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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
EXPERIMENTAL SIMULATIONS OF DYNAMIC STRESS BRIDGING
IN PLASTIC BONDED EXPLOSIVES
Keith M. Roessig and Joseph C. Foster, Jr.
Air Force Research Laboratory/Munitions Directorate
101 W. Eglin Blvd. Ste. 135
EglinAFB, Florida 32542
Abstract. This work investigates the role of the particle/binder interface in the formation of stress
bridges within bonded particulate materials. The photoelastic technique is exploited to examine the
dynamic stress states within three systems: a binderless particle bed, a particle bed with binder, and a
particle bed with a binder bond strength of zero. In a binderless system, stress concentrations form
readily due to the fact that the stress must be transferred through specific contact points. The particle
bed with binder is shown to have a much more diffuse stress state because shear stresses are transferred
at the interface between crystal and binder. In the system with bond strengths of zero, stress
concentrations redevelop due to stress transfer only near the contact point between disks. Stress chains
are seen to develop in front of the bulk wave in the zero bond strength condition.
plasticized binder holds all the particles together.
At the mesoscale, there are three distinct
components of the particulate plastic bonded
explosive: the crystal, the binder, and the
crystal/binder interface. The contact mechanics
within the mesoscale of this material are such that
under a compressive loading, shear stresses will
develop. Damage to the material will include
debonding of the crystals and binder, crystal
fracture and binder tear. The damage accumulation
is not uniform, but is concentrated along certain
paths.
INTRODUCTION
Initiation of energetic materials by mechanical
loading is important for many safety and
performance issues. Of specific interest in this
work is the ignition behavior of cure cast plastic
bonded explosives (PBXs), particulate materials
consisting of explosive crystals in a plastic binder.
Heat generation through bulk mechanical shear
comes from the yield strength of the material,
which is very low in the PBXs of interest (<10
MPa), and not thought high enough to start a
reaction. But within single crystals, localization of
the shear deformation within the crystals on certain
slip planes can lead to an increased heat generation
within the particles due to dislocation motion.
Stress bridging has been shown to increase stresses
in crystals loaded in a dry particle bed by localizing
the volume of material loaded to a small percentage
of the whole (1,2).
A micrograph of a post-test impact test specimen
made of a modified PBXN-109 cure cast plastic
bonded explosive is shown in Fig. 1. Energetic
HMX crystals, substituted for the RDX crystals
found in the standard PBXN-109 formulation, are
surrounded by small aluminum particles. A
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FIGURE 1. Damage pattern of a modified PBXN-109 after an
unconfined impact test.
This work examines the effect of the
particle/binder interface on the stress state within a
participate material. Of specific interest is the
ability of the material to form stress concentrations
along specific paths within the material. Three
conditions are examined: a binderless system, a
particle/binder system with an approximate acoustic
wave speed ratio of one in which both compressive
and shear stresses can be transferred at the
boundary, and a system where shear stress is not
allowed to be transferred at the boundary. This last
case corresponds to a system with bond strength
equal to zero.
RESULTS AND DISCUSSION
Dynamic fringe patterns for the particle bed
without a binder are shown in Fig. 2. In this
configuration, the load must be distributed through
the disks due to the lack of a binder. This allows
for large stress concentrations to develop at the
contact points between the disks, shown by the
number of fringes occurring at these points. The
fringe patterns are symmetric, showing that there is
little to no shear being transferred across the contact
points (4).
There are two distinct paths that
support the most load. Other disks have a much
lower loading state, while two disks remain
completely unloaded.
By adding the acrylic binder, the stress state
changes dramatically. The fringe patterns in Fig. 3
show a much more diffuse stress state than that of
Fig. 2. The general circular wavefront of the fringe
patterns is due to the loading conditions described
in the experimental methods section and the fact
that the binder can support the load. Each disk is
loaded in this case, and the stress concentrations do
not develop. Towards the front of the fringe
wavefront, the contact points are seen to be
supporting shear by the antisymmetry of the fringe
patterns. Similar fringe shapes are seen in simpler
geometries designed to promote shear deformation
(3). The fringe patterns are also seen to be
propagating at a much faster speed than the case
without binder. This is not a change in the acoustic
properties of the materials, but rather an increase in
the maximum shear at points farther away from the
impact at a later time. Photoelasticity is sensitive to
shear stresses, and with the disk/binder interface
allowing the transfer of shear, fringe patterns form
earlier in disks further from the impact.
The final case with the acrylic binder and greased
interfaces is shown in Fig. 4. While the distributed
circular wave pattern forms as in the bonded acrylic
binder case, stress concentrations form at the disk
contact points as in the binderless case. The fringes
are symmetric, indicating a low level of shear stress
at the contact points. The stress must be transferred
as compressive stress near the contact points as the
disks now act as wave guides. The fringe pattern
propagation speed has decreased. As shear stress
cannot be transferred through the disk/binder
interface, one would expect the shear stress states to
develop at later times.
EXPERIMENTAL METHODS
The determination of dynamic stress at the
mesoscale within an explosive during an impact
event is very difficult, if not impossible, with
current testing methods. Therefore, both temporal
and spatial scalings must be performed to
understand the mechanics at this scale (3). The
simulation of explosive crystals in a hard binder
took the form of circular PMMA disks 6.4mm
thick, 50mm diameter in an acrylic binder with
similar acoustic properties to PMMA. This binder
material forms a bond to the disks to allow the
transfer of both compressive and shear stresses
across the disk/binder interface. To examine the
case where only compressive stress is transferred
across the interface, silicon grease was placed on
the edges of the disks to prevent bonding of the
binder to the disks during the curing process. Both
of the binder systems are compared to a binderless
system consisting exclusively of PMMA disks.
The loading cell for the tests consists of a 4340
steel frame. Dynamic tests were conducted by
placing the load frame into a compression
Hopkinson bar apparatus. The loading pulse could
then be controlled by the striker bar length and
velocity. A 6.4 mm thick loading pin transfers the
loading pulse from the Hopkinson bar to a 1018
steel bar that spans the entire specimen impact
surface. In this way, planar impact conditions could
be simulated. The photoelastic technique was used
to generate fringes of constant maximum shear
stress within the disks. High speed photographs of
the fringe patterns were taken with an Imacon 460
digital camera.
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FIGURE 2. Stress chains form in particle beds without any binder, Concentrations occur at each contact point between disks.
pictures were taken at 200 and 600|j,s after impact.
The
FIGURE 3. By adding an acrylic binder, the dynamic stress state changes dramaticlaly. There are very few concentrations and the load is
distributed across both the binder and the disks. Pictures were taken 45 and 115|as after impact.
FIGURE 4. Placing grease interfaces between the disks and binder creates a different stress state than shown in Figure 4 because shear
stress is not allowed to pass across the interfaces. While a more distributed stress state develops, stress concentrations at disk contact point
appear in the pictures. The pictures were taken 45 and 150|as after impact.
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The interesting aspect of the second picture in
Fig. 4 is the stress bridge that propagates three disks
in front of the bulk fringe pattern. Figure 5
schematically shows the stress chain. This behavior
has been seen in computer simulations by
Bardenhagen and Brackbill (5). In the simulations,
the mechanical wave speed ratio of the binder to
disk material causes the stress chain. In this
experiment, the chain is due to the interface
condition of pure compressive stress transfer similar
to the binderless case. There are two different
mechanisms that both inhibit transfer of shear from
one crystal to the next causing these stress chains.
In the experiments described in this work, shear
stress is prevented from traveling across the
boundary. In the computer simulations, shear is not
transmitted due to the long transit time of the shear
wave within the binder. In either case, shear
information is not transferred to interact with the
compressive wave at the contact points between
disks.
The stress chain also uses the binder in this case
to propagate. The arrow in Fig. 5 goes through the
binder to the last disk. There are no fringes at the
contact between the last two disks in the chain,
implying no contact load between the two. This
behavior is seen in real explosives also. The
fracture in the specimen in Fig. 1 many times
propagates from crystal to crystal but also
propagates along the crystal/binder interface.
is seen to be essential in eliminating the stress
concentrations within the sample.
FIGURE 5. The stress chain formed in the acrylic binder
system with greased interfaces extends out from the fringe wave
front in the rest of the material.
REFERENCES
1. Foster, Jr., J. C., Christopher, F. R., Wilson, L. L.,
Osborn, J., "Mechanical Ignition Of Combustion In
Condensed Phase High Explosives,"
Shock
Compression of Condensed Matter 1997, edited by
S.C. Schmidt et al., AIP Conference proceeding 429,
pp. 389-392.
2. Roessig, K.M., and Foster, J.C., Jr., "Dynamic Stress
Chain Fracture in Particle Beds," in Plastic and
Viscoplastic Response of Materials and Metal
Forming, edited by A.S. Khan et al., proceedings of
Eighth International Symposium on Plasticity and Its
Current Applications, July, 2000, pp. 437-439.
3. Roessig, K.M., "Mesoscale Mechanics of Plastic
Bonded Explosives," Shock Compression of
Condensed Matter 2001.
4. Shukla, A. and Higam, H., Journal of Strain Analysis
20,241-245(1985)
5. Bardenhagen, S.G. and Brackbill, J.U., Journal of
Applied Physics 83, 5732-5740 (1998).
CONCLUSIONS
Dynamic photoelasticity is used to examine the
stress states within three systems: a binderless
particle bed, a particle bed with binder, and a
particle/binder system with a bond strength of zero.
In the binderless system, stress concentrations form
readily at the specific contact points between the
crystals. The particle bed with binder is shown to
have a much more diffuse stress state because shear
stresses are transferred at the contact points as well
as along the rest of the interface. In the system with
bond strengths of zero, stress concentrations
redevelop. In this case, only purely compressive
stress may be transferred anywhere along the
interface. Stress chains are seen to develop in front
of the bulk wave in the zero bond strength
condition. The transfer of shear along the interface
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