Photographic study of a dead -pressed explosive

G. M. Swallowe, J. E. FieldCavendish Laboratory, University of CambridgeMadingley Road, Cambridge, CB3 OHE, England

Abstract

High speed photography in conjunction with electron microscopy and a pressure measuringtechnique have been used to investigate the differences between dead -pressed and non -dead-pressed samples of the primary explosive Mercury Fulminate (Hg Ful). Photographs of reactionpropagation were taken in transmitted light using a specially adapted drop- weight machinewith transparent anvils. The results of these experiments suggested a mechanism for dead -pressing in Hg Ful based on the microscopic internal structure of the compacted explosive.

Introduction

Dead -pressing is a phenomenon which occurs when Powdered explosives are highly compressedto form compacts whose density approaches that of the single crystal density. A dead -pressedexplosive does not initiate as easily as normally and may fail to cause detonation in thesucceeding explosive in an explosive train. This latter point is particularly importantsince many materials which dead -press are primary explosives used in detonators. Althoughthe phenomenon has been known for some time it has attracted increasing interest in recentyears. This is because the current trend towards miniaturisation requires the use ofdensely packed explosive to maintain the power output of smaller devices at a level similarto that of the older larger designs.

The work described here has been carried out on Mercury Fulminate (Hg Ful). This mater-ial has been used as a detonator filling since 1864 and has long been known to suffer fromthe problem of dead -pressing. In preliminary experiments the relative outputs of pelletsof Hg Ful pressed to various densities in a 3 mm die were measured using a strain gaugedanvil. These results, illustrated in Figure 1, show a considerable decrease in the outputof the pellet as the density rises above 4.0 gm /cc. It is this reduced output and theresultant failure of the main charge to detonate which is the major difficulty with a dead -pressed explosive.

Experimental

Samples of Hg Ful were prepared by pressing 20 mg lots in a "Specac" 3 mm die at variousloads up to 1.5 tons and determining the densities of the resulting pellets by measuring andweighing. The samples were impact initiated by dropping a weight of 5.5 kg onto them froma height of 1 m. The weight was suspended from an electromagnet and its fall was guided by3 cylindrical rods. Impacts were observed by using a transparent toughened glass anvilsystem similar to that described by Heavens and Field'. The system is illustrated inFigure 2.

Photographic sequences were recorded with an A.W.R.E. C4 rotating mirror camera at aframing rate of 1.5 x 105 f.p.s., and a Hadland Imacon 790 camera at framing rates up to100. All experiments used backlighting by means of an MFT 218 flashtube. Synchronisationfor the C4 camera was simply achieved by running the camera with open shutter in a darkenedroom and triggering the flash by means of a pin attached to the weight which made contactwith a piece of spring steel attached to the anvil assembly. The length of the flash wasadjusted to correspond to just less than one complete exposure of the film. When usingthe Imacon the flash was triggered in the same way as described for the C4 and the camerawas triggered by applying thin layers of copper foil to the upper and lower glass anvils.The weight and guide rods were electrically insulated from the rest of the system and96 Volts applied between the weight and lower anvil. Contact between the two foils duringimpact caused a short circuit and the resulting voltage pulse produced across a seriesresistor was used to trigger the camera. The time during impact at which camera triggeringoccurred could be controlled by varying the foil thickness.

Results and Discussion

The photographic sequences show considerable differences in the behaviour of dead -pressedand non -dead -pressed samples. Selected frames from Imacon sequences are shown in Figures3,4 and 5. It can be seen that in the "normal" samples (Figures 3 and 4) the reactionfront starts from a single site and advances uniformly and rapidly ( ti 400 m s -1) throughoutthe whole sample. In the dead -pressed sample (Figure 5) however, although the reaction

484 / SPIE Vol 348 High Speed Photography (San Diego 1982)

Photographic study of a dead-pressed explosive

G. M. Swallowe, J. E. FieldCavendish Laboratory, University of Cambridge Madingley Road, Cambridge, CBS OHE, England

Abstract

High speed photography in conjunction with electron microscopy and a pressure measuring technique have been used to investigate the differences between dead-pressed and non-dead- pressed samples of the primary explosive Mercury Fulminate (Hg Ful). Photographs of reaction propagation were taken in transmitted light using a specially adapted drop-weight machine with transparent anvils. The results of these experiments suggested a mechanism for dead- pressing in Hg Ful based on the microscopic internal structure of the compacted explosive.

Introduction

Dead-pressing is a phenomenon which occurs when powdered explosives are highly compressed to form compacts whose density approaches that of the single crystal density. A dead-pressed explosive does not initiate as easily as normally and may fail to cause detonation in the succeeding explosive in an explosive train. This latter point is particularly important since many materials which dead-press are primary explosives used in detonators. Although the phenomenon has been known for some time it has attracted increasing interest in recent years. This is because the current trend towards miniaturisation requires the use of densely packed explosive to maintain the power output of smaller devices at a level similar to that of the older larger designs.

