Abstract:
A warhead includes a body, a patterned liner made of plastic, and an explosive charge disposed within the liner. The liner pattern is formed of gaps and liner elements. The explosive charge includes a first set of sections that are disposed adjacent to the liner gaps and a second set of sections that are disposed adjacent to the liner elements. Upon detonation of the explosive charge and because of the temporal delay in transmitting the detonation energy between these two sets of sections, the warhead body is caused to shear and break into fragments with controlled size. The use of plastic as the liner material also provides a welcome safety feature for this warhead. In the event of unwanted heat ignition, the plastic (which is also low melt temperature material), would melt to seal the explosive and would also flow. Because of the plastic, neither sudden pressure nor heat/ignition inside the round, would therefore be as catastrophic.

Description:
U.S. GOVERNMENTAL INTEREST 
     The inventions described herein may be manufactured, used and licensed by or for the U.S. Government for U.S. Government purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to the field of ballistics, and in particular to casings of explosively formed projectiles, shaped charges, and other munitions. More specifically, the present invention pertains to the controlled fragmentation of the munition casing or warhead body. 
     BACKGROUND OF THE INVENTION 
     Warhead fragmentation effectiveness is determined by the number, mass, shape, and velocity of the fragments. By using a controlled fragmentation design, warhead fragmentation could generally be achieved quickly and cost effectively. Exemplary controlled fragmentation techniques are described in U.S. Pat. Nos. 3,491,694, 4,312,274, 4,745,864, 5,131,329, and 5,337,673. 
     In general, conventional designs use “cutter” liners that form fragments by generating a complex pattern of high-velocity “penetrators” for fragmenting the shell. Although these conventional fragmentation designs have proven to be useful, it would be desirable to present additional functional, cost and safety improvements that minimize the warhead weight, reduce manufacture expenses, and advance current United States Insensitive Munition (IM) requirements. 
     What is therefore needed is a controlled fragmentation technique through the use of a patterned liner which introduces shear stress into the warhead body and creates the desired fragmentation pattern. Fragment size, fragment numbers, and patterns thereof may be influenced through a novel liner configuration. The need for such a controlled fragmentation technique has heretofore remained unsatisfied. 
     SUMMARY OF THE INVENTION 
     The present invention satisfies these needs, and presents a munition or warhead such as a projectile, and an associated method for generating a controlled fragmentation pattern (herein referred to as “the invention” or “the present invention”). 
     According to the present invention, warhead fragmentation is achieved more efficiently and more cost effectively than conventional techniques, through the use of a relatively inexpensive formed plastic liner (or liners) with a predetermined pattern of cutouts. According to the present invention, the “shear” and “stamp” liner cutouts generate contours of localized transitional regions with high-gradients of pressures, velocities, strains, and strain-rates acting as stress and strain concentration factors. Unstable thermoplastic shear (adiabatic shear) eventually transfers the entire burden of localized strain to a finite number of shear planes leading to the shell break-up and formation of fragments. 
     According to one embodiment of the present invention, the warhead includes a liner that is disposed inside the warhead body. The liner includes a predetermined pattern that is created with gaps filled with the warhead&#39;s explosive, such allowing the detonation shock wave to directly propagate into the fragmenting case without passing through the liner. As a result, the explosion produces a complex pattern of shear planes in the warhead body, causing the case break-up and formation of fragments with predetermined sizes. This design is distinguishable from existing fragmentation liner technologies that attempt to score or cut the warhead body. 
     One of the advantages of the present embodiment compared to existing technologies is the cost effectiveness of the manufacturing process of the present design, in that it is faster and more economical to fabricate and to pattern a plastic liner, as opposed to notching or cutting the steel warhead body itself. An advantage of the present invention is that the use of plastic material reduces the overall weight of the warhead compared with use of other materials, as do the gaps which reduce the weight of the liner. But, more important, the use of plastic is a great safety feature. An unwanted ignition of the explosive due to the heat of launch would normally be catastrophic as well as fratricidal. But the plastic of this invention covers the explosive inside the casing body. In the event of unwanted heat/ignition, the plastic (which is also low melt temperature material), would melt to seal the explosive and would also flow. The melted plastic would push out overflows that are usually provided in these rounds. Because of the plastic, neither sudden pressure nor heat/ignition inside the round, would therefore be as catastrophic. Therefore, choice of low-melt temperature plastic as the liner material in this invention, adds safety to the round. This benefit is favorable, consistent with current Insensitive Munition (IM) requirements in minimizing accidental ammunition explosion due to fire hazards. 
