WO1988002470A2

WO1988002470A2 - Method for detonating an explosive charge

Info

Publication number: WO1988002470A2
Application number: PCT/GB1987/000677
Authority: WO
Inventors: Peter Christian Shann
Original assignee: Explosive Developments Limited
Priority date: 1986-09-29
Filing date: 1987-09-25
Publication date: 1988-04-07
Also published as: GB8623365D0; WO1988002470A3; JPH02500383A; AU8033887A

Abstract

The invention identifies a serious defect in the detonation of solid explosives against target surfaces, particularly against convex surfaces and proposes a new method for detonating a solid explosive to increase the efficiency thereof. In one embodiment, for cutting a tube (63) circumferentially, the explosive mass (61) is applied to the target surface (63a) and a layer of a second explosive material (62) is applied to that surface of the mass of explosive most remote from the target (63). The second explosive material (62) has a detonation speed faster than that of the mass (61) and is detonated first to transmit detonation front into the mass (61) whereupon the detonation front in the mass (61) is always forwardly inclined.

Description

METHOD FOR DETONATING AN EXPLOSIVE CHARGE

This invention relates to explosives.

It is well known in the art to cut or deform a target, such as a metal plate or sheet, by placing a mass of explosive material on the surface of the target to be affected and detonating the explosive mass. A shock wgve front generated by the detonation of the explosive mass enters the target through that surface area in contact with the explosive material and travels through the target to affect the desired target modification. The mass of explosive material is generally detonated by a primer of an explosive material, selected for its sensitivity and ease of detonation, and the primer is itself detonated by a detonator. Thus, in effect, the detonator is fired to detonate the primer which generates a detonation front in the mass of explosive material. With this arrangement for detonation the detonation front travels through the primer and the mass of explosive material in all directions away from the point of initiation of the detonation (the detonator) and when the mass of explosive material is homogeneous the speed at which the detonation front travels through the mass of explosive material is relatively uniform. Thus, the detonation front can be considered to be hemispherical with its centre generally at the single point of initiation and its surface expanding away from said point of initirtion at uniform speed, then the mass of explosive material is detonated from an initiation point on part of the explosive mass remote from the target, which is the usual practise, the detonation front will only reach that surface of the explosive mass in contact with the target at right angles to the said surface at the smallest thickness through the explosive mass between the point of initiation of detonation and the surface of the explosive mass in contact with the target surface. Thereafter, as the hemispherical detonation front travels transversely through the explosive mass away from said smallest thickness, the angle of the detonation front to the surface of the explosive mass in contact with the target increases. As the shock wave front developed by the detonated mass is parallel to the detonation front at the target surface and the target surface will reflect the shock wave in dependance upon the angle at which the shock wave front strikes the target surface, it will be seen that the maximum affect of the explosive will be at that small area where the detonation front is generally parallel to the interface between the explosive mass and. the target surface and, thereafter, the increasing reflection of shock wave front with the resultant reduction in the "magnitude" of said shock wave front transmitted to the target rapidly reduces the effects of the shock wave front in the target. A further difficulty with explosive materials, well recognised in the art, is experienced when the explosive mass is applied externally to a curved target surface such that the detonation front must run along a curved path. Uith the conventional modes of detonation the explosive mass is virtually ineffective on such targets. It can now be seen from the έbove propositions that the failure of an explosive on a curved surface is wholly due to the angle of the detonation front relative to the target surface. If detonation of a mass of explosive applied to a curved surface is initiated from a single point the detonation front will run down through the thickness of the explosive material to the target engaging surface and at that point the detonation front at the surface of the explosive mass remote from the target will be in advance of the detonation front adjacent the target. However, as the detonation front travels through the explosive material at uniform speed and the detonation front adjacent the target has less distance to travel around any given arc than the detonation front at the explosive material surface remote from the target, the plane of the detonation front will approach and pass through a radial plane based on the axis of curvature of the target and will thereafter lie in a reverse plane with the detonation front adjacent the target leading. With such an arrangement the target surface is subjected to a less severe but more prolonged shock wave affect which, as the reverse plane of the detonrtion front increases, becomes less effective on the target.

The present invention seeks to provide a more efficaceous method for detonating an explosive material and an explosive arrangement capable of practising said method. According to the present invention there is provided a method for detonating a msss of explosive material characterized by the steps of applying to said mass of explosive material a layer of a second explosive material having a faster detonation speed than said mass of explosive material and detonating said layer of second explosive material to effect detonation of said mass of explosive material.

