The Thermomechanical Response of Particulate Composite Energetic Materials Under Mechanical Vibration
thesisposted on 06.02.2020 by Allison R Range
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Polymer-bonded, particulate composite energetic materials are widely used in the defense and energy sectors for munitions systems and explosive devices, yet their thermomechanical behavior is not well understood. In this work, the macroscale, thermomechanical response of energetic materials to contact mechanical excitation was studied at frequencies spanning 1 Hz to 100 kHz. The effects of formulation variation, thermal boundary conditions, excitation amplitude, and intentional stress concentrations within large composite plate samples were examined when the samples were excited at various resonances. Initial experiments focused primarily on the first structural resonance of a plate geometry, as determined experimentally using laser Doppler vibrometry. Excitation at frequencies up to 1000 Hz resulted in heating, attributable to the viscoelastic nature of the polymeric binder, on the order of 2-3\degree C/hr, and the propagation of structural defects such as cracks. At frequencies above 10 kHz, the heat generation within the material increased, potentially due to a combination of viscoelastic heating, particle / binder interactions, and conduction with exterior mounting components. Experiments were performed utilizing laser Doppler vibrometry and infrared thermography on both inert and fully-loaded energetic material variants in order to analyze the thermal and mechanical response due to the mechanical excitation using a piezoelectric shaker. The thermal analysis of various sample formulations revealed temperature changes on the order of 4\degree C/min, with several samples reaching 100\degree C within a 15 min window. Due to significant material softening observed during the thermal cycling, the resonant frequency was found to shift over the course of the experimental analysis. Potential applications may require the energetic materials to sustain extreme temperature environments coupled with vibratory loads. In an effort to characterize this effect, a custom enclosure was fabricated to maintain an ambient air temperature around the sample of up to 70\degree C. Distinctions between ambient and elevated temperature environment experiments on a preliminary sample set were noted in regard to rate of change of temperature, and several samples exhibited maximum surface temperatures in excess of 100\degree C. Finally, an analytical model was developed to estimate the heat generation associated with the observed experimental results. The cylindrical sample geometry was modeled as a longitudinal rod subjected to harmonic base excitation, and the resulting mechanical response was used to estimate the rate of mechanical energy expended per unit length at the predicted resonant frequencies. The thermal response was obtained by numerical integration of the heat transfer equation accounting for energy generation in the system, and the resulting axial temperature profiles predicted maximum temperatures on the order of 120\degree C after 60 seconds of excitation at the first predicted resonant frequency.