CP620, Shock Compression of Condensed Matter - 2001edited by M. D. Furnish, N. N. Thadhani, and Y. Horie2002 American Institute of Physics 0-7354-0068-7

EMBEDDED ELECTROMAGNETIC GAUGE MEASUREMENTS ANDMODELING OF SHOCK INITIATION IN THE TATB BASED

EXPLOSIVES LX-17 AND PBX 9502f

R. L. Gustavsen, S. A. Sheffield, R. R. Alcon1 J.W. Forbes, C.M. Tarver, and F. Garcia2

1 Los Alamos National Laboratory, Los Alamos, NM 875452 Lawrence Livermore National Laboratory, Livermore, CA 94550

We have completed a series of shock initiation experiments on PBX 9502 (95 weight % dry aminatedTATB explosive, 5 weight % Kel-F 800 binder) and LX-17 (92.% wet aminated TATB, 7.5 % Kel-F800). These experiments were performed on the gas/gas two stage gun at Los Alamos. Samples wereprepared with ten or eleven embedded electromagnetic particle velocity gauges to measure the evolutionof the wave leading up to a detonation. Additionally, one to three shock tracker gauges were used totrack the position of the shock front with time and determine the point where detonation was achieved.Wave profiles indicate little delay between formation of hot-spots in the shock front and release of hot-spot energy. In other words, a great deal of the buildup occurs in the shock front, rather than behind it.Run distances and times to detonation as a function of initial pressure are consistent with publisheddata. The Ignition and Growth model with published parameters for LX-17 replicate the data very well.

INTRODUCTIONThe TATB based explosives LX-17 and PBX

9502 are among the most thoroughly studied highexplosives in current use. References 1-6 provideexamples of work on these explosives, which wewill refer to throughout this paper. The reasons forinterest in these explosives are their insensitivity toinitiation by shock and heat, their high power, andtheir manufacturability. By manufacturability, wemean that uniform density pressings can be madeand from these, parts are easily and safely machined

One of the finest methods for measuring theshock initiation of detonation in explosives is theembedded multiple electromagnetic particle velocitygauge method developed and used at Los Alamos.Until recently, this method could not be applied tothese insensitive TATB based explosives. The rea-son for this is that very high shock pressures areneeded to initiate these explosives. These pressurescan be achieved with ~ 2 km/s projectile velocitiesachievable with powder guns and high impedancemetallic impactors such as copper or stainless steel.However, electromagnetic particle velocity gaugemeasurements cannot be made using metallic (con-

ducting) impactors or projectiles because of theeddy currents produced as the metal moves throughthe magnetic field.

About 5 years ago, we started doing shots on theLos Alamos 50 mm bore gas driven two-stage gun.This gun is capable of the 2 -3 km/s projectile ve-locities needed to initiate TATB based explosivesusing plastic impactors. Unfortunately, being theonly gun of its kind, it has had a lengthy shakedownperiod. We have encountered projectiles whichdisintegrated when launched, huge impact tilts,gouged barrels, and projectile velocity measuringsystems that refused to work. Fortunately, this isnow behind us. Hereafter, we present our progresson making embedded magnetic particle velocitygauge measurements in these insensitive explosives.Throughout, we will try to show that our work is inaccord with, and complementary to, the fine previ-ous studies.

EXPERIMENTAL DETAILSThe overall configuration for the initiation ex-

periments is shown in Figure 1. This is the sameconfiguration described in great detail in Ref. 7. A

Work performed under the auspices of the U.S. Dept. of Energy.

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FIGURE 1. Experimental setup. Explosive sample installed ingun target chamber and magnetic field.

Lexan projectile is faced with an impactor diskmade of Kel-F, a high-density plastic, and coinci-dentally, the binder material for the explosives stud-ied. Because of our problem with projectiles disin-tegrating when launched, we have till now been re-luctant to use impactors made of anything but plas-tic.

In all but one of the experiments, the projectilevelocity was measured using optical methods. Wemust confess, however, that the optical methodchanged from one experiment to the next as "im-provements" were made. Nevertheless, all themethods we used gave standard errors in the veloc-ity of < lOm/s.

When the impactor strikes the explosive sample,a planar shock wave is generated which begins theinitiation process. The stress of the initial shock isdetermined using the impact velocity, the Hugoniotof the Kel-F impactor,8 the Hugoniot of the explo-sive,9 and standard impedance matching techniques.

Electromagnetic particle velocity gauges are em-bedded in the explosive sample at ten or eleven dif-ferent depths. These vary from the impact surfaceto about 11 mm into the sample. These, of course,produce voltages proportional to the local mass(particle) velocity at the Lagrangian position of thegauge. Three "shock trackers" also allow construc-tion of distance - time (x-f) plots of the position ofthe shock front with time as it moves through thesample. These x-t trajectories are used to determinethe position and time where detonation is achieved.

