AU3244097A - Improvements in or relating to shaped charges - Google Patents

Description

1031AO

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION

ORIGINAL

Name of Applicant: Actual Inventor: Address for Service: DYNO NOBEL ASIA PACIFIC LIMITED Anthony Leonard Ey H.R. HODGKINSON CO.

Patent Trade Mark Attorneys Level 3, 20 Alfred Street MILSONS POINT NSW 2061 IMPROVEMENTS IN OR RELATING TO SHAPED

CHARGES

Invention Title: The following statement is a full description of this invention, including the best method of performing it known to us: r r r c I 4 1 5 2 THIS INVENTION relates to an improved shaped charge.

The term shaped charge is generally applied to explosive charges which conventionally incorporate a container which is normally cylindrical and an inner liner located within the container. The explosive is placed within the container in a space not occupied by the liner which is usually of conical shape. This results in shaping the explosive to form a cavity at one end and the resultant shaped charge (also known as a lined cavity charge) will also usually include a detonator located in a detonator well in a top wall of the container axially opposite to an apex of the conical liner.

The shaping of the explosive in the manner as described above means that when the explosive is detonated the optimum efficiency of the charge is realised by utilising and enhancing the directional effect of the detonation. When a detonation occurs a detonating wave front moves through the explosive at velocities between 1500 metres per second and 9000 metres per second. The wave applies a greater force in the direction in which it is travelling. This force is increased by shaping or hollowing the end or head of the charge as described above. This has the effect of increasing the surface area of the exposed explosive, and allows the detonation wave to arrive at this surface progressively. If the cavity or hollow created in the charge comprises two or more surfaces at an acute angle to each other the forces released at these surfaces will converge and will be reinforced at their point of intersection. The result is the 1~ :::i5

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U 3 formation of a jet stream which has a greater velocity than that of the detonation of the explosive. As the jet stream moves down the cavity ahead of the detonation of the explosive. As the jet stream moves down the cavity ahead of the detonating wave it leaves behind it a low pressure area into which the rest of the charge which is moving down the periphery of the cavity tends to expend itself providing a penetrating effect. Providing the cavity is perfectly symmetrical, the container structurally homogeneous, and the explosive uniformly distributed in density, the forces will be equal and the results optimum.

The introduction of the abovementioned liner greatly increases the penetrating effect. The liner is usually formed from metal and facilitates efficient energy transfer to the target.

Under the influence of explosive forces the liner S collapses and flows towards the charge axis where its walls collide and divide into two parts; the smaller part moving forward into the jet front; the larger part to the rear to form a slug. In forming up with the jet, the metal, which may be either particulated or continuous, adds to the momentum and kinetic energy condition of the detonating head. But though the slug is of greater mass, the maximum kinetic energy per mass is at the extreme front of the jet. The slug is thus not a feature in the penetrating process.

In order for the jet to achieve maximum penetration an optimum standoff or gap between the base of the charge and the target is usually required. The velocity of the jet may cr~ -n I -I 4 be increased or decreased by altering the apex angle of the cavity. If the angle is reduced the speed of the jet is greater but the mass travelling to the target is less. A jet may not form if the cone angle is too narrow or too wide, or if the explosive is unsuitable, eg., it does not produce a high enough detonating pressure.

In "The Science of High Explosive" (Melvin A Cook, Robert E Krieger Publishing Co Inc New York (1971) it is noted that cast TNT with a steel cone will normally not produce a jet over 120 degrees; whereas other explosives, eg., Composition B, will produce jets between 20 degrees and 150 degrees. There appears to be a direct relation between the mass of the detonation head and the mass of the liner, with a maximum energy being transferred to the liner when the mass of !.13 the liner equates with that of the detonating head at their rj|; point of collision. If the mass of the head is much higher than that of the liner then only a fraction of the explosive energy may be imparted to the liner resulting in a marked decrease in penetration (Cook p. 242). Nevertheless the metal type is important, and as a general rule the less dense metal liners such as copper, brass and aluminium are more effective than steel and cast iron in promoting penetration. The liner thickness is often calculated from the charge diameter (normally 2k% of the charge diameter). The charge standoff is also calculated from charge diameter (normally from one to three charge diameters). For maximum end effect in a conical lined cavity the charge height should be equal to three charge diameters. However Cook notes that charges of intermediate I length are more economical and their loss in effect is minimal. Clearly there are a considerable number of variables to be taken into account when designing a shaped charge. The guidelines are dependant upon such factors as the target material; its thickness; density; tensile strength; backing; and the effect desired.

