Rolled homogeneous armour (RHA) is a type of

vehicle armour Military vehicles are commonly armoured (or armored; see spelling differences) to withstand the impact of shrapnel, bullets, shells, rockets, and missiles, protecting the personnel inside from enemy fire. Such vehicles include armoured fightin ...

made of a single steel composition hot-rolled to improve its material characteristics, as opposed to layered or cemented armour. Its first common application was in tanks. After

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...

, it began to fall out of use on main battle tanks and other armoured fighting vehicles intended to see front-line combat as new anti-tank weapon technologies were developed which were capable of relatively easily penetrating rolled homogeneous armour plating even of significant thickness. Today, the term is primarily used as a unit of measurement of the protection offered by armour on a vehicle (often composed of materials that may not actually contain steel, or even contain any metals) in equivalent ''millimetres of RHA'', referring to the thickness of RHA that would provide the same protection. Typically, modern

composite armour Composite armour is a type of vehicle armour consisting of layers of different material such as metals, plastics, ceramics or air. Most composite armours are lighter than their all-metal equivalent, but instead occupy a larger volume for the sam ...

can provide the same amount of protection with much thinner and lighter construction than its protective equivalence in RHA. Likewise, the term is also used as a unit of measurement of penetration capability of armour-piercing weaponry, in terms of the millimetres of RHA that the weapon system can reliably penetrate.

Composition

Armoured steel must be hard, yet resistant to shock, in order to resist high velocity metal projectiles. Steel with these characteristics is produced by processing

cast steel Steel casting is a specialized form of casting involving various types of steel cast to either final/net or near-net shape. Steel castings are used when iron castings cannot deliver enough strength or shock resistance.Oberg, p. 1332 Examples ...

billets of appropriate size and then rolling them into plates of required thickness. Hot rolling homogenizes the grain structure of the steel, changing the crystalline structure of the steel and normalizing it. RHA is homogeneous because its structure and composition are uniform throughout its thickness. The opposite of homogeneous steel plate is cemented or face-hardened steel plate, where the face of the steel is composed differently from the substrate. The face of the steel, which starts as an RHA plate, is hardened by a heat-treatment process.

History

From the invention of tanks through to the Second World War, tank armour increased in thickness to resist the increasing size and power of anti-tank guns. A tank with sufficient armour could resist the largest anti-tank guns then in use. RHA was commonly used during this period (combined with other plate alloys and

armour), and the power of anti-tank guns was measured by the thickness of RHA they could penetrate. This standard test has remained in use despite the modern usage of many other types of armour, some of which do not include steel or any other metals. RHA was in common use as primary armour until after

, during which a new generation of anti-tank rounds using

shaped charges A shaped charge is an explosive charge shaped to form an explosively formed penetrator (EFP) to focus the effect of the explosive's energy. Different types of shaped charges are used for various purposes such as cutting and forming metal, init ...

came into use instead of heavy high-velocity projectiles.

Current use

Since World War II, because of a reduction in effectiveness against new weapons (mainly shaped charges and improved

kinetic energy penetrators A kinetic energy penetrator (KEP), also known as long-rod penetrator (LRP), is a type of ammunition designed to penetrate vehicle armour using a flechette-like, high-sectional density projectile. Like a bullet or kinetic energy weapon, this typ ...

), RHA has largely been superseded by

, which incorporates air spaces and materials such as

ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...

s or

plastic Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...

s in addition to steel, and

explosive reactive armour Reactive armour is a type of vehicle armour that reacts in some way to the impact of a weapon to reduce the damage done to the vehicle being protected. It is most effective in protecting against shaped charges and specially hardened kinetic ener ...

. For the testing and calibration of anti-tank guns, the term ''rolled homogeneous armour equivalency'' (''RHAe'') is used when giving an estimate of either the penetrative capability of a

projectile A projectile is an object that is propelled by the application of an external force and then moves freely under the influence of gravity and air resistance. Although any objects in motion through space are projectiles, they are commonly found in ...

or the protective capability of a type of armour which may or may not be steel. Because of variations in armour shape, quality, material, and case-by-case performance, the usefulness of RHAe in comparing different armour is only approximate. Currently, most armoured vehicles have their basic structures formed from RHA to lend general strength and toughness.

Specifications

For current

United States Army The United States Army (USA) is the land service branch of the United States Armed Forces. It is one of the eight U.S. uniformed services, and is designated as the Army of the United States in the U.S. Constitution.Article II, section 2, cla ...

use, RHA is produced to

military standard In military organizations, the practice of carrying colours (or colors), standards, flags, or guidons, both to act as a rallying point for troops and to mark the location of the commander, is thought to have originated in Ancient Egypt som ...

MIL-DTL-12560 by several manufacturers. Another standard is MIL-DTL-46177; however, this standard has been inactivated, and all new designs use MIL-DTL-12560. MIL-DTL-46177 RHA is nearly identical to AR500 in terms of tensile and yield strength.

References

External links

Bibliography of Tank Armor
* {{cite web , url=http://www.intlsteel.com/PDFs/armor.pdf , title=Plate Steels for National Defense , date=September 2003 , publisher=ISG Plate (

International Steel Group International Steel Group (ISG) was an American steel company, headquartered in Cleveland, Ohio, which was established by the New York investment firm WL Ross & Co LLC to acquire the assets of bankrupt steel companies and combine them together in ...

) , location=Coatsville, Penn., archive-url=https://web.archive.org/web/20060320134139/http://www.intlsteel.com/PDFs/armor.pdf , archive-date=March 20, 2006 , access-date=July 3, 2010

Internet Archive The Internet Archive is an American digital library with the stated mission of "universal access to all knowledge". It provides free public access to collections of digitized materials, including websites, software applications/games, music, ...

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