This application claim the priority of U.S. Provisional Application No. 60/967,232 filed on Aug. 31, 2007 naming Linda R. Pinckney, Huan-Hung Sheng, Steven A. Tietje and Jian Zhi (Jay) Zhang as inventors and entitled “MULTI-HIT TRANSPARENT, MULTI-STACK ARMOR SYSTEM.”
The invention is directed to a hybrid laminated transparent armor system, and in particular to a composite armor system containing a glass-ceramic material and a conventional glass material.
Transparent materials that are used for ballistic protection (armor) include (1) conventional glasses, for example, soda lime and borosilicate glass which are typically manufactured using the float process; (2) crystalline materials such as aluminum oxy-nitride (ALON), spinel, and sapphire; and (3) glass-ceramic materials (“GC”). In the last category, a transparent lithium disilicate GC from Alstom, known as TransArm, has been studied by several groups. Due to its superior weight efficiency against ball rounds and small fragments, TransArm has the potential to increase performance of protective devices such as face shield; studies of the shock behavior of these materials have shown that the GC has a high post-failure strength compared to that of amorphous glasses. See GB 2 284 655 A; PCT International Patent Publication WO 03/022767 A1; and J. C. F. Millett, N. K. Bourne, and I. M. Pickup, The behaviour of a SiO2—Li2O glass ceramic during one-dimensional shock loading, J. Phys. D: Appl. Phys. 38, 3530-3536 (2005). Other prior art includes  U.S. Pat. No. 5,060,553 and  U.S. Pat. No. 5,496,640 which describe, respectively,  armor material based on glass-ceramic bonded to an energy-absorbing, fiber-containing backing layer, and  fire- and impact-resistant transparent laminates comprising parallel sheets of glass-ceramic and polymer, with intended use for security or armor glass capable of withstanding high heat and direct flames. Additional patent or patent application art includes U.S. Pat. No. 5,045,371 titled Glass Matrix Armor (describing a soda-lime glass matrix with particles of ceramic dispersed throughout, the ceramic not being grown in situ in the glass) and U.S. Patent Application US 2005/0119104 A1 (2005) titled Protection From Kinetic Threats Using Glass-Ceramic Material (describing an opaque armor based on anorthite (CaAl2Si2O8) glass-ceramics).
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In one aspect, using ballistics testing of various combinations of glass, glass-ceramic, and spall-resistant layering, we have discovered that the combination of a hard transparent GC strike face with one or more intermediate layers of glass and a spall-resistant backing layer provides significantly better ballistics performance as a function of areal density than does an all-GC or all-glass design. We have seen no reference in the prior art to the benefits of this particular configuration.
In another aspect, the invention is directed to the use of laminations of transparent GCs with glass for various armor systems; for example, armor systems for ground vehicles and aircraft as well as for personal protective devices. The optical properties of these armor systems meet the visible transparency as well as near IR transparency requirements of military armor systems, and their moderate density combined with a higher ballistics limit offers either of two important attributes or a combination of both attributes which are:
(1) The ability to achieve ballistics performance equivalent to that of glass, with lower thickness, thereby providing critically-needed lower weight for armor systems; and
(2) The ability to achieve superior ballistics performance with the same laminate thickness used for current transparent armor systems.
In one embodiment the invention is directed to a transparent armor laminate system, said laminate system comprising one or a plurality of glass-ceramic material layers, one or a plurality of glass layers and a backing or spall layer; wherein said glass-ceramic layer has a crystalline component and a glass component, the crystalline component being in the range of 20-98 Vol. % of the glass ceramic and the glass component being in the range of 2-80 Vol. %. In another embodiment the crystalline component is in the range of 50-98 Vol. % and the glass component is in the range of 2-50 Vol. %. The layers in a stack of the foregoing materials may be bonded together using an adhesive material in any form, for example, a paste, gel or a sheet material. The adhesive material is placed between the layers of each stack. When the glass-ceramic, glass and spall layers are bonded together using a polymeric sheet material, bonding is achieved by application of heat and/or pressure. Adhesives in gel, paste or other fluid or semi-fluid form may be cured and bonding achieved by application of heat or radiation. The bonding materials, adhesive or polymer material, should match the refractive index of the other materials as closely as possible so as not to lessen optical performance. In preferred embodiments the adhesive and polymeric material should be transparent to infrared radiation.
