Abstract:
A pre-stressed curved plate comprising a curved plate having at least one concave surface, the curved plate being enveloped and adhesively bonded with tensioned reinforcing fibers, whereby the reinforcing fibers are first wound around the plate under tension being spaced apart from the concave surface and subsequently subjected to pressure to stretch and bond the reinforcing fibers to the surfaces of the plate, where upon bonding, the tensile strain of the fiber introduces stress in the plate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Phase Application under 35 U.S.C. 371 of International Application No. PCT/IL2015/050107 filed on Jan. 29, 2015 and published as WO 2015/114632 A1 on Aug. 6, 2015, which claims priority to Isreal Patent Application No. 230775 filed on Feb. 2, 2014. The entire disclosures of all of the above applications are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to pre-stressed curved ceramic plates that have improved ballistic protection characteristics and improved survivability under static and dynamic loading. The invention describes the structure of the plate, pre-stressed by an external envelope of stretched fibers bonded to the plate, and method of producing them. 
       BACKGROUND OF THE INVENTION 
       [0003]    Ceramic materials are widely used for ballistic protection with the purpose of overcoming the high power penetration of projectiles, such as armor piercing (AP) projectiles, due to their relatively low density, high rigidity and high compression strength. See C. Barry Carter, M. Grant Norton, Ceramic Materials Science and Engineering, Spinger Science, 2007. Usually, ceramics are used in structures for ballistic protection in the form of plates, tiles, pellets or specially shape ceramic elements. 
         [0004]    The significant disadvantage of these ceramic components is their low fracture toughness and the formation of a wide net of relatively long cracks during fracture. Such behavior of ceramic components reduces their ability to protect against not only around the local impact zone, but also around the wide area, which is affected by propagated cracks. 
         [0005]    The impact on the ceramic component by a projectile generates the propagation of compression stress waves, which are reflected back from the free surfaces of the protection plate as tensile waves. 
         [0006]    The ceramic materials have low tensile strength and usually these tensile stresses lead to the fracture of the ceramic component. It is therefore reasonable to assume that pre-stressing (initial compression) the ceramic component in a ballistic protection structure, should decrease the level of reflected tensile stresses after impact, and improve the protective ability of the ceramics component in a protection structure. 
         [0007]    D. Sherman, J. PHYS IV FRANCE 7 1997, showed that inducing the compression stresses in the ceramics by confined frame reduces significantly their damage from impact. 
         [0008]    T. J. Holmquist, International Journal of Impact Engineering, November 2003, evaluated the response of ceramic targets with and without pre-stress subjected to projectile impact. The results demonstrate that ballistic performance of confined ceramic targets can be improved by pre-stressing the ceramic. 
         [0009]    U.S. Pat. No. 6,389,594 describes that an applying of the high isostatic pressure on a monolith ceramics wrapped by a composite materials impregnated with adhesives to cure it, induces the internal stresses into the ceramics and improves the multi hit capabilities of the armor. 
         [0010]    U.S. Pa. No. 6,601,497 discloses an armor component that includes a tile having a perimeter, with optional recess to encase a wrapping material wrapped around the perimeter of the tile. The purpose of wrapping the tile is to prevent lateral expansion of the tile if it is hit. The wrapping material pre-stresses the tile, however the technique used here limits the degree of tensioning of the wrapping material by relatively low ability of tensioners and delivery system for the wrapping materials, and does not take an advantage of the tensile ability of them. The level of pre-stressing of the tile depends on the perimeter geometry of the tile, where the circular perimeter provides a uniform pressure on the tile recess. The straight sections of the tile perimeter will not supply any pressure on the tile, so this technology shouldn&#39;t be effective for monolithic ceramic plates that usually have straight perimeter lines, like personal protection ceramic plates. 
         [0011]    U.S. Pat. No. 7,300,893 describes an armor including a strain rate hardening elastomers. An option to wrap the ballistic clothes or fibers around the ceramic tiles is suggested in this invention as a way to encapsulate the elastomer, and not for pre-stressing of the ceramic plate. 
         [0012]    U.S. Pat. No. 7,827,899 describes an armor panel to protect a body from an incoming projectile, comprising the ceramic or glass-ceramic plate and backing polyethylene clothes, encapsulated within by a wrapping layer and bonded together by applying an external pressure on this wrapping layer. An armor panel, produced by this way, contains the externally wrapped layer, but this layer doesn&#39;t induce any significant pre-stressing of the internal ceramic plate. 
         [0013]    The above prior art relates to pre-stressing anti-ballistic flat ceramic articles. 
         [0014]    It is an object of the present invention is to provide anti-ballistic pre-stressed curved monolithic plates/tiles and a method of producing same. 
