a. chakravarty et al- factors affecting shock sensitivity of energetic materials

8/3/2019 A. Chakravarty et al- Factors Affecting Shock Sensitivity of Energetic Materials

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CP620, Shock Com pression of Condensed Matter - 2001edited by M. D. Furnish, N. N. Thadhani, and Y. Horie 2002 American Institute of Physics 0-7354-0068-7/02/$19.00

FACTORS AFFECTING SHOCK SENSITIVITY OF ENERGETICMATERIALS

A. Chakravarty, M.J. Gifford, M.W . Greenaway, W.G. Proud, J.E. FieldPCS, Cavendish Laboratory, Madingley Road, Cambridge, CB S O H E . U K .

Abstract. An extensive study has been carried ou t into the relationships between the particle size of acharge, the density to which it is packed, the presence of inert additives and the sensitivity of the chargeto different initiating shocks. The critical parameters for two different shock regimes have been found.The long duration shocks are provided by a commercial detonator and the short duration shocks areimparted using laser-driven flyer plates. It has been shown that the order of sensitivity of charges todifferent shock regimes varies. In particular, ultrafme materials have been shown to be relativelyinsensitive to long duration low pressure shocks and sensitive to short duration high pressure shocks. Thematerials that have been studied include HNS, RDX and PETN.

INTRODUCTIONWhen a shock-wave is incident on an energeticcharge, a number of parameters must be consideredwhen determining whether detonation is likely to

result. The nature of both the charge and the shock-wave are important.In a very simplistic way a shock can bedescribed by its pressure and duration (ignoringshock profile at this stage). For a shock to causeinitiation it must be capable of creating sufficientchemical reaction to sustain it. Acting against thischemical reaction, to weaken the shock, arerarefactions due to the expansion of the materialwhich, due to the subsonic flow of the materialfollowing the shock, will eventually reach the front.The relationship between the required pressure andduration for initiation is such that the shock levelmust be high enough to cause sufficient reaction tosustain the shock before the initial shock decays. Ifthis criterion is met then a detonation will propagatein the charge.The magnitude and duration of a shock requiredfor a particular charge to be initiated are dependenton the microstructure and chemistry of the charge.The microstructure is crucial in determining the

nature of hot-spots that are created in the charge andthe chemistry is important in determining theresponse of the material to the presence of the hot-spots.A large number of researchers have attempted toelucidate the role of hot-spots in the shock initiationof detonation. The reviews of the field given byKhasainov et al.^ an d Dremin2 give a very completeaccount of the state of the literature on this subject.The present study has focussed on varying thedensity and grain size of the charges and the natureof the imparted shock in an attempt to alter the hot-spot parameters and so determine the critical factorsassociated with them.

MATERIALS USEDBoth the pentaerythritol tetranitrate (PETN) andcyclotrimethylene trinitramine (R DX) were supplied

in ultrafine and conventional forms by ICI NobelEnterprises, Ardeer, U.K. The ultrafine powdershave a primary particle size of ~1 pm and areproduced by a proprietary process. The

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conventional grain material has a particle size ofabout 18 0 urn.The hexanitrostilbene (HNS) used in thesestudies was supplied by DERA, Fort Halstead andcame originally from Bofors AB in Sweden. Theultrafme form is known as HNS IV and has a grainsize of less than a micron. The HNS IV wassupplied both in a pure form and with pressingadditives. In the case where zinc stearate andgraphite were added to act as pressing agents, theadditives contributed approximately 1% to the totalmass of the material. The coarse grain HNS (knownas HNS II) had a grain size that was typically of theorder of 25 um .

EXPERIMENTAL METHODTwo principal experimental methods were usedduring the course of the research described here.Fo r the imparting of relatively long duration shocks,a gap testing geometry was used. When short highpressure shocks were required a system forgenerating laser-driven flyer plates was used.

