Introduction

This materials report looks at the historic and contemporary use of material in the production of personal protection. The history of armour is extensive and to cover it completely in-depth would be a gargantuan challenge. Therefore, this report will focus on how armour evolved around the materials available, material processes available and the military necessity for protection. As well as how the economics of armour manufacture also played a large role in the armour used at the time. This report will be focusing on largely European armour and techniques.

Armour developed with the human knowledge of the materials used and techniques in developing it, as humans learnt through experience, trade and battle our command over the materials around us increased.

The earliest use of armour was organic material, hides, leather and fabric. Which then slowly developed into mail and plate metal armour as we learnt to harvest resources from the earth and convert them into materials we could use. However, all the time our forging capabilities increased as did our offensive strategy and science. Full plate armour, with the introduction of firearms, became redundant and so ushered in the use of soft body armour and modern composites.

The Bronze age

The start of metal armour began around 1400 BC in ancient Greece with bronze plate armour. The Greeks used overlapping bronze plates to allow for movement and to prevent arrows or swords from penetrating. The Egyptian military, however, initially avoided this approach most likely due to the hot climate (De Backer, 2012). Many middle-eastern militaries instead used linen and leather scales. The difference between bronze’s thermal conductivity standing at 26.0 W/m-C (Engineers Edge, 2021) and leather conducting 0.14 W/m-C (Engineering ToolBox, 2003). This low thermal conductivity allowed soldiers to fight in the arid climate. This is an early example of the trade-offs made in order to be more effective in combat as the likely hood of overheating and exhaustion for Egyptian soldiers was more dangerous than the strength and durability of their armour.

Bronze was used in most ancient civilisations due to its improved properties and it being easier to manufacture than iron. Bronze was much harder, with a Vickers hardness of 60-258 compared to irons 30-80 (Gale and Totemeier, 2003), and could hold an edge better than copper and withstand greater trauma. Once alloyed with tin the different sizes of the metal atoms mean that the metallically bonded layers cannot slide across each other as easily. This increases the force required to dislocate or disrupt the metallic bonds and therefore protects the wearer from injury. The change in grain size shown {include image} also decreases the chance of dislocations or cracks.

Bronze is also easier to work and cast than its components which allowed more complicated and effective pieces of armour to be produced at the scales necessary. Bronze’s composition varied historically, although typically it contained 10% tin and 90% copper. Even though iron was more abundant, the difficulty in extraction from raw hematite and the high melting temperature of iron at 1538C made iron much more expensive to produce than tin at 231.93C and copper at 1084.62C (American Elements, 2021).

Smiths at the time would have simply used the metal available to them, resulting in the widely varying composition and content of trace elements. Content of elements such as Arsenic and Antimony in recycled metal would fluctuate due to their low melting and boiling points (Modlinger, 2017).

The use of iron was sporadic throughout the bronze age due to its inferior properties to bronze. However, it became much more widespread once steel could be smelted and the scale at which it could be produced increased. Steel is produced by combining carbon with iron, and while the reasons why steel is stronger than iron was unknown at the time, smiths recognised its superior properties and moved from bronze to steel.

Plate armour took two common forms, scale and lamellar constructions. Lamellar armour was originally produced in the East and used extensively in Asia and eastern Europe. the armour consisted of small sheets of metal punched and sewn together resulting in rows of over lapping plates or ‘lamellae’ (Blair, 1959). The individual plates allowed for improved movement as each sheet could slide independently across each other. The lamellae were often lacquered to avoid ferrous metal corrosion from exposure to water and air. Scale armour which used much smaller scales sewn together, was overtaken by the more mobile lamellar armour construction. Scale armour however offered better protection from piercing blows.

Plate armour fell out of popular use after the fall of the Roman Empire, mail became the most common form of armour. Plate armour only regained popularity in the 13th century in the form of single plates of iron or steel worn over a mail hauberk - a chainmail shirt (Blair, 1959).

Full plate armour

Full plate armour was formed over the gradual introduction of individual pieces to protect the body, by 1330 early full plate armour had been developed (Blair, 1959). During this time period Iron and steel were most used. Iron is common and available close to the surface; it is also easy to work due to its malleability after refining or smelting. Iron could be worked and formed at high temperatures because of its high ductility. As iron can delaminate and split into thin layers if worked at low temperatures due to its grain structure, to produce effective pieces of armour smiths at the time would have had to forge the plate at sufficiently high temperatures (Duperas, 2012).