In one preferred embodiment in accordance with the invention the method includes the step of applying the layer of second explosive material to the surface, or a surface, of the mass of explosive material remote from the target engaging surface of said mass of explosive material.

It will now be seen that by this method of detonction the detonator initiates detonation of the primer, the primer initiates detonation of the layer of second explosive material and the said layer of second explosive material detonates the mass of explosive material. As the detonation speed of the layer of second explosive material is greater than that of the mass of explosive material the detonation front within said layer will always be in advance of the detonation front in that surface of the mass in contact with the said layer and when the explosive mass is of uniform thickness and is applied to a flat target surface the detonation front will be established at a substantially constant detonation front angle within the explosive material. The plane of such a detonation front will preferably be at the smallest angle possible to the plane of the interface between the explosive mass and the target, to obtain maximum absorption of the shock wave front v/ithin the target, and the greater the relative speeds of detonation between the said layer and the said mass the smaller will be the angle between the detonation front in the mass of explosive and the target surface.

As the efficiency of the mass of explosive in affecting a desired modification of a target is directly related to the efficiency at which the shock wave front is transmitted into the target the method proposed by the present invention will be superior to prior art methods of detonation and a saving of explosive material can be achieved thereby.

In another embodiment in accordance with the invention the said layer of second explosive is applied to the target engaging surface of the mass of explosive material so as to lie between the target and said mass. When the detonation proceeds from the detonator and primer to the layer of said second explosive material the detonation front travels through said layer initiating detonation of the mass from that surface adjacent the target. By this means the target surface is subjected to a rapid, but relatively small shock wave front from the detonating layer and then to a relatively long pressure pulse as the mass of explosive material detonates outwardly from the target. This form of prolonged pressure application to the target is most effective for certain applications, such as deforming targets to a desired configuration.

The invention also envisages an explosive arrangement for practising the above methods and comprising a mass of explosive material with a layer of a second explosive material, of faster detonation speed than said mass of explosive material, applied to one surface of the said mass.

Preferably the said layer of second explosive material has a thickness less than 5 mm and more preferably less than 1mm.

In one embodiment in accordance with the invention a shock wave delay element presents a surface intended to face a target, a mass of explosive material is applied to a surface or surfaces of said element other than the target facing surfrce and said layer of a second explosive material is applied to the surface or surfaces of the mass of explosive material most remote from said target facing surface of said element. Preferably said shock wave delay element is of elongate form and said explosive mass and said layer of second explosive material extend continuously along the length direction of said element.

The layer of second explosive material preferably comprises a continuous sheet but in other embodiments the layer of second explosives material may be discontinuous and may for example include apertures throvgh which the mass of explosive is exposed or the said layer may comprise strips of explosive material applied to the surface of the mass to achieve a desired detonation pattern over said surface. In a still further embodiment in accordance with the invention the mass of explosive material may comprise a plurality of discrete masses with a common layer of second explosive material applied to all said discrete masses. By this means particular detonation patterns of the mass of explosive material can be obtained.

In a still further embodiment in accordance with the invention a plurality of layers of explosive material in stacked relationship are applied to the mass of explosive material and the layers have different detonation speeds to allow desired patterns of detonation on that surface of the mass of explosive material in contact with said layers.

In one embodiment in accordance with, this aspect of the invention each said layer of explosive material may be of uniform thickness but in another embodiment selective layers may be of non-uniform thickness.

The invention will now be described further by way of example with referei.ce to the accompanying drawings in which;

Fig. 1 shows, diagrammatically, the progression of the detonation front through a mass of explosive material detonated by the prior art method of detonation.

Fig. 2 shows, diagrammatically, the progress of the detonation front around an arcuate mass of explosive material detonated by the prior art method of detonation. Fig. 3 shows, diagrammatically, the progress of the detonation front through an explosive arrangement detonated in accordance with the method of the invention. Fig. 4 shows a cross section through a second embodiment in accordance with the invention. Fig. 5 shows a cross section through a third embodiment in accordance with the invention. Fig. 6 shows a transverse cross section through a two-shock wave cutting arrangement in accordance with the invention. Fig. 7 shows, diagrammatically, the progress of the detonation front through the arrangement illustrated in Fig. 6 when bent longitudinally to an arcuste form.

