METHOD AND APPARATUS FOR DETONATING EXPLOSIVES

Abstract: This invention relates to explosives and, more particularly, to a method of and apparatus for detonating secondary explosives. Slapper-type detonators are well known in the art but all such prior art devices teach the use of thick slapper-plates, which require heavy and precision machined parts for the detonator. The present invention proposes a method for detonating secondary explosives, and a detonator for practising the method, wherein a microfine primary charge (25) of readily deflagratable and detonatable material is fired in a chamber (23, 28), one thin wall (24) of which is spaced from the secondary explosives (22) to be detonated. When the pressure in the chamber (23, 28) reaches a predetermined value the thin wall (24) separates from the chamber (23, 28), the pressure drop sensitises the detonatable elements of the primary charge (25) and said elements detonate and accelerate the thin wall (24) against the secondary explosives (22) to impart a self-sustaining detonation front therein.

Description: METHOD AND APPARATUS FOR DETONATING EXPLOSIVES This invention relates to explosives and, more particularly, to a method of and apparatus for effecting detonation of an explosive mass. It is well known in the art that most conventional bulk explosives, often referred to as and hereinafter called "secondary" explosives, are resistant to detonation even when provided with so-called sensitizers, which may comprise void forming means, such as glass spheres, or other sensitizing materials. For safety purposes it is an increasing practise to make such secondary explosives more resistant to, rather than more sensitive to, detonation. / Secondary explosives materials can only be detonated by an externally-generated detonation wave, of sufficient magnitude as to induce a self-sustaining detonation wave in the said secondary explosives material, or by a shock wave induced by impact and of sufficient magnitude as to generate a self- sustaining detonation wave in said secondary explosives material. In all the prior art methods for inducing detonation in a secondary explosives mass the detonation means include a primary charge of rapidly deflagratable and/or detonatable material or materials, such as lead azide, lead styphnate, salts of fτ___minic acid, gun powder or other such easy to deflagrate/detonate materials, capable of being fired by heat and/or a shock wave of. lower value than that required to detonate a secondary explosives material. The above recited prior art primary charge materials are capable of detonation and/or include a high explosive material, such as RDX, PETN or TNT, in particle form to render the primary charge more sensitive to detonation. Such a readily deflagratable and detonatable charge is, hereinafter, referred to as "a primary charge of the type defined". Aproblem has always existed in translating the energy generated by the primary charge to a self-_n__3ta__ning detonation front in the secondary explosives material. One well . known prior art method of detonating secondary explosives materials comprises a primer and a detonator. The detonator for such method comprised a primary charge of readily deflagratable material, or a primary charge of the type defined, and the said primary charge was fired by a flame front, such as is generated by a slow fuse, or electrically, using a spark or heating element, or by a shock wave generated by, for example, a detonation cord. The primer comprised an explosives material, having a mass many times greater than the primary charge, capable of being detonated by the fire front and/or the pressψre or shock wave generated by the firing of the primary charge. Thus, on firing, the detonation proceeded in three steps, the primary charge fired to initiate detonation of the primer and the primer, on detonation, initiated detonation of the - 2 - secondary explosives material. This method of detonation is clumsy in practise, partially because of the problems in keeping the elements in the desired intimate contact to ensure good transmission of the energy generated at each stage, the detonator or primer often mis-fired and failed to detonate the secondary explosives, and partially because, for obvious reasons of safety, it was necessary for the detonators the primers and the secondary explosives to be stored remote from one another and brought into their respective positions only at the last possible moment. Further, the system was limited in its applications . In another prior art arrangement for detonation of a secondary explosives material the primary charge is fired to drive a so-called "flyer" or "slapper" plate at high speed against the secondary explosives material. Detonators proposed for practising this method of detonation are disclosed in, for example, U.S.A. Patent No. 2883931, U.S.A. Patent No. 3062143 and U.S.A. Patent No. 3978791. In all the prior art slapper-plate type detonators described in the above identified prior art the primary charge is contained in a chamber one wall of which is defined by an annular plate