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

INSTRUMENTATION OF SLOW COOK-OFF EVENTS

H. W. Sandusky and G. P. Chambers

Energetic Materials Research and Technology DepartmentNAVSEA Indian Head Division, 101 Strauss Ave., Indian HeadMD 20640-5035

Abstract. An arrangement was developed for validating models of slow cook-off. Experiments wereconducted on the explosive PBXN-109 with measurements of temperature, pressure, and volume untilthe onset of reaction; and measurements of case velocity and blast overpressure during reaction. Thegoal is to relate changes in the energetic material during heating with time and position for onset ofreaction plus reaction violence as a function of sample size? confinement, gas sealing, and heatingprofile. A mild range of reactions occurred as evidenced by fragmentation of the confinement intomostly large pieces; however, at the highest confinement no sample was recovered.

INTRODUCTION

Cook-off is both complex and quite dependenton a variety of environmental factors, such as therate of heating, which is fast in a fuel fire andorders of magnitude slower from indirect heating.In addition to heating rate, the variables includesample size (diameter and length-to-diameterratio), radial and axial confinement, initial ullage,and sealing of pyrolysis products during heating.Since only a limited number of full-scale tests canbe conducted, which limits the number ofvariations in environmental factors, it would beadvantageous to predict cook-off response todifferent hazard scenarios with computer models.Models at the Sandia National Laboratory,Albuquerque (SNLA) (1,2) and the LawrenceLivermore National Laboratory (3) were evaluatedagainst small-scale screening tests, such as theVariable Confinement Cook-off Test (VCCT). Itwas recognized that model validation requiresbetter controls on the tests and more measurementsin each test. In addition, the properties of heated

explosives are being characterized (4,5) to supportthe modeling effort.

Suitable metrics for comparing models andexperiments include the rate of expansion of theenergetic material, temperature at various locationswithin the energetic material and at various pointson and in the apparatus, strain in the confinementand pressure buildup within the confinement as afunction of time, evolution of gaseousdecomposition products, time at which cook-offoccurs, and the violence of that reaction. In someexperiments, it would be advantageous to stop theheating at some point in the cycle and remove theexplosive for evaluation of thermal damage anddecomposition. To meet these requirements, anexperimental arrangement was developed that isdifferent than the usual closed pipe in most small-scale cook-off tests. An initial series ofexperiments was conducted on the explosivePBXN-109, which is RDX and aluminum in arubber binder. A companion program (6) with aclosed pipe is being conducted at the Naval AirWarfare Center/China Lake (NAWC/CL).

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EXPERIMENTAL ARRANGEMENT

The apparatus shown in Figure 1 consists of atest cell mounted between flat springs in a loadframe, which is simply two pieces of 152 mm widesteel channel connected by 25 mm threaded rods.Between each spring and base there can be a springstop in which is mounted a potentiometer-baseddisplacement transducer. Spring displacement hasalso been measured by a strain gage mounted onthe spring, which is calibrated by replacing theexplosive samples with hydraulic oil andpressurizing the test cell. There is a clear field ofview around the test cell for measurements of rapidexpansion and fragmentation of the confinementwith flash radiography and high-speedphotography. The other measurement of reactionviolence is blast overpressure by a transducerwithin a meter of the apparatus.

Details of the test cell are shown in Figure 2. Acylindrical sample of the same dimensions as thatin the VCCT? 25.4 mm diameter by 63.5 mm long,is radially confined in a seamless mechanical tubeof 1018 steel with variable thickness and axiallywith spring-loaded rams. Confinement isdependent on the thickness of the tube and thestrength of the springs. The springs reduce theinternal pressure buildup - expansion from heatingand damage in the sample and pyrolysis products -so that the seals on the rams are not breached. Therams have axial ports for instrumentation withinthe samples, which to date have beenthermocouples sealed by epoxy. Each ram also hasan O-ring seal with the confinement tube. Thetube is heated by resistance wire with minimalinsulation so that the field of view is not obscured.The rams are somewhat thermally isolated fromthe springs to reduce the heat loss from the ends ofthe sample and thereby maintain more uniformtemperatures over the sample length.

Ram Cap withBlunt Knife EdgeInterior

Thermocouple s - - -"-

Resistance Wireover ConfinementTube of VariableThickness

ControlThermocouple,

m\Midplane w

Phenolic Insulator

FIGURE 1. Overall experimental apparatus. FIGURE 2. Details of test cell.

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TABLE 1. Summary of PBXN-109 Cook-off Experiments

Tube WallThickness (mm)

1.271.902.54

Heating Profile,Start & Ramp

130°C,then6°C/hr150°Cthen3°C/hr150 °C. then 3 °C/hr

Ullage(%)

-3.6-3.6-1.0

Gradient(°C/25 mm)

181111

Cook-offTemp. (°C)

172170165

% ExplosiveRecovered

3560

No. of TubeFragments

436

The displacement of the springs was measuredeither by a potentiometer-based displacementtransducer, as shown in Figure 1, or with a straingage on each spring calibrated for displacement.For both techniques, spring displacement wasrelated to pressure on the rams by hydraulicallypressurizing the tube before each experiment.Temperatures on the confinement and within thesample were measured by copper-constantanthermocouples from 0.25-mm diameter wire withTeflon insulation. The tube typically had a straingage at the midplane and a break-wire, bothcircumferentially mounted. During heating, tubestrain is a second measure of interior pressure,calibrated by hydraulically pressurizing the tube.If a high-elongation strain gage of annealedconstantan is used, this gage can also follow theinitial tube expansion during the onset of cook-off.Along with the break-wire, the strain gage alsoserves as trigger probe for the dynamic diagnosticsduring cook-off. The apparatus and the associatedinstrumentation were evaluated experimentally andcomputationally (7) with inert samples of Teflon.