To increase the strength and toughness of the material, carbon could be alloyed with the iron creating steel. Steel is stronger than iron due to the microstructures of pearlite and cementite formed when carbon is introduced. The carbon in the steel allows the plate to be hardened by process of heat treating, quenching and tempering (Duperas, 2012).

Heat treatments involve heating, holding the metal at temperature (soaking) and cooling the material down in water or oil (quenching). The steel is hardened by heating the metal to a temperature which forms microstructures of pearlite and austenite, it is then rapidly cooled in the process of quenching which transforms the austenite in the steel to martensite. Tempering is the process of reheating the steel to a high temperature but below its melting point. This process slowly changes the microstructure of the martensite into spheroids of cementite. Tempering increases the toughness of the material by reducing brittleness and internal stresses (Digges, Rosenberg and Geil, 1966).

The process hardens steel by creating a finer pearlite, cementite and ferrite microstructure (Digges, Rosenberg and Geil, 1966). This increases the strength as Dislocations move through a materials grain structure creating deformations, a finer grain in a metal increases the chance of a split to be stopped at the grain boundary (Gedeon, 2010). High carbon content can, however, make the material vulnerable to cracking if not worked at the correct temperature. This decrease in ductility is due to cracks being transmitted across microstructures of pearlite even if their critical strain is relatively high (Bhadeshia and Honeycombe, 2017).

By the end of the 14th century smiths could produce complex and intricate armour designs, improving physical properties and aesthetics for the wearer. Fluted armour became popular due to its increased complexity and its enhanced strength to weight ratio. Corrugations or flutes in armour increase strength as it prevents the metal from folding or bending against the corrugations. Fluting could also have been used to deflect points of blades, using the valleys to carry the momentum of an oncoming attack away from essential organs. Armour was also often designed to imitate the fashionable clothing of the time using fluted metal to imitate pleats and folds of fabric.

Traditional medieval armour reached its commonly depicted form during the late 15th and 16th century with full suits of gothic styled full plate armour. The helm, breast plate and back plate were produced with the thickest steel as they protect the vital organs (Kraner et al, 2019) and although this would have been consistent with most armourers, different smiths would have had different approaches to creating a full suit of armour. Many smiths at this time were highly experienced and skilled so while the composition of the steel alloy was not intentional, they would have been aware of superior steels. The steels used contained elements manganese and cobalt that produced a finer grain in the metal increasing the strength (Kraner et al, 2019).

By the late 17th century armour had been reduced to a breast plate due to the development of firearms that rendered most armour useless. The cuirass or breast plate used was thick and heavy, designed to with stand musket fire and cavalry wore an iron skull cap underneath their hat to defend from sword blows.

Metallic armour remained in use decades after its decline, being used in the American civil war and world war 1 in varying capacities. However, with the increase in fire power, steel and iron armour were soon overtaken by the increased mobility and effectiveness of ceramic and soft body armour.

Modern Day

Contemporary armour production is driven by the desire to produce lighter weight structures, increase wearer comfort and to create economical solutions. With the development of advanced analytical and processing techniques the chemical composition and physical construction of armour can now be honed. However, although armour systems have been greatly improved this development has been mirrored in weaponry, ever increasing the standard for wearable protection (hazell, 2016).

Modern armour generally consists of a soft woven fabric and a stiff ceramic or metallic plate or a combination of the two. The metals most used are steel, aluminium, magnesium and titanium. Steel and aluminium are used because of their price and their ease of use in fabrication, aluminium’s light weight is also prized in wearable protection (hazell, 2016).

Titanium is common in armour application as it maintains a high relative strength and hardness compared to its low relative density. Although magnesium alone is quite brittle, once alloyed with aluminium or zinc the material can be strengthened with heat treatments. This makes it attractive for future military applications as magnesium has never been field tested (hazell, 2016).

Soft body armour, a relatively recent development in protection, consists of multiple layers of woven fabrics layered and sewn together. Silk, cotton and nylon were the first materials used to produce soft body armour, using woven fibres to absorb energy from projectiles. The interwoven fibres transfer the forward energy from the projectile in to a laterally moving force across the material (Mawkhlieng and Majumdar, 2019).

Bullet proof vests are designed to spread the kinetic energy from a projectile across the vest and prevent penetration. The material used must also be able to respond to the ballistic impact rapidly as the event lasts between 50-200 microseconds (Crouch, 2019). As such, fibres such as nylon and Kevlar are used due to their low density, high specific strength and lower elongation at break. These factors improve a materials ability to absorb energy and reduce deformation.

