Lightweight ceramic armor with improved blunt trauma protection

[0001] 1. Field of the invention

[0002] The present invention relates generally to the field of armor for personal protection of the body. The invention relates more specifically to armor that is intended to withstand and provide protection against blunt trauma or ballistic impact from a projectile or the impact of a stabbing weapon. Such impacts on the armor may transmit a high localized load to the body of the wearer. The invention reduces the intensity of this loading thereby reducing tissue trauma. The invention is intended for use with helmets or protective vests typical of those worn by police and soldiers and with hard armor upgrade plates that are used in conjunction with flexible armor vests to increase protection to rifle bullet level.

[0003] 2. Background Art

[0004] Traditional soft armor used in protective vests is comprised of layers of flexible fabric or non-woven textile. The fibers used in these textiles are typically aramid (such as Kevlar or Twaron) or polyethylene (such as Spectra Shield or Dyneema). Other advanced fibers are also used. When a bullet strikes these layered protective armors, the impact load causes a bulge to develop which deforms the back surface of the armor. Since the armor is worn adjacent to the body, this bulge or deformation can extend into the body of the wearer. If the deformation is large, tissue damage or trauma may occur. The National Institute of Justice (NIJ) has established test methods and standards for this deformation. A clay material, contained in an open faced box, is utilized as a back support for the armor system. After the bullet impact, a depression is preserved in the face of this clay. The depth of this depression is measured and is referred to as the back face signature (BFS). The NIJ requires that this deformation not exceed 44 mm in depth. Certain other specifications, including those used by the military, may be slightly different, but with the general goal of achieving deformations below the NIJ maximum value. It is widely held that trauma resulting from back face signature (BFS) can be severe and debilitating. While this may be of reduced consequence for a law enforcement officer that will receive prompt medical attention in the event of a shooting, it is of serious concern for soldiers that may not receive prompt medical treatment as a result of battlefield conditions. The invention employs a flexible layer which can be utilized as a behind armor addition that substantially reduces deformation while adding very little weight or other negative side-effects.

[0005] U.S. Pat. Nos. 5,918,309 and 6,253,655 each describe armor systems that are relevant to the present invention. However, neither such patent discloses the use of peak load reduction material as a trauma reduction layer in a behind armor configuration. U.S. Pat. No. 5,534,343 discloses the use of an inner layer of flexible cellular material. However, the patent relates to flexible armor. The most relevant prior art appears to be U.S. Pat. No. 5,349,893 which discloses a ceramic armor having an inner layer of cellular material. However, that material is disclosed as being rigid, semi-flexible or semi-rigid, as opposed to flexible.

[0006] In the past, semi-rigid plates of plastic or metal have been used as an insert behind the armor to help spread the deformation across a larger body surface, thereby reducing trauma. These methods add significant weight and also reduce the efficiency of the armor with respect to dispersal of the bullet energy by restricting the movement of the armor back surface during the impact event. The invention is very light in weight and does not reduce the performance of the basic armor material. This is achieved by the use of one or more layers of an energy absorbing material that contains a large volume fraction of fluid filled free space, distributed throughout an internal cellular structure that accommodates deformation on one surface without passing on the same degree of deformation to its adjacent surface. It is, in effect, a highly compressible and flexible layer that disperses a dynamic load applied normal to its surface (resulting from the bullet impact) into a more laterally extensive pressure distribution. When this material is combined with reinforcing fiber material layers, the deformation reduction is further enhanced.

[0007] The flexible cellular layer is used to best advantage when placed at the rear surface of a ceramic/composite armor upgrade plate of the type commonly used to enhance the protection of a flexible ballistic vest armor. When incorporated into the design of the upgrade plate, the deformation passed on to the soft vest armor and in turn to the body of the wearer, is significantly reduced. When applied to the USMC Interceptor body armor system SAPI (Small Arms Protective Insert) plate, the invention reduces deformation from an average of 41 mm to only 35 mm. This reduction is considered highly beneficial to the armor wearer.

[0008] The preferred embodiment of the invention comprises a flexible, resilient, cellular honeycomb structure where the cell walls and principal skin surfaces are made of a thermoplastic polyurethane (TPU) material. The open spaces of the honeycomb may be filled with a fluid, although air is most commonly used. The thickness of the TPU layer is typically between {fraction (3/16)} and {fraction (5/16)} inches in a body armor. The thickness utilized in a vehicle armor, primarily to enhance multihit ballistic performance, may be significantly greater depending on the projectile size and the associated magnitude of impact energy. The dimension of the internal honeycomb structure and the thickness of material comprising the face skins and cell walls may be varied over a wide range depending on the severity of the deformation to be mitigated. The preferred embodiment also employs one or more layers of a strong fiber reinforcement fabric or non-woven textile that is bonded to the face skins of the honeycomb sheet. This reinforcement adds strength and enhances load spreading without defeating the essential dynamic response of the trauma reduction mechanism.