In another embodiment the invention is directed to a transparent armor laminate system having a plurality of “n” sub-stacks containing a glass-ceramic strike-face layer, at least one intermediary glass layer, and a backing layer, where n is an integer having a value of greater than one, and wherein each sub-stack is separated by an interlayer. The interlayer includes a filling material comprising a gas (including air), a fluid, a polymeric material, a gel; or a combination thereof. The GC layer has a crystalline component and a glass component, the crystalline component being in the range of 20-98 Vol. % and the glass component being in the range of 2-80 Vol. %. In another embodiment the crystalline component is in the range of 50-98 Vol. % and the glass component is in the range of 2-50 Vol. %. The layers in a stack of the foregoing materials may be bonded together using an adhesive material in any form, for example, a paste, gel or a sheet material. The adhesive material is placed between the layers of each stack. When the glass-ceramic, glass and spall layers are bonded together using a polymeric sheet material, bonding is achieved by application of heat and/or pressure. Adhesives in gel, paste or other fluid or semi-fluid form may be cured and bonding achieved by application of heat or radiation. The bonding materials, adhesive or polymer material, should match the refractive index of the other materials as closely as possible so as not to lessen optical performance. In preferred embodiments the adhesive and polymeric material should be transparent to infrared radiation.
In a further embodiment the invention is directed to a transparent armor laminate system having plurality of “n” of sub-stacks containing a glass-ceramic strike-face layer, one or a plurality of glass layers and a backing layer, where n is an integer greater than one, and wherein each sub-stack is separated by an interlayer comprising a gap filled with a gas such as air or inert gas, and wherein the glass-ceramic layer has a crystalline component and a glass component, the crystalline component being in the range of 20-98 Vol. % crystalline component and the glass component being in the range of 2-80 Vol. %. In another embodiment the crystalline component is in the range of 50-98 Vol. % and the glass component is in the range of 2-50 Vol. %.
A transparent armor laminate system comprising a plurality of sub-stacks separated by an interlayer; the front or first sub-stack comprising
a plurality of layers including a strike-face layer comprising a glass-ceramic,
a backing layer of a spall-resistant material, and
at least one intermediate layer comprising glass and laminated between the strike-face layer and the backing layer;
the remaining sub-stack(s) comprising a plurality of layers including a strike-face comprising at selected one from the group consisting of a glass-ceramic layer and a glass layer,
a backing layer of a spall-resistant material, and
at least one intermediate layer comprising glass and laminated between the strike-face layer and the backing layer; and
an interlayer between at least the front or first stack and the adjacent sub-stack comprising an isolating material selected from a group consisting of gas, a fluid, a polymeric material, a gel, and combinations thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
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FIG. 1 is an illustration of a typical commercially available armor system composed of glass and a polycarbonate backing.
FIG. 2 is an illustration of the invention generally illustrating the use of a glass-ceramic strike face, one or a plurality of glass layers and a polycarbonate backing.
FIG. 3 illustrates a lightweight glass-ceramic/glass as compared to an all float-glass system as is commercially available.
FIG. 4 is a graph of ballistic velocity vs. areal density illustrating the superiority of a glass-ceramic/glass armor system of the invention over other types of systems.
FIG. 5 is a graph illustrating the weight savings that can be achieved using a glass-ceramic/glass laminate as opposed to an all-glass laminate.
FIGS. 6A, 6B, and 6C illustrate three configurations embodying sub-stacks separated by an interlayer or gap of or containing a material such as a polymer, air, an inert gas, a gel or other fluid that does not impair the transparency of the armor laminate.
