SUMMARY

To quantify and better understand how certain electromechanical properties of energetic materials 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 electromechanical properties. The attainment of sufficient E-field intensity within the material is then used as part of the input for the subsequent analysis, wherein dielectric breakdown and exothermic reaction processes are simultaneously resolved over the nanosecond timescale based on an electrodynamic-chemical-thermal framework. Dielectric breakdown is modeled as the irreversible transition of the material from dielectric phase into conductive phase wherever the local E-field exceeds the breakdown strength. The resistive heating caused by breakdown triggers the formation of hotspots that serve as critical sites for the progression of exothermic reactions. The chemical reaction is modeled as a single-stage, forward kinetic process involving the catalyzed decomposition and direct pyrolysis of the PVDF-TrFE binder, followed by the exothermic fluorination of the Al particles. The reaction rates are characterized using the 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, with the determination of conditions for ignition being of particular interest. Further, the individual contributions of the flexoelectric and piezoelectric properties to the overall ignition behavior are delineated using poled and unpoled specimens.