Challenges in Understanding the Dynamic Behavior of Heterogeneous Materials
389
shock compression process. These processes are discussed in detail in Meyers [54]
and Horie and Sawaoka [42] and remain of critical importance in understanding the
response of explosives and reactive materials.
3.2 Shock-Induced Chemical Reactions
The uniaxial strain configuration sets up a much different loading state than the
uniaxial stress configuration. Due to the lateral confinement of the powders, a shock
wave develops in the material which ramps up the pressure within the particles
and compacts the powder, setting up local deformation fields limited by the bulk
compressibility of both the individual constituents and the bulk surrounding powder.
Of note is the ability of a flyer-plate gas gun experiment to measure the thermodynamic shock-compressed state of the powder. If the measured shock response
deviates from the predicted thermodynamic state of an inert compacted powder, it
can be inferred that some event occurred which caused the deviation, namely, a
shock-induced chemical reaction. The Ballotechnic model [11, 17, 27, 30, 33] can
be used to infer the shock-induced chemistry event. The crush-up to full density
of a distended powder mixture can greatly influence the chemical reactivity of the
mixture due to local hot spot generation and particle friction, which needs to be
accounted for in the reaction product equation of state.
Shock compression experiments on Ti+2B compacts at 50% TMD were performed to assess the equation of state and shock compression response of the
mixture to assess the baseline performance of the mixture. The Hugoniot was
measured from the equations of state and Rankine-Hugoniot jump conditions.
Figure 13 casts the shock compression response of the Ti+2B powder mixture in
P-V space, along with predicted equations of state. The Ballotechnic curve (Eq. 36)
is also included, and it can be observed that a number of experiments fall within the
Ballotechnic, which indicates a potential shock-induced chemical reaction.
Meso-scale simulations were performed to identify the possible precursor mechanisms for the observed chemical reaction. Synthetic microstructures were generated using the same methods as the uniaxial stress simulations, and shock
compression simulations were performed as 1:4 scale models of the actual experiment. Individual stress traces at the backer “simulated gage” are plotted in Fig. 14
which show a distinct distribution in possible rise times and peak pressures, owing
to the adjacency of B or Ti particles relative to where the measurement was taken.
This manifests as a dispersed two-wave structure, reminiscent of the wave-splitting
that occurs within elastic-plastic shock wave propagation.
This distinct heterogeneity influences how both the EOS and wave profiles are
interpreted. It is also unique to heterogeneous materials and demonstrates how the
probe smears the actual local response of the shock-breakout event. Gonzales [29]
provides an assessment of measurement uncertainty in light of this heterogeneity.
Of note is the observation that the bulk temperatures remain comparatively low,
as shown in Fig. 15 [25]. However, the intimate mixing of the reactants and large
389
shock compression process. These processes are discussed in detail in Meyers [54]
and Horie and Sawaoka [42] and remain of critical importance in understanding the
response of explosives and reactive materials.
3.2 Shock-Induced Chemical Reactions
The uniaxial strain configuration sets up a much different loading state than the
uniaxial stress configuration. Due to the lateral confinement of the powders, a shock
wave develops in the material which ramps up the pressure within the particles
and compacts the powder, setting up local deformation fields limited by the bulk
compressibility of both the individual constituents and the bulk surrounding powder.
Of note is the ability of a flyer-plate gas gun experiment to measure the thermodynamic shock-compressed state of the powder. If the measured shock response
deviates from the predicted thermodynamic state of an inert compacted powder, it
can be inferred that some event occurred which caused the deviation, namely, a
shock-induced chemical reaction. The Ballotechnic model [11, 17, 27, 30, 33] can
be used to infer the shock-induced chemistry event. The crush-up to full density
of a distended powder mixture can greatly influence the chemical reactivity of the
mixture due to local hot spot generation and particle friction, which needs to be
accounted for in the reaction product equation of state.
Shock compression experiments on Ti+2B compacts at 50% TMD were performed to assess the equation of state and shock compression response of the
mixture to assess the baseline performance of the mixture. The Hugoniot was
measured from the equations of state and Rankine-Hugoniot jump conditions.
Figure 13 casts the shock compression response of the Ti+2B powder mixture in
P-V space, along with predicted equations of state. The Ballotechnic curve (Eq. 36)
is also included, and it can be observed that a number of experiments fall within the
Ballotechnic, which indicates a potential shock-induced chemical reaction.
Meso-scale simulations were performed to identify the possible precursor mechanisms for the observed chemical reaction. Synthetic microstructures were generated using the same methods as the uniaxial stress simulations, and shock
compression simulations were performed as 1:4 scale models of the actual experiment. Individual stress traces at the backer “simulated gage” are plotted in Fig. 14
which show a distinct distribution in possible rise times and peak pressures, owing
to the adjacency of B or Ti particles relative to where the measurement was taken.
This manifests as a dispersed two-wave structure, reminiscent of the wave-splitting
that occurs within elastic-plastic shock wave propagation.
This distinct heterogeneity influences how both the EOS and wave profiles are
interpreted. It is also unique to heterogeneous materials and demonstrates how the
probe smears the actual local response of the shock-breakout event. Gonzales [29]
provides an assessment of measurement uncertainty in light of this heterogeneity.
Of note is the observation that the bulk temperatures remain comparatively low,
as shown in Fig. 15 [25]. However, the intimate mixing of the reactants and large