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

DEVELOPMENT OF THE LANL SANDWICH TEST*L.G. Hill

Los Alamos National Laboratory, Los Alamos, New Mexico 87545 USA

The Sandwich test is a slab-variant of the ubiquitous copper cylinder test, and is used to obtain highexplosive product equation-of-state information in the same manner as its predecessor. The motiva-tion for slab geometry is 1) better high-pressure resolution, and 2) the ability to accommodate initialtemperature extremes for solid explosive samples. The present design allows initial temperaturesfrom -55 C to 75 C. The pros and cons of the two geometries are discussed, followed by a descriptionof the mechanical design and instrumentation. Sample data for several ambient PBX 9501 testsdemonstrates excellent data quality and repeatability.

INTRODUCTIONFor over 35 years the copper cylinder test has

been the primary source of high explosive (HE)product equation-of-state (EOS) data. Tradi-tional instrumentation comprised electrical pinsto measure detonation velocity, and a streakcamera to measure liner motion. The adventof velocity interferometry in the early 1970's al-lowed the two-dimensional "slab" variant to beinstrumented1'2. The Sandwich test is a carefullyengineered slab test that accommodates initialtemperatures between -55 C and 75 C.

GEOMETRY: SLAB VS. CYLINDERSlab and cylindrical geometries both have in-

herent pros and cons. Which is most desirablewill depend on the situation. The primary ad-vantage of slab geometry is better high-pressureresolution. This is because the flow expands inonly one lateral direction, so that pressure fallsoff slower with axial distance. Moreover the linercan be made thinner, and of a higher impedancematerial, to give a finer shock ring-up structure.

In cylindrical geometry the liner stretches andthins, so that only a very ductile material likepure annealed copper can expand sufficientlywithout tearing. In slab geometry the liner onlybends. This allows its material to be chosen by

* Work performed under the auspices of the United StatesDepartment of Energy.

other criteria; it also allows the liner to be thin-ner to start with. This aside there is the issue offabrication, and the difficulty in accurately ma-chining very thin-walled soft copper tubes.

The second advantage of slab geometry is thatit is conducive to designs that accommodate awide range of initial charge temperatures. Differ-ential thermal expansion between HE and linermake this prospect very difficult for cylindricalgeometry—particularly in the cold case, as thecylinder test is quite intolerant of gaps.

An optimal Sandwich test with thin liners re-quires a machined HE sample to maintain a pre-cise assembly. This precludes other physical HEforms such as liquid, putty, or powder. Of coursethese could be accommodated by a design withthicker, rigid walls, but that would largely defeatthe advantages of slab geometry. Such materialsare better suited for the cylinder test.

Another practical limitation of the Sandwichtest is that it must use a velocity interferometer,rather than a streak camera, to measure liner ve-locity. A cylinder test can use both instrumentssimultaneously. Velocity interferometers have amodest depth-of-field, which limits the test sizeto that comparable to a traditional 1-inch cylin-der test. For insensitive HEs a larger test is desir-able to minimize reaction zone effects. A streakcamera can measure arbitrarily large cylinderssimply by adjusting the magnification (e.g., 3).

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Side View

YISAE Probe

Support fleets, 4 ea,Shldkb, 2 ea,

SilkoneO-Rings

4. -

FIGURE 1. Schematic diagram of the Sandwich test, with nominal dimensions (see detail, Figs. 2 & 3).

SIZE & MATERIALSThe design goal is to develop a slab alterna-

tive to the standard 1-inch cylinder test. To do sowe must first set the corresponding dimension—the slab thickness. Detonation Shock Dynamicstheory4 shows that detonation curvature effectsare nominally equivalent when the cylinder ra-dius is equal to the slab thickness, in this case1/2 inch. (This is why the failure thickness of anHE is very close to half its failure diameter.)

Next we must choose a liner material andthickness. The detonation transmits a shock intothe liner that reverberates downstream throughits thickness 61, the wall velocity jumping inincrements of the axial round trip distance A.Hence, A represents the effective spatial resolu-tion. In the Mach angle approximation A is

= , (i)where DQ is detonation velocity, c is sound speed,ma is mass/ area, p is density, and I denotes theliner. For EOS analysis it is desirable that theGurney approximation be satisfied. This requiresmai to be >l/3 of the HE mass/area5. Havingfixed mai , A is minimal if the remaining expres-sion is minimal. This favors stiff, high densitymetals such as Mo, Ta, and W. Other mate-rial factors are acoustic impedance (higher valuestend to provide better confinement and DO closerto DCJ) and toughness (which resists tearing andspall). Overall, Tantalum seems the best choice.

Additional factors affect optimal liner thick-ness. A should exceed the reaction zone length,otherwise the detonation loses confinement anddeviates more from DCJ . The liner must be thickenough not to break during observation, and itsRMS thickness variation should be small com-pared to the mean (which favors thicker sheets).Finally, the present design needs a sheet thick-ness that is flexible, but not prone to creasing.

The other dimensions scale with the max-imum measured wall expansion, E. Existingtests used a commercial (VALYN™ FOP-1000-60mm) VISAR probe, with 30 < E < 40 mm.Edge effects should be minimized by heavy tamp-ing plates with half-width E or greater.

The distance from probe beam to downstreamcharge end is such that the detonation breaks outwhen the liner has traveled a distance E. Conse-quently, late liner motion is not affected by thetermination. The same run is allowed betweenthe line wave initiator and the probe beam, al-lowing the liner to reach its steady trajectoryprior to observation. Computations have con-firmed this length to be sufficient6.

The slab width is governed by two issues.First, the product wake at the edges must notblock the probe beam during the measurement.Second, the test should be wide enough that,with sides heavily tamped, the detonation waveand following flow are two-dimensional near thecenter line. These criteria require a slab half-width of order E. Figure 1 shows a schematicdiagram with nominal dimensions.

