly small fragments with low
performance. Preformed fragments could provide
the required perforation capacity, but their use is
precluded by another consideration: the high launch
accelerations of gun-tube weapons or piercing
decelerations of hard target defeat weapons militate
against the survivability of preformed fragment
warheads.
Controlled fragmentation is an alternative
approach to provide the desired terminal effects
while retaining survivability. Pearson (1,2) used a
grid system which is usually machined into the inner
surface of the warhead case. But these families of
NOVEL TECHNOLOGY
In the present paper, a novel technology is
described which avoids the problems mentioned
above. Figure 1 shows the principle on a cylindrical
test charge. Between the unimpaired shell case and
the high explosive, a grid layer with premade,
optimized notches is inserted. When the charge
detonates, this grid layer "just in time" provides
grooves in the case which decompose it in a con-
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trolled way, analogous to the method by Pearson.
This grid system layer is designed corresponding
with material and geometry of the case and in
accordance with the intended controlled fragment
mass and maybe further system frame conditions.
With this model, a variety of notch geometries
and material combinations were tested. The material
strength properties were also varied to enable
investigation of their effect on the groove formation. After some 3.0 usec, the detonation shock
wave arrives at the grid layer. After some 7.0 ps
grid layer and shock wave interactions with the shell
case have subsided so that the groove creation in the
casing is completed after mere 4.0 psec. Compared
with the fragmentation of the casing (2), this
process is very short and is achieved indeed "just in
time".
The casing grooves are created by the penetration of some kind of flying plate from the notch,
followed by the fast high explosive particles which
considerably increase the local stress in the casing
and thus, result in shock waves which propagate
into the material causing spall stresses near the free
surface of the case. For the example given in Figure
2, a relatively low spall strength was selected so that
these stresses entail spallation effects.
FIGURE 1. Novel technology of controlled fragmentation
with grid layer between casing and high explosive (from left to
right: shell case, grid layer, high explosive, explosive train).
Diamond shaped grids with an angle a of the
diamond of some 60° (Figure 1) have proved to
deliver satisfactory results.
The optimised grid design comminutes the case
into diamond shaped fragments, the theoretical mass
of which is the following:
Rj-R? pR
f \ i
a }
\~2
(1)
R0: outside casing radius
R^: inside casing radius
n: number of fragments per circumference
To better understand the mechanism during
fragmentation of the casing, hydrocode simulations
and experimental investigations have been carried
out.
FIGURE 2. HULL simulation of a typical groove formation
The picture of Figure 2 shows the shattering of the
case in the place of the notch with a material plot. In
the experimental investigations to be described
hereafter, the spallation effect was found to be far
less accentuated.
HULL SIMULATION
The model was set up in 2D. Only the creation
of the grooves, supported by the notches from the
grid layer was intended to be simulated. The tensile
stresses vertical to the created grooves which are
formed by the hoop stress during the expansion of
the cylinder case were not considered.
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EXPERIMENTAL INVESTIGATION
Groove Analysis
To validate the results from the hydrocode
simulations, an analogous experimental "2D set-up"
was chosen in Figure 3 on which no hoop stresses
are created by cylindrical expansion. The intention
was to test a variety of material combinations and
notch designs. The applied casing materials were
steel and Ti alloy. The grid layers were provided
from steel, zirconium (incendiary amplifier) and
plastics (PVC). The grid layer thicknesses used
were t=3 mm and t=6 mm. The various notches
were machined into the grid layer under an angle of
a/2 = 30°. Both the notch width and the "flying
plate" thickness were varied.
Measuring Point
FIGURE 4. Typical groove analysis: hardness and groove
depth measurement
Notch Width b [mm]
FIGURE 3. Test set-up
FIGURE 5. Influence of notch width on groove depth in
casing for a 8 mm Ti alloy casing
This test set-up allowed to vary several
parameters simultaneously. After the trial, the
grooves created in the casing plates were investigated in more detail. A typical example is shown in
Figure 4. The groove depth in the casing was
measured and the hardness relative to the cross
section. A range with strong plastic deformation and
increased Vickers hardness around the created
groove can be recognized.
Figure 5 presents as an example result the influence the notch width as a varying parameter
exerts with the different grid layer materials and
thicknesses on the groove depth in a 8 mm Ti alloy
casing. This example shows that the groove depth in
the casing increases while the notch width in the
grid layer also rises. Metals provide deeper grooves
than plastics. The same applies in the case of steel
for a 6 mm thick layer compared to a 3 mm layer.
Comparing the premade grooves by Pearson
(1,2) who already achieved good fragmentation
results with about 1 mm deep grooves in 6.4 mm
thick steel cases (groove depth: about 10-20% of
the case thickness), groove depths of several mm
thickness can be provided using the help of the grid
layers.
Concerning the casing material and thickness, an
optimum grid layer design can be achieved.
Additional requirements can still be considered with
this. For example, it is possible to double the
number of effective fragments when combining a
steel layer with a steel casing. Using plastics, the
weight can be reduced. It is also possible to control
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the ratio of high explosive mass C to total mass
C+M using the grid layer mass and thus, the kinetic
energy and impulse of the complete casing mass M
can be maximised. Finally, the grid layer material
can be selected in a way to allow for easier
machining of the notches into this material than it is
possible on high strength casing material.
Figure 7 shows a comparison of the performance
achieved with natural and controlled fragments
from a 8 mm thick high strength (1.2 GPa) Ti alloy
casing on the same charge. The controlled
comminution of the lower charge half was achieved
with a 3 mm thick Zr grid layer. The advanced
perforation performance in the lower section of the
10 mm thick mild steel target plate in 2 m stand-off
can be clearly recognized.
APPLICATIONS
On cylindrical test charges, the process was
successfully applied with a wide range of variation
parameters. The parameters comprised the
following:
• Low to high strength casing materials: steel
types and Ti alloy
• Casing calibers 50-150 mm
• Casing wall thicknesses: 3-15 mm
• Grid layer materials: steel, zirconium, plastics
(PVC)
• High performance explosives, also with blast
effects (with aluminum)
Grid
Layer •
FIGURE 7. Comparison of natural and controlled
fragments with advanced perforation performance of controlled
fragments
Casing: 8 mm Ti alloy, Grid layer: 3 mm Zr
Figure 6 shows the photo with the comminution
of a 11 mm thick high strength (tensile strength:
1.2 GPa) steel casing, caliber 145 mm with a steel
grid layer of 5.4 mm. The controlled fragments are
shown from either of the two sides (outside and
inside view). The additional fragments supplied by
the steel grid layer are not depicted.
CONCLUSION
As a summary of the above it is possible to
conclude that according to application purpose and
requirement the best suited grid layer can be determined while the casing structure does not have to
be weakened.
REFERENCES
1. Pearson, J., "The Shear Control Method of Warhead Fragmentation" in Proceedings of the 4th International Symposium
on Ballistics, Monterey, CA, 17-19 Oct. 1978
2. Pearson, J., "A Fragmentation Model Applied to Shear-Control Warheads", Naval Weapons Center, China Lake, CA, NWC
TP 7146, May 1991
3. Held, M., "Fragmentation Warheads" in "Tactical Missile
Warheads", Progress in Astronautics and Aeronautics, AIAA,
Volume No. 155, 1993.
Outside View
Inside View
FIGURE 6. Controlled fragments of a 1.2 GPa high
strength 11 mm thick steel casing
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