         [0015]    It is a further object of this invention to provide a method of preparing pre-stressed anti-ballistic curved plates/tiles. 
         [0016]    Anti-ballistic articles containing pre-stressed curved plates/tiles according to the present invention have superior ballistic characteristics including significant reduction of damages to the plate when hit, thus, providing armor plate with an enhanced multi-hit capabilities. In addition, anti-ballistic articles containing pre-stressed curved plates of the present invention are of relatively lower weight and decreased thickness than conventional ballistic articles for achieving the same affect. 
         [0017]    Further objectives, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the following figures. 
       SUMMARY OF THE INVENTION 
       [0018]    In accordance with the present invention, there is provided a pre-stressed curved plate comprising a curved plate having at least one concave surface, said curved plate being enveloped and adhesively bonded with tensioned reinforcing fibers, whereby the reinforcing fibers that were wound around the concave side of the plate and spaced apart from the concave surface were subjected to pressure at elevated temperature to stretch and bond the reinforcing fibers to the surfaces of the plates and maintain the tension after cooling, thereby stressing the plate, said bonding being effected by either applying an adhesive to the surfaces of the curved plate prior to winding the reinforcing fibers or coating the reinforcing fibers prior to or during winding. 
         [0019]    The bonded fibers maintain the internal tension due to their stretched status and induce compression stress in the plate. 
         [0020]    It is to be understood that the term plate used herein includes single curve or multi-curve plates or tiles and shells containing at least one concave zone. 
         [0021]    The invention also provides methods for preparing stressed curved ceramic plates, as follows. 
         [0022]    a. a curved plate is coated with adhesive. 
         [0023]    pre-tensioned reinforcing fibers are wound around the ceramic plate, contacting the convex surface of the plate, and spanning across the concave surface of the plate, leaving a distance between the fibers and the convex surface. 
         [0024]    external pressure, such as by autoclave or press is then applied, optionally at elevated temperature, to the wound fiber on the concave side of the plate, to stretch it until it comes in contact with the concave surface of the plate and is bonded thereto by curing the adhesive. 
         [0025]    cooling the plate and releasing the external pressure, whereby the bonded tensioned fibers induce compression stress on the plate. 
         [0026]    b. the same procedure as a., with the exception that the reinforcing fibers are pre-impregnated with adhesive prior to winding or are impregnated during winding. 
         [0027]    c. Where the tensile strength of reinforcing fibers is limited and will not allow the fibers to stretch sufficiently under applied pressure and elevated temperature, to contact the concave surface, one can place a removable spacer on the concave surface so that the reinforcing fibers will be wound around the spanner, increasing the length of the fiber loop, so that when the spacer is removed the reinforcing fibers will have enough length and tenacity be pressed against the concave surface and adhere thereto. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    In accordance with the present invention, the curved plate is preferably a ceramic plate and can be made of aluminum oxide (Al2O3), boron carbide (B4C), silicon carbide (SiC), boron silicon carbide (BSC), titanium diboride (Ti B2), aluminum nitride, silicon nitride, sintered silicon carbide, sintered silicon nitride, and glass-ceramic or combinations thereof. It is preferred that the plate have a thickness of between 2-30 mm. Furthermore, the plate, tile or shell may also be multi-curved containing at least one concave zone. 
         [0029]    The reinforcing fibers are selected from aramid, poly(p-phenylene-2,6-benzobisozazole), glass, carbon, thermoplastics (polyamide, polyethylene, polypropylene) or metal (boron, steel, aluminum) fibers. 
         [0030]    Suitable adhesives for bonding the fibers to the curved ceramic plate are preferably hot melt thermoplastic adhesives like polyethylene and polypropylene films or thermosetting adhesives like epoxy with optionally high or low curing temperatures, rubber or elastomer based adhesives, ceramic adhesives. 
         [0031]    The fibers are preferably impregnated with an adhesive material either before or during the winding, but before subjecting them to expansion by external pressure. Alternatively, the plate can be coated with an adhesive prior to winding the reinforcing fiber. 
         [0032]    The pre-stressed curved plate can form an insert for either personal body armor, or antiballistic panels for vehicles, ships, aircrafts and helicopters, or for a helmet. 
         [0033]    One advantage of the pre-stressed curved plate of the present invention is that it has increased multi-hit capability. 
         [0034]    A second advantage of the pre-stressed curved plate anti-ballistic articles containing according to the present invention is that they achieves the same ballistic characteristics with reduced weight. 
         [0035]    During the winding procedure the fibers should have some initial tension and not be slack to assure tight winding. This can be achieved by tensioners, see S. T. Peters, W. D. Humphrey, R. F. Foral, Filament Winding, Composite Structure, Fabrication, SAMPE, ISBN 0-938994-81-6. 
