Science Engineering Technology at AWE
Discovery25
September 2014
This issue:
Explosive Materials
Development
Micromechanical
Modelling
Personal Protective
Equipment Testing
Slapper Detonator
Modelling
Hazard and Safety Studies
AWE’s Outreach
Discovery
Contents
Overview2
Explosive Materials Development
4
Micromechanical Modelling
10
Personal Protective Equipment Testing
14
Slapper Detonator Modelling
22
Hazard and Safety Studies
28
AWE’s Outreach
34
Science Engineering Technology at AWE
Discovery25
September 2014
This issue:
Explosive Materials
Development
Micromechanical
Modelling
Personal Protective
Equipment Testing
Slapper Detonator
Modelling
Hazard and Safety Studies
AWE’s Outreach
Cover image:
Explosive microstructure representation;
colours depict stress contours on particles
(courtesy of Imperial College London).
25
I am delighted to be introducing the 25th issue of Discovery which is dedicated to
explosives research at AWE.
AWE plays a major part in
materials science and I have
enjoyed seeing some of the
pioneering research that is
undertaken as part of my role in
supporting the long established
AWE Cranfield University
Strategic Alliance.
AWE’s collaboration with
Cranfield has an enviable history,
an alliance of which I am proud
and one that has evolved and
developed steadily over the last
decade. The collaboration between
AWE and Cranfield University
brings mutual benefits through
educational programmes and
the delivery of research and
consultancy services.
I had the pleasure of attending
the AWE Cranfield Technical
Showcase in October 2013 at AWE
Aldermaston and was impressed
by the extent of the collaborative
work being undertaken. Over 100
AWE and Cranfield University
scientists were present, clearly
using the opportunity to improve
knowledge and understanding
of activities under the Strategic
Alliance and their value both to
AWE and Cranfield University. In
addition, four new areas of future
collaboration were identified.
This further endorses the strong
relationship between the two
parties allowing continued access
to unique facilities at the Cranfield
and Shrivenham campuses.
that AWE does working with UK
academia. I also hope the articles
give you an appreciation of the
esteem in which AWE’s researchers
and scientists are held and their
contribution to the UK deterrence
programme, so vital to protecting
the UK and keeping us all safe.
This gives me great confidence in
the future of the Alliance and all
the people at AWE and Cranfield
University involved that make
it work through their expertise,
commitment and dedication.
I would particularly like to thank
my Cranfield University colleagues
Professor Ian Wallace (Head of
Cranfield Defence and Security),
Professor Dimitris Drikakis and
Professor Rade Vignjevic (both
AWE William Penney Fellows)
for their continued and excellent
contribution to the collaboration.
I hope you enjoy reading the
scientific articles and get a sense
of the range of explosive science
1
Professor Jacqueline Akhavan
Head of the Centre for
Defence Chemistry,
Cranfield University
Discovery
Discovery25 • The Science and Technology Journal of AWE
Overview
AWE's interest in explosives
is very broad and covers the
formulation of new materials for
future applications through to the
manufacture, in-service support
and ultimately breakdown and
disposal of explosive components.
AWE is not only focused on
explosive components but
provides assessments, backed
up with experimental and
modelling data, to support process
and facility safety cases for its
operations. Such a wide range
of activities presents a number
of technical challenges for the
explosives community at AWE
and the articles contained in this
edition touch on some of those
key areas.
High explosives are compact
sources of chemical energy that
function by decomposition to
produce gaseous products at very
high temperatures and pressures.
It is the rapid expansion of these
gases that produces the blast
effects that are characteristic
of explosives. High explosives
can explode in a matter of
microseconds and it is this very
rapid release of energy that
causes damage.
In order to use the energy created
from a detonating explosive in an
efficient manner, it is necessary
to form the explosive into an
appropriate shape. Unfortunately,
many high explosives exist as
crystalline powders. In the Second
World War, ‘melt cast’ explosives
were used widely. High powered,
high melting explosives such as
RDX or HMX were dispersed
in molten TNT (trinitrotoluene)
at lower temperature and then
poured into a mould, cooled and
solidified into a desired shape.
2
Technology has since moved
forward and has resulted in a type
of explosive formulation known
as a Polymer Bonded Explosive
(PBX). The advantages of PBXs
over melt cast explosives are that
a high power explosive can be
incorporated into the formulation,
resulting in higher energy output
per unit volume, and better
safety properties are noted; this
is due largely to the fact that a
protective layer of non-explosive
binder prevents frictional contact
between crystals if subjected to
external stimuli. This balance
between explosive performance
and safety is critical for modern
explosives where volume
constraints are experienced.
AWE re-established a formulations
team in circa 2006 to develop
new PBX formulations for future
applications. The Explosives
Material Development article
describes ongoing activities
of this team and the tools and
techniques developed to support
development in this area.
results of these trials are described
in the PPE Testing article.
One of the challenges to the
explosive synthetic chemist is to
try and design molecules that
have high explosive performance
in a safe manner. Trying to predict
the properties of new explosive
materials is therefore difficult.
At AWE we always start new
material development at a very
small scale to protect the operator
and scale up in a staged process,
carrying out safety testing as
we progress.
Once explosive composites have
been developed they must be
subjected to a suite of tests to
understand their engineering,
physics, performance and safety
through life. This testing is
essential to support material and
system qualification activities
and a large range of tests are
carried out at AWE and at
offsite explosive range facilities.
The Hazard and Safety article
provides an overview of the
testing undertaken to determine
if an explosive is safe to use.
Recently we have undertaken
a number of explosive trials
on various items of Personal
Protective Equipment (PPE) to
provide further protection to the
operator in the unlikely event of an
abnormal occurrence during these
high hazard experiments. The
In the Comprehensive Test Ban
Treaty (CTBT) era a move towards
a model-based approach to
understanding system performance
is becoming increasingly important.
The modelling requirements
for explosive initiation trains
are multi-facetted. The Slapper
3
Detonator and Micromechanical
Modelling articles are examples of
this model based approach.
AWE has undertaken to invest
significantly in its explosive
facilities. The strategy is to refurbish
current facilities and for the long
term, a series of new builds is
planned that will provide enduring
UK capability. One example of this
is the High Explosives Fabrication
Facility (HEFF), that will rationalise
a number of production facilities,
spread across AWE, under one roof.
This edition of Discovery provides
an overview of some of the work
undertaken at AWE related
to explosives.
Discovery
Discovery25 • The Science Engineering and Technology Journal of AWE
Explosive Materials
Development
The majority of High Explosive (HE) materials used in
modern munitions are Polymer Bonded Explosive (PBX)
composites. These consist of explosive particles (filler)
which are bound together by a polymeric binder. PBX
material properties such as performance, safety,
processing and ageing can be targeted by purposeful
selection and formulation of the constituents [1]. This
includes the physical properties of the explosive particle
filler, the polymeric binder component and density of the
finished component.
binder undergoing tensile testing;
Figure 2 shows the results of
binder testing.
This is the principal reason for
carrying out development work
on the binder component before
any composite formulation work
is started. Figure 1 is a picture of a
Energetic binders in explosive
compositions have been the subject
of much research over recent years.
Conventional polymer binders can
offer well characterised physical
attributes to the formulation;
energetic polymer binders can also
contribute to the overall energy
release. The benefit is enhanced
safety of the formulation by
trading the explosive filler with
energetic binder whilst delivering
FIGURE 1
FIGURE 2
Modulus/Failure Stress (MPa)
Modulus
Explosive particle characterisation
and modification consists of the
physical analysis and processing of
the raw explosive materials. This
aims to produce and characterise
particles that are optimised in
terms of particle size, shape
and distribution for the chosen
application.
