The thermomechanical response of composite energetic materials under high- and low-frequency mechanical excitations
The preferential generation of heat within hidden explosive materials would be highly valuable to currently-existing explosives detection technologies, due to the associated increase in vapor pressure and thus concentration of detectable vapors. In this work, the thermomechanics of plastic-bonded explosives, propellants, and surrogates thereof are investigated due to their widespread adoption in improvised explosive devices. These classes of material consist of energetic crystals held within an inert plastic binding material. Understanding the composite nature of the material, two methods of heat generation are investigated: low-frequency excitation with wavelengths on the structural-scale and ultrasonic excitation with wavelengths on the crystal-scale. It is observed that direct, electrodynamic shaker-produced excitation at structural resonances of the bulk explosive structure leads to viscoelastic-like heating, with results in good agreement with classical viscoelastic theory in both beam and plate geometries. In this case, scanning laser Doppler vibrometry and infrared thermography were employed to capture the mechanical and thermal response of the structures, respectively. Typically, this type of heating was seen to induce a 2 degrees C surface temperature rise over 60 min with 2 g of base excitation. Further related work included the development of an experimental methodology to determine the wavespeeds of common binder materials. Excitation at ultrasonic frequencies was investigated first in fully-loaded energetic composites, revealing the possibility of heat generation in response to direct ultrasonic stimulus. In this frequency regime incident energy is expected to interact not with the bulk structure but with the individual crystals of the plastic-bonded explosive. Further studies involved the creation of unique single-crystal samples excited with piezoelectric ultrasonic patch transducers. This method elicited very strong heating after just 2 s, up to approximately 10 degrees C and larger. In certain cases, chemical decomposition of energetic crystals has occurred as a result of this heating. To compare between samples, a 1-dimensional semi-infinite thermal model was fit to the transient surface temperature profiles to estimate the heat generation strength and source depth. Crystalline inclusions were observed to generate significantly more heat than spherical inclusions, with no significant dependency on energetic content. Further studies suggest that more irregular crystals exhibit a greater probability of exhibiting significant heating. Finally, initial work on high solids loading 'particles-of-particles' is discussed, highlighting a possible link between solids loading and susceptibility to chemical decomposition in response to ultrasonic stimuli.
Rhoads, Purdue University.
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