NSWCDDfTR-92/218

AD-A264 482

THE MEASUREMENT OF ELECTRICALCONDUCTIVITY IN DETONATINGCONDENSED EXPLOSIVES

BY DOUGLAS G. TASKER, RICHARD J. LEE, and PAUL K. GUSTAVSON

WEAPONS RESEARCH AND TECHNOLOGY DEPARTMENT

MARCH 1993

,DTICELECTE

SMAY19 2MD

Approved for public release, distribution is unlimited

$sw 1 NAVAL SURFACE WARFARE CENTER

Dohlgren, Virginia 22445000 Silver Spring, Maryland 20903-5000

&• 5 18 09 1 93-1140-- Nmu mI!, 1 ;II1"jllll ,,1' I '

NSWCDD/TR-92/218

THE MEASUREMENT OF ELECTRICAL CONDUCTIVITYIN DETONATING CONDENSED EXPLOSIVES

BY DOtlGLAS G. TASKER, RICHARD J. LEE, and PAUL K. GUSTAVSON

WEAPONS RESEARCH AND TECHNOLOGY DEPARTMENT

MARCH 1993

Approved for public release, distribution is unimited

NAVAL SURFACE WARFARE CENTERDahlgren, Virginia 22448-5000 * Silver Spring, Maryland 20903-5000

NSWCDD/TR.92/218

FOREWORD

The electrical conductivity of a number of detonating explosives are reported. Theresults show the conductivity to be sensitive to detonation wave instabilities. Theconductivity profile observed in the aluminized explosive PBXN-111 (formerly PBXW-115)is interesting and unusual. The results are probably due to a combination of waveinstabilities and late reaction effects, in either aluminum or ammonium perchlorate, or both.The results demonstrate the power of conductivity measurement as a diagnostic tool; waveinstabilities are apparent that could not be determined by other means.

We are indebted to Dr. A. Roberts of the Office of Naval Research for his supportunder Code 123, Project 02403SB, "Electrically Enhanced Detonation."

The excellent scientific contributions by R. Granholm and P. Gustavson to thesestudies are acknowledged. Grateful thanks to R. Hay, N. Snowden, B. Snowden, H. Gillum(deceased), C. Sorrels (deceased), R. Baker, and many others for their fine experimentalwork. Special thanks to Dr. J. Forbes for many helpful discussions and encouragement,especially related to the roles of ammonium perchlorate and aluminum in detonationreaction.

Approved by:

WILLIAM H. BOHLI, Acting HeadEnergetic Materials Division

Aaession For

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NSWCDD!TR-92/218

ABSTRACT

The time-resolved measurement of electrical conductivity provides a unique meansof studying detonating explosives. The results of a large number of experiments arereported. Two types of experiments were performed; they measured bulk resistance andtime-resolved conductivity. Some interesting effects, due to detonation wave instabilities,were observed when the explosives' dimensions were close to criticality, i.e., close to failurediameter or thickness.

The conductivity profile observed in the aluminized explosive PBXN-111 (formerlyPBXW-115) is interesting and unusual. The results are probably due to a combination ofwave instabilities and late reaction effects. This demonstrates one of the advantages of usingconductivity measurement as a diagnostic tool. Wave instabilities are apparent that couldnot be determined by other means.

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CONTENTS

Chapter Page

I INTRODUCTION ............................. 1-1

2 ELECTRICAL MEASUREMENT TECHNIQUES ........... 2-1

3 CONDUCTIVITY-DISTANCE DATA OBIAINED FROMBULK RESISTANCE MEASUREMENTS ................. 3-1

4 CONDUCTIVITY OF DETONATING ALUMINIZED 4-1EX PLOSIV E .................................

5 TIME RESOLVED CONDUCTIVITY MEASUREMENTS .... 5-1

6 DISCUSSION AND CONCLUSIONS ................... 6-1

REFERENCES ............................... 7-1

Apendix

A THE VERIFICATION OF DETONATION IN SHEETS OFPBXN-1 . .................................. A-1

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ILLUSTRATIONS

Fipgure PAne

2-1 BASIC CONDUCTIVITY MEASUREMENT CIRCUIT ........ 2-2

3-1 COAXIAL RESISTANCE MEASUREMENT ................ 3-3

3-2 VOLTAGE AND CURRENT DATA FOR PBX-9404 IN THECOAXIAL EXPERIMENT ......................... 3-5

3-3 VOLTAGE AND CURRENT DATA FOR PBXW-113(I) IN THECOAXIAL EXPERIMENT ......................... 3-6

3-4 VOLTAGE VS. CURRENT AND di/dt VS. TIME FORPBXW-113(I) IN THE COAXIAL EXPERIMENT ............. 3-7

3-5 PLANAR RESISTANCE MEASUREMENT EXPERIMENT .... 3-9

3-6 VOLTAGE AND CURRENT DATA FOR PBX-9501 IN THEPLANAR EXPERIMENT .......................... 3-10

4-1 APPARATUS TO MEASURE CONDUCTIVITY STRUCTURE INPBXN-1 11 USING GUARD-ELECTRODES, A. TOP VIEW OFELECTRODE, B. END VIEW FROM BOOSTER ............. 4-3

4-2 PBXN-111 RESISTANCE MEASUREMENT APPARATUS,EXPLODED VIEW .............................. 4-4

4-3 VOLTAGE AND CURRENT MEASUREMENTS INDETONATING PBXN- 111, FIRST EXPERIMENT .......... 4-6

4-4 VOLTAGE AND CURRENT MEASUREMENTS INDETONATING PBXN-1l1, SECOND EXPERIMENT ......... 4-7

5-1 TIME RESOLVED CONDUCTIVITY EXPERIMENT SHOWINGCONDUCTION ZONE MOVING FROM LEFT TO RIGHT .... 5-2

5-2 A. CLOSE-UP OF CONDUCTION PATHS AROUND SLITB. CONDUCTIVITY PROFILE ADJACENT TO SLIT ......... 5-3

5-3 PBX-9501 CONDUCTIVITY STRUCTURES FOR THICKNESSESOF 1 mm AND 2 mm ............................ 5-6

A-1 DETONATION VERIFICATION EXPERIMENT ............ A-2

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TABLES

Table Page

3-1 COAXIAL TUBE DIMENSIONS........................ 3-2

3-2 COAXIAL RESISTANCE MEASUREMENTS................3-4

3-3 PLANAR CONDUCTIVITY aeA PRODUCTS................ 3-11

A-1 TEST CONFIGURATIONS AND RESULTS................. A-i

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CHAPTER 1

INTRODUCTION

The time resolved measurement of electrical conductivity provides a unique means ofstudying detonating explosives. These measurements offer new i.isights into the physics andchemistry of detonation.

Much work has been done in an effort to understand the electrical conduction indetonating explosives that are subjected to high electric fields. 1 The methods that have beendeveloped are used as tools for probing the detonat':,n process; in particular, they can be usedto study the effects of detonation wave stability an, the roles of carbon and aluminum duringthe detonation process.

In this report the results of a large number of experiments are reported. The data aresummarized for two types of experiment, bulk resistance and time resolved conductivitymeasurements, and are interpreted in the light of our existing understanding.

DETONATION CONDUCTION MODELS

Various models of conduction have been considered during the course of these studies.The three main candidates for conduction that have been considered are: shock inducedconduction in the unreacted explosive due to band gap reduction, shock induced conduction inthe reaction products, and conduction in coagulated carbon behind the reaction zone.

