Perisai Chobham

Perisai Chobham merupakan nama tidak rasmi bagi perisai komposit yang dibangunkan pada 1960-an oleh pusat penyelidikan kereta kebal Britain bagi Chobham Common. Nama ini kini menjadi istilah umum bagi perisai kenderaan seramik.

Sungguhpun perincian mengenai pembinaan perisai Chobham Common masih dirahsiakan, ia telah digambarkan sebagai sebatian yang terdiri daripada jubin seramik diselaputi dengan matriks logam dan dilekatkan pada kepingan belakang dan beberapa lapisan anjal. Disebabkan seramik yang digunakan amat keras, ia menawarkan ketahanan lebih terhadap peletup bentuk (shaped charge) seperti peluru peletup kuat anti kereta kebal (High Explosive Anti-tank) (HEAT) dan ia menghancurkan perobek tenaga kinetik (kinetic energy penetrator).

Struktur 

Jubin seramik mempunyai masalah "keupayaan hentaman berganda" di mana ia tidak mampu menahan hantaman bertalu tanpa kehilangan nilai perlindungannya dengan pantas. Untuk mengurangkan kesan ini jubin dihasilkan sekecil mungkin, tetapi unsur matrix mempunyai ketebalan minima pratikal sekitar satu inci / 25 mm. dan nisbah perlindungan diberikan jubin akan menjadi tidak diingini, meletakkan had pada diameter sekitar sepuluh sentimeter. Jubin kecil segi empat atau hexagon dikelilingi matris samaada melalui tekanan secara isostatik kedalam matrix yang dipanaskan, atau dengan mengamkannya menggunakan resin epoxy. Sejak awal sembilan puluhan ia telah diketahui bahawa meletakkan jubin dibawah tekanan sekata oleh matricnya meningkatkan ketahanan kepada perobek kinetik dengan banyak, yang sukar dicapai apabila menggunakan gam.

Matrix perlu dilapis dengan kepingan, bagi memperkukuhkan jubin seramik di belakang dan menghalang matrix logam reyuk oleh hentaman kinetik. Biasanya kepingan pelapis adalah separuh dari jisim matrix sebatian. Pemasangan ini kemudian dipasang pada lapisan anjal. Ini menyerap sebahagian hentaman, tetapi fungsi utamanya adalah bagi meningkatkan hayat perkhidmatan matrix dengan melindungnya dari getaran. Beberapa pemasangan boleh ditindan, bergantung kepada ruang yang ada; dengan ini perisai boleh dibentuk secara modular, boleh disesuaikan dengan keadaan. Ketebalan pemasangan biasa masa kini adalah antara lima dan enam sentimeter.

The armour configuration of the first western tanks using Chobham armour was optimised to defeat shaped charges as guided missiles were seen as the greatest threat. In the eighties however they began to face improved Soviet kinetic energy penetrator rounds of various sorts, which the ceramic layer was not particularly effective against: for the original ceramics the resistance against penetrators was about three times, for the newest composites it is about ten times less than against HEAT rounds. For this reason many modern designs include additional layers of heavy metals to add more density to the overall armour package.

The introduction of more effective ceramic composite materials allows for a larger width of these metal layers within the armour shell, given a certain protection level provided by the composite matrix. They typically form an inner layer placed below the much more expensive matrix, to prevent extensive damage to it should the metal layer strongly deform but not defeat a penetrator. They can also be used as the backing plate for the matrix itself, but this compromises the modularity and thus tactical adaptability of the armour system; furthermore, due to their extreme hardness, they deform insufficiently and would reflect too much of the impact energy to the ceramic tile. Metals used include a tungsten alloy for the Challenger 2 or, in the case of the M1A1HA (Heavy Armor) and later American tank variants, a depleted uranium alloy.

