Abstract:
A protective wall for use in a structure such as a ship comprises a first and second metal layer, with an armor plate in between and layers of elastomeric material between the armor plate and the first and second metal layer respectively. The first and second metal layer having a higher ductility than the armor plate, the armor plate being mounted between the first and second metal layer in a way that allows the first and second metal layers to stretch relative to armor steel plate at least in response to forces resulting from impact by fragments from an explosion. When an explosion occurs in a space bounded by the protective wall, the first and second metal layer deflect under the pressure pulses due to blasts, stretching relative to the armor plate. The armor plate blocks high speed fragments. The elastomeric material has a double function: it increases the resistance of the armor plate to fragments and it allows for separation of the armor plate from the first and second metal layer, allowing them to stretch.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to blast and fragment resistant wall sections used inside structures like vehicles such as ships. 
       BACKGROUND 
       [0002]    U.S. Pat. No. 6,706,406 discloses a ship hull design with hull plates formed from a pair of parallel ductile steel layers with an elastomer bonded to the steel layers in the space between the two layers. Voids may be provided in the elastomer, filled with rigid foam or light gauge steel. The elastomer arrests the development of cracks, which reduces hull damage during grounding or collisions. 
         [0003]    When a ship is hit by an explosive projectile, the most noxious damage is due to the effect of explosion within the ship. The explosion leads to a pressure surge and high velocity fragments that can kill ship&#39;s personnel and create extensive damage to the ship&#39;s infrastructure. 
         [0004]    One possible solution to the high velocity fragments problem is the use of armored bulkheads. Various technologies for manufacturing armored plates are known from the field of armored vehicles, such as use of hard grade steel, ceramics, fabrics of strong fibers or composites thereof. For example, an armor plate comprising a hard armor plate with an elastomer coating on the outside is known from an article titled “Elastomer-steel laminate armor”, by C. M. Roland et al., published in Composite structures, 92 pages 1059-1064. By coating the plate, a layer is formed that has no rigidity of its own, but is attached everywhere to the armor plate, so that it stretches everywhere together with the armor plate. The coating serves to increase penetration resistance. Roland et al note that the origin of blast and ballistic mitigation remains to be fully understood, but they mention energy absorption by rubber and strain delocalization Roland et al note that a multi laminate structure may be used, which can be extended to using multiple layers may be introduced into the coating. Roland et al give examples of very thin layers in the coating like 0.25 mm aluminum and 0.33 thick low modulus PU-1. With such an thin aluminum layer in the coating, the coating attaches the aluminum layer everywhere to the armor plate. 
         [0005]    However, many of these technologies are unattractive for use in ships, in view of the vast number of plates required in ships, which leads to concerns about weight and costs. This further means that it is desirable that the protective plates as much as possible perform structural functions in the ship, which may conflict with the behavior of conventional armor. For example hard materials are often brittle, and hence vulnerable to pressure surge. Moreover, it may be difficult to weld armor plates to other structures in a way that the welds support a structural function in a reliable way. 
       SUMMARY 
       [0006]    Among others, it is an object to provide for improved blast and fragment protection in ships. 
         [0007]    According to one aspect a protective wall for use in a structure such as a ship is provided, the protective wall comprising
       a first metal layer,   an armor plate adjacent the metal layer, the first metal layer having a higher ductility than the armor plate, the armor plate being mounted in a way that allows the first metal layer to stretch relative to armor plate at least in response to forces resulting from the pressure surge from an explosion;   a layer of elastomeric material between the armor plate and the first metal layer.       
 
         [0011]    When an explosion occurs in a space bounded by the protective wall, the first metal layer flexes under the pressure pulses due to blasts, stretching relative to the armor plate. The armor plate blocks high speed fragments. The elastomeric material has a double function: it increases the resistance of the armor plate to fragments and it allows the armor plate to be kept in position relative to the first and second metal layer under normal circumstances, without causing significant load transfer from the layers to the armor plate during blast or impact loading. The elastomeric material need not fill the entire space between the armor plate and the first metal layer. 
