Patent Publication Number: US-2004055495-A1

Title: Tin alloy sheathed explosive device

Description:
TECHNICAL FIELD  
       [0001] This invention relates generally to linear explosive devices, such as detonating cords and linear shaped charges, having a lead-free tin alloy sheath or liner and related systems using such devices. The invention further relates to a lead-free tin alloy sheathed devices and related systems using them.  
       BACKGROUND OF THE INVENTION  
       [0002] Certain types of energetic linear shaped explosive devices, such as detonating cords and linear shaped charges, can be used for severing, cutting, fracturing, or impacting a target material or structure. These devices have a wide range of potential uses, such as automotive and aircraft safety systems, aircrew escape systems, military weapon ignition systems, and commercial blasting.  
       [0003] One example of how such devices may be used is an aircraft canopy fracturing system. These systems help reduce bodily impact with and/or injury from the aircraft canopy during seat ejection. As an ejection seat is jettisoned from an aircraft cockpit, it passes through the portion of the aircraft where the aircraft canopy is located during normal flight conditions. Preferably, during an emergency event that requires evacuation of the aircraft during flight, the canopy is jettisoned before seat ejection. But in some instances seat ejection begins before the canopy has been jettisoned.  
       [0004] When seat ejection begins prior to jettisoning the aircraft canopy, the ejection seat must be able to blast entirely through the canopy in order to escape from the aircraft. A canopy fracturing system helps reduce the risk to the pilot or occupant of the aircraft as the ejected seat forces its way through the canopy by creating an opening in the canopy or by weakening the structure of the canopy so that the canopy breaks apart under lower impact forces. In these systems, an explosive or pyrotechnic material inside the liner or sheath of a of a linear explosive device near the inside of the canopy propels the liner outward at a high velocity, thereby penetrating or shattering the canopy before the ejected seat can collide into the canopy.  
       [0005] In general, the construction of energetic linear products historically involves a malleable outer metallic sheath and an inner core of reactive material. This class of products is normally produced by filling a length of relatively large diameter metal tubing with the chosen energetic material, and subsequently reducing the diametral cross-section by metalworking processes such as swaging and drawing. The resultant product is greatly reduced in diameter and substantially elongated. The choice of metal tubing from which to produce these products is predicated upon the physical properties of the material, which, among other properties, must possess exceptional ductility and low work hardening.  
       [0006] Tubing manufactured from a variety of metallic lead (Pb) or lead alloys has historically been used to produce these types of products. One such example can be found in U.S. Pat. No. 3,147,707 to Caldwell. In the past, the use of lead to form the metallic sheath was desired because of its high density, which in turn yielded good penetration and shattering forces and also because of its good metalworking properties. However with the increasing awareness of environmental contamination and human health concerns regarding lead poisoning, the use of lead based products has been prohibited in many applications. Accordingly, it is desirable to find a sheath or liner material suitable for replacing metallic lead in order to address human health and welfare concerns that also possesses acceptable metalworking properties.  
       [0007] Rodney et al., U.S. Pat. No. 5,333,550, which issued on Aug. 2, 1994 discloses a “lead-free” tin alloy sheath comprised of mostly tin with the balance antimony. The alloys described in Rodney &#39;550 include a binary composition (97 percent tin and 3 percent antimony), a ternary composition (96.5 tin, 1.5 percent copper, and 2 percent antimony), and a quaternary composition (98.5 percent tin, 1 percent bismuth, 0.25 percent copper, and 0.25 percent silver). Rodney &#39;550 also allows lead to be present in the compositions as an impurity in amounts up to 1.42 percent.  
       SUMMARY OF THE INVENTION  
       [0008] The present invention is directed to tin alloys that may be used to make a sheath or liner for a wide variety of explosive devices. One embodiment of the present invention involves linear explosive devices having a sheath that is substantially lead-free. In this embodiment, the sheath is formed from a tin alloy having from about 1 to about 6 percent zinc by weight and from about 99 to about 94 percent tin by weight. The sheath at least partially encases a reactive material, such as explosive materials, deflagrating materials, pyrotechnic materials, and mixtures thereof.  
       [0009] In one embodiment, the percentage of zinc in the tin alloy sheath is about 4 percent by weight, while in another embodiment it is about 6 percent by weight. The linear explosive device may be a wide variety of devices, such as an ignition cord, a linear shaped charge, or a mild detonating cord.  
