Diagnostic techniques in deflagration and detonation studies
© Proud et al 2015
Received: 11 September 2014
Accepted: 9 September 2015
Published: 28 September 2015
Advances in experimental, high-speed techniques can be 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 on small-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 differentiating between reactive response and purely mechanical behaviour.
Energetic materials give high energy-release rates resulting in phase transformation, rapid temperature rise, producing mechanical and other work. These dramatic changes occur on a sub-millisecond to sub-nanosecond timescales, making them inaccessible to many standard techniques. Initially, studies focused on post-reaction effects e.g., crater size in metal blocks, or impulse given to ballistic pendulums. These studies should not be underestimated, however, the interpretation applied to results was quite broad, so the resulting theories reflect the limitations in understanding the high-speed processes.
Increased use of explosives resulted in a number of explosive accidents and poisonings , so legislation was introduced with the aims of reducing risk to property and people. This resulted in a series of qualification and classification protocols to address common hazard scenarios . They also produced a series of reference scales to allow the comparison of data from different sources. While this ranking is often useful, given the nature of the test fundamental processes may be masked by the presence of complicating factors.
With an increasing importance of environmental issues, safety and handling and reliability, numerical modeling and prediction has become a major area offering the promise of wide applicability, a shorter timescale and lower cost than a large experimental series. However, accurate prediction requires accurate knowledge of fundamental behaviour coupled with well-designed experiments. This increases interest in physical understanding of the reaction processes.
Modern data capture techniques; which give increased capture rates and sensitivity have allowed increased understanding and the availability of nanosecond time-resolved data is increasingly common [e.g., in reference 3]. As a result small-scale tests and sensitive techniques have been developed to populate and validate predictive models.
This paper presents a number of techniques to give an overview of some important material parameters and processes. This is not an exhaustive account nor are the techniques applicable to all cases; however, they show the need for well-controlled stimuli, giving clear results. In some cases the understanding of previously unknown or secondary processes has led to another cycle of development.
High strain rate regimes
High strain-rate regimes, the associated equipment and stimulus duration
100 s of seconds
10 s milliseconds
Generally used to determine impact ignition thresholds
100 s microseconds
Compression, tension and torsion loading. Extensively used for PBX formulations. Constitutive models
Miniature Hopkinson bar
10 s microseconds
For fine grain materials or single crystals. Generally metals
10 s microseconds
Sometimes used for metal jacketed energetic samples
Pressures and durations similar to that of gap tests. Laser driven flier plates have sub-microsecond duration high-intensity shocks
In general yield and fracture stresses increase with strain rate  in particular at strain rates >103 s−1, which is also the regime where the effect of sample inertia becomes significant. Ultimately under shock-loading the sample response changes from one of stress equilibrium to of a wave-controlled process where the material is severely compressed in the principal loading direction but does not have the time to move laterally.
Techniques and diagnostics
Quasi-static loading forms the bedrock of all material characterization. Properties traditionally measured in this way are density, heat capacity, melting point, ignition point, molecular and crystal structure. Advances in X-ray techniques such as tomography have allowed the internal structure of powders and crystals to be determined in great detail . Atomic force microscopy permits chemical composition, hardness and topology to be measured at a nanometer level . Environmental scanning electron microscopy allows insight into variations on a sub-crystal length scale. The analysis of data from many areas rely on these properties which often need to be experimentally determined as they are not easy to predict e.g. the density of a polymer-bonded explosive.
Polymer bonded explosives have been subject to much research into understanding the effect of fracture [9–12]. If fractures move through the polymer binder they will be important in weakening the material but are not so obviously related to increasing material sensitivity. However, cracks that run through the energetic filler particles, opening up new interfaces in energetic crystals which can then rub against each other are a more obvious route to increased sensitivity. Data from moiré interferometry and other image correlation techniques, combined with micromechanical models, show the importance of the strain rate behaviour of the polymer binder. In general polymers become harder with decreasing temperature or with an increase in strain rate . The adhesion between crystal and binder is also very important in strain localization process and the production of critical hot-spots .
Deflagration to detonation studies
Localisation of energy into inhomogeneities within an energetic material produces hot spots [13, 14] from which reaction can build and spread. Several mechanisms such as void collapse have been identified as important. These mechanisms act simultaneously, sometimes it is difficult to identify which is dominant, however, once sufficient critical hot spots are produced, deflagration will start to spread. The resulting deflagration has been studied in a variety of ways using high-speed photography.
Plastics cylinders, in general do not provide the required level of confinement  for a deflagration to detonation (DDT) event. When a photographic record was required  the confinement shown in Fig. 2b, developed by Luebcke , was used, based on previous designs by Korotkov  and Griffiths and Groocock . The confinements consisted of sections of square steel bar, a 5 mm channel drilled along the centre, and a 1 mm wide polycarbonate window laid into the steel. The window allows imaging while being sufficiently thin that the level of confinement is not overly compromised.