FIGS. 7A-7H illustrate a ballistic test using a two stack transparent armor system of the invention with an air gap between the stacks.
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As used herein the term, strike-face, is used to signify the face of the laminate armor that receives the incoming projectile.
It is generally recognized that a material's hardness and fracture toughness contribute to its ballistic performance, although the exact correlation between static material properties and ballistic performance is still elusive after decades of research (see J. J. Swab, Recommendations for Determining the Hardness of Armor Ceramics, Int. J. Applied Ceram. Technol., Vol. 1  (2004), pages 219-225). One hypothesis is that an ideal armor material needs to have sufficient hardness to break up the projectile, but above a certain threshold value, hardness no longer dictates performance. If optimization of other mechanical properties such as fracture toughness can be achieved while the hardness is above the threshold value, armor performance can be optimized as well.
As illustrated in FIG. 1, a typical commercial transparent armor system 10 consists of one or more sheets (first four layers) of glass 12 or transparent crystalline ceramic laminated into a composite layered structure with a backing layer 14 or “spall catcher.” The gray arrow 11 in FIG. 1 indicates the path of an incoming projectile. The number of layers and order of layers in the composite structure depend upon the threat types the armor system is designed to defeat. Transparent crystalline materials are usually ALON (aluminum oxynitride), spinel and sapphire. The typical transparent glass materials used for these layers are conventional glasses, such as soda lime and borosilicate glasses, typically manufactured using conventional float glass processing.
While transparent crystalline ALON, spinel and sapphire have all demonstrated weight efficiencies more than three times better than glass so that the armor system can stop the same projectiles with less than one-third the total weight of a glass-based system, these crystalline materials require the use of expensive powder processing (ALON and spinel) or crystal growth (sapphire) methods to make the materials. These methods are intrinsically very expensive, have low product yields, result in materials that are very costly to finish/polish, and are not conducive to making large size sheets of transparent materials that are required for uses such as windows. In addition, if curved sheets are required for a particular application, this requirement would add further complexity and cost. As a result, these high performance materials are mainly used in research laboratories, and are rarely used in real-world situations.
Glass offers significant cost benefits over crystalline materials that require high temperature processing. However, in order to increase the ballistic performance of glass armor, more layers and/or thicker glass has to be added. As a result, the overall armor weight has become more and more unbearable to the “user” whether a person or a vehicle. There is consensus that a fundamental solution lies in the use of innovative materials, not more of the same glass.
As a class of material, GCs combine the manufacturability of glass with many of the property benefits of crystalline materials. GCs offer significant advantages over conventional glass in resisting the penetration of projectiles that include armor piercing (hard steel core) bullets. In ballistics testing of various combinations of layers of glass, GC, and a spall-resistant backing, we have discovered that the combination of a hard transparent glass-ceramic strike face with one or more intermediate layers of glass and a backing layer comprising a spall-resistant material provides significantly better ballistics performance as a function of areal density than does an all-glass-ceramic or an all-glass design with or without a spall-resistant backing layer. FIG. 2 is an illustration of a laminated armor 20 of the invention having a hard glass-ceramic strike face layer 26 (first or front-most layer), a plurality of glass layers 22 (next three layers) and a backing layer 24 (back-most layer) comprising a spall-resistant material. The spall-resistant material may comprise polycarbonate or other tough polymer. An advantage of the system represented by 20 is that in addition to stopping projectiles (represented by arrow 21) at a preset velocity (e.g., muzzle velocity for certain type of bullets) they would require less material—in thickness or areal density—than conventional glass laminates and even glass-ceramic/glass-ceramic laminates. The gray arrow 21 in FIG. 2 indicates the path of an incoming projectile.