Traditional methods of producing
explosive particles, such as
crystallisation and milling,
may be used to produce a wide
distribution of particle sizes.
Whilst the particle size and
chemical content are more easily
controlled, the morphology and
density of the particle produced
are strongly influenced by the
preferred crystal habit and the
method of producing particles.
Crystallisation and milling can
produce angular particles that are
complex in shape.
Failure Stress (MPa)
Failure Strain (%)
1000
2.5
900
800
2
700
600
1.5
500
400
1
300
200
0.5
Failure Strain (%)
The choice of polymer binder
component is important as it has
the potential to offer increased
explosive performance over
traditional melt-cast and wax
bonded composites in terms
of energy, higher melting
temperatures, improved
mechanical strength and ageing
properties.
the same overall explosive
performance.
100
0
0
A
B
C
D
E
F
Isocyanate Samples
Effect of different isocyanate samples on the mechanical properties
Image of a binder under tensile test.
of a Polyurethane using the same base polyol.
5
Discovery
Discovery25 • The Science and Technology Journal of AWE
The relationship between the
morphology of HE particles
and the inherent safety and
performance properties has been
widely studied. Recent work
has shown that a significant
improvement in the shock
sensitivity of two widely used
explosives, HMX and RDX, can
be made by tailoring the chemical
and physical properties of the HE
crystals [2, 3].
It was suggested that shock
sensitivity is related to sharp
edges, surface cracks, crystal shape
as well as inclusions or impurities
inside crystals and the crystal
size. Reduced sensitivity HMX,
for example, is now available
commercially and as shown in
Figure 3, has a more rounded
nature and contains less
internal porosity.
Spherical particles produced
with a smooth surface are ideal
from a powder handling and
ageing perspective. They have
the minimum surface area to
mass ratio. A spherical shape also
provides scope for designing and
tailoring the particle packing
properties and surface area for any
given application i.e. as a high or
low density filler. The production
of reduced sensitivity HE materials
and a desire to gain more control
over the shape and morphology
of HE particles is driving research
at AWE and academic outreach
partners.
Recent work with Cranfield
University has been undertaken to
process the explosive HNS (2, 2’,
4, 4’, 6, 6’-Hexanitrostilbene) into
spherical particles. The method
has proven to be a reliable and
controlled means of producing a
range of fine particles in narrow
size distributions.
The challenge was to establish the
optimum conditions for control
over the particle sizes whilst
maintaining a spherical shape. The
use of the spherical HNS particles
in its desired application requires
them to be in a compacted and
geometrically confined state. HNS
has been traditionally processed
by wet chemistry techniques and
the particles were highly irregular
in nature, as shown in Figure 4,
and were not ideal for pressing to
high densities.
FIGURE 3
Figure 5 shows the surface of a
high density pellet pressed from
spherical HNS particles. HNS
spherical particles press readily to
a high density and the compacted
solids produced are robust enough
to be handled. This technique is
being developed and applied to
other explosives including PETN
(pentaery thritol tetranitrate)
and HMX.
Reduced sensitivity HMX (top), and HMX type A (bottom).
6
The science and technology
of PBX formulation involves
the careful combination and
processing of the explosive
filler with polymer binders and
other ingredients. The result is
a precursor for manufacturing;
explosive moulding powders
contain the essential ingredients of
the formulation prepared into low
density granules and are ready for
pressing and machining into the
finished PBX components.
FIGURE 4
Spherical HNS particles (left) compared to HNS type IV (right) (courtesy of
It is possible to control the
product quality in terms of size,
distribution and strength of the
granule in the moulding powder
to favour manufacture. This can be
achieved through selection of the
mixing technology and the right
operating conditions appropriate
to the product required.
Examples of typical processing
techniques available include:
• Solvent-paste mixing
• Water-slurry (PBX process)
Cranfield University).
•Emulsion/coating
• Resonant Acoustic mixing
• Extrusion (paste explosives)
Newer mixing technologies such
as acoustic mixers (Figure 6) are
attractive as they use non invasive,
low-frequency, high-intensity
sound energy that is capable of
mixing high viscosity materials.
They do not possess many of the
FIGURE 5
hazards associated with traditional
mixing technologies, particularly
those that may be attributed to shear
of the HE from the mixing blades.
The development of the mixing
process involves stages of scalingup and maturing the processing
steps by a controlled increase in
the mass of explosive material.
This ensures consistency in
safety, mechanical and explosive
performance properties at each
FIGURE 6
Surface of die pressed pellet produced from spherical HNS (courtesy of
Resonant acoustic mixer.
Cranfield University).
7
Discovery
Discovery25 • The Science and Technology Journal of AWE
stage from laboratory scale, to pilot
plant and finally to manufacturing
plant. Figure 7 shows a test setup
during manufacture and Figure
8 shows some of the equipment
that is used in the scale up
manufacturing.
FIGURE 7
It is important to produce pressed
compacts that are close to the
theoretical maximum density
(TMD) of that composition in order
to maintain performance [4] and
at the same time strive towards
reducing the sensitiveness of the
composition.
An example of a test carried out on new explosive compositions to
measure detonation velocity.
The development of such highly
filled compositions that are
pressed close to their TMD is a
relatively unique situation when
compared to other industries.
FIGURE 8
1 litre (left) and 25 litre (right) planetary mixer used to scale-up explosive compositions using the solvent paste
mixing process.
8
FIGURE 9
20 mm
Small particles coating larger particles.
The choice of average particle
size and particle size distribution
used in a composition will affect
the formation of granules in the
moulding powder, as shown in
Figure 9 and hence the processing
behaviour of the resulting powder
and so are essential when
optimising the density of the
final pressing.
Summary
The challenge to produce well
characterised explosive particles,
to formulate them into tailored
compositions and eventually
manufacture into components
intended for a desired purpose
involves the use of a wide
variety of techniques, utilising
some established processes
and some more state-of-the-art
concepts. Only by developing an
understanding of raw materials,
processing and its influence on
final product throughout its service
life, will total control be possible.
Optimisation of the particle
modification processes for
easier processing and targeted
properties, such as producing
specific sizes of spherical HNS
particles, has been achieved.
Particle characterisation
techniques are available to
confirm the desired effect.
Formulation techniques aim
to produce explosives with
the appropriate power, safety
and mechanical properties to
satisfy the requirements of the
application. It also supports
the manufacturing process to
produce homogeneous mixtures
with granule properties that
make subsequent pressing and
machining processes more
efficient.
9
References
[1] J. Akhavan, The Chemistry of
Explosives, The Royal Society
of Chemistry, 2004.
[2]
Ruth M. Doherty, Relationship
Between RDX Properties and
Sensitivity, Propellants,
Explosives, Pyrotechnics 33,
No. 1 (2008).
[3]
RDX and HMX with Reduced
Sensitivity Towards Shock
Initiation –RS-RDX & RS-HMX,
Øyvind Hammer Johansen, Jørn Digre Kristiansen,
Richard Gjersøe, Alf Berg,
Terje Halvorsen, Kjell-Tore
Smith, Propellants, Explosives,
Pyrotechnics 33, No. 1 (2008).
[4]
E. C. Abdullah, A. M. Salam
and A. R. Aziz, Cohesiveness
and Flowability Properties of
Silica Gel Powder, Physics
International, vol. 1, no. 16, 2010.
Discovery
Discovery25 • The Science Engineering and Technology Journal of AWE
Micromechanical Modelling
The micromechanical model is a material science finite
element analysis (FEA) model that has been developed
through a strategic partnership between Imperial
College London, University of Cambridge and AWE. It is
a predictive capability to support the understanding of
material safety, particularly relating to the mechanical
integrity of energetic materials.
