JET BREAK-UP CHARACTERIZATION OF
MOLYBDENUM SHAPED CHARGE LINERS
E.L. Baker*, A. Daniels, J. Pham, T. Vuong, S. DeFisher
U.S. Army, Armament Research, Development and Engineering Center
Picatinny Arsenal, NJ 07806-5000
ABSTRACT
The development of molybdenum lined shaped charges is a relatively new area of investigation, with recent emphasis placed
in producing increased jet ductility. Excellent jet ductility has now been successfully produced from a variety of molybdenum
shaped charge liners. Conventional forging and high energy rate forming (HERF) have been used to produce a variety
molybdenum shaped charge liner preforms including hemispheres, cones and trumpets. Most liner forgings exhibited good
formability using both conventional forging and HERF processing. Liner forgings have been fabricated from arc cast and
powder metallurgy molybdenum, as well as one from a single crystal. Grain size and geometry have been investigated using
scanning electron microscopy. Neutron diffraction has been used to investigate material texture through samples taken from
the base and apex regions for some of the liner forgings. Tensile tests have been used to investigate low rate material
strength and ductility. Hopkinson bar testing has been used to develop Johnson-Cook, Zerilli-Armstrong and MTS material
models. Long standoff triple flash radiography was used to evaluate overall jet ductility. All the designs produce experimental
jet tip velocities in close agreement with modeling, typically near 12.0 Km/s. Experimental trends of jet ductility were observed
with proper material selection and thermomechanical conditioning producing excellent jet ductility.
INTRODUCTION
The development of molybdenum lined shaped charges is a relatively new area of investigation. Copper has traditionally been
the liner material of choice for shaped charges. Molybdenum is an attractive material for shaped charge liners due to a high
sound speed (5.124 Km/s versus 3.94 Km/s for Cu) and a relatively high material density (10.2 g/cc versus 8.93 for Cu). The
high sound speed is desirable in order to achieve high velocity coherent jet tips [1,2]. The high material density is desirable for
penetration capability. Molybdenum has proven benefits for warhead precursor applications, where high coherent jet tip
velocities are extremely important and deep penetration capability is of lesser importance. In order for a shaped charge to
produce deep armor penetration, the stretching shaped charge jet must achieve a very long length before particulating. For
this reason, significant emphasis is placed on producing increased jet ductility [3]. The determination of desirable molybdenum
liner material properties for improved jet ductility could lead to improved performance and production processes for liner
fabrication.
DESIGN
In order to take advantage of potential molybdenum benefits, appropriate shaped charge design is vital. The differences in
material properties make the design of molybdenum shaped charges quite different than for copper lined shaped charges. The
resulting shaped charge geometries are significantly different than traditional copper lined shaped charge geometries. We
have concentrated on a number of different design types, depending on application: material investigation or warhead
application.
In order to investigate the effects of liner material properties and processing, rather than shaped charge design, a relatively
simple design that produced a robust jet with a broad velocity distribution was desired. A 73mm diameter extreme shaped
charge design with relatively simple fabrication requirements was developed using formal numerical optimization incorporated
into the analytic shaped charge model PASCC1 (Picatinny Arsenal Shaped Charge Code, 1 dimensional analysis) [4,5]. A
constant thickness conical liner with a simple truncated apex was selected for ease of fabrication. The PASCC1 numerical
optimization capability was used to determine the wave shaping axial position and diameter required to maximize the jet tip
velocity. Inequality constraints specifying a maximum allowable 1.22 collapse Mach number and a maximum allowable wave
shaper diameter of 60mm were imposed. An additional inequality constraint of no jet inverse velocity gradient was also used.
Figure 1 presents the resulting optimal design. Although the optimization result predicted a high velocity jet tip (12.5 Km/s), the
jet tip region was predicted to be extremely thin. In order to overcome this deficiency, a second optimization was performed
that maximized the smallest jet radius at 50 µs with the collapse Mach number constrained to equal 1.22. The previous
maximum allowable wave shaper diameter inequality constraint was again imposed. Figure 2 presents the resulting final
optimized design. The resulting final design was predicted to produce a robust jet with a 12.5 km/s jet tip velocity. Figure 3
presents the PASCC1 predicted jet velocity versus initial liner axial position for the two optimized designs. Figure 4 presents
the PASCC1 predicted jet radius versus jet axial position at 50µs for the two optimized designs.