As mentioned, a perfect symmetry and structural homogeneity of the charge container as well as a uniform distribution of explosive density is required for achieving the charges optimum efficiency. This requirement, together with that of an explosive possessing a high detonating velocity and corresponding high detonating pressure and density make the shaped charge an expensive proposition and it has been a-long felt desire in the industry to decrease the 15 cost of conventional shaped charges.

Both conical and wedge (ie. a triangular prism liner having opposed open ends located in a shallow cuboidal container with each open end of the liner abutting an adjacent end face of the container) lined cavities have been extensively tested. But in the application of lined cavity charges to fragmenting rock such cavity configurations present problems. In view of the need to fragment as well as Spenetrate the rock target over the widest possible area, the ass o the detonation head needs to be less localised (ie. it needs to be part of a thicker, and a shorter, jet carrying more mass to the target). If it is too localised it merely bores a hole. As noted, the wider cone anxgles transfer a less lo:a I ied mass to the target and aI-e rtlhs more effective in producing fragmentation- lhowever also as note above, jet formation in the wider angles is not obtainable with steel liners except when used with explosives with very high i 5 detonating pressure- At angles of less than 120 degrees ~the

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~CI~ ll~esC~AeO~YIILI~Y IY r effectiveness of the charge in fragmenting rock diminishes.

Reference may also be made to representative prior art such as described for example in French Patent 1 273 936, European Patent 0 157 902, PCT specification PCT/EP.84/00313 (WIPO 85/01572), European specification 0 252 385, United States specification 3 478 685 and United States specification 3 759 182.

French patent 1 273 936 refers to a conventional shaped charge having a conical liner in a cylindrical container. European patent 0 157 902 is not relevant to the present invention and specification PCT/EP/00313 discloses a hybrid explosive unit not relevant to the present invention.

European specification 0 252 385 refers to the provision of specially designed insert for hollow charges having a lower convex shape and a higher conical shape separated by a neck or transition part so as to provide an initial detonation wave followed by a later or main detonation wave which may discharge into a crater in the target formed by the initial detonation wave.

20 United States specification 3 176 613 refers to a shaped explosive charge incorporating a hemispherical liner and high explosive material located about a convex surface of the liner and adjacent thereto. A tamping case is disposed about an external surface of the high explosive and includes a detonator in contact with the high explosive. The thickness of the high explosive adjacent the detonator is smaller than the thickness of the high explosive at a base surface of the tamping case. This latter thickness of high explosive Y~n- -loccupies an annular gap surrounding the hemispherical liner.

United States specification 3 478 685 refers 'to a projectile with high initial velocity which includes a conical cavity and a conical shaped liner and thus is exemplary of the prior art previously described.

United States specification 3 759 182 discloses a hollow shaped charge including an outer container of frusto conical shape and an inner shell or liner of conical shape. A detonation cylinder is located in a top part of the outer container and thus coincides with a longitudinal axis of the liner and container. This specification is thus also exemplary of the prior art previously described.

Reference also may be made to other prior art illustrative of background technology relevant to this invention which includes German specifications 2 840 362, 2 559 179, 1 901 472, 3 019 948 and 2 724 036, French specifications 1 327 804, 1 922 472, 1 022 350, 2 512 539, 2 332 514 and E-95214 by Alsetex. Reference also may be made to United Kingdom specifications 1 142 915, 1 195 641, 2 081, 851 and 2 039 008, Swiss specification 475 543 and United States specifications 232680, 2 494 256, 2 616 370, 2 629 325, 2 667 836, 2 782 715, 2 833 215, 2 856 850, 2 932 251, 2 974 595, 2 797 892, 3 183 836 and 3 224 368. None of these S specifications are relevant to the present invention.

German specification 1 571 283 which is also referred -to in EP specification 0 252 385 is of interest in that it discloses a hollow charge having a lower tulip shaped insert with a conical apex located in a container having a 8 base wall, a cylindrical lower side wall, a frusto conical upper side wall and a top wall including a detonator. There is also included an upper insert or conical body for deflecting the detonation wave and an annular gap surrounding the tulip shaped lower insert and located inwardly of the lower side wall of the container adjacent the base wall which is filled with explosive. However it is stated in EPO 252 385 that the hollow charge of German Specification 1 571 283 is deficient in that it produces a detonation wave having a stretched tip and thus is useful only in destroying a ji protective layer of a target but not the main armour plate of !i the target.