Polymer Bonded Explosives (PBX)
are energetic composites that
are designed to meet safety and
performance requirements. Not
only are they exploited for their
explosive power but their role is as
an engineering component when
subjected to in situ stresses and
strains during the manufacturingto-disassembly lifecycle. The
explosive and engineering safety
of PBXs depends on acceptable
strength to resist sudden cracking
or damage.
A typical PBX comprises of stiff
crystalline explosive particulates
bonded within a polymer matrix,
called the binder, see Figure 1.
The rigid particles reinforce and
stiffen by imparting rigidity to the
composite and in turn limiting
the amount of stretch before
failure occurs.
The required mechanical strength
of the composite can be tailored
by varying the type of materials
and ratios of material ingredients.
In order to achieve target material
mechanical properties, a cycle of
trial and error that can result in
the need to produce numerous
formulations. This repetition
raises safety issues regarding the
increased exposure, handling
and cost.
FIGURE 1
Modelling Capability
The micromechanical model
capability was developed
to support the explosive
formulations effort at AWE. The
aim is to predict a composite’s
mechanical response from
properties of the constituents and
their interactions; reducing the
need for experimentation and
improving safety.
The concept behind a simulation
of PBX composites is essentially
to represent the microstructure
(i.e. < 150 μm in length) as a
representative volume element
(RVE) to simulate the response
of a continuous material. The
particles are assigned properties
attributable to that of the explosive
crystal and the material between
boundaries as binder properties.
Stresses and strains (tensile or
compressive) can be applied to
the boundaries and using finite
element modelling methodology
the resulting cracks or weak areas
can be realised.
There are two input methods to
determine the geometry of the
RVE, either a simulated structure
or representative structure in the
form of an image.
Cracked explosive microstructure (courtesy of University of Cambridge).
11
Representative images of PBXs are
produced by Scanning Electron
Microscopy (SEM). However,
by using this method not all
of the particles are accounted
for. The very fine particles are
not always captured using the
microscopy methods; additional
detail from the images is lost in
the binary image conversion. The
representative structure results
in an approximate 60% packing
Discovery
Discovery25 • The Science and Technology Journal of AWE
particles are normally assumed to
be simply elastic, as they are
orders of magnitude less compliant
than the associated binder material.
FIGURE 2
The University of Cambridge has
provided AWE support on
explosive safety for a number
of years through bespoke
experimentation. One success
has led to characterisation of the
adhesive interaction between the
binder and fill particles.
Scanned (left) and simulated (right) RVE (courtesy of Imperial College
London).
fraction, which is significantly
less than the intentional packing
fraction in PBX materials (90% ‒
95%). An alternative is to simulate
the structure using particle
packing software; in this case a
higher packing fraction can be
simulated at the cost of idealised
particle shapes. Figure 2 shows the
difference between scanned and
simulated RVE.
Once an RVE structure is created
and meshed, various material
models are applied that describe
the individual constituent material
properties and their interactions.
These material and interaction
models are derived through
bespoke methods from our
strategic partners Imperial
College London and University
of Cambridge.
A number of binder constitutive
rheological models have been
developed by Imperial College
London. Such material models are
often complex and are dependent
on a variety of factors such as
strain rate and temperature. They
are enhanced to make up for
the ‘lost’ volume fraction in the
images and as a result stiffen the
rheological properties of the pure
binder, for example using the
Mori-Tanaka method. Material
models for the crystalline fill
They have developed two
methods; the first is a method
called the Wilhelmy Plate Method
which introduces binder coated
surfaces into reference liquids
characterising their surface
chemistry. From such
values, a thermodynamic work of
adhesion, between the binder and
crystalline fill value, can be derived.
This effectively equates to the
FIGURE 3
Position-sensitive
Photodetector
Laser Diode
Cantilever Spring
Sample
Tip
Atomic force microscopy binder pull off (courtesy of University
of Cambridge).
12
FIGURE 4
Failure propagation within the microstructure.
energy requirement to separate
the two surfaces from each other.
The second method employs
Atomic Force Microscopy
(AFM) to measure the adhesive
interaction by binder pull off
experiments. This method uses
cantilevers that have been doped
with a hemisphere of binder
material at one end, see Figure 3.
These cantilevers are fully
calibrated then slightly pressed
into samples of the crystalline fill.
The direction of the cantilever
is then reversed and the force
required to remove the samples is
recorded, thus fully characterising
the interaction.
Both methods lend themselves
to characterise the interaction
between any crystal or binder
system without the need to
physically formulate the composite.
The Model in Action
Analyses have been carried out
using finite element software.
Once all the material model and
interaction specific variables
have been inputted, test specific
boundary conditions
that mimic real life mechanical
test regimes are applied to the
model and it is set to run over a
specified time.
This is being achieved by the
incorporation of new materials,
new failure mechanisms and
additional interactions such as
friction between the binder and
particulate fill. The extra data will
refine the predictive capability
and choice of material models for
numerous explosive compositions
of interest.
Simulations have provided
excellent agreement both visually
and numerically for a variety of
materials. Figure 4 demonstrates
various methods of failure,
including adhesive debonding and
material failure which coalesce to
mimic composite fracture, features
known to occur in real life PBXs.
Due to such promising results and
the effective working partnership
between AWE, University of
Cambridge and Imperial College
London, the micromechanical
modelling capability is being
further developed.
13
Discovery
Discovery25 • The Science Engineering and Technology Journal of AWE
Personal Protective
Equipment Testing
The management of hazards by a hierarchy of control measures is a widely accepted
system promoted by the Health and Safety Executive. The use of Personal Protective
Equipment (PPE) is the least effective control in this hierarchy and is usually
implemented as a final defence. PPE includes gloves, safety glasses and work wear,
the selection of which is of critical importance in an explosives laboratory. In the
laboratory, hands-on techniques are employed to prepare new explosives at the
small (i.e. gram) scale. In addition to minimising the hazards through other controls,
it is vital that the PPE provides suitable and sufficient protection against personal
injury should an unintended explosion occur.
The main hazard from an
unintended small scale
explosion in the laboratory is the
fragmentation of the experimental
apparatus such as glass or ceramic
flasks, funnels etc. A review of the
literature on this subject revealed
that there was minimal evidence
to support the selection of PPE
to protect against this threat [1].
Consequently, a series of realistic
trial scenarios were designed
to test a range of PPE and thus
provide data to enable appropriate
PPE to be selected.
As part of the safe system of
work for the development of new
explosive materials, the process
begins with very small quantities.
Quantities are scaled up once the
new material has been shown
to possess the required level of
safety to a range of stimuli such as
impact, friction, heat etc.
In order to mimic the hazards
associated with the scale up
process, explosive charge masses
of 0.3 g, 1.0 g and 7.5 g were used.
Additional charge masses of 0.05 g
and 5.0 g were used to simulate
specific experimental scenarios.
All charge masses given in this
paper except the 0.05 g charge
were in addition to the explosive
mass in the detonator.
Deliberate firings of high
explosives are initiated by a
detonator but these are not
representative of an accident
scenario in a chemical laboratory.
Care was taken to select a
detonator arrangement that
minimised the release of metal
fragments, as these could
otherwise adversely influence the
test results. A Reynolds Industries
RP-2 detonator, mounted within a
thick cardboard tube, was shown
to be effective in this regard. This
detonator also has an extremely
low explosive mass of 0.05 g, thus
any impact on the trial results
were minimal.
The fragment source consisted
of selected items of laboratory
apparatus, with a small PETN
(pentaerythritol tetranitrate)
explosived charge placed inside.
This charge was then deliberately
initiated by the RP-2 detonator, to
break up the apparatus and propel
the resulting fragments.
