william g. proud et al- diagnostic techniques in deflagration and detonation studies

8/3/2019 William G. Proud et al- Diagnostic techniques in deflagration and detonation studies

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New Trends in Research of Energetic Materials, Czech Republic, 2010 [Content]

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Diagnostic techniques in deflagration and detonationstudies

William G. Proud*, David M. Williamson, John E. Field,

and Stephen M. Walley

Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom* currently at - Institute of Shock Physics, Imperial College London, United Kingdom

[email protected]

Abstract:

Advances in a number of experimental, high-speed techniques have been used to explore

the processes occurring within energetic materials. This review describes techniques used

to study a wide range of processes: hot-spot formation, ignition thresholds, deflagration,

sensitivity and finally the detonation process. As this is a wide field the focus will be onsmall-scale experiments and quantitative studies. It is important that such studies are

linked to predictive models, which inform the experimental design process. The stimuli

range includes, thermal ignition, drop-weight, Hopkinson Bar and Plate Impact studies.

Studies made with inert simulants are also included as these are important in differentiat-

ing between reactive response and purely mechanical behaviour.

Keywords: high-speed; quantitative; experimental; diagnostic; characterisation

1 IntroductionEnergetic materials are characterized by high energy-release rates that result in phase trans-

formation, rapid temperature rises and mechanical or other work. These dramatic changes takeplace on sub-millisecond to sub-nanosecond timescales making them difficult to study usingstandard techniques. For many years, studies tended to focus on post-reaction effects, e.g. thesize of craters in metal blocks, ignition thresholds, the impulse given to pendulums. The valueof these studies should not be under estimated as they form the basis of this field of study.However, the interpretation of these results was wide and the resulting theories reflect the lim-ited understanding of the high-speed processes.

Industrialization and increased use of explosives resulted in a number of unfortunate ex-plosive accidents and poisonings [1]. Legislation was imposed which required the reduction inrisk to property and people. This drove the development of a series of qualification and classi-

fication techniques to address the variety of common hazard scenarios [2]. These also servedthe purpose of producing a series of reference scales that allowed the broad comparison of datafrom different sources.

With increased emphasis on factors such as environmental issues, safety and handling andreliability, modeling has become a major area of development: the promise of wide applicabil-ity, prediction of material properties and the description of whole systems. However, accurateprediction requires good knowledge of fundamental behaviour and well-controlled validationexperiments. This drives increased interest in physical understanding of the reaction processes.While many of these properties and trends are present in legally required qualification studies,often it is in the presence of complicating factors.

The advent of modern data capture techniques has allowed increased physical understand-

ing and the ability to study effects that have previously been masked. A range of small-scale
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tests and techniques have been developed to isolate particular processes, both to populate andvalidate predictive models. High-speed data capture has made nanosecond time-resolved dataincreasingly common, advances in opto-engineering have produced cameras and lasers whichoperate at speeds and sensitivities not possible a few decades ago [e.g. 3].

In this paper a number of techniques will be discussed which give an overview of someimportant material parameters and processes. It should be emphasized that this is not an ex-

haustive account nor are the techniques applicable in all cases; however, they do show that tostudy these systems there is a need for simple, well-controlled stimuli, giving clear results. Inmany cases these studies show the importance of previously unknown or secondary processes,resulting in much improved understanding and another cycle of development.

2 High Strain Rate RegimesThe range of strain rates, along with the corresponding techniques is listed in Table 1. A

review on these techniques applied to a wide range of materials has been published by the au-thors elsewhere [4]. In this review the techniques will be discussed specifically in relation toenergetic materials; drop weight studies are generally conducted with respect to ignition levels,

Hopkinson bar studies with relation to accident and impact scenarios, while plate impactprobes the region leading to detonation. The main effect of increasing the strain rate is that thetransient stress levels increase and the sudden delivery of energy allows processes with highactivation energies to be accessed. Processes, which operate on long time scales, e.g. thermaldiffusion, which are significant under quasi-static loading, do not have time to occur.

It is generally found that yield and fracture stresses increase with increasing strain rate [5].The increase in failure stress is very marked at strain rates above 103 s-1 and the effect of inertiabecomes significant. Ultimately the response changes from one where the sample can be as-sumed to be in stress equilibrium to one of a wave with associated 1-D strain moves so quicklythat the material does not have time to move laterally, as seen in shock waves.

Table 1: High Strain Rate Regimes and the Associated Equipment.

