CHARACTERIZATION OF MOCK PLASTIC-BONDED ENERGETIC MATERIALS UNDER CYCLIC COMPRESSION AT LOW STRAIN RATES

This work seeks to characterize the mechanical response and material properties of polymer-bonded energetic materials (PBXs) when acted upon by a low-frequency, high-amplitude excitation. These particle-binder composites have been shown to incur damage when samples are deformed, even down to around 0.01% strain, via a phenomenon known as the Mullins effect. The presence of damage in PBX materials has been shown to erratically inhibit performance. By garnering an understanding for how energetic materials respond to a mechanical excitation, a better prediction of the material properties can be made. The goal of this research was to experimentally measure energy dissipation in mock energetic materials and then use a constitutive model based in Ogden hyperelasticity and yield-surface-free endochronic plasticity to determine material properties from the experimental data. Cyclic compression tests were performed in order to produce low-frequency, high-amplitude, mechanical excitation, and stress relaxation tests were carried out to determine steady-state, stress relaxation times under a constant strain. The cyclic compression test results were then passed through the constitutive model to reproduce stress data and estimate material parameters that describe stress-softening, stiffness, and damage. The end goal of this research is to begin connecting the research related to quasi-static uniaxial loading experiments with frequency-based studies.

It should be noted that low-frequency, high-amplitude cyclic compression of energetic materials has yet to be thoroughly explored. For example, continuum mechanic studies report a connection of strain rate to stress response, but given their steady-state nature, they do not account for material damage that occurs over time, which is pivotal in understanding how a material may act when a variable force with respect to time is placed on the material. Forcing, such as a simple sinusoidal excitation or a general white noise excitation, can yield differing results over the duration of the test. The work described here takes the findings of similar materials and continues on by determining the transient response when a variable, cyclic force is enacted on a specimen in order bridge the gap between unidirectional compression and frequency-dependent mechanical excitation.