located against an annular shoulder whereupon, on firing of the primary charge, there is a pressure build up behind the annular plate until the unsupported central regions of the plate shear away from the annular shoulder-supported rim thereof and the sheared central portion, comprising the slapper-plate, is driven along a passage towards a target secondary explosives,in similar manner to a bullet or shell being driven along a gun barrel. All the prior art slapper-plate type detonators proposed todate stress the importance of the leading face of the slapper-plate being parallel to the surface of the target secondary explosives to be impacted at the moment of impact and, to avoid deflection of said leading face from the desired plane, all the . prior art proposals teach the use of thick slapper-plate discs, and whereupon the sheared central portion has a thickness equal to or greater than a third of its diameter. The use of such thick slapper-plates requires that the pressure necessary to shear the slapper-plate away from the rim and to drive the thick slapper-plate against the target secondary explosives with the desired velocity is substantially, often greater than 50,000 p.s.i., and the walls of the chamber containing the primary charge and the walls of the passage along which the slapper-plate is driven must be capable of withstanding such pressure without failure. In order to so contain the high pressures developed by the primary charge the prior art proposals teach the use of thick chamber and passage walls and, for this reason, most parts of the detonator are thick- walled, heavy and require precision macihiiiing which makes them expensive and thereby unattractive for general commercial practise. Further, and because of the necessity for containing the detonating primary charge until the slapper-plate has sheared, the prior art proposals have been limited to gas- sealed combustion chambers and the firing of the primary charge has been limited to electrical firing means, which limits their range of application. In another proposed arrangement for effecting detonation of a secondary explosives, disclosed in for example the U.S.A Patent No 4727808, a thin-walled detonator includes a primary charge of fine particles separated from the secondary explosives by a thin wall, generally having a hole therethrough, and the primary charge is fired at that end remote from the secondary explosives. - On detonation of the primary charge a shock wave therefrom ' accelerates through the said hole to effect detonation of the secondary explosives. It should be noted that in this latter method of detonation the secondary explosives is detonated directly and wholly by the shock wave generated by the primary charge In this specification (a) the term "thin" when applied to a wall shall mean that the thickness of the wall is equal to or less than one fifth of its major dimension, (b) the term "thin-walled" when applied to a tubular element or sleeve shall mean that the thickness of the wall of the element is equal to or less than one fifth of the diameter of the bore of said element, and (c) the term "microfine" when applied to a solids material in divided form shall mean that the greater part of the particles in said material have a major dimension less than 5 microns. An object of the present invention is to provide a method for effecting detonation of a secondary explosives material and which method is reliable in operation and relatively inexpensive to practice. According to the present invention there is provided a method for detonating a secondary explosives material comprising the steps of confining a primary charge of the type defined in a chamber, locating the said chamber adjacent to but spaced from the secondary explosives material to be detonated, and initiating deflagration of the primary charge, characterised by the steps using a microfine primary charge, forming the said chamber to define one thin wall, locating said thin wall to be directed towards said secondary explosives material and spaced therefrom by an air gap between 5mm and 20 mm, and arranging the said thin wall, in whole or in part or parts, to separate from the chamber when the pressure against said wall, generated by the deflagrating primary charge, has reached a predete__mined value to cause said separated part or parts of said thin wall to be accelerated across said air gap whereupon to strike said secondary explosive material and thereby induce a detonation front in the secondary explosive material. Preferably said method is characterised by the steps of weakening the chamber and /or said thin wall to define a predetermined line or lines of weakness along which the said wall or a wall part or parts will separate from the said chamber when subjected to said predetermined pressure. The said predetermined pressure is conveniently defined as the pressure at which the said wall or wall parts separate from the chamber and the said pressure can be varied by the manner in which the said lines of weakness are determined and applied to the container. The invention also envisages a