Interfacial failure in plastic bonded explosives

Abstract

Plastic bonded explosives (PBXs) materials are a form of particulate composite materials consisting of stiff energetic crystals with different particle sizes. These crystals are randomly distributed inside a soft polymer binder material with volume fraction greater than 85%. The binder material holds the crystal particles together, provides means of dissipating energy in cases of accidental load, increases the material’s storage life and enables safe handling without deterioration of the explosive performance. The mechanical properties of PBX materials profoundly depend on the mixture ratio formulation, the constituents’ material properties, environmental conditions (temperature, pressure, humidity), and loading rate. Mechanical characterisation of the polymer bonded explosives (PBXs), though very costly, is therefore crucial for their safe handling during storage and transportation while preserving the optimal explosive performance. The modulus of the PBX binder is five orders of magnitude lower than the modulus of the explosive crystals. Despite the low volume fraction (5% - 15%), the binder material influences the PBX material properties significantly, hence characterisation of the binder material is vital. A rheological constitutive law model can capture the pronounced time-dependent and temperature-dependent behaviour of the binder over a large deformation range. In this project, the material properties of the binder were determined using constant shear strain rate, shear stress relaxation and monotonic tensile test results obtained over a wide range of temperature and strain rates. A visco-hyperelastic model was parameterised using the derived test data. In addition, a methodology is proposed for extracting valid test data from rheological testing of soft solid materials where the storage modulus is higher than the loss modulus. The PBX materials fracture predominantly by interface debonding between the binder and explosive crystals, at temperatures above the glass transition temperature of the binder. Crystal to crystal friction, even with an insignificant external load, can lead to an accidental detonation of the PBX material. This interfacial debonding can be described by cohesive zone laws. In this project, the cohesive zone material properties, namely the linear stiffness (𝑘1), the interface cohesive stress (𝜎𝑖𝑛𝑡 𝑚𝑎𝑥) and the interface cohesive energy (𝛾𝑖𝑓) were determined using fracture testing coupled with Digital Image Correlation (DIC) to capture the deformation and strain fields around the crack tip. According to the experimental results, the cohesive zone parameters for the particle-binder interface are strain rate-independent, whereas as temperature rises, the cohesive zone properties drop significantly, especially the interface cohesive stress and the interface cohesive energy. The mechanical properties of the PBX composite were also determined experimentally; the test results showed that PBX-1 (the material under this study, filler is crystalline cyclotetramethylene tetranitramine (HMX) and the matrix is nitrocellulose-based polymer, volume fraction 88%) has better mechanical properties, i.e. higher Young’s modulus, failure stress and failure strain, under compressive and flexural loading, as compared to tensile loading at the same temperature and load rate.In addition, PBX microstructure models were constructed using SolidWorks and MacroPac software. Simulations based on regularly packed microstructures, i.e. body-centred cubic, face-centred cubic, hexagonal-closed packed and simple cubic, with volume fractions of 10%, 20%, 30%, 40% and 50%, were conducted in order to determine the effect of the microstructure on the bulk properties. The lower volume fractions for arbitrary representative volume elements were chosen for the parametric study, as it enabled the microstructure meshing easier and simulation results could be validated. The effects of spatial distribution and number and size of particles were also studied while keeping a constant 30% volume fraction. Two types of virtual PBXs materials were analysed, a PBX material (PBX-A) with an elastic-plastic binder, and a PBX material (PBX-B) with a visco-hyperelastic binder. For the elastic-plastic binder PBX-A, the correlation between the crystals dispersion within the binder (nearest-neighbour distance, mode distance, volume disorder) and the PBX properties (Young’s modulus, failure stress, yield stress and plateau stress) were investigated. The bulk Young’s modulus, yield stress and plateau stress increased as the volume fraction increased, whereas the micro-yield stress decreased as the volume fraction increased. Plateau stress, macro-yield stress and Young’s modulus were a function of the particle mode distance, whereas micro-yield stress and tress triaxiality were a function of minimum nearest neighbour distance. For PBX-B, the instantaneous shear modulus and failure stress increased as the volume fraction increased, whereas the failure strain decreased as the volume fraction increases. The instantaneous initial shear modulus was a function of mode and minimum nearest-neighbour distance. The study showed that the mechanical properties of PBX materials could be tailored by controlling the particles’ spatial distribution, morphology, the volume fraction, and the binder system.