         [0012]    In an embodiment, the wall comprises a second metal layer, the armor plate being located between the first and second metal layer, the first and second metal layer having a higher ductility than the armor plate, the armor plate being mounted between the first and second metal layer in a way that allows the first and second metal layers to stretch relative to armor steel plate at least in response to forces resulting from impact by pressure surge from an explosion; the protective wall comprising layers of elastomeric material between the armor plate and the first and second metal layer respectively. Thus, the wall provides protection in spaces on either side of the wall against explosions on the other side of the wall. The elastomeric material need not fill the entire space between the armor plate and the metal layers. 
         [0013]    The first and second metal layer may be fixed parts of the structure, which may be a ship or a land vehicle for example, the first and second metal layer taking up at least part of the normal operational forces that arise due to use of the structure. The wall may be a bulkhead of a ship for example. 
         [0014]    In an embodiment the protective wall comprises a plurality of layers of armor plate and layers of elastomeric material. This increases protection against fragments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0015]    These and other objects and advantageous embodiments will become apparent from a description of exemplary embodiments using the following figures. 
           [0016]      FIG. 1  shows the position of a bulkhead in a ship 
           [0017]      FIG. 2  illustrates a response of the bulkhead to an explosion 
           [0018]      FIG. 3  shows a cross-section of a bulkhead 
           [0019]      FIG. 4  shows a detail of an attachment of the bulkhead 
           [0020]      FIG. 5  shows a cross-section of a further bulkhead 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0021]      FIG. 1  shows the position of a bulkhead  10  in a ship, between two longitudinal wall sections  12 , which may be hull sections. The plane of bulkhead  10  extends perpendicularly to the keel line of the ship. Bulkhead  10  is designed to provide for protection in a space  14  adjacent bulkhead  10  on one side of bulkhead  10  against the effects of the explosion of a projectile with high explosive within the ship on the other side of bulkhead  10 . 
         [0022]      FIG. 2  illustrates the desired effect of the pressure surge associated with such an explosion: bulkhead  10  flexes from the side A on which the explosion occurs to the other side B, absorbing explosive energy in the form of stretching energy. 
         [0023]      FIG. 3  shows a cross-section of bulkhead  10 . Bulkhead  10  comprises a pair of ductile metal layers  30 ,  32  with elastomeric material  34  between ductile metal layers  30 ,  32  and an armor steel plate  36  “floating” between ductile metal layers  30 ,  32 . As used herein, “floating” means that the coupling between armor steel plate  36  and ductile metal layers  30 ,  32  allows ductile metal layers  30 ,  32  to stretch independently of armor steel plate  36  under influence of forces with force amplitudes as induced by explosions. A floating presence may be realized by the absence of a direct fixed attachment of armor steel plate  36  to ductile metal layers  30 ,  32 , or at least by a limitation of direct fixed attachment to a point or line. A floating presence of armor steel plate  36  may be realized when substantially all contact between armor steel plate  36  and ductile metal layers  30 ,  32  is mediated via elastomeric material  34 . A floating armor steel plate  36  may rest on a rib of a ductile metal layer  30 ,  32  for example, without being fixedly attached to the rib. As used herein, fixedly attached includes attachment by welding, rivets, bolts etc. that resist forces that arise during explosions, whereas not fixedly attached includes contacts that allow for relative sliding motion at least when exposed to forces that arise during explosions. Spot welds or small bolts (e.g. M3 bolts or less) may be used 
         [0024]    Ductile metal layers  30 ,  32  may be made of steel of a known standard ship building steel quality, for example of steel types Steel 37 or Steel 52 (as is well known the standards for Steel 37 etc, specify the material properties and many manufacturers supply steel that meets these specifications; in the more modern EN 10028-3 standard Steel 37 and Steel 52 are also known as S235 and P355 respectively, but the terms Steel 37 and Steel 52 are still well known to skilled persons in the ship building industry). 