Strain rate Equipment Stimulus duration Comment

10-6 10-2 Instron 100s of seconds Quasi-static loading10

+2 10

+3Drop weight 10s milliseconds Generally used to determine

impact ignition thresholds10

+2 10

+3Hopkinson bars 100s microseconds Compression, tension and tor-

sion loading. Extensively usedfor PBX formulations. Constitu-tive models.

10+4

10+5

Miniature Hopkin-

son bar

10s microseconds For fine grain materials or sin-

gle crystals. Generally metals10+3 10+6 Taylor impact 10s microseconds Sometimes used for metal jack-

eted energetic samples10+5 10+8 Plate impact microseconds Pressures and durations similar

to that of gap tests. Laser drivenflier plates have sub-microsecond duration high-intensity shocks
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3 Techniques and Diagnostics3.1 Quasi-static techniques

Quasi-static loading and standard property determination form the bedrock of all materialcharacterization. Properties traditionally determined are density, thermal capacity and conduc-

tivity, melting points, ignition temperature, spectroscopic signatures, molecular and crystalstructure. Advances in x-ray techniques and especially tomography have allowed the internalstructure of composites and crystals to be determined in a non-destructive fashion [6]. Atomicforce microscopy allows chemical composition, crystal hardness, facet variation to be seen at ananometer level [7] while environmental scanning electron microscopy gives insight to thecrystal scale variation to be observed. Many high rate studies are rationalized by reference tothese properties, which need to be determined experimentally they are not always simple topredict e.g. the theoretical maximum density of a polymer-bonded explosive.

X-ray diffraction, over a range of sample temperatures, allows the structure to be deter-mined in a series of sequential experiments. It has been widely confirmed that crystal orienta-tion is significant in defining material sensitivity through the crystal structure and the crystal-lographic slip planes. Temperature controlled x-ray diffraction has been useful in showing thesensitivity of molecular and crystal-cell properties, estimating the heat capacity and highlight-ing a wide range of phenomena, phase transitions, the movement of water of crystallization andultimately when long-range order breaks down and the material decomposes [8]. Figure 1(a)shows the structure of copper(II) styphnate tetrahydrate, while figure 1(b) shows the tempera-ture variation of the crystal parameters. The crystal expands in a regular fashion until ~300K.Above this, the crystal structure starts to break down as initially the water of crystallizationmoves out of the structure. This is rapidly followed by bulk breakdown of the material. For thismaterial, this shows the limitation of its practical use, but also it indicates the fundamentalcause of the marked change in the crystal structure.

(a) (b)Figure 1. (a) Crystal structure and (b) crystal parameters of copper(II) styphnate tetrahydrate.

Taken from [8]

Moving beyond the crystal to consider polymer bonded systems much effort has been di-rected into understanding the movement of cracks through the system [9-12]. For polymer-bonded systems, fractures, which move within the polymer binder alone, are considered impor-tant in weakening the material but are not obviously related to increased ignition sensitivity.

Cracks which move through the energetic crystals opening up an interface where crystal frag-ments can the rub together and become hot are a more obvious route to increased sensitivity.
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The use of data from moir interferometry or image correlation, in combination with microme-chanical models, has shown the importance of the marked strain rate behaviour of polymers.Polymers become harder and more crystal-like with small downward changes in temperature orincreases in strain rate [12]. The adhesive strength between crystal and binder is very importantin the strain localization process [11].

3.2 Deflagration to Detonation StudiesThe localisation of energy into small regions of an energetic material to produce hot spots

has been widely studied and reported [13, 14]. Several mechanisms, e.g. void collapse, havebeen identified as important in this process. These mechanisms tend to act simultaneously andso it is sometimes difficult to identify which is the most important. However, once a sufficientnumber of hot spots have been produced by heating or impact, deflagration will start to spread.The dynamics of this process can be studied in a number of ways; one way is to use high-speedphotography and combine this with other techniques.

Amongst the simplest experiments are those, which use metal cylinders as shown in figure2. The channel along the centre of the cylinder had a diameter of 5 mm and varied in length

between 70 and 100 mm (total length 100 - 130mm). Initial analysis of what had occurredwhile reaction had taken place in a column was performed by a system of post-mortem exami-nations. Test shots were carried out using cylinders made of copper, steel and brass. It wasfound that copper cylinders gave the best record of the event that had taken place. Brass provedtoo brittle to survive a detonation in one piece and steel although providing some record of theevent did not distort in the ductile manner of copper and as such provided a less clear picture ofthe processes that had occurred.