     According to another embodiment of the present invention, the warhead includes a stepped liner that is disposed inside the warhead body. While, as explained earlier, the liner in the previous embodiment includes a pattern that is created with gaps that allows the explosive to expand therein during detonation, the liner of this alternative embodiment, includes a step liner that includes either a uniform or an alternating pattern of raised material on a matrix. This alternative embodiment includes a thin layer of material that replaces the gaps of the previous embodiment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features of the present invention and the manner of attaining them will be described in greater detail with reference to the following description, claims, and drawings, wherein reference numerals are reused, where appropriate, to indicate a correspondence between the referenced items, and wherein: 
         FIG. 1  is a party cutaway, cross-sectional view of a warhead incorporating a liner according to the present invention, to effect controlled fragmentation; 
         FIG. 2  is an elevational, cross-sectional view of a preferred embodiment of the warhead of  FIG. 1 , according to the present invention; 
         FIG. 3  is an elevational, cross-sectional view of another embodiment of the warhead according to the present invention; 
         FIG. 4  is an elevational, cross-sectional view of yet another embodiment of the warhead according to the present invention; 
         FIG. 5  is an elevational, cross-sectional view of still another embodiment of the warhead according to the present invention; 
         FIG. 6  is comprised of  FIGS. 6A and 6B  that illustrate the differential shear force application on the warhead body upon detonation of the explosive charge; 
         FIG. 7  is comprised of  FIGS. 7A ,  7 B,  7 C,  7 D,  7 E, and  7 F that illustrate various exemplary patterns of the liner of the present warhead; 
         FIG. 8  is a perspective view of the liner according to one embodiment of the present invention; and 
         FIG. 9  is an elevational, cross-sectional view of another embodiment of the warhead of  FIG. 1 , showing a stepped liner. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  illustrates an exemplary embodiment of a warhead, a munition, or a projectile  100  (collectively referenced herein as warhead  100 ) utilizing a liner  101  that is selectively patterned to effect control fragmentation of a warhead nose  103  or body  102  according to the present invention. The warhead  100  generally comprises the body  102  that houses the liner  101 , an explosive or explosive charge  104 , back plates (not shown), and an initiation mechanism assembly (not shown). The warhead liner  101  generally takes the shape of the warhead body  102 . An exemplary shape of the liner  101  is the cylindrical shape, as illustrated in  FIG. 8 . 
     The explosive charge  104  comprises, for example, LX-14, OCTOL, hand packed C-4, or any other solid explosive, that can be machined, cast, or hand-packed to fit snugly within the inside of liner  101 . 
       FIG. 2  is an elevational, cross-sectional view of the warhead  100  of  FIG. 1 , according to the present invention, showing a controlled fragmentation pattern  200  of the liner  101 . As is illustrated in  FIG. 1 , while the solid explosive charge  104  does not need to be patterned but just usually plain cylindrical, it could be pre-patterned as one piece but with bumps that fit in the open gaps in correspondence to match the pattern  200  (such as  FIGS. 7A-7G ) of the liner  101  (such as in  FIG. 8 ). 
     As further illustrated in more detail in  FIG. 7 , the pattern  200  of the liner  101  could be formed by any known or conventional method, such as by stamping, or by stereo lithography. The liner  101  could be made of any suitable low-melt temperature material such as HDPE (High Density Poly Ethylene), or Accura SI 40 stereo lithographic material mimicking Nylon 6:6. The pattern  200  includes openings, gaps, or cutouts  700  (collectively referred to herein as gaps  700 ) that are interposed among a plurality of patterned liner elements  704 . The pattern will be described in greater detail in connection with  FIG. 7 . 
