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United KingdomLAtomic Energy Authority,()%hL

AWRE, Aldermaston

L'MRE REPORT NO. E2/!5

*~ 55 FZSIEWP -rrtn 54The Effect Of Light Casings on the Blast Parameters from a

Spherical Charge Of RDX/TNT 60/40

£ya.w J. Bishop 09JD**James

(Work carried out for RARDE under Contract No. MINTECH KV/B/431.)

Recomended for issue by

D.E.J. Samuels, Superintendent

Approved by

N.S. Thumpaton, Senior Superintendent

623.565. 23:662.2-3(RDX/TNT);1662.2-3(P.DxImT) :623.565.23

DISTRIBUTION STATENIE1T AApproved for Public9 Re!ease

* Distribution Urlirnited

UNCLASSIFIED

TABLE OF CONTENTS

PAGE

SUMMARY 3

1. INTRODUCTION 3

2. OBJECT OF THE EXPERIMENTS 4

3. EXPERIMENTAL DETAILS 4

3.1 Charges 43.2 Layout 43.3 Recording Equipment 4

3.4 Firing Schedule 5

4. PIEZO-ELECTRIc GAUGE RESULTS 5

5. HIGH SPEED CINE CAMERA RESULTS 5

5.1 Aluminium Cased Charges 65.2 Le&d Cased Charges 6

5.3 Magnesium Cased Charges 6

5.4 Steel Cased Charge 7

6. DISCUSSION 7

6.1 Aluminium Cased Charges 762 Lead Cased Charges 7

6.3 Magnesium Cased Charges 86.4 Steel Cased Charge 9

7. COMPARISON WITH OTHER WORK 9

7.1 Blast Parameters 97.2 Fragment Velocities 9

8. CONCLUSIONS 10

9. RECOMMENDATIONS 10

10. ACKNOWLEDGEMENT 11

REFERENCES 12

TABLES 1 - 3 13

FIGURES 1- 14 16

2

UNCLASSIFIED

SUMMARY

The report examines the blast wave parameters obtained from an8 lb spherical charge of RDX/TNT cased in aluminium, lead, magnesium orsteel. Three different case thicknesses of aluminium were examined, twoof lead, two of magnesium and one of steel. The shock wave profile wasrecorded with side-on piezo-electric pressure gauges placed between 4and 20 ft away from the charge. High speed cine films were taken of thefireball growth.

Peak overpressure and impulse enhancement over the values from anuncased RDX/TNT charge were obtained only from the thickest lead casing.All the aluminium, magnesium and steel casings gave a reduction in pressureexcept at the furthest distance from the charge. The thinnest casings ofaluminium, lead and magnesium all produced impulses of the same valueas those from an uncased charge.

1. INTRODUCTION

The present work, carried out under an RARDE contract, was designedto see if there was an enhancement of blast parameters from lightly casedcharges compared with the parameters from the same mass of explosiveuncased.

Very little work has been done in the past on the effect of lightcasing on the blast parameters from spherical charges. Dewey, Johnson andPatterson [1] have investigated the effect of light powder surrounds andcasings on the blast parameters from Pentolite and HBX - 6 explosives.Among the surrounds and casings employed were aluminium powder, castaluminium and magnesium and stainless steel. The charges, which werespherical, ranged between 2 oz and 1 lb in weight and they were firedboth at atmospheric and reduced pressures. Normally reflected impulseswere measured with piezo-electric gauges. The results of these experimentsshowed that surrounds of aluminium powder gave the largest increase inreflected impulse close to the charge and increases in peak pressure andreflected impulse at all distances. Spun steel casings reduced the reflectedimpulses at atmospheric pressure. At reduced ambient pressures all casingsgave increased reflected impulses. Cast aluminium and magnesium casingsincreased the reflected impulse at reduced pressure more than an equalmass of explosive and did not reduce the reflected impulse at atmosphericpressure.

RARDE requested the measurement of blast parameters over the rangefrom 100 - 10 psi from RDX/TNT charges cased in shells of aluminium, lead,magnesium and steel of various thicknesses. Measurements of the casingfragment velocity were also made.

3t'1("1 1 IIIIIIII I*

CONFIDENTIAL DISCREET

2. OBJECT OF THE EXPERIMENTS

(a) To determine whether there is an enhancement of air blast when ahigh explosive charge is clad with a thin metallic shell.