Long Duration ShocksThe charges used in these experiments wereincrementally pressed columns of either RDX orPETN. The confinements used were 25 mm long 25mm diameter PMMA cylinders. The explosive

columns were 5 mm in diameter.The donor charge used during the experimentswas a PETN boosted C8 detonator which was foundto have a reliable output in terms of the shockpressure produced.

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The gaps that were used to mediate the shockpressure were discs of PMMA placed between thedetonator and the surface of the column . A thinlayer of silicone grease was used between all threecomponents of the test in order to aid thereproducibility of the testing. PV DF gauges placedbetween the PMMA gap and another piece ofPMMA in the charge position were used to obtainan indication of the shock pressure during a test. Atypical trace from a PVDF gauge is shown in figure1. Both photographic streak recording and brasswitness plates were used to determine whether adetonation event had occurred during a test.

Short Duration ShocksThe HNS charges used for the short-durationshocks were 5 mm long, 5m m diameter cylinderscontained within 25 mm diameter PMMAconfinements. The charges were incrementally

pressed into the confinements. The surface of thecharges was polished with 2500 grade SiC paper toprovide a consistent surface finish. The quality ofthe surface finish was checked using a SloanDekTak II surface profilometer.

FIGURE 1. Typical trace from a PVDF gauge.

0 0.5 1.0Distance along scan (mm)

FIGURE 2. Profilometer traces from the Sloan DekTak II.Th e laser-driven flyer launching system is

described fully in previous publications from thislaboratory-*"^ and details can also be found in thepaper by Greenaway et al. in these proceedings.Th e system uses a Nd:YAG laser to accelerateflyers 1 mm in diameter and 5 um thick to velocitiesup to 8 mm us'1. On impact these flyers provideintense shocks lasting approximately 1 ns. The

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energy of the pulse imparted to the flyer iscontrolled in order to determine the velocity of theflyer. Energies between 50 and 400 ml wereaccessed during this study.A Hadland Imacon 790 high speed imageconverter camera was used to provide streakphotographs of the initiation events. The camerawas triggered from the signal that fired the laserwith a suitable delay added. These photos allowedcalculation of the position of the initiation eventwithin the column.

RESULTS

Long duration shocksFigure 3 shows the results of the experiments

which used long duration shocks in a gap testgeometry. These experiments were carried out onPETN and RDX in both ultrafme and conventionalgrain sizes. As can be seen the density was alsovaried in the RDX study in order to determine theeffect that increased porosity has on the sensitivityof the charges. Although there is some overlap inthe go/no go gaps for some of the densities, ingeneral the experimental reproducibility wasextremely high.

Ultrafine goo Ultrafine no go Conventional gon Conventional no go

60 70 80Density ( %TM D )FIGURE 3. Results of gap testing on RDX. Thresholds forPETN are also indicated.

The ultrafme PETN at a density of 90% TMDhad a critical gap of 3.68 0.01 mm compared witha gap of 5.57 0.02 mm for the conventional grainsize material. These were shown to correspond toshock pressures of approximately 4.1 and 2.1 GPa

as measured using the PVDF gauges describedpreviously.The gap required to prevent initiation of theRDX charges increased significantly in both theultrafme and conventional m aterials as the porosityincreased, but the ultrafine material was consistentlyless sensitive to this form of initiation.

Short Duration ShocksThe findings of this study into initiation by shortduration shocks have been explained in some detailin the paper by Greenaway et al . within theseproceedings. The results of this study involvinglaser-driven flyer plates are that HNS II could no tbe initiated with very short duration shocks at the

energies available in that system, but that the HNSIV could be readily initiated with a go/no gothreshold of about 250 mJ of laser pulse energy.The presence of zinc stearate and graphite asadditives in some of the HNS IV acted to increasethe flyer energy required for initiation of the chargesto approximately 350 m J.

^

:; 0 * 0 o

HNSJV5':70%TMD

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* "-paD D DnD

D

DO

D

DHNSIV+addfives65% TM D

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78%TMD :(Densfesare only approximate)

FIGURE 4. Results of the laser-driven flyer tests. Filled objectsdenote a "go" result.The results of this study indicate a strongcorrelation between pressing density and sensitivity.

