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Kerr Cell Photography of High Speed PhenomenaE. M. Pugh, R.v. HeineGeldern, S. Foner, and E. C. Mutschler Citation: Journal of Applied Physics 22, 487 (1951); doi: 10.1063/1.1699989 View online: http://dx.doi.org/10.1063/1.1699989 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/22/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in HOLOGRAPHIC PHOTOGRAPHY OF HIGHSPEED PHENOMENA WITH CONVENTIONAL AND QSWITCHED RUBY LASERS Appl. Phys. Lett. 7, 92 (1965); 10.1063/1.1754327 HighSpeed Photography Phys. Today 9, 32 (1956); 10.1063/1.3059894 High Speed Photography Phys. Today 4, 32 (1951); 10.1063/1.3067307 A Kerr Cell Camera and Flash Illumination Unit for Ballistic Photography J. Appl. Phys. 21, 995 (1950); 10.1063/1.1699545 An Inexpensive Stroboscope for High Speed Photography Rev. Sci. Instrum. 14, 273 (1943); 10.1063/1.1770186
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JOURNAL OF APPLIED PHYSICS VOLUME 22, NUMBER 4 APRIL, 1951
Kerr Cell Photography of High Speed Phenomena * E. M. PUGH, R.V. HEINE-GELDERN, S. FONER, AND E. C. MUTSCHLER
Department of Physics, Carnegie Institute of Technology, Pittsburgh, Pennsylvania (Received November 15, 1950),
Visible light photographs have been obtained of metal jets squirted from the lined conical cavities of high explosive charges. Since these jets travel through air at nearly meteoric velocities (7 to 12X 105 cm/sec), their front ends are heated to incandescense and vaporized. The remainder of the jet is relatively nonluminous and is photographed by synchronizing a Kerr cell shutter (exposure time < 1 J.'sec) and an exploding wire light source (peak intensity 4XI08 candlepower) with the phenomenon. Schlieren type photography cannot be used because of the luminosity accompanying the phenomenon. Detail is obtained in the photographs by using a large lens with a seven-in. focal length. To maintain the effective aperture atf/4 requires a large Kerr cell which in turn requires a 25,000 volt pulse for its operation.
The technique is used in studying the action of these jets on steel, water, glass, and Plexiglas. The technique is also used in studying the propagation of shock waves in solids and liquids when these phenomena are due to the action of the metal jets or to the more direct action of explosives.
INTRODUCTION
T HE metal jets which are squirted from high explosive charges with lined cavities1 travel at
velocities exceeding 7000 m/ sec. Since they are relatively nonluminous compared to the intense luminosity of the accompanying explosion, their photography by visible light presents some unusual problems. For that reason, previous attempts at photographing these jets have been unsuccessful, the only satisfactory results having been obtained by flash x-ray methods.2
Visible light silhouette photographs have now been obtained by synchronizing a very brilliant light source (electrically exploded wire of peak intensity 3 to 5 X 108
candlepower) and a Kerr cell shutter with the phenomenon. To obtain detail of the fast moving jet at an object-to-Iens distance of 3 to 6 feet, the exposure time must be one microsecond or less. Since fast (and therefore large grain) film must be used, the negative must be large enough to preserve detail. This is accomplished by
Obsenlolion Room
using an aerial lens having a focal length of seven inches and an aperture of 1/2.5. This aperture is reduced slightly by the Kerr cell, so that pictures are taken at an effective aperture of 1/4.
Figure 1 shows a schematic and simplified diagram of the experimental setup. The explosion takes place in a bomb proof separated from the observation room by a heavy, reinforced concrete wall. A port hole protected by bullet proof glass allows the camera to take pictures of phenomena occurring in the bomb proof. The phenomenon is silhouetted against a diffusing screen of tracing paper illuminated from the back by the light of the electrically exploded wire. The Kerr cell with its crossed polaroids transmits light only during the microsecond interval when a 25-kv pulse is applied to the electrodes of the Kerr cell.
JETS TRA VEUNG IN AIR
Early attempts to photograph the high speed jets from shaped charges in visible light were usually
Diffusing Seteen '------' (tissue paper I
Bomb Proof
FIG. 1. Simplified schematic diagram of experimental setup used for Kerr cell photography.
* This work was carried out under a research contract with the Office of the Chief of Ordnance, Contract No. W-36-061-0RD-291O, and has been released for publication by the Office of Public Information, Department of Defense.
1 Birkhoff, MacDougall, Pugh, and Taylor, J. App!. Phys. 19,563 (1948). 2 J. C. Clark, J. App!. Phys. 20, 363 (1949).
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488 PUGH, HEINE-GELDERN, FONER, AND MUTSCHLER
FIG. 2. Jet from a copper cone with flat apex. Only a small portion of the jet is visible, the rest being surrounded by an opaque shroud of spall particles which are vaporized by their rapid motion through air.
unsuccessful,3 showing even less jet than can be seen in Fig. 2. While a small portion of the jet can be seen near the upper portion of that picture, most of the jet is obscured by an opaque shroud. In fact the pictures did not show any jet at all until the light source was made so brilliant that some light was transmitted by the nearly opaque cloud surrounding the jet and until the exposure time was made short enough to stop the motion.