detonator for carrying out the method according to the invention, characterised by a tubular element, means supporting an ignition means closing one end region of said tubular element, an internal thin wall formed separately from said tubular element located in the bore of said tubular element intermediate the ends of said bore, and a primary charge, of the type defined, located between said means supporting the ignition means and said internal thin wall. Preferably the said tubular element is of of thin- walled construction and of substantially uniform cross section throughout its length. In a prefered embodiment the detonator is characterised in that said means supporting an ignition means comprise a second tubular element, of thin-walled construction, located in one end region of the first tubular element, one end of said second tubular element has said ignition means supported therein, the other end of said second tubular element locates said internal wall within the first tubular element and said second tubular element contains the said primary charge. Preferably the detonator is characterised in that said second tubular element is a sliding fit in the bore of said first tubular element. In a prefered embodiment in apcordance with the invention the detonator is characterised by abutment means for limiting the penetration of the second tubular element into said first tubular element. In one embodiment in accordance with the invention said internal wall located by said second tubular element is separately formed from said second tubular element and comprises a metal element, a rigid plastics element or an explosives material. In another embodiment said internal thin wall is formed integral with said second tubular element. Preferably said ignition means comprise flame-front producing means, conveniently defined by a tubular plastics element having its bore coated with a deflagratable or detonatable material. In another embodiment said ignition means comprise electrical ignition means. In a prefered embodiment the detonator according to the invention is characterised in that the said first tubular element is open at that end remote from said means supporting an ignition means and a target secondary explosives material is located in said open end and is spaced from said internal wall by an air gap having an axial length between 5mm and 20 mm. With such an embodiment, and contrary to the prior art proposals, the open end of said tubular element is insertable directly into a mass of secondary explosive to be detonated and the surface of the secondary mass exposed within the bore of the first tubular element comprises the surface of said secondary explosives material in which detonation is to be initiated. In another embodiment that said end of said first tubular element remote from said ignition means is closed by an end wall,- a target secondary explosives material is disposed in that end of said first tubular element closed by said end wall and the said first and second tubular elements are so axially related that an air gap, having an axial length between 5mm and 20 mm, separates the said thin internal wall from the free surface of said target secondary explosives material. Preferably the said air gap has an axial length between 7 mm and 12 mm. In one form the detonator according to the invention is characterised by a delay element located between said ignition means and said primary charge. In a preferred embodiment the said second tubular element and/or said internal thin wall is weakened to define a predetermined line or lines of weakness along which the said internal thin wall or a part or parts thereof will separate from the second tubular member when the internal thin wall is subjected to said predetermined pressure. The present invention recognises that whilst the sensitivity to deflagration of a primary charge of the type defined increases with increase in temperature and pressure the charge is rendered less sensitive to detonation by a gradual increase in pressure . However, if a pressure build-up in the deflagrating charge is rapidly released the charge, and/or its detonatable component or components, is rendered very sensitive to detonation and is readily driven into detonation by the sharp pressure drop-Che detonation of the primary charge elements contributes greatly to the acceleration of the internal wall or the internal wall parts towards the secondary explosives material. Thus, in the method and the detonator proposed by the present invention the internal thin wall can separate from the container, in whole or in parts, at relatively low pressures, sometimes as low as 1000 p.s.i. and in some cases as low as 500 p.s.i. depending upon the composition of the primary charge, and still effectively strike the secondary charge with sufficient energy as to ensure detonation thereof. The present invention also recognises that, contrary to the teachings of prior art slapper- type detonators, it is not essential for the slapper-plate to impact the second-try explosives material in one piece and, equally, that it is not essential