Unfortunately due to the nature of the pressingtechnique employed and the powder, it was difficultto accurately reproduce a given density of charge.Within the limits of the study, it can be said that thecharges pressed to a density of 65% TMD appear tobe less sensitive than those pressed to 70% TMD.

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Without performing a larger study, however, it isimpossible to say what the exact nature of thedependence on density of the sensitivity is for thisform of initiation.DISCUSSION

This study together with previous studies carriedout within this group has shown that simpleorderings of m aterials by sensitivity cannot be done.It is not even possible to do this for sensitivity toinitiation by shock as has been demonstrated here.The results of this study have shown that for agiven situation, the sensitivity of the material isdependent on the chemistry, the grain size, thedensity of the charge and the nature of the shockitself. The way in which all of these variablesdetermine the likely response of a charge to aninsult can be linked to their effect on thedistribution, nature and form of the hot-spots thatare caused by the shock.

It has been shown7"9 that the effect of increasingthe shock pressure is to change the relativeimportance of the jetting and the gas compression inthe process of pore collapse. As small pores aremore effective for the rapid formation of hot-spotsby jetting and large pores are more effective in thecase of gas compression it seems that it may be thepore size rather than the grain size that is critical. Inthe case of the high pressure short duration shocksimparted by laser-driven flyers, the incident shock isno t sufficiently large for it to cover an entire pore inthe coarse material and so the releases will act tohinder collapse. These short shocks are, however,of sufficiently high pressure for jetting to besignificant and this may well be the dominantmechanism in the ultrafine charges where the poresare extremely small.With the longer duration, lower pressuredetonator-supplied shocks, the shock is sufficientlylarge to encompass whole gas spaces in both theultrafine and the conventional powders. Due to thereduced pressure, jetting is a less importantmechanism for hot-spot production than gascompression and as a result the larger pores that arefound in the conventional charges are moreconducive to the creation of hot-spots capable ofcausing reaction.

Th e effect of density on the sensitivity of thecharges is caused by the change in the relativedensity of hot-spot nucleation sites compared to thedensity of material available for reaction. It appearsfrom the results of the gap testing of the RDX that itis the number of available sites for hot-spots thatdetermines the sensitivity (at least down to thedensity of 40% TMD that was used in this study).It is not so clear from the laser-driven flyer studythat the same is true in this regime. Th e importanceof good coupling between the energetic material andthe hot-spot is more pronounced due to the shortduration of the shock, so this may account for whatappears to be a higher sensitivity of the moredensely packed charge. Further research wouldhave to be carried out with more emphasis ondensity in order to determine the optimum densityfor charge sensitivity in this shock regime.

ACKNOWLEDGEMENTS

The authors would like to acknowledge ICINobel Enterprises (Ardeer), U.K. and DERA, FortHalstead fo r their support of this research. Dr. M.Cook of DERA is particularly thanked for usefulcomments that he has made.REFERENCES

1 B. A. Khasainov, A. V. Attetkov, and A. A. Borisov,Chem. Phys. Rep. , 15, 987-1062 (1996).^ A. N. Dremin, Toward Detonation Theory Springer-Verlag, Berlin, 1999.

3 S. Watson, PhD Thesis, University of Cambridge,1998.4 S. Watson and J. E. Field, J. Phys. D:Appl Phys. 33,170-174(2000).5 S. Watson, M. J. Gifford, and J. E. Field, J. Appl. Phys.88 , 65-69 (2000).6 S. Watson and J. E. Field, J. Appl. Phys., 88, 3859(2000).7 J. P. Dear, J. E. Field, and A. J. Walton, Nature 332,505-508(1988).8 N. K. Bourne and J. E. Field, Proc. R. Soc. Lond. A,435 ,423-435(1991) .9 J. E. Field, Accounts Chem. Res., 25, 489-496 (1992).

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a. chakravarty et al- factors affecting shock sensitivity of energetic materials

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