From the abovementioned prior art summary it will be appreciated that the use of a hemispherical liner and an annular gap surrounding the base of the liner is suggested in United States patent 3 176 613 to Godfrey and thus this reference is considered the most pertinent to the present invention. However it was essential in Godfrey that the thickness of the high explosive was greatest at the base edge 20 of the container and smallest adjacent the detonator. This was necessary to ensure that after detonation the liner converged on the focal point of the shaped charge simultaneously such that almost all of the liner material is S" projected along the jet path as a slug having a length only several times its diameter.

However to achieve a differential thickness of the liner in Godfrey it was necessary to utilise a container of similar dimensions to the bowl shaped or hemispherical shaped ~ig~Ll~i~i~~ ~C~L~CPQ~RI~RL~ ~BPI~Oh~BB~ 9 liner and thus there was no requirement for the detonator to be located at a certain minimum axial height above the liner.

It would also seem apparent in Godfrey that for simultaneous convergence of the liner at the focal point that the charge would require a multiple of detonation points rather than a single detonation point as described.

For best effect as explained in detail hereinafter the upper part of the hemispherical liner should converge at the focal point slightly ahead of the lower part so that the forces from the lower part are residual rather than primary.

Also it would seem that the theoretical requirements of a detonation wave require not only a minimum effective explosive height above the cavity but also a greater potential energy above the cavity rather than laterally and for these reasons :w 5 it would seem that the Godfrey proposal is deficient.

It has now been discovered in regard to the present invention that lined hemispherical or bowl shaped cavities which heretofore have not been adopted commercially can now be utilised efficiently and effectively ir relation to shaped charges having regard to certain criteria referred to hereinafter. This means that to a large degree undue experimentation can be substantially avoided and shaped charges using such criteria can be manufactured on an economic basis.

The shaped charge of the invention therefore includes a structurally homogeneous container, a detonator and a substantially hemispherical liner defining a bowl shaped cavity in the container with uniformly distributed explosive

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being located in the container in a space not occupied by the liner and surrounding the liner with the liner defining a concave inner surface and a convex outer surface and having a peripheral edge characterised in that said peripheral edge is spaced from an internal surface of the container to define an annular gap which has a width of at least five percent of the radius of the hemispherical liner and said detonator is located on a longitudinal axis of the container at a height of at least 1.9 times the height of the bowl shaped cavity with said container and explosive being symmetrical about said longitudinal axis whereby in use said shaped charge is mounted on a target with no stand off present.

The present invention has been devised on the basis of experimental tests which have now ascertained that if a stand off is present in regard to use of hemispherical liners that the effect of the explosion or penetration of the .i explosion is substantially reduced. Also, the effective surface area of the cavity is substantially reduced. If the annular gap is less than five per cent of the radius of the hemispherical liner then this creates a situation similar to one in which a stand off is present. In any event, suitably the minimum width of the annular gap is in the order of 12mm.

The above experimental tests have also confirmed that the minimum distance between the detonation point and the liner measured on the longitudinal axis of the shaped charge is suitably equal to or greater than 1.9 or more suitably twice the radius because if this relationship is not followed during manufacture of the shaped charge then again the effect I1 of the detonation and penetration of the blast will be substantially reduced.

The detonator for use in the present invention may be of any suitable type. For example, the detonator may include a base charge usually consisting of PETN (pentaerythritol tetra-nitrate), a priming charge normally comprising a mixture of A.S.A. lead axide, lead styphnate and a small amount of aluminium powder, a flash chamber and a fusehead extending into the flash chamber. The fusehead may comprise of two thin brass foils, separated by an insulation board, and joined together, or bridged, by a very fine, high resistance wire which is located inside a head of ignition compound. The ignition compound may be comprised of several layers, an innermost layer comprising a normally highly sensitive compound such as mono-nitro-resorcinate and potassium chlorate with some nitro-cellulose. This layer ignites when the bridgewire becomes hot. Another surround layer of potassium chlorate and charcoal promotes the ignition into a hot flame. An outermost layer may comprises cellulose which binds and strengthens the head. All of the above components may be contained in a tubular housing or tubular part of the container having a crimped closure plug formed S. from neoprene with brass foil of the fusehead extending thereto and linking with a detonator lead wire. The fusehead wire and foil usually forms part of an electrical circuit which when the circuit is closed causes the fusehead to flash 1 which in turn ignites the priming charge which then initiates detonation of the base charge.