The first model, due to Crawford, assumes that conduction occurs in the unreactedexplosive in the von-Neumann spike. 2 The unreacted explosive at atmospheric pressure behavesas an insulator with a relatively large energy gap, E9, between the valence and conduction bands;for instance, in TNT Eg = 6.3 eV. Estimates were made of the reduction of the band gap dueto pressure which suggest that TNT becomes an "organic metal" when compressed to 50 percentof its initial volume. This work was not pursued because the experimental data demonstratedthat the conduction continued throughout the reaction zone and beyond, as the results belowshow.

The second model was proposed by Griem. 3 For relative simplicity, he assumed thatthe reaction zone could be represented by a mixture of ionized, atomic species of carbon.hydrogen, nitrogen, and oxygen, C, H, N, and 0. Here again, it was assumed that Lie

detonation pressure reduced the band gaps of the atomic species to allow ionization to occur attemperatures of 3000 to 5000'K (i.e., less than 0.5 eV). The preliminary results cf this work

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suggested that H ions were primarily responsible for conduction by liberating electrons that arefree to move in a conducting plasma. However, later work showed that at near solid densitiesH- ions may not exist. The electrons may "hop" from one H atom to the next behaving as"virtual negative ions." This preliminary study did successfully predict conductivitiescomparable with experimental results.

Hayes has produced convincing evidence that the conduction may be due to carbon in theproducts of the detonation gases. 4 He propagated a non-planar detonation wave into a divergentelectric field at the end of a coaxial probe. The interpretation of the data was hampered by thedivergent geometry but this classic work was probably the first to demonstrate the use ofconductivity measurements to probe the structure of the detonation process. Hayes showed thatthe peak cond.,ctivity, which is perhaps equivalent to the spike reported here in Chapter 5, wasstrongly correlated to the calculated carbon content of the products.

OVFRVIEW OF RESULTS

Bulk Res tance Measurements

The dynamic electrical resistances of detonating explosives were obtained fromsimultaneous measurements of voltage across 'he explosives, and current flowing in them, asfunctions of time. The resistances, R, were obtained from the slopes of voltage, V, versuscurrent, I, i.e., using V-I plots. From these resistances the products of the explosives'conductivities and conduction-zone widths were determined. These V-I plots were typicallylinear and the lines could be extrapolated through the origin. However, small departures fromthese lines which occurred at the time the detoration wave first entered the electrodes weredetected. These departures were determined to be due to the finite time duration for theconduction zone to enter the electrodes.

It was found that the products of the explosives' conductivities and conduction-zonewidths were typically independent of geometry. However, results for PBX-9404 and PBX-9501showed significant differences. These effects were found to be due to detonation waveinstabilities that occur when the explosives' dimensions are close to criticality, i.e., close toiai,'oire diameter or thickness.

Conductivity in Aluminized Explosive PBXN-111

The conductivity profile observed in PBXN-I11 (formerly PBXW-115) is interesting andunusual. At first the results were interpreted as evidence of late reactions in the explosive.However, there are large variations from experiment to experiment which suggest that thestructure is strongly affected by detonation wave instabilities. The results are probably due toa combination of wave instabilities and late reaction effects. This demonstrates one of theadvantages of conductivity measurement as a diagnostic tool. Wave instabilities are apparent

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that could not be determined by dent testing or ionization pin data; these can be observed in finedetail using the present technique.

Conductivity Structure in PBX-9501

The conductivity structure of PBX-9501 reported in Chapter 5 is particularly interesting.It has been estimated from critical diameter measurements that the reaction zone thickness isapproximately 100 pm; 5 this is equivalent to a time duration of only 11 ns. Yet the datapresented in Chapter 5 of this report showed conduction spikes of at least 100 ns durationfollowed by 1 jis duration tails. Several researchers have noted that their models of detonationphenomena do not adequately match observed data unless a slow-burn term for the combustionof carbon is included. 6"' 58 Johnson 7 has suggested that carbon slowly coagulates into largeclusters behind the deionation front in the product gases. This coagulation provides anadditional, slow energy release after the main reaction. These findings are consistent with theconductivity measurements by Hayes and of the work reported here, i.e., that conduction is dueto the presence of carbon behind the shock front.

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CHAPTER 2

ELECTRICAL MEASUREMENT TECHNIQUES

The conductivity measurements have been performed in two mnain classes of experiments,those that measure the bulk resistance, reported in Chapters 3 and 4, and those that measureconductivity as a function of time and distance in Chapter 5. A critical review of these variousmethods was published previously. 9 In this report the results for four explosives compositionsPBX-9404, PBX-9501, PBX-9502, and PBXN-1 11 are reported. Both sets of experiments sharea common electrical circuit which is described below.

BASIC CIRCUIT

The fundamental electrical circuit for all the resistance measurement experiments isshown in Figure 2-1. The main circuit was comprised of a capacitor C, an electrical Closingswitch, S; a transmission line inductance and resistance, Ls and r; and the explosive loadinductance and its resistance, LX and RX. The capacitor, C, represents the power source.Electrical diagnostics were used to measure the current, i; the rate of change of current, di/dt;and the voltage, V. To eliminate contact resistance errors the voltage was measured using afour-probe technique.'

The switch was either an explosively-activated closing switch,' 0 a thyratron, or atriggered spark gap. The switch protected the diagnostic circuits and explosive from prolongedexposure to the electric field. The transmission line was either an array of parallel coaxialcables or a parallel plate stripline.

The choice of circuit components depended on the scale and type of the experiment. Thecapacitor value ranged from 50 to 1500 F; it was charged to the voltage, Vc (2.5 to 40 kV),prior to the start of each experiment.

There are several advantages to using high voltages. First, the contact potentials thatexist between the metal electrodes and the conducting explosives, which are typically only a fewvolts, are too small to affect the high voltage measurements. Second, the large currents that areobtained are well-suited to high speed, high-linearity current measurements using Rogowski'coils or current transformers (marked CT in Figure 2-1). Rogowski coils are essentially air-cored current transformers; the output voltage is directly proportional to the rate of change ofmagnetic flux ,D in the circuit.

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VoltageProbe

L xx

xx

AI

CurrentProbe

FIGURE 2-1. B3ASIC CONDUCTIVITY MEASUJREMENT CIRCUIT

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The capacitor, the transmission line, and the explosive load form a classic LCR seriesresonance circuit. When the switch is closed the circuit will resonate if it is not sufficientlydamped or dissipative. For simplicity we replace the total inductance with L, and the totalresistance with R;

R = r + R, L = Lt + l. (2-1)

The state of damping is expressed by the quality factor, q. Z0 is the pulse impedancefrom classical circuit theory, where

Z= -, then q=.§ (2-2)R

Using conventional circuit theory we equate the sum of the voltages in the circuit to thevoltage on the capacitor, V. In this analysis the impedance of the voltage probe, R,, isconsidered to be too large to affect the circuit. If time t = 0 represents the time when theswitch is closed we have

i idt + L di + iR K (2-3)CfJ d,0

The classic solution to this series LCR circuit can be expressed as

V -_

i(t) = - sin(w t) e 2L (2-4)

where / 1

( 34q 2 RT- (2-5)

Zo = ±OoL, and Zo = woL.