Fig. 8 shows, diagrammatically, a further embodiment in accordance with the invention and

Fig. 9 shows, diagrammatically one further embodiment in accordance with the invention.

In the prior art arrangement illustrated in

Fig. 1 a mass of explosive material 11 has a surface 11a in direct contact with a surface 12a. of a target 12 to be modified. The mass 11 is detonated by a primer 13 resting on the surface 11b of mass 11 remote from the target 12 and a detonator 14 has its lower end (as viewed in Fig. 1) inserted into the parts of primer 13 remote from the surface 11b. With such an arrangement the detonator 14 on being fired initiates detonation of the primer 13 and the detonation front travels through primer 13 to initiate detonation of the mass 11. As and when the detonation front within the mass 11 reaches surface 11a a shock wave front is transmitted to the surface 12a of the target 12. Thus, the point of detonation of the system may be accepted as the point A centrally within that part of detonator 14 within primer

15. When detonator 14 is fired to initiate detonation of the primer 13 the detonation front travels through primer 13 at uniform speed and in all directions away from point A so that at a very short time after detonator 14 fires the configuration of the detonation front within primer 15 will be hemispherical, as shown by the broken line 15.

The detonation front from primer 15 can pass into the mass 11 only at the interface between primer 15 and mass 11 and, assuming a detonation front speed through the primer 13 equal to the detonation front speed through mass 11, as the detonation front reaches the regions of primer 13 most remote from the point A the detonation front within mass 11 will have the configuration identified by numeral 16 with the curvature thereof based on point A.

The detonation front continues to flow through mass 11 with the curvature, still based on point A, reducing until said front reaches surface 11a of mass 11, at which point the front will have the unbroken configuration identified by numeral 17 and at this point a shock wave is transmitted to the surface 12a of the target 12. The detonation front 17 will first reach surface 11a through that thickness of the mass 11 directly beneath the point A, as viewed in Fig. 1, that is through the mininal thickness of mass 11 from point A to point B and, at that point of the detonation front first reaching the surface 11a, the detonation front 17 will be parallel to the surface 12a and the shock wave front will enter the target surface 12a exactly parallel thereto so that the reflection of shock wave front from surface 12a will be minimal. From point B the detonation front continues to travel outwardly through the mass 11 but, as the curvature of the detonation front is still based on point A, the angle of the tangent of the detonation front at surface 11a increases as the radius of the detonation front increases, the angle of the shock wave front to surface 12a increases and therefore the reflection of shock wave front from surface 12a increases and the "magnitude" of the shock wave entering surface 12a reduces.

Assuming a distance "X" from point A to point B, then after the detonation front has travelled a distance X from point A to point B the plane of the detonation front at the surface 11a will be parallel to surface 11a. After travelling a distance 1½X the detonation front will be making an angle of 45º with the surface 11a and after travelling a distance 2X the detonation front 19 will be making an angle of some 61 to the surface 11a.

As stated hereinbefore the angle at which the detonation front meets the surface 11a is the angle at which the shock wave meets the surface 12a and the above simple illustration clearly shows how the shock wave front can become ineffective in the target 12 a very short distance from point B.

In the prior art arrangement shown in Fig. 2 a target 20 comprises a pipe of inner radius R1 and outer radius R2 and a mass 21 of explosives is applied to radius R2 and is of uniform thickness so that its outer surface defines a radius R3. The explosive mass 21 is to be detonated by a primer 22 placed on the radius R3 of the explosive mass 11 and detonation of primer 22 is initiated by a detonator 23 inserted into primer 22 to define a point of detonation C. When detonator 23 is fired a hemispherical detonation front is generated within primer 22 until the detonation front passes into the mass 21. The configuration of the detonation front at that point where the detonation front reaches the remote parts of the primer 22 is still based on point C and is identified by numeral 24.

\\hen the detonation front has passed through the thickness of the mass 21 to reach the radius R2, identified by numeral 25, the said front 25 still has a curvature based on point C, the shock wave transmitted to the radius R2 of the target 20 is tangential to the curvature of radius R2 at the point E on radius R2 and at this point the detonation front 25 will have travelled to point E on radius R3.