Some companies offer titanium carbide modules. These metal modules (typically employing perpendicular rods) have many perforations or expansion spaces reducing the weight up to about a third while keeping the protective qualities fairly constant. The depleted uranium alloy of the M1 has been described as "arranged in a type of armour matrix" and a single module as a "stainless-steel shell surrounding a layer (probably an inch or two thick) of depleted uranium, woven into a wire-mesh blanket".

Such modules are also used by tanks not equipped with Chobham armour. The combination of a composite matrix and heavy metal modules is sometimes informally referred to as "second generation Chobham".

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Mutu perlindungan 

Disebabkan oleh kekerasan terlampau seramik yang digunakan, ia memberika rintangan terhadap jet shaped charge dan memecahkan penembus tenaga kinetik. The (pulverised) ceramic also strongly abrades any penetrator. Against lighter projectiles the hardness of the tiles causes a "shatter gap" effect: a higher velocity will within a certain velocity range (the "gap") not lead to a deeper penetration but destroy the projectile itself instead . Because the ceramic is so brittle the entrance channel of a shaped charge jet is not smooth — as it would be when penetrating a metal — but ragged, causing extreme asymmetric pressures which disturb the geometry of the jet, on which its penetrative capabilities are critically dependent as its mass is relatively low. This initiates a vicious circle as the disturbed jet causes still greater irregularities in the ceramic, until in the end it is defeated. The newer composites, though tougher, optimise this effect as tiles made with them have a layered internal structure conducive to it, causing "crack deflection". This mechanism using the jet's own energy against it, has caused some to compare the effects of Chobham to those of reactive armour. This should not be confused with the effect used in many laminate armour of any kind: that of sandwiching an inert but soft elastic material such as rubber, between two of the armour plates. The impact of either a shaped charge jet or long-rod penetrator, after the first layer has been perforated and the rubber layer is being penetrated, will cause the rubber to deform and expand, so deforming both the back and front plates. Both attack methods will suffer from obstruction to their expected paths, so experiencing a greater thickness of armour than is there is nominally, thus lowering penetration. Also for rod penetrations, the transverse force experienced due to the deformation may cause the rod to shatter, bend, or just change its path, again lowering penetration.

Keberkesanan perisai Chobham dapat dilihat dalam Perang Teluk pada 1991 dan 2003, di mana tiada sebuah pun kereta kebal pihak bersekutu yang berjaya dimusnahkan oleh pasukan armor atau peluru berpandu anti kereta kebal Iraq. Dalam beberapa kes, kebanyakan kereta kebal yang berjaya dirosakkan oleh tembakan berulang kali menggunakan peluru dengan penembus tenaga kinetik atau peluru letupan kuat anti kereta kebal. Ini kerana jenis peluru yang digunakan oleh Iraq pada kereta kebal T-72, T-55 dan T-62 tidak berupaya untuk menembusi perisai hadapan kereta kebal pihak bersekutu. Harus diingatkan juga, tentera Iraq sebenarnya amat jarang berjaya menembak kereta kebal pihak bersekutu disebabkan oleh kekurangan latihan dan sistem optik dalaman. Sehingga kini, hanya 5-10 kereta kebal yang dilindungi oleh perisai Chobham berjaya dikalahkan oleh tembakan musuh dalam pertempuran, termasuk sebuah kereta kebal M1 Abrams yang ditembak di skirt sisi, di bawah cincin turet oleh RPG-7VR. Jet dari senjata RPG telah menembusi skirt perisai sisi dan perisai badan sisi, kemudian merentasi bahagian dalam kereta kebal dan akhirnya menembusi 1.5 hingga 2 inchi perisai badan di bahagian sebalah satu lagi.

Keburukan 

Perisai ini sangat berkesan jika dibandingkan secara relatif ke atas kosnya dan maka berpotensi menjadi senjata perang – iaitu dalam perang ekonomi. Dalam Perang Iraq, pihak pejuang telah mensasarkan kereta kebal dengan menggunakan RPG dan bom buaatan sendiri, dan walaupun tidak berjaya memusnahkannya, kos pembaikan telah membawa tekanannya sendiri kepada pihak tentera.