Now the voltage across the unknown resistance is iRX so

V R . - 2TIiR. = - sin (w' t) e (2-6)

zo,

In these experiments the inductance L. was small, typically less than 1 nH. Small

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corrections were made for the voltage across L, by measuring the inductance prior to theexperiment, and then sutudcting L, di/dt from the measured voltage, V , . (L. actuallydiminishes as the detonation wave reduces the length of the current path between the source andthe conduction zone; this effect was allowed for by assuming a linear reduction of inductancewith time. Reference 1 describes this correction in more detail.)

At the time I = 0 the detonation wave, with its conduction zone, enters the electrodesand current i starts to flow. (In other experiments it has been shown that the arrival of thedetonation wave shock front is coincident with the start of the conduction zone within 0.5 ns. 1)Consequently, the explosive itself acts as the main closing switch and the voltage V(t) across theexplosive becomes:

V(t) L, di -ri(t) - fi(t)dtdt C 0(2-7)

-i(t)R. + L. &-dt

At t = 0, i(0) 0, so

dil - from Equation (2-4)d t 1 =0 L ( 2 -8 )

and V(0) - from Equation (2-7).L

Notice that at t = 0 the voltage across the load is independent of R, because i = 0. Atthat time all the applied voltage appears across the inductances in the circuit. The current thenrises approximately sinusoidally and the voltages and currents are recorded. By plotting voltageas a function of current, with the appropriate corrections for Lxdi/dt, the resistance of theexplosive is obtained.

VOLTAGE MEASUREMENTS

It is difficult to measure voltage accurately in explosives experiments because of the rapidchanges of current, i.e., di/dt > 1010 A/s. These errors and their elimination are treatedelsewhere."11 1 The rapid changes occur due to the short time durations typical of theexperiments. Consequently, stray inductances in both the explosive circuits and the voltageprobes can cause large errors. Large eirors can also arise from the mutual inductances betweenthe explosive's circuit and the voltage probes. These errors are minimized by careful designincluding the use of parallel striplines. In these experiments, the mutual inductances werereduced to < 1 nil by careful design.

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Commercial oscilloscope voltage probes are unsuitable for this work; they are poorlyscreened from magnetic disturbances and exhibit large errors due to the mutual inductancebetween the main circuit and the voltage probe circuit. Moreover, the direct connection of theground wire of a voltage probe invariably produces 'arge errors due to ground loop effects.Ground loops are eliminated by the indirect measurement of current, in a relatively large shuntresistor, Rv, using a current transformer. See Figure 2-1. The resistor, R , used here rangedfrom 100 1) to 10 kfQ and was made from a solution of copper sulphate (CuSO 4) in apolyethylene tube; details for construction and use of such shunt resistors is given elsewhere. 1

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CHAPTER 3

CONDUCTIVITY-DISTANCE DATA OBTAINEDFROM BULK RESISTANCE MEASUREMENTS

The experiments described here were all typified by either a planar or coaxial geometry,in which a high explosive was placed between two metal electrodes; the electrodes extendedpart-way along the length of the explosive sample. A detonation wave was initiated at one endof the explosive outside the region confined by the electrodes. The detonation wave then enteredthe explosive between the electrodes so that the velocity vector D and the electric field E weremutually perpendicular; i.e., D was parallel to the length of the electrodes.

The electrodes were connected to a high voltage capacitor bank at a voltage in the rangeof 2.5 to 40 kV. The explosive acted as a good insulator with a breakdown strength of

20 kV/mnm in its unreacted state; however, when detonating the explosive reaction zone hada conductivity of the order of 100 mhos/m. The detonated explosive thus initiated current flowbetween the electrodes, and the subsequent electrical current and voltage were. monitored as afunction of time. By plotting voltage, V, as a function of current, I, i.e., the V-I plot, thedynamic nature of the conduction process was determined.

The resistance RX of the explosive was then obtained from the slope of the V-I plot.Now in these experiments we could not measure conductivity, a(x), directly but rather theintegral of o(x) along the conduction zone. We define a conductivity-zone-width product, oA,where A was the effective conduction zone width.1 The uA is an average value, and it isassumed that R. is constant, then:

A= f o(x)dx (3-1)

The aA product was then related to R for the two geometries thus:

Planar: o A - (3-2)WR.

where h was the explosive thickness and W the electrode width, and

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ln(r1 /r2)Coaxial: G A = -______ (3-3)

where r, and r2 were the radii.

The resistance RX was obtained from the slope of the V-I plot. The plot was typicallylinear and the line could be extrapolated through the origin. However, a small departure fromthe line was detected at the time the detonation wave entered the electrodes- this curvature willbe discussed in Chapter 5.

COAXIAL EXPERIMENTS

The resistance of the conduction zone in a number of explosives was measured in acoaxial configuration similar to that reported by Demske et al.12 General details of theexplosives compositions can be found in the literature. 5, 13

Each explosive was pushed into a brass or copper tube and a brass electrode was insertedin the center of the explosive; see Figure 3-1 The explosive and the tube were slightly tapered(1 pm diameter per mm length) to insure intimate contact between them. This minimizedpossible contact resistances, due to air gaps, that can exceed the explosive's resistance." Theexplosives were 127 mm long and detonation was initiated at one end. Two sizes ofconfiguration were used; see Table 3-1.

TABLE 3-1. COAXIAL TUBE DIMENSIONS

Inside diameter Outside diameter Explosive mass Tube length(mm) (mm) (g) (mm)

3.1 14.0 33.0 100.0

11.5 30.2 140.0 75.0

COAXIAL RESULTS

The results of the coaxial experiments are shown in Table 3-2 for both the large(indicated by a t) and small configurations. Typical voltage and current data are shown inFigure 3-2 for PBX-9404. For Figure 3-2 the sequence of events is as follows. Just prior tothe time t = 0 usec the detonation wave enters the space between the coaxial electrodes. Whenthe wave enters current begins to flow and, because of the inductance of the power supply, L,the voltage drops by Ldi/dt, where di/dt is the rate of change of current in the circuit. Thevoltage across the electrodes is then the sum of the voltage across the explosive's resistance, iR:,

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CH-6Booster

Brass.... ..Electrode

DetonatorCopper Explosive under TestTube

FIGURE 3-1. COAXIAL RESISTANCE MEASUREMENT

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and the voltage across the electrode inductance, L. di/dt, as shown in Equations (2-7) and (2-8).By correcting for L. di/dt the true voltage can be plotted versus the current, so that theresistance may be obtained.

The PBX-9501 results show that oA was independent of charge size. Results suggestthat, provided that the explosive's dimensions exceeded the critical diameter, the current flowin the explosive was Ohmic. The explosive performance properties of PBX-9404 and PBX-9501are virtually identical, 5 differing only in their sensitivities, so they are usually treated as identicalexplosives. This is also true for electrical conductivity; no significant differences have beenfound in this study.