(b) To measure the fragment velocities of the various metallic shells.

3. EXPERIMENTAL DETAILS

3.1 Charies

The charges, which were supplied by RARDE, each consisted of amachined sphere of RDX/TNT 60/40 nominally 8 lb in weight, drilled toits centre to take a No. 8 electric detonator. The charge was surroundedby a metal case which was made in two hemispheres with a half inch radiussemicircle cut out of each hemisphere so as to leave the detonator holeclear (see Figure 1).

3.2 Layout

It was necessary to fire the charges at a minimum height of 17 ftfrom the ground in order to ensure that reflections from the groundwould not be superimposed on the positive phase of the blast wave profiles.The charge was suspended in a string net between two masts at a height of25 ft above the ground. This height was chosen as 20 ft masts were readilyavailable on which to mount the blast wave recording gauges. Guys wereattached to a wooden cross from which the net was suspended (see Figure 2)so that the charge could be arranged to have the seam of the metal casingat right angles to the blast gauges. This is to ensure that any enhancementof parameters measured could not be due to possible jetting at the seam.

3.3 Recording Equipment

The hydrostatic overpressure in the shock wave was recorded byquartz piezo-electric transducers with B12 type omnidirectional baffles.These gauges were mounted along a horizontal line, with their sensitivesurface at the same height as the charge centre. The distances variedfrom approximately 4 to 20 ft. A dummy gauge was erected 2 ft in front ofthe first gauge in order to deflect the fragments, keeping the damagesustained by the first gauges to a minimum. Measurements were obtainedfrom eight of these B12 type gauges for each charge fired.

The gauges were connected to Southern Instruments Mini-rackrecording equipment and the signals obtained were photographed by rotatingdrum cameras. The gauge cables had to be well protected from the effectsof blast and fragments on firing. Since the cable protection was withinseveral feet of the gauges it was streamlined, in order not to distortthe records obtained. Figure 3 shows the experimental gauge positions.The distance from the charge centre to the centre of each gauge wasdetermined from a photograph taken immediately before firing. Themeasurements were made from a 12 in. x 10 in. enlargement printed on KodakBromide Foil Card. The gauge baffle diameters were used to calibratedistance.

4CONFIDENTIAL DISCREET


A 16 mm Fastex high speed cine camera was used to film the fireballand its growth, in colour. The camera was run at approximately 12000 halfframes per second.

3.4 Firing Schedule

Table 1 lists the charges fired during these experiments. Thefirst firing was an uncased charge of RDX/TNT. The shock wave parametersthat were obtained from this firing were compared with those from eachof the cased rounds. These consisted of aluminium casings of 0.250,0.548 and 0.786 in. thickness, two lead casings of 0.078 and 0.218 in.thickness (two rounds were fired of the latter thickness due to recordingdifficulties on the first shot), two thicknesses of magnesium, 0.437 and0.788 in. and one steel casing of 0.105 in.

4. PIEZO-ELECTRIC GAUGE RESULTS

Since the object of the experiments was to determine if there wasany enhancement of blast parameters due to casing a spherical bare chargewith a metallic shell, the blast parameter-distance graphs are thereforepresented in which comparison is made between:-

(a) Peak overpressure for a cased charge and for the same chargemass uncased (see Figures 4 - 7).

(b) Positive impulse intensity for a cased charge and for thesame charge mass uncased (see Figure 8).

The above two parameters are those primarily concerned in considera-tions of target damage. Similar comparisons are also made for the positiveduration (Figure 9).

The results are sunnarised in Table 2.

5. HIGH SPEED CINE CAMERA RESULTS

A high speed cine colour film was obtained of the fireball growthfor each round fired. An example is shown in Figure 10 which is a blackand white reproduction of frames from the magnesium cased charge (0.788 in.thick) film. From Figure 10 it can be seen that in the initial stagesthe fireball is spherical in shape but after about 0.5 ms it begins totake on a "spikey" appearance. The spikes are the luminous fragments ofthe casing which surrounded the charge. The luminous aluminium andmagnesium fragments showed up clearly in the cine films, they were a brightyellow-white colour. The smaller fragments from the steel cased chargewere not so distinct and were a dark yellow-orange shade. For all the leadcased charges the fireball after its initial white appearance during thefirst 0.5 ms after detonation, soon changed to a dark greyish orangecolour. There were no luminous fragments such as were obtained from theother three casing materials but the fireball did have a slightly spikeyshape. In assessing the fireball growth from the colour films the leading



edge of the fireball was measured on each frame. Since in the later stagesof the fireball growth the leading edge was composed of the spikes of theluminous casing fragments the plot of fireball growth with time can beused to determine the fragment velocity at any point. The fireball growthcurves for the four types of metal casing are given in Figures 11 - 14.The fragment velocities calculated from these curves are shown in Table 3.

