j.e. field et al- the shock initiation and high strain rate mechanical characterization of ultrafine...

8/3/2019 J.E. Field et al- The Shock Initiation and High Strain Rate Mechanical Characterization of Ultrafine Energetic Powders

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The Shock Initiation and High Strain Rate Mechanical Characterization of Ultrafine

Energetic Powders and Compositions

J.E. Field, S.M. Walley*, W.G. Proud, J.E. Balzer, M.J. Gifford, S.G. Grantham,M.W. Greenaway, C.R. Siviour

Physics and Chemistry of Solids Group (PCS), Cavendish Laboratory,Madingley Road, Cambridge CB3 0HE, United Kingdom

* Corresponding author. email: [email protected]

ABSTRACT

This paper reviews the techniques that have been developed at the Cavendish Laboratoryfor the study of the mechanical and ignition properties of energetic materials.

HIGH-SPEED PHOTOGRAPHY

A number of techniques have been developed and applied in our laboratory for theinvestigation of the properties of energetic materials. One method we have used in a wide rangeof such studies has been high-speed photography e.g. refs [1-11]. The advantage is that it ispossible to see directly what is going on in, for example, hot spot initiation of energeticmaterials.

In recent years, it has increasingly been desired to model the impact response of structurescontaining energetic materials using numerical methods. If meaningful numerical results aregoing to be obtained for, say, munitions or rockets, it is of vital importance that constitutiverelations be constructed which describe the mechanical response of unreacted energetic materialsover the temperature and strain rate ranges of interest. With this in mind, we have developed arange of techniques for obtaining the mechanical properties of energetic materials over a wide

range of strain rates and temperatures. Examples of publications where such data have beenpublished include refs [4, 7, 12-20].

One problem with conventional mechanical testing methods is that they only allow themeasurement of the global response of a specimen. Energetic materials usually consist of aviscoelastic binder heavily loaded with explosive crystals. In order to develop realistic andphysically-based constitutive models for such unusual composites, it is vital to determine howthese materials deform on the mesoscale. To this end, we have developed a range of optical andmicroscopy techniques. These have been used for both quasistatic [14, 15, 18, 21-23] anddynamic studies [19, 20, 24-26].

HOPKINSON BARS

The split Hopkinson pressure bar (SHPB) is one of the most widely used machines forobtaining the stress-strain properties of materials at high rates of deformation (103 - 104 s-1) [27,28]. Our initial work in this area was to develop a miniaturised direct impact system (DIHB)allowing strain rates between 104 and 105 s-1 to be accessed [29, 30]. A larger DIHB was laterdeveloped to obtain the high rate mechanical properties of polymer-bonded explosives (PBXs)and polymers [7, 31]. In recent years, we have constructed a suite of conventional SHPBs ofvarious mechanical impedances so as to be able to measure the high rate properties of materials

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ranging in strength from a few MPa to a few GPa. Inconel is used as a bar material inexperiments where the specimen is held at more than 300K above room temperature (theimpedance of Inconel is only a weak function of temperature [32]).

The effect of grain size on the high rate of strain mechanical properties of PBXs was firstnoticed by Field and coworkers [13]. Since that data was published, it has been observed that the

effect is made more prominent by lowering the temperature at which the experiments are carriedout [33] (see also figure 1). The ammonium perchlorate (AP)/hydroxyl-terminated polybutadiene(HTPB) PBX used in the work reported here consisted of 66% AP and 33% HTPB by mass. The

AP was available in four different crystal sizes: 3m, 8m, 30m and 200-300m. The lowtemperature study was carried out using our Inconel bar system. Cooling was performed bysurrounding the ends of the bars with a chamber into which helium gas was passed that had beencooled using liquid nitrogen. The temperature was monitored using chromel-alumelthermocouples. Figure 2 shows that the effect of particle size on the flow stress of the material is

linear in 1/ d where dis the particle size.

0

20

40

60

80

100

120

0 0.05 0.1 0.15 0.2 0.25

3m, 4400300 s-1

8m, 3700200 s-1

30m, 3700250 s-1

200-300m, 3100350 s-1

TrueStress/MPa

True Strain

Figure 1. Plot of the stress-strain responses ofan AP/HTPB with four different AP crystalsizes at -60 C obtained using an SHPB.

50

60

70

80

90

100

110

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Flowstress/MPa

(Particle size)-0.5

/m-0.5

Figure 2. Plot of the flow stress data of figure1 versus the reciprocal of the square root ofthe AP crystal size.