TABLE 3-2. COAXIAL RESISTANCE MEA3UREMENTS

Resistance RxExplosive (52) (mhos)

PBX-9404 0 .12t 1.2

PBX-9404 0.20 1.2

PBX-9501 0.20 1.2

PBX-9502 0.02 10.2

PBXW-108 0.12 2.0

PBXW-113(I) 0.09 2.6

tLarge configuration

The-Effect of a Long Conduction Zone in PBXW-113(I)

The typical V-I plot is linear throughout the duration of the experiment, provided thatthe explosive's resistance is constant from the beginning of current flow. However, explosiveswith a long conduction zone clearly take a finite time for that zone to enter the electrodes, andthe resistaiice may take a few microseconds to fall to its final value. Materials likePBXW-113(1) exhibit this type of behavior. The PBXW-113(I) electrical data are shown inFigures 3-3 and 3-4. The data show that the corrections for LX di/t did not reduce the initialvoltage to zero; it is speculated that this was because of the finite length of the conduction zonein PBXW-1 13(I). A similar but much shorter duration effect has been observed in PBX-9501;this is described in Chapter 5. The data for PBXW-1 13(I) show that the resistance does not fallto its final value for z 1.8 Ms. Assuming a detonation velocity of 8.39 mm/ps this is equivalentto a conduction zone of = 15 mm in length.

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2Vv 1.9 .. .. .

kV - -1.6

1.4

1.2 +- Voltage

1 -

.6

.4

.2 t psecs

0' 1 2 3 4 5 6 7 8 9 10

11 T

10±k.A 9

8-:

761 Current

St

2t psecs

5 1 3 4 5 6 7 8 9 10

FIGURE 3-2. VOLTAGE AND CURRENT DATA FOR PBX-9404 INTHE COAXIAL EXPERIMENT

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.9

.8

VtkV

.6-.

.4 , Voltage3 i j1

.2-

.1 I

S 2 4 6 8 10 12 14 16t ptsecs

101

91 8 /kA7

7/

4 4-Current

2t

0 2 4 6 8 10 12 14 16t .isecs

FIGURE 3-3. VOLTAGE AND CURRENT DATA FOR PBXW-113(I) IN THECOAXIAL EXPERIMENT

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1.8

1.6

1.4di-dt 1.2 I

GA/s

.8

.6

.4

.2

0 26 8 10 1 4 1t j.Lsecs

.7-±v kvt

.6

.3

.21

1 - - I ! l I

00 1 2 3 4 5 6 7 8 9 10I kA

FIGURE 3-4. VOLTAGE VS. CURRENT I.ND di/dt VS. TIML FOR PBXW-113(I)IN TH~E COAXIAL E-Xi'PIMEiNi'

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PLANAR EXPERIMENTS

The planar experiments were essentially simplified versions of the Ershov experimentsdescribed in Chapter 5. A sheet of high explosive was placed between two flat, parallel brasselectrodes; see Figure 3-5. A detonation wave was initiated in the explosive sheet with a linewave generator and booster (not shown).

Current and voltage data were measured while the detonation wave travelled along, andparallel to, the electrodes. The electrodes were 12.7 mm wide, 12.7 mm thick, and 150 mm.long. The explosives sheets ranged from I to 25.4 mm thick.

PLANAR RESULTS AND DISCUSSION

The conductivity zone-thickness products, obtained using Equation (3-2) for the planarexperiments, are shown in Table 3-3; for comparisor, the measured detonation velocities, D, andthe infinite-diameter detonation velocities, D., are also included. The thicknesses of theexplosive sheets are shown as h.

A parent Inconsistencies Between Coaxial and Planar Results

Voltage vs. time, current vs. time, and voltage vs. current (V-I) data are shown inFigure 3-6 for PBX-9501 in the planar configuration. It is evident that the conductivity re3ultsfor PBXs 9404 and 9501 are quite different than the results for the coaxial configuration ofTable 3-2. In the coaxial experiments the oA products appear to be independent of chargedimensions, but these dimensions are much greater than their critical diameters of = 1 mm. Butin the case of the thin sheets there is a clear thickness dependance, probably because these sheetsare close to failure. The failure thickness is expected to be half the failure diameter, 14 i.e.,=0.6 mm for PBXs 9404 and 9501 provided that the wave can propagate 15 to 25 times the

failure thickness, i.e., -z 12 mm.

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Conduction Zone Current Path

Electrode 1

~~I '.\-

Electrode 2--- Detonation Explosive

FIGURE 3-5. PLANAR RESISTANCE MEASUREMENT EXPERIMENT

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10}V

8 T

Voltage6 :1/4-

it .secs

0 2 4 6 8 10 12 14

70

kA .60 /

50

40 - ~//

30 Current

20 1/10 t Isecs

" . I /~~--

0 2 4 6 8 10 12 14

FIGURE 3-6. VOLTAGE AND CURRENT DATA FOR PBX-9501 IN THEPLANAR EXPERIMENT

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TABLE 3-3. PLANAR CONDUCTIVITY a, PRODUCTS

Explosive h (mm) T a& (mhos) D (nm/s)

PBX-9404 t 1.00 0.20 8.30

PBX-9501t 1.00 0.24 8.27

Pi3X-9501 2.00 0.54 8.52

PBXN-111tt 12.7 0.20-0.40 5.27

PBXN-111 25.4 0.04-0.40 5.37

tDo = 8.82 mm/Ms

ttD = 6.20 mm/Ms

The observed dependance of the oA product on thickness, for explosives sheets close tofailure, is clearly an important finding. The results given in Chapvr 5 explain these apparentinconsistencies, where the measurement of conductivity in PBX-9501 as a function of time isreported.

Departures from Linearity

Small departures from the linear V-I plots at the beginning of the traces have beendetected, i.e., when the detonation wave first entered the electrodes; see Figure 3-4. Theseeffects were first thought to be due to errors in voltage measurement arising from variousmagnetic effects. In particular, it was believed that the corrections for electrode inductance,Lxdi/dt, were in error. Despite exhaustive tests these departures could not be eliminateC. Thetrue origin of these errors was not identified until time-resolved conductivity measurements weremade. The departure was found to be a real physical effect due to the existence of a tail in theconductivity profile; this is described in Chapter 5.

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CHAPTER 4

CONDUCTIVITY OF DETONATING ALUMINIZED EXPLOSIVE

The measurement of conductivity may provide valuable insight related to details of thecombustion of aluminum in plastic-bonded Navy explosives. The Navy's plastic bondedexplosive PBXN-1 11 studied here contains 25 percent aluminum by weight.

The technique for measuring resistance in aluminized explosives is essentially the sameas the measurement in thin sheets of explosive discussed in Chapter 3. Attempts to measure thePBXN-1 11 conductivity structure using the Ershov experiment were unsuccessfil because of thelarge failure thickness of the explosive compared to the sizes which could be practically usedin these experiments.

DETONATION STABILITY IN SHEETS OF PBXN-111

A series of experiments were performed to establish the thickness of PBXN-1 11 thatcould sustain a stable detonation. Square slabs of PBXN-111, 75 mm in length and width, wereinitiated in various thicknesses by PBX-9501 boosters. The results showed that the explosivecould be detonated in thicknesses of 12.7 rrmt or greater if it were confined by heavy brassplates; see Appendix A. The measured failure diameter of PBXN-111 is 37.1 mm, 15 whichagrees favorably with our data if a two to one ratio between failure diameter and thickness isassumed.14 With the benefit of the conductivity results discussed later, it is known that thedetonation may have been unstable in these tests, i.e., the wave may not have been fullydetonating. These observations are consistent with the calculations by Kennedy which indicatethat the chemical reaction of PBXN-111 may be only 12 percent complete at the sonicsurface, 16 whereas for an ideal explosive reaction would be complete.