         [0036]    To maintain the tension in the wound reinforcing fibers and to induce the compression in the plate, the wound plate should be cured under applied external pressure and optionally at an elevated temperature. The external pressure should be sufficiently high to maintain full contact between the fibers and the concave surface of the plate during the curing of adhesive and final bonding of the stretched fibers to the plate. The degree of compression of the plate will depend on the curvature of the plate, the direction of winding trajectory of the fibers, and elastic modulus of the fibers. Thus, the same fibers will induce different compressions in plates with different curvatures. 
         [0037]    In cases where the ultimate tensile strain of a fiber is so low that even under pressure and elevated temperature it would not stretch sufficiently to reach and bond to the concave surface of a ceramic plate, then it is possible to interpose a removable spacer between the concave surface of the plate and the wound fiber. The removable spacer should have an appropriate geometry to increase the length of the fibers spanning the concave surface of the curved plate, in effect transforming the concave side to convex with respect of the winding. Thus, when pressure is applied to the wound plate and the spacer is removed the fibers will be sufficiently stretched to be pressed against the concave surface of the plate and be bonded thereto. After bonding the tensile strain of the fibers will induce stress to the plate, which is maintained upon cooling. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]      FIG. 1  illustrates a curved ceramic plate before winding with reinforcing fibers. 
           [0039]      FIG. 2  is a cross-sectional view of a curved ceramic plate wound initially with reinforcing fibers in accordance with the present invention. 
           [0040]      FIG. 3  is a cross-sectional view of a curved ceramic plate with wound fibers stretched and bonded to the surfaces of the plate. 
           [0041]      FIG. 4  is a cross-sectional view of a curved ceramic plate with spacer wound initially with reinforcing fibers prior to applying pressure at elevated temperature. 
       
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
       [0042]      FIG. 1  is a perspective view of a curved ceramic plate  10  having a concave surface  12  and a convex surface  14 . In  FIG. 2  the curved plate  20  is wound with adhesive coated reinforcing fibers  16  under tension. Fibers  16  thus come in direct contact with the convex surface  14  and loops around the concave surface  12  forming a taught straight line between the edges  17  and  19  of the plate, leaving a space  15  between the tautly wound fibers  16   a  and the concave surface  12  of the plate.  FIG. 3  shows fibers  16 , 16   a  wound around a curved ceramic plate  30  after being subjected to elevated temperature of about 180° C., while applying pressure (see the arrows) against the tensioned wound fibers  16 , 16   a,  pressing the fibers against the surfaces  12 ,  14  and curing it. Curing can be done by means of a hydroclave, autoclave, isotactic press or other suitable apparatus as described in U.S. Pat. No. 6,389,594. The tensioned wound fibers  16 , 16   a  are thus bonded to the concave surface  12  and compress the curved plate. 
         [0043]      FIG. 4  illustrates in cross-section a curved ceramic plate  40  having a concave surface  12  and a convex surface  14 . A removable spacer  42  is placed on the concave surface  12 . Reinforcing fibers  44 ,  44   a  having low tensile strain, which would prevent the fibers from being pressed against the concave surface  12  under pressure were they stretched taut across line x-x, are wrapped around a spacer  42  raising fibers  44   a  over the concave surface  12  above the line x-x. This increases the length of fibers  44   a  on the concave side sufficiently so that when the temperature is raised and pressure is applied, and the spacer  42  is removed, fibers  44   a  will stretch sufficiently to reach the concave surface  12  to be bonded to the plate and cured. 
         [0044]    The type of fiber should be chosen according to its possible elongation and maximum elastic modulus to increase the compression stresses in the plate. 
         [0045]    It should be noted that by using different fibers and/or different adhesives and/or other bonding conditions such as different processing temperature or pressure, one may vary the degree of impregnation and/or the degree of tensioning of the fibers and/or the degree of compression, which may affect the degree of stress in the ceramic plate/tile. 
         [0046]    In practice, ceramic plates of this invention are generally backed with an anti-ballistic cloth in anti-ballistic body protecting articles. 
       EXAMPLE 
       [0047]    A number of ceramic body armor articles were prepared as follows. Aluminum oxide double curved ceramic plates 7.1 mm thick and weighing an average of 2100 g were wound with 200 g of T700 carbon fibers under tension, treated under pressure and 180 degrees temperature to pre-stress the plates, and backed with 23 layers of polyethylene cloth, with average weight of 460 g including adhesive. These articles had a general weight of 2760 g. 
         [0048]    These articles were tested against APM 2  projectiles with velocity range 868-883 m/s and provided good protection (two bullets were stopped by these articles) whereas articles with the same ceramic plates without pre-stressing have to include at least 40 layers of polyethylene cloth with average weight of 2900 g to provide the same ballistic protection.