-7 12 The container which contain's the liner is suitably substantially cylindrical, substantially conical or frustoconical or other similar shape. Preferably the container has a tapered. top wall which contains a detonator wall for retention of the detonator. Suitably the top wall tapers upwardly from an adjacent continuous side wall toward the detonator well and toward the longitudinal axis of the container. However this is not essential and the top wall may be substantially planar or flat and extend in a horizontal plane when in use if desired.

The container may also contain an open end portion comprising a cylindrical tubular part surrounded by the Itapered top wall. The detonator well may extend into the cylindrical tubular part and one suitable way of forming this wall may comprise a closure plug or cap which may engage with the open end portion having a central recess or aperture for retention of the detonator.

The shaped charge of the invention also includes explosive which is located in the container interior surrounding the liner. The explosive may be of any suitable type and thus comprise Emulite (an emulsion slurry), TNT (tri nitro toluene) RDX (ie. cyclonite), nitromethane, Semtex, Composition B or mixtures thereof. Emulite is preferred because of its relative low cost.

If desired, in some situations where the explosive *has a highi figure of insensitivity the shaped charge nay also contain a booster which is contained in the open end portion of -the container and which also extends into the container 13 interior surrounded by the explosive. The booster may comprise PETN or PETN and TNT or other suitable explosive.

The hemispherical liner of the invention may be formed from metal but is preferred that it be formed from plastics such as PVC, polypropylene or polyethylene or ceramics or glass because these materials are cheaper than metal.

EXPERIMENTAL

A series of tests took place at a blasting range owned by Dyno Wesfarmers Ltd at Airforce Road, Helidon, Queensland, These tests were carried out by Anthony Ey of Dyno Wesfarmers Ltd and Patrick Zegenhagen and William Parker of Titan Blasting Pty Ltd.

Reference will now be made to the drawings which illustrate a preferred embodiment of the invention wherein: FIG 1 is a schematic representation of a shaped charge constructed in accordance with the invention; FIG 2 is a diagrammatic representation of FIG 1 showing the direction of detonation of the explosive and the '"02D focal point of the target; FIG 3 illustrates the direction of propagation of the detonation wave after detonation of the explosive and FIG 4 illustrates the direction of collapse of the liner after detonation of the explosive.

In FIG 1 the shaped charge 10 filled with uniformly distributed explosive 9 includes a substantially cylindrical container 11 having a continuous side wall 12 having an upper itapered or frusto conical part 13 and detonator well 14.

14 There is also included a base wall 15 sealed to flange 16 of wall 12 by bonding agent. There is also shown a hemispherical liner 17 having a peripheral edge 17A spaced from internal surface 18A of wall 12 by an annular gap 18. There is.also an open end part 19 comprising a cylindrical tubular part 19A.

Also shown is a closure plug 20. There is also provided a detonator 8, target 22 and bowl shaped cavity 24.

The initial tests were proposed by Titan Blasting Pty Ltd and based on a diagram which appears in the Commercial Diving Manual by Richard Harn and Rex Whistler which described conical liner having an apex angle of 800. Titan Blasting Pty Ltd constructed six shaped charge units at a cost of $619.00.

A model A was utilised having a cylindrical container of 86 mm radius and 158 mm height and a conical liner having an 86 mm radius and height and 860 apex angle. A model B was also utilised having substantially the same dimensions as model A but having a larger apex angle of 900 to increase the potential for breakage. Both model A and model B had 140 mm standoff and model A had 3mm thickness in regard to the conical liner and model B had 5 mm liner thickness.

The first test was conducted on the February 1 1990 test began with model B. The target was sandstone. The explosive (Emulite) was loaded to a distance of twice the height of the conical cavity. The result was a neat 50 mm hole with little radial breakage of rock. The hole appeared to be about 900 mm deep. Clearly there was too much focussing of the detonating head.

Test number 2 with model A (using a 3 mm steel cone, a charged with Emulite and boosted with a 51b HDP booster placed central and on the apex of the cone) gave an improved fragmentation with less penetration. The jet hole was not apparent and 300 m was dug out.

Test number 3 was carried out with model B using the same characteristics as Test 2 but having the standoff reduced to half (ie. 70 mm). This was the best result so far.

Although the penetration was only about 500 mm, the neat hole had opened up to 64 mm and the break extended out to about 400-500 mm dia. Nevertheless, there was still too much focussing of the jet. So far the tests were not encouraging.

Test number 4 was carried out using an inverted model B arrangement. The target was located with the inverted cone apex 10 mm away. The result was unsatisfactory.