that the leading face of the slapper-plate be parallel to the target surface of the secondary explosive on impact. The said internal wall may conveniently comprise a metal or rigid plastics material. Preferably the first and second tubular elements include cooperating means for varying the axial length of the air spacing between the said internal wall and the end of the first tubular element and, conveniently, such cooperating means may comprise screw threads or bayonet fittings between the first and second tubular elements which allow the relative positions of said elements to be adjusted. Preferably the primary charge comprises a solids material in divided form and may conveniently comprise or include RDX, PETN, HMX, gunpowder, flash powder or any other material capable of sustaining deflagration when confined. Preferably the axial length of the primary charge is less than twice the diameter of the bore of the tubular element containing the said charge and, more preferably, the axial length of the primary charge is less than the diameter of the bore of the tubular element containing the said charge. The invention will now be described further by way of example with reference to the accompanying drawings in which, Fig. 1 shows, diagraiπmatically and in longitudinal cross section, one simple detonator in accordance with the invention, Fig. 2 shows, diagrammatically and in longitudinal cross section, a second embodiment of a detonator in accordance with the invention, Fig. 3 shows, diagrammatically and in longitudinal cross section, a third detonator in accordance with the invention, Fig. 4 shows, diagrammatically and in longitudinal cross section, a fourth detonator in accordance with the invention and, Fig. 5 shows, diagrammatically, a longitudinal cross section through the detonator shown in Fig. 4 with the detonator in a condition for use. In the example illustrated in Fig. 1 the detonator comprises a tubular element 11, having ends 11a.and lib, with a separately formed internal wall 12, which may comprise a metal or rigid plastics material, closing the mid-regions of the bore lie of the element 11. The internal wall 12 is thin, as hereinbefore defined, and may be a friction fit in the bore lie or held by an adhesive in said bore, thus to maintain said wall 12 in a desired position during normal handling and render said wall 12 resistant to displacement up to a predetermined pressure value. The resistance of said thin wall 12 to displacement from its "held" position determines said predetermined pressure. The end wall 11a of the element 11 is closed by means supporting an ignition means, in this case an annular element 13 of a resilient material which has its outer diameter tigjht fitting in the bore lie of element 11 and its bore region firmly receiving the end of a detonator tube 14. A restrictor plate 15 is firmly inserted into the bore lie ahead of the leading radial face of the annular element 13 and the plate 15 has a bore 15a., aligned with the bore of the detonating tube 14 but of smaller diameter than the bore of tube 14. The detonation tube 14 may comprise a slow fuse or a detonation tube, as defined hereinafter. That bore lie between the plate 15 and the internal wall 12 is filled with a microfine primary charge 16 of the type defined. In use, the end lib of the tubular element 11 is inserted into a mass of secondary explosive M to be detonated and the depth of insertion of the tubular element 11 into the mass M is sufficient to leave an air volume 17 between the internal wall 12 and the free surface ML of the mass M within the bore lie. Die axial length of the air volume 17 is important, as will become apparent hereinafter. In order to judge the correct depth of penetration for the tubular element into the secondary explosives M, to establish a desired air gap, that part, of the external cylindrical surface of the tubular element 11 to be inserted into the mass M may be differently coloured from the remainder of the element 11. In an alternative arrangement a flange 18, shown in broken line in Fig.l, is located on the tubular element 11 and serves to abut the mass M to limit the penetration of tubular element 11 into the mass M. When the detonation tube 14 is fired a flame front is delivered through the aperture 15a_ in the wall 15 to initiate deflagration of the primary charge 16. In the initial stages of deflagration the restrictor plate 15, backed by the annular element 13, contains the primary charge 16 so that during the initial part of the deflagration of charge 16, the loss of pressure gases through the aperture 15a is small. Because the primary charge 16 is microfine the rate of deflagration through the charge 16 is very rapid and as the pressure and temperature generated by the microfine deflagrating charge 16 increase the rate of deflagration increases until the pressure on that face of the wall 12 facing the charge 16 reaches said predetermined pressure to cause the internal wall 12 to breaks away from its securement with the tubular element 11. At this stage the restrictor plate 15 may have been blown out