(a) (b)Figure 2. Cylinders for use in (a) post-mortem type experiments (b) photographic study of

DDT. Taken from [15]

Plastics such as polycarbonate and polymethylmethacrylate (PMMA) were not tried as ithad been shown [15] that the level of confinement offered by these materials was insufficientto withstand the sustained pressures required for a DDT event in PETN or RDX. In experi-ments where a photographic record of the events was required, it was necessary to use a con-finement that afforded direct optical access to the charge whilst retaining the pressure levels ofthe simple metal confinement.

The confinement shown in figure 2(b) was developed by Luebcke [16] based on previousdesigns by researchers such as Korotkov [17] and Griffiths and Groocock [18]. The confine-

ments used consisted of sections of square steel bar with a 5 mm channel drilled along the cen-tre for the charge and a 1 mm wide polycarbonate window laid into the steel. The window af-
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fords an optical viewing point whilst being thin enough that the level of confinement affordedby the steel is not overly compromised. The length of the charges used varied from 45 to 80mm depending on the nature of the event that was being investigated.

The explosive charges were pressed incrementally using a hydraulic press. Care was takento ensure that the height of each individual increment never exceeded more than half of the di-ameter of the column. Charges below densities of 50% of theoretical maximum density (TMD)

were also incrementally pressed. A system whereby static weights were placed on top of thepressing rod was used in these cases. A pyrotechnic was used to ignite the columns of explo-sive. The pyrotechnic was ignited using a nichrome wire through which was passed an electri-cal current. The mixture consisted of 80% potassium dichromate and 20% boron as developedby Dickson [19]. It has the advantage of producing few gaseous products, so little pre-pressurisation of the charge occurs. The burning temperature is far higher than the ignitiontemperature of the materials used in the charges. The pyrotechnic was tamped in order to en-sure as few gas pockets as possible so as to reduce rapid expansion during reaction. The pyro-technic was added after the main charge had been pressed into the column. Finally a smallconical aluminium piece was placed around the electrical wires and pushed into the confine-ment to reduce rearward venting of the charge to a minimum. As a result pressure generated

during the early stages of reaction could be sustained.Often, a thin piece of copper foil was placed between the pyrotechnic charge and the ex-

plosive charge. This was done in order to prevent light emitted during the slow burning of thepyrotechnic charge from being transmitted down the column to the optical fibres and as a resultprematurely triggering the experimental diagnostics. The brass tube into which the pyrotechniccharge was placed also acts as a light stop as well as an easy check on the amount of pyrotech-nic that is used.

The type of data recorded from this experiment is shown in figure 3. A full description ofthe processes can be found in the papers of Gifford [20-22]. However, the streak image showsconvective burning B, the step from deflagration to detonation D and the movement of the

waves including a retonation wave F. Addition of high-speed thermocouples [21] to the systemallowed the internal states at point along the column to be known prior to detonation.

Figure 3. Normal or type I DDT event in ultrafine PETN. Taken from [21]

3.3 Drop Weight StudiesA standard technique for sensitivity studies, the drop-weight is a system that allows long,

low-level pressure pulses to be applied to small samples. In combination with high-speed pho-tography, photodiodes and stress transducers it is a powerful system for both qualitative andquantitative analysis of material response.

The standard way of analyzing the output of a drop weight assumes the weight behaves asa rigid body and hence that one can simply apply Newtons laws of motion. This is used in de-

termining the calibration of a drop weight force transducer dynamically, by directly relating the
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measured force both to the deceleration of the weight also to position. A typical signal from acalibration experiment is presented in figure 4.

Figure 4. Typical output from a drop-weightstress transducer used in these studies. Takenfrom [4]

Figure 5. The ignition under impact of athermite composition. Taken from [31]

In practice, the output signal from a drop weight machine often has oscillations comparablein size to the signal produced by the mechanical resistance of the specimen. This is particularlytrue if the drop weight itself is instrumented, e.g. with accelerometers. The reason is that im-pact excites the weight below its resonance frequency [23]. Elastic waves, therefore, reverber-ate around inside until the momenta of the constituent parts of the weight have been reversed.Rebound then occurs and the specimen is unloaded. Recent work [24] has demonstrated that itis possible to obtain high quality data from such machines either by the use of a momentum

trap in the weight, if the weight itself has to be instrumented [25] or by careful design of a sep-arate force transducer placed below the specimen e.g. in [26,27].

One modification to the drop weight apparatus, which has proved invaluable in the elucida-tion of explosives ignition mechanisms, is to machine a light-path through the weight and toperform the deformation between transparent glass anvils [14, 2831]. This allows the event tobe captured using high-speed photography. An example of a classic high-speed photographicsequences obtained using this apparatus is given in Fig. 5.