Using a hydraulic press, the explosive charges were incrementally pressed to 75 % of theoretical maximum density (TMD); care was taken to ensure that each pressing increment never exceeded more than half of the diameter of the column to reduce density variation along the column. Charges with densities of ~50 % TMD were also incrementally pressed but required more modest force to be applied in this case static weights were placed on top of the pressing rod.
In many cases, a thin piece of copper foil was placed between the ignition section and the explosive charge. This prevented light from the burning of the ignition charge from being transmitted to the optical fibres used to trigger the experimental diagnostics.
Pyrotechnic ignition was used, the mixture being 80 % potassium dichromate and 20 % boron as developed by Dickson . The pyrotechnic was added after the main charge had been pressed into the column. The pyrotechnic was tamped in order to ensure as few gas pockets as possible and was ignited using a nichrome wire heated by an electrical current. This system has the advantage of producing few gaseous products and the burn temperature is far higher than the ignition temperature of the energetic materials used.
To keep a pressure seal on the column a small aluminium cone was placed around the electrical wires and pushed into the confinement to reduce rearward venting of the charge. As a result pressure generated during the early stages of reaction could be sustained.
Drop weight studies
The drop-weight is a standard device used for sensitivity studies that allows millisecond long, low-level pressure pulses to be applied to small samples. If high-speed photography, photodiodes and stress transducers are used as diagnostics this gives a powerful system for qualitative and quantitative understanding.
During impact a drop weight machine shows stress wave oscillations comparable in magnitude to the mechanical resistance of the specimen as the impact excites the weight below its resonance frequency . Elastic waves reverberate to bring the weight to rest and in many cases cause the weight to rebound. Recent research [24–27] shows that high quality data can be obtained from such machines.
Second harmonic generation
In some cases the test material may possess properties that allow specific techniques to be applied. One of the optical properties of HMX was used to probe its ignition under drop-weight impact . Second harmonic generation (SHG) was used to study the β-δ phase transition in HMX in the late 1990s [33–36]. A second harmonic is produced when radiation interacts with molecular crystals of appropriate symmetry; sometimes this is called ‘frequency doubling’. β-HMX has a “chair” configuration and is centrosymmetric: forbidden from generating second harmonics. However, δ-HMX, has a “boat” configuration, lacks a centre of inversion and efficiently generates second harmonics. To allow these second-order processes to be observed the incident radiation has to be intense, laser illumination is required. A Nd:YAG, (YAG = yttrium aluminum garnet) laser, operating at 1064 nm, was used allowing the detection of the second harmonic at 532 nm (green), in the region of many cameras peak sensitivity. The second harmonic generation is near instantaneous process and so a 9 ns laser pulse, can probe for δ-HMX during impact.
A 5.5 kg mass dropping from 39 cm gave an impact velocity of 2.75 m s−1 and would reliably ignite a 50 mg β-HMX sample. The form of the stress history was a half sinusoid of peak pressure 107 Pa and 10−4 s duration. A light gate was used to trigger the diagnostics just before impact, while a photodiode monitored ignition. The cameras were set such that the visible light camera started to record just before the laser while the first frame on the filtered camera coincided with the laser pulse.
Using β-HMX pellets with one surface converted to δ-HMX tested the sensitivity of the system to δ-HMX. SHG was visible independent of the δ phase being on the top or the bottom of 1 mm thick pellet with excellent spatial resolution.
Field  saw localized heating and attributed this to shear band formation. It appears that the heating associated with impact is sufficient to cause small, localized patches of δ-HMX (~1 mm across) in the region of shear bands. After ignition, the heating caused by reaction produces larger patches, ~3 mm across, before the sample was completely consumed.
Digital speckle photography
Micro-mechanical models are being developed and require validation data that includes the motion of the particles and binder as well as the development of fractures. In this case the technique of digital speckle photography can be used. This is a method that compares the movement of random patterns to produce strain maps. These patterns may be naturally present, as in the case of large grain-size PBXs, or can be painted onto the surface.
Specimen diameter was independently measured using the shadow cast during loading by illuminating a line wider than the sample diameter with a line laser and measuring the intensity of the light arriving at a photodiode. As the sample expands the amount of light seen at the photodiode decreases.
Digital speckle radiography
In the last three examples, shock waves will be considered. In this case mechanical behaviour of an inert simulant, followed by a study of energetic sensitivity and finally detonation studies used to understand processed occurring within the detonation front.
Digital Speckle Radiography (DSR) is the x-ray equivalent to Digital Speckle Photography (DSP)  relying on the cross-correlation of image subsections. DSR makes use of X-ray images combined with the placing of a lightly populated, <30 %, layer of X-ray opaque particles within the sample. The benefit of DSR over DSP is that it allows for the measurement of the internal deformations. Short duration of flash X-rays, 70 ns pulse width, are ideal for ‘freezing’ fast processes such as those found in high-speed impact .