of the element 11 and the element 11 may be failing under the internal pressure but the very rapid rate of deflagration ensures that the wall 12 will break away from the element 11 under pressure and be directed towards the surface Ml. With the breakaway of the wall 12 from the element 11 the pressurized deflagrating charge 16 is released towards the target secondary explosives material M and, with the rapid fall in pressure, the unconsumed elements of the primary charge are rendered sensitive to detonation and detonate to drive the wall 12 into high velocity impact with the surface Ml to induce a self-sustaiiiing detonation front in the secondary explosives material M. In the eπ_b-K_iment illustrated in Fig. 2 the detonator comprises a tubular element 21 with ends 21a, 21b and a bore 21c. That end region of the element 21 adjacent end 21b_ is closed by a plug 22 of a target secondary explosive material. A second tubular element 23, having one end closed by an integral end wall 24, has an annular recess R adjacent the end wall 24 defining a line of weakness. The element 23 is inserted into the open end 21a_ of tubular element 21, with the internal wall 24 leading. The bore 23ji of tubular element 23 is charged with a microfine primary charge 25 and the open end 23b of the element 23 is closed by a means supporting an ignition means and which may, as illustrated, conveniently comprise an assembly identical with that described above with reference to Fig. 1 and thus comprises an annular element 26, a detonation tube 27 and a restrictor plate 28. In use the detonator illustrated in Fig. 2 is pushed, end 21b leading, into a mass of secondary explosive material to be detonated. On firing of the detonator tube 27 a flame-front passes through the aperture 28a. in restrictor plate 28 to initiate deflagration of primary charge 25. The deflagration of charge 25 continues, increasing the temperature and pressure within the element 23, until the pressure reaches that point at which the element 23 fails along the line of weakness R and the wall 12 is free to displace towards explosive plug 22. As the wall becomes free of the element 23 unconsumed primary charge is released towards the plug 22 and, with the sudden release of pressure, said charge goes into detonation to drive the wall 24 at very high velocity against the plug 22 to initiate detonation of said plug 22. Further, and as with the Fig. 1 erobodiment, the tubular element 21 is of thin-walled construction, as defined hereinbefore, as also is the tubular element 23. As will be seen from Fig. 2 the end 23b of the second tubular element 23 is deflected outwardly, in the form of a bell-end, to define an abutment engageable with the end 21a of the first tubular element 21 and serves to limit the penetration of the element 23 into the element 21. By this means, and with the axial length of the second tubular element known, the axial length of the tubular element 21 between end 21a and the free face of the plug 22 within the element 21 can be calculated to obtain the desired air gap between the said plug 22 and the internal wall 25. With the abutment of end 23b against end 21a the tubular elements 21 and 23 may be secured against relative axial displacement, as for example by crimping. In the detonator illustrated in Fig. 3 a tubular element 31 is closed at one end by an integral end wall 32 and that end of the bore 31a of the element 31 adjacent the end wall 32 is charged with a target secondary explosive material 33. The open end 31b of element 31 is closed by a second tubular element 34 having an end wall 35 formed integral therewith. The end wall 35 has a circular recess C in its exposed face defining a weakened aimulus in the wall 35. The element 34 is slideably inserted into the bore 31a of element 31, end wall 35 leading, and the open end 34a of tubular element 34 is closed by an annular element 36, the cylindrical surface of which is tight fitting in the bore 34b_ of the second tubular element 34, and the bore of the annular element 36 firmly retains a delay element 37. The volume within tubular element 34 is charged with a primary charge 38. The delay element 37 conveniently comprises a tubular element 39, the bore 39ja of which opens to the primary charge 38. Electrical ignition means, illustrated as a coil 40, are entered into the bore 39a of tube 39 remote from the second tubular element 34 and the bore 39a ^ of the tubular element 39 is charged with an exotheιrπύc-l__rning composition 41, preferably a gas-less mixture of solid oxidising and reducing agents. Such exotheπrάc-burning materials are well known in the art and may for example comprise such mixtures as boron-red lead, boron-red lead silicon, boron-red lead-dibasic, lead phosphite, aluminium-cupric oxide, magnesium-barium peroxide- selenium and silicon-red-lead. With this arrangement the firing is initiated by an electrical impulse to the coil 40 within the delay material and which initiates firing of the said delay material. The ignition front of the delay material travels slowly along the tube 39 and, after the predetermined