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Composite Armor

One method of providing armor that is lighter and stronger is to use composite armor. Composite armor consists of different materials such as metals, plastics, or ceramics that together provides an armor that is stronger and lighter than traditional pure metal armor. A relatively famous form of composite armor is so called "Chobham armor," that sandwiches a layer of ceramic between two plates of steel armor, and is used on main battle tanks such as the Abrams, where it has been proven to be highly effective in defeating high explosive anti-tank (HEAT) rounds. When the British introduced Chobham armor in 1965 a asignificant step was taken in armor technology. Chobham armor is basically a laminate armor, with ceramic, steel and titanium sandwiched together between ballistic nylon.

The Chobham armor was used in the beginning of the 1980's in the Valiant and the prototypes of the British Challenger tank. The further developed combination bulkhead armor using armor steel and other metals was used in the Leopard 2. The sudden progress in the area of protective technology had the result in a few Western nations that the race in ammunition effectiveness, which had been abandoned temporarily, was taken up again. In the Ml, Leopard 2 and Challenger, it can be expected that no penetration will occur from a large part of today's antitank weapons at up to medium ranges, at least not from the front or the side (up to certain side angles). Although Chobham armors details are still a closely held secret, its existence provides significant protection to the crew of any tank protected by it. However, while "Chobham armor" is well suited for use placement on a main battle tank, it is too heavy and expensive for use on lighter fighting vehicles or transports.

There are four main considerations concerning protective armor panels. The first consideration is weight. Protective armor for heavy but mobile military equipment, such as tanks and large ships, is well known. Such armor usually comprises a thick layer of alloy steel, which is intended to provide protection against heavy and explosive projectiles. However, reduction of weight of armor, even in heavy equipment, is an advantage since it reduces the strain on all the components of the vehicle. Furthermore, such armor is quite unsuitable for light vehicles such as automobiles, jeeps, light boats, or aircraft, whose performance is compromised by steel panels having a thickness of more than a few millimeters, since each millimeter of steel adds a weight factor of 7.8 kg/m2.

Armor for light vehicles is expected to prevent penetration of bullets of any type, even when impacting at a speed in the range of 700 to 1000 meters per second. However, due to weight constraints it is is difficult to protect light vehicles from high caliber armor-piercing projectiles, e.g. of 12.7 and 14.5 mm, since the weight of standard armor to withstand such projectile is such as to impede the mobility and performance of such vehicles.

A second consideration is cost. Overly complex armor arrangements, particularly those depending entirely on synthetic fibers, can be responsible for a notable proportion of the total vehicle cost, and can make its manufacture non-profitable.

A third consideration in armor design is compactness. A thick armor panel, including air spaces between its various layers, increases the target profile of the vehicle. In the case of civilian retrofitted armored automobiles which are outfitted with internal armor, there is simply no room for a thick panel in most of the areas requiring protection.

A fourth consideration relates to a common problem with ceramic armor concerns damage inflicted on the armor structure by a first projectile, whether stopped or penetrating. Such damage weakens the armor panel, and so allows penetration of a following projectile, impacting within a few centimeters of the first. The ceramic plates used for personal and light vehicle armor, which plates have been found to be vulnerable to damage from mechanical impacts caused by rocks, falls, etc. . Indeed, one of the significant drawbacks to the use of ceramic materials in armor applications is that they lack repeat hit capability. In other words, ceramic materials tend to disintegrate upon the first hit and cease to be useful when subjected to multiple projectiles. For a more effective utilization of ceramic materials in armor applications, it is necessary to improve the impact resistance of this class of materials.

Fairly recent examples of armor systems are an armor plate composite including a supporting plate consisting of an open honeycomb structure of aluminium; and an antiballistic composite armor including a shock-absorbing layer. Also of interest is spaced armor including a hexagonal honeycomb core member. Other armor plate panels use sintered refractory material, as well as the use of ceramic materials, are described.

Ceramic materials are nonmetallic, inorganic solids having a crystalline or glassy structure, and have many useful physical properties, including resistance to heat, abrasion and compression, high rigidity, low weight in comparison with steel, and outstanding chemical stabiity. Such properties have long drawn the attention of armor designers, and solid ceramic plates, in thicknesses ranging from 3 mm. for personal protection to 50 mm. for heavy military vehicles, are commercially available for such use.

One common type of modern composite armor includes a layer of ceramic between steel armor plates, which has proved to be effective in protecting tanks. One advantage of the use of a ceramic layer with steel armor plates is that the ceramic material absorbs projectile penetration by fragmentation, diminishing the penetration. There is a need to provide reduced weight composite armor with the capability of providing protection against multiple ballistic impacts for use on vehicles lighter than tanks, buildings, and even as personal body armor by individuals. However, it has been found that following an initial ballistic impact the effectiveness of conventional ceramic armor can quickly deteriorate significantly due to the inherent fragmentation of ceramic armor when subjected to shock waves or shear forces of a ballistic impact.