ATTEMPTS TO MEASURE PBXN-111 CONDUCTIVITY VS. TIME

Measurements of PBXN-111 conductivity were attempted using a modified Ershovexperiment. In experiments with small failure diameter explosives used in thin, flat sheets, e.g.,PBX-9501, the ratio of width, W, to thickness, h, was relatively large, W/h > 5.Consequently, errors caused by the divergent fields at the edges of the electrodes werenegligible. (We verified this with guard-electrode experiments where the elimination of fielddivergence had no detectable effect on the measured bulk resistance of PBX-9404.)

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Narrow electrodes are necessary for the Ershov experiments because of problemsassociated with wave curvature, these are described in Chapter 5. The detonation wavefrontmust be precisely parallel to the slit, or otherwise the measured conductivity profile will beperturbed as the front sweeps across the slit. But the large failure thickness of PBXN-111caused h to be large and therefore, W/h to be small. Consequently, guard-eltctrodes were usedto eliminate field divergence and to provide confinement; see Figure 4-1.

In this experiment a center electrode, 12.7 mm square cross-section, was used to measureconductivity in a 12.7 mm thick sheet of PBXN-111. Parallel guard-electrodes, 12.7 mm thickand 25.4 mm wide, provided confinement and the elimination of fringe fields. Theguard-electrodes were insulated from the center electrode with thin strips of Teflon (PTFE).Rogowski coils, marked RC in Figure 4-1, were used to monitor currents in the center electrode,and all three electrodes combined. The guard-electrodes were connected with heavycopper-braids, and the voltage was measured as described in Chapter 2. Figure 4-1(A) showshow one set of electrodes were configured. Figure 4-1(B) shows how the electrodes were placedabove and below the explosive; this view is from the booster end of the explosive, i.e., fromthe right side of Figure 4-1(A).

It was found that the Teflon insulation was quickly destroyed by the detonation wave, thismade it impossible to measure current in the center electrode. Consequently, the Ershovexperiments were abandoned for PBXN-111, and simpler bulk resistance measurements usingwide, single electrodes were performed

BULK RESISTANCE MEASUREMENTS IN PBXN-111

The bulk resistance of detonating PBXN- 111 was measured in the apparatus shown inFigure 4-2. The explosive was confined by 25.4 mm thick brass plates and initiated by thebooster system described above. Prior to detonation an electric field E was applied across theexplosive. Unreacted PBXN-111 conducts in electric fields exceeding =1 kV/mm.Consequently, it was necessary to insulate the explosive with a 25 um thick film of Kapton typeH (polyimide) insulation. This insulation has a dielectric strength of - 3 x 108 V/m; however,it conducts readily when shock-loaded above =5 GPa.10 The Kapton thus prevented theexplosive from conducting and igniting prior to arrival of the detonation wave, When shockedto the detonation pressure, : 12 GPa, the Kapton resistance became negligibly small, hence thetrue resistance of the explosive could be measured; see Figure 4-2. (A second voltage probe wasused to measure the potential difference across the film to verify that the Kapton's resistance wasindeed negligible; the probe was connected to a short length of copper foil as shown inFigure 4-2.)

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i Voltage oti < -- etonation

Braids I Gur-letoe

BadGuard-electrode

Center electrode "Slit \PTFE- Insulator

y Guard-electrode:

I[center 7

RC to

R -IA. Top view of oneCurrentin] set of electrodes

Electrodes

B. End view as seenLBN- i1from booster

I_ K -

Electrodes

FIGURE 4-1. APPARATUS TO MEASURE CONDUCTIVITY STRUCTURE INPBXN-111 USING GUARD-ELECTRODES, A. TOP VIEW OFELECTRODE, B. END VIEW FROM BOOSTER

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Copper FoilElectrode +

Kapton '__-

- - __I

PBXN-111 ' iV

Detonation __ VoltageS Probes

Booster __ __]

Electrode

FIGURE 4-2. PBXN-111 RESISTANCE MEASUREMENT APPARATUS,EXPLODED VIEW

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PBXN-111 RESULTS AND DISCUSSION

Figure 4-3 shows the measured voltage and current from one experiment performed on25.4 mm thick PBXN-1 11. Electrodes of 75 mm width and 150 mm length were used. Theinitial voltage on the capacitor was 5 kV.

The structure of this current waveform is unusual, perhaps unique; we have not observedit in any other explosives. From ionization pin data the detonation wave took 26 As to sweepthe length of the electrodes, i.e., it exited the electrodes at 26 As, which is off the graph.However, the current rose to 2.9 kA, and ther decayed to 500 A within 8 /is; the decaycontinued for the remainder of the experiment. Another apparently identical experiment gavedifferent results: the first peak was only I kA, followed by a similar decay and then a rapid riseafter 10 ps to 4 kA; see Figure 4-4.

These results were at first interpreted as evidence of late reactions in the explosive.However, there are large variations from experiment to experiment which suggest that thedetonations were unsteady and that these strongly affected the measurements. The results areprobably due to a combination of variations from experiment to experiment and late reactioneffects. It is certain that 25.4 mm thick sheets of PBXN-1 11 are close to the critical thickness.Therefore, the initial current peak may be due to the over-boostering of the explosive. In 1helight of these results, the experiments should be repeated with greater lengths of explosive toallow the detonation waves to stabilize prior to the measurements of resistance.

Leiper 17 has analyzed the performance of a 'highly non-ideal explosive' similar toPBXN-111. Using a slightly divergent flow-code he found that less than 70 percent of theformulation reacted within the subsonic part of the flow; even at several times the critical chargediameter, reaction was not complete before the sonic plane. He suggested that a similar effectmay account for the results for PBXN-I 11.

Leiper's findings are certainly in keeping with our anomalous PBXN-11 data. Manyother researchers have observed, or suggested, that the detonation process in even idealexplosives requires relatively long distances (many charge diameters) to stabilize. Notablereferences to these effects include Ramsay, 14 Jones, 16 Mader, 8 and Bdzil.19 Clearly non-ideal explosives, such as Leiper's, are likely to take even longer to stabihize. Clairmont et al.observed that explosives loaded with ammonium perchlorate (AP) need relatively large lengthto diameter ratios for detonation to stabilize, they observed a detonation that failed after thewave had travelled 7.5 diameters, 20 this is discussed further by Price. 21 All these effects canbe interpreted as evidence of late reactions behind the sonic surface.

The role of aluminuni in non-ideal explosives is particularly interesting. The reactionof the aluminum is quite slow; detonation performance data suggest that the energy release takestens or hundreds of As. This reaction should have little effect on the detonation velocity and

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6

5.5V 5

W 4.5

4

3.5

3

2.5 Voltage

2

1.51

.5 t v.secs

0 2 4 6 8 10 12 14 16 18 20

3

2.8kA. 2.6

2.42.221.8 Current

1.61.41.21.8{ _ _ _ _ _ _ _ _ _ _

.4

.2 t vsecs

2 4 6 8 10 12 14 16 18 20

FIGURE 4-3. VOLTAGE AND CURRENT MEASUREMENTS IN DETONATINGPBXN- 111, FIRST EXPERIMENT

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kV 4

3.5

2. rVoltage

2

1,5

.5

0 2 4 6 8 10 12 14

t psecs

1 CurrentkA

.8 /

.6 /

.4 "

.2

0 2o 4 6 8 10 12 14

t psecs

FIGURE 4-4. VOLTAGE AND CURRENT MEASUREMENTS IN DETONATING

PBXN-111, SECOND EXPERIMENT

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pressure. Yet Akimova 22 has observed that the addition of small particle size aluminum to castTNT compositions actually decreases the failure diameter, and other workers includingGimenez 23 and Price 24 have observed the effects of aluminum upon the reactions occurringwithin as little as 2 As of the detonation front. Flickiger observed that the effects of aluminumreaction were observed about 5 As after detonation in the cylinder test. 25 The results show thatthe effects of aluminum reaction continue for some time behind the detonation front. Clearly,the electrical conductivity is sensitive to the degree of reaction in PBXN-1 11 and can thereforebe used as a reaction diagnostic.