delay, produces a flame-front at that end of the tube 39 exposed to the primary charge 38, thus causing the primary charge 38 to fire. Once the charge 38 fires the deflagration continues, in exactly the same way as that described with respect to Figs. 1 and 2,until the end wall 35 fails along the weakened annulus C, releasing the central part of the end wall defined by the recess C. The central part of the end wall 35 is accelerated towards the free surface of the explosive plug 33 and, again as in the examples illustrated in Figs 1, and 2, with the failure of the wall 35 the unconsumed primary charge 38 is driven into detonation to accelerate the separated end wall part against the free surface of the target secondary plug 38 to induce a self-sustaining detonation front in said plug. In the embodiment illustrated in Figs. 4 and 5 the detonator comprises a tubular element 51 having a screw thread 51a_, rolled into its cylindrical wall. One end of the element 51 is closed by an integral wall 52, having a central opening 52a therethrough and the other end of the element 51 is closed by a wall 53 which is tight fitting in the element 51. A microfine primary charge 54 is packed in the tubular element 51 between the walls 52 and 53 and a detonation tube 55 is tightly inserted through the central opening 52a whereupon the bore of the detonation tube 55 is open to the primary charge 54. In the illustrated embodiment the external diameter of the detonation tube 55 is smaller than the internal diameter of the sleeve 51 and thus, when the charge 54 is fired, the escape of gases down the tube 55, and the loss of gases through the central opening 52a should the tube 55 be blown out of the central opening 52a is restricted. An outer tubular element 56 has an screw thread 56a formed thereon,cooperating with the screw thread 51a_ on the element 51, and element 56 includes a radial wall 57 at one end thereof which is pierced by an aperture 56b to allow the detonation tube 54 to pass therethrough and, at its end remote from the radial wall 57, the element 56 is closed by a plug 58 of a secondary explosives material. As will be seen from the illustrations the internal axial length of the tubular element 56 between the end wall 57 and the plug 58 is greater than the axial length of the element 51. Thus, the element 56 can be rotated relative to the element 51 and, via the cooperating screw threads 51a and 56a, the element 51 can be displaced axially displaced relative to the element 56 between a "safe" condition, as illustrated in Fig. 4 and wherein the plug 58 is immediately adjacent the plate 53, and a condition for firing, as illustrated in Fig. 5, wherein the end wall 52 abutts the wall 57 and the plug 58 is spaced a predetermined distance from the plate 53. As with the embodiments illustrated in Fig. 1, 2 and 3, when the detonator is in a condition for firing an air gap is established between the plate 53 and the explosive plug 58. It will be noted that in the Figs. 4 and 5 embodiment, with the tubular element 56 axially displaced to the safe position illustrated in Fig. 4, on accidental firing of the primary charge 54 and on build up of the pressure generated thereby to blow the wall 53 out of the element 51, the wall 53 will immediately strike the plug 58 and will not have accelerated sufficient to generate the energy necessary to cause detonation of the plug 58 and, because the pressure generated by the deflagrating charge 54 will not be rapidly reduced towards the plug 58 to sensitise the charge 54 to detonation, the plug 58 of secondary explosives material will not be detonated. To prepare the detonator illustrated in Fig. 4 for use the element 56 is simply rotated, to effect axially displaced of the element 51 to the position shown in Fig. 5, whereupon the plate 53 is axially spaced from the plug 58. Thereafter the detonator is inserted into a mass of secondary explosive, which may be a solids, plastics or liquid explosives, and on firing of the primary charge 54 by the combustion/detonation front released from the adjacent end of the detonation tube 55, the primary charge 54 is driven to deflagration, the rate of deflagration increases as the pressure generated by the deflagrating charge 54 increases and, on the internal pressure reaching the pressure necessary to blow the wall 53 out of the sleeve 51 said wall is driven towards the plug 58 and accelerated by detonation of the primary charge 54. The detonation tubes 15 and 55 referred to above may comprise commercially available detonation tubing comprising a hollow tube of a plastics material with a dusting of microfine co bustible/detonatable material on the surface of its bore. Such detonation tubing is normally fired by detonating a percussion cap, at that end of the tube most remote from the detonator and, on firing of the percussion cap, the flame front therefrom ignites or detonates the dusting of material on the bore of the tube and the combustion/detonation of said material continues along the tube to ignite the primary charge. As stated hereinbefore the axial spacing of the internal wall from the target secondary explosives material is of great importance in practising the present invention and the following experiment was used to determine said limits. 