         [0025]    Steel 37 and Steel 52 have a limited tensile strength (plates fail at engineering stress of less than 550 MPa and hence also less than 600 MPa) and a large deformability (more than 25% engineering strain (elongation) before rupture). Standard measurements for measuring tensile strength are known per se. The strength of a material is expressed in terms of the stress at which the material fails. This involves subjecting a piece of material to a tensile force, the force defining a tensile stress on the piece as the force divided by the cross-sectional area of the piece in a plane perpendicular to the direction of the force. The strength is the maximum stress that the material is able to resist, i.e. the stress reached before the piece of material fails e.g. by necking. Because no high demands are placed on this strength, other building materials such as aluminum or titanium may also be considered. Similar performance can be achieved when the outer ductile steel layers are replaced with ductile metals such as aluminium and titanium. This may be applied to land vehicles requiring a similar combination of blast and fragment resistance. Ductile metal layers  30 ,  32  may have a thickness between 1-15 millimeter, and preferably between 4-7 millimeter. The material of armor steel plate  36  may be of material with the yield stress and tensile strength of known commercially available ARMOX 500 or ARMOX 600 for example. Such materials have a strength to resist at least 1000 MPa and up to 2500 MPa local tensile stress, which may be realized for example by metallurgical techniques such as adding carbon processing the steel to increase internal stresses. Armor steel plate  36  may have a thickness between 5-15 millimeter or, more narrowly, between 1-10 millimeter. The overall thickness of bulkhead  10  may be in the range of 5-150 millimeter and preferably between 60-120 millimeter. Various types of elastomeric materials may be used, for example one of the types described in the article by C. M. Roland et al. cited in the preceding. A rubber layer may be used for example. 
         [0026]      FIG. 4  shows a cross-section of a detail of an attachment of bulkhead  10  to a structural element  42  of the ship, such as a longitudinal wall section  12 , a girder in longitudinal wall section  12  or similar structural elements such as a ceiling or floor (or decks) or girders in the ceiling or floor. More generally, a structural element is any elements of the ship&#39;s structure that mediates forces that keep the ship from deforming or falling apart during normal operation. Structural element  42  may be made of ductile steel, of a material similar to that of ductile metal layers  30 ,  32 . The attachment comprises a connection piece  40  with a T-shaped cross-section, which may be of the same material as ductile metal layers  30 ,  32 . The foot of the T of connection piece  40  is welded to structural element  42 . In an embodiment wherein bulkhead  10  has an elongated shape, at least the parallel pair of sides of bulkhead  10  that are closest to each other are attached to the structure of the ship are attached according to this T profile. In an embodiment the top of the Tmay be an insert plate with a thickness that is necessarily larger than the sum of the thickness of layers  30  and  32 . For example when the floor to ceiling distance is smaller than the wall to wall distance, the T may be attached to the floor and ceiling. This prevents excessive deformation during explosions. Ductile metal layers  30 ,  32  are welded to the top bar of the T connection piece  40 , at a single or few points only for positioning, in a manner not suitable for significant load transfer. Preferably, armor steel plate  36  is only coupled to connection piece  30  via elastomeric material  34 . Armor steel plate  36  is not fixedly attached directly to connection piece  40 , or at most at a single point. 
         [0027]    Bulkhead  10  may be assembled in situ when the ship is built. In embodiment ductile metal layers  30 ,  32  are first welded to a structural element  42  via connections pieces  40 , leaving a space between the layers. Elastomeric material  34  is applied to armor steel plate  36  (this may be done at a prefab stage) and the armor steel plate  36  with applied elastomeric material  34  is inserted between the ductile metal layers  30 ,  32  that have been welded into the ship&#39;s structure. In this way heat damage to elastomeric material  34  during welding can be avoided. 
         [0028]    In another embodiment, one of the ductile metal layers  30  is first attached to a connection piece like that of  FIG. 4 , for example by welding. Next armor steel plate  36  or a plurality of such plates  36  is or are placed, for example on supporting ridges on the ductile metal layer  30 . Armor steel plate  36  may be placed between stiffeners on ductile metal layer  30  for example. A layer of armor steel plate  36  may be constructed in situ from a series of strips of armor steel plate  36 . Subsequently, the other ductile metal layer  32  is attached to the connection piece, for example by welding. 
         [0029]    In a further embodiment the other ductile metal layer  32  is formed from a series of strips that are each welded to the connection piece and/or to stiffeners on the first attached ductile metal layer  30 . In these embodiments a prefab armor steel plate  36  may be used, which is provided with elastomeric material  34 . Alternatively, the elastomeric material  34  may be added during assembly as well. The connection piece may subsequently be welded to the ship&#39;s structure, or this may be done at an earlier stage, e.g. before the first ductile metal layer  30  is attached to the connection piece. 