3.4 Second Harmonic GenerationIn a recent paper the use one of the more unusual optical properties of HMX to probe the

ignition process [32]. The relatively new technique of Second Harmonic Generation (SHG)was used to study the phase transition in HMX by many authors in the late 1990s [3336]A second harmonic is generated when intense radiation is incident on molecular crystals with

appropriate symmetry properties. -HMX, the chair configuration, is a centrosymmetricmolecule and as such is forbidden by its symmetry from generating second harmonics. How-

ever, -HMX, a boat configuration, has no centre of inversion and generates second harmon-ics very efficiently. For these second-order processes to be observable the incident radiationneeds to be very intense and so a laser is required. In order to produce second harmonics nearthe peak sensitivity of the high-speed cameras, a Nd:YAG, (YAG = yttrium aluminum garnet)laser operating at 1064 nm was used to irradiate the sample allowing the detection of the sec-

ond harmonic at 532 nm (green). This technique can be applied in a transparent anvil drop
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weight apparatus. The second harmonic generation is near instantaneous process and so a 9 ns

laser pulse, can a probe for -HMX during impact.The apparatus allowed separate light paths for the laser light to travel to the sample and for

the visible light (both light from reaction and SHG) to travel from the sample to the cameras. Adiagram of the drop weight setup is shown in Figure 6.

Figure 6. Drop weight modified to detectSHG in HMX crystals during impact. Taken

from [32]

Figure 7. Four images showing SHG. Thefield of view of each image is 10 mm wide.

All of the images have enhanced and reversedcontrast (black indicates the presence of

SHG). Taken from [32]

The cameras used were Imacons 790 and 792, and a notch filter in front of one of them

was used to isolate SHG, while the other followed the visible light from reaction. A dropheight of 39 cm was used, giving an impact velocity of 2.75 m s -1. The height was chosen asthe lowest at which the sample would reliably ignite on impact. The stress-time output was ahalf sinusoid with a peak pressure of 107 Pa and duration of the order of 104 s. A light gatebroken by the falling drop weight just before impact was used to trigger the diagnostics, and aphotodiode monitored the time of ignition relative to that time reference. While precise impacttimes are not known for individual experiments, the photodiode gave a good reference as towhen reaction began. The system was such that one frame on the filtered camera coincidedwith the laser pulse. The visible light camera was set to record a sequence starting just beforethe laser pulse. Each pulse deposited 0.8 J of energy over an 8 mm diameter spot.

The sensitivity of the system to the presence of-HMX, in the sample bulk, was shown by

using -HMX pellets converted to -HMX through selective heating. SHG was clearly visible

whether the phase regions were on the top or the bottom of the pellet and so -HMX at anydepth in the sample could be detected with excellent spatial resolution in pellets up to 1 mmthick.

The time delay between the trigger provided by a light gate broken by the falling drop

weight and the laser pulse was varied, in order to probe for -HMX throughout the impact. Se-lected images showing SHG are presented in Fig. 7. The images showed the second harmonic

in narrow regions of the sample. In the experiments all the impacted pellets reacted but no -

HMX was observed any earlier than 13 s before ignition. This implies that any transition to

the phase occurs late in the reaction sequence. It could be the case that either the -HMX re-gion grows rapidly before ignition or that after ignition the higher temperature of the burningmaterial converts the rest of the sample before it is completely consumed.
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In earlier research, Field [37] saw localized heating along lines and at their junctions andattributed this to shear band formation. It appears that the heating on impact is sufficient to

cause conversion to phase under these stress conditions before ignition, the patches of -HMX observed were small (1 mm across) and very localized. After ignition, larger patches (3

mm across) of-HMX were seen before the sample was completely consumed.

3.5 Digital Speckle PhotographyHopkinson bars are used to probe rates of 102 104 s-1. This is an important region in strain

rate characterization as here the dynamic strength of the material increases dramatically. Ratesensitive materials, e.g. many polymers, become very brittle in this strain-rate regime . A fullerdescription of these techniques can be found in [4]. In general the output of Hopkinson Barstudies takes the form of stress-strain curves (figure 8). However, this is based on assumptionsabout sample integrity, no cracking, and homogenous material flow. For polymer-bonded ex-plosives (PBXs), this may not be the case [38]. It is important to measure the movement of thesample during the test procedure in order to ensure the results are valid.

To do this, a technique called digital speckle photography was developed. This is a method

that, in broad terms, compares the movement of random patterns, either naturally present, as inthe case of large grain-size PBXs, or painted onto the surface. In the study summarized heresamples of an inert PBX simulant, a polymer-bonded sugar (PBS), were sprayed with a dilutesolution of silver dag, to give a fine, silver speckle pattern. High-speed photographs weretaken using an Ultra-8 camera, and analyzed using software described in [39, 40]. The proce-dure is described in more detail in [41] and the result shown in figure 9.