70 detonators, identical with the embodiment illustrated in Fig. 1 were made up, each detonator contained a primary charge of microfine PETN of 0.3gms at a density of 0.23gms/cm 3 10 of said detonators were inserted into a bulk explosive and the air gap was adjusted to an axial length of 4mm and the detonators were fired. Nine of the detonators failed to detonate their respective bulk explosives and only one detonation was successful. 10 of said detonators were inserted into a bulk explosive and the air gap was adjusted to an axial length of 18mm and the detonators were fired. 8 of the detonators detonated their respective bulk explosives 10 of said detonators were inserted into a bulk explosive and the air gap was adjusted to an axial length of 20mm and the detonators were fired. All 10 detonators failed to detonate their respective bulk explosives 10 of said detonators were inserted into a bulk explosive and the air gap was adjusted to an axial length of 10mm and the detonators were fired. All 10 of the detonators detonated their respective bulk explosives 10 of said detonators were inserted into a bulk explosive and the air gap was adjusted to an axial length of 17mm and the detonators were fired. All 10 of the detonators detonated their respective bulk explosives 10 of said detonators were inserted into a bulk explosive and the air gap was adjusted to an axial length of 5mm and the detonators were fired. Nine of the detonators detonated their respective bulk explosives. 10 of said detonators were inserted into a bulk explosive and the air gap was adjusted to an axial length of 7mm and the detonators were fired. All 10 of the detonators detonated their respective bulk explosives. Thus, on the basis of such experiments all the detonators arranged with an axial spacing between 5mm and 17 ran successfully fired their respective bulk explosives. In all the above described embodiments the primary charge comprised PETN but in other experiments said charge comprised or included RDX, HMX, gunpowder, and/or flash powder and all such materials showed themselves capable of sustaijiing deflagration when confined and detonation when subject to a rapid pressure drop. The tubular elements 11, in the above experiments conveniently comprised thin walled aluminum tubing having a wall thickness less than one tenth of the bore diameter. Whilst it may be held that the presently proposed relatively weak tubular elements will readily fail and burst as the pressure generated by the deflagrating primary charge increases it has been shown in practice that the rate of deflagration through said charge is so rapid that, on failure of the internal wall, sufficient of the charge is always released towards the target secondary explosives material as to accelerate the slapper component sufficient ensure detonation of the target explosive and the detonators proposed by the present invention have proved in practice to be more reliable than the prior art slapper type detonators. As the prior art slapper-plate disclosure stress the necessity of m_ι_ _taining the face of the slapper parallel with the face of the target explosive experiments were conducted to test this teaching. In these experiments 40 detonators were constructed identical to the embodiment illustrated in Fig.l. 20 of such detonators were cut to define an axial length of lQmm between the internal wall and the end of the element 11. All 20 detonators were fired against a sheet of aluminium and the indentations caused by the internal walls were examined. In only three cases were the indentations indicative of the internal wall striking the aluminium plate parallel to said plate and in all the other cases the indentations were indicative of the internal plates striking the plate at an angle to the plate and in at least 4 cases indicative of the wall being fragmented. The rema__ning detonators were placed on bulk explosives, the air space was adjusted to an axial length of 10mm and the detonators were fired. In all 20 cases the detonators successfully detonated their respective bulk explosives. Thus, from these experiments, it was clearly shown that a thin wall, in whole or in part, efficiently detonated a secondary explosives material and that the detonation of the primary charge was responsible for accelerating the thin wall to such a velocity as to impact the secondary explosives material with the energy necessary to impart a self-sustaiiiing detonation front therein. In all the above described embodiments the primary charge had an axial length equal to the diameter of the bore of the cliarge-containing tubular element. ■The density of the primary charge will, to a degree, be dependant upon its composition but, using PEΪN or HMX as the charge or the principal component of the charge, the density can vary between 0.1 and 1.96 gms/cm , preferably between 0.2 and " 1.0 gms/cm? and most pre ferably between 0.2 and 0.6 gms/cm .