If distance CD. is equal to X and the curvature at E is still based on the centre C then distance C.E. will equal X. From point D the detonation front runs outwardly from the line D.E., transversely through the mess 21, but with uniform detonation speed the curvature of said front will still be based on the centre C. When the detonation front, identified by numeral 26, has travelled a distance 2X from point C, the curvature of front 26 will be intersecting radius R2 at point F and radius R3 at point G and, as will be seen from Fig. 2,the tangent T1 of the curvature of detonation front at point F is making a substantial angle with the tangent T2 of the radius R2 at point F whereby the reflection of the shock wave front, parallel to the tangent T1, will be such as to render said shock wave substantially ineffective on the target 20. For the proportions of the elements illustrated the angle of the detonation front 26 to the tangent T2 at point F will be some 72 and that is achieved in traversing an arc, from point D to point F, of only some 22½º.

When the detonation front, identified by numeral 27 at point H has travelled a distance of 3x from point C the tangent T3 to the detonation front 27 will have passed through the radial plane based on the axis of the target 20 and will be making an angle of some 91° with the tangent T4 of radius R2 at point H.

Thus, at point H, the plane of the detonation front 27 is directed away from the target 20 and any shock wave transmitted to the target 20 at point H will be completely ineffective on the target.

As the detonation front continues to travel through the mass 21 from point H the plane of the detonation front continues to be increasingly inclined, in the reverse direction, to the target 20.

It is for the above reasoning that explosion materials are relatively ineffective on arcuate targets.

In the embodiment illustrated in Fig. 5 in accordance with the present invention a mass of explosive material 31 on a target 32 has a layer of explosive material 35 applied to its surface 31a remote from the target 32. A primer 34 rests on layer 33 and is detonated by a detonator 35. The layer 33 has a detonation speed faster than that of the explosive mass 31 an d for the purposes of this example consider a detonation speed 10% faster than that of the mass 31 and of the primer 34. With this arrangement firing of the detonator 35 defines the centre of detonation J which generates a hemispherical detonation front 36 in primer 34. As the detonation front expands, layer 33 detonates at point K and the detonation produces a detonation front in the layer 33 which travels through said layer 33 in the plane of said layer 33. As the detonation front passes through layer 33 said front will initiate detonation of the surface 31a of mass 31 but the detonation speed across surface 31a will be 10% higher than that through the mass 3 1 .

Thus, at the highest point where the detonation front would normally have continued to the parts of primer 34 remote from the detonation centre J, the detonation front will have the configuration identified by reference 37 and whereby the faster detonation froht through the layer 33 has extended regions of the hemispherical shape transversely adjacent surface 31a. When the detonation front has travelled a distance X from detonation centre J and whereby said front has arrived at surface 31b of mass 31 adjacent the target 32 said front will have the configuration identified by numeral 38 and wherein the lover regions of the detonation front (as viewed in Fig. 3) will be hemispherical but the upper parts have extended transversely, due to the advanced detonation of the layer 33.

Thus, the width of the detonation front in mass 31 at surface 31a will be equal to twice the radius of the hemispherical lower regions of said front 38 plus 20% of twice said radius. At this point the tangent to the detonation front at surface 31b is parallel to surface 31b and therefore the shock wave front transmitted to the target 32 will have minimum reflection. When the shock wave front would normally have travelled a distance 2X from the centre of detonation J, identified by numeral 39, the said detonation front should be intersecting the surface 31a at point K but, due to the faster detonation through layer 33 and the resultant detonation of the mass 31 along surface 31a, the detonation front will be a substantially straight front, as shown by numeral 40, and with the proportions illustrated the front 40 wall make an angle of some 54½º with surface 51b. Once the straight line front has been established the faster detonation alone layer 33 will continuously reduce the angle of said front to the surface 31b, whereupon the reflection of shock wave front from the target 32 will continuously reduce.

Fig. 4 shows a second embodiment in accordance with the invention and wherein a mass of explosive material 41 rests on a target 42 and the mass 41 is to be detonated via a detonator 43 a primer 44 and a layer 45 of explosive material having a higher detonation speed than the mass 41. In this embodiment a layer 46 of an explosive material having a slow detonation speed, slower than the detonation speed of mass 41, lies between the mass 41 and layer 45.

The layer 46 is of non-uniform thickness, the upper surface being domed, and by this means on firing of detonator 43 to detonate primer 44 the layer 45 is detonated by the primer 44 and the detonation front in layer 45 expands radially from the first contact of the layer 45, at point M.

