Many of the materials used to protect a vehicle’s occupants are 'single-use'. After a serious event, partial penetrations in armoured glass, steel, polycarbonate or aramid mean that panels should be replaced, but they rarely are. Effective repairs can be made by patching some materials, but not armoured glass. Patching aramids and polycarbonate requires some skill. Welding hard armour panels that have broken may severely change the properties of the surrounding material, so should not be attempted. The low-probability of a threat striking in precisely the same place twice makes some users reluctant to replace expensive armouring, but all armour is only as good as its weakest point.
Obviously, it is desirable that whatever materials are used for protection they should not add to the threat when they fail. Because armoured glass does add to the threat when it fails – with high-speed glass spalling resulting from any penetration or near-penetration, armoured glass is usually made to exceed the protection needed. Generally, materials that fail progressively (rather than catastrophically) are preferred but the exception may be in the use of hard steels as armour plating for use against blast forces. Hardened steels will not bend when subjected to intense blast forces, and this can be appropriate, (when they protect vital engine parts, for example). When the forces involved overmatch the steel’s properties, it will fail by cracking or shattering. When the threat is of a predictable scale, it can make compelling sense to deliberately use a hard steel armour that overmatches the threat. When the threat is unknown, some designers prefer to use mild steel that will bend and stretch before finally failing.
Steel hardness is relevant to its performance when protecting against fragmentation. All steel armour can produce 'spalling', which is when small fragments on the inside of the armour break away because of an impact on the other side which does not result in a penetration. Because softer steels bend more readily, they can also produce spalling from low-energy fragmentation impacts. Hard armour steels tend not to produce spalling unless the impact has sufficient force to crack or penetrate. The pressure exerted by shaped charge armour penetrators can oblige steel of any hardness to flow aside (as if molten) even when a considerable thickness of steel is used. Because the penetrator tends to break up on leaving an obstruction, the use of armour with cavities inside can provide a measure of protection against penetrators and also against spalling from the outer plate. In combat vehicles, reactive armour is used to try to counter the effects of a munition detonating against the armour and to prevent the forming of any shