Synchrotron-based high-pressure research in materials science Page: 4 of 8
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Recent experiments highlight the importance of
cracks and voids in the ignition, combustion, and
reaction violence of PBX 9501. The Steven 'lest
determines the critical impact velocity of a lightly
confined energetic material to the low-speed impact
of a blunt steel projectile. Radial cracks emanating
from the impact point are apparent for a test where
no sustained reaction occurred (10). Idar et a!. (10)
find that damaged fPX 9501 has a significantly
lower impact threshold for violent reaction than
L -~ -I
Figure 4. Schematic of radial burn experiment.
Sample is ignited in center using a coiled nichrome
wire. Pressure is measured in the center of th
Henson et al. have conducted shear impact
experiments using thin samples of PBX 9501 (11).
A rectangular steel plunger is driven into the lightly
confined sample at about 100 n/s. Plunger
intrusion causes both shear and non-shear fracturing
with reaction initiated along fracture zones.
Skidmore et al. (12) have used microscopy to study
damaged samples recovered from the shear impact
experiments and find that the HMX along the
fracture zones shows clear signs of heating and
quenched reaction (12).
Evidence of the importance of crack-sustained
combustion also appears in elevated-temperature
experiments, such as the Mechanically Coupled
Cookoff (MCCO). Dickson et al. slowly heat a,
confined sample of PBX 9501 to a well-defined
temperature field, then ignite the center of the
sample. They detect reaction, indicated by luminous
emission, throughout cracks that are caused by
pressurization due to production of reactive gases
(13). The fast reactive waves, indicated by the
luminosity, propagate through the cracks at
velocities on the order of 500 m/s. An interesting
question is whether reaction is spread into these
cracks via convective processes, or whether crack tip
dissipation ignites the material. Crack tip
dissipation is expected to provide only a small
amount of energy. However, in the MCCO
experiments the material is already at an elevated
temperature so very little energy is needed for
reaction to occur. Recent initial experiments have
been perfonned on pristine materials that show
reactive cracks. These results are shown below.
In recent carefully heated cook-off (thermal
Figure 5. Images from radial burn experiment. Part (a) shows the
initial ignition in the center and pat (b), 0.5 ms later, shows
illuminated cracks that extend from the center ignition site to the
explosion) experiments involving larger-scale
explosive charges in an annular configuration (14),
nearly symmetric compression of the inner wall was
observed, although thermocouple records indicate
ignition occurred asymmetrically. Consequently, it
is surmised that some physical mechanism must
spread reaction around the annular explosive charge,
followed by violent reaction yielding a nearly
symmetric compression of the inner walls.
Spreading of ignition sites must occur at a rate on
the order of 1000 m/s. Connected porosity is
expected in this material because it is heated for
several hours at elevated temperatures.
Consequently, convective burning is one possible
mechanism to provide this spreading of ignition.
Convective burning in this material has not been
observed before at speeds near 1000 m/s in PBX
9501. Recent experiments have shown that these
rates are achievable in PBX 9501. Initial results are
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Chen, Bin; Lin, Jung-Fu; Chen, Jiuhua; Zhang, Hengzhong & Zeng, Qiaoshi. Synchrotron-based high-pressure research in materials science, article, June 1, 2016; (https://digital.library.unt.edu/ark:/67531/metadc934796/m1/4/: accessed March 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.