In addition to offering lower weight (as compared to glass-only laminate) and lower cost (as compared to crystalline materials), the hybrid configuration in the present invention requires much less total glass-ceramic thickness; for example, 10-20 mm of the laminate of the present invention is functionally comparable to a 30 mm laminate of glass-ceramic-only. This lower material requirement greatly facilitates manufacturability of the glass-ceramic from an optical transmission standpoint. Many glass-ceramics are prone to absorption problems because a small amount of impurity present in the glass, such as iron oxide, tends to react with TiO2 (a typical nucleation agent) to cause absorption in the blue end of the visible spectrum. FIG. 3 illustrates the difference, and hence the weight savings through layer reductions that can be obtained using a GC/glass laminate 50 (right side of figure) as compared to an “all float-glass” system 40 (left side of figure).
Glass-ceramics are microcrystalline solids produced by the controlled devitrification of glass. Glasses are melted, fabricated to shape, and then converted by a heat treatment to a partially-crystalline material with a highly uniform microstructure. Thus, glass-ceramics contain a crystalline component and a glass component. The basis of controlled crystallization lies in efficient internal nucleation, which allows development of fine, randomly oriented grains without voids, micro-cracks, or other porosity. Like glass and ceramics, GCs are brittle materials which exhibit elastic behavior up to the strain that yields breakage. Because of the nature of the crystalline microstructure, however, mechanical properties including strength, elasticity, fracture toughness, and abrasion resistance are higher in GCs than in glass. Glass-ceramics found useful for transparent armor application contain 20-98 Vol. % crystalline component and 2-80 Vol. % glass component while maintaining their transparency. In another embodiment the crystalline component is in the range of 50-98 Vol. % and the glass component is in the range of 2-50 Vol. %.
As noted above the exact correlation of static material properties and ballistic performance is poorly understood. One hypothesis is that an ideal armor material must have sufficient hardness to break up the projectile, but above a threshold value hardness no longer dictates performance. This hypothesis is supported by the moderate, but by no means impressive, Knoop hardness values of 700-730 that are obtained, for example, with spinel GCs. The microstructure of transparent GCs themselves—typically 10-50 nm crystals dispersed uniformly throughout the material, often in a continuous, “softer” residual glass—can provide enhanced ballistics protection. Hasselman and Fulrath (“Proposed fracture theory of a dispersion-strengthened glass matrix, J. Am. Ceram. Soc., 49 (1966), pp. 68-72) proposed a fracture theory wherein hard spheroidal crystalline dispersions within a glass will limit the size of flaws which can be produced on the surface, thereby leading to an increase in strength. The microstructure, strength and moderate hardness of GCs may explain their efficacy as a strike face in glass-GC hybrid laminates.
Ballistic results for a variety of glass and GC laminate configurations are illustrated in the graph in FIG. 4. In all laminates used in FIG. 4, a one-half inch, (˜1.27 cm (0.5 inch)) polycarbonate backing was used in conjunction with the glass and/or glass-ceramic materials. FIG. 4 is a plot of the AP ballistic limit, that is, the ability to stop armor-piercing bullets in units of ft/sec, against laminate areal density (in units of lbs/ft2). The black circles represent various GC-glass configurations. Corresponding data for commercial glass laminates are taken from the literature (Ceramic Armor Materials by Design, ed., J. W. McCauley Ed., Ceramic Transactions, Vol. 134 (2002). High ballistics limit at the lowest areal density is what is desired. FIG. 5 is a graph illustrating the weight savings of a hybrid GC-glass laminate compared to that of an all-glass laminate. The boxes to the right illustrate the relative thickness of the GC and glass (grey and white, respectively) for each data point. Boxes 1-4 represent laminates of comparable total thickness and areal density. Box 1 has the greatest thickness of glass-ceramic material and Box 3 has the smallest thickness of glass-ceramic material. Box 4 is all glass. Box 5 represents an all glass laminate of greater thickness than that of Box 4.