The containers selected to act as
fragment sources included glass
test tubes, glass round bottomed
flasks and ceramic Buchner
funnels. The different fragment
threats produced by the varying
containers and charge masses
allowed the degree of protection
FIGURE 1
Casting of ballistic gelatine hands inside protective gloves.
15
Discovery
Discovery25 • The Science and Technology Journal of AWE
FIGURE 2
Example of high speed video stills of a 0.3 g explosive charge inside a ceramic Buchner funnel.
afforded by the following items of
PPE to be evaluated:
• Four types of glove
• Two types of wrist protector
• One type of face shield
• Two types of safety spectacles
• Two types of safety screen
The items of PPE were placed at a
representative stand-off from the
fragment source. Ballistic gelatine
(see Figure 1) was used to simulate
human tissue in order to assess
the damage done to ‘hands’ both
with and without PPE.
TABLE 1
Container type
Explosive mass (g) Peak observed velocity (ms-1)
Buchner funnel
0.3
85
Buchner funnel
1.0
162
Buchner funnel
7.5
590
Round bottom flask
0.3
200
Round bottom flask
1.0
475
Round bottom flask
7.5
1230
Test tube
5.0
1260
Approximate peak fragment velocities.
16
smaller mass of explosive was also
assessed. As part of the early stage
explosives formulation process, a
very small quantity of explosives
and binder (0.02 g) in solvent
may be hand-mixed in a nickel
crucible, using the heat of the
hand to drive off the solvent from
the mixture.
FIGURE 3
Damage to gelatine hands from 0.3 g (Left), with the palm protected, and
1.0 g (Right) explosive in a glass round bottom flask.
Inspection of the gelatine enabled
an assessment of the damage done
to ‘hands’ inside protective gloves.
Fragment flight and impact was
visualised using a Phantom V710
camera recording at 25,000 frames
per second.
Hands
The high speed videos of the
various firings revealed the speed
and relative sizes of the fragments
produced, see Figure 2.
Fragment velocities and sizes were
measured using high speed video
footage and the highest recorded
velocities are shown in Table 1.
The glass round bottom flask
firings resulted in a large number
of small, high velocity fragments.
At 0.3 g and 1.0 g, the ceramic
Buchner funnel produced fewer
fragments, which were generally
larger in size and with lower
velocities. At 7.5 g the ceramic
Buchner funnel was extensively
fragmented, resulting in many
small fragments; however the
fragment velocities were still
significantly lower than for the
round bottom flask with the same
charge mass.
Trials to assess the damage of
unprotected gelatine ‘hands’
showed a large number of fragment
penetrations from the glass round
bottom flask fragment source even
with a 0.3 g charge. At the 1.0 g
scale, the injuries to a real hand
would be extensive, see Figure 3.
The hazard to unprotected hands
posed by the detonation of a much
Two gelatine filled nitrile gloves
were arranged, one supporting
a nickel crucible and the other
suspended above the crucible
holding a bone spatula, see
Figure 4. The test configuration
represents a significant over-test,
as the charge mass is more than
double that which would be present
in a real accident (the 0.05 g in a
RP-2 detonator was the smallest
charge that was capable of being
fired), the explosive is a consolidated
charge (rather than lower density
powder) and full detonation occurs,
which is extremely unlikely in this
accident scenario.
Examining the gelatine ‘hand’
after the nickel crucible firing
revealed a shallow area of damage
FIGURE 4
Nickel crucible with a 0.05 g explosive charge (RP-2 detonator only).
17
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Discovery25 • The Science and Technology Journal of AWE
(Figure 5), which, if considered
on its own, could lead people to
assume that this scenario would
only cause a shallow surface
injury to an unprotected hand.
FIGURE 5
The high speed video showed the
nickel crucible being forced down
into the gelatine hand, which
would likely result in soft tissue
damage, bruising and potentially
broken bones. The recovered
crucible was found to be severely
damaged with the end splayed
open, resulting in sharp metal
edges which were driven into the
gelatine hand, see Figure 6.
This highlighted the need for a
holistic assessment of the data
gathered, as focusing on just one
element created an unbalanced
view of the full effect.
Damage to gelatine hand from Nickel crucible test.
Gloves
FIGURE 6
Splaying of Nickel crucible.
FIGURE 7
Filter Paper
Explosive Charge
Built in Ceramic Gauze
Explosive located in Buchner funnel, not in contact with external walls.
18
Low levels of surface damage
were observed on gloves and wrist
protectors using 0.3 g and 1.0 g
charges, with the ceramic Buchner
funnel as the fragment source.
Extensive fragment damage to the
surface of the PPE was observed
under the same conditions with a
glass round bottom flask fragment
source, furthermore at the 1.0 g
scale it was clear that penetration
of some test articles had occurred.
The same observation was made
when repeating the experiment
with a 7.5 g charge; the glass
round bottom flask produced
significantly more damage than
the ceramic Buchner funnel.
The degree of damage caused by
the different fragment sources
under the same conditions can
Table 2
Penetrations, number/depth of deepest penetration
Test article
Unprotected hand
0.3 g ceramic
Buchner funnel
1.0 g ceramic
Buchner funnel
7.5 g ceramic
Buchner funnel
4/moderate
18/moderate
Severe/deep
Red glove
No penetration
No penetration
6/deep
White glove
No penetration
No penetration
2/shallow
Blue glove
No penetration
No penetration
9/shallow
Grey glove
No penetration
No penetration
11/shallow
Yellow wrist protector
No penetration
18/shallow
Severe/deep
Grey wrist protector
No penetration
No penetration
18/shallow
Summary of results from Buchner funnel trials.
Table 3
Penetrations, number/depth of deepest penetration
Test article
0.3 g glass round
bottom flask
1.0 g glass round
bottom flask
7.5 g glass round
bottom flask
Unprotected hand
56/moderate
Severe/deep
Severe/deep
Red glove
No penetration
26/moderate
80/deep
White glove
No penetration
57/moderate
Severe/deep
Blue glove
No penetration
16/moderate
65/moderate
Grey glove
Not tested
6/shallow
45/moderate
Yellow wrist protector
2/moderate
17/deep
Not tested protector
Grey wrist
Not tested
10/shallow
Severe/deep protector
Summary of results from glass round bottom flask trials.
19
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Discovery25 • The Science and Technology Journal of AWE
FIGURE 8
Example of high speed video stills of a liquid filled Aluminium bath with a glass round bottomed flask containing
a 7.5 g explosive charge.
be attributed to two main factors.
Firstly, there is a significant air
gap between the explosive and
the relevant external structure of
the ceramic Buchner funnel (see
Figure 7), whereas the geometry
of the round bottom flask means
that the flask wall is in very close
proximity to the explosive that
was projected towards the PPE.
The air gap in the Buchner funnel
attenuates the shock and reduces
the effect of the explosive on the
fragment source. Secondly, the
walls of the ceramic funnel are
much thicker and more massive
than those of the glass flask and
are therefore not accelerated to the
same degree.
This explains the results in Table 1,
where the peak observed fragment
velocities from the glass round
bottom flask are higher than those
for the ceramic Buchner funnel at
all charge masses.
An indicative summary of the
protection offered by the gloves
and wrist protectors against the
ceramic Buchner funnel and glass
round bottom flask fragments
is given in Tables 2 and 3. The
relative depth and number of
penetrations is shown, with
‘Severe’ indicating multiple
areas of conjoined penetration
that could not be divided into
individual impact points.
Aluminium Pan
Aluminium pans are often used
in oil baths to allow explosives
synthesis processes to be heated. The
ability of a oil filled aluminium
pan to contain fragments from a
glass round bottom flask using
1.0 g and 7.5 g charges was
assessed (see Figure 8).