         [0030]    In operation, bulkhead  10  combines the effect of the deformability of ductile metal layers  30 ,  32  on protection against pressure pulses with fragment protection by armor steel plate  36 . When a fragment hits bulkhead, it will typically penetrate one of ductile metal layers  30 ,  32  and cause armor steel plate  36  to exhibit “plugging”, i.e. local shearing through the thickness. Because armor steel plate  36  “floats” between ductile metal layers  30 ,  32 , its inhibiting effect on deformation of ductile metal layers  30 ,  32  during pressure pulses is reduced. Elastomeric material  34  makes it possible to mount armor steel plate  36  between ductile metal layers  30 ,  32  with a minimal effect on the deformability. Alternatively, armor steel plate  36  may be kept in place by means of local stiffeners, such as ribs on ductile metal layers  30 ,  32  (not shown), without being directly fixedly attached to these stiffeners. In addition elastomeric material  34  increases the resistance of armor steel plate  36  against explosion propelled fragments. Part of this may be due to reduction of notch effect on armour steel plate  36  by fragments, reducing the otherwise predominant failure mechanism through thickness shearing. 
         [0031]      FIG. 5  shows a cross-section of a further bulkhead, with a plurality of layers of armor steel plates  36   a - c  between ductile metal layers  30 ,  32  with elastomeric material  34  between ductile metal layers  30 ,  32  and armor steel plates  36   a - c  and between the layers of armor steel plates  36   a - c.  By splitting armor steel plate  36  of the embodiment of  FIG. 3  into multiple layers with intermediate elastomeric material a higher protection can be realized at the same or lower weight. 
         [0032]    Armor steel plate  36  of the embodiment of  FIG. 3  or each layer of armor steel plates  36   a - c  of the embodiment of  FIG. 5  may be provided in the form of a plurality of strips parallel to the main plane of bulkhead  10  and separated by elastomeric material. In this way a better deformability may be realized. The strips in different layers of armor steel plates  36   a - c  may be offset relative to each other, so that spaces between strips in one layer are backed up by strips in the other. Horizontal or vertical strips may be used. A plurality of strips may be used over the height or width of the wall. As used herein, “strips” may be elongated rectangular elements, but the term strip cover any shape and height/width ratio. 
         [0033]    In an experiment a wall of 6 millimeter thick ductile metal layers  30 ,  32  of Steel 52, and a double ARMOX-600T double sheet of armor steel  36  was made, with volume between the ductile metal layers  30 ,  32  and armor steel  36  filled with polyurethane rubber adhered to armor steel  36 . Fragments Simulating Projectiles (known in the art as FSP&#39;s) with a mass of 39 grams were fired at the wall with a 29 millimeter canon with speeds up to 1850 m/sec to simulate fragments. Up to this speed penetrations of the wall could not be obtained. At 1831 m/sec the first ductile layer was penetrated. The sheets of ARMOX were dented and cracked but the cracks opened insufficiently to allow remains of the projectile to pass through. The back ductile layer was only deformed. 
         [0034]    This compared favorably with experiments with a wall of 20 millimeter Steel 52, which was penetrated at speeds higher than 1096 m/sec. A combination of two walls of 10 millimeter Steel 52 at 32 mm spacing, was penetrated at speeds higher than 1512 m/sec. A combination of 6 mm Steel 52-7 mm Armox600T 6 mm Steel 52, with 9 and 16 mm spacing was penetrated at speeds higher than 1196 m/sec. 
         [0035]    Although embodiments have been described wherein ductile metal layers  30 ,  32  are provided on both sides of armor steel 36, it should be appreciated that in other embodiments a ductile metal layer on one side may be omitted. This may means that less protection is provided against fragments from the side where no ductile metal layer is present. But this may not be needed when this space itself is protected against projectiles, or contains other means to slow down fragments. 
         [0036]    In an embodiment the structure is assembled by attaching a first metal layer to a connection piece, mounting an armor plate coated on both sides with elastomeric material between on the first metal layer attached to the connection piece, attaching a second metal layer to the connection piece and/or the first metal layer and over the armor plate, the first and second metal layer having a higher ductility than the armor plate. This makes it possible to assemble the wall easily within a ship for example. 
         [0037]    In another embodiment structure is assembled by at least part of the structure with a wall comprising a first and second metal layer attached to elements of the structure, and inserting an armor plate coated on both sides with elastomeric material between the first and second metal layer, the first and second metal layer having a higher ductility than the armor plate. This makes it possible to assemble the wall on site.