6.1 Introduction
Metals have been used extensively in armour. Generally speaking, there are only four practical metallic contenders for armour applications: aluminium, magnesium, steel and titanium. Steel and aluminium are the most common metals in use today mainly due to the price and their ability to be worked and welded. However, magnesium and titanium, although expensive, have some desirable properties that will be discussed in Sections 6.3.3 and 6.3.4.
Figure 6.1 shows a summary of how most armour is made. The vast majority of armour is wrought plate. That is to say that it is processed by rolling or pressing. Some armour is cast, although this has been mostly reserved for the turrets of tanks such as the Chieftain (see Figure 6.2).
To protect against ballistic attack, it is necessary to use a solid homoge-neous plate, although there are one or two exceptions, as will be seen later.
To protect against blast, this is not necessarily the case. In fact, it has been shown that hollow porous structures can aid in providing protection against blast waves.
6.2 Properties and Processing of Metallic Armour 6.2.1 Wrought Plate
Wrought plates are mechanically worked either through hot-working or cold-working the material. The typical ways that this is achieved are as follows:
• Forging: where the piece is subjected to successive blows or by con-tinuous squeezing of the metal.
• Rolling: by far the most common method of processing wrought armour plate mainly because the desirable thicknesses and proper-ties can be achieved through this process.
Metallic armour
Aluminium CastWroughtWrought
Steel CastWroughtDual property
Titanium Wrought
Magnesium FIGURE 6.1 Classifications of materials that are used in armour applications.
• Extrusion: where the piece is forced through a die to produce the desired shape.
• Drawing: where the metal is pulled through a die that has a tapered bore. Rod and wire are commonly fabricated in this way (not used for armour plate).
6.2.2 Cast Armour
Casting metal structures for armour applications, where molten metal is solidified in a mould, has become less attractive in recent years as the strength that can be offered by wrought plates is far superior. However, before and during World War II (WWII), there was a significant amount of cast armour produced. Casting metallic structures (say, for example, for tur-rets on tanks) can provide some geometric and cost advantages. Accordingly, the Chieftain MBT employed such a turret. However, castings are notorious for containing porosity and generally possess low toughness values.
Some improvements in casting of steels occurred in the 1970s where it was shown that cooling the metal in such a way that heat was extracted on one surface led to improvements in properties. This process resulted in columns of grains extended from the chill surface completely through the casting thereby giving the casting microstructural ‘texture’. The end result was a casting that had superior ductility and ballistic performance than conven-tional castings (Papetti 1980).
FIGURE 6.2
Chieftain Main Battle Tank (MBT) employing a cast steel turret.
6.2.3 Welding and Structural Failure due to Blast and Ballistic Loading Welding is a very common way of joining metallic plates, and it is particu-larly important for armoured fighting vehicles (AFVs) that may be exposed to blast loading. Joins are often a source of weakness in the structures, and it is these that tend to fail first if the whole structure is subjected to a dynamic stimulus (such as a blast wave). Therefore, it is necessary to get the technique right.
During the welding process, there is diffusion of the metal so that the join is metallurgical rather than mechanical. Arc and gas welding occurs by a process of melting the work pieces and a filler material (i.e. the welding rod), whilst they are all in contact with one another. When all materials solidify, the filler material provides a join between the work pieces. Unfortunately, there will be a material that is adjacent to the weld that experiences microstruc-tural changes due to being subjected to elevated temperatures. This will lead to change in the localised properties of the material, and this can turn out to be a source of weakness. This area of property alteration is called the ‘heat-affected zone’ and is sometimes abbreviated as ‘HAZ’. There are several rea-sons why properties are changed in the HAZ, as summarised by Callister (2007):
1. If the material was previously cold-worked, the temperature increase due to the welding process may lead to grain growth or recrystalli-sation. This process weakens the material and can lead to a reduc-tion of strength, hardness and toughness in this zone.
2. On cooling, the material experiences residual stresses due to differ-ent cooling rates through the thickness of the weld. These residual stresses can lead to a weakness in the joint.
3. For steels, the material may have been heated sufficiently by the welding process to form austenite. On cooling, the phases that are produced are dependent on the cooling rate and the carbon con-tent of the steel. For plain carbon steels, normally, pearlite will form on cooling. However, for alloy steels, one possible product that is formed is martensite. This is undesirable as it is brittle. This will be discussed briefly in Section 6.3.1.1.