When the tip of the shroud is magnified by several diameters, it appears to consist of a large number of high speed particles traveling nearly parallel to the jet and vaporizing continuously by their rapid passage through air. This conclusion was verified by taking a photograph of a jet passing through an evacuated glass tube. At pressures below 10-3 mm Hg, no shroud at all is seen, the particles being too small to be visible in the photograph, and no metal vapor being formed in the absence of air.
3 In 1943, investigators at the Explosive Research Laboratory in Bruceton, Pennsylvania (MacDougall, Paul, Eyster, and Messerly) did obtain some photographs of jets in a vacuum, using a flash bulb made of high explosive. These pictures have never been shown in an unclassified publication.
The jet shown in Fig. 2 resulted from a copper cone with a fiat apex. (In the commercial production of these cones it is impractical to make liners truly conical down to a pointed apex.) However, the lack of a pointed apex was found to be responsible for the production of most of the shroud seen in Fig. 2, for when nearly perfect brass cones were made on a lathe, the photograph shown in Fig. 3 resulted. There almost all of the jet is visible, only a small portion at the tip of the jet being obscured by the metallic vapor (incandescent near the tip) produced by its rapid motion through air. (F. Zwicky and F. Whipple have proposed using these jets for the study of "artificial meteors." They have not previously been photographed at close range.)
Figures 4 and 5 are intended to give some idea of the physical setup used for the photographs shown so far. They show the setup and the action picture, respectively, of two metal jet charges detonated simultaneously by equal lengths of prima cord fuse running to a single "distributor" charge.
In a later paragraph pictures of jets penetrating certain target materials are discussed. To facilitate interpretation of those pictures, two other topics are considered first: the detonation of explosive charges, and the shock waves and cracking damage produced in
FIG. 3. Jet from a machined brass cone with truly conical apex. The shroud accompanying the jet shown in Fig. 2 has been nearly eliminated.
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FIG. 4. Two metal jet charges standing side by side on a sheet of cardboard and fused with primacord for simultaneous detonation.
FIG. 5. Jets resulting from the simultaneous detonation of the two charges shown in Fig. 4.
FIG. 6. Detonation of a cylinder of pentolite. The detonation wave is traveling downward at a velocity of 7510 m/sec.
various materials when placed in contact with a detonating charge.
DETONATION PHENOMENA
The following photographs were obtained with both pentolite, a high explosive having a detonation velocity
FIG. 7. Simultaneous detonation of two pentolite cylinders. Note region of Mach interaction between the two expanding. cones of explosive products.
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490 PUGH, HEINE-GELDERN, FONER, AND MUTSCHLER
FIG. 8. Appearance of the mild steel plate on which the two charges shown in Fig. 7 had been standing.
of 7510 m/sec, and with primacord (a fast explosive fuse), having a detonation velocity of 6420 m/sec.
Figure 6 shows a one-inch diameter cylinder of pentolite in the process of exploding, the detonation wave proceeding downward. The upper portion of the cloud of explosive products is caused by the initiating mechanism of the charge. Measuring the angle between the conical envelope of explosive products and the charge axis (37.5°), and using the known detonation velocity of 7510 m/sec for pentolite, the calculated initial velocity of radial expansion of the explosive products is approximately 5800 m/sec.
Interference effects between two envelopes of explosive products intersecting at an angle are shown in Figs. 7 and 9. In Fig. 7 two one-inch diameter pentolite cylinders were placed on a steel plate with axes parallel,
FIG. 9. Cylindrical pentolite charge detonated simultaneously from both ends.
FIG. 10. Detonation of primacord under water. Detonation wave proceeded downward at a velocity of 6420 m/sec.
2t in. between centers, and detonated simultaneously. A region of very high luminosity, caused bi.1Mach
FIG. 11. Detonation of a small pentolite charge standing on the top face of a 6X6Xli-in. slab of Plexiglas. Note original shock wave, reflected tension wave and dark region of shattering.
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interference, is clearly visible. The photograph of the 4X4-in. mild steel plate on which the charges had been standing is shown in Fig. 8. Besides the two circular indentations produced by the flat ends of the charges, a well-defined cut was produced in the plate by the Mach stem which indicates the order of magnitude of the pressure within it. The depth of indentation (3 to 4 mm) due to this interference effect was approximately equal to that produced by the charges themselves. For Fig. 9, a long cylinder of pentolite was detonated simultaneously from both ends. Here a ring-shaped luminous region of Mach interference was produced along the circle of intersection of the two expanding cones of explosive products.