Figure 1. Maximized jet tip velocity wave shaper geometry (1st Optimization).
Figure 2. Final optimized wave shaping design to produce a robust high velocity jet (2nd Optimization).
Figure 3. Jet velocity versus initial liner axial position.
Figure 4. Jet radius versus jet axial position at 50µs.
The final optimized design was computationally verified using the arbitrary Lagrange Eulerian program CALE. Figure 5
presents CALE predicted material boundaries at 0 and 32µs. The CALE calculations indicated a jet tip velocity of 11.6 Km/s.
Figure 5. Material boundaries at 0 and 32µs.
A variety of high performance shaped charge designs have been completed and experimentally tested. In particular, trumpet
shaped charge designs have been tested in a number of different design configurations. Figure 6 presents a photograph
comparing a small angle conicial molybdenum liner to a trumpet molybdenum liner. Figure 7 present three different trumpet
shaped charges that have been used in experimentation. All three designs produce similar jet tip velocities (~12. Km/s).
Detonation wave shaping is commonly used in order to prevent ultra-thin liner thicknesses which are particularly difficult to
fabricate.
Figure 6. Photograph comparing conical and trumpet molybdenum shaped charge liners.
Figure 7. Trumpet shaped charge designs.
MATERIAL PROCESSING AND JET CHARACTERIZATION
A number of different material processing methods have been used for the fabrication of molybdenum shaped charge liner
preforms including single crystal, high energy rate forming (HERF), conventional forging and hot isostatic pressing (HIP). The
emphasis of these material processing investigations has been to produce high ductility shaped charge jets, as well as to
address potential production liner manufacture methods for high performance molybdenum shaped charge warheads.
A single crystal shaped charge liner was fabricated and provided to TACOM-ARDEC by R. Sullivan of BWX Technologies,
NNFD Product Development. The monocrystal preform was machined to a ½ scale version of the conical design previously
developed for molybdenum material investigation. The orientation of the monocrystal liner was intended to be 111 oriented
along the vertical axis of liner, but analysis showed that it was about 9 degrees off axis. The warhead was loaded with Octol
70/30 and a ½ scale diameter waveshaper with full scale axial thickness was fabricated into the warhead. Long stand-off flash
radiography was used to characterize the experimental jet ductility. The resulting jet demonstrated extremely brittle behavior,
with a large amount radial particulate. Complete ductility data reduction was not completed due to the extreme jet dispersion
exhibited by the jet. Figure 8 presents a long stand-off jet x-ray result.
Figure 8. Long stand-off jet x-ray from monocrystal molybdenum lined shaped charge.
The ability to quickly tailor final liner material properties and characteristics for small numbers of liner forgings has
made high energy rate forming (HERF) an extremely valuable liner forging method for materials investigation. ARDEC has
previously used HERF for copper and tantalum warhead liner material properties investigation and processing development
[6]. Due to the success of these previous efforts, a HERF material processing investigation of molybdenum shaped charge
liners was conducted. The investigation concentrated on the effect of molybdenum liner material properties and processing on
jet ductility. Significant increases of jet ductility were achieved. High energy rate forming was used to produce liner forgings
from arc cast barstock. The near net liner preforms were made with a total of five forging blows. Although the molybdenum
liner forgings exhibited excellent formability at the high strain rates produced by the HERF process, care had to be taken to
avoid tensile cracking. A recrystallization study yielded thermal conditioning required to obtain fine grain (~12 micron)
structured recrystallized material. The liner forgings grain structures were investigated using traditional optical microscope
techniques. The unrecrystallized grains were extremely long and stringy compared to the nearly equal axis grains produced by
recrystallization. Four different thermal and mechanical conditioned liner forgings were subsequently produced using different