       [0010] While the present invention may have several uses and work in a wide array of systems, one embodiment involves an aircraft canopy fracturing system for creating an opening in the canopy. This embodiment involves a transparent material that forms at least part of the aircraft canopy and a linear explosive device. In one embodiment, the linear explosive device of the canopy fracturing system has a sheath formed from a tin alloy having from about 1 to about 15 percent zinc by weight and from about 80 to about 99 percent tin by weight. The reactive material of the linear explosive device of this embodiment may include explosive material, deflagrating material, pyrotechnic material, and mixtures thereof.  
       [0011] In one embodiment, the sheath of the aircraft canopy fracturing system has from about 1 to about 5 percent zinc by weight, while in another the amount of zinc present is from about 7 to about 13 percent of zinc by weight.  
       [0012] In yet another embodiment of the present invention involves a linear explosive device having a sheath that is made from a tin alloy comprising from about 1 to about 15 percent zinc by weight and from about 80 to about 99 percent by weight of tin. One embodiment is further directed toward using the tin alloy in a shaped charge explosive. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0013]FIG. 1 shows a cross-sectional view of a linear shaped charge having a tin alloy sheath contemplated by the present invention;  
     [0014]FIG. 2 shows a cross-sectional view of a mild detonating cord or of an ignition cord having a tin alloy sheath contemplated by the present invention;  
     [0015]FIG. 3 shows a cross-sectional side view of a shaped charge including a tin alloy sheath contemplated by the present invention;  
     [0016]FIG. 4 is a graph of penetration vs. standoff for the linear shaped charge of Example 2;  
     [0017]FIG. 5 is a graph of penetration vs. standoff for the linear shaped charge of Example 3; and  
     [0018]FIG. 6 is a graph of the optimum standoff distance in relationship to the coreload of the linear shaped charges of Examples 2 and 3. 
    
    
     DETAILED DESCRIPTION OF THE SUBJECT INVENTION  
     [0019] The present invention is directed toward a tin alloy material that can replace traditional lead-based materials in order to meet environmental health standards while also being compatible with metalworking requirements. In general, the invention relates to a wide variety of energetic linear or shaped products using a tin alloy metallic sheathing or liner. Referring to FIG. 1, the tin sheath  10  in which an explosive core load  12  is disposed is comprised of an alloy having 87 to 99 percent by weight tin and the balance zinc. The amount of tin in the alloy may be less than the preferred range stated above. For instance, the amount of tin in the alloy may be at least about 80 percent, or alternatively may have at least about 83 percent tin. Likewise, the upper range stated above may also be lower in some alternative embodiments. For instance, the amount of tin in the alloy may be less than about 97 percent, or even further limited to about 94 percent or less. These upper and lower ranges can also be combined in any fashion to create different ranges for the amount of tin in the alloy, such as ranges from about 80 percent to about 97 percent, from about 83 percent to about 94 percent, and so on.  
     [0020] A second component of the alloy used in the present invention is zinc. In general, higher amounts of Zinc in the alloy increases the rigidity of the material. Thus, by varying the amounts of Zinc and Tin in the alloy, it is possible to obtain different physical properties of the alloy. This allows the sheath material to be tailored to the particular application in which the device will be used.  
     [0021] In one embodiment of the present invention, the amount of zinc added to the alloy causes the rigidity of the sheath to be increased by about 5 percent or greater than the rigidity of a similarly constructed sheath made substantially of tin. More preferably, the rigidity of the sheath is increased by about 10 percent or more from the addition of zinc. Even more preferably, the addition of zinc to the alloy increases the rigidity of the sheath by about 15 percent.  
     [0022] The amount of Zinc added to the alloy also may be described in terms of its percentage by weight. For instance, the alloy forming the sheath may comprise from about 1 to about 15 percent zinc by weight and more preferably comprise from about 1 to about 13 percent zinc by weight. For purposes of this application, a high zinc alloy may have from about 7 to about 13 percent zinc or more, while a low zinc alloy may have from about 1 to about 5 percent zinc alloy. Alternatively, a high zinc alloy may have from greater than about 8 percent zinc by weight. Likewise, a low zinc alloy may have less than about 3 percent zinc. As a preferred minimum for any range of zinc stated herein, the lowest amount of zinc present is greater than about 0.1 percent zinc by weight.  
     [0023] The tin/zinc alloy of the present invention has the advantages of being compatible with metalworking requirements and also meeting human environmental health standards. In order to meet human environmental health standards, it is preferred that the alloy is essentially free of lead. The presence of other impurities or additives, however, may be possible without departing from the invention. This invention principally applies to the composition of linear energetic products, which can take many forms. For example, this invention can be applied to linear shaped charges, mild detonating cords, ribbon cords, linear time delays, pyrotechnic ignition cords, thermal-detonating cords and a variety of other related products.  