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CHAPTER 5

TIME RESOLVED CONDUCTIVITY MEASUREMENTS

Ershov 26 has reported an ingenious technique which can be used to measureconductivity in a plane wave geometry with a near parallel electric field. The technique iscapable of providing high time-resolution data; it has been adapted to provide the measurementsreported here.

A thin sheet of high explosive was placed between two flat, parallel brass electrodes;see Figure 5-1. A narrow insulated slit was positioned against the explosive in the center of thebottom, ground electrode; a conducting loop bridged the slit in the electrode. A high voltagepower supply (not shown) was connected to the electrodes on the right side of the testconfiguration shown in Figure 5-1. The rate of change of electric current in the loop wasmeasured with a Rogowski coil. Detonation was initiated on the left hand side of the explosivesheet with a line wave generator and booster (also not shown). The detonation wave travelledin a direction perpendicular to the slit, as shown, so that the wave front was parallel to it.

The sequence of sketches in Figure 5-1 shows the progression of the conduction zonefrom left to right in the explosive. The beginning of the experiment is shown in the top of thefigure. The current travelled along the top electrode, down through the conduction zone, alongthe bottom electrode and around the loop, and then back to the power supply; the Rogowski coildetected the total current, i. Later the conduction zone straddled the slit and current flowed onboth sides of it; see the central figure and enlarged version in Figure 5-2(A). Finally, in thebottom of Figure 5-1 the current travelled along the top electrode, down through the conductionzone, along the bottom electrode, and back to the power supply; no current flowed in the loopafter passage of the detonation conduction zone.

As Ershov demonstrated, the electric field E behind the conduction zone of conductivity,j, is noi perturbed by the current flow, provided that the vectors E and curl a are perpendicular.The current density, j, and conductivity, a, can now be expressed as functions of time, t, forthe direction parallel to the applied field E,

j(t) = o(t)E (5-1)

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Conduction Zone Current Path

Electrode/ 1Detonjt io

Electrode Eixplosive

Conducting

Rogowski Coil di/dt Loop

FIGURE 5-1. TIME RESOLVED CONDUCTIVITY EXPERIMENT SHOWINGCONDUCTION ZONE MOVING FROM LEFT TO RIGHT

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Current Paths/\

Conduction_Zone

o(x) 7 W ----

/ , A. Close-up ofNarrow Slit conduction

around slit

ConductionZone Slit

Position

B. Conductivity

FGURE 5-2. A. CLOSE-UP OF CONDUCTION PATHS AROUND SLITB. CONDUCTIVITY PROFILE ADJACENT TO SLIT

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NSWCDD/TR-92/218

Now j(t) is the current i(t) per unit area. The area of conduction in the direction of Eis the conductor width W multiplied by the length in the x direction, so

X

i(t) = WfEo(x)dx (5-2)0

The power supply is connected to the ends of the electrodes furthest from the detonator.The wave enters the explosive between the electrodes at x = 0 and t = 0, and it arrives at theslit at x = x', t = x'/D where D is the detonation velocity; see Figure 5-2(B).

If the field E and the velocity D are independent of distance, x, then the substitution ofx = Dt and E = V/h is valid, where V is the voltage between the electrodes, and h is theirseparation. Then:

D

W) = WDV fr(t)dt (5-3)h 0

di(t) = a(t) WDV (5-4)dt h

The last expression demonstrates the elegance of the Ershov technique: di/dt follows theexact profile of the required quantity, a(t). Moreover, accurate di/dt data are easy to obtainusing a Rogowski coil. Since di/dt is directly proportional to d4/dt, then the output is alsoproportional to a(t) from Equation (5-4).

EXPERIMENTAL DETAILS

The explosive sheets were approximately 150 mm square, 1.00 mm or 2.00 mm thick,and extended beyond the edges of the 12.7 mm wide electrodes. A 200 /Am wide slit (or gap)was cut across the width of one electrode so that it was parallel to the detonation front andinterrupted current flow along the surface of the electrode. A low inductance, low resistanceloop maintained current flow around the slit; the Ldi/dt and iR voltage differences werenegligible for this experiment. The loop was formed by a 5.2 nun diameter hole drilled in theelectrode at the end of the slit; see Figure 5-1.

The slit was inisulated with Teflon film to delay breakdown across the slit. Thebreakdown could be due to dielectric failure in the slit, or mechanical closure by hydrodynamicflow of the metal electrodes. When such breakdown occurs the current no longer flows in theloop and the di/dt measurement is terminated.

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NSWCDD/TR-92/218

The main electrical circuit is similar to those reported in Chapters 2 and 3 for the planarexperiments. The data were recorded on transient digitizers and 1 GHz bandwidth analogoscilloscopes.

A Rogowski coil, with a time resolution of <_ 1 nsec, was placed in the hole adjacent tothe slit to measure the rate of change of current di/dt. To test the validity of the Rogowski coildi/dt data, the records were integrated numerically and compared with independe:nt current datafrom a calibrated current transformer. The data were in good agreement. For a slit of 200 Amwidth, the time resolution, given by the detonation transit time across the slit, was 22 ns.

If the detonation front were not straight and parallel to the slit then the resultant di/dtrecord would be perturbed by this wave structure, i.e., the wave would not cross the slit at allpoints across its width simultaneously. Measurements of shock wave curvature using a streakcamera showed a maximum uncertainty of = 10 ns due to wave curvature.

TIME RESOLVED MEASUREMENTS IN PBX-9501

Conductivity data are shown for 1 mm and 2 mm thick sheets of PBX-9501 inFigure 5-3. Typically the currert and voltage at the time of conductivity measurement were 4.0kA and 1.5 kV. The peak conductivities for both thicknesses were -220 mhos/m and thedurations of the initial conduction spikes were t 100 ns. The initial spikes were followed bylower level, longer duration tails; these tails were markedly different for the two thicknesses.

The data shown in Figure 5-3 for the 2 mm thickness have an abrupt break, at = 900 ns,which was most probably due to closure of the 200 Am slit. When the slit closes current flowsacross the slit instead of in the loop; such breaks are quite common and unavoidable.Measurements of current and voltage taken at the same time as di/dt confirmed that conductioncontinued for an additional 1 or 2 As in the explosive.

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NSWCDD/TR-92/218

30

2501

. 2M0

1501lmm - -

100' 2 mm Slit

50 .Closure

50

-200 0 200 400 600 800 1000 1200time, nsecs

FIGURE 5-3. PBX-9501 CONDUCTIVITY STRUCTURES FOR THICKNESSES OF1 mm AND 2 mm

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NSWCDD/TR-92/218

CHAPTER 6

DISCUSSION AND CONCLUSIONS

BULK RESISTANCE MEASUREMENTS

From the dynamic resistances of detonating explosives the products of the explosives'conductivities and conduction-zone widths were determined. The V-I plots were typically linearand the lines could be extrapolated through the origin. However, departures from these lineswere detected which had occurred at the tim. the detonation wave first entered the electrodes,i.e., at time t = 0, position x = 0. These departures were determined to be due to the finitewidth of the c",' ... tion zone. Essentially the electrodes see a conduction zone profile enteringfrom one eitd. .1.is can be illustrated by estimating the resistance at the beginning of theexperimen.