SHOCK WAVES AND CRACKING IN TARGET MATERIALS
The Kerr cell method of high speed photography has been applied to studies of shock waves and fracturing in various materials. The shock produced by a length of primacord detonated under water is shown in Fig. 10, the dark central region consisting of the (relatively) slowly expanding explosive products. Due to the airwater interface at the top of the tank, a definite curvature of the shock wave, corresponding to that predicted by a Huygens construction, is visible. From such photo-
FIG. 12. Detonation of a pentolite cylinder laid horizontally along the upper face of a piece of Plexiglas.
FIG. 13. Metal jet perforating a !-in. steel plate. Note intense spalling from the bottom surface of the plate.
graphs it is found that this shock wave propagates in water at a velocity ranging from 2200 m/sec close to the primacord to about 1700 m/sec some distance away from it. At still longer distances, the shock wave velocity would probably approach the velocity of sound in water (1450 m/sec). A similar picture (not shown) of a pentolite charge detonated under water indicates an initial shock wave velocity of 3200 m/sec.
The shadow technique described so far is useful for observing high speed phenomena in transparent solids. For this purpose, Plexiglas targets of dimensions 6X6 X 1 t in. were used.
A cylinder of pentolite was placed on such a Plexiglas target and detonated to produce the results shown in Fig. 11. The cylinder of explosive was placed to the right of the center on the top 6X it-in. face, so that one observes a reflected tension wave from the right-hand face in addition to the original compression wave. One also observes the extent of shattering at the time the exposure was made.
Since the detonation velocities of most explosives are well known, shock wave velocities in transparent solids can be determined conveniently by laying an explosive charge on one face of the material and measuring the angle between the supporting face and the shock wave. Figure 12 shows the shock wave produced in this
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492 PUGH, HEINE-GELDERN, FONER, AND MUTSCHLER
FIG. 14. Metal jet penetrating water. The jet itself is not visible, because it is surrounded by a region of intense turbulence.
manner by pentolite laid on the upper face of the Plexiglas target. The shock wave varies from 3400 m/sec for the most intense shock, to 2400 m/sec as the shock intensity decreases. This lower velocity agrees well with the sound velocity measured by Protzman.4
The last five figures show photographs of metal jets in the process of penetrating or perforating various target materials. In the case of Fig. 13, the target material was a steel plate i-in. thick. At the top of the picture the cloud of explosive products can be seen moving downward with the rear part of the jet. Just above the steel plate, near the point where it is struck by the jet, another cloud consisting of a particle spray coming from the target can be seen. The pear-shaped cloud underneath the plate, consisting mostly of particles spalled from the steel plate, is sufficiently dense to obscure the tip of the jet, while the remainder of the jet is now passing through the hole in the plate without interference.
Figure 14 shows a jet shot into a glass tank filled with water. The velocity of penetration of water under these circumstances is known to be about 5000 m/sec. Since the shock wave makes an angle of nearly 30° with the jet, its velocity must be close to 2500 m/sec, or about
4 T. F. Protzman, J. App!. Phys. 20, 627 (1949).
1. 7 times the velocity of sound in water. The jet itself is not visible in the picture, since it is completely surrounded by a region of violent turbulence in the water.
A jet penetrating a piece of Plexiglas of dimensions 6X 6X It in. is shown in Fig. 15. The area above the tip of the jet is opaque because the shock wave has reached the front and back surfaces and the resulting reflected tension wave has shattered the Plexiglas in that region. The picture indicates that no shattering is produced by the intense compression wave at the tip of the jet.
Figure 16 shows a jet penetrating into a It-in. thick piece of glass after it has perforated a similar slab of plexiglas, the two slabs being separated by a i-in. air space. The Plexiglas plate shows a reflected tension wave traveling upward from the bottom surface, in addition to a rather complicated cracking pattern. The jet inside the glass is surrounded by a transparent region which in turn is bounded by a dark line. This dark line cannot be a shock wave since the shape of a shock wave would be conical rather than cylindrical. It seems probable that the transparent region consists of molten glass which has been melted by the high pressure produced by the tip of the jet: energy considerations indicate that the glass in that region should be heated to a temperature between 1000 and 1500°C. If this is correct, then the black line surrounding the jet repre-
FIG. 15. Metal jet penetrating through Plexiglas.
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FIG. 16. Metal jet penetrating into a glass plate after having perforated a similar slab of Plexiglas, separated from the glass by a i-in. air space.
sents a boundary between glass in its solid and its liquid phase.
Figure 17 shows the jet entering a glass target at an angle of 20° to the target surface. The boundary between the liquid and solid phases is again evident. Cracking has begun at the rear surface directly opposite the point where the jet entered the glass, indicating that
FIG. 17. Jet penetrating a glass plate at an angle of incidence of 20° to the surface of the plate.
the shock wave has reached that surface and has been reflected. This photograph clearly demonstrates that the jet is not deflected when it impinges at a glancing angle on the surface of a hard material like glass. Experiments also show that it is not deflected by the hardest steel.
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