While the above discussion is focused on armor performance against a single bullet impact, the real armor system needs to protect against multiple hits as is typically encountered in typical hostile situations. The GC/glass/backing sub-stack shows significant weight savings for a single bullet impact compared to conventional all-glass or all-GC laminates. The inventors have discovered that a system comprising a plurality of sub-stacks laminated with an interlayer between the sub-stacks can withstand multiple bullet hits. The interlayer isolates cracks between sub-stacks and may include an isolating material and/or a gap. The isolating material may comprise an energy absorbing material such as, for example, a polymer or gel. The first GC/glass/backing sub-stack would stop the first bullet impact. The interlayer reduces significant damage in the second sub-stack, thereby permitting the second sub-stack to stop a second impact. The key benefit of the system is that the whole lay-up can be manufactured in one autoclave process step.
FIGS. 6A, 6B, and 6C illustrate three exemplary system designs where weight savings observed in single hit cases should be readily translated into multi-hit benefits. The direction of impact of an incoming projectile is represented by arrows 71, 81 and 91 FIG. 6A illustrates a system 70 comprising two sub-stacks 73, 75, where each sub-stack includes a laminate having a glass-ceramic strike-face layer 76, at least one intermediate layer 72 comprising glass, and a backing layer 74. An interlayer 78 separates the first sub-stack 73 from the second sub-stack 75. The system presents a strike-face layer 76 to an incoming projectile.
A variation of the design is illustrated in FIG. 6B, in which the system 80 comprises two sub-stacks 83, 85, and each sub-stack includes a laminate having a glass-ceramic strike-face layer 86, at least one intermediate layer 82 comprising glass, and a backing or spall catching layer 84. An interlayer 88 separates the first sub-stack 83 from the second sub-stack 85. The system presents a strike-face layer 86 to an incoming projectile. The interlayer 88 in this embodiment includes a gap between two sub-stacks. The gap may be filled with air, an inert gas, or with a clear gel to reduce interfacial reflective losses. A gap may better ensure that cracking in the first sub-stack 83 does not cause damage in the second stack 85, thereby potentially improving multi-hit performance further.
A further variation of the design is illustrated in FIG. 6C, in which the system 90 comprises two sub-stacks 93 and 95. The first or front sub-stack includes a laminate having a glass-ceramic strike-face layer 96, at least one intermediate layer comprising glass 92, and a backing layer or spall catching layer 94 of, for example, polycarbonate. This system presents a glass-ceramic strike face to an incoming projectile. The second sub-stack includes a laminate having a glass strike-face layer 92, at least one intermediate layer 92 also comprising glass, and a backing or spall catch layer 94. An interlayer 98 separates the first sub-stack from the second sub-stack. The interlayer 98 in this embodiment includes a gap between the two sub-stacks. The gap may be filled with air, inert gas, or with a clear gel or polymer. The gap may insure that cracking in the first sub-stack does not cause damage in the second sub-stack, thereby improving multi-hit performance further.
In the system designs of FIGS. 6A 6B, and 6C, the sub-stacks are separated by an interlayer. The interlayer will be filled by an isolating material that can serve to isolate cracks between sub-stacks as a result of a projectile impacting the armor laminate. Isolating materials include polymers, air, inert gases, gels, and fluids. The isolating material should (a) allow the first sub-stack to deform relatively independently of the presence of the second sub-stack, (b) have an index of refraction that matches or substantially matches that of the sub-stack so as to reduce optical distortion, and (c) be substantially transparent so as to reduce the loss of light transmission through the system. In most transparent armor systems, for example those used for shields, windows (for example, vehicles and buildings), and goggles, transmission in the near-infrared (e.g., 700-1000 nm) should be as unimpaired as possible. However, losses in the infrared can be tolerated if the laminate system will only or primarily be used in daylight. Conversely, if the system will only or primarily be used in conjunction with infrared equipment, then some transmission losses in the visible range can be tolerated. If the system is to be used in conjunction with ultraviolet equipment, then transmissions losses in the ultraviolet region should be reduced as much as is possible or feasible. Thus, the exact tolerance for light (visible, infrared and/or ultraviolet) losses will be dictated by the use to which the laminate will applied. The materials used in the laminate system should thus be selected in accordance with the intended use.