Measurements of the specimen diameter during the experiments was also obtained bymeasuring the shadow cast by the specimen, which for an opaque un-cracked specimen with asmooth surface is the same as the specimens load bearing diameter.

Figure 8. Stress-strain curvesfor PBS9501 at three Hopkin-son bar strain rates. Taken from[40]

Figure 9. Evolution of dis-placement at 1420 s-1. Ex-posure time 2 s, frametimes shown relative to ar-rival of stress wave.

Figure 10. Stress-time plotfor the specimen of PBS 9501in Fig. 9. Photographs in 2are indicated by crosses.

The evolution of displacement on the surface of a typical specimen, relative to the unde-formed frame, is given in figure 9. The arrow on the frame at 24 s shows the onset of strainlocalization, before surface cracks are visible in the next frame. Comparison to the stress-

strain curve in figure 10 shows that the first sign of localization corresponds to the peak stress.This means that that localization, or cracking, precedes strain softening in this material.
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The speckle data were, where possible, also used to calculate global surface strain fields inthe specimens in longitudinal and radial directions, not shown, and thus apparent Poissons ra-tio at different strains, figure 12.

The line laser output was used successfully to calculate the radial strain of a number ofspecimens. The scatter between specimens is shown neatly in figure 11, where two experi-ments at the same strain rate give overlapping longitudinal, but very different radial, strain time

curves. Figure 12 shows the results of calculations of apparent Poissons ratio, defined as ra-dial strain divided by longitudinal strain, from 6 experiments, compared to those from thespeckle analysis and a typical stress-strain curve. The small strain Poissons ratio, calculatedfrom ultrasonic measurements, is ~0.30. The figure shows that the values calculated fromspeckle data are consistently below those of the line laser.

Figure 11. Comparison ofapparent Poissons ratios

from line laser and speckle (3specimens) to the small strainPoissons ratio (X) and a typi-

cal stress-strain curve.

Taken from [40]

Figure 12. Comparison of lon-gitudinal strains from the stan-dard Hopkinson bar analysis toradial strains from the line la-

ser for two specimens.

Figure 13. Comparison ofstress-strain curves calcu-

lated assuming volume con-servation in the specimen,

and using the apparent Pois-sons ratio.

Overall these measurements show during deformation the early onset of specimen frac-tures, which produce pockets of material and of air: the area of the specimen is not the same asthe load bearing area of material. Therefore, the true stress in the material may be better repre-sented by assuming volume conservation in the material not volume conservation of the speci-men. To measure volume conservation in this way would require significant technique devel-opment in dynamic tomography for example.

Therefore, a true understanding of the physical behaviour of the material will require amore complete evaluation. However, it is encouraging to note that, as shown in figure 13, be-cause the key features of the mechanical behaviour occur at small strains, the difference be-tween the stress-strain curve calculated assuming volume conservation, and that calculated us-ing the line laser measurement of specimen radius shows less scatter than that between differ-ent specimens.

3.6 Digital Speckle RadiographyDigital Speckle Radiography (DSR) is equivalent to Digital Speckle Photography (DSP)

[42] in that the technique relies upon the cross-correlation of image subsections. However,DSR makes use of X-ray images and the placing of a lightly populated,


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ternal deformations of optically opaque materials. The use of short duration of flash X-rays, 70ns, is ideal for freezing fast processes such as those that occur in ballistic impact [43].

Here the DSR technique has been applied to a series of experiments designed to simulatethe effects of setback and set-forward upon the explosive fill of modern munitions [44]. Thatis, the high inertial forces exerted during gun launch (setback) and impact/penetration (set for-ward).

An inert polymer bonded explosive simulant, a PBS, based on granulated sugar in a hy-droxy-terminated polybutadiene (HTPB) matrix, was designed to mimic the fillings of gun-launched energetics. The speckle field was created within the target samples by the seeding oflead particles within a central plane perpendicular to X-ray axis and parallel to the impact axis.The lead particles were 500 m in size and the coverage was approximately 20 % by area.

The X-ray system used was a Scandiflash 150 keV unit producing a flash of x-rays in a 70ns pulse. The X-rays impinged upon medical grade intensifier screens and film, which wassubsequently digitally scanned.

The targets were shock-loaded by plate impact [45] of polycarbonate. Additionally, an ex-periment was performed in which the sample was doubly shocked, first by polycarbonate andthen by copper a few microseconds later. Double shock scenarios have important implications

for explosive initiation [46]. Experimental parameters are velocity at impact and time delaybetween impact and triggering the flash X-ray system.