The liquid (silicone oil in a
laboratory environment, water
in a test) transfers the explosive
20
shock directly to the aluminium
pan. With a 1.0 g charge the
pan remained intact providing
effective radial fragment
protection from the glass flask,
whereas a 7.5 g charge was
sufficient to fragment the pan.
Safety Spectacles, Face
Shields and Screens
Safety spectacles and a standard
polycarbonate laboratory face
shield were tested against a 1.0 g
charge in a glass round bottom
flask, with complete protection
being provided. A 5.0 g charge
was subsequently tested with a
combination of safety spectacles
and face shield using a test tube as
the fragment source.
In this situation the fragments
which impacted the PPE were
directly in contact with the
explosive charge, maximising
the energy of the fragments. The
against realistic explosive
laboratory accident scenarios.
AWE has used this evidence to
select more appropriate PPE and
has thus significantly improved
the level of protection against
these hazards. In general,
protection, or at least a significant
reduction in harm, was possible
for the smaller charge sizes
providing an appropriate degree
of stand-off was maintained.
FIGURE 9
References
Glass test tube firing of a 5.0 g explosive charge with face shield, safety
glasses (left hand side), 3 mm safety screen (right hand side) and 6 mm
[1] Klapötke et al, Safety Science,
48 (2010), 28 – 34.
safety screen (front).
Acknowledgements
face shield suffered multiple
penetrations; however fragments
that defeated the face shield were
stopped by the safety spectacles,
which were behind the face shield.
The same combination of PPE
was tested with a 7.5 g charge in
the glass round bottom flask. The
face shield failed catastrophically
and although the safety spectacles
were not penetrated by fragments,
the arms of the spectacles were
fractured at the hinges.
A safety screen constructed from
3 mm polycarbonate was subjected
to fragments generated by glass
round bottom flasks with 1.0 g
and 7.5 g charges and a glass test
tube with a 5.0 g charge.
The screen provided complete
protection against the 1.0 g charge;
however it failed catastrophically
when subjected to the much more
severe 5.0 g and 7.5 g threats,
resulting in large fragments of
polycarbonate with velocities
of up to 66 ms-1. A 6 mm
polycarbonate screen provided
complete protection against the
5.0 g charge in the test tube.
Figure 9 shows still images from
the high speed video of the 5.0 g
explosive charge in a glass test
tube. On the left hand side of the
experiment are the face shield and
safety specticals, on the right is the
3 mm polycarbonate safety screen
and at the bottom is the 6 mm
thick polycarbonate safety screen.
The initial stages of failure can
be observed for the 3 mm safety
screen in the lower middle image
shown in Figure 9.
The authors would like to thank
the AWE Media Group for their
support and use of their high
speed video facilities and the
Electro-Explosives Team for the
use of their firing chambers and
all the preparation work needed
to conduct these trials.
Conclusions
These trials have allowed
quantitative assessment of the
mitigation provided by PPE
21
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Slapper Detonator Modelling
Detonators are an essential component for the safe, repeatable and reliable
operation of an explosive train. Slapper detonators (or Exploding Foil Initiators)
were first developed in Lawrence Livermore National Laboratory in 1965 by John
Stroud. Slapper detonators were designed mainly for military applications but have
since been adopted by several fields including explosive welding and hardening, oil
and mining.
At AWE several modelling
techniques are employed to
help design Slapper detonators.
Traditionally these models
included empirical fits to past
experimental data but with recent
leaps in computational power
more predictive, first principle
based techniques are being
developed.
The functioning of a Slapper
detonator can be sectioned
into three parts: the electrical
system, the flyer launch and the
shock initiation of the explosive
pellet. This article describes the
techniques used in the empirical
based approach and provides
a brief description of future
activities for more predictive
techniques.
plasma. Inertial forces restrain
the expansion of the metal for a
period of time, after which the
metal expands rapidly; this point
is known as the point of burst.
The expanding plasma punches
out a flyer that travels down a
short barrel. At the end of this
barrel the flyer shock initiates
an explosive pellet such as HNS
(hexanitrostilbene) or PETN
(pentaerythritol tetranitrate).
Slapper detonators require an
electrical firing system, called
the Fireset, to supply the correct
electrical input to the bridge.
The Fireset and detonator can
be carefully designed to achieve
the correct timing and output
pressure from the device.
A Semi-analytical
Slapper Detonator Model
The bridge has large variations
in resistance due to changes
in the thin foil when a large
amount of current is passed
FIGURE 1
Bridge
Lands
Dielectric material
Functional Overview
Figure 1 provides a side schematic
Tamper
view of a typical Slapper detonator
and the 3D representation of a
chip slapper detonator. Typically
a few thousand amps of current
Explosive (usually HNS or PETN)
are passed through the bridge
Explosive (usually HNS or PETN)
Flyer travels down barrel
material, causing a high current
Barrel
density and electromagnetic
Air
fields. This in turn causes Joule
Flyer travelsDielectric
down barrel
material
Lands
Heating within the metal and
Barrel
Air
subsequently the resistance
Tamper
Dielectric
Bridge
significantly increases.
material
Lands
While the metal heats it rapidly
changes state from a solid to
Schematic representation of a Slapper detonator.
Tamper
Bridge
23
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Discovery25 • The Science and Technology Journal of AWE
through it. Modelling this
behaviour from first principles
is extremely computationally
expensive as thermodynamic
and electromagnetic effects
would need to be taken into
account. Empirically fitting to
past data is a much faster method
of determining the electrical
response of a bridge, providing a
semi-empirical model rather than
a first principles approach.
specific action, and therefore
accumulated energy, has been
shown to increase as a function of
initial current rate of rise [2]. This
suggests that there are inertial
forces which initially restrict
bridge expansion.
Several authors have tried to
understand and characterise
the point at which the bridge
bursts in Slappers detonators.
Anderson and Neilson were the
first to use the concept of action
which is based loosely on classical
mechanics [1]. Action is defined as
FIGURE 2
Exp current
Exp voltage
Sim current
Sim voltage
Current (A)
Voltage (V)
t
A(t) = ∫ tI 2 (t) dt
A(t) =0 ∫ I 2 (t) dt
Past experimental data can be
used to construct specific
action-resistivity curves for a
particular bridge material. Simple
scaling laws can then be used
to adjust the shape and position
of this curve to different Fireset
conditions. The electrical current
within the circuit can be modelled
using a first order differential
equation which is supplemented
with the appropriate specific
action-resistivity curve. A
prediction can be made of the
0
A more general term is defined as
specific action
t
2
I (t) 2
g(t) = ∫ t I (t) dt
g(t) =0 ∫ a
dt
a
0
Time (s)
Validation of electrical response prediction made by the model.
Velocity (ms -1)
where I(t) is the current flowing
FIGURE 3
through the wire and a is its area.
Action is a convenient quantity to
characterise the1point of burst
E �as
=
J
=
voltage and resistance
1 are difficult
E�
=E c + � c =� Ec + E � c
to obtainJ experimentally.
EE c + �� c � Ec + E � c
�
E
Anderson and Neilson found
from several experiments that
the specific action at burst as
a function of current density
was invariant to within 10% for
Simulation
each respective bridge material.
Experiment
Therefore, specific action can
Distance travelled (mm)
potentially be used to find
the point of burst for a bridge
material, irrespective of the
Prediction of flyer velocity and comparison with experimental trace.
bridge geometry. In practice
24
Using the point of burst and
energy data, the velocity of the
flyer can be calculated. The flyer
is pushed down the barrel by the
rapidly expanding bridge which
has changed state. The model
assumes that the density of the
expanding gas is uniform, there is
an infinite tamper behind the flow,
the flyer only has kinetic energy
and the gas flow is ideal.