The glass-ceramic part of the laminate system should have good transparency and minimal light transmission losses or distortion in the selected transmission regions (for example without limitation, in the visible, infrared and ultraviolet ranges). The exact percentage of the phases, crystalline and glass, depend on the composition of the glass before ceramming and the precise heat treatment used to crystallize the glass. Any glass material that can be cerammed according to the foregoing teachings and the teachings elsewhere herein can be used as the glass-ceramic component of the armor laminate. In addition the glass-ceramic material should have a Knoop hardness of at least 600. The desired microstructure and crystallinity level in the glass-ceramic will likely depend on the types of threat that will be encountered and the multi-hit pattern that is being sought. Examples of the glass-ceramics include, without limitation, glass-ceramics in which the crystalline component is beta-quartz, a spinel and mullite.
The glass component of the armor laminate may consist of at least one glass layer having a thickness in the range of 5-50 mm. In one embodiment, each individual glass layer has a thickness in the range of 10-20 mm. The glass material can be any glass meeting the criteria of transmissivity and low distortion as described elsewhere herein. Examples of such glass include but are not limited to soda-lime glass; silica glass, borosilicate glass; and aluminoborosilicate glass.
The backing layer, also known as the “spall catcher,” comprises a spall-resistant material such as a polymeric material. Suitable polymeric materials include polyacrylates, polycarbonates, polyethylenes, polyesters, polysulfones and other polymeric materials as used in currently available transparent armor. As with the glass-ceramic materials and the glasses used in the armor laminates of the invention, the spall-resistant material must meet the criteria of transmissivity and low distortion as described elsewhere herein.
In one embodiment transparent armor laminate has a strike-face layer comprising glass-ceramic, at least one intermediate layer comprising glass, and a backing layer. The individual layers have a thickness in the range of 10-20 mm so that the thickness of the armor laminate is 30-60 mm. The Knoop hardness of the glass-ceramic material is greater than 600 and preferably greater than 700.
In another embodiment the transparent armor system has a plurality of sub-stacks, that is, a plurality of “n” sub-stacks where n is an integer having a value of greater than one, that is, n=2, 3, 4, 5 . . . etc. FIGS. 6A, 6B, and 6C illustrate three embodiments in which n=2. In FIGS. 6a and 6B, each sub-stack is a laminate of a glass-ceramic 76, 86, at least one layer of a glass 72, 82 and a backing layer 74, 84. Projectile impact is illustrated by the arrow 71, 81. In FIG. 6C, the first sub-stack is a laminate of a glass-ceramic, at least one layer of a glass, and a backing layer, while the second sub-stack is a laminate of at least one layer of glass and a backing layer. The two or more sub-stacks have interposed between them an interlayer 78, 88, the interlayer being filled with an isolating material. The interlayer will have a thickness sufficient to isolate adjacent sub-stacks. The thickness of the interlayer will typically be at least about 0.050 inch (about 0.127 centimeter) and up to about 0.500 inch (about 1.27 centimeters). The isolating material may include a gas, a fluid, a polymeric material, a gel, or combinations thereof. The gas may include air or an inert gas, and preferably will be dry, that is, low humidity, so as to reduce fogging. A polymeric material should resist brittle failure and preferably will be tough, that is, resistant to shock or impact and capable of absorbing energy before fracture. Examples of tough polymers include polycarbonate, polyurethane, and certain acrylics. The isolating material will be selected depending on the intended use, and will preferably reduce distortion and transmissivity losses.
Of course, additional stacks may be used depending on the application. In a variation of this embodiment, the number of sub-stacks is three or more and the thickness of each strike-face layer, intermediate layer, and backing layer ranges from 5-15 mm, so that the total thickness of a sub-stack ranges from 15-45 mm. In the first sub-stack the glass-ceramic strike-face layer is the most forward, that is, directed toward the incoming projectile, in the sub-stack so that in the event of multiple hits the strike-face layer will receive any projectile that penetrates the preceding sub-stacks.
