WO1991019952A1

WO1991019952A1 - Hollow charge device

Info

Publication number: WO1991019952A1
Application number: PCT/GB1991/000965
Authority: WO
Inventors: Brian Gerard Twomeny; Salim Taky Sadiq Al-Hassani
Original assignee: The University Of Manchester Institute Of Science & Technology
Priority date: 1990-06-19
Filing date: 1991-06-14
Publication date: 1991-12-26
Also published as: AU8073791A; GB9013687D0

Abstract

A hollow charge device comprising an explosive charge (1) in the form of an elongate bar which defines a groove in which a liner (3) is supported. The explosive charge (1) is curved such that it defines a portion of a volume of revolution about a predetermined axis (2) and the groove opens radially outward away from the axis (2). An initiating means comprising a single detonator (10) is arranged so as to generate a detonation front propagating radially outwards from the axis (2) towards the explosive charge (1). The explosive charge (1) is thereby detonated and the detonation front reaches the liner (3) substantially simultaneously over the whole area of the liner (3). The liner is thus projected radially outwards relative to the axis (2) in the form of a high penetration cutting jet.

Description

HOLLOW CHARGE DEVICE

The present invention relates to a hollow charge device.

Hollow charge devices are well known and are generally but not exclusively used for cutting structures upon which they are placed. Generally hollow charge devices comprise a mass of explosive shaped to define a concave recess which is lined with a ductile metal liner. Detonation of the explosive charge rapidly compresses the ductile liner so as to form it into an outwardly-projected jet of metal. The jet is capable of penetrating and cutting through structures located in its path.

The cutting effectiveness of hollow charge devices is a function of a number of variables, including the shape of the lined cavity, the spacing or stand-off distance between the charge and the structure to be cut, and the way in which detonation of the charge is initiated. Charge shape and stand¬ off distance can generally be readily determined by experiment but charge initiation is a more significant problem. The charge is generally initiated by a single detonator from which a spherical detonation front propagates through the explosives. It has not proved possible to produce a simple charge structure initiated by a single detonator to achieve a linear cut.

Various attempts have been made to solve this problem. For example in British Patent Specification GB 2138111A, a linear hollow charge device is disclosed in which a main charge is detonated by a transfer plate which is physically projected across a gap by detonation of an initiating charge. The result is not entirely satisfactory as a complex pattern of initiation is created leading to poor cutting performance.

Another known device is described in European Patent Specification EP-0317294A. The hollow charge device consists of a bar of high explosive having a planisymmetrical groove along one side lined with a hollow charge liner. A pair of backing charges connected by a bridging charge are disposed along the opposite side of the bar. Each backing charge is separated from the bar by a gap which tapers towards the common longitudinal periphery of the bar. The surfaces of the backing charges facing the gaps are provided with liners. A linear initiating charge is separated from the bridging charge longitudinally of the device by a gap. The facing surface of the charge is lined with a metal strip which, when the charge is detonated, is projected across the gap to initiate the bridging charge linearly. A detonation wave then propagates from the bridging charge down each backing charge towards its peripheral region, projecting the liners across the gaps to initiate the bar. A detonation wave front is thereby initiated along the length of the bar which then propagates through the bar to form the hollow charge liner into a linearly-projected cutting jet of high penetration efficiency.

The structure described in European Patent Specification EP0317294A is such that the hollow charge device must be of relatively small length. A high degree of accuracy and precision is also required to ensure perfect symmetry. Furthermore, although the device can achieve a very powerful penetration, the length of cut provided by each device is about the same as the length of the device itself. Therefore, in order to effect a long cut in a plate or around a pipe a number of charges have to be connected together end to end like a chain or bracelet. Given a typical charge length of 15 cm, ten charge devices would be required to produce a 1.5 m long cut in a plate. When used for cutting a pipe from outside, the radial stand off requires even more devices per unit length of cut.

It is an object of the present invention to provide an improved hollow charge device which obviates or mitigates the problems outlined above.