As the detonation front within layer 45 expands from point M it detonates the top surface of layer 46 but from the point M the detonation front must travel downwardly through the thickest point of slow speed layer 46 before initiating detonation of the mass 41. As the detonation front in layer 45 travels at high speed, detonating the surface layer 46, the further the detonation front in layer 45 is from point M the thinner is the thickness of layer 46. Thus, the layer 46 comprises a detonation front delay element and the degree of delay in passage of the detonation front through the layer 46 is dependant upon the distance from point M. By this means a hemispherical detonation front is obtained in mass 41 with a curvature which is not based on the centre of detonation and which is based on the speed of detonation through the layer 45, the layer 46 and the curvature of the top surface of layer 46. By carefully balancing these factors the shock Wave front may be transmitted to the whole of the target surface beneath mass 11 with a minimum of reflection from the target 42.

In the embodiment illustrated in Fig. 5 a mass of explosive material 50 has a layer of explosive material 51, of faster detonation speed than the mass 50, between mass 50 and a target 52. A tubular liner 53 of a detonation delay material, for example rubber tubing, extends upwardly from the layer 51 to the upper surface 50a of mass 50 and said liner 53 is filled with an explosive material 54 of high detonation speed.

At the surface 50a of mass 50 the explosive filling 54 extends upwardly through a central aperture in a sheet liner 55 of detonation delay material upon which a primer 56 is supported so as to be insulated from mass 50. The primer is detonated by a detonator 57.

With this arrangement, on firing of detonator 57, the primer 56 is detonated but extends detonation only to the explosive material 54, being insulated from the mass 50 by the sheet liner 55. The detonation front travels down the explosive material 54, without effecting detonation of mass 50 due to liner 53, and detonates layer 51. Thus, the first detonation of the combination of mass 50 and layer 51 will occur at the junction of layer 51 with explosive core 54.

The detonation of layer 51 thereby runs radially from the contact with explosive core 54 and as said detonation proceeds through layer 51 the mess 11 is detonated from its surface in contact with layer 51 upwardly through the mass 11. The shock wave front transmitted to the target 52 will comprise an initial shock wave front which will strike surface 52a of the target directly beneath the explosive core 54, this shock wave front will be parallel to the surface 52a so that the reflection of shock wave from surface 52a will be minimal, but the explosive core 54 may be of relatively small cross sectional area so that the effects of the shock wave developed therefrom will be relatively small.

Thereafter, the target surface 52a will be subjected to the shock wave front developed by the detonation of layer 51 which, if layer 51 is relatively thin, will be relatively small. Thereafter, the target surface 52a experiences a slow application of shock wave front from the upwardly detonating mass 11.

It will be seen that with this arrangement, and contrary to the sharp or severe application of shock wave to the target, the arrangement illustrated in Fig. 5 allows a relatively long slow pressure pulse to be applied to the target 52, an effect difficult if not impossible to achieve with conventional methods of detonation, and such an effect offers great advantages in, for example, the modification of relatively large target areas.

It will be appreciated that whilst the above description has ignored detonation of the mass 11 through the liner 55 and liner 53 and such detonation will affect the detontion front configuration within mass 50 the above description correctly describes the detonation transmission to the mass 50 during the greatest part of the detonation of mess 50.

The cross section of the device illustrated in Fig. 6 has been found to be most advantageous in practising the so-called "two shock wave" cutting of targets. In. the illustrated embodiment an elongate shock wave delay element 60, having the cross-section of a truncated isosceles triangle,conveniently made of extruded rubber or rubber substitute, has a mass 61 of explosive material applied tc its sloping surface 60a, 60b and its top surface 60c. The base surface 60d of the element 60 constitutes the target engaging surface of the device.

The mass 61 of explosive material has its top surface 61a (as viewed in Fig. 6) substantially parallel to the surface 60d of element 60 and a layer 62 of an explosive material having a detonation speed faster than that of mass 61 is applied to said surface 61a.

The device illustrated rests on a target surface 63a of a target 63.

With the above arrangement, and on detonation of the fast layer 62, the detonation front travels down through the explosive material 61 until, on reaching surface 60c the progress of the mid-region of the detonation front is arrested by the surface 60c of the delay element 60 and a shock wave first enters the delay element 60 through surface 60c. The detonation of the explosive mass continues on each side of surface 60c towards the deepest part of the mass 61 adjacent the side edges of surface 60d, the detonation front generating shock wave front within element 60 through sides 60a and 60b.