The PBXN-109 is from the same batch as thatused in the NAWC/CL tests. (6) The sample, witha total mass of-52 g, is in three pieces with 1-mmdiameter holes drilled for the thermocouples.

RESULTS AND DISCUSSION

The conditions and results from threeexperiments are summarized in Table 1. Themajor input condition varied was the wallthickness of the confinement tube. Heatingprofiles replicated those at NAWC/CL (6). Therelatively high starting temperatures prior to theslow heating permitted completion of eachexperiment in 8 hrs. The 3.6% ullage in the first

two experiments was from the sample being 0.33mm smaller in diameter than the tube and slightlyoversized holes for thermocouples. The 1% ullagein the last experiment was achieved by eliminatingthe clearance between the sample and tube.

With the -3.6% ullage, there was no significantspring deflection from thermal expansion;however, during the three hours prior to cook-offthere was an exponential increase from thermaldamage and pyrolysis. This is illustrated in Figure3 for the second experiment, where the springdeflection at cook-off corresponds to 1.9% increasein sample length and a 2.7 kpsi (18.7 Mpa) samplepressure. With the minimal ullage in the lastexperiment, thermal expansion was recorded asshown in Figure 4; however, the confinementleaked at 4 kpsi. The last pressure drop wasprobably from seepage around a thermocouplewhose signal was lost.

Heat losses through the rams caused thethermal gradients listed in Table 1 between themidplane thermocouples and the one 25.4 mmfrom the midplane. The gradient is significant inthat the levels of thermal damage and self-heating

-0.50 50 100 150 200 250 300 350

Time (min)

FIGURE 3. Spring response in second experiment.

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170

O

110-50 50 100 150 200 250 300 350

Time (min)FIGURE 4. Control temperature and spring response inthird experiment.

in the samples are reduced near the rams. Internaltemperatures near cook-off are shown in Figure 5for the second experiment, with the thermocouplelocations shown in Figure 2. Despite theuniformity of temperature for the two midplanelocations up to one hour before cook-off, self-heating only appeared on the axis. At 12.7 mmbelow the midplane where temperatures are 6 °Clower, no self-heating appeared even on the axis.

The amount of explosive recovered decreasedwith increasing tube wall thickness. The explosiverecovered in the second experiment was onlymillimeter size pieces, indicating an axial reactionthat fractured the surrounding annulus once theconfinement failed. In the experiment with thethickest wall, an overpressure of 12.6 psi wasmeasured 0.60 m away; there were several smalltube fragments; and at a circumferential wall strainof 8%, the wall velocity was 42 m/s. Perhaps thiswas an explosion, whereas the reactions in theprevious experiments were deflagrations.

SUMMARY AND CONCLUSIONS

Simultaneous mechanical and thermalmeasurements were made during slow cook-off ofan explosive designed to be insensitive to varioushazardous stimuli. Cook-off violence for PBXN-109 was similar to that observed in other small-and full-scale tests. There is significant pressurefrom thermal expansion when ullage is essentiallyeliminated, but only expansion without a pressureincrease for several percent of ullage. At a heating

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I

140200 220 280 300240 260

Time (min)FIGURE 5. Interior temperatures in second experiment.

rate of 3 °C/hr, self-heating occurs on the axis in asmall, <10 mm zone.

ACKNOWLEDGEMENTS

This program was promoted by a number ofindividuals, most notably Art Ratzel at SNLA,Alice Atwood at NAWC/CL, and Ruth Doherty atthis Center. Ken Schebella when visiting thisCenter from DSTO in Australia, Kevin Gibson,Richard Lee, and Vasant Joshi assisted variousaspects of the experimental development. TheOffice of Naval Research provided funding.

REFERENCES

1. Baer, M. R. et al., in Proceedings of EleventhInternational Detonation Symposium, Office of NavalResearch, ONR 33300-5,1998, pp. 852-861.

2. Schmitt, R. G. et al., op. cit., pp. 434^42.3. Nichols, A. L. m et al., op. cit., pp. 862-871.4. Maienschein, J. and Chandler, J. B., op. cit.,

pp. 872-879.5. Renlund, A. M. et al., op. cit., pp. 127-134.6. Atwood, A. I. Et al., in Proceedings ofJ,4NNAF 19th

Propulsion Systems Hazards Subcommittee Meeting,CPIA Publ. 704, Vol. I, 2000, pp. 205-220.

7. Bill Erikson, SNLA, private communication.

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