The Woodruff School

SUMMARY

Extensive research was performed in the past to investigate how energetic materials (EM) ignite via thermomechanical dissipation produced by viscoplastic deformation, friction, and fracture. Yet, little is known regarding the other types of excitation (e.g., electric sparking) responsible for causing ignition. Electromechanically induced dissipation is one notable example of such mechanisms that has become increasingly vital to understand in the recent development of a novel, multifunctional stimulus capable of sensitizing and triggering ignition.

To better understand how certain electromechanical properties of EMs can alter their ignition behavior, a two-step, multiphysics framework spanning multiple timescales is developed. The numerical simulations first track the development of electric field (E-field) in the material under external mechanical load over the microsecond timescale. The model uses a coupled mechanical-electrostatic framework for computing the stress, strain gradient, and E-field distributions of P(VDF-TrFE)/nAl possessing flexoelectric and piezoelectric properties.

The attainment of sufficient E-field intensity within the material is then used as part of the input for the subsequent analysis, wherein the dielectric breakdown and exothermic reaction processes are simultaneously resolved over the nanosecond timescale based on a electrodynamic-chemical-thermal framework. Here, the breakdown process is explicitly modeled as the irreversible transition of the material from dielectric phase into conductive phase wherever the local E-field exceeds the dielectric breakdown strength. The chemical reaction is modeled as a single-stage, forward kinetic process involving the decomposition and direct pyrolysis of the PVDF-TrFE, followed by the exothermic fluorination of the Al particles. The reaction rates are characterized using a temperature-dependent Arrhenius equation. The species transport is modeled as diffusion and advection driven by the pressure gradient.

The analyses focus on the effects of load intensity and microstructural attributes, such as Al particle size, particle volume fraction, void size, and porosity level, on the mechanical-electrical-chemical-thermal processes with the determination of conditions for ignition being of particular interest. Further, the individual contributions of the electromechanical properties to the overall ignition behavior are delineated using poled and unpoled specimens.

SUBJECT:	Ph.D. Proposal Presentation

BY:	Ju Hwan Shin

TIME:	Wednesday, May 10, 2023, 1:00 p.m.

PLACE:	Love Building, 210

TITLE:	Predicting Ignition Behavior of Heterogeneous Energetic Materials with Electromechanical Properties

COMMITTEE:	Dr. Min Zhou, Chair (Mechanical Engineering) Dr. Ting Zhu (Mechanical Engineering) Dr. Jerry Qi (Mechanical Engineering) Dr. Yuhang Hu (Mechanical Engineering) Dr. David Kittell (Sandia National Laboratory)

SUMMARY Extensive research was performed in the past to investigate how energetic materials (EM) ignite via thermomechanical dissipation produced by viscoplastic deformation, friction, and fracture. Yet, little is known regarding the other types of excitation (e.g., electric sparking) responsible for causing ignition. Electromechanically induced dissipation is one notable example of such mechanisms that has become increasingly vital to understand in the recent development of a novel, multifunctional stimulus capable of sensitizing and triggering ignition. To better understand how certain electromechanical properties of EMs can alter their ignition behavior, a two-step, multiphysics framework spanning multiple timescales is developed. The numerical simulations first track the development of electric field (E-field) in the material under external mechanical load over the microsecond timescale. The model uses a coupled mechanical-electrostatic framework for computing the stress, strain gradient, and E-field distributions of P(VDF-TrFE)/nAl possessing flexoelectric and piezoelectric properties. The attainment of sufficient E-field intensity within the material is then used as part of the input for the subsequent analysis, wherein the dielectric breakdown and exothermic reaction processes are simultaneously resolved over the nanosecond timescale based on a electrodynamic-chemical-thermal framework. Here, the breakdown process is explicitly modeled as the irreversible transition of the material from dielectric phase into conductive phase wherever the local E-field exceeds the dielectric breakdown strength. The chemical reaction is modeled as a single-stage, forward kinetic process involving the decomposition and direct pyrolysis of the PVDF-TrFE, followed by the exothermic fluorination of the Al particles. The reaction rates are characterized using a temperature-dependent Arrhenius equation. The species transport is modeled as diffusion and advection driven by the pressure gradient. The analyses focus on the effects of load intensity and microstructural attributes, such as Al particle size, particle volume fraction, void size, and porosity level, on the mechanical-electrical-chemical-thermal processes with the determination of conditions for ignition being of particular interest. Further, the individual contributions of the electromechanical properties to the overall ignition behavior are delineated using poled and unpoled specimens.