Thursday, 9 August 2012

Introduction to armor

This treatise will define the different types of armor materials including their basic structure, strengths and weaknesses and primary reason for use in armor. It will contain some metallurgical terms which are unique to the field, and can be deciphered through the use of the key at the bottom of this work.

Ceramics are defined in many ways depending on the text you read but a widely accepted, not necessarily all inclusive, definition is any inorganic covalently or ionically bonded material containing at least 2 elements. The last part of this definition excludes elements such as silicon or carbon from being termed as ceramics though this is an area of debate and many materials such as graphite and diamond are best described by typical ceramic properties. Bonding in these materials is of great importance as it lays the foundation for much of the characteristic properties of ceramics such as hardness and lack of ductility. Most ceramics are of mixed bond type, pure covalent bonding is hard to find and in most cases the bonds are at least part ionic, however the bond type is usually said to be whichever type is dominant making salts such as sodium chloride or lithium fluoride ionic and refractories such as silicon carbide and boron nitride covalent. An important property of these bonds is their directionality, both covalent and ionic bonds are known as directional bonds which, unlike metallic bonds, have rigid bond locations and directions making them brittle since movement or flexibility of these bonds are rather limited in comparison to the electron sea in metals. Another limiting characteristic to the ductility of ceramics is due to the requirement to maintain local and overall electronic neutrality.

This means that for every positive charge there must be a negative charge or for every cation a corresponding anion. So thinking about plastic deformation of these materials with this requirement should cause the reader to realise that the movement of an atom from its neutral position in the material would require it to be electronically compensated, which in most cases results in fracturing since its often easier to maintain the electronic state through this method then moving whole neutral systems within the material. While brittleness of these materials is undesirable in many cases there are also very positive aspects of ceramics, as the rigidity of these directional bonds generally makes ceramics very good high temperature insulators as well as making them relatively high elastic modulus materials (a good thing in many structures since large elastic deformations are not usually wanted). However the most important properties for armor are their hardness and compressive strength, even the hardest metals don't match the hardness of many ceramics such as boron carbide or boron nitride and bond rigidity gives these materials amazing compressive strengths.

Important aspects of ceramic armor are that they are generally lighter than metals and have a higher ballistic efficiency for first shot protection. It can be seen in the table that, with the exception of the titanium based ceramics, all are under 4 g/ccm making them at least half the weight of armor steels and aluminums since they are lighter than steels and require less volume to stop rounds than aluminums. High hardness also makes ceramics prime candidates for high performance armors since they are able to deform almost any type of penetrator and continued penetration into ceramic results in further breakdown of the bullet from grinding against the comminuted target material. The downside is of course the lack of multi-hit capacity which is related to the massive fracture zones that occur upon impact. However these fracture zones typically have ordered patterns of cracking which can be categorised into several types of cracks: Tensile, radial, conical, and lateral. As explained by Carlucci and Jacobson [4], the first stage of fracture is the formation of tensile cracks as a result of rarefraction waves in the material combined with low tensile strength, these cracks form on the principal stress planes typically 25°-75° from the surface normal. The tensile cracks are circular about the impact center and they propagate out until eventually coalescing into a conical fracture region in the ceramic making a plug with internal free surfaces. If the plug is held in place by a backing material as is typical for most armors then the stress is redistributed into radial cracking out from the impact center. After radial cracking comes lateral cracking in the plane of the impact, at this point the plug is still held in place and the material cannot leave the impact zone (with some exception of material ejected around the bullet) and so micro-cracking starts to break down the larger fractured pieces into smaller and smaller particles which make up the comminuted zone.
The afore mentioned fracture illustrates a normal impact against a ceramic with backing plate, however it should be mentioned that while complete fracture is unavoidable in these materials, minimization of this cracking can be accomplished via proper backing material or confinement. Somewhat unintuitively the increased V 50 of a ceramic armor with backing plate is not due to the strength of the backing plate but rather the effect the plate has on prohibiting plug ejection and its effect on lowering the strength of rarefraction waves. While it would first appear that a ballistic impact is a purely compressive in nature it is in fact a multi-stage event starting with a compression or shock wave which reflects off the targets boundaries and any internal free surfaces to create rarefraction or tensile waves. As we know ceramics have exceptional compressive strengths and are therefore somewhat immune to the initial waves generated, but the rarefraction waves of the material (when in a state of internal tension) combined with its relatively low tensile strength is where ceramics start to fail, hence the first stage of crack formation being tensile cracks as mentioned above.
The extent of cracking is largely based on the energy in these reflecting internal waves which continue to move about in the material until their energy has been dissipated, in the form of plastic work or fracture since they cannot be transmitted past the external free surface of the target due to the high impedance mismatch between ceramic and air. Application of a properly bonded backing plate can alleviate some of this wave energy, though, by allowing at least part of the initial compression waves to transmit from the ceramic across into the backing plate, usually aluminum or fiber reinforced epoxy. This transmission then lowers the reflected rarefraction waves energy since the initial and reflected wave energies are roughly proportional, minus some loss to heat formation. This requires a well engineered interface since the acoustic impedance of ceramics are generally much higher than aluminum or polymer composites, and special bonding techniques including ceramic glues and graded material boundary layers are being researched for optimisation.