5.1 Aluminium Cased Charies

The fireball growth curves for the three thicknesses of aluminiumare shown in Figure 11. From this graph can be seen that the thinner thecasing thickness the faster the rate of fireball growth and consequentlythe higher the initial fragment velocity. Also plotted in Figure 11 forcomparison is the fireball growth curve obtained from the uncased RDX/TNTcharge. Initially, because it is unrestrained by a casing, the uncasedRDX/TNT charge fireball expands more rapidly but after 0.4 ms its velocityis decreasing rapidly compared with that of the cased charges.

The first 400 us of the cased charge curves are very nearlystraight lines consequently it is possible to determine the fragmentvelocity for each of the case thicknesses over this portion of the growthfrom the slope of the line. The velocity of the 0.250 in. aluminium casefragments is 9400. ft/s and for the 0.548 and 0.786 in thick cases thevelocities are 7700 and 7200 ft/s respectively. At 6 ft from the chargecentre the velocity of the heaviest casing fragments had dropped to5600 ft/s while the velocities of the 0.250 and 0.548 in. case fragmentshad become 7400 and 7000 ft/s respectively.

5.2 Lead Cased Charges

Figure 12 shows the plot of fireball growth for the lead casedcharges. As for the aluminium cased charges the thinner lead casingproduced the faster fireball growth. The shape of the lead cased chargecurves however are more like that of the uncased RDX/TNT charge which isshown as a dotted line in Figure 12. There is a much more rapid drop invelocity of the fireball growth for the lead casings compared with thosefrom the aluminium. The colour films of the lead cased firing showed thatthe fireball remained spherical during the whole of its measured growth.There were no luminous fragments visible, as spikes, as there were withthe aluminium cased charges. This coupled with the fact that no leadfragments were found in the vicinity of the layout after the firingleads one to conclude that both lead case thicknesses were completelyvapourised by the explosion. The lead fireball growth curves arepractically straight over the first 250 us and a calculation of velocityover this period gives values of 9100 and 7400 ft/s for the 0.078 and0.218 in. thick cases respectively. At 6 ft from the charge centre thevelocities had dropped to 2800 and 2500 ft/s.

5.3 Magnesium Cased Charges

The plot of fireball growth with time for the 0.437 and 0.788 in.thick magnesium case charges are given in Figure 13. Here again the



lighter cased charge produced the largest fireball growth. The averagevelocity of the thinner casing over the first 400 us was 8600 ft/s whilethat of the thicker case was 7700 ft/s. At 6 ft from the charge centrethe velocity of the thinner case has decreased very little being 8000 ft/swhile that of the thicker case has become 5800 ft/s.

5.4 Steel Cased Charge

The fireball growth obtained from the single firing of a steelcased charge is shown in Figure 14. The fireball behaviour is similar tothat from the thinnest aluminium and magnesium cases. The velocity ofthe fragments over the first 400 ps is 8600 ft/s, at 6 ft this decreasesonly slightly being 8100 ft/s.

6. DISCUSSION

6.1 Aluminium Cased Charies

Examination of Figures 4, 8(a) and 9(a) shows that although the0.548 in. thick aluminium casing gave the smallest reduction in peakoverpressure nearest the charge compared with the uncased RDX/TNT chargesit produced the largest reduction in positive duration. The 0.786 in. casegave the lowest peak overpressures but the durations from this chargewere similar to those from the thinnest casing.

Figure 8(a) showed that the positive impulses decreased withincreasing case thickness. From the impulse results there is an indicationthat a casing thinner than 0.250 in. might produce an impulse enhancementover the uncased charge results. However the effect of a case < 0.250 in.thick on the peak overpressures and durations cannot be deduced from theresults obtained so far.