The work described here has been carried out on Mercury Fulminate (Hg Ful). This mater­ ial has been used as a detonator filling since 1864 and has long been known to suffer from the problem of dead-pressing. In preliminary experiments the relative outputs of pellets of Hg Ful pressed to various densities in a 3 mm die were measured using a strain gauged anvil. These results, illustrated in Figure 1, show a considerable decrease in the output of the pellet as the density rises above 4.0 gm/cc. It is this reduced output and the resultant failure of the main charge to detonate which is the major difficulty with a dead- pressed explosive.

Experimental

Samples of Hg Ful were prepared by pressing 20 mg lots in a "Specac" 3 mm die at various loads up to 1.5 tons and determining the densities of the resulting pellets by measuring and weighing. The samples were impact initiated by dropping a weight of 5.5 kg onto them from a height of 1 m. The weight was suspended from an electromagnet and its fall was guided by 3 cylindrical rods. Impacts were observed by using a transparent toughened glass anvil system similar to that described by Heavens and Field*. The system is illustrated in Figure 2.

Photographic sequences were recorded with an A.W.R.E. C4 rotating mirror camera at a framing rate of 1.5 x 10 5 f.p.s., and a Hadland Imacon 790 camera at framing rates up to 10 . All experiments used backlighting by means of an MFT 218 flashtube. Synchronisation for the C4 camera was simply achieved by running the camera with open shutter in a darkened room and triggering the flash by means of a pin attached to the weight which made contact with a piece of spring steel attached to the anvil assembly. The length of the flash was adjusted to correspond to just less than one complete exposure of the film. When using the Imacon the flash was triggered in the same way as described for the C4 and the camera was triggered by applying thin layers of copper foil to the upper and lower glass anvils. The weight and guide rods were electrically insulated from the rest of the system and 96 Volts applied between the weight and lower anvil. Contact between the two foils during impact caused a short circuit and the resulting voltage pulse produced across a series resistor was used to trigger the camera. The time during impact at which camera triggering occurred could be controlled by varying the foil thickness.

Results and Discussion

The photographic sequences show considerable differences in the behaviour of dead-pressed and non-dead-pressed samples. Selected frames from Imacon sequences are shown in Figures 3,4 and 5. It can be seen that in the "normal" samples (Figures 3 and 4) the reaction front starts from a single site and advances uniformly and rapidly ( °° 400 m s"" 1 ) throughout the whole sample. In the dead-pressed sample (Figure 5) however, although the reaction

484 / SPIE Vol. 348 High Speed Photography (San Diego 1982)

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3.8

4.0

4.124.17

4.23

light

Figure 1. Relative pressure outputs from Figure 2. Experimental arrangement at theHg Ful pellets of the indicated instant of impact: W, drop -weight;densities (gm /cc). G, glass blocks; M, mirror; P,prism;

S, sample. The upper glass block isattached to the weight.

velocity may be locally high (' 250 m s -1) it proceeds in a more directional manner (ratherthan uniformly from the initiation point) and usually from more than one site. Carefulexamination of say, Figure 5, frame 5, shows unreacted "islands" of sample. These burnrelatively slowly. In summary the "normal" samples react quickly and uniformly withinti 5 ps while the dead -pressed samples react unevenly and take tens of microseconds for com-plete consumption of material. The rate of energy release in dead -pressed material is there-fore much lower and as shown in Figure 1 the output pulse is considerably reduced. A seriesof photographs taken with samples in the density range 3.24 - 4.25 gm /cc has shown asteady decrease in the local reaction velocity from 400 - 200 m s-1, and this velocity de-crease together with the discontinuous type of burning is concluded to be the cause of thelarge drop in output pressure.

Although the high speed camera work suggests why the output pressure is so much lower ina dead -pressed sample it raises the question of why the burning mode should be so differentin the different density ranges. To investigate this problem samples of dead -pressed(p = 4.21 gm /cc) and non -dead -pressed (p = 3.74 gm /cc) Hg Ful were sectioned and examinedin an electron microscope. This examination (Figure 6 and 7) showed that both types ofsample had voids in the microstructure which would presumably give pathways for reactionproducts. However, there were distinct differences. As Figure 6 shows, the dead -pressedmaterial has essentially uni- direction voiding,whereas the lightly- pressed sample, Figure 7,has a network of connected voids. Clearly for a reaction front propagating downwards themicrostructure would hinder in the dead -pressed case and assist in the other.

The above observations probably also explain the burning patterns observed in the high-speed photographic work. The void structure in the non -dead -pressed sample would allowhot gases from any ignition point to flow quickly away from the reaction zone and assistreaction in all directions. In the dead -pressed sample, however, the narrower and lessbranched crack pattern would restrict the movement of gases to more specific directions andlower velocities.

Conclusions

High speed photography and electron microscopy have provided valuable insights into theprocesses involved in the dead -pressing of explosives. The results indicate that, at leastfor Hg Ful, the internal microstructure of the compact is of primary importance in deter-mining its dead -pressing properties. These results provide firm evidence for the specula-tion of Muraour2 that the lack of gas penetration into the bulk is a major factor in ex-plaining the dread -pressing of Mercury Fulminate.

SPIE Vol. 348 High Speed Photography (San Diego 1982) / 485

-3.8

-4.0

-4.12

-4.17

-4.23

light

NJ I ....---to camera

Figure 1. Relative pressure outputs from Hg Ful pellets of the indicated densities (gm/cc).