Test number 5 had the standoff further reduced in model A to 20 mm having a 450 gm GDP booster. No jet Sformation was evident at all. Over all, tests 1-5 had been so far discouraging. It seemed that contradictory goals were being pursued. It seemed that what was required was a certain amount of concentration of forces and also a certain amount of dissipation of forces. It became clear that a cone may not be S the perfect cavity shape. It became apparent that a change to a hemisphere cavity may both provide a concentration and dispersal of forces. Subsequently some plastics hemispherical bowls and some stainless steel bowls which were hemispherical except for a flattened base were utilised in subsequent tests.

a On 13 February 1990 another series of tests at Helidon were carried out.

16 Test number 1 employed a plastic bowl marketed under the trade mark DECOR (200 mm OD, including the lip of the bowl). The diameter of the hemisphere of the bowl was 165 mm.

This was fitted into a PVC cylinder with an ID of 165 mm. The lip of the bowl was thus protruding beyond the external diameter of the cylinder. The height of the bowl (OD) was mm. Using this arrangement the explosive at the base of the cylinder is reduced to a minimum.

The unit was fired with no standoff, with 4 kgs of Emulite. The results of this experiment were the best to date (the depth penetrated was about 400 mm; the area broken, about 1 metre diameter). The result was the first real encouragement on the project.

Test number 2 used a flat bottomed, stainless steel bowl. The diameter of the base of the bowl, to the outer lip was 191 mm; the diameter of the bowl, internally, was 178 mm; the height 66 mm. The bowl was inserted in a model "A" section of cut-off cylinder.

Test 2 was fired also without a standoff; The results were similar to test 1. Although there appeared to be slightly more penetration (ie. more than 400 mm) the break diameter was less (about 75 mm).

Test 3 employed the Decor bowl with 50 mm standoff.

Poor penetration, minimal break.

Test 4 used a stainless steel bowl (as in test 2) but with a 30 mm standoff (the outer container was made from cardboard). Results: not impressive.

On 26 February 1990 another series of tests was Test 1 again used the Decor bowl in a 165 mm PVC cylinder loaded with 3.5 x 32 mm plugs of Emulite and 3.5 Kgs of Tovex Extra. The shot was covered with dirt to.-provide confinement. Results: Only 300 mm was penetrated with localised radical cracking and surface disruption over 500 mm radius.

Test 2 consisted of 4 plastic bowl charges, also in 165 mm PVC cylinders, fired simultaneously in an attempt to determine the effects of reinforcement. The results were poor. The 4 charges were initiated with detonating cord and contained 4 kgs of emulite per charge. Each unit was covered with dirt. They were spaced 1 metre apart (square).

Penetration -was only about 200-250 mm. The ground was not s15 broken between the charges.

The results so far were then evaluated. After the encouragement of the first shot of the 13th we had failed to gain any improved results.

We concluded that owing to the narrowness of the 0Z annular gap between the base of the hemisphere and the i" cylinder walls the charge was ineffective in this area. In effect we were losing a significant surface area of the cavity and at the same time introducing a standoff by the S ineffectiveness of the explosive in the lower cavity. We decided to modify the charge by increasing the amount of explosive in the lower cavity area.

Test 3 had the charge diameter increased to 200 mm (picking up the outer lip of the bowl). The charge container

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Iwas made of cardboard; the filler, 4 kgs of Emulite.

that the lower cavity was effective.

It seems that there is a critical dimension at the base of the hemisphere that must be equalled or exceeded for optimum perf ormance. In the case of the bowl in the 165 mm diameter, the loss of 35 mm of diameter or 212 metres. of the radius was critical. As noted, it seems that a reduction in this region both reduces the surface area of the cavity and creates a standoff.

However, whether 21% represents the exact minimum critical limitation of the annular gap has not been established. It would seem that a smaller critical distance may be used which may be at least 5% and more suitably 10% of the radius of the hemisphere. Nevertheless, clearly one needs to increase the container size to over 200 mm in this specific S diamexperiment. It was noted that the amount of explosive was not 0 was increased when the charge was increased to include the lip of rthe bowl. At this point we began to cost the units on the Sbasis of Emulite filled containers made up of whisking bowls and pot plant holders. However, we became concerned that the charge could be too easily copied by non-technically minded people. This reason and other motivated us towards the idea of having the units custom made and factory filled. we so decided that we ought to test the unit in the wet. We had hoped to be able to test it in Uranga Harbour, but had F L 19 difficulties in getting clearance at short notice. We thus decided to test the charge under a 200 litre drum of water at Helidon.

On 8 May 1990 we tested the unit utilised .in shot number 3 with the exception that the charge container was formed from plastics material and also using a sealed cavity in a drum of water. It produced a most favourable result.

