A Lagrangian framework for analyzing microstructural level response of polymer-bonded explosives

Abstract

A cohesive finite element method (CFEM) framework for quantifying the thermomechanical response of polymer-bonded explosives (PBXs) at the microstructural level is developed. The analysis carried out concerns the impact loading of HMX/Estane at strain rates on the order of 104–105 s−1. Issues studied include large deformation, thermomechanical coupling, failure in the forms of microcracks in both bulk constituents and along grain/matrix interfaces, and frictional heating. The polymer matrix is described by a thermo-elasto-viscoelastic constitutive formulation, accounting for temperature dependence, strain rate sensitivity and strain hardening. The HMX crystals are assumed to be elastic. The CFEM framework allows the contributions of individual constituents, fracture and frictional contact along failed crack surfaces to heating to be tracked and analyzed. Digitized micrographs of actual PBX materials and idealized microstructures with Gaussian distributions of grain sizes are used in the analysis. The formation of local hot spots as potential ignition sites is primarily due to the viscoelastic dissipation in the matrix in early stages of deformation and frictional heating along crack surfaces in later stages of deformation. The framework is a useful tool for the design of energetic composites and the results can be used to establish microstructure–response relations that can be used to assess the performance of energetic composites.