OPTICAL TECHNIQUES

When the standard SHPB equations are used to calculate sample stress and strain from theoutput of gauges on the bars, the values calculated have the feature that the strain is calculated

using the calculated displacement of the sample/input bar interface. This means that the strain iseffectively an average through the specimen thickness. There is therefore no way of knowingwhether there are any inhomogeneities in the specimen deformation. Other assumptions that aremade in deriving the SHPB equations are that the specimen is in force equilibrium and that thelubrication is perfect so that there is no barrelling [27]. Equilibrium can be checked bycomparing the force-time traces on both sides of the specimen. Also lubrication is available thatreduces friction of polymer-based materials to zero [34-36]. However, it would be advantageousto be able to obtain data from inhomogeneous, or brittle or foam specimens. Such materials do

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not necessarily reach a constant strain state during an experiment. It would also be useful to beable to measure strain fields at low levels of deformation so that elastic properties might possiblybe determined.

For these reasons, we are in the process of developing and applying various dynamicoptical techniques [20] to the measurement of specimen deformation in our compression split

Hopkinson bar [19]. Digital Speckle Metrology (DSM) combined with high-speed photographyhas been used to follow the through thickness deformation of a conventional cylindricalcompression specimen made from a sugar simulant of a PBX (PBS9501) (figure 3). Goodagreement was found with the overall strain computed using the standard Hopkinson barequations. DSM has also been used to follow the dynamic loading of PBS9501 in the Brazilianconfiguration (diametral loading of a disc) (figure 4). Such techniques are particularly valuablefor materials where it is suspected that a significant volume change may take place duringdeformation (the standard Hopkinson bar equations assume volume conservation). It should beremembered that the SHPB is an excellent, and versatile, piece of apparatus. The quality ofresults produced by the system is high, and if the limitations are known, then very useful datacan be obtained.

Figure 3. Displacement quiver plots for anSHPB compression experiment onPBS9501. The length of the arrows isproportional to the displacement at theirbases. Note that there are arrows on boththe input bar and the specimen.

Figure 4. Displacement quiver plots for aBrazilian experiment on PBS9501 in ourSHPB

The arrows in figures 3 and 4 are examples of quiver plots. These have been presented inthe zero momentum frame, i.e. the specimens frame of reference. The results are veryencouraging: there is no strain concentration in the specimens, and the deformation in theBrazilian test is exactly as seen in quasi-static experiments. In particular, if the values of strainare taken across the centre of the specimen perpendicular to the loading direction, then the strainacross the centre of the sample should be twice the overall strain according to theory. This wasfound indeed to be the case.

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High-speed photography has often been applied to Hopkinson bar testing, for example toexamine specimen deformation [29, 37, 38]. The use of an optical wedge can improve sensitivityfor detecting barrelling and plastic wave activity [29]. This technique can give, for example,useful confirmation of strains calculated using the Hopkinson bar equations. It can also giveindications of failure mechanisms and sample diameter. However, photography can only be used

to measure overall deformations. DSM by contrast is a technique that allows surface deformationfields to be measured. Speckle techniques require a random pattern to be observed on a surface.The random pattern can then be tracked with time using a correlation algorithm providing ameasure of both components of displacement between a reference and a deformed image. Theresolution of the measurements depends on the size of the speckle pattern, and the resolution ofthe camera. The correlation algorithm used in these experiments was originally developed bySjdahl and co-workers [39-41].

In the experiments reported above, the random speckle pattern was applied to the samplesurface using an airbrush [17]. The Brazilian test produces tensile deformation in a compressionapparatus [42-44]. It has the advantage of requiring much smaller samples than a traditionaltensile test, which is important when working with explosives. The disk shaped specimen is held

between two curved anvils. The curved nature of the anvils prevents localised strains at thepoints of contact, and instead ensures that the sample undergoes tensile strain along a linerunning down its centre.

A Hadland Ultra-8 digital high-speed camera [45] was used for the experiments, with aPallite VII radial light providing illumination. Use of a digital camera removes the need forfiducial markers to align the different photographs for comparison. The Ultra-8 duplicates theimage onto 8 areas of a CCD array, each of which is sampled independently so that each of the 8images can have completely different delays and exposure times. Before each experiment a set ofreference images was taken. Each deformed image was compared to the reference image fromthe same part of the CCD array. This procedure removes the effect of any possible distortion inthe images due to the internal optics of the camera. The exposure time and interframe time

between the dynamic images were both set to 20s.

DROP-WEIGHT

The dropweight is a widely used machine to determine the impact sensitivity of energeticmaterials e.g. ref. [46]. A modification of this apparatus made in our laboratory many years agowas to machine a lightpath through the falling weight so that the deformation of materialimpacted between glass anvils can be observed using high-speed photography [2]. Theconfinement conditions are different to those in say a Rotter apparatus [13, 47], but it has thedistinct advantage that it is possible to see what is going on during the formation of hot spots andsubsequent deflagration. A number of classic papers based on work performed on energetic

materials in this apparatus have been published; see, for example, refs [2-4, 6, 7, 11].Figure 5 presents selected frames from a high-speed photographic sequence taken using an

AWRE C4 rotating mirror framing camera [48] with an interframe time of 7s of thedeflagration caused by the impact of our dropweight apparatus on three energetic powders.Burning can be seen to start at the interface between the AP and PETN powders. The flamegenerated initiated the burning of the HMX. The last to deflagrate is the AP. Note especially theprominent fracture patterns in the sintered disc of AP. The largest of these provides an easy pathfor the escape of gaseous products.