When the detonation front reaches the lowermost regions of the explosive mass 61, adjacent the side edges of surface 60d, a shock wave front is transmitted to the target surface 63a and in fact two shock wave fronts enter the target surface 63a simultaneously adjacent the side edges of the element 60. As the shock waves first entering the target 63a progress through the target 63 the delayed shock waves, delayed by passage through delcy element 60, progressively enter the target surface beneath element 60 and, at that point whereat the first entering shock wave front reach the undersurface of the terget63, the shock wave front will, for the example illustrated, have the configuration identified by numerals 64a, 64b. The two shock wave fronts 64a and 64b meet within the target to effect the two wave cutting at the plane of collision within the target 63.

The use of the shock wave delay element and explosive, with the layer of high detonation speed explosive as proposed by the present invention, offers particular advantage when the two shock wave cutting device is applied to an arcuate target, as shown in Fig. 7.

For convenience, Fig. 7 comprises a side view of the delay device and the progress of the detonation front will be described in relationship to the exposed side and it will be appreciated that the actual cross sectional detonation fronts will be as described with, reference to Fig. 6.

As will be evident from Fig. 7 the shock wave delay device is applied to the external surface 63a of an arcuate target 63, the surface 60d of the shock wave delay element 60 being applied directly to surface 63a of the target 60 so that the layer 62 is radially outermost.

The two shock wave cutting device is to be detonated by a primer 64 resting on laypr 62 intended to be detonated by a detonator 65 partially inserted into the primer 64.

On firing of the detonator 65 to initiate detonation of primer 64. a centre of detonation N is established and a hemispherical detonation front, with N as its centre, progresses through primer 64 until said front contacts layer 62, layer 62 then detonates circumferentially in both directions snd as it so dentonates it detonates the explosive material 61 directly adjacent thereto.

Thus, when the detonation front has travelled a distance X to the plane of surface 60d the first two zones of detonation immediately adjacent the side edges of surface 60d transmit shock waves into the surfi.ce 63a of target 63 as described above with reference to Fig. 6. At this point the detonation front in layer 62 will be leading and the detonation front v/ill have the configuration identified by numeral 66. At this point the shock wave transmitted to target surface 63a will be parallel to N tbe tangent of the surface 63a. at that point.

When the detonation front has travelled a distance of 2X through the mass 61 it would, in conventional detonation methods, have the configuration shown in broken line and identified by numeral 67 but, due to the faster detonation along layer 62, the actual detonation front will be forwardly inclined plane 68 making an angle of some 28 with the radian Rr1 from the axis of target 63.

After travelling a distance 3X the detonation front 69 is still in a forwardly inclined plane making an angle of some 26 with the radian Rr2 from the axis of target 63. After travelling a distance 4X the detonation front 70 is making an angle of some 24 degrees with radian Rr3 from the axis of target 63.

Thus, in the specific example in the proportions as illustrated, the angle of the detonation wave front changes from 28 at radian Er1 to 24 at radian Rr3, 4º in some 27½º between radians Er1 and Rr3, and thus at this rate of change the plane of the detonation front will not pass through the plane of a radian from the axis of the target 63 until the detonation front has travelled through some 165º beyond radian Rr3, which is some 214 from the radian passing through the detonation centre N. As a detonation front will travel clockwise from the radian passing through the detonation centre N and a second detonation front will travel anti-clockwise from said radian the two detonation fronts, travelling at the same speed, will meet at that radian

180º from the radian passing through point N, whereby the detonation fronts will always be forwardly inclined with the detonation of layer 62 leading.

Referring to Fig. 6 it will be seen that the forward inclination of the detonation front, with the detonation front in the layer 62 leading, is the ideal detonation pattern for the cross section of The two shock wave cutting device. The detonation front leading through layer 62, detonating the mass 61 from its upper surface, ensures that the detonation progresses downwardly and the masses of explosive on sides 60a and 60b of delay element 60 detonate simultaneously to achieve the ideal conditions for two shock wave cutting. In the event of the regions of the mass adjacent the target leading, as was the case with the prior art constructions, not only was the greatest part of the detonation front, and thereby the resultant shock wave front, directed away from the target but the two explosive material volumes separated by the delay element 60 could suffer differential detonation front progression wuereby the two shock wave fronts entering the target on either side of the element 60 would not enter the section simultaneously, whereby the actual cut would deviate from the intended line of cut. Fig. 8 shows a cross-section through a further embodiment in accordance with the invention for obtaining a more uniform detonation of a mass of explosive material 80.