An impact absorbing layer formed of a fragmenting material typically undergoes spalling when subjected to the shock waves and shear forces of a ballistic impact. At least one containment layer is provided covering at least a portion of the impact absorbing layer to minimize and contain fragmentation of the impact absorbing layer, such as a primary containment envelope covers at least a portion of the impact absorbing layer to minimize and contain fragmentation of the impact absorbing layer. The fragmenting material may be a ceramic formed of a material such as silicon carbide, carbon/carbon composites, carbon/carbon/silicon carbide composites, boron carbide, aluminum oxide, silicon carbide particulate/aluminum metal matrix composites, or combinations thereof.

Silicon carbide and boron carbide are typically used in body armor because they have what is known as high hardness, meaning they are very good at stopping projectiles. However, they exhibit low fracture toughness, meaning that they are extremely brittle and are not good at resisting fracture when they have a crack. Therefore, although tiles made from these materials can slow down and stop a high velocity projectile, such as a bullet, they often shatter in the process and are only good for a single hit.

It is desirable to form a material that is harder, but that also is higher in fracture toughness. However, that concept is a contradiction is terms. Currently, the higher the fracture toughness of a material, the more that material becomes metal-like, which means less brittle and more ductile. The higher the hardness of the material, the lower the ductility and the higher the brittleness. Aluminum comprises a high fracture toughness of 10, but a low hardness value of 130. In comparison, a material that is formed from micron-sized silicon carbide or boron carbide powder that has been put through a conventional sintering process exhibits a high hardness value of 2000, but a low fracture toughness value of between 2 and 4.

The trend line supports the concept that the harder a material becomes, the lower the fracture toughness it comprises, and the higher fracture toughness a material has, the softer that material becomes.

Composite materials have also been prepared for use as lightweight armor for lighter fighting vehicles. A relatively common vehicle that has been protected using lightweight composite material is the M1114 High Mobility Multi-Purpose Wheeled Vehicles (HMMWV). The composite used to armor the HMMWWV is called HJ1. This material includes high-strength S-2 Glass fibers (Owens Corning) and phenolic resin that complies with MIL-L-64154 requirements, and is laminated into hard armor panels that offer significant protection against fragmented ballistic threats when compared to monolithic systems on an equivalent weight basis. However, relatively simple fiber-based composite armors have difficulty protecting vehicle occupants against many common ballistic and blast threats. Armor piercing (AP) ammunition is designed to penetrate the hardened armor of modern military vehicles. It typically includes a sharp, hardened steel or tungsten carbide penetrator covered with a guilding metal jacket that adds mass and allows the projectile to conform to a rifled barrel and spin for accuracy. When an AP round hits armor, the guilding is rapidly deformed and drops away, leaving the sharpened penetrator traveling with a high velocity to bore its way through the armor. Studies indicate that sharp-nosed projectiles tend to move the fibers within the composite laterally away from the advancing projectile, resulting in kinked fibers around the penetration cavities but with little energy absorption. Thus, the primary reason why armor-piercing projectiles are so effective against fiber-based composite armor is that neither the fiber nor matrix material of the composite is hard enough to cause deformation of the sharp, hardened penetrator nose.

Ceramic faced armor systems were thus developed to defeat AP ammunition by breaking up the projectile in the ceramic material and terminating the fragment energy in the backing plate that supports the ceramic tiles. During impact, the projectile is blunted and cracked or shattered by the hard ceramic face. Fragmentation and comminution are produced in the ceramic and the projectile, resulting in fine ceramic rubble traveling with the projectile. The incident momentum of the initial projectile is thus transferred to fragments of shattered projectile and the ceramic rubble. The ceramic rubble typically has a mass comparable to the initial projectile; hence, the final shattered projectile and ceramic rubble exhibit a much lower impact velocity on the backing plate.

Unfortunately, during this process, the armor system is typically damaged. In order for such systems to defeat additional impacts of the threat that are near to previous impacts, the size of the damaged area produced in the armor system needs to be controlled and minimized. With better damage control, the damage size produced is smaller and more closely spaced hits can be defeated by the armor. Armor systems containing segmented ceramics in the form of "tiles" solve a part of this problem because cracks cannot propagate from one tile to another. However, strong stress waves can still damage tiles adjacent to the impacted tile by propagating through the edges of the impacted tile and into adjacent tiles. Ceramic tiles can also be damaged by the deflection and vibration of the backing plate. In addition, impact from the lateral displacement of material during ceramic fracturing can crush and damage adjacent tiles. These armor damage mechanisms must be suppressed in order to provide armor with the ability to reliably defeat multiple projectile impacts.




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