EXPERIMENTAL RESULTSWave profiles of particle velocity vs. time and

x-t plots of the shock trajectories were successfullyobtained for eight experiments on PBX 9502 and

three experiments on LX-17. In addition, therewere many additional experiments that failed forone reason or another. Impact stresses of 10.8 to15.4 Gpa were created with projectile velocities of2.4 to 3 km/s. This produced run distances of 4.4 toover 14 mm.

Figure 2 shows wave-profiles from shot 2S-47where LX-17 was impacted with a Kel-F impactor ata velocity of 2.951 ± 0.004 km/s producing an inputof 14.96 GPa. The data quality is seen to be ex-ceptional in this figure. Surprisingly, this qualitywas typical for experiments that worked. Experi-ments that didn't work failed in an unmistakablefashion. Nine good wave profiles from gauges lo-cated at depths of 0.0 through 6.7 mm were re-corded. The first profile is from a gauge on thefront of the sample and the remaining 8 from em-bedded gauges. The input particle velocity is 1.45km/s and this grows to a full detonation well beforethe wave reaches the last gauge.

All wave profiles other than the first show thatthe amplitude of the wave at the shock front is in-creasing as the wave travels into the sample. This isobserved in all heterogeneous explosives. Addi-tionally, there is a small hump behind the shockfront. This is consistent with some delay betweenthe shock passing through un-shocked material andthe release of energy.

Additionally, as observed by Wackerle, Stacyand Seitz,6 the slope of the particle velocity imme-diately behind the shock front is positive. This per-

Time (|is)

FIGURE 2. Particle velocity wave profiles from shot 2S-47 onLX-17. The input is 14.96 GPa and was created by impactingKel-F on the LX-17 at 2.951 km/s.

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sists and is observable up until detonation isachieved, and only then can it not be resolved. Thisis indicative variously of a slow initial reaction or aninduction time before reaction begins. The modelsof Tarver and also of Pier Tang account for thisusing a weak or slow initial reaction rate.

When one accounts for the ~ 40 ns time re-sponse of the embedded gauges, the detonationwave profiles are consistent with those observed inRefs. 4 and 6 using Fabry-Perot Velocimetry. Thatis, after the initial shock, the time to go from ~ 2.2km/s to 1.5 km/s is roughly the same for both typesof experiments. Additionally, in Refs. 4 and 5, thetime to drop from the spike at ~ 2.65 km/s to ~ 2.2km/s, where our records begin, is very short ~ 40 -50 ns which is roughly the time response of ourgauges.

The x-t plot showing the position of the shockfront with time is shown in Figure 3. This again isfor shot 2S-47 in LX-17. Points shown are from allthree shock tracker gauges and the particle velocitygauges. The slope of the line through the last fewdata points indicates the detonation velocity.

From plots such as those shown in Figure 3,there are a number of ways to determine the rundistance and time to detonation. Because the ap-proach to detonation in these TATB based explo-sives is so gradual, we have found it useful to usethe point where the data closely approach the deto-nation line as the transition point. Even so, it isoften helpful to have the guidance of corresponding

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75

Wave Arrival Time (|is)

FIGURE 3. x-t plot obtained from shock arrival at shock trackerelements and particle velocity gauge elements. The shot is #28-47on LX-17.

wave profiles such as those in Figure 2.Figure 4 shows "Pop - Plots" of distance to

detonation as a function of the initial shock pres-sure. Historic work for PBX 9502 (Ref. 2) and LX-17 (Refs. 1,3,5) is shown as well as data from thepresent study. Note first that PBX 9502 is slightlymore sensitive than LX-17. This is due to; 1)slightly more explosive and less binder in the PBX9502; 2) For the case of 1.900 g/cm3 LX-17, PBX9502 and LX-17 have nearly identical void contentof ~ 2.6 - 2.7%. The reason for the decreased sen-sitivity in LX-17 in the work of Dallman and Wack-erle is that the explosive they used had a lower voidcontent ~ 2.0% (density 1.913 g/cm3).

PBX 9502 data shown in the Pop - Plot of Fig-ure 4 is from three powder lots. Dick et. al.2 usedexplosive pressed from "Virgin" PBX 9502 mold-ing powder. Our work used two different lots of"Recycled" PBX 9502 molding powder. Recycledmolding powder is made from 50% Virgin moldingpowder and 50% ground up machining scraps. Allsets of data lie on the same curve. The Pop - Plotfor PBX 9502 is thus,log(X) = 4.26 ±0.09-(3.06± 0.08)log(F). (1)This work and Jackson et. al.1 used LX-17 pressedto 1.900 g/cm3 from two different molding powderlots. The Pop - Plots for this density of LX - 17 islog(X) = 4.53±0.12 + (3.22±0.10)log(F). (2)Dallman and Wackerle7 used LX-17 pressed atPantex to a density of 1.913 g/cm3. The Pop - Plotfor this material is

PBX 9502 - Dick• Log(X) = 4.26 - 3.06Log(P)

PBX 9502 - This workLX-17 -1.90 g/cm3 - JacksonLog(X) = 4.53 - 3.22 Log(P)LX-17 -1.913 g/cm3 DallmanLog(X) = 5.34 - 3.83 Log(P)LX-17 1.900 g/cm3 - This work

Initial Shock Pressure (GPa)

FIGURE 4. Pop - plots for TATB based explosives. Names offirst authors identify data sets.