[0009] It is a primary object of the invention to reduce the trauma experienced by the wearer of ballistic armor vests and ceramic/composite upgrade plates when struck by a projectile that is subsequently defeated and captured within the armor. This objective is accomplished by reduction of the magnitude of deformation transmitted to rear most surface of the armor system (the surface closest to the body of the wearer) using a highly compressible and highly flexible layer of cellular material.

[0010] It is a further object of the invention to enhance the multiple hit capability of a ceramic/composite armor upgrade plate by mitigating the armor damage caused by a bullet impact. This is accomplished by the load spreading nature of the trauma reduction layer which dynamically disperses load and creates a counter-load during the impact event. This counter-load reduces the deflection of the rigid armor plate during the impact event which, in turn, reduces the degree of flexure-induced cracking experienced by the plate. The protective value of ceramic/composite armor to multiple hits is directly related to the degree of cracking experienced by the plate.

[0011] The aforementioned objects and advantages of the present invention, as well as additional objects and advantages thereof, will be more fully understood hereinafter as a result of a detailed description of a preferred embodiment when taken in conjunction with the following drawings in which:

[0012] FIG. 1 is a schematic of a typical prior art body armor construction showing hard armor plate on a soft ballistic fabric without the addition of the present invention;

[0013] FIG. 2 is a schematic of a hard armor plate, and ballistic composite backing with the invention added to decrease back face deformation;

[0014] FIG. 3 is a schematic of body armor construction showing a hard armor plate on a soft ballistic fabric with a trauma reduction layer added to decrease back face deformation;

[0015] FIG. 4 is a detailed schematic of the preferred embodiment of the cellular material layer employed in the invention;

[0016] FIG. 5A is a schematic showing a frontal view of a typical test set-up to measure deformation;

[0017] FIG. 5B is a schematic showing a side view of a typical test set-up prior to projectile impact;

[0018] FIG. 5C is a schematic showing a side view of a typical test after projectile impact and defining deformation,

[0019] FIG. 6 is a schematic of typical test results after impact and the deformation value registered in the clay (D₁) for a standard armor system without the invention;

[0020] FIG. 7 is a schematic of typical test results after impact and the deformation value registered in the clay (D₂) for a standard armor system with the invention;

[0021] FIG. 8 is a schematic of load dispersion prior, during and after compression of the invention, demonstrating how load is transmitted from a local impact area to adjacent areas and how the trauma reduction layer actually creates a reactive load adding to the overall regional support;

[0022] FIG. 9a is a schematic showing hard armor plate with out the trauma reduction layer prior to impact;

[0023] FIG. 9b is a schematic showing how conventional support can allow the ceramic to flex;

[0024] FIG. 9c is a schematic showing large crack propagation when the ceramic has flexed from the lack of support;

[0025] FIG. 10a is a schematic showing hard armor plate with the trauma reduction layer prior to impact;

[0026] FIG. 10b is a schematic showing how the added support of the trauma reduction layer can reduce ceramic flex;

[0027] FIG. 10c is a schematic showing reduced crack propagation when the ceramic has flexed less because of the trauma reduction layer;

[0028] FIG. 10d is a schematic showing the trauma reduction layer of the invention prior to point loading;

[0029] FIG. 10e is a schematic showing the trauma reduction layer of the invention after point loading and resultant reactive support force;

[0030] FIG. 11 is a graph showing the peak deformation with and without the trauma reduction layer. The graph also shows how the invention laterally distributes the load more uniformly over a larger area; and

[0031] FIG. 12 is a graph comparing the average deformation values of a standard armor system of the same design the only difference being:

[0032] Awithout the trauma reduction layer,

[0033] Bwith the trauma reduction layer,

[0034] Cwith the invention modified with reinforced Kevlar skins.

[0035] Referring first to FIG. 1, it will be seen that in typical high grade body armor construction, a soft ballistic fabric vest 2 is upgraded with a ceramic/composite body armor upgrade plate 1 comprising a layer 11 of a ceramic material and a layer 12 of ballistic backing material, the latter typically made of a bonded or unbonded, woven or non-woven textile comprising strong fibers such aramid, polyethylene, nylon, glass, PBO or other strong fibers. Layers 11 and 12 may be enclosed by a spall suppression cover material 13.