This embodiment utilizes a plurality of sheets 81 of explosive material and a plurality of sheets 82 of a detonation insulating, or delay material, such, as rubber sheets. The sheets 81 and 82 are in alternative stacked relationship with an insulating sheet 82a on the top of the stack and a sheet 81d. of explosive material at the bottom of the stack. The sheet 81d rests on the mass 80 and is of an explosive material having a faster detonation speed than that of the mass 80.

The assembly is to be detonated by a primer 83 fired by a detonator 84.

When detonator 84 is fired to detonate primer 83, seated on insulating sheet 82a, the primer 83 sends a detonating wave front into the top layer of explosive sheet 81a only through a central aperture 82a1 in sheet 82a so that sheet 81a detonates outwardly from the base of aperture 82a1 .

The detonation front runs throurh sheet 8l£ and sends a detonating front to sheet 81b only through two apertures 82b1 and 82b2 in insulating sheet 82b. The apertures 82b1 and 82b2 are equally spaced from aperture 82a1 so that two detonation wave fronts are transmitted to sheet 81b through apertures 82b1 and 82b2 simultaneously.

The two detonation fronts started in sheet 81b run transversely through sheet 81b from apertures 82b1 and 82b2 and send detonation wave fronts to sheet 8l£ only through four apertures 82c1, 82c2, 82c3 and 82c4 in sheet 82c apertures 82c1 and 82c2 being equally spaced from the aperture 82b1 and apertures 82c3 and 82c4 being equally spaced from aperture 82b2, with exactly the same spacing as that between aperture 82b1 and the apertures 82c1 and 82c2 so that the four detonation front directed to the layer 81c arrive at sheet 81c simultaneously.

The sheet 81c detonated at four spaced apart points sends eight detonation wave fronts through sheet 82d to sheet 81d via apertures 82d1 and 82d2 equally spaced from aperture 82c1 , apertures 82d3 and 82d4 equally spaced from aperture 82c2, apertures 82d5 and 82d6 equally spaced from apertures 82c3 and apertures 82d7 and 82d8 equally spaced from 82c4, the said spacings of said apertures being equal so that eight equally spaced apart detonation fronts pass simultaneously through sheet 82d to initiate eight detonation fronts in fast detonation speed sheet 81d.

Each detonation front started in fast detonation sheet 81d runs rapidly through said sheet initiating detonation of the top surface of the mass 80. It will be appreciated that using the above "manifold" system it is possible to add further layers of sheet 81 and further layers of sheet 82 to multiply the number of detonrtion fronts initiated simultaneously in the sheet 81d and, thereby in the top surface of the mass 80. Whilst the initiation of a number of detonation fronts simultaneously into the mass 80 will afford very obvious advantages the use of the fast detonation sheet layer 81d greatly increases the efficiency of the system. Thus, without layer 81d each detonation point started on the upper surface of mass 11 would produce a hemispherical detonation front in mess 80 with the centre of the detonation front at that point of initiation. With the layer 81d of faster detonation speed than the mass 8θ the detonation fronts established in mass 80 are mushroom shaped, the radius of the detonation front from the initiation centre always being greater at the upper surface adjacent sheet 81d than the vertical radius into the mass 80. By this means the general configuration of each detonation front reaching the target engaging surface of the mass 80 is flatter, that is less inclined to the target engaging surface of mass 80, the reflection of shock wave from the target surface is less than would be the case with prior art methods of detonation, and the effect of the explosive mass 80 on the target is greatly increased. Fig. 9 shows an alternative arrangement for obtaining multiple initiation points on the surface of a mass of explosive material

90. In this embodiment a sheet of detonation insulating material, or detonation delay material, 91, has a number of apertures 92 therein each intended to initiate detonation in the surface of the mass 90 beneath the sheet 91. By printing, stencilling, or other means known in the printing/photographic arts, explosive material 93 is deposited on the surface of sheet 91 and is of such configuration as to provide paths of equal length from each aperture 92 to a detonation centre 94.

With the arrangement, on detonation at point 94, the detonation fronts run to all the apertures 92 by paths of equal length so as to arrive at apertures 92 simultaneously. The detonation fronts arriving at aperture 92 simultaneously initiate detonation in a sheet 94 of an explosive material of faster detonation speed than the mass 90, between the sheet 91 and the mass 90, thereby to afford the initiation of "mushroom'' detonation fronts in mass 90 to obtain the advantage described above.