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Figure 5. Selected frames from a high-speed photographic sequence of dropweight impact ofpowders of: AP (left), PETN (right), HMX (bottom). Time from moment of impact: 0, 406, 420,

455 s. From ref. [49].

PLATE IMPACT AND SMALL GAS GUNS

Laboratory gas guns are widely used to produce well-characterised 1D shock waves inmaterials. Our laboratory has designed and built a single-stage 50mm bore gas-gun systemcapable of performing impacts at up to 1.2 km s -1 [50]. Recently a smaller 19mm bore facilityhas been constructed for studying the dynamic properties of energetic materials [51, 52]. Thishas been used to study hot spot formation and quenching in thin beds of ammonium nitrate (AN).The impact cell has a 2mm thick copper plate on the impact side. Behind this plate is the bed ofmaterial of interest. High-speed photography is performed through the rear window whichconsists of a 25mm thick glass window. An optic fibre is used to take any light emitted to aphotodiode (time resolution 1 ns) and to a UV/visible spectrometer (capture time 1 ns). The cell

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was shocked by a 2mm thick copper flyer mounted on the front of a nylon sabot. This produces ashock of 600 ns duration. Figure 6 presents two frames from a sequence taken using an Ultra 8

camera [45] of an impact performed on a 0.4mm thick layer of AN, grain size 150-210 m. Theexposure time was 30 ns and the interframe time was zero i.e. the second frame began to betaken immediately the first frame was captured. Two regions can be identified and these are

ringed in figure 6. In one region the glowing hot spots are close together and glow moreintensely in frame 2. In the other region the hot spots are further apart and glow less intensely inframe 2: they are in the process of being quenched. More details of this experiment, includingspectroscopic results, may be found in refs. [51, 52].

Figure 6. Selected frames from a high-speed photographic sequence of the shockloading of a0.4mm thick layer of AN. Interframe time 30ns. From ref. [52].

LASER-FLYER

A system has been developed in our laboratory for the launch of miniature flyer plates atvelocities approaching 10 km/s. Laser-induced plasma is used to drive these flyers which aretypically less than 1 mm in diameter and a few microns thick. High-speed streak photographyhas been used to measure the key parameters of the flyers as a function of launch energy, massand distance travelled. A fibre-optic beam shaping system has proved highly effective atmanipulating the shape of the flyers and excellent reproducibility is found.

Using a laser as the driver to launch flyers has numerous advantages in terms of repetition,reproducibility and control. The dimensions and performance of the flyer are controlled by theoptical and physical properties of the laser pulse and launch target. Precise measurement of flyerperformance is achieved using high-speed streak photography. Characterisation experimentshave focused on all aspects of the flyer behaviour including diameter, velocity, planarity andintegrity. Beam shaping, using a fibre-optic system, enables the shape and profile of the flyer tobe altered [53]. In most applications, flat planar flyers are desirable and to achieve this, a top-hat light profile is necessary.

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Low light fluences are required for flyer launch, typically less than 20 J/cm2 and high-speed diagnostics ensure good characterisation. The system is suitable for producing single orarrays of flyers of well-defined size, shape and velocity [54, 55]. We have used this system tostudy the detonics of explosives [56-58]. A Nd:YAG laser is used, producing 9 ns (full-wave halfmaximum) duration pulses at its fundamental wavelength, 1064 nm. The laser energy is

concentrated onto a substrate-backed metal film. The optical radiation is readily absorbed at thesubstrate-film interface to produce hot plasma (~ 8000 K). The rapid expansion of this plasma issufficient to launch the remaining depth of film as a flyer plate.

The flyers reach their maximum velocity within 100 ns and can traverse a few hundred min air. The effect of friction heating, velocity gradients within the flyer and heating from theplasma cause the flyer to effectively burn up. The first of these is believed to be the dominantmechanism so that in a vacuum, the flyers would be expected to be able to travel much further.Flyer characterization has been performed using high-speed streak photography [55, 59, 60]. Theflyer was allowed to impact a transparent window held on the optical axis and parallel to thelaunch substrate. The gas trapped between the flyer and window is compressed adiabatically.This process liberates light which is photographed by a high-speed camera looking back down

the optical path. Streak photography was used to obtain a record of the flyer impact and a time-of-flight measurement. In this technique, the impact event was viewed through a 100 micron slit,which was moved at 10 ns/mm across a photographic film to produce the streak records.Example flyers produced with this system are shown in figure 7.