The fireball growth curves in Figure 11 suggest that a casing lessthan 0.250 in. thick would give a faster fireball growth than any so farobtained, this in turn would mean higher fragment velocities.

6.2 Lead Cased Charges

From Figures 5, 8(b) and 9(b) it can be seen that the 0.078 in.thick lead shell had very little effect on the blast parameters. The peakoverpressures and positive impulses are almost identical to those fromthe uncased RDX/TNT charge over the range recorded. The positive durationswere reduced by 1OZ at the closer in distances to the charge but thisreduction is barely significant as the standard deviation of the curvesis * 52.

For the 0.218 in. thick case however there was an average enhancementof 302 on peak pressure and 15% on positive impulse. The durations at thenearest distances to the charge were reduced by 25% but further out theywere the same as the uncased charge. The enhancement in peak pressuresand positive impulses is probably due to a combination of the followingfactors:-



(a) Lead with its ductile property and low melting point hasbeen seen to become molten when in contact with a detonatingexplosive [2]. Detonation caused the lead shell surface tobecome covered with closely packed, fast growing "spikes".These "spikes" which are molten spray will no doubt quicklybecome vapourised by the explosive gases. The additionof lead vapour in the RDX/TNT explosion products will producea change in density and in y the ratio of the specific heatsfor the gases and products.

(b)' The shock strength Y and the shock velocity U arerelated by the Rankine-Hugoniot formula

U2 _Y -1)+Y(y+l)a2 2y '

where a is the velocity of sound in the undisturbed airin front of the shock. An increase in shock strength at a givendistance from the charge could be produced if there was a loweringin the value of y for the air in front of the gaseous productsof the explosion or an increase in shock velocity.

(c) The increased density of the explosive gas products dueto the presence of lead vapour will bring about an increasein the hydrostatic overpressure in the portion of the shockwave behind the contact surface. An increase in overpressurein the latter portion of the shock wave will increase thepositive impulse in the wave as a whole.

From the two case thickness of lead so far fired, it seem possiblethat a case thicker than 0.218 in. would produce an even greater enhancementthan that already obtained. However the weight of the 0.218 in. thick leadcase was 11.25 lb which means that any increase in shell thickness willproduce an overall charge that is extremely heavy and cumbersome.

6.3 Mannesium Cased Charses

The peak overpressure, positive impulse and positive durationgraphs for the magnesium cased charges shown in Figures 6, 8(c) and 9(c)show that the behaviour of the pressures and durations from these chargesare different to that of the aluminium cased (section 6.1). The thickestcasing gave the greatest reduction in peak pressure and duration close into the charge. The positive impulses however follow a similar pattern tothe aluminium cases, the thinnest casing having the same impulse valuesas the uncased charge and the thicker casing - 0.788 in. having impulsescomparable with the 0.548 in. thick aluminium case. Examination of the casemasses in Table 2 shows that the 0.548 in. thick aluminium and the 0.788 in.magnesium cases are almost the same weight being 7.97 and 8.01 lbrespectively. Similarly the 0.250 in. aluminium and the 0.437 in. magnesiumcases have the same order of weight, they are 3.22 and 4.00 lb respectively.Whether from the aluminium and magnesium cased charges any of the impulsein the shock wave behind the contact surface is due to some vapourisationof the metal cases, as discussed in section 6.2, is open to question.The colour films of the fireball certainly indicate that there is burning



of the aluminium and magnesium. Since at least 50% of the aluminium andmagnesium casings in a given zone were found after the explosion theburning can only be partial. Consequently the amount of aluminium andmagnesium vapour present in the explosive gases will be small comparedwith the lead where no fragments were recovered.

The fireball growth graph in Figure 13 indicates that a magnesiumcase thinner than 0.437 in. would possibly produce fragments of highervelocity than those from the two thicknesses already examined.

6.4 Steel Cased Charge

Figures 7, 8(d) and 9(d) show that there was a reduction in allthree blast parameters for the one steel cased charge that was fired.Close in to the charge the reduction in pressure, impulse and durationcompared with an uncased charge is greatest, this difference decreasesuntil at 20 ft the charge the pressures and durations from the casedand uncased charges are the same and the impulses reduced by only 5%.The weight of the steel 0.105 in. casing was 3.88 lb (Table 2), this iscomparable to the 0.250 in. aluminium, 0.078 in lead and the 0.437 in.magnesium casings. An examination of Figure 8(a) - (d) shows that theimpulse curves for these four casings are, with the exception of thesteel, almost idefitical. The energy required to expand the metal casesof these charges is negligible when compared with the total energy ofthe explosive. This suggests that there is burning of the aluminium,lead and magnesium cases which contributes to the impulse in the mannerdiscussed in section 6.2.