It will be appreciated that the foregoing examples have been shown in proportions most suited to illustrate the invention and are not intended to illustrate desirable proportions of explosive to target thickness. It will also be appreciated that whilst an arbitrary figure of 10% has been used in the specific example as the proportion by which the speed of detonation through the layer of fast detonation speed exceeds the detonation speed of the mass of explosive the invention is not limited thereto and in practise it can generally be stated that the higher the proportion by which the detonation speed of the layer of fast explosive exceeds that of the mass of explosive the greater will be the benefits. In the case of arcuate explosive masses, illustrated for example in Fig. 7, the minimum proportional difference in the detonation speeds of the layer 62 and the mass 61 will be dependant upon the circumferential lengths of the target surface 63a and the layer 62.

As stated hereinbefore the object of the layer of faster detonation speed is to accelerate the detonation of the surface of the mass in contact with the layer rather than to add to the "magnitude" of the detonation and for this reason the layer may be less than 3mm in thickness and in most cases less than 1mm in thickness.

Whilst the present invention has been described by way of example with reference to specific embodiments the invention is not restricted thereto and many modifications and different applications of the invention will be apparent to persons skilled in the art. By way of example, it will be apparent to persons skilled in the art that in the Fig. 5 embodiment a small detonator/primer may be inserted down the tube 53 to directly initiεte detonation of the high, speed layer 51, or, using the arrangement as illustrated in Fig. 5 the tube 53 and explosive 54 may be upstanding in an inverted conical recess in the mass 52 thus providing on air space surrounding the upper regions of tube 53 to prevent detonation initiation of the upper regions of mass 50 through the tube 53.

Claims

CLA IMS

1. A method for detonating a mass of explosive material characterized by the steps of applying to said mass of explosive material a layer of a second explosive material having a faster detonation speed than said mass of explosive material and detonating said layer of second explosive material to effect detom tion of said mass of explosive material.

2. A method as claimed in claim 1 characterized by the steps of applying the layer of second explosive material to the surface, or a surface, of the mass of explosive material remote from the target engaging surface of said mass of explosive material.

3. A method as claimed in claim 1 characterized by the steps of applying the layer of second explosive material between the target surface and the mass of explosive material.

4. An explosive arrangement, for use in the methods of claim 1, 2 or 3, characterized by a mass of explosive material with a layer of a second explosive material, of faster detonation speed than said mass of explosive material, applied to one surface of said mass.

5. An explosive arrangement as claimed in claim 4 characterized in that said layer of second explosive material has a thickness of less than 3 mm.

6. An explosive arrangement as claimed in claim 4 or 5 characterized in that said layer of second explosive material has a thickness of less than 1 mm.

7. An explosive arrangement as claimed in claim 4, 5 or 6, characterized in that said layer of second explosive material is continuous.

8. An explosive arrangement as claimed in any one of claims 4 to 7 inclusive characterized in that a shock wave delay element presents a surface intended to face a target, a mass of explosive material is applied to a surface or surfaces of said element other than the target facing surface and said layer of a second explosive material is applied to the surface or surfaces of the mass of explosive material most remote from said target facing surface of said element.

9. An explosive arrangement as claimed in claim 8 characterized in that said shock wave delay element is of elongate form and said explosive mass and said layer of second explosive material extend continuously along the length direction of said element.

10. An explosive arrangement as claimed in claim 4, 5, 6 or 7 characterized in that a plurality of discrete masses of explosive material are contacted by a common layer of second explosive material.

11. An explosive arrangement as claimed in any one of claims 4 to 10 inclusive characterized in that the detonation speed of said layer of second explosive material is at least 5% greater than the detonation speed of the mass of explosive material.

12. An explosive arrangement as claimed in claim 4, 5, 6, 7 or 8 characterized in that said layer of second explosive material is discontinuous.

13. An explosive arrangement as claimed in claim 12 in which said second layer of explosive material includes apertures or comprises strips of shapes intended to impose a desired detonation pattern in the mass of explosive material.

14. An explosive arrangerent as claimed in any one of claims 4 to 13 inclusive characterized in that a plurality of layers of second explosive materials of different detonation speeds are arranged in stacked relationship on said mass of explosive material.

15. An explosive arrangement as claimed in claim 14 characterized in that said layers of different detonation speeds are of different thicknesses or of non-uniform thicknesses.