Pembangunan dan kegunaan 

Sejak awal 1960-an, A.S membuat banyak penyelidikan ekstensif bagi mencari prospek untuk menggunakan bahan-bahan seramik komposit sebagai perisai kenderaan.. Penyelidikan ini tertumpu akan penggunaan logam matriks aluminum yang diperkuat dengan juraian silikon karbida, yang akan dikeluarkan dalam bentuk helaian besar. Helaian logam ringan yang diperkuat ini kemudian diapitkan di antara lapisan keluli. Aturan ini memiliki kebaikan dan berupaya untuk menerima tembakan berulang dan boleh dibentuk. Ini membolehkan perisai utama dibentuk berdasarkan bentuk perisai bercerun. Bagaimanapun, tujuan utama perisai komposit dengan kandungan logam yang tinggi ini adalah untuk meningkatkan perlindungan untuk bertahan dari tembakan peluru penembus tenaga kinetik berdasarkan berat perisai yang diberi; prestasi perisai ini dalam bertahan terhadap tembakan peluru letupan berbentuk adalah tidak berapa baik dan perlu diperbaiki.

Teknologi pilihan yang dibangunkan di A.S berdasarkan penggunaan modul kaca yang akan dimasukkan ke dalam perisai utama; meskipun susunan ini menawarkan peningkatan ke atas pelindungan dari peluru letupan berbentuk, keupayaannya untuk menerimaa tembakan berulang sangat rendah. Sistem sama yang menggunakan kaca yang dimasukkan ke dalam perisai utama keluli berdasarkan teknologi 1950-an untuk prototaip kereta kebal Soviet Obiekt 430 bagi program kereta kebal T-64; this was later developed into the "Combination-K" type, having a ceramic compound mixed with the silicon oxide inserts, which offered about 50% better protection against both shaped charge and KE-penetrator threats, compared with a steel armour of the same weight. It was, later in improved forms, part of all subsequent Soviet main battle tank designs. After an initial period of speculation in the West as to its true nature, the characteristics of this type were disclosed when the dissolution of the Soviet Union and the introduction of a market system forced the Russian industries to find new customers by highlighting its good qualities; it is today rarely referred to as Chobham armour.

Meanwhile in the United Kingdom another line of ceramic armour development had been started, meant to improve the existing cast turret configuration of the Chieftain that already offered excellent heavy penetrator protection; the research by a team headed by Gilbert Harvey of the Fighting Vehicles Research and Development Establishment, was thus strongly oriented at optimising the ceramic composite system for defeating shaped charge attack. The British system consisted of a honeycomb matrix with ceramic tiles backed by ballistic nylon, placed on top of the cast main armour. In July 1973 an American delegation, in search of a new armour type for the XM815 tank prototype, now that the MBT-70 project had failed, visited Chobham Common to be informed about the British system. It was very impressed by the excellent shaped charge protection combined with the penetrator impact damage limitation, inherent to the principle of using tiles. The Ballistic Research Laboratory at the Aberdeen Proving Ground that year initiated the development of a version, named Burlington, adapted to the specific American situation with a much higher projected tank production run and the use of a thinner rolled steel main armour. The increased threat posed by a new generation of Soviet guided missiles armed with a shaped charge warhead — as exemplified by the events of the Yom Kippur War of October 1973, when even older generation missiles caused considerable tank losses on the Israeli side — made Burlington the preferred choice for the armour configuration of the XM1 (the renamed XM815) prototype.