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DESIGN & INSTRUMENTATIONThis section discusses critical implementation

issues, starting with the liner. The 20-mil thickTa sheet is manufactured by Rembar Corp. Itis commercially pure and annealed, with meangrain size ~25 /mi. As-received sheets are pre-cisely cut to length and width, and paired asclosely as possible according to mean thickness.

The liner should conform to the HE withoutglue, as 1) a glue gap is not easily analyzed, and2) glue joints may fail at extreme temperaturesdue to differential thermal expansion. A glue-less assembly is achieved as follows. First, theliners are tacked to the HE with RTV siliconealong their narrow ends, and clamped while itcures. This sandwich is then assembled with thetamping plates, an RTV bead is formed alongthe four longitudinal joints (see Fig. 2), and theassembly is clamped while it cures. The sand-wich is centered upon a ridge on the tamper, thechamfered edges of which form a channel at eachcorner. This channel is evacuated, sucking theliner tight to the HE. A gap between liner andtamper allows for differential thermal expansion.

Clap

tJS In

FIGURE 2. Edge detail (cf. Fig. 1).

The velocity pins shown in Figs. 1 & 2 are de-tailed in Fig. 3. Sharp pins are epoxied insidePEEK capillary tubing, which is glued into holesin the tampers. The installed point locations aremeasured by optical comparator. The pins arecapped by a spring steel shim strip insulated byKapton tape, recessed to be flush with the tam-per ridge and tight against the HE. The deto-nation drives the shim through the Kapton intothe pin, completing an electrical circuit. In hotor cold shots the pins move with the tamper, sotheir ambient spacing may be adjusted by thetamper thermal expansion. The standard errorin velocity so-obtained is within 5 m/s.

HE Main ChargeSpring Steel (GND)

Kapton TapeBpoxy FillerPEEK Capillary TubingSharp Sewing NeedleSteal Tamping Plate

FIGURE 3. Velocity pin detail (cf. Figs. 1&2).

The Sandwich test is challenging for VISARbecause the plate travels through a large distanceand angle. The difficulty lies in both absolutelight level, and the limited dynamic range of fastscopes. Happily, one may compensate by sandingthe target region of the liner (using a diamondfile for Ta) in the lateral direction; this forms agrating that fans scattered light in a plane thatremains coincident with the probe as the linerturns7. One may also place the probe at an ini-tial stand-off distance and angle </> that generatesthe most uniform light return. For a steady flowwith specified DO and 0, an exact transformationthen yields the liner trajectory.

Fiber optic fiducial pins (see Fig. 1) preciselypositioned along the probe beam path generate alight flash, and hence an x-t point, when struckby the liner. These give an independent measureof the VISAR fringe constant, allowing correc-tion of a small 2D aberration that arises due tothe finite probe beam size. A small error from theincreased refractive index behind the air shock isessentially eliminated by firing in Helium.

FIGURE 4. Sandwich test photograph (Shot# 8-666).

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FIGURE 5. Framing camera pictures of a detonating Sandwich test (Shot^ 8-592). Liner grid is 1 cm square.

SAMPLE RESULTSFigure 5 shows electronic framing camera pic-

tures of a PBX 9501 Sandwich test detonating inair. The left side of each frame is the end view,with detonation approaching the viewer. Theright side is the side view (via mirror), with det-onation traveling to the right. The liner jump-off is visualized by light from the air shock, andshows that the detonation is flat. The productwake develops at about a 15° angle, leaving aclear view for the VISAR probe beam. Both lin-ers hold together during the measurement.

Figure 6 shows a composite of six VISARrecords for four shots (two of which had dualVISARs with differing sensitivities). The re-peatability is essentially within the signal noise.The bottom trace is a fiber optic fiducial pin sig-nal from one of the tests. This can be interpretedto a time resolution less than 10 nsec.

Six Superimposed VISAR Records

Fiber Optic Pin Signal -

^ 0 2 4 6 8 10 12 14t (MS)

FIGURE 6. Composite VISAR records for PBX 9501.

ONGOING DEVELOPMENTThe Sandwich test is in a mature stage of re-

finement and validation. While there is evidencethat edge effects are negligible, an experimentalwidth-convergence study is necessary to be sure.The design was successfully tested at -55 C, butmore experience at initial temperature extremesis desirable. An effort to assemble the chargefrom multiple segments was partially successful,but indicated (as expected) a strong sensitivityto joint quality. 2D VISAR aberrations, whilelargely correctable via the optical fiducial pins,should be further minimized by an optimal probedesign that minimizes the spot size at jump-off.

ACKNOWLEDGEMENTSI thank T. Aslam, J. Bdzil, R. Catanach,

J. Echave, R. Gustavsen, B. Harry, W. Hemsing,D. Murk, P. Quintana, and H. Stacy for designand test support; G. Buntain, P. Howe, D. Idar,S. Larson, and J. Stine for funding support.

REFERENCES1. Lee, E., Breithaupt, D., McMillan, C., Parker, N.,

Kury, J., Tarver, C., Quirk, W., & Walton, J.; 8th

Symp. (Int.) on Detonation (1985).2. Tarver, C.M., Tao, W.C., & Lee, C.G., Propellants,

Explosives, Pryotechnics, V. 21 (1996)3. Davis, L.L., Si Hill, L.G.; These Proceedings.4. Aslam, T.D., Bdzil, J.B., & Hill, L.G.; IIth Symp.

(Int.) on Detonation (1998)5. Kennedy, J.E., Explosive Effects & Applications,

Ch. 7, J. Zukas & W. Walters, Eds. (1998)6. Bdzil, J.B., Unpublished calculations (2000)7. Hemsing, W.F., Private communications (1999)

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