Claims: CLAIMS 1 A method for detonating a secondary explosives material comprising the steps of ccmfining a primary charge ofthe type defined in a chamber, locating the said chamber adjacent to but spaced from the secondary explosives material to be detonated and initiating deflagration of the primary charge, characterised by the steps of using a microfine primary charge, for iiig the said chamber to define one thin wall, locating said thin wall to be directed towards said secondary explosives material and spaced therefrom by an air gap between 5mm and 20 mm, and arranging the said thin wall, in whole or in part or parts, to separate from the chamber when the pressure against said wall, generated by the deflagrating primary charge, has reached a predetermined value to cause said separated part or parts of said thin wall to be accelerated across said air gap whereupon to strike said secondary explosive material and thereby induce a detonation front in the secondary explosive material. 2. A method according to claim 1 characterised by the steps of weakening the chamber and /or said thin wall to define a predetermined line or lines of weakness along which the said wall or wall parts will separate from the chamber when the wall or wall part or parts are subjected to said predetermined pressure. 3. A detonator, for carrying out the method according to claims 1, or 2, characterised by a tubular element, means supporting an ignition means closing one end region of said tubular element, an internal thin wall formed separately from said tubular element located in the bore of said tubular element intermediate the ends of said bore, and a primary charge, of the type defined located between said means supporting the ignition means and said internal thin wall. 4. A detonator according to claim 3 characterised in that said tubular element is of thin-walled construction and of substantially uniform cross-section throughout its length. 5. A detonator according to claims 3 or 4 characterised in that said means supporting an ignition means comprise a second tubular element, of thin walled construction, located in one end region of the first tubular element, one end of said second tubular element has said ignition means supported therein, the other end of said second tubular element locates said internal thin wall within the first tubular element, and said second tubular element contains the said primary charge. 6. A detonator according to claim 5 characterised in that said second tubular element is a sliding fit in the bore of said first tubular element. 7. A detonator according to claim 6 characterised by abutment means for limiting the penetration of the second tubular element into said first tubular element. 8. A detonator according to claims 5, 6 or 7, characterised in that said internal thin wall located by said second tubular element is separately formed from said second tubular element and comprises a metal element, a rigid plastics element or an explosives material. 9. A detonator as claimed in claim 5, 6 or 7, characterised in that said internal thin wall is formed integral with said second tubular element. 10. A detonator as claimed in any one of claims 3 to 9 inclusive, characterised in that said ignition means comprise flame-front producing means. 11. A detonator according to claim 10, characterised in that said flame-front producing means comprise a tubular plastics element having its bore coated with a deflagratable or detonatable material. 12. A detonator according to any one of claims 3 to 9 inclusive, characterised in that said ignition means comprise electrical ignition means. 13 . A detonator according to any one of claims 3 to 12 inclusive, characterised in that the said first tubular element is open at that end remote from said means supporting an ignition means and a target secondary explosives material is located in said open end and is spaced from said internal wall by an air gap having an axial length between 5mm and 20 mm. 14. A detonator according to any one of claims 3 to 12 inclusive, characterised in that said end of said first tubular element remote from said ignition means is closed by an end wall, a target secondary explosives material is disposed in that end of said first tubular element closed by said end wall and the said first and second tubular elements are so axially related that an air gap, having an axial length between 5mm and 20 mm, separates the said internal thin wall from the free surface of said target secondary explosives material. 15. A detonator according to claims 13 or 14, characterised in that the said air gap has an axial length between 7 mm and 12 mm. 16. A detonator according to any one of claims 3 to 15 inclusive characterised in that a delay element is located between, said ignition means and said primary charge. 17. A detonator according to any one of claims 5 to 16 inclusive, characterised in that the said second tubular element and/or said internal thin wall is weakened to define a predetermined line or lines of weakness along which the said internal thin wall or a part or parts thereof will separate from the second tubular member when the internal thin wall is subjected to said predetermined pressure. 18. A detonator according to claim 15, 16 or 17, characterised in that said first and second tubular elements include cooperating means for varying the axial length of the said air gap. 19. A detonator according to any one of claims 3 to 18 inclusive, characterised in that the axial length of the primary charge is less than twice the diameter of the bore of the tubular element containing the said charge. 20. A detonator according to claim 19, characterised in that the axial length of the primary charge is less than the diameter of the bore of the tubular element containing the said charge.

Inventors: SHANN PETER CHRISTIAN, SHANN, PETER, CHRISTIAN
Applicants: EXPLOSIVE DEV LTD, EXPLOSIVE DEVELOPMENTS LIMITED
Application year: 1990
Main classification: F42D1
IPC codes: F42B3, F42D1