However, on 11 December 1974 a Memorandum of Understanding was signed between the Federal Republic of Germany and the USA about the common future production of a main battle tank; this made any application of Chobham armour dependent on the eventual choice for a tank type. Earlier in 1974 the Americans had asked the Germans to redesign the existing Leopard 2 prototypes, considered by them too lightly armoured and had suggested to adopt Burlington for this purpose; the Germans however in response that year initiated a new armour development programme of their own. Having already designed a system that in their opinion offered satisfactory protection against shaped charges, consisting of multiple laminate spaced armour with the spaces filled with ceramic polystyrene foam as fitted to the Leopard 1A3, they put a clear emphasis on improving KE-penetrator protection reworking the system into a perforated metal module armour. A version with added Burlington was considered, including ceramic inserts in the various spaces, but rejected as it would push vehicle weight well over sixty metric tonnes, a weight then seen as prohibitive by both armies. The US Army in the summer of 1974 faced the choice between the German system and their own Burlington, a decision made more difficult by the fact that Burlington offered, compared with steel armour, no weight advantage against KE-penetrators: the total armour system would have a RHA equivalence against them of about 350 mm (compared to about 700 mm against shaped charges). No consensus developing, General Creighton Abrams himself decided the issue in favour of Burlington. Eventually each army would procure its separate national tank design, the project of a common tank failing in 1976. In February 1978 the first tanks protected by Burlington left the factory when the first of eleven pilot M1 tanks were delivered to the US Army.

Di United Kingdom, penggunakan perisai Chobham agak terlambat disebabkan kegagalan beberapa projek kereta kebal termmaju: pertama usahasama kereta kebal Jerman-Britain; kemudian program MBT-80 Britain. Kerajaan Iran telah memesan sejumlah 1,225 buah kereta kebal Chieftain yang ditingkat upaya, dan dinamakan sebagai Shir-2 (FV 4030/3), dengan memasang perisai Chobham pada perisai utama kereta kebal, menjadikan berat keseluruhannya meningkat kepada 62 tan metrik. Apabiba pesanan ini dibatalkan pada Februari 1979 setelah berlakunya Revolusi Iran, kerajaan Britain dengan tekanan bagi memodenkan angkatan kereta kebal mereka agar lebih hebat secara relatif berbanding angkatan kereta kebal Kesatuan Soviet, telah memutuskan untuk menggunakan keupayaan lebihan pengeluaran bagi membeli sejumlah kenderaan yang hampir menyamai rekabentuk kereta kebal Shir-2, yang dikenali sebagai Challenger 1. Pada 12 April 1983, kereta kebal pertama Britain yang dilindungi dengan perisai Chobham telah diserahkan kepada Royal Hussars. 
 
Versi terkini perisai Chobham telah digunakan pada kereta kebal Challenger 2 (dikenali sebagai "perisai Dorchester"), dan (komposisi bahan mungkin berbeza) kereta kebal siri M1 Abrams, yang mana berdasarkan sumber-sumber rasmi menyatakan bahawa kereta kebal ini kini dilindungi dengan kepingan silikon karbida. Given the publicly stated protection level for the earliest M1: 350 mm steel equivalence against KE-penetrators (APFSDS), ia seolah-olah dilengkapi dengan kepingan alumina.

Walaupun didakwa sebaliknya, dari segi fakta kereta kebal Leopard 2 tidak menggunakan perisai Chobham, tetapi perisai berlubang asli, untuk mengelakkan membuat pembelian, penyelenggaraan dan kos pengantikan tinggi ke atas sistem perisai seramik yang berasaskan alumina yang tidak berkesan. Bagi kebanyakan kereta kebal moden, seperti kereta kebal Type 90 Jepun dan Ariete Itali, jenis perisai yang akan digunakan belum diputuskan. Terdapat aliran umum yang beralih dari penggunaan perisai seramik kepada perisai berlubang.

Nota

Rujukan

Bacaan lanjut 

 Jeffrey J. Swab (Editor), Dongming Zhu (General Editor), Waltraud M. Kriven (General Editor); Advances in Ceramic Armor: A Collection of Papers Presented at the 29th International Conference on Advanced Ceramics and Composites, January 23-28, 2005, Cocoa Beach, Florida, Ceramic Engineering and Science Proceedings, Volume 26, Number 7; ISBN 1-57498-237-0

Pautan luar 
 Article on DSTL/QinetiQ Chertsey and Longcross Test Track (Chobham Tank Research Centre)