     [0024] The embodiment of FIG. 1 illustrates an external lead-free tin alloy sheath  10  having a chevron cross section that surrounds the explosive core load  12 . Preferably, the angle of the chevron shape is from about 80 to about 100 degrees, and more preferably is from about 85 degrees to about 95 degrees. In one embodiment, the angle of the chevron is about 90 degrees with a tolerance of approximately 3 degrees. The chevron may have other angles and configurations depending upon the particular application in which the device will be used.  
     [0025] In addition to the applicability of the present invention to a linear shaped charge device as shown in FIG. 1, a skilled artisan would appreciate that the tin/zinc alloy may be used in different types of explosive products. For example, FIG. 2 illustrates the tin/zinc alloy discussed herein used with an ignition cord or with a mild detonating cord. Other, shaped devices, such as the generally conical device illustrated in FIG. 3, may also benefit from use of a tin/zinc alloy. These illustrations, however, do not limit the applicability of the tin/zinc alloy to other devices, such as the ones described above.  
     [0026] Thus, the present invention may be used in the three types of linear explosive products described by FIGS. 1 and 2, but also may also be used in applications other than linear devices. A general description of each of these devices is provided below.  
     [0027] A linear shaped charge (LSC), such as shown in FIG. 1, may have a secondary detonating type of explosive, such as PETN, RDX, HNS, DIPAM, HMX, CH- 6 , PBXN- 5 , and HNS II, is loaded into tube made of a tin/zinc alloy as described herein and then processed by mechanically swaging and roll forming or stationary die swaging into a chevron shape that is capable of cutting various target materials using the Monroe effect of penetration or severence.  
     [0028] A mild detonating cord (MDC) and an ignition cord, illustrated in FIG. 2, also may have a secondary detonating type of explosive as described above that is similarly loaded into a tin/zinc alloy metallic tube and then processed by swaging and drawing into a round circular cross-section containing any specified coreload (grains/ft).  
     [0029] An ignition cord may have various fuel/oxidizer mixes of pyrotechnic material loaded into a tin/zinc alloy tube as described herein and then processed by a mechanical reduction method of swaging and drawing, so as to produce a linear product that can be used as a deflagrating ignition source for all types of propellant gas generators or solid propellant. The coreload can range from a fraction of a grain per foot to several hundred grains per foot depending upon the application.  
     [0030] As shown in FIG. 3, the tin/zinc alloy of the present invention also may be applied to shaped charges. In general, a shaped device may have a concave tamper which receives explosive material and a tin/zinc alloy liner that holds the explosive material in place and which defines and maintains the explosive material in a concave configuration to focus the energy of detonation. Upon detonation, the liner forms a penetrating jet that propels the tamper at a high velocity toward a target. The tamper need not be made of the same tin/zinc alloy as the liner, and in fact, need not be made of a tin/zinc alloy at all.  
     [0031] A skilled artisan would recognize that the present invention may be used with other types of applications than described herein, and that other cross-sectional shapes for linear devices may also be used, such as rounded, semi-circular, polygonal, or the like.  
     [0032] The hollow center of the sheath  10  may be filled with pyrotechnics either in the form of ignition powder, detonating powder, or any suitable explosive composition. Several examples of explosive compositions have been provided above, and additional compositions may be used in place of the ones described without departing from the spirit and scope of the present invention. The selection of a suitable explosive composition ultimately depends upon the application or intended used of the device.  
     [0033] The following examples further illustrate that the advantages and features of the present invention. These features include a linear shaped explosive device having a lead-free sheath or liner that is effective for use in an aircraft canopy fracturing system.  
     EXAMPLE 1  
     Metal Sheathed Detonating Cords  
     [0034] A tube was made from an alloy having a nominal composition of 92 percent tin and 8 percent zinc, and having an outside diameter of 0.825 inches and an inside diameter of 0.500 inches. The tube was filled with HNS II explosive. The ends of the tube were sealed, and the loaded tube was subsequently reduced in diameter by a swaging and drawing process. The final diameter of the linear detonating cord product was 0.074 inches.  
     [0035] A 12-inch length of the detonating cord was spiral-wrapped around a 0.250-inch diameter mandrel to examine the flexibility of the product. The cord was undamaged and demonstrated good strength and flexibility.  
     [0036] A second 12-inch length of the detonating cord was tested to determine the velocity of detonation. A velocity of 6500 meters per second was recorded, which is regarded as comparable to lead (Pb) sheathed detonating cords.  