Fit- we make a simplifying assumption that there is no electric field divergence at theedges of the electrodes. Although this assumption introduces significant errors, it is justifiedbecause our purpose is to illustrate the effect of the wave entering the electrodes, not to provideaccuracy. (The field divergence could be minimized in the manner described in Chapter 5.)If we combine Equations (3-1) and (3-2) for a planar geometry, and write G for the conductance,i.e., I/R, then G can be expressed as

x

G - _ fo(x)dx (6-1)R h0 0

If we relate distance to time, assuming a steady detonation velocity, D, then x = Dt, and wecan differentiate to find the rate of change of conductance with time

dG - DW (6-2)dt h

So, the resistance is constant (and, therefore, the V-I plot is straight) when dG/dt and thus o(t)is zero. This can only occur after the full width of the c(t) profile has entered the electrodes.As was seen f, PBXN-1 11 in Chapter 4, this profile can be tens of microseconds long.

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NSWCDD/TR-92/218

It was generally found that the products of the explosives' conductivities andconduction-zone widths were independent of explosive geometry. However, the results forPBX-9404 and PBX-9501 showed significant differences due to detonation wave instabilities thatoccur when the explosives' dimensions are close to criticality, i.e., close to failure diameter orthickness.

MEASURE, iENTS OF CONDUCTIVITY STRUCTURE

As re,orted in earlier work the peak conductivity for PBX-9501 was ==220 mhos/m. 9

The conductivity data reported here have improved time resolution; moreover, the techniqueshave been refined to observe longer duration structures. The new data show initial spikes inconductivity of =-100 ns duration for all experiments, followed by tails that are stronglydependent on the state of detonation in the explosive. These are believed to. be the firstpublished observations of two-step conductivity structures, i.e., a short duration spike followedby a long duration tail.

The conductivity structure of PBX-9501 is particularly interesting. It has been estimatedfrom critical diameter measurements that the reaction zone thickness is approximately 100 Jtm, 5

which is equivalent to a time duration of only 11 ns. Yet the data here show longer than 100ns conduction spikes followed by at least 1 us-duration tails. Several researchers have noted thattheir models of detonation phenomena do not adequately match observed data unless a slow-burnterm for the combustion of carbon is included. 6.7 '8 Johnson7 has suggested that carbon slowlycoagulates into large clusters behind the detonation front in the product gases. This coagulationprovides an additional, slow energy release after the main reaction. These findings aresupported by Hayes' work.4

It is estimated that the critical thickness of sheets of PBX-9501 is L0.6 m, hence, thepropagation of detonation in these 1 mm and 2 mm thick sheets will be influenced by lateralrarefactions. This is confirmed by detonation velocities measured here (8.27 and 8.52 mm/ls)which are less than published velocities, i.e., 8.83 mm/jis. Further studies should be done toobserve the effects of confinement and explosive thickness.

From this work, it is speculated that the tails of the conduction profiles in PBX-9501 aredue to the slow coagulation of carbon in the products. Moreover this coagulation appears to bestiongly influenced by the thickness of the explosive sheets. In other words, the arrival oflateral rarefactions, in the wake of the reaction zone, perturbs the coagulation process. Thespikes are unaffected because they occur too rapidly. Therefore, detonation failure may be dueto the control )f the energy release during carbon coagulation in the tail. The measurement ofconductivity may be the only wiy of observing these phenomena.

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NSWCDD/TR-92/218

ALUMINIZED EXPLOSIVE PBXN-111

The aluminized explosives data show the conductivity measurement to be a valuable too,for the analysis of reaction and wave stability during detonation. The results, discussed inChapter 4, suggest that the structure of the conductivity profile is strongly affected by detonationwave instabilities. The results are probably due to a combination of wave instabilities and latereaction effects. This technique should lead to the formulation of better metallized explosives.

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REFERENCES

1. Tasker, D.G., The Properties of Condensed Explosives for Electromagnetic EnergyCoupling, NSWC TR 85-360, Oct 1985, NSWC, White Oak, Maryland.

2. Crawford, O.H., "Electrical Conductivity in the Reaction Zone of High Explosives,"Oak Ridge National Laboratory, May 1985, private communication.

3. Griem, H.R., "Properties of Dense Plasmas Generated by Discharges Behind ShockWaves," University of Maryland, Laboratory for Plasma and Fusion Energy Studies,College Park, MD, Final Report, 28 Jul 1986 - 28 Jul 1987.

4. Hayes, B., "On Electrical Conductivity ir. Detonation Products," Proceedings of theFourth Symposium (International) on Detonation, Office of Naval Research,Washington D. C., Oct 1965, p. 595.

5. Dobratz, B.M., LLNL Explosives Handbook, UCRL-52997, 16 Mar 1981, Lawrence

Livermore National Lab.

6. Mader, C.L., Numerical Modeling of Detonations, Univ. California Press, 1979.

7. Johnson, J.D., "Carbon in Detonations," in Proceedings of the 1989 APS TopicalConference in Condensed Matter, Elsevier Science Publishers, B. V., 1992, p.697.

8. Tang, P.K., "A Study of Detonation Processes in Heterogeneous High Explosives," J.Appl. Phys., Vol. 63, 1988, p. 1041.

9. Tasker, D.G., Granholm, R.H. and Lee, R.L., "The Fast Measurement of ElectricalConductivity Structure Within The Detonation Zone of a Condensed Explosive," inShock Waves in Condensed Matter, Elsevier Scierce Publishers, B.V., 1988, p. 923.

10. Tasker, D. G., Lee, R. J., and Gustavson, P.K., An Explosively-Actuated ElectricalSwitch Using Kapton Insulation, NSWCDD/TR-92/124, Feb 1992, NSWCDD, SilverSpring, MD.

11. Tasker, D.G. and Lee, R.J., 'High Current Electrical Conduction of PressedCondensed Detonating Exploives," Shock Waves in Condensed Matter, PlenumPublishing Corporation, 1986.

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12. Demske, D.L., Forbes, J.W., and Tasker, D.G., "High Current Electrical Resistanceof PBX-9404," Shock Waves in Condensed Matter-1983, Elsevier Science PublishersBV., 1984.

13. Hall, T.N. and Holden, J.R., Navy Explosives Handbook, Explosion Effects andProperties--Part Ill. Properties of Expiosives and Explosive Compositions,NSWC MP 88-116, NSWC, White Oak, MD.

14. Ramsay, J.B., "Effect of Confinement in 95 TATB/5 KEL-F, Proceedings of theEighth Symposium (International) on Detonation, Albuquerque, NM, Jul 1985, p. 3 7 2 .

15. Forbes, J.W., Lemar. E.R. and Baker, R.N., "Detonation Wave Propagation inPBXW- 115," Ninth Symposium (International) on Detonation, OCNR 113291-7,Portland, Oregon, 1989, 8.806.

16. Jones, D.A. and Kennedy, D.L., Application of the CPex non-ideal explosive modelto PBXW-115, Materials Research Laboratory Report MRL-TR-91-40, Oct 1991.

