Abstract:
Disclosed is a passive armor assembly for protecting a body disposed behind it from an impact of a long rod penetrator (hereinafter LRP). The armor assembly includes an armor surface that is capable of exerting asymmetric forces on the oncoming LRP and an armor member being disposed behind the armor surface. The armor member is made of a high compression strength, low density, brittle material, and its thickness along the direction of the impact exceeds the length of the LRP. Preferably, the thickness is at least 1.5 the length of the LRP. In a preferred embodiment the armor member is made of a material exhibiting the combination of fracture toughness smaller than 3 MPam 1/2 , density of less than 2 g/cc, and compression strength of from 10 2  to 10 3  MPa.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to passive armor assemblies for protecting a body disposed behind them from an oncoming long rod penetrator (hereinafter LRP). 
       BACKGROUND OF THE INVENTION 
       [0002]    A long rod penetrator (LRP) is a type of ammunition which uses kinetic energy as the primary means of penetrating armor. A penetrator is considered long if it has a length to diameter (L/D) ratio of from about 5 to about 35. Such penetrators are usually fired from tank guns or other guns having diameters of between 20 and 120 mm, particularly 30 mm. 
         [0003]    Long rod penetrators fired by guns are commonly just 6-25 mm in diameter, and 100-600 mm long. To maximize the amount of kinetic energy released on the target, the penetrator is normally made of a hard and heavy material, such as steel, tungsten alloy or depleted uranium. 
         [0004]    It is generally accepted that long rod penetrators are one of the most effective ammunitions in penetrating armor today. They are typically fired at velocities of between 1000 and 1800 m/s. 
         [0005]    EP 943886 describes a lightweight armor assembly resistant against the penetration of firearm projectiles. This assembly includes a front body of a lightweight material slanted relative the expected trajectory of an oncoming firearm projectile. Examples of materials used for the front body according to this document are glass, glass ceramics, polymethyl metacrylate (hereinafter PMMA), polycarbonates, PVC, Kevlar™, and Spectra™. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides a passive armor assembly for protecting a body disposed behind said armor assembly from an impact of a long rod penetrator (hereinafter LRP), said armor assembly comprising (i) an armor surface that is capable of exerting asymmetric forces on the oncoming LRP and (ii) an armor member made of a high compression strength, low density, brittle material, said armor member being disposed behind said armor surface and having a thickness along the direction of said impact, which exceeds the length of the LRP. Preferably, the thickness is at least 1.5 or more the length of the LRP. 
         [0007]    The armor member may be made of a single body (monoblock), may be made of several layers of smaller thickness adjacent to each other, or may be made of separate sub-members. Such layers may be attached to each other, for instance, by a suitable adhesive, but this is not a necessity. Preferably, the thickness of each of the plurality of layers composing the armor body is at least about 20 mm. 
         [0008]    The density of the armor member should be low and its compression strength should be high as to ensure that the damage velocity in the member will be lower than the velocity of the LRP inside it to allow the penetrator to continuously penetrate into non-damaged portions of the armor member. It was found that materials exhibiting the combination of (a) fracture toughness smaller than 3 MPam 1/2  or elongation to fracture of less than 5%, (b) density of less than 2 g/cc, and (c) compression strength 10 2  to 10 3  MPa are suitable for composing the armor member of the invention. Some deviations from these values may be permitted, as long as the functionality of the assembly, as this is described below, is retained. As one material that was found to exhibit these properties is PMMA, such materials will be referred to hereinafter as PMMA-like. Non-limiting examples for such materials are polyester, epoxy resin, and various polymeric resins with brittleness increasing agents, such as ceramic powders. Non-limiting examples to ceramic powders that may function as brittleness increasing agents are alumina powder and silica powder. Non-limiting examples to polymeric resins, the brittleness of which may be increased by such agents is epoxy. 