     [0037] A third 12-inch length was bonded to the surface of a 12-inch by 12-inch square piece of 0.250-inch thick stretched acrylic, of the type used in construction of aircraft canopy transparencies. The acrylic fractured properly when the cord was detonated.  
     EXAMPLE 2  
     Linear Shaped Charges  
     [0038] As in Example 1, a tube was filled with HNS II explosive and the ends sealed. The loaded tube was progressively swaged until arriving at a linear shaped charge (LSC) geometry per FIG. 1. This geometry yielded a coreload of 20 grains per foot of explosive. A 12-inch length of the LSC was attached to a 0.50-inch thick, 2024-T4 aluminum target plate. The LSC was angled in relation to the target plate surface, so that one end was resting in direct contact with the plate (i.e., zero standoff distance) and the opposite end was spaced 0.125-inches off the surface of the plate. This tapered standoff provides a means of assessing the distance between the LSC and the target required for optimum performance. The LSC was detonated and the depth of the resultant groove in the plate was measured at one-inch intervals. The plotted data appears in FIG. 4. These data confirm the anticipated performance of a high efficiency linear cutting charge.  
     EXAMPLE 3  
     Linear Shaped Charges  
     [0039] As in Example 2, a loaded tube was processed by swaging until arriving at the dimensions shown in FIG. 1. This geometry yielded an LSC having a coreload of 40-grains per foot of explosive. The LSC was tested in the manner described in Example 2, except that the standoff from the target ranged from zero to 0.250 inches. The plot of resultant target penetration vs. standoff distance is presented in FIG. 5. These data confirm the anticipated performance of a highly efficient linear cutting charge.  
     [0040] The data from Examples 2 and 3 were then integrated to produce the plot presented in FIG. 6, showing the optimum standoff distance in relationship to the coreload of the LSC.  
     EXAMPLE 4  
     Linear Shaped Charges  
     [0041] A tube made from an alloy having a nominal composition of 96 percent tin and 4 percent zinc was loaded and processed in the manner described in Examples 2 and 3. From this loaded tube, linear shaped charges were produced in a variety of coreloads, nominally between 10 and 60 grains per foot. These various LSC specimens were then performance tested by conducting penetration tests into 2024-T351 aluminum target plates in the manner described in previous examples. The resultant data is presented in FIG. 6. It was determined that the performance of these specimens is consistent with the known performance of high efficiency lead (Pb) sheathed LSC.  
     [0042] Certain of these LSC specimens were performance tested for penetration into mild steel target plates at zero standoff, a routine quality control lot acceptance test. It was shown that these specimens performed comparably with high efficiency lead (Pb) sheathed LSC.  
     EXAMPLE 5  
     Linear Shaped Charges  
     [0043] As in Examples 1-4, a variety of tubes having a wide range of tin/zinc composition were loaded and tested for mechanical as well as detonation properties. These consisted of the following tin/zinc alloys:  
                       TABLE 1                       Sample No.   Tin (% by wt.)   Zinc (% by wt.)                                            1   about 92   about 8       2   about 94   about 6       3   about 96   about 4       4   about 98   about 2                  
 
     [0044] Each of these compositions was found to process and perform substantially equally, although it was noted that as the amount of Zinc in the composition was increased, the alloy exhibited greater rigidity. As discussed above, this feature allows the sheath of a device to be customized to the particular application at hand. It can be seen therefore that the physical requirements of the specific application can be tailored by the choice of the specific tin/zinc alloy chosen.  
     [0045] If there are any additional constituents of the tin/zinc alloy, it is preferred that they do not adversely affect the metalworking properties of the alloy. It is further preferred that any additional constituents do not raise significant human health and welfare concerns. For instance, it is preferred that the alloy used remain substantially lead-free, such as by using an alloy having no more than about 2 percent lead by weight either as an impurity or as a constituent part of the alloy. More preferably, however, the substantially free alloy has less than about 1 percent lead, and even more preferably has no lead. One embodiment of the present invention addresses these two desired features of health and workability by forming the sheath or liner from an alloy consisting essentially of tin and zinc in any of the amounts described herein. Thus, any other constituents that may be present in the alloy do not adversely affect human health and welfare concerns or desired metalworking parameters.  
     [0046] While the invention has been described in detail with reference to particular preferred embodiments, persons skilled in the art will appreciate that various alterations may be made to the invention as described without departing from the intent, spirit, and scope of the invention. As such, it is intended that these variations, adaptations, modifications, and equivalent arrangements of the present invention fall within the scope of the invention and the appended claims.