Figure 2. Experimental arrangement at theinstant of impact: W, drop-weight; G, glass blocks; M, mirror; P,prism; S, sample. The upper glass block is attached to the weight.

velocity may be locally high (^ 250 m s" 1 ) it proceeds in a more directional manner (rather than uniformly from the initiation point) and usually from more than one site. Careful examination of say, Figure 5, frame 5, shows unreacted "islands" of sample. These burn relatively slowly. In summary the "normal" samples react quickly and uniformly within ^ 5 ys while the dead-pressed samples react unevenly and take tens of microseconds for com­ plete consumption of material. The rate of energy release in dead-pressed material is there­ fore much lower and as shown in Figure 1 the output pulse is considerably reduced. A series of photographs taken with samples in the density range 3.24 - 4.25 gm/cc has shown a steady decrease in the local reaction velocity from 400 - 200 m s~l, and this velocity de­ crease together with the discontinuous type of burning is concluded to be the cause of the large drop in output pressure.

Although the high speed camera work suggests why the output pressure is so much lower in a dead-pressed sample it raises the question of why the burning mode should be so different in the different density ranges. To investigate this problem samples of dead-pressed (p = 4.21 gm/cc) and non-dead-pressed (p = 3.74 gm/cc) Hg Ful were sectioned and examined in an electron microscope. This examination (Figure 6 and 7) showed that both types of sample had voids in the microstructure which would presumably give pathways for reaction products. However, there were distinct differences. As Figure 6 shows, the dead-pressed material has essentially uni-direction voiding,whereas the lightly-pressed sample, Figure 7, has a network of connected voids. Clearly for a reaction front propagating downwards the microstructure would hinder in the dead-pressed case and assist in the other.

The above observations probably also explain the burning patterns observed in the high­ speed photographic work. The void structure in the non-dead-presse'd sample would allow hot gases from any ignition point to flow quickly away from the reaction zone and assist reaction in all directions. In the dead-pressed sample, however, the narrower and less branched crack pattern would restrict the movement of gases to more specific directions and lower velocities.

Conclusions

High speed photography and electron microscopy have provided valuable insights into the processes involved in the dead-pressing of explosives. The results indicate that, at least for Hg Ful, the internal microstructure of the compact is of primary importance in deter­ mining its dead-pressing properties. These results provide firm evidence for the specula­ tion of Muraour2 that the lack of gas penetration into the bulk is a major factor in ex­ plaining the dread-pressing of Mercury Fulminate.

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Figure 3. Frames from Imacon sequenceshowing reaction propagation innon -dead -pressed Hg Ful. Sampledensity 3.54 gm /cc, interframetime 5 us.

1

3

5

5

Figure 4. Frames from Imacon sequenceshowing reaction propagation innon -dead -pressed Hg Ful. Sampledensity 3.24 gm /cc. Interframetime 5 us. Initiation site isarrowed.

Figure 5. Frames from Imacon sequenceshowing reaction propagationin dead -pressed Hg Ful. Sampledensity 4.15 gm /cc, interframetime 5 us. Unreacted islandsare arrowed in frame 5.

486 / SPIE Vol 348 High Speed Photography (San Diego 1982)

Figure 3. Frames from Imacon sequenceshowing reaction propagation in non-dead-pressed Hg Ful. Sample density 3.54 gm/cc, interframetime 5 ys.

Figure 4. Frames from Imacon sequenceshowing reaction propagation in non-dead-pressed Hg Ful. Sample density 3.24 gm/cc. Interframe time 5 ys. Initiation site is arrowed.

Figure 5. Frames from Imacon sequence showing reaction propagation in dead-pressed Hg Ful. Sample density 4.15 gm/cc, interframe time 5 ps. Unreacted islands are arrowed in frame 5 .

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Figure 6. Electron micrograph of dead -pressed Hg Ful showing thenarrowness and directionalcharacter of the voids.

40p

Figure 7. Electron micrograph of non -dead-pressed Hg Ful showing voidbranching.

Acknowledgements

This work was supported by the Procurement Executive, Ministry of Defence. One of us,G.M.S., also wishes to acknowledge St. Edmund's House, Cambridge, for the provision of aResearch Fellowship.

References

1. Heavens, S.N. and Field, J.E., Proc. Roy. Soc. Lond. A338, 77 (1974).2. Muraour, H. Mem. Artill. France, 18, 895 (1939).

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PHiLi;.^:,:

%

40 H

Figure 6. Electron micrograph of dead- pressed Hg Ful showing the narrowness and directional character of the voids.

Figure 7. Electron micrograph of non-dead- pressed Hg Ful showing void branching.

Acknowledgements

This work was supported by the Procurement Executive, Ministry of Defence. One of us, G.M.S., also wishes to acknowledge St. Edmund's House, Cambridge, for the provision of aResearch Fe11ows hip.

References

1. Heavens, S.N. and Field, J.E., Proc. Roy. Soc. Lond. A338, 77 (1974).2. Muraour, H. Mem. Artill. France, 1.8, 895 (1939).

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