     With further reference to  FIG. 6 , upon detonation of the explosive charge  104  of the warhead  100 , in the areas of liner cutouts  700 , the momentum of the shock wave propagating through the explosive  104  is transmitted directly to the sections  600  of the interior of the warhead body  102 , as illustrated by force F 1  ( FIG. 6A ). In the case of the sections  604 , which are disposed adjacent to the liner elements  704 , the detonation wave momentum is transmitted first to the liner elements  704  and then to the interior of the warhead body  102 , as illustrated by force F 2  ( FIG. 6A ). 
     The time delay between the moments when the shock waves reach sections  600  and  604  is determined by the differences between the detonation velocity of the explosive  104  and the shock wave propagation speed of the liner material  101 , respectively. Since the motion of the section  600  should typically start earlier than that of the section  604 , the transitional region between section  600  and  604  is subjected to intense “stretching” by the force differential F 1 −F 2 , F 1 &gt;F 2 . This generates a high gradient of pressures, velocities, and strains between sections  600  and  604 , acting as stress and strain “concentration factors”. 
     As shown in  FIG. 6B , unstable thermoplastic shear (adiabatic shear) eventually transfers the entire burden of localized strain to a finite number of shear planes leading to the shell break-up and formation of fragments. As a result, a predetermined pattern of liner cutouts  700  “stamps out” a pattern of localized transitional regions  600 - 604 , so as to cause the warhead body  102  to shear and break into fragments with controlled size. 
     For given choices of materials of explosive  104  and liner  101 , the thickness of the liner  101  helps determine the time delay between forces F 1  and F 2 , and, subsequently, the magnitude of the required gradients of stresses, strains, and strain-rates in transitional regions  600 - 604 . In a preferred embodiment, the thickness of the liner  101  varies between approximately a fraction of a millimeter and several millimeters, in order to cause a time delay that varies between approximately a hundred of nanoseconds and two microseconds. 
     The selectively controlled pattern  200  comprises sections of equal size or, alternatively, sections ranging in size from a relatively large size to smaller sections. The larger size of the intact (non-gap) sections is selected for more heavily armored targets, while the smaller size of intact (non-gap) sections is applicable for lightly armored or soft targets. Consequently, the pattern  200  efficiently enables variable and selective lethality of the warhead  100  that can range from maximum lethality for more heavily armored targets to a maximum lethality for lightly armored or soft targets. 
       FIG. 2  is an elevational, cross-sectional view of a warhead  100  according to one embodiment of the present invention, with the dashed lines illustrating the shearing locations upon detonation of the explosive charge  104 , as presented herein. According to this embodiment, the explosive charge  104  completely fills the liner plus is also allowed to fill in, in the gaps  700  of the liner  101  during the manufacture process, so that the explosive charge  104  is in direct contact with the liner  101 . Since explosive  104  in gaps  700  is in direct contact with the interior surface of the warhead body  102 , the shock wave pressures in sections  600  will be significantly higher than that of sections  604 , wherein the shock pressures have been “buffered” with liner elements  704 . Accordingly, the material of the warhead body  102  in sections  600  will strain-harden more, attaining higher values of “post-shock” yield strengths than those of sections  604 . In general, this will result in high probabilities of fractures occurring in the transition regions  600 - 604 , so as to cause the warhead body  102  to shear and break into fragments in a controlled, predetermined manner. 
       FIG. 3  is a cross-sectional view of the warhead  300  according to another embodiment of the present invention, with the dashed lines illustrating the shearing locations upon detonation of the explosive charge  104 , as presented herein. According to this embodiment, the explosive charge  104  is just generally cylindrical and is not allowed to fill the gaps  700  of the liner  101 , so that the gaps  700  isolate the explosive charge  104  from the warhead body  102 , and form a pattern of internal chambers that can be void, or filled with air, or, alternatively, filed with any other light-density inert gases. 
     In this embodiment, the gaps  700  extend throughout the depth of the liner  101 . Since in this embodiment of the invention the material density of sections  700  is significantly lower than that of sections  704 , the shock wave pressures transmitted to sections  600  will be significantly less than that of the neighboring sections  604 . Accordingly, the material of the warhead body  102  in sections  604  will have higher “post-shock” yield strengths than that of “void” or “air-gap-buffered” sections  600 , so as to cause the warhead body  102  to shear and to break along the transition regions  600 - 604 . 