According to the present invention there is provided a hollow charge device comprising an explosive member in the form of an elongate bar defining a groove, a liner supported on the surface of the groove, and means for initiating the explosive member to project the liner as a cutting jet, characterised in that the explosive member is curved to define a portion of a volume of revolution about a predetermined axis, the groove opening radially outwards away from said axis, and the initiating means comprises a single detonator arranged to generate a detonation front propagating radially outwards from said axis towards the explosive member.

Preferably a pair of secondary explosive members extend from the detonator along respective sides of the elongate bar, the secondary explosive members tapering down from the bar to the detonator and supporting initiation liners facing but spaced from the bar. The spacing is selected to be such that when the detonator is fired, a detonation front propagates through the secondary explosive members and projects the initiation liners such that the initiation liners strike the bar on the side thereof remote from the groove simultaneously along the full length and width of the bar.

The required spacing may be achieved simply by leaving a void within the hollow charge structure between the main explosive member and the secondary explosive members. Essentially this requires the provision of a tapered space within the structure. Alternatively tapered initiation liners may be provided such that the voids within the structure can be of simple rectangular form.

According to a second aspect of the present invention, there is provided a hollow charge device in which a primary explosive charge is initiated by secondary explosive charges spaced from the primary explosive charge by a gap, an initiation liner being provided on the secondary explosive members and arranged to be projected across the gap to initiate detonation of the primary explosive member, characterized in that the initiation liners are tapered to achieve simultaneous detonation of the primary explosive member.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a sectional view through a first embodiment of the present invention;

Fig. 2 is a section on the line 2-2 of Fig. 1;

Fig. 3 is a section through a second embodiment of the present invention;

Fig. 4 is a view on the line 4-4 of Fig. 3; and Fig. 5 is a perspective view of the second embodiment of Figs. 3 and 4 with outer components of the structure removed.

Referring to Figs. 1 and 2 of the accompanying drawings, the illustrated device comprises a primary explosive charge 1 which is a volume of revolution about an axis 2. The explosive charge defines a groove facing radially away from the axis 2, the groove being lined by a liner 3. In use the explosive charge 1 is detonated and the detonation front reaches the liner 3 substantially simultaneously over the whole area of the liner 3. The liner 3 is thus projected radially outwards relative to the axis 2 in the form of a high penetration cutting jet. Thus the primary explosive charge 1 is in the form of an elongate bar defining a groove, the bar being curved such that the groove faces radially outwards.

Secondary explosive charges or backing charges 4 are spaced from the side of the explosive bar 1 remote from the groove by voids 5. The voids are defined on one side by protective liners 6 supported by the explosive bar 1 and on the other side by initiation liners or flyer plates 7. The voids 5 taper down from a support bar 8 which is also a volume of revolution about the axis 2 towards side plates 9. The flyer plates 7 are in intimate contact with the backing charges 4 which extend on opposite sides of the support bar 8 to a detonator 10. The backing charges 4 are supported between confining plates 11. A protective casing 12 is sandwiched between the confining plate 11 and the backing charges 4 and thin metal sheets 13 provide a seal around the assembly. Bolts 14 secure the sheets 13 to the support bar 8, the side plates 9 to the confining plates 11, and the liner 3 to the side plates 9.

As is apparent from Fig. 2, the entire device is visually equivalent to a cut portion of a pie. Because of this configuration the cutting jet is projected radially outwards so that the length of the cut produced is no longer limited to the length of the explosive bar 1 as in the case of linear hollow charges. The length of the cut is a function of the stand off gap between the hollow charge device and the surface to be cut.

When the detonator 10 is fired, a detonation front propagates from what is in effect the geometrical centre of the device. There is no need for still further charges to achieve a linear detonation front in the backing charges. Because the upper portion of the backing charges is a sector of a circle the detonation front emanating from the detonator will naturally flow radially and impinge simultaneously on the flyer plates 7, causing them to initiate the explosive bar 1 instantaneously and simultaneously.

The angle of taper of the voids 5 is selected as in prior art devices to ensure that the flyer plates 7 are parallel to the facing surfaces of the explosive bar 1 as the flyer plates impinge upon the explosive bar.