4. Some stainless steels may become sensitive to inter-granular corro-sion in the HAZ. In essence, they begin to corrode due to the for-mation of a chromium-free zone adjacent to the grain boundary as chromium carbide precipitates are formed. Therefore, the grain boundaries become susceptible to corrosion.
The HAZ in the weld can be the origin of failure, as depicted in Figure 6.3, both through structural collapse and through perforation (caused by the local failure of weld material).
Care should be taken when welding armoured steel – and this is all the more important to consider when carrying out field repairs. Cracking can occur along the weld if inappropriate methods are used. Alkemade (1996) examined the weldability of high-hardness steel armour plate – specifically for the Australian light armoured vehicle (LAV) 25 that is made from welded high-hard steel plate.
The purpose of his work was to examine the susceptibility to cracking after fusion welding of Bisalloy 500 armour plate (0.2% proof strength = 1580 MPa).
He showed that for this armour, hydrogen-induced cracking was seen in the hardened region of the HAZ where the heat input was 0.5 kJ/mm, and the pre-heat was 75°C or less, whereas no cracking was observed at this pre-heat input when the preheat was raised to 150°C. Additionally, when the heat input was raised to 1.2 kJ/mm, no cracking was observed even when preheat was not used.
A good weld between two armour plates can be achieved by the use of lase beam welding (LBW). LBW uses a highly focused and intense laser beam as a heat source to selectively melt the materials. Often, there is no need to supply a separate filler material, and the process can be employed in a highly automated fashion. The resulting HAZ is usually small due to the fact that the total energy input into the work pieces is small, and the welds are precise. With LBW, it is entirely possible to achieve porosity-free welds with strengths at least equal to the parent metal.
6.3 Metallic Armour Materials
Four of the more important metal alloys that are used in protection will now be reviewed: steel, aluminium, magnesium and titanium. The United States publish MIL specifications that define the minimum ballistic behaviour of
(a) (b)
FIGURE 6.3
Failure of an armoured vehicle subjected to blast loading showing (a) structural collapse through failure of the welds and, (b) perforation of the hull bottom by explosively accelerated debris.
these materials against certain threats, and these are summarised in Table 6.1.
A quick Google search will provide the text.
6.3.1 Steel Armour
Steel is by far the most commonly used material in armoured vehicles to date mainly because steel is a good ‘all-rounder’. Toughness, hardness, good fatigue properties, ease of fabrication and joining and its relatively low cost make steel a popular choice for armoured vehicle hulls. Steel has been used extensively over the centuries and found its first use in armoured vehicles in the tanks of World War I (WWI), and it is still used extensively today.
6.3.1.1 A Quick Word on the Metallurgy of Steel
Steels are Fe–C alloys. Introductory materials science books discuss in detail the iron–carbon phase diagrams (e.g. see Ashby and Jones 1986). This is a dia-gram that shows how the microstructure of the steel will develop if allowed sufficient time during cooling such that near-equilibrium conditions are maintained at all times. Unlike many non-ferrous alloys, the cooling rate plays a large part in how the microstructure (and the resultant mechanical prop-erty) is formed. Cooling a steel quickly will have a different effect than cooling it slowly – depending on the composition of carbon. For example, slowly cool-ing a plain carbon steel from around 850°C will result in body-centred cubic (BCC) α grains (ferrite) being formed and nodules* of pearlite. Pearlite is an alternate plate-like mixture of α and iron carbide (Fe3C). The iron (α) phase is quite soft with a local hardness of about 90 VHN, whereas pearlite is stronger and harder with a typical hardness of 250 VHN. Slow cooling is done in air, a process known as normalising. Rapid cooling or ‘quenching’ is done in water
* Note that pearlite is a mixture of two separate phases, and therefore, it is not referred to as a
‘grain’ but rather a ‘nodule’.
TABLE 6.1
US Military Standards for Steel, Aluminium, Magnesium and Titanium Alloys
Standard Date Name
MIL-DTL-12560J 24 July 2009 Homogeneous wrought armour plate MIL-DTL-46100E 24 October 2008 Armor plate, steel, wrought, high hardness MIL-DTL-46177C 24 October 1998 Armor, steel plate and sheet, wrought,
homogeneous (1/8 to less than 1/4 in. thick) MIL-DTL-46027K 31 July 2007 Armor plate, aluminium alloy, weldable 5083,
5456 and 5059
MIL-DTL-46063H 14 September 1998 Armor plate, aluminium alloy, 7039
MIL-DTL-32333 29 July 2009 Armor plate, magnesium alloy, AZ31B, applique MIL-DTL-46077G 28 September 2006 Armor plate, titanium alloy, weldable