         [0009]    Without being bound to theory, it may be assumed that disruption of an LRP by an armor assembly according to the present invention may be caused by the combination of at least the following factors:
       1) The armor surface causes the penetrator to impact on the armor member asymmetrically, creating in the armor member an asymmetrical penetration crater.   2) The brittleness of the armor member preserves the asymmetrical nature of the penetration crater.   3) Due to low damage velocity of the armor member and its relatively large thickness, non-damaged portions of the armor member constantly exert on the penetrator asymmetric forces. Since the penetrator is constantly in an asymmetrical crater, the forces acting on it all along the way are asymmetrical, such that they eventually cause it deformation or breakage.
           Since the penetrator is long, the armor member exerts different (asymmetric) forces at different times on different portions of the penetrator, and thus tends to enhance the deformation of the penetrator as it advances inside the armor member, and even brings it to breakage. The longer the penetrator, the greater is the number of its potential breaking points.   
               
 
         [0014]    The main mechanism that causes the LRP to loose momentum is increase of surface area in the impact direction, caused upon deflection, deformation or breakage of the LRP 
         [0015]    To increase the volume efficiency of an armor assembly according to the invention it may be advisable to include in the armor assembly, behind the armor member, a backing layer of ductile material in order to adsorb momentum of the LRP or its pieces as they exit from the back of the armor member, or make them ricochet from it. This may allow using armor member of smaller thickness without compromising the degree of protection. In some cases, this may also allow to have armor assemblies with higher weight efficiency than may be designed without such a ductile backing layer. In some cases, the function of such a backing layer may be fulfilled by a wall of the body to be protected. 
         [0016]    Non-limiting examples of ductile materials, as this term is used all along the present description and claims include ductile metals, such as steel and aluminum, and composite materials, such as high-density polyethylene or aramid fibers or fabric, as those commercially available under the trade-names Kevlar™ or Dyneema™. 
         [0017]    Ricocheting from the backing material may happen with armor assemblies in accordance with the present invention almost irrespectively of the angle at which the penetrator impacts the armor surface, thanks to the deflection, deformation, and/or breakage the penetrator suffers during its penetration into the armor member. 
         [0018]    To exert asymmetric forces on the oncoming LRP, the armor surface may be inclined with respect to the impact direction of the LRP. Other examples of suitable armor surfaces are surfaces that are not necessarily slantingly disposed, but have different local densities across their surface, for example, a net with variable density, a bumped board, and the like. 
         [0019]    In case a slantingly oriented layer is used as an armor surface, the angle between it and the expected trajectory of the oncoming penetrator is preferably between 5° and 60°, angles of 10° to 50° are preferable, and most preferable are angles of about 30°. 
         [0020]    The armor surface may be, for example, the outer surface of the armor member, attached thereto, or separated therefrom. 
         [0021]    According to one embodiment, the armor surface is disposed in front of the armor member, preferably parallel thereto, and the distance between the armor surface and the armor member is at least 5 mm, preferably 10 mm or more. 
         [0022]    In such an embodiment, if the armor surface is made of PMMA-like material or composite material, the effect of the armor surface being separate from the armor member is an increase of the asymmetric nature of the forces exerted on the penetrator prior to penetration deep into the armor member. Yet, if the armor surface is made of a metallic material, the effect the assembly has on the penetrator is particularly strong. This is so although the ductile surface is thin enough not to have a protective value by itself. Without being bound to theory, the explanation may lie in that forces exerted on the penetrator upon exiting from the armor surface are mainly perpendicular to the armor surface, while forces exerted thereon upon impacting the armor member are mainly parallel to the armor member. Thus, the penetrator suffers forces of opposite directions, and may be caused to split. This may be of particular usefulness if the penetrator, from which protection is sought, has a hemispherical or flat head. 