Nylon was used is standard until the 1970s when Kevlar was introduced. Since then, numerous aramid fibres and ultra-high molecular weight polyethylene fibres (UHMWPE) have been produced (Mawkhlieng and Majumdar, 2019), with Crouch (2019) stating the creation of UHMWPE’s have been one of the most significant developments in armour materials in the last 20 years.

Typical soft body armour contains packs of layered fabrics, a laminated backing material consisting of cross-plied UHMWPE-based fabric and a strike face material of hard armour panel. Hard armour panels consist of reaction sintered silicon carbide, used for its high hardness; this allows the panel to break oncoming projectiles into pieces and prevent them from penetrating the wearer (Mawkhlieng and Majumdar, 2019).

The Future

The human race throughout the ages have found as many effective and innovative ways to protect themselves as they have to injure others. Starting our development with what was readily available on the surface and slowly improving processes for developing material properties.

Just as armour has always been influenced by the advance of weaponry and science available at the time, the future of wearable protection is in novel and experimental materials. As has happened with the introduction of UHMWPE materials, new materials will allow for increasingly advanced systems. UHMWPE fabrics drastically reduced the weight of body armour systems allowing for new stacked systems to be utilised. Stacked systems, where different armour materials are layered together, allow for greater protection against a wider range of rounds (Crouch, 2019).

With the drive for inventing new materials for use in combat comes the development of smart materials. Materials that allow fabrics used to have multiple purposes, such as health monitoring, as well as protection. Or the development of transparent armour technologies, using polycrystalline glass and transparent polymers to provide protection while also increasing visibility (Subhash, 2012). The future of materials will allow us to avoid compromises and limit the trade-offs increasingly frequently.

References

Abedini, O., Behroozi, M., Marashi, P., Ranjbarnodeh, E. and Pouranvari, M., (2019) Intercritical heat treatment temperature dependence of mechanical properties and corrosion resistance of dual phase steel. Materials Research, 22(1).

American Elements (2021) Available at: Ma, X., Huang, C., Moering, J., Ruppert, M., Höppel, H.W., Göken, M., Narayan, J. and Zhu, Y., 2016. Mechanical properties of copper/bronze laminates: Role of interfaces. Acta Materialia, 116, pp.43-52.

Bhadeshia, H. and Honeycombe, R., (2017) Steels: microstructure and properties. Butterworth-Heinemann.

Blair, C., 1959. European armour (pp. 58-59). London: Batsford.

Crouch, I.G., (2019) Body armour–New materials, new systems. Defence Technology, 15(3), pp.241-253.

De Backer, F (2012) Scale-armour in the Mediterranean area during the early Iron Age: from the IXth to the IIIrd Century BC. Revue des études militaires anciennes, 5, pp.1-38.

Digges, T.G., Rosenberg, S.J. and Geil, G.W., 1966. Heat treatment and properties of iron and steel. NATIONAL BUREAU OF STANDARDS GAITHERSBURG MD.

Dupras, N., 2012. Armourers and their workshops The tools and techniques of late medieval armour production.

Engineer’s Edge (2021) Available at: https://www.engineersedge.com/materials/metal_melting_temperatures_13214.htm (Accessed: 25 April 2021)

Engineering Toolbox (2003) Available at: https://www.engineeringtoolbox.com/melting-temperature-metals-d_860.html (accessed: 25 April 2021)

Gale, W.F. and Totemeier, T.C. eds., (2003) Smithells metals reference book. Elsevier.

Gedeon, M (2010) ‘Grain Size and Material Strength’, Technical Tidbits

Hazell, P.J., (2015) Armour: materials, theory, and design. CRC press.

Kraner, J., Lazar, T., Zorc, B., Fajfar, P. and Knap, M., (2019) Archaeometallurgical characterization of a 16th century suit of armour made by Valentin Siebenbürger. Journal of Cultural Heritage, 39, pp.49-56.

Mansoor, P (2019) ‘Armour’. Encyclopedia Britannica https://www.britannica.com/topic/armour-protective-clothing. Accessed 27 April 2021.

Mawkhlieng, U. and Majumdar, A., (2019) Soft body armour. Textile Progress, 51(2), pp.139-224.

Mödlinger, M., (2017) Protecting the body in war and combat: metal body armour in Bronze Age Europe. Austrian Academy of Sciences Press.

Subhash, G., (2012) Transparent armor materials. Experimental Mechanics, 53(1), pp.1-2.