7. COMPARISON WITH OTHER WORK

7.1 Blast Parameters

Comparison with the work of Dewey, Johnson and Patterson (1] isdifficult because their measurements of the effects of light surroundsand casings was carried out almost entirely using reflected impulses,not with side-on impulses as in the present experiments. Their conclusionswere that the effect of a casing lighter than the explosive was not todecrease the reflected impulse but to increase it. Whether the casingwas chemically reactive, acoustically "matched" to the explosive, simplya plastic loaded with inert material or a powdered aluminium surround,its effect was usually about the same as that of adding an equal mass ofexplosive to the charge. However, compared with an uncased charge of thesame weight as the explosive filling as much as a doubling of reflectedimpulses was found with aluminium and magnesium casings and in the side-onimpulse from a powdered aluminium surround.

In the present work comparing the side-on impulses for thealuminium and magnesium cased charges with those from an uncased chargethere is no increase for any charge to case ratio so far examined.

7.2 Fragment Velocities

Gurney [3] worked out a formula for predicting the velocity ofthe fragments produced by detonating a spherical charge of high explosive



surrounded by a spherical shell of metal. His formula was

V a /E +3 c/5 m' 0.*.....(2)

where E is the energy per unit mass of explosive, c is the mass of theexplosive, m the mass of the case and v the initial velocity of thefragments.

Using equation (2) the theoretical fragment velocity for the casedcharges fired have been calculated and are listed in Table 3, column (4).E was taken as 1200 cal/gm. Comparing the theoretical values in column(4) with those measured from the fireball graph in column (5) it can beseen that at the lower initial velocities the agreement is excellent.With the higher velocities the theoretical values are some 10 - 14% abovethe experimental values.

8. CONCLUSIONS

(a) For the peak overpressures an enhancement over the valuesfrom an uncased RDX/TNT charge was obtained from a 0.218 in.thick lead case and similar values to the uncased chargefor the'0.078 in. thick lead casing. All the aluminium, magnesiumand steel casings gave a reduction in pressure at all distancesfrom the charge except the furthest distance recorded.

(b) For positive impulses an enhancement was obtained overthe uncased charge from the 0.218 in. thick lead casing. The0.250 in. aluminium, the 0.078 in. lead and the 0.437 in.thick magnesium cases all produced impulses of the samevalue as the uncased charge. The remaining casings gave areduction in impulse.

(c) For positive durations all the casings gave a reductionin values compared with the uncased charge. This reductionwas greatest nearest the charge and became negligible atthe furthest distance recorded.

(d) The initial fragmentation velocities agreed well withtheory for the lower velocities and were 10 - 14% in errorat higher velocities.

9. RECOMMENDATIONS

The work carried out to date indicates the possibility of obtainingan enhancement in positive impulse with an aluminium casing less than0.250 in. thick or a magnesium casing less than 0.437 in. thick. In orderto ascertain this fact it would be useful to fire at least two moremagnesium cased charges of different thicknesses and one round ofaluminium.

American work [1] has shown that the greatest impulse enhancementfrom lightly cased charges was obtained at reduced ambient pressures.Consequently it would probably be informative to repeat the present workreported here at low atmospheric pressure.



10. ACKNOWLEDGMENT

The authors would like to thank those members of the Applied BlastSection who assisted with this work and also the members of the PhotographicSection who took the high speed cine film.



REFERENCES

1. J. M. Dewey, 0. T. Johnson and J. D. Patterson, II: "Some Effectsof Light Surromds and Casings on the Blast from Explosives".BRL Report 1218

2. P. M. B. Slate, M. J. W. Billings and P. J. A. Fuller: "TheRupture Behaviour of Metals at High Strain Rates". J. Inst. Metals,95 (1967)

3. R. W. Gurney: "The Initial Velocities of Fragments from Bombs, Shelland Grenades". BRL Report 405



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FIGURE 6. PEAK OVERPRESSURE vs DISTANCE FOR MAGNESIUM

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