A one-dimensional,
incompressible Navier-Stokes
formula is used to express the
pressure in the gas and the ideal
gas equation of state is used to
relate the pressure of the gas
and input energy. As a result, an
analytical expression can be found
for the acceleration of the flyer
which is solved numerically using
a Runge-Kutta algorithm.
Experimental flyer velocities can
be obtained from a technique
called Photonic Doppler
Velocimetry (PDV) and can be
used to validate the model.
Figure 3 shows that the model
can achieve a very good fit to
experimental PDV traces. This
gives confidence that the correct
impact velocity is being used
for the prediction of shock
initiation within the detonator
explosive pellet.
The final part of the model
predicts whether the energy from
the impact of the flyer is sufficient
to cause a detonation within the
explosive pellet. The one
dimensional initiation threshold
of explosives can be quickly
evaluated using the James
Initiation Criteria [3] developed at
AWE. The James Criteria uses the
observation that many explosives
require a minimum amount of
energy to initiate. This phenomena
is taken into account using a
critical specific energy, ∑ c , where
P is the pressure imparted into the
explosive and τ is the duration of
the shock.
Experimental
data have shown
t
that
for=a given
there is a
A(t)
I 2 (t)explosive
dt
critical value
of E, denoted E c and
0
Σ c , below which no detonation
occurs, as illustrated in Figure 4.
∫
This means that a single value,
t
2
J, can be calculated
I (t) and used
= whether ordtnot the
tog(t)
predict
a
explosive0pellet within the
detonator will detonate or not.
The James Criteria parameter is
defined as:
∫
∑c = u 2/ 2
and u is the particle velocity in
the shock transmitted to the
explosive by the flyer impact. The
energy per unit area of the shock
imparted into the explosive is
also needed and is defined as
J=
1
Ec � c
+
�
E
=
E�
� Ec + E � c
The semi-analytical Slapper
detonator model developed at
AWE calculates the threshold
firing voltage needed for the
E = Puτ
FIGURE 4
Increased shock pressure
dynamic electrical response of
the bridge and hence the energy
supplied to the flyer when
launched. Figure 2 shows a
comparison between experimental
data and predictions made by
the model. Typically electrical
predictions are made within
15% error with experiment using
this technique.
Σ
Det.
J=1
Initiation
Cut-off
Σc
Non-Det.
Ec
E
Increasing shock duration
Critical values found for a typical explosive below which no detonation
is seen when impacted with a thin flyer.
25
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Discovery25 • The Science and Technology Journal of AWE
FIGURE 5
Magnetohydrodynamics model showing temperature.
explosive HNS and showed
within 10% agreement with
experimental data.
High Fidelity
Predictive Models
The techniques described so far
are empirical in nature making
them dependent on existing
experimental data. The first
principles approach demonstrates
application of our understanding
of the detonator function and
could provide greater accuracy
in the predictions of Slapper
detonator performance over
the semi-empirical approach.
However, high fidelity modelling
of Slapper systems is not trivial as
electromagnetic, thermodynamic
and hydrodynamic processes
are all prevalent and these also
demand experimental data.
Much like the semi-analytical
approach described above,
the physical phenomena in
a Slapper detonator can be
broken down into stages.
Magnetohydrodynamics (MHD)
has long been used in astrophysics
to model magnetic fields in stellar
objects. This technique can also
be applied to Slapper detonators
as extremely large electromagnetic
fields are seen that also need to
be coupled to a thermal
conduction model.
AWE has recently been using
a code to successfully perform
MHD simulations, as illustrated in
Figure 5. In future this technique
will be applied to Slapper
detonators to further facilitate the
design of new Slapper detonator
geometries and materials.
In addition to an MHD component,
predicting the initiation of
the explosive is also required.
Presently there are very few
models which could simulate
the build up of detonation in
either PETN or HNS. A technique
which can simulate the build
up to detonations is called
reactive flow modelling and
although more predictive than
the James criterion, still requires
experimental data and certain
assumptions to supplement the
equation of state.
26
It is understood that the important
reaction stages of the detonator
running to detonation takes place
over multi-length scales. In the
future a multiscale modelling and
experimental approach may be
needed to obtain a fundamental
understanding of the explosive, as
illustrated in Figure 6.
FIGURE 6
Hydrodynamics
Metres
Seconds
Mesoscale
Micrometres
Microseconds
Molecular dynamics
Nanometres
Nanoseconds
Equation of state,
P(V, E, T) needed for
continuum simulations
0.1 nm
P(V, E, T) of
crystalline
high explosive
obtained from
simulations
0.1 nm
A multi-scale modelling approach to understanding the buildup to an explosive detonation.
Summary
References
Acknowledgements
Slapper detonators can provide
both safety and performance
benefits to an explosive system.
They are often part of a complex
initiation train which requires
both precise timing and output
pressure from the detonator.
Modelling these systems gives
a great deal of insight into
performance, given its complex
and coupled internal processes.
Currently AWE uses empirical
methods to explore the design
space and parameters needed
to achieve a specific outcome.
However, in future more high
fidelity physics based approaches
will be used to obtain a more
predictive capability.
[1]
The authors would like to thank
Hugh James for the use of his
diagram to explain the James
Criteria. Also the authors would
like to thank Jonathan Baker
for the images showing
magnetohydrodynamic
simulations. Many thanks
also to Scott Aitken for
the experimental data used to
validate the semi-analytical
Slapper detonator model.
Anderson, G.W. and Neilson,
F.W. - Use of the “Action Integral”
in exploding wire studies.
Exploding Wires, Plenum Press
Inc., first edition, 1959.
[2] Lee, R.S. - Fireset. Lawrence
Livermore National Laboratory
Report, UCID 21322, 1988.
[3]
James, H.R.- An extension to the
critical energy criterion used to
predict shock initiation
thresholds. Propellants,
Explosives, Pyrotechnics,
vol. 21, p 8 – 13, 1996.
27
Many thanks to Peter Bolton,
Colum O’Connor and Adam
Hazelwood for providing an SEM
image of PETN.
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Discovery25 • The Science Engineering and Technology Journal of AWE
Hazard and Safety Studies
To determine whether an explosive is safe to use, it is subjected to UK standard tests
to determine the materials sensitiveness (a measure of the stimulus required to
ignite or initiate an explosive material) and explosiveness (a measure of the degree
of violence shown by an explosive material in response to a given stimulus). UK
standard tests for both approval and qualification form part of AWE’s capability to
demonstrate the hazard and safety characteristics of explosive materials. These are
in addition to bespoke experiments, simulation and material modelling efforts.
The tests commonly used at
AWE examine the response to
impact, heat and Electro Static
Discharge (ESD). Tests include
impact sensitiveness (Rotter test),
response to incendive sparks
(Ease of Ignition test), response to
prolonged flame exposure (Train
test), heat stability (Temperature
of Ignition test), sensitiveness
to impact between a variety
of surfaces (Mallet Friction
test), response to exposure
to electrostatic discharges of
different energies (Spark test),
explosiveness response when
impacted and ignited (LabSET
test), impact with crush and
pinch (Steven test) and response
due to penetration by a metal
rod (Small Scale Spigot test). The
tests are undertaken on either
powder or relatively small pressed
charges. Figure 1 is a picture of
an explosive powder undergoing
exposure to a prolonged flame,
also know as the Train test.
If the material passes the tests
then it may undergo qualification
tests designated by the Defence
Ordnance Safety Group (DOSG)
for approval. These tests use
pressed charges to examine
the response of the explosive to
potential threats that the material
may encounter during its lifecycle.
The two most likely threats are
impacts e.g. during transportation
or as a result of drops or falls and
thermal e.g. fires.
A number of approval tests have
their origins in the conventional
munitions arena. Examples of
some of the threats and stimuli
FIGURE 1
that the qualification tests subject
the explosives to are; glancing
blows against gritted surfaces
(Oblique Impact test), crush,
pinch and extrusion (Susan test),
intrusion (Spigot Impact test),
localised ignition whilst confined
(Tube test – Internal Ignition)
and bulk heating whilst confined
(Tube test – Fast Heating).
Performing these approval tests
on a variety of explosive materials
offers an opportunity to ‘rank’
materials in terms of sensitiveness
and explosiveness.
The extent of the reaction in
the explosive is dependent
on a number of factors such
as the composition of the
explosive (explosive type, binder
concentration and material etc),
nature of the threat (for impact
tests speed, mass, shape, material
and orientation) and degree of
confinement around the explosive
in the region of the reaction.
Confinement can protect the
explosive from the threat;
generally higher levels of
confinement give more violent
reactions than bare or lightly
confined tests. For bare or lightly
confined test vehicle impacts, the
explosives are generally disrupted
by the impact to such an extent
that the sites of ignition are either
extinguished or that the extent
Explosive powder burning steadily in the train test.
29
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Discovery25 • The Science and Technology Journal of AWE
FIGURE 2
Frames from a video camera showing the reaction of some explosives, encased within a lightly confining test
vehicle, impacted by a bullet.
of the reaction does not grow
past the burning stage. Heavy
confinement tends to keep most
of the explosive intact within the
test vehicle and traps the gasses
produced from the reaction of the
explosives.
The trapped gas increases the
pressure within the test vehicle
and causes the burn rate to
increase, which increases the
pressure rapidly and which
further increases the burn rate. If
the explosive has extensive cracks
throughout its structure, as a
result of the damage caused by
the impact, then the flame front
can rapidly travel throughout the
explosive causing the majority
of the volume of the explosive
to contribute to the rapidly
increasing pressure within
the confinement. Usually, this
rapid rise in internal pressure,
which can occur in just a few
milliseconds, will overcome the
strength of the confinement and
the test vehicle will disassemble
in a manner that can appear to
resemble an explosion. If the
confinement provided by the test
vehicle is sufficiently high, this
rapid increase in internal pressure
can transition from a rapid burn to
detonation.
30
AWE's hazard and safety
capability is extended by the
work with Cranfield University.
The effect of insult type and
severity on the reaction growth
and subsequent violence has been
researched and implemented at
a firing facility at Cranfield. This
work was undertaken under
the AWE technical outreach
programme.
The research contract has enabled
both impact and thermal threat
type tests to be developed. Tests
have ranged from drop weight
impacts, bullet impacts (see Figure
2) and projectiles fired from
FIGURE 3
Stills from a video showing a gas gun projectile impacting a spigot that causes a reaction in a heavily confined
explosive powder.
gas guns into bare or confined
explosives. Figure 3 shows
stills from a video showing the
response of gas gun projectile
impacting a spigot that causes a
reaction in a heavily confined
explosive powder.
beneath, through the glass surface,
by high speed video cameras and
any ignition of the explosive is
recorded as a flash of bright light,
Figure 4 shows schematics of the
Trolley test and some target plates
after impact.
A recent development in impact
testing (Trolley test) complements
the Oblique Impact Qualification
test which is used at approval.
Whereas the Oblique Impact test
examines the explosiveness of
a large explosives test sample
by dropping it at an angle onto
a gritted surface from different
heights, the Trolley test examines
the sensitiveness of a small sample
of the explosive to a similar threat
by using a gas gun to propel
the trolley, with the explosive
hung beneath, down a track so
that the explosive impacts an
inclined, gritted toughened glass
surface. The impact is filmed from
Thermal tests developed as part
of the research contract with
Cranfield University have enabled
explosive test samples to be
heated, in different test vehicles,
which provide various levels of
confinement, at different heating
rates until the explosive reacts.
Blast gauges, thermocouples and
video cameras are used to record
the development and growth of
any reaction produced by the
explosives. In one test a disc of
explosive was lightly confined in
a hermetically sealed test vehicle
and subjected to a propane fuelled
flame on the top surface of the
test vehicle. It was found that the
The trapped gas increases the pressure
within the test vehicle and causes the
burn rate to increase, which increases
the pressure rapidly and which further
increases the burn rate.
31
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test vehicle separated relatively
benignly and most of the explosive
was recovered unreacted.
FIGURE 4
a.
To model the response of
explosives to impact, a predictive
capability called HERMES (High
Explosive Response to Mechanical
Stimulus) has been developed.
The HERMES material model
comprises of several sub models
including a constitutive model
for strength, porosity and surface
area through fragmentation, an
ignition model an ignition front
propagation model, and a model
for burning after ignition.
b.
Ignition is based on a purely
mechanical criterion depending
on a time integral of a function
of the shear, equivalent stress,
pressure and strain rate as follows:
c.
Ign = ∫
d.
t
0
2–
27 s1s2s3
2Y 3
5
p + s2 2
P0
1/2
έp dt
Here s 1,2,3 are the principal stress
deviators, Y is the equivalent
stress, p is the pressure, P0 is
a prescribed constant value of
pressure, and έp is the plastic
strain rate. The mathematics
are designed to capture the
cumulative effects of shearing,
compressing and rapid straining
through the respective terms in
the time integral.
e.
The Trolley test. (a) A schematic of the test layout with the target, trolley
and test track on the left of the wall and the gas gun on the right of the
wall. (b) A schematic of the explosives under the trolley impacting the
inclined target surface. (c) The target plate after impact with HE smeared
across its surface. (d) Shows the remains of the explosive attached to the
trolley after impact. (e) Frame from the high speed video showing
ignition (the bright flash of light in the upper centre of the picture) of
the explosive as it impacts the target.
32
Ignition is deemed to commence
when I gn reaches a particular value
called the ignition criteria. The
ignition criterion vary between
different types of explosives and
are determined from observations
of experiments with these
different explosives.
FIGURE 5
Projectile
Cover Plate
Test Vehicle Base
The predicted ignition location (left image) for explosives in the “Steven” test. The projectile is deforming the
test vehicle cover plate which is crushing the explosive against the test vehicle base. A “Steven” test vehicle
base, recovered from a test (right image) shows annular scorch patterns on the base plate.
In order to validate the results
obtained from HERMES, a
simulation was constructed of an
impact test (Steven test) with the
results obtained being compared
to the results of experiments.
For a range of impact velocities,
a number of model parameters
were varied and their effect on the
predicted explosive response
was noted.
The predicted onset of ignition
due to extreme shear and material
deformation occurs in an annular
region similar to those seen in
experiments. Figure 5 show the
predicted ignition locations and
the base plate for a Steven test.
For the prediction of explosives
response to thermal threat, there
have been a number of heat flow
codes which have been modified
to include chemical reaction
routines. These codes have ranged
from the relatively simple early
codes such as TRUMP and TOPAZ
to more recent and considerably
more complex coupled codes
such as CALORE and ALE-3D.
These codes are constantly under
development with incorporations
such as more complex chemical
reaction kinetics for the explosive
materials.
Summary
AWE maintains a modern hazard
testing capability and the ability to
develop new test tools to examine
the response of explosives to
particular threats. This is needed
to continue to assure that the
explosives that are used at AWE
are suitable and safe to use
throughout their lifecycle.
Future Work
The models are planned to be
improved by carrying out tests
that determine the physical
mechanisms which determine
the onset of reaction and its
subsequent growth. This will
entail the inclusion of improved
diagnostic techniques in
current tests and the design and
implementation of bespoke
test methods.
The development of an enhanced
predictive capability based on
computer modelling enables better
understanding of the behaviour
of the explosives when subjected
to threats as well as providing
the opportunity to reduce the
requirement of fielding expensive,
large scale trials.
33
Discovery
Discovery25 • The Science Engineering and Technology Journal of AWE
AWE’s Outreach
Major Events and
Collaborative Activities
In this section, we cover a number of high-profile
events and conferences in which AWE has been
involved. They represent a wide range of disciplines
and areas of expertise and also put AWE’s relationship
with key stakeholders into context.
UK Policy Conference
Some 120 newcomers and experts
in the field of nuclear weapons
policy gathered at the third
AWE sponsored annual Project
On Nuclear Issues (UK PONI)
Conference in London. The
event, held on 6 June 2013 at the
Royal United Services Institute
(RUSI), created a forum which
combined fresh perspectives on
nuclear policy with insights and
experiences from established
experts to explore the future of
nuclear policy, both at home
and abroad.
The conference attracts
representatives from across the
nuclear community including
the UK government, industry
and academia. There were also
international representatives
from the US, Russia, France and
China giving attendees a thoughtprovoking and lively debate.
Other representatives included
FCO, NATO, MOD, Rolls-Royce,
King’s College London, Lockheed
Martin and the Centre for
Strategic International Studies.
Former Secretary-General
NATO, the Rt Hon Lord
George Robertson, delivered a
keynote address during which
he described his journey on
deterrence issues and some of
the political challenges when
he was UK Defence Secretary
and during his time in office at
NATO. He said Continuous at
Sea Detterrence (CASD) is the
ultimate deterrent to safeguard
our country.
Rt Hon Lord George Robertson describing his nuclear journey
Institute of Mathematics
Nearly 50 scientists, industry
experts, academics and students
had their first glimpse of the
world leading Orion laser facility
when the delegation gathered at
the Institute of Mathematics and
its Applications (IMA) Employers’
Forum, hosted by AWE
Aldermaston on 4 March 2013.
The one-day session centred on
academic technical outreach,
including some of the advantages
and challenges that are faced
by employers and universities
when engaging in outreach. As
well as AWE, other organisations
represented included Dstl, Thales,
EDF Energy, Schlumberger,
Qinetiq, Heriot-Watt University,
University of Reading, University
of Cambridge, Loughborough
University, University of Warwick,
University of Stirling and
University of Manchester.
Inaugural Linear Solver
Workshop
A two-day workshop was held at
Warwick University on 8-9 July
2013 to discuss a key challenge for
exploiting the rapidly evolving
high performance computing
(HPC) architectures to solve the
complex science and engineering
models which underpin the
techniques used by AWE for
certification of the stockpile.
Feedback from the meeting
attendees highlighted the
importance of bringing together
experts in these diverse areas
together in the UK to discuss
the common challenges faced in
35
Discovery
Discovery25 • The Science and Technology Journal of AWE
exploiting the next generation of
hardware platforms. To exploit
the next generation of hardware,
the workshop highlighted the
importance of developing new
algorithms which remains one
of the main challenges hence the
involvement of numerical analysts.
This will be driven by the
competing demands for greater
power efficiency and increased
performance.
International
Radionuclide Migration
Conference
Over 300 international delegates
gathered at The Brighton Centre,
for the Migration 2013 conference.
This was the first time the
conference had been held in the
UK and gave our scientists the
opportunity to promote their
research to a wide number of
universities, research institutions
and government bodies.
"the UK has a long history of
leadership in nuclear forensics. While
important in its own right, it also
gives us the edge in a diplomatic and
political context through raising the
UK profile and enhancing credibility.”
The Migration conference series
provide an international forum
for the exchange of scientific
information on chemical processes
controlling the migration
behaviour of actinides and fission
products in natural aquifer
systems. AWE hosted a number
of posters at the evening sessions
and, throughout the event, an
AWE exhibition stand showcased
the core business areas and breadth
of the science conducted at AWE.
AWE presence at Migration 2013 Conference
36
The main focus of the conference
was the development of actinide
chemistry to enable the
characterisation of activation and
fission products in natural aquifer
systems and the subsequent
assessment of geochemical species
and their migration in the
geosphere.
Presenters provided lectures
on experimental investigations,
method development and
predictive modelling. A special
session was held to discuss the
programme in place for waste
disposal in the UK.
AWE supports the 14th international Conference on Radionuclide Migration
Nuclear Forensic
Workshop
Nearly 80 experts in nuclear
forensics from around the
world gathered for the Nuclear
Forensics Workshop held on
7-9 January 2014 at Lancaster
House, London. The three-day
event was co-sponsored by AWE,
MOD, the Home Office and the
Foreign Office under the aegis of
the Global Initiative to Combat
Nuclear Terrorism (GICNT).
The event shared knowledge in
nuclear forensics between the
GICNT Member States to support
global security efforts.
The mission of the GICNT is to
strengthen global capacity to
prevent, detect and respond to
nuclear terrorism by conducting
multilateral activities that
strengthen the plans, policies,
procedures and interoperability
of partner nations. The GICNT is
co-chaired by the US and Russia.
Minister for Crime and Security,
James Brokenshire MP, said: “In
recognition of the threat of nuclear
terrorism, the UK is developing
advanced nuclear forensics
capabilities and practices. These
capabilities will be embedded
and integrated into our existing
law enforcement and operational
systems to provide a seamless end
to end capability for managing
nuclear security incidents.
Chief Scientific Adviser to the
Foreign Office, Professor
Robin Grimes, said: “It is well
appreciated that the UK has a
long history of leadership in
nuclear forensics. While important
in its own right, it also gives us the
edge in a diplomatic and political
context through raising the UK
profile and enhancing credibility.”
capability that allows the UK
to investigate criminal acts
involving nuclear materials.
The Conventional Forensics
Analysis Capability can recover
fingerprints, fibres, DNA and
other traditional trace forensics
markers from material that
have been contaminated with
radiological, nuclear or explosive
materials.”
As part of the forum, a structured
table-top exercise called Blue
Beagle was discussed by a panel
composed of leaders in nuclear
forensics from law enforcement
and other agencies. The panel
discussed various stages of
a radiological crime scene
investigation.
“Building upon the knowledge
and capabilities of AWE we have
created a dedicated world-class
nuclear forensics analytical
37
Discovery
Discovery25 • The Science and Technology Journal of AWE
Discovery
Editor:
Doctor Graeme Nicholson
Editorial Board:
Contributors:
David Chambers
Doctor David Geeson
Doctor Norman Godfrey
Doctor Katherine Grant
Rashad Hussain
David Murray
John Roberson
Overview
Doctor Claire Leppard
Doctor Steve Trussell
Explosives Materials Development
Doctor Peter Bolton
Doctor Adam Hazelwood
Michael Hopkins Till
Graphic Design and Illustration:
Micromechanical Modelling
Doctor Claire Leppard
Daniel Lewis
AWE Media Group
Photography:
AWE Media Group
Find out more about AWE at our website:
www.awe.co.uk
Comments and suggestions regarding this
journal, please email:
discovery@awe.co.uk
For further copies of this journal and details
of other AWE publications, please write to:
Corporate Communications Office
Building F161.2
AWE Aldermaston
Reading
Berkshire
RG7 4PR
38
PPE Testing
Doctor Peter Jenkins
Stephen Miller
Doctor Chris Murray
Slapper Detonator Modelling
Doctor Mary-Ann Maheswaran
John Richardson
Hazard and Safety Studies
Doctor John Curtis
Andrew Jones
25
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AWE Aldermaston, Reading, Berkshire, RG7 4PR
Discovery 25 • The Science Engineering and Technology Journal of AWE
The Science & Technology Journal of AWE • Issue 25 • September 2014