Here the DSR technique is applied to experiments simulating the effects of setback and set-forward on the explosive fill of a munition. These effects result from the high inertial forces exerted during gun launch (setback) and impact/penetration (set forward) .
An inert polymer bonded simulant of sugar in a hydroxy-terminated polybutadiene (HTPB) matrix, represented the energetic filling. The speckle field was created by seeding lead particles on the central plane perpendicular to X-ray axis, parallel to the impact axis. The lead particles were 500 μm in diameter and the coverage was ~20 % by area.
The X-ray system was a Scandiflash 150 keV unit producing a 70 ns X-ray pulse. Medical grade intensifier screens and film were used to capture the images on film, which was subsequently digitally scanned.
In these experiments most targets were loaded by single shock wave . However, an experiment was performed in which the sample was doubly shocked. Double shock scenarios have important implications for explosive initiation .
Using displacement curves reported in , and knowledge of the time delays between the experiments, the shock wave velocity was 1.6 km s−1 for impact at 300 m s−1 and 2.4 km s−1 for impact at 600 m s−1. These data is also used to determine the strain associated with the shock: 4.0 ± 0.1 % for 300 m s−1 impact and 7.3 ± 0.1 % for 600 m s−1.
Small scale gap test
In the gap test the shock sensitivity of a material is determined by placed it in contact with a barrier and exposed to a shock pulse produced on the other side of the barrier, the ‘gap’, by an explosive charge. As the shock from the charge moves through the barrier it dissipates: the thicker the barrier the lower the shock felt by the sample. Repeating the experiment with different barrier thicknesses allows a 50:50 or go: no-go threshold to be established.
The camera was triggered when light was detected in a fibre optic fitted near the end of the detonator.
The cylinder test
However, if the aim is to track changes within the detonation front a more time-resolved technique may be required. In this the diagnostics were supplemented by the use of a velocity interferometer (VISAR)  developed by Barker et al. in the late 60’s and early 70’s .
VISAR uses the Doppler shift associated with reflecting light from an accelerating surface. The reflected light is captured and split into two beams, one of which passes through a glass cylinder known as an “etalon”. The glass slows the light and when it emerges it is delayed in time with respect to the other beam, which has passed through air. If acceleration has occurred combining the two beams will produce interference, a beat frequency, which allows the surface velocity to be determined. The time resolution of a VISAR system is of the order 2 ns.
A streak camera recorded the radial expansion of the cylinder while VISAR measured the velocity history of the outer surface of the cylinder . Copper cylinder expansion tests were carried out on nitromethane/aluminium (NM/Al) compositions containing between 20, to 60 % weight aluminium particles. The NM was Analar grade and the aluminium particles were spheres with a mean diameter of 10.5 μm. The copper cylinders were 304 mm long, with an inner diameter of 25.4 mm and a wall thickness 2.6 mm, sealed at the bottom. A small booster pellet initiated the charges.
These examples show a wide range of techniques applied to energetic systems, ranging from low to high velocity impact but they are far from exhaustive. However, it is possible to draw some general conclusions. Using simple experimental geometries is useful as it allows data to be extracted with limited recourse to complex analysis. Attention to sample preparation, particularly the use of small increments to minimize sample density variation, gives a marked reduction in error bars allowing processes to be clearly observed. In energetic materials, it has been known for over 50 years that ignition and initiation are multi-variable processes, so experimental design is of paramount importance.
In some cases like HMX, the specific material properties allow techniques such as second harmonic generation to be used.
While many of the techniques listed can involve complex equipment, some advances, like the use of line lasers to monitor material expansion require modest resources.
Most importantly it is essential to have a clear idea of the desired output from the study. Where the requirement for the study is a legal one, such a qualification test for placing of a material on the market or transporting it, variation in procedure may invalidate the results. However, if it is a study to evaluate material properties, experiments can be adapted to lead to new data, which can reveal the complex and fundamental behaviour of reactive materials.
WGP contributed the sections dealing with gap testing and cylinder expansion studies. DMW contributed the sections on digital speckle radiography. JEF is an expert on ignition and growth of reaction in energetic materials and contributed to the understanding of the results of the DDT studies and provided essential guidance on other sections of the paper. SMW is an acknowledged expert in drop weight studies and contributed significantly towards the sections on drop-weight studies. As a review paper we also acknowledge the contributions made by our students and colleagues. All authors read and approved the final manuscript.
The authors acknowledge a large number of colleagues, students and collaborators, at the Cavendish Laboratory 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. Luebcke, H. Czerski contributed significantly to many of the techniques. The late Dr. Avik Chakravarty made a singularly effective contribution to the small-scale gap test. Many technicians have 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-speed cameras. 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 energetic 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. Govier all provided stimulus, guidance and much discussion. This long list is not exhaustive and the author (WGP) apologises for any omissions. WGP and the Institute of Shock Physics acknowledges the support of AWE and Imperial College London.
The authors declare that they have no competing interests.
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