[0036] In FIG. 2, the body armor upgrade plate 3 has been modified as compared to the plate 1 of FIG. 1. More specifically, a trauma reduction layer 7 has been added to a ceramic layer 5 and ballistic composite backing layer 6 within the surrounding spall cover 4. This improved ceramic/composite upgrade plate 3 is shown in FIG. 3 adjacent a soft ballistic vest 2 of the same type shown in FIG. 1. FIG. 4 illustrates the preferred characteristics of the trauma reduction layer 7. It will be seen that trauma reduction layer 7 comprises a cellular or honeycomb material 14 having face skins 13 and preferably covered on both of its external surfaces by reinforcement layers 16 which are preferably bonded to the cellular or honeycomb material. The cellular honeycomb material may for example be a thermoplastic urethane ether material (TPU) supplied by Supracor of San Jose, Calif. under the trademark STIMULITE. In the preferred embodiment, the TPU material has a thickness of 0.1875 inches, a cell diameter of 0.25 inches, a cell wall thickness of 0.006 inches and a cell face or cover surface thickness of 0.005 inches. The presently preferred TPU material comprises honeycomb cells containing air. However, honeycomb cells having other gases and even certain liquids such as water, may be useful to spread the impact load and reduce blunt trauma-causing deformation. Moreover, other TPU material thicknesses, cell dimensions, and cell wall thicknesses may be advantageous depending upon the armor and deformation requirements. Additionally, other forms of cellular material besides honeycomb cellular, may be suitable as a trauma reduction layer as long as each cell is hermetically isolated from cells adjacent to it so that as some cells are compressed others are forced to expand from the resulting transfer of pressure. The optional reinforcement layers 16 are preferably made of a ballistic fabric such as Kevlar in relatively thin layers, i.e., less than about 0.01 inches.

[0037] Referring to FIG. 5 it will be seen that in order to test the resulting armor in accordance with methods required by the National Institute of Justice (i.e., NIJ Standard 0101.04) the ceramic/composite body armor upgrade plate 3 and soft armor panel 2 are affixed to a clay structure 8 within a box 9 and a bullet 10 is fired at the armor. The extent of deformation of the clay 8 is then measured as an indication of blunt trauma. FIG. 6 illustrates this test for high grade body armor comprising a ceramic layer 5, composite backing 6 and soft armor fabric 2 while FIG. 7 illustrates the same test for armor with the additional trauma reduction layer 7. The respective measured deformations D₁ and D₂ of clay 8 may then be compared such as shown graphically in FIG. 11. As shown in FIG. 11, D₁ is significantly greater than D₂. Moreover, FIG. 11 shows that use of a trauma reduction layer in accordance with the present invention spreads the impact load over a larger expanse (X₂ versus X₁) but decreases the peak value of deformation, the latter being the key to reducing blunt trauma to the armor-wearing personnel. The reason for this impact load spread and reduction of peak deformation is believed to be due to the interaction of cells in the TPU layer of the trauma reduction layer. This interaction is illustrated in FIGS. 8a, 8b and 8c. FIG. 8a shows the composite backing and TPU layer interface before projectile impact; FIG. 8b shows the interface during initial impact; and FIG. 8c shows the interface after initial impact. It can be seen that during initial impact, the composite backing begins to compress the TPU material causing cell walls to compress and creating a lateral force vector transmitted to adjacent cells. Moreover, as the backing continues to deform from the force of the projectile, the collapse of impacted cells continues creating both lateral and oppositely-directed vectors in adjacent cells due both to mechanical deformation and to reduction of cell volume at the point of impact.

[0038] The trauma reduction layer of the present invention has still another benefit, namely reducing crack propagation in the ceramic layer and thereby increasing impact resistance to second shots. This benefit is illustrated in FIGS. 9 and 10. FIG. 9a shows the conventional upgrade armor plate prior to projectile impact. FIG. 9b shows the same plate during impact and demonstrates substantial ceramic layer flexure due to a lack of underlying support. FIG. 9c illustrates substantial crack propagation in the ceramic layer due to the large extent of flexure. FIG. 10a shows the improved upgrade armor plate prior to projectile impact. FIG. 10b shows the improved plate during impact and demonstrates reduced ceramic layer flexure because of added support from the underlying layers. FIG. 10c shows significantly reduced crack propagation in the ceramic layer. The added support results from a reactive support force generated by cellular layer. FIG. 10d shows that layer before impact. FIG. 10e illustrates how the reactive force results from compression at the impact point and the inflation of adjacent cells caused by that compression. Thus, the present invention provides two significant benefits. It reduces blunt trauma and it improves performance against a second impact.

[0039] Finally, FIG. 12 shows actual data of tests conducted using the test setup of FIG. 5a wherein three shorts were fired into three personal armor configurations. Each A bar shows deformation of a conventional upgrade plate. Each B bar shows deformation of an improved plate with commercial TPU as a trauma reduction layer. Each C bar shows deformation of an improved plate with commercial TPU modified with Kevlar face covers on the front and back surfaces of the cellular layer.

[0040] The graph of FIG. 12 clearly demonstrates the significant reduction in deformation with the addition of a TPU-based trauma reduction layer. It also shows the even greater reduction in deformation with the addition of a Kevlar reinforced TPU-based trauma reduction layer.

[0041] Having thus disclosed preferred embodiments of the invention, it will be understood that those having the benefit of the teaching herein will now perceive various modifications and additions to the basic features revealed. Therefore, such modifications and additions are deemed to be within the scope hereof which is to be limited only by the appended claims and their equivalents.