Interfacial failure in plastic bonded explosives
File(s)
Author(s)
Iqbal, Muhammad
Type
Thesis
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.
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.
Version
Open Access
Date Issued
2020-12
Date Awarded
2021-04
Copyright Statement
Creative Commons Attribution NonCommercial NoDerivatives Licence
Advisor
Charalambides, Maria
Taylor, Ambrose
Sponsor
Atomic Weapons Establishment (Great Britain)
Engineering and Physical Sciences Research Council (EPSRC)
Publisher Department
Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)