The second method of increasing the penetration resistance of ceramic materials is by radial confinement, a technique used in many laboratory experiments but not well implemented in the field. The idea behind confinement is to create lateral compression which increases the energy required to open internal cracks since crack opening is tensile in nature and closing is compressive. This resistance to crack opening is the same as the idea behind many forms of glass such as tempered or Gorilla glass, where surface layers are engineered to be under compression and thus resist the growth of critical surface flaws. However this method requires some way to keep the ceramic under confinement, which in experiments is generally done by steel casings (unrealistic for practical applications). Commercial implementation of this method has been accomplished more commonly by coating of ceramic parts by molten metal under pressure, which can cause fairly extreme compressive pressures upon cooling since the metal coating shrinks much more than the ceramic during solidification.

Other ways have been studied to take advantage of the high hardness of ceramics while still maintaining a reasonably high toughness such as adding reinforcements to metals in the form of particles, transformation toughening the ceramics (to a lesser extent), and optimisation of layered composites. While the toughness of ceramics will always be low it is important to realise that this is really a bulk material property related to the number and size of flaws in the material. These flaws such as pores or cracks act as stress concentrators and while the applied stress at material failure may be relatively low under tension, the actual resolved tensile stress at the crack tip is fairly high, so low tensile strength is a somewhat relative property instead of a perfect material property. This idea has been well represented in experiments with silica based glass fibers and rods formed in vacuum, since in general a strength of around 2 GPa with almost no ductility is found in typical glasses due to surface flaws acting as stress concentrators. When formed in vacuum, where the glass surfaces are not subjected to air particles constantly impacting and forming flaws, strengths of near 29 GPa have been found with large deformations and no fracture. While these experiments lack a practical solution to the issue of critical flaws, they do illustrate important ideas relative to the field of ceramics and the reason for the Weibull Modulus. 

Key Words
  • Anion: This is an ion with more electrons than protons, giving it a net negative charge (since electrons are negatively charged and protons are positively charged).
  • Cation: This is an ion with fewer electrons than protons, giving it a positive charge.
  • Comminuted: The process in which solid materials are reduced in size, by crushing, grinding and other processes.
  • Rarefraction Waves: A decrease in the density and pressure of a medium, such as air, especially when caused by the passage of a wave, such as a sound wave.
  • V 50: This is the velocity at which 50 percent of impacting projectiles will penetrate the armor, and the other 50 percent will be stopped.
Further Reading
1. Ballistics: Theory and design of guns and ammunition, Donald E. Carlucci, Sidney S. Jacobson. CRC Press 2008.
2. Ballistic performance of confined 99.5%-Al 2 O 3 ceramic tiles, C.E. Anderson Jr, S.A. Royal-Timmons. Journal of Impact Engineering, vol 19, No 8, (1997) 703-713.
3. An experimental study of penetration resistance of ceramic armour subjected to projectile impact, V. Madhu, K. Ramanjaneyulu, T.B. Bhat, and N.K. Gupta. Journal of Impact Engineering 32 (2005) 337-350.
4. Advances in Ceramic Armor, Ceramic Engineering and Science Proceedings, vol. 1-7, The American Ceramic Society, Wiley Publishing.
5. Fundamentals of Ceramics, M.W. Barsoum. IOP Publishing Ltd 2003.


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