         [0023]    The armor surface of the invention may be advantageously covered with a front non-armor layer for protecting it from environmental hazards, such as blows, humidity, irradiation, and extreme temperature. Such a non-armor member may be made, for instance, from a thin layer of 4 mm aluminum, 10 mm Kevlar™, or steel with similar areal weight. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    In order to better understand the invention and to see how it may be carried out in practice, several illustrative embodiments will now be described, by way of non-limiting examples only, with reference to the accompanying drawings, in which: 
           [0025]      FIGS. 1-7  are schematic descriptions of passive armor assemblies according to 7 different embodiments of the present invention; 
           [0026]      FIGS. 8-11  are each a set of X-ray photographs taken when a passive armor assembly according to four different embodiments was penetrated by an LRP. The contours of the penetrators and their pieces are outlined to ease the understanding of the photographs. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]      FIG. 1  schematically shows a passive armor assembly  2  according to one embodiment of the invention for protecting a body  4  disposed behind the armor assembly from an oncoming LRP  6 . The armor assembly  2  includes an armor surface  8  that is slantingly oriented in respect of the trajectory of the LRP  6 , and thus capable of exerting asymmetric forces thereon. The assembly  2  also has an armor member  12 , the front surface thereof is the armor surface  8 . The armor member  12  is made of a high compression strength, low density, and brittle material. The armor member  12  has along the LRP impact direction a thickness τ that is greater than the length L of the LRP  6 . 
         [0028]    Similar parts shown in the following figures will be referred to using the same reference numerals as hereinbefore. 
         [0029]      FIG. 2  schematically shows a passive armor assembly  2  according to another embodiment of the invention, where the armor member  12  is made of a plurality of mutually adjacent layers  20 ,  21 ,  22 , and  23 . It was found that in such an embodiment, it is preferable that each of the several layers  20 ,  21 ,  22 , and  23  is at least 20 mm thick. The several layers may be attached to each other, for instance, by a suitable adhesive, but this is not a necessity. 
         [0030]      FIG. 3  schematically shows a passive armor assembly  2  according to another embodiment of the invention, where the armor surface  8  is made of three discrete sub-members  12 A,  12 B, and  12 C, having thickness of τ A , τ B , and τ C , respectively, such that τ A +τ B +τ C =τ. τ should be at least equal to the length of a penetrator from which protection is sought. Upon impacting an armor assembly according to such an embodiment, an oncoming LRP meets a slanted armor surface a plurality of times, and asymmetric forces act on it once and again. Such an embodiment may be of particular advantage in protecting against LRPs with spherical or flat heads. 
         [0031]      FIG. 4  schematically shows a passive armor assembly  2  according to another embodiment of the invention, wherein the armor surface  8  is separate from the armor member  12 . In this figure, the surface  8  is a 30° slanted surface. 
         [0032]      FIG. 5  schematically shows a passive armor assembly similar to that shown in  FIG. 4 , but the armor surface  8  is metallic, and parallel to the outer surface of the armor member  12 . The distance between the armor surface  8  and the armor member  12  should be such that the penetrator has sufficient time to react to the forces acting thereon at the exit from the surface  8  before it meets the front surface of the member  12 . In practice this is at least 5 mm, preferably at least 10 mm. 
         [0033]    Each of the armor assemblies described above may have a backing layer for adsorbing momentum of the LRP or its pieces as they exit from the back of the armor member and/or a front protective layer for protecting the armor member from environmental hazards. 
         [0034]      FIGS. 6A and 6B  show schematically front and side views, respectively, of an armor assembly  2  according to another embodiment of the invention having an increased multiple hit capability. The assembly  2  comprises a plurality of discrete modules  24  that preferably have size of between 30×30 cm and 60×60 cm, for instance, 30×60 cm. Each of the modules  24  may be envisaged as a mini armor assembly  2 ′ encased in a box  26 . The box  26  may be made of any material that is strong enough to support the mini assembly, allow its attachment to a body to be protected, and protect the mini assembly from environmental hazards. One non-limiting example to such material is 2 mm thick steel. Any gap between a box  26  and a mini assembly inside it may be filled, for instance, with molded rubber. The mini assembly  2 ′ may be in accordance with any embodiment of the invention. Furthermore, it is possible that the mini assemblies will include only the armor member, while a backing layer, a front protective layer, and/or an armor surface may be common to several modules or to the entire assembly  2 . 
