(458h) Hot-Spot Formation in Composite Energetic Materials Due to Ultrasonic and Impact Excitation As a Function of Young’s Modulus and the Work of Adhesion

Energetic
materials such as propellants and explosives fulfill a variety of roles in
addressing modern engineering and scientific challenges. Of primary interest
for the purposes of increasing the safety of handling these materials as well
as improving performance is the response of the energetic material to a wide
variety of mechanical insults. It is widely accepted throughout the energetics
community that the initiation of energetic materials due to mechanical insults
such as impact or shock is primarily due to thermomechanical processes. In
particular, the localization of energy within the material leads to the
creation of small regions of greatly increased temperatures which can
subsequently grow due to chemical decomposition of the material. These
localized temperature fields or “hot-spots” allow for ignition or initiation of
energetic materials at energies far below what would be required to bring the
bulk material to temperatures sufficient for thermal decomposition.
Consequently, the susceptibility to various mechanical insults or sensitivity
can be understood to be a function of chemical composition, intensive
properties such as thermal conductivity, and physical parameters including
heterogeneity of the material and the inclusion of air bubbles. The dependence
on the exact nature of the mechanical insult further complicates a full
description of the sensitivity of a material. Traditionally, sensitivity of a
given material has been characterized by the response to friction, impact, and
electrostatic shock. Further tests in more specialized setups may quantify the
response to mechanical shock, but these three tests have been the standard for
determining the safety of a material. Recent developments including the
expectation of operating at more extreme conditions and demands for performance
have led to the discovery of a sensitivity to acoustic excitation or vibration.
This new excitation mode has been proven to be sufficient to force energetic
materials to violently decompose over relatively short timeframes demonstrating
an immediate need for the reevaluation of sensitivity testing and the
incorporation of designs to mitigate the possibility of rapid unplanned
decomposition of energetic materials. This study aimed to explore the role of adhesion
and stiffness as two material properties that might substantially contribute to
the heating observed during acoustic excitation. With early promising results,
further impact excitation tests were carried out in order to explore how
changing one material property may result in different outcomes in the
sensitivity response.

High
frequency acoustic excitation was provided in the form of ultrasonic excitation
and non-shock mechanical excitation was applied via drop weight impact. In
order to examine the possible mechanisms of hot-spot formation for ultrasonic
excitation, a mechanically-delaminated inclusion of HMX in polymer binder was
subjected to ultrasonic excitation utilizing an ultrasonic transducer. With
weak coupling provided via ultrasound gel, these experiments allowed for
exploration of a low incident energy regime unexplored by other works in the
field. For the investigation of the role of adhesion and in order to control
the mechanical properties of the materials, various modifications were made to
the polymer components of the samples. The ultrasonic excitation experiments
utilized samples consisting of a block of polymer binder enclosing a crystal at
a set distance from the top surface of the sample. The thermal properties of
each polymer material were also evaluated in order to evaluate the energy
dissipation at the embedded crystal. Hydroxyl terminated polybutadiene (HTPB)
and polydimethylsiloxane (PDMS) polymer binders were utilized with particular
additives or modifications to the curative ratios in order to provide a range
of adhesive and mechanical properties for testing. As current
state-of-the-field experimental methods are insufficient for evaluating the
true mechanical and adhesive properties of composite polymer materials under
high frequency excitation, the mechanical and adhesive properties at static
conditions and room temperature were utilized as figures of merit to determine
if any overt relationship existed between these material properties and the
energy dissipation due to ultrasonic excitation.

While the ultrasonic excitation
experimental results suggest that the work of adhesion has no effect on the
heating rate at the inclusion, a positive correlation exists between the
Young’s modulus of the polymer material and the heating rate at the inclusion. The
variation of the heating rate with the Young’s modulus is shown in Figure 1.
While slight, the positive correlation may suggest either less energy is lost
due to viscoelastic effects or that the energy dissipation mechanism at the internal
interface is modulated by the stiffness of the binder.

Drop
weight impact tests were performed on samples containing 85% HMX crystals by
mass in order to investigate if a similar relationship existed between the
studied material properties and non-shock impact sensitivity. Specific heights
were determined through utilization of the Neyer Sentest software which uses a
D-optimal approach to quickly determine the drop height as well as the shape of
the distribution assuming that the sample responses are normally distributed. The
L50 or 50% probability of explosion of the polymer-HMX systems tested so far
are displayed in Table 1. There is some indication of possible bimodal
behavior in the results, but further experimentation is necessary in order to
determine if this is simply an artifact of inherent sample variability. While
such a possibility would certainly be an interesting indicator of two distinct
mechanisms being responsible for the behavior, further refinement of the
statistical method used would be required to account for the novel shape of the
distribution. Regardless, early results generated from this study suggests that
stiffer composite energetic materials are more sensitive to impact than more
pliable materials which superficially corresponds to results from the
ultrasonic heating experiments but is likely driven by a different mechanism.

These results suggest that the
stiffness of the material plays an important role in the energy dissipation
mechanisms responsible for hot-spot formation in these materials and that more
compliant materials exhibit a lesser degree of sensitivity. Further
investigations will be necessary to fully map the parameter space and determine
if the behavior becomes dominated by different properties for greater excitation
parameters. Modification of the drop weight impactor could also allow for high
speed imaging of hot-spot formation providing insight into possible bimodal
behavior.