Referring now to Figs. 3 to 5, these illustrate a second embodiment of the present invention. The same reference numerals are used in Figs. 3 to 5 as are used for equivalent elements in Figs. 1 and 2. The only significant difference between the two illustrated embodiments is that in the arrangement of Figs. 3 to 5 the voids 5 are of simple rectangular cross-section rather than tapered cross-section. In the case of the embodiment of Figs. 3 to 5, the flyer plates are themselves tapered to provide the appropriate delay matched to the time taken for the detonation front in the backing charges 4 to propagate from the support bar 8 to the side plates 9. Thus the tapered flyer plates shown in Figs. 3 act effectively as delay elements. Such an arrangement is advantageous as the tapered void which is difficult to accurately achieve during assembly of the device is replaced by a simple rectangular void which is relatively easy to achieve. The tapered flyer plates of the embodiment of Fig. 3 can easily be accurately achieved by appropriate manufacture of those components. The thicker parts of the flyer plates 7 in the embodiment of Fig. 3 are launched with a lower velocity than the thinner ones. Thus by matching the explosive thickness, the flyer thickness, and the detonation velocity of the backing charge, it is possible to get the flyer to move in a straight plane and impinge simultaneously on all the facing surface of the main explosive charge 1. Thus the embodiment of Fig. 3 to 5 is more robust and easier to assemble, which is important particulary for underwater applications.

A significant advantage of this invention is the fanning out of the radial cutting jet. The immediate gain in the length of cut is determined by the angle of taper of the device (apparent from Figs. 2 and 4) and the stand off. The gain in the length of cut is the product of the stand off and the angle of taper. For example, for a device of 15 cm length and a segment angle of 45 degrees on a target of stand off 0.1 m there will be a 34% reduction in the number of charges required as compared with a simple linear device. Thus for a given weight of charge a longer cut can be achieved. It is to be expected however that the penetration, or cutting efficiency, will decrease from the centre of the device towards the edges of the cut. This is because the radial fanning out of the cutting jets spreads a unit mass of that jet across a progressively greater length of cut the further one moves away from the centre of the device. A more uniform type of cut could be obtained however by using multiple charges arranged in a line with the fanned jets overlapping on or just before the target surface is impacted.

When used under water, a space defined by twice the volume of the groove lined by the liner 3 must be clear of water to allow effective jet formation. This can be achieved by extending the angled side plates 9 to meet at the central plane of the device and hence seal the resultant curved square section cavity.

Claims

1. A hollow charge device comprising an explosive member in the form of an elongate bar defining a groove, a liner supported on the surface of the groove, and means for initiating the explosive member to project the liner as a cutting jet, characterised in that the explosive member is curved to define a portion of a volume of revolution about a predetermined axis, the groove opening radially outwards away from said axis, and the initiating means comprises a single detonator arranged to generate a detonation front propagating radially outwards from said axis towards the explosive member.

2. A hollow charge device as claimed in claim 1, wherein a pair of secondary explosive members extend from the detonator along respective sides of the elongate bar, the secondary explosive members tapering down from the bar to the detonator and supporting initiation liners facing but spaced from the bar.

3. A hollow charge device as claimed in claim 2, wherein the spacing between the initiation liners and the bar is such that when the detonator is fired, a detonation front propagates through the secondary explosive members and projects the initiation liners such that the initiation liners strike the bar on the side thereof remote from the groove simultaneously along the full length and width of the bar.

4. A hollow charge device as claimed in claim 2 or claim 3, wherein the said spacing comprises a void defined within the hollow charge structure between the main explosive member and the secondary explosive members.

5. A hollow charge device as claimed in claim 4, wherein a tapered space is provided within the hollow charge structure.

6. A hollow charge device as claimed in claim 4, wherein the initiation liners are tapered.

7. A hollow charge device as claimed in claim 6, wherein the said voids within the hollow charge structure are substantially rectangular in shape.

8. A hollow charge device in which a primary explosive charge is initiated by secondary explosive charges spaced from the primary explosive charge by a gap, an initiation liner being

SUBST8TUTE SHEET provided on the secondary explosive members and arranged to be projected across the gap to initiate detonation of the primary explosive member, characterized in that the initiation liners are tapered to achieve simultaneous detonation of the primary explosive member.

9. A hollow charge device substantially as hereinbefore described with reference to the accompanying drawings.

SUBSTITUTE SHEET