         [0035]      FIG. 7  is a schematic illustration of an armor assembly designed in accordance with the present invention to protect targets such as an armor personnel carrier (APC) from a tungsten heavy alloy penetrator, with diameter of 8 mm and length of 160 mm. The designed armor assembly has good weight efficiency and acceptable volume efficiency. 
         [0036]    The assembly  2  has a 30° slanted PMMA surface  8  that is a part of a slanted PMMA armor member  12 , having a thickness of 140 mm (280 mm thickness along the line of impact, τ). The PMMA surface  8  is covered with a front protective layer  16  of 10 mm Kevlar™. The layer  16  may be replaced by a board of 4 mm Al-2024, thin steel layer of similar areal weight, or any similar material, that is known in the art to be useful for protecting PMMA from environmental hazards, such as blows, humidity, irradiation, and extreme temperature. The assembly  2  also has a backing layer  14 , made of 6 mm HH steel. Attaching such an assembly to a wall of an armed vehicle made pf 10 mm HH steel may provide protection for a special angle of 60° (calculated on the base of 30° to the horizon). The total thickness of the assembly is 150 mm and its weight is 450 kg/m 2  (which is equivalent to 58 mm steel). A penetrator fired at velocity of 1400 m/s did not penetrate the assembly, and thus, the weight efficiency of the assembly is about 2. This assembly has better weight efficiency than any other passive assembly known to the inventors, either metallic or ceramic. 
         [0037]      FIG. 8  shows X-ray photographs taken when an armor assembly of the kind shown schematically in  FIG. 2  is hit by an LRP. 
         [0038]    The LRP in the photograph is an APFSDS-like penetrator having 8 mm diameter and 135 mm length. The penetrator was shot at 1400 m/s. The assembly included 180 mm thick PMMA layer, oriented 30° to the line of impact, such that τ=360 mm. 
         [0039]    The LRP is shown 100 μs (I) and 400 μs (II) after the hit. In position I the LRP is deformed, with its nose going upwards, and in II it is broken and turned around, with a nose piece behind a tail piece. 
         [0040]      FIG. 9  shows X-ray photographs of an armor assembly according to the embodiment schematically shown in  FIG. 3 , where each of the sub-members is made of a plurality of mutually adjacent layers. The sub-members are each 60 mm thick PMMA plates and the gaps between them are each 40 mm thick. The sub-members are oriented 30° to the line of impact. 
         [0041]    The photographs were taken when the assembly was hitted by an LRP made of heavy tungsten alloy with a hemispherical head and L/D=20, L=160 mm, fired at 1400 m/s. The photographs were taken 150 μs after the hit (I), where it is shown that the penetrator starts deforming (nose slanted upwardly); 350 μs after the hit (II), and 530 μs after the hit (III), where progressive deformations are observed. 
         [0042]      FIG. 10  is a set of X-ray photographs taken when a passive armor assembly according to the embodiment schematically shown in  FIG. 4 , with armor member made of a plurality of mutually adjacent layers was penetrated by an LRP similar to the one described in the context of  FIG. 9  above, shot at 1420 m/s. As may be seen in the figure, 100 μs from the hit (I) the penetrator was deformed at the nose area, at 400 μs from the hit (II), the head was deformed and broken, and 570 μs from the hit (III) pieces of the LRP exit the back of the assembly. 
         [0043]      FIG. 11  is a set of X-ray photographs taken when a passive armor assembly according to the embodiment schematically shown in  FIG. 5  (length in impact direction τ=520 mm) was penetrated by an LRP similar to the one described in the context of  FIG. 9  above, at 1410 m/s. The front armor surface ( 8  in  FIG. 5 ) was made of 5 mm thick HH steel. As may be seen in the figure, 150 μs from the hit (I) the penetrator was deformed at the nose area in the downward direction, at 350 μs from the hit (II), the front portion of the penetrator was broken to pieces, and 490 μs from the hit (III) the entire penetrator is broken to pieces, with only a small portion thereof continuing to move along the impact direction.