Penetration was about 700-800 mm, with 800 m radius on the surface and about 250 mm radius at the base of penetration.

This was exactly what we wanted. It met the requirements of Uranga Harbour, and offered the potential of being sealed up or down for increased or reduced performance, depending on S requirements.

Tests with a slightly scaled up version of the hemispherical charge were carried out in 1991 in the Torres Strait under operational conditions. The units were employed in an underwater excavation. They consisted of Decor 247 mm whisking bowls (APP DES REG 87410): internal height of bowl: 112 mm; internal diameter of base: 216 mm; bowl thickness: 3 mm (high density plastic). The lip of the bowl provided an annular gap of 12.5 mm or 11% of the bowl's internal radius.

S. The bowls were inserted in plastic terra pots (24cm); internal diameter: 250 mm; external diameter: 270 mm; height 225 (external); 215 mm (internal, owing to a depression in the base). The top diameter (actually the inverted pot's base): 188 mm. The cavity was hermetically sealed by means of a 3 mm plastic cap ringed with co-polymer sealant. To offset s the buoyancy of the cavity the unit was given approximately g^+ 2Kgs of ballast consisting of concrete cast 300-350 mm square and 100 mm high around its base. The explosive charge was Enulite (7kgs); loaded to the extend of the pot. The unit was primed and boosted with a 12 roll, 10gm knot at the rear axis, which was secured in place by Araldite epoxy-resin on both sides of the base of the pot. The latter had been cut 360 degrees and opened to provide access for loading the Emulite and securing the primer. It was subsequently replaced and secured with light electrical wire. The det-cord primer had a 350 mm tail. The units were very easy to handle and place in the water. The target was limestone reef; the depth of water averaged 3 metres.

The first unit was tested on a rock shelf, approximately 4 metres long, 3.6 metres wide and 3 metres deep. The charge was positioned roughly centre of the platform, providing a natural face on 3 sides, ie. 3 metres in one direction, and 2 metres in each of the other two directions.

The charge appeared to have smashed and pulverised the whole of the rock platform to the full extent of its depth, although this could not be ascertained. Fragmentation averaged 20 mm 40 mm. The remaining units were employed in Sconjunction with no cavity focal charges in the successful destruction of a large reef (over 200m3).

In early July 1991, 6 units were applied to productive work at the Mt Cotton Quarry, Mt Cotton. Four of 1 the charges were essentially the same as those used in the i Torres Strait- All employed the Decor Whisking Bowl and a 7kg 21 Emulite load, and were used to break up a large outcrop of rock on the quarry face. The rock was very hard blue quartz with white vein and iron content. Three of the charges were placed around the base of the outcrop and were directed horizontally inwards. The fourth charge was placed halfway up the side of the 60m3 outcrop and was also directed horizontally inwards. All the units were covered with dirt to suppress the noise of the blast, and all were fired on instantaneous electric detonators. The ratio of the charge per volume was about 0.47kg/m3, which is extremely light for a non-borehole application.

The result was quite satisfactory. Besides local pulverising of the rock, numerous stress cracks were evident Sradiating from the points where the charges were placed. The rock yielded easily to the quarry's 988 loader. The second task was a rock of about 17m3 [2.7m x 3.5m x 1.8m deep). We were asked to break up this rock into handleable sizes. The charge broke the rock completely.

Another task was a high spot on the quarry floor 30 (blue quartz, very hard). A copper coated, tin liner, 3mm thick was tested on this task. Height 100mm; base 175mm; annular gap 25mm or 28% of radius. 7.15kgs of Emulite was loaded to a height of 220mm. The charge container was a plastic bucket; base (inverted): 250mm; height 2 diameter: 200mm. The liner was centred on a piece of cardboard onto which the bucket was glued. The charge was covered with dirt to suppress noise, and was initiated by an electric detonator. The shot penetrated about 650mm, possibly more, over an area of about Im diameter.

It should be noted that all of the above tests employed lined cavities which were not strict hemispheres.

The whisking bowl liners had an internal height of 112mm and internal base diameter of 216mm; the aluminium liner was high and 140mm in diameter, and the copper and tin liner had a height of 100mm and a base diameter of 175mm. Although not true hemispheres they were close to such. But it is apparent that some flexibility does exist for varying the height to base ratio with minimal loss of effect. However a true hemisphere is the optimum cavity shape, and unless one were aiming at a reduced effect the variation should be kept to a S minimum.