The three illustrations shown as FIG. 6a, 6B, and 6C are examples of possible system designs and it should be understood that permutations of the three material types in the sub-stack are possible. Optimal designs will likely depend on the types of threat that will be encountered and the multi-hit pattern that is being sought. Various configurations of a glass-ceramic strike-face backed by lower modulus glasses, which in turn are turn are backed by a very low modulus polymer layer, may provide optimal ballistics efficiency in breaking up and slowing down a projectile, while affording a lower-weight package (armor system) than is possible with either an all-glass or all-glass-ceramic configuration.
Two sub-stacks were made with each sub-stack comprising a glass-ceramic strike face, a glass intermediate layer, and a polycarbonate backing layer. Each sub-stack was capable of stopping a 0.30 cal. APM2 projectile. One sub-stack was placed on top of the other sub-stack with one-quarter inch spacers separating the sub-stacks. Using an autoclave, the sub-stacks were bonded into a system having an interlayer comprising an air gap. A first projectile was directed at the strike-face of the first sub-stack. As expected, the first sub-stack stopped the projectile and the second sub-stack sustained no damage. A second, third, and fourth projectile were fired in a T-pattern at the system. The system stopped all four projectiles without penetration. The specimen had an areal density of 40.5 lb (18.37 kilograms)/sft. In contrast, the US Army reports an areal density of greater than 50 lb (22.68 kilograms)/sft for a glass-only solution.
Two sub-stacks were made with the front sub-stack comprising a glass-ceramic strike face, a glass intermediate layer, and a polycarbonate backing layer. The second sub-stack comprised a glass strike face, a glass intermediate layer, and a polycarbonate backing layer. One sub-stack was placed on top of the other sub-stack with one-quarter inch spacers separating the sub-stacks. Using an autoclave, the sub-stacks were bonded into a system having an interlayer comprising an air gap. A first projectile was directed at the strike-face of the first sub-stack. As expected, the first sub-stack stopped the projectile and the second sub-stack sustained no damage. A second, third, and fourth projectile were fired in a T-pattern at the system. The system stopped all four projectiles without penetration. The specimen had an areal density of 38.5 lb (17.5 kg)/sft.
Two systems, A and B, were prepared as in Example 1 except that the interlayer was filled with polyurethane, and the interlayer thickness was only 0.100 inch (0.254 centimeter) and 0.050 inch (0.127 centimeter), for the systems A and B, respectively. In both cases the second stack saw damage from the first impact. In system A, the damage was apparently small enough that the armor sample was able to stop all four projectiles fired in a T-pattern. System B stopped the first three projectiles, but the fourth projectile passed completely through the system.
Systems were produced as in Example 2 but with a UV-curable, translucent polymer replacing the polyurethane. Both the 0.100 inch (0.254 centimeter) and 0.050 inch (0.127 centimeter) interlayer samples stopped only three of four projectiles fired in a T-pattern.
Systems were produced as in Example 2 but with a silicone polymer replacing the polyurethane. Both the 0.100 inch (0.254 centimeter) and 0.050 inch (0.127 centimeter) interlayer samples stopped only three of four projectiles fired in a T-pattern.
FIGS. 7A-7H illustrate a ballistic test using a two stack transparent armor system of the invention with an air gap between the stacks. The first or front stack has a glass ceramic strike face. The first or front layer of the second stack has a glass strike face. FIGS. 7A and 7B are front and back views, respectively, of the two stack system after a single projectile has hit the first stack. FIG. 7C is a back view as in FIG. 7B, but with a sheet of paper inserted in the gap between the first stack and the second stack. The absence of crack or fracture line in FIG. 7C shows that the projectile did not strike the front layer of the second stack (the second stack strike face). Thus, the second stack is undamaged. FIGS. 7D, 7F and 7G are front views shown the first stack after it has been hit by the second, third and fourth projectiles. FIG. 7G clearly shown the inverted T-pattern made by the four projectiles. As back view 7E and 7H (after the second and fourth shots, respectively) shown cracking of the second stack, there is no penetration through the second stack.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.