This laser-driven flyer plate system was found to be capable of initiating fine grain HNSand PETN. HNS was found to be more insensitive to this type of initiator. In both cases, thesensitivity of the material was heavily dependent on particle size. Neither the coarse grainedHNS nor the PETN equivalent could be detonated [58].

Figure 7. Examples of two laser-drive flyerplates moving at ca. 2 km/s. The LH one was

produced using beam shaping. The RH one wasproduced without beam shaping.

Figure 8. Schematic comparison of the widthof the shock produced by a laser flyer withconventional (LH) and ultrafine (RH)energetic crystals. From ref. [58].

The large dependence on particle size is explained by the size of the critical hot spots. Theshock width is of the order of a few microns, which is comparable to the grain size (and hencegas spaces) of the fine materials but is much smaller than the grain size of the coarse grain

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materials. Thus, in these materials, the gas spaces cannot be entirely collapsed during the shortduration of the shock (see figure 8). In addition, there is better mixing of the hot spots andexplosive particles in a fine grain material. These differences offer an explanation as to why thefine grain materials are more sensitive to these very short shocks.

A number of possibilities for increasing the mode generation within the fiber were

investigated: bending and looping the fiber, improving the injection optics, incorporating a modescrambler, and increasing the fibre length [53]. Bending and looping alters the angle of theinternal reflections within the fiber and thus can generate more modes if a multimode laser isused. However, as we are using a single-mode laser, this method was found not to work.

Some examples of the emerging spot of light are given in figure 9 (increasing top hat factorfrom the left- to the righthand side). The profile with the lowest top hat factor (lefthand side)shows the largest fluctuations and profile with the highest top hat factor (right-hand side) showsa much more even distribution of energy.

1m straight 10m coiled 20m coiledFigure 9. Effect of pulse shaping on the intensity distribution of light across a laser pulse. Fromref. [53].

DEFLAGRATION-TO-DETONATION TRANSITION (DDT)

Two types of deflagration-to-detonation transition (DDT) have been identified in columnsof explosive materials composed of small crystals [61-63]. Which one occurs has been found todepend upon the density of packing (porosity). We have found that classic type I DDT occursin columns of PETN with a density greater than 50% TMD (see figure 10). What happens is thatconvective burning from one end compresses the material ahead of it into an impermeable plugwhich at some critical pressure and velocity shocks the material the other side into detonating.

Figure 11 shows a streak record of type II DDT process occurring in a charge of ultrafine

PETN pressed to a density of 29 1% TMD. A schematic of the confinement is shown next to

the streak record at the same scale as the photograph. The light output detected using opticalfibres and photodiodes showed that the initial convective burning stage took some 600 s topropagate along the length of the column. The column then continued to burn for a period of

around 70 s before the detonation broke out. The streak record gives detail of the events in the

final 40 s prior to the detonation and the following 30 s.

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Figure 10. A type I DDT event in ultrafine PETN. From ref. [64].

At the time indicated by the dashed line A, it can clearly be seen that reaction is takingplace at a number of points which appear stationary on the time scale of the streak record. As thecamera can only image the surface of the column it can be surmised that these reaction sites arein the part of the charge in contact with the window. The luminosity of the reaction at these sitescontinues to increase until at site B the conditions reach the critical parameters for initiation.There appears to be no shock wave associated with the build up to the initiation event. A

detonation wave (C) then propagates downstream (relative to the ignition site) at an averagevelocity of 5.4 0.2 mm s-1. There is also an upstream retonation wave (D), that propagatesfrom the point B, but the proximity of the initiation to the upstream end of the column makes

any calculation of the velocity of this wave difficult. A reflected shock (E) travelling at 2.23

0.09 mm s-1 can also be seen propagating upstream from the point at which the detonation (C)reached the witness plate at the downstream end of the column.

E

C

D

A

B10s

20mm

O

O

P

I

S

Figure 11. Streak photograph of a type II DDT event in ultrafine PETN. A schematic of thecolumn configuration is shown to scale to the right of the photograph. P - PETN column; O -optical fibres; F - Fuse. From ref. [65].

Other experiments in which type II DDT occur are all similar in the way in which thereaction proceeds, but the point at which the detonation breaks out can be anywhere along thelength of the column. The first step appears to be the formation of a channel in the low densitymaterial during the convective burn stage. There appears to be no correlation between eithercolumn length or density and the position of the detonation break-out.

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ACKNOWLEDGEMENTS

We would like to thank the following for financial support of this work: QinetiQ, [dstl],AWE, EPSRC, European Office of the US Army, Airforce, and Navy.

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AA5.4.12

j.e. field et al- the shock initiation and high strain rate mechanical characterization of ultrafine...

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