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log(X) = 5.34±0.26-(3.83 ±0.23)log(P). (3)

NUMERICAL MODELINGWe used the Ignition and Growth "Reactive

Burn" model to simulate these experiments. As withall reactive burn models, equations of state are re-quired for the unreacted explosive and for the reac-tion products. We used the JWL model for theseequations of state

Table 2. Ignition and Growth reactive burn parameters.

P = 0 ) + B exp(-R2V I F0 ) (4)

Table 1 gives EOS parameters for LX-17 and PBX9502 products and unreacted explosive. Note thataside from the initial volume, both explosives haveidentical Equations of State.Table 1. EOS Parameters for LX-17 and PBX 9502

ParameterEq.(4)

A (Mbar)B (Mbar)

RiR2CO

CF*10-5(Mbar/K)V0 (cm3/g)£XMbar)D(cm/us)

PcXMbar)

Unreacted EOS(PBX 9502)

632.07-0.04472

11.31.13

0.89382.487

0.525(0.531)

Product EOS13.4540.6727

6.22.20.51.0

0.0690.75960.2714

The Ignition and Growth reactive burn modelwas used, where A, is the fraction reacted is

Table 2 lists Parameters used for the reaction mod-eling. Note again that most parameters were thesame for both explosives. The lower value of a forPBX 9502 indicates that this explosive starts react-ing at a slightly lower pressure or compression.There was not room in this paper to include com-parison between the measured and calculated waveprofiles, but these parameters have been found tomodel all of the available data very well. A morecomplete paper including all the data and thesecomparisons will be published later.

Parameter Eq. (5)

«l/|is)G^Mbar"1 [is"1)(7XMbar"z jis"1)

aXz

LX-17

4.4(105)0.6

4000.227.03.0

PBX 9502

4.4(105)0.6

4000.214

7.03.0

ACKNOWLEDGEMENTSRobert Medina operated the gas gun for all of

the experiments presented here.REFERENCES

1. R.K. Jackson, L.G. Green, R.H. Barlett, W.W. Hofer, P.E.Kramer, R.S. Lee, EJ. Nidick, Jr., L.L. Shaw, and R.C.Weingart m Proceedings of the Sixth Symposium (Interna-tional) on Detonation, Office of Naval Research, ReportACR-221,p.755, 1976

2. Jerry J. Dick, C.A. Forest, J.B. Ramsay, and W.L. Seitz,, /.Appl Phys., 63, 4881, (1988)

3. K. Bahl, G. Bloom, L. Erickson, R. Lee, C. Tarver, W. VonHolle, and R. Weingart in Proceedings of the Eighth Sym-posium (International) on Detonation, Office of Naval Re-search, Report NSWC MP 86-194, p. 1045, 1986

4. W.L. Seitz, H.L. Stacy, Ray Engelke, P.K. Tang, and JerryWackerle in Proceedings of the Ninth Symposium (Interna-tional) on Detonation, Office of Naval Research, ReportOCNR 1 13291-7, p. 657, 1989

5. J.C. Dallman and Jerry Wackerle in Proceedings of theTenth Symposium (International) on Detonation, Office ofNaval Research, Report ONR 33395- 12, p. 130, 1995

6. Jerry Wackerle, H.L. Stacy, and W.L. Seitz in Proceedingsof the Tenth Symposium (International) on Detonation, Of-fice of Naval Research, Report ONR 33395-12, p. 468,1995

7. S.A. Sheffield, R.L. Gustavsen and R.R. Alcon, in ShockCompression of Condensed Matter — 1999, American Insti-tute of Physics (AIP) Conference Proceedings 505 (1999), p.1043.

/?0=2.14g/cm3, £7^=1.99 + 1.76

S.A. Sheffield and R.R. Alcon, in Shock Compression ofCondensed Matter - 1991, S.C. Schmidt, R.D. Dick, J.W.Forbes, D.G. Tasker (editors), Elsevier Science Publishers(1992), p. 909.

£^=1.90 + 3.001^:1^0.82Us = 2.90 + l.78up:up> 0.82C.A Forest, unpublished (1995). This Hugoniot for PBX9502 is based primarily on the data of Ref. 2.

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