Figure 14 shows the X-ray image which captures the shock-loading process. The projectilehas just begun to penetrate the target. Also visible in the image is a fiducial speckle field whichis used to remove any rigid body motions introduced during the scanning process. This is doneby fitting planes to the u- and v-components of displacement in the fiducial region and sub-tracting these from the total displacements measured from the film. Also visible is the silhou-ette of a UK 1 pence piece, which acts as a scale bar (diameter 20.32 mm).

Figure 14: Flash X-ray ofimpact experiment Projectiletraveling left to right. Takenfrom [44]

Figure 15: u-component datafrom plate impact experiment.

Figure 16: v-component datafrom plate impact experiment.

In the analysis, the u-axis is parallel to the projectile axis, and the v-axis is perpendicular.Figure 15 shows the u-component of displacement obtained from the DSR analysis within thetarget area, the shock wave is clearly visible. The curved nature of the shock front is due to thelateral release of material from a shocked to an unshocked state. The lateral release wavesoriginate from the radial edges of the plate projectile where the material is no longer loadeduniaxially, and will eventually negate the propagating shock wave. Information about the lat-

eral release is contained within the v-component data, figure 16. It should be remembered that
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the data in figures 15 and 16 represent the 2-dimensional displacement fields within the planeof the lead seeded layer, and that the target response is really 3-dimensional in nature.

From spatial shift of the parallel displacement curves reported in [44], and knowledge ofthe time delays between the experiments, estimates can be made of the shock wave speeds: 1.6km s

-1at for an impact at 300 m s

-1. A second series of experiments showed a shock speed of

2.4 km s-1 for an impact at 600 m s-1. This data can also be used to determine the strain associ-

ated with the shock: 4.0 0.1 % for an impact at 300 m s-1

and 7.3 0.1 % for an impact at600 m s-1.

By using a composite impactor it was possible to put a stepped shock wave through thetarget. The displacement shown in figure 17, displays this. Region I is unshocked material. Re-gion II is material subject to shock loading by polycarbonate: the strain in this region is 5.5 0.1 %. Region III is material subject to double shock loading, first by polycarbonate and thencopper: the strain in this region is 60 5 %. This data has been used both to populate and vali-date material models.

Figure 17. Displacement measured in PBS material subject to a stepped shock wave.

3.7 Small Scale Gap TestIn the last two examples, shock waves will be considered; a detonation is by definition a

reactive shock wave. These examples are complementary. In the small-scale gap test a shockimpinges onto a column of energetic material, while in the cylinder test an energetic materialpushes on an inert surround cylinder of inert material.

The gap test is a shock sensitivity technique in which a sample of the material of interest isplaced in contact with a barrier. On the other side of the barrier an explosive charge is deto-nated. The shock from the charge moves trough the barrier, losing energy as it goes, and, ulti-

mately passes into the test material. The thicker the barrier, the more the pulse from the chargeis reduced and the lower the shock felt by the sample. Experiments are repeated with differentbarrier or gap thicknesses until a 50:50, or other probability, go: no-go threshold is established.

In this study small samples were used [47]. The test samples were pressed into PMMAholders. The cylindrical holders were 25 mm long, 25 mm in diameter with a central bore of 5mm. The columns were incrementally pressed with a step height of 0.5 mm to give a very ho-mogeneous column. A variety of pressing densities were studied from 60% - 90%. The holderswere then placed in the arrangement as shown in figure 18. The holder has a PMMA gapplaced above it and a C8 detonator placed on top. The assembly was placed in an armouredbox fitted with an observation window. The camera was triggered by a fibre optic that was fit-ted near the end of the detonator on top of the gap plate. When light was detected from thissource, the camera was triggered. At the lower end of the gap arrangement, was a brass witness
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plate. All contacting surfaces were coated in a thin layer of silicone grease in order to allowgood acoustic transmission between the layers.

The streak records from a series of experiments on 90% TMD PETN presented in figure19. The PETN had an ultrafine grain size. The gaps were 3.51, 3.63, 3.67 and 3.71 mm. It canbe seen that in the top image the detonation is slightly overdriven and was prompt i.e. at the topof the column. The detonation speed then settled down within 5 mm in the column length. In

the second image, there is a slight hooking at the top of the column. This is due to the detona-tion not starting at the top of the column but rather a short distance down it. At 3.67 mm thehooking is severe and the detonation wave breaks out about 8 mm down the column. Finally at3.71 mm there is no light associated with the detonation process. The brass witness platedented in the area exposed to the explosive column indicating a detonation pressure in all casesexcept 3.71 mm. For 3.71 mm, only a small dent was seen indicating the more modest pressureassociated with deflagration.