Test number 2 on 13 February 1990 used a stainless steel bowl with a flattened base. The test was fired without a standoff. The results were similar to the previous test which employed a whisking bowl. Again there is clearly a scope for varying the cavity shape from a true hemisphere. It appears that providing that providing the cavity is concave 2.q and symmetrical, and providing the cavity is not unduly elongated or flattened, minimum loss of effect will result.

1 It is also apparent that such variations may be employed to achieve a specific result.

From the foregoing tests it is therefore apparent that shaped charges constructed in accordance with the invention are far superior to the prior art shaped charges using conical liners. So far tests have been mainly carried out with Emulite, which has a density of 1.25g/cc; a Velocity 23 of Detonation of 5000m/sec, and a detonating pressure of 100 kBar. Tests have also been mainly confined to 3mm plastic liners. But one would expect that other inert materials would be economically suitable and more effective. Glass for example may be cheaply produced and adapted to plastic containers and would increase the mass of the liner. Ceramic liners are another option.

The necessity of a) eliminating the standoff, and b) Fproviding an annular gap of sufficient thickness, is directly related to the fact that, unlike a cone, the distance from all points of the hemispherical liner to the focal point is equal.

Moreover, unlike a cone, there is a convergence at the focal point of the liner over a short distance. Consequently, there is a relatively short jet and slug. This accounts for the I *i~i need to eliminate the standoff.

As noted, in the wider cone angles the jet velocity is lower but the mass travelling tothe tage is rat.

The effect with a hemisphere is similar. Consequently the resultant hole is large but not as deep. Godfrey (US Patent 3 176 613) notes that the hemispherical cavity liner arrives at the focal point "simultaneously" or "almost simultaneously" But for simultaneous convergence at the focal point to occur the charge would need to be initiated at multiple points above the hemispherical cavity. It is also noted that it would seem that no commercial usage or experimentation has taken place which supports the Godfrey proposal.

With rear axial initiation, the upper part of the hemispherical liner (approximately 33% of the surface area) 24 converges at the focal point slightly ahead of the lower part (approximately 66%) of the liner. Thus the forces from the lower part of the charge are significantly residual rather than primary. Therefore, the potential energy of the .annular gap need not be as great as the upper part of the charge (contrary to Godfrey's mod-l). Moreover, the characteristics of a detonation wave (its "reaction phase" or the time it takes from initiation to reach its steady state velocity; its directional effect, etc.) result in the need to have A minimum effective explosive height above the cavity; and A greater potential energy above, rather than to the sides of the cavity (taking into consideration, for example, the greater force being applied by the wave :'51 front in the direction it is moving).

Godfrey's model is insufficient in this regard.

As noted, the energy from the annular gap is significantly residual rather than primary (as opposed to Godfrey's model which has "simultaneous" convergence at the 2D focal point).

Cook refers to "residual flow or secondary lateral S penetration" of a target which results in a hole, in the case of a lined conical cavity, many times greater than diameter of the jet (page 253) Yadav observed that the impact of the jet produce' a hole whose diameter is larger than the jet through the hole (Yadav H.S. Propellents, Explosives and Pyrotechnics, Vol., 14, Feb 1989, p12-18).

Of course .a non-stood-off hemispherical cavity i: ~sl~ i eI

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I I 2* I 2 neatly defined charge does not produce a A hole Nevertheless residual forces acting laterally after the jet front has been expended would contribute to fragmentation and would account for the necessity of a sufficient width of explosive around the base of the hemisphere.

The minimum width of this annular gap, with regards to the present invention, up to the maximum size tested, is of the radius of the hemispherical liner and more suitably No such gap is required for conical cavity shaped charges in normal applications.

All units tested, with the exception of those used in the Torres Strait, were initiated on the charge axis at a point more than twice the height of the cavity. The Torres S Strait units, owing to the need to seal the top end of the S Terra Pot containers, were initiated at 1.919 times the height S of the cavity.

Besides providing a significant cost advantage over lined conical cavity shaped charges, the hemispherical lined cavity charge is more efficient for breaking up rock and earth S material.

With the current environmentally orientated emphasis S on minimising peak kPa pressure in underwater blasting, a further reason for optimising charge efficiency and minimising charge quantity exists. The effectiveness of a single 7kg ;5 unit on an underwater limestone reef of approximately 40m3 has been demonstrated.

