Stopping all danger

The materials technologies developed to protect against weapons and explosive devices have become exceptionally sophisticated. Since attacks come in many forms, the ideal armour protection has to be able to cope with blast waves, as well as armour-piercing ammunition and projectiles associated with improvised explosive devices. Clearly, though, the value of being able to absorb blast and impact is not limited to the military sector. Equally, the combination of protection and lightweight materials has many uses in, for instance, structural and automotive applications. No one type of protection copes with all types of threat, with the result that successful armour protection combines a variety of materials to do different things. Britain's Chobham armour, for instance, is said to be based on ceramic tiles within a metal matrix, bonded to a backing plate with several elastic layers. Considerable advantage can also derive from relatively simple innovations such as the steel cages placed around armoured vehicles to protect against improvised explosive devices. Equally, it was found in World War II that asphalt worked well on merchant navy ships in stopping projectiles because, unlike concret, it did not produce lethal splinters when struck. Taking this idea a stage further, a coating of polyurea on the inside of a structure or armoured vehicle is invaluable in preventing spalling – the breaking off of splinters from the inside of the armour. Although the outer structure may fail, the polymer deforms plastically and retains the debris. The idea comes originally from the US Air Force Research Laboratory, which in 1999 began evaluating a commercial spray-on truck bed liner on concrete walls. Pure polyurea worked best and this led to the development of Paxcon, which has since been applied to the insides of military vehicles and individual armour plates. It has also seen extensive use in civilian and industrial applications where blast mitigation is desirable. Armour is not necessarily hard. Canadian Cymat Technologies promotes stabilised aluminium foam under the brand name SmartMetal. Also used in automotive applications to improve crashworthiness, this absorbs blast by progressively collapsing and compressing without failing. However, protection against fast moving projectiles does require something hard to shatter them, which is why Morgan Technical Ceramics makes ceramic rosettes for incorporation into personal body armour. However, there must also be some means of absorbing impact energy, and many armour systems incorporate something relatively soft between layers that are hard and strong. The soft material may in some cases be rubber, although Barry Pegrum, who uses bullet absorbing systems in Sector Associates' training facilities, uses layers of different thickness steel and plywood. Permali in Gloucester, however, has developed a ceramic armour in which energy is absorbed by reinforcing fibres being pulled out of matrix. The systems are too complex to model properly and everyone seems to have their own preferences, however. For example, one company selling multilayer titanium armour claims that an ideal combination is hard on the outside and tough and ductile on the inside, while an MSc thesis written by Min Huang, under Dr Tomasz Wierzbicki of the Impact and Crashworthiness Laboratory at MIT, recommends the opposite.