A similar series of results was obtained for conventional grain size material where the gapthickness for failure was shown to be 5.54 mm of PMMA. In a final series of experiments, Dy-nasen PVDF gauges [48] were placed between a gap plate and a block of PMMA 100 mmsquare and 25 mm thick. The output from a PVDF gauge is shown in figure 20. The output

voltage can be converted to stress. For the 5.54 mm gap used in the experiment here this corre-sponded to a stress level of 2.1 GPa. For a PMMA thickness of 3.67 mm, the stress level wasof the order 4.1 GPa. It should be pointed out that the PVDF gauge is recording the stress forseveral microseconds, much longer than it takes detonation to start.

Figure 18. Experimental arrangements. Top,for gap test, Bottom, for PVDF gauge study

Figure 20. PVDF Output from a gauge sepa-rated by 5.54 mm PMMA gap from

a C8 detonator.

Figure 19. Streak records of fine-grainedPETN 90% TMD. Variation in gap thickness;top - 3.53 mm, second 3.63 mm, third 3.67

mm, bottom 3.71 mm
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3.8 The Cylinder TestThe cylinder test is used to measure detonation performance. In concept it has a very sim-

ple geometry: a tube filled with explosive. The material is detonated and, if possible, the de-formation of the tube is measured and then related to the detonation pressure and velocity.High-speed recording systems, such as streak photography, allow the rate of expansion of thecylinder to be measured relatively straightforwardly.

However, if the aim is to track gradual changes in detonation behaviour of a system, due tothe presence of additives, especially in the width of the detonation front, this requires a moretime-resolved technique. In this case streak photography was supplemented by the used of ve-locity interferometer (VISAR) [49]. In this study it was used to look at the effect of aluminiumadded to sensitized nitromethane [50].

In these experiments the streak camera recorded the radial expansion of the cylinder whilethe particle velocity-time history of the outer surface of the cylinder during detonation was de-termined using VISAR. It is important that the surface is not polished to an optical finish be-cause such a finish is unlikely to be preserved at the high stress and strains associated with thecylinder test. Barker et al. developed this technique in the late 60s and early 70s [49]. The

reflected laser light is captured and split into two beams, one of which is passed through a glasscylinder known as an etalon. Because the refractive index of glass is larger than that of air,the light in this beam is slowed down and hence delayed with respect to the other beam, whichpasses through air. If the target is stationary, the interference pattern will not change with timewhen the two beams are recombined. For an accelerating target, however, the reflected beamwill be Doppler shifted. The time resolution of the VISAR data is 2 ns and the noise on thephoto-multiplier produces an error of 5 %.

Copper cylinder expansion tests were carried out on nitromethane / aluminium (NM/Al)compositions containing 20, 30, 40, 50 and 60% by weight aluminium. The NM used wasAnalar grade and the aluminium particles were spherical, with a mean size of 10.5m. Themixtures were prepared by adding 5% by weight of polyethylene oxide to the NM to thicken it,

thus helping suspend the aluminium, then adding the appropriate percentage of aluminium. Thecopper cylinders (304mm long, 25.4mm ID with a wall thickness of 2.6mm) were mountedvertically, sealed at the bottom end and initiated by a small booster pellet at the top.

The overall expansion was viewed with a Cordin-132 streak camera writing at 5.999mm/s. An argon flash bomb and diffusing screen were used to provide backlighting. Therewas a decrease in expansion speed as the particle loading density was increased with a markedchange between 40% and 50% as shown in figure 22.

The accuracy of measurement of the initial expansion phase is limited by the resolution ofthe film. Analysis of the VISAR traces gave the velocity histories shown in figure 23. Thetraces are offset for clarity. The velocity history in these traces tends to flatten after 10 s but

this is due to the loss in sensitivity when the fibre is close to the expanding copper surface. Theinitial acceleration can be seen to take the form of a series of steps. These are due to the reflec-tion of waves within the copper cylinder. This can be explained with reference to figure 24.

The detonation of the NM/Al mix causes the cylinder to expand. A shock wave is sentfrom the inner wall to the outer wall of the cylinder. This reflects at the outer wall as a release,which moves back towards the centre of the tube. When it reaches the centre it is reflected ascompression due to the sustained pressure of the detonation products. Overall, the outer wall ofthe cylinder accelerates as a series of steps. The cylinder walls become thinner, ultimatelyshredding into fragments. This effect tends to obscure the step-like structure at later times.Hence the system with the most rapid expansion shows the least resolution of the steps whilethe highest loading of Al caused the most easily resolved steps.