       FIG. 4  is a cross-sectional view of the warhead  400  according to another embodiment of the present invention, with the dashed lines illustrating the shearing locations upon detonation of the explosive charge  104 , as presented herein. According to this embodiment, and similarly to the warhead design of  FIG. 3 , the explosive charge  104  is not allowed to fill the gaps  402  of the liner  101 , so that the gaps  402  isolate the explosive charge  104  from the liner  101 . In addition, unlike the gaps  700  that extend throughout the depth of the liner  101 , the gaps  402  extend only partially through the depth of the liner  101  and form a stepped configuration with an internal chamber. 
       FIG. 5  is a cross-sectional view of the warhead  500  according to another embodiment of the present invention, with the dashed lines illustrating the shearing locations upon detonation of the explosive charge  104 , as presented herein. According to this embodiment, the explosive charge  104  is not allowed to fill the gaps  700  of the liner  101 , so that the gaps  700  isolate the explosive charge  104  from the liner  101 . In this embodiment, the gaps  700  extend throughout the depth of the liner  101 , and are isolated from the explosive charge  104  by means of a metallic liner  505 , to form a void, or a gas filled chamber. In a preferred embodiment, the gas is air. The metallic liner  505  is made of a suitable metal, such as aluminum, steel, or copper. 
     Referring now to  FIG. 7 , it illustrates various exemplary embodiments of the liner  101 . The liner  101  of  FIG. 7A  is patterned in a checkerboard, circumferential/longitudinal configuration wherein the orientation of the square-shaped gaps  700  is in parallel to the axis  705  of the munition. The gaps  700  and the liner elements  704  are illustrated as being uniform and equal in size. It should be clear that according to another embodiment, the gaps  700  or the liner elements  704  could have different sizes. Though the gaps  700  and liner elements  704  are square-shaped, other shapes are contemplated by the present invention. 
     The liner  101  of  FIG. 7B  is similarly patterned in a checkerboard, configuration, as in  FIG. 7A . However, in  FIG. 7B , the configuration is a diagonal configuration wherein the orientation of diamond-shaped gaps  700  is at the angle with axis  705  of the munition. The gaps  700  and the liner elements  704  are illustrated as being uniform and equal in size. It should be clear that according to another embodiment, the gaps  700  or the liner elements  704  could have different sizes. Though the gaps  700  and liner elements  704  are diamond shaped, other shapes are contemplated by the present invention. 
     The liners  101  of  FIGS. 7C ,  7 D,  7 E, and  7 F illustrate variations to the liner patterns of  FIGS. 7A and 7B , by varying the sizes of the gaps  700  relative to the liner elements  704 . 
       FIG. 9  illustrates another embodiment of the present invention, and shows a warhead  900  with a stepped liner  901 . The warhead  900  includes a stepped liner  901  that is disposed inside the warhead body  102 . As explained earlier, the liner  101  in the previous embodiment includes a pattern that is created with gaps  700 . The stepped liner  901  is formed of raised sections  904 , sunken sections  910 , and gaps  700 . To this end, the stepped liner  901  is formed of a first liner  101  as described earlier and a second liner  902  that could be either integrally secured to the first liner  101 , or integrally formed part thereof similarly to the embodiment of  FIG. 4 , or have a sliding connection with liner  101 . 
     Also, according to another embodiment, the liner  901  is comprised of two liners such as  101  (with gaps  700 ), whereas the outer liner can slide between the inner liner  101  and the interior surface of the warhead body  102  in the circumferential direction, as to allow a selectable fragmentation pattern with the desired fragment size that can be “dialed in” by rotation of the outer liner immediately prior to the deployment of the munition. 
     It is to be understood that the specific embodiments of the invention that have been described are merely illustrative of certain applications of the principles of the present invention. Numerous modifications may be made to the munition with a controlled fragmentation pattern described herein without departing from the spirit and scope of the present invention.