Papers by Author: Ananda Barua

Abstract: This paper focuses on the ignition of polymer-bonded explosives (PBXs) under conditions of non-shock loading. The analysis uses a recently developed ignition criterion [ which is based on the quantification of the distributions of the sizes and temperatures of hotspots in loading events. This quantification is achieved by using a cohesive finite element method (CFEM) developed recently and the characterization by Tarver et al. [ of the critical size-temperature threshold of hotspots required for chemical ignition of solid explosives. Calculations are performed on PBXs having monomodal grain size distributions with grain volume fractions varying between 0.72 and 0.90. The impact velocities considered vary between 100 and 200 ms^-1. Results show that the average distance between the hotspots is dependent on the grain volume fraction. As the grain volume fraction increases, the time to criticality (t_c) decreases, signifying increases in the ignition sensitivity of PBX to impact loading. The microstructure-performance relations obtained can be used to design PBXs with tailored performance characteristics and safety envelopes.

Abstract: A framework for quantifying the thermomechanical response of polymer bonded explosives (PBX) at the microstructural level is developed using a cohesive finite element method (CFEM). This framework allows the contributions of individual constituents, fracture and frictional contact along failed crack surfaces to heating to be analyzed and tracked. Digitized micrographs of actual PBX materials and idealized microstructures with various distributions of grain sizes are used in the analysis. The analysis concerns impact loading of HMX/Estane with 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 Estane 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 under the conditions analyzed. Energy localization leading to 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. Microstructure-response relations that can be used in the design of soft energetic composites are established.