The above experimental tests have established that a hemispherical or near hemispherical shape for lined cavity j S26 shaped charges is the best shape in terms of economic advantage and optimum results where a focal effect is desired 1 in the excavation of rock and earth materials. However, the hemispherical or near hemispherical shape must be employed in conjunction with an annular gap around the base of the hemisphere of not less than 5% and more suitably 10% of the liner radius For optimum effect the lined hemispherical cavity charge must not be stood off or raised above the target as is normal practice with conical cavity charges. Finally, the distance between the point of initiation and the liner, measured along the (longitudinal) base axis of the shaped charge, must not be less than 1.9 times the height of the cavity. (see FIG 2).

"The above discussion is illustrated in FIGS 2, 3 and .s 4. FIG 2 shows the direction of detonation or direction of propagation of a detonation wave 21A after detonation of the detonator 8 located in detonator well 14. The focal point 21 is also illustrated which coincides with the centre of the target 22.

2 As shown in FIG 3, when the detonation wave 21A having a direction of propagation as illustrated by arrows 23 hits the cavity 24, approximately one third of the liner 17 collapses inwardly towards the focal point 21 forming a short i but low velocity jet stream 25. The apex or point 26 of the jet stream enters the target 22 and penetrates to a working depth which is dependent on the nature and composition of Ai the liner material; (ii) the thickness of the liner; (iii) the type of explosive used, and (iv) the nature and composition of i

'U

27 the target.

As shown in FIG 3 by the time the initial detonation wave has stabilised the top one third 27 of the liner 17 has collapsed and is carrying out its desired task. The remaining two thirds of the liner 28 bursting inwardly behind the foremost jetstream and its residual energy clears out the already weakened target area.

The cavity charge is suitably in contact with the target area and also for underwater use it is preferred that the cavity 24 should be sealed from ingress by water. Thus for underwater use the container may be provided with a base wall although this may not be necessary for uses other than underwater use such as for example rock and earth blasting.

eeo In a preferred embodiment the detonator may be mounted on the longitudinal axis at a height of at least three times the height of the bowl of the cavity. In another aspect S• the invention provides a method of detonating a shaped charge including the steps of: "r i obtaining or otherwise forming said shaped charge including locating a substantially hemispherical liner in a structurally homogenous container so as to define a bowl shaped cavity and uniformly distributing explosive in said container in a space not occupied by the liner and surrounding the liner with the liner defining a concave inner surface and a convex outer surface and having a peripheral edge whereby said peripheral edge is spaced from an internal surface of the r container to define an annular gap which has a width of

I

at least five percent of the radius of the hemispherical 28 Sliner and placing a detonator in said container so that the detonator is located on a longitudinal axis of the container at a height of at least 1.9 times the height of the bowl shaped cavity and wherein the container and explosive is n, 5 symmetrical about said longitudinal axis; and Cii) mounting said shaped charge to a target with no standoff present.

I eres *Z e

Claims (8)

1. 3. A shaped charge as claimed in claim I or 2 wherein the detonator is located on said longitudinal axis at a height of at least twice the height of the bowl shaped cavity.
2. 4. A shaped charge as claimed in claim 3 wherein the detonator is mounted on said longitudinal axis at a height of at least three times the height of the bowl shaped cavity.
3. 5. A shaped charge as claimed in any preceding claim wherein said container has a base wall which is integral with or sealed to a substantially continuous side wall of the container.
4. 6. A shaped charge as claimed in any preceding claim wherein the detonator is located in a II detonator well in a top wall of the container wherein said well is located on said longitudinal axis, 1 I
5. 7. A shaped charge substantially as herein described with reference to the accompanying drawings.
6. 8. A method of detonating a shaped charge including the steps of obtaining or otherwise forming said shaped charge including locating a substantially hemispherical liner in a structurally homogenous container so as to define a bowl shaped cavity and uniformly distributing explosive in said container in a space not occupied by the liner and surrounding the liner with the liner defining a concave inner surface and a convex outer surface and having a peripheral edge spaced from an internal surface of the container to define an annular gap therebetween which has a width of at least five percent of the radius of the hemispherical liner and placing a detonator in said container so that the detonator is located on a longitudinal axis of the coitainer at a height of at least 1.9 times the height of the bowl shaped cavity and wherein the container and explosive is symmetrical about said longitudinal axis; and (ii) mounting said shaped charge to a target with no standoff present.
7. 9. A method as claimed in claim 8 wherein said annular gap has a width of at least percent of the radius of the hemispherical liner.
8. 10. A method as claimed in claim 8 substantially as herein described with reference to the accompany drawings. Dated this 28th day of July 1997 DYNO NOBEL ASIA PACIFIC LIMITED BY: Patent Attorney for the plicant |Pte ttomey.-