17. Leiper, G.A., discussion of paper by Tasker, D.G. and Lee, R.J., "The Measurementof Electrical Conductivity in Detonating Condensed Explosives," Ninth Symposium(International) on Detonation, OCNR 113291-7, Portland, Oregon, 1989, p.405.

18. Mader, C.L., "Chemistry of Build-Up of Detonation Wave," Actes du SymposiumInternational sur le Comportement des Milieux Denses sous Hautes PressionsDynamiques, Commissarat a i'energie atomique, Paris, 1978.

19. Bdzil, J.B., Fickett, W., and Stewart, D.S., "Detonation Shock Dynamics: A NewApproach to Modeling Multi-Dimensional Detonation Waves," Ninth Symposium(International) on Detonation, OCNR 113291-7, Portland, Oregon, 1989, p. 7 30 .

20. Clairmont, A.R., Jaffe, I. and Price, D., The Detonation Behavior of AmmoniumPerchlorate as a Function of Charge Density and Diameter, NOLTR 67-71, 20 Jun1967, NSWC, White Oak, Maryland, p.20.

21. Price, D., Review of Information on Composite Explosives: IV, ATR-TR-90-35, Aug1990, Advanced Technology and Research Corporation, Maryland.

22. Akimova, L.N. and Stesik, L.N., "Detonation Capacity of Perchlorate Explosives,"Combustion, Explosion and Shock Waves, Vol. 12, 1976, p. 2 4 7 .

23. Gimenez, P. et al., "F.P.I. Velocimetry Technique ...." Ninth Symposium(International) on Detonation, OCNR 113291-7, Portland, Oregon, 1989, p. 1 37 1 .

24. Price, Donna, Review of Infjormation on Composite Explosives: I. GeneralBackground, ATR-TR-88-0046, Nov 1988, Advanced Technology and ResearchCorporation, Maryland, pp. 1-2.

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25. FlIckiger, R., "Characterization and Perfonnance Tests of Hexals," Proc. Int. Symp.Py;.,. and Expls., Beijing, 1987, pp.270-275.

26. Ershov, A.P., Zubkov, P.I. and Luk'yanchikov, L.A., "Measurements of theElectrical Conductivity Profile in the Detonation Front of Solid Explosives,"Combustion, Explosion and Shock Waves, Vol. 10, No. 6, 1974, p.776.

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APPENDIX A

THE VERIFICATION OF DETONATION IN SHEETS OF PBXN-111

EXPERIMENT

These tests were conducted to verify that detonation could be sustained in thethicknesses of PBXN-1 11 used in the planar conductivity studies; see Chapter 4. Fourexperimei. s were performed to determine the thickness required for stable detonation inrectangular slabs of PBXN-111. Figure A-1 illustrates the basic configuration for theexperiments. The configurations for each test are shown in Table A-1. The 50.8 mm thickcharge was made from two 25.4 mm charges. The confinement was comprised of 76.2 mmsquare, 12.7 mm thick brass plates placed against the top and bottom faces of the confinedtest charge, as shown in Figure A-1.

TABLE A-1. TEST CONFIGURATIONS AND RESULTS

Thickness Dent(mm) Conditions (mm)

12.7 No confinement 0.00

12.7 Confined 1 2.54

25.4 No confinement 1.55

50.8 No confinement 3.30

The initiation system consisted of a 114.3 mm wide, 25.4 mm long, 12.7 mm thickPBX-9501 booster and a 1.27 mm thick triangular Detasheet linewave generator. Thelinewave generators were cut into equilateral triangles, 76.2 mm on a side, from largerlinewave generators. A 63.6 mm thick steel witness plate was used to detect detonation atthe edge of the PBXN-111 slab furthest from the booster.

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NSWCDD/TR.92/218

LZWitness Block

Detonator

/Booster

~\ Linewave Generator

Confinement Plates

FIGURE A-1. DETONATION VERIFICATION EXPERIMENT

A-2

NSWCDD/TR-92/218

RESULTS

The 12.7 mm thick charge with no confinement was the only configuration that didnot detonate. This configuration produced very little indentation in the witness plate andseveral pieces of the explosive were recovered after the shot. The other experimentsproduced dent depths of 1.55 mm, 2.54 mm, and 3.3 mm as shown in Table A-1.

CONCLUSIONS

The steel witness block data showed that the explosive could be detonated inthicknesses of 12.7 mm (or greater) using the brass confinement. This configuration wasused for the studies described in Chapter 4.

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NSWCDDfTR-92/218

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REPORT DOCUMENTATION PAGE MNo07418P.ub, reporting brdi..en for this Collection of information it estimated to average I hou~r per resfporre. iriici~ng the time for revokwang instru~torI. searching existng datatougcet. g9atherinj Ind maintaining the data needed, and completing andl reviewi~ng the collectioin of informationr Send comrments regarding thritbrurden es.timrate or any othere~selr Of th, Collfeti.ri Of information. in~cluding "rg4.t-bons for reducng this orldie'r. to Watshirrgton Headqu~arter% Services. Directorate for Information Operations andkeportI. 120 ti elfttO Davis Highway, Suite 1204. Arlington, VA 22202-4302. and to the Office of Management and tlr.dget. Paperneork Kedrindrort Project (0 704-O 4018),*.sh,l.tonlKI 20S03

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4. TITLE AND SUBTITLE I ac 19 . FUNDING NUMBERSThe Measurement of Electrical Conductivity in DetonatingCondensed Explosives

6. AUTHOR(S)

Douglas 0. Tasker, Richard J. Lee, and Paul K. Gustavson

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) B. PERFORMING ORGANIZATIONREPORT NUMBER

Naval Surface Warfare Center Dahlgren DivisionWhite Oak D~etachment NSWCDD/TR-92/2 1810901 New 11ampshire AvenueSilver Spring, MI 20903-5000

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) '10 SPONSORING/MONITORINGAGENCY REPORT NUMBER

Off-ice ofChief of Naval ResearchDr. It. A. Roberts800 N. Quincy Street, IICT IArlington, VA 22217-5000

11. SUPPLEMENTARY NOTES

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

Approved for public release-, distribution is unlimited.

13. ABSTRACT (Maximum 200 wurds)

The time-resolved measurement of electrical conductivity provides a unique means of studyingdetonating explosives. The results of a large number of experiments are reported. Two types ofexperiments were performned; they measured bulk resistance and time-resolved conductivity. Someinteresting effects, due to detonation wave instabilities, were observed when the explosives' dimensionswere close to criticality, i.e., close to failure diameter or thickness.

The conductivity profile observed in the aluminized explosive PBXN-1 I11 (formerly PBXW-1 15) isinteresting and unusual. The rerults are probably due to a combination of wave instabilities and latereaction effects. This demonstrates one of the advantages of uiting conductivity measurements as adiagnostic tool. Wave instabilities are apparent that could nut be determined by other means.

14. SUBJECT TERMS IS. NUMBER OF PAGESExplosives D~etonation wave PBX-9501 PI3XW-1 13 61Aluminized explosives instabilities PBX-9502 PBXW- 115 16. PRICE CODEElectrical conductivity PBX-9404 PB XW-108 PLIXN-1 11I

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OfOF REPORT OF THIS PAGE OF ABSTRACT ABSTRACTU NC LASSI FIE 1) UNCLASSirIED IUNCLASSIFIED) SA I

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