While the VISAR data measure the rapid acceleration of the outer surface of the coppercylinder in the first few microseconds, the steps are not seen in the resulting streak displace-
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ment history. Part of the reason for this is that the stepped region corresponds to the first 2 sof the process, where the displacements are very small.

From this simple analysis it can be seen that while the streak camera system is excellent forlarge displacements it would be difficult to capture the detail of the early acceleration usingsuch a system. The displacements measured using the streak and VISAR can be combined asshown in figure 25.

Figure 21. The experimental set-up

Figure 22. Streak camera records. Cylinderexpansion radius time plots for various per-centages mass fractions of 10.5 m Al in ni-

tromethane.

Figure 23. Velocity histories from VISAR forthe loading densities studied

Figure 24. Schematic of deformation of thecopper cylinder.

Figure 25. Combined VISAR (solid) andstreak (dashed) records for 60% Al loading
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4. ConclusionsWhile the above series of examples shows a wide range of techniques applied to energetic

systems, ranging from low to high velocity impact, it is for from exhaustive. However, it ispossible to draw some simple conclusions. Keeping experimental geometries and interfaces assimple as possible is useful as it allows data to be extracted with limited recourse to complexanalysis. Attention to pressing, the use of small increments gives a marked reduction in error

bars which allows other processes to be more clearly observed. In energetic materials, it hasbeen known for over 50 years that ignition and initiation are multi-variable processes and soexperimental design is of paramount importance. In some cases like HMX, the specific crystalproperties allow second harmonic generation to be used to look at the ignition process.

While many of the techniques listed can involve complex equipment, some advances, likethe use of line lasers to monitor material expansion can be done with modest resources.

Finally, the desired output from the study is important, where it is a legal requirement, var-iation in procedure may invalidate the results. However, if it is a study to evaluate materialproperties, experiments can be adapted to lead to new data, which can reveal the complexity ofreactive materials.

5. AcknowledgementsThe authors acknowledge a large number of colleagues, students and collaborators, who

have supported and developed the techniques in this review. Amongst these Drs. P. Dickson,P.J. Rae, S.G. Grantham, C.R. Siviour, H.T. Goldrein, D.J. Chapman, M.J. Gifford, P.E. Lueb-cke, H. Czerski contributed significantly to many of the techniques. The late Dr. Avik Chak-ravarty made a singularly effective contribution to the small-scale gap test. Many technicianshave provided indispensable assistance over the years, D. Johnson, R. Marrah, R. Flaxman, K.Fagan, and others in the Cavendish Laboratory Workshop. EPSRC supported the high-speedcameras. QinetiQ, [dstl], Orica, AWE, MoD, provided the support for much of this research.

The UK-E consortia, supported by MoD, provided the framework for much of the recent ener-getic materials research. Finally, I.G. Cullis, A. Cumming, P. Gould, P. Collins, P.D. Church,D. Mullenger, P. Haskins, M. Cook, I. Kirby, M. Braithwaite, P. Collins, S. Wortley, R. Govierall provided stimulus, guidance and much discussion. This long list is not exhaustive and theauthor (WGP) apologises for any omissions.

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and IM tests. Proc. 37th Int. Ann. Conf. of Institut fr Chemische Technologie. Karlsruhe,Germany: paper 68. 2006

[3] Smith GW, Riches MJ, Huxford RB Smith PA, Seymour CLG, and Bell JD, Ultra: Anew approach to ultrahigh-speed framing cameras. Proc. SPIE4183: 105-118. 2001

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[5] Meyers, MA, Dynamic Behavior of Materials. New York, Wiley Interscience. 1994[6] Greenaway MW, Laity PR and Pelikan V, X-ray microtomography of sugar and HMX

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M. D. Furnish, M. Elert, T. P. Russell and C. T. White eds. Melville, NY, American In-stitute of Physics: 1279-1282. 2006

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sives and propellants. Philos Trans R Soc Lond A;339:26983. 1992[30] Walley SM, Field JE, Palmer SJP. Impact sensitivity of propellants. Proc R Soc Lond

A;438:57183. 1992[31] Walley SM, Balzer JE, Proud WG, Field JE. Response of thermites to dynamic high

pressure and shear. Proc R Soc Lond A;456:1483503. 2000[32] Czerski H, Greenaway MW, Proud WG, and Field JE, - phase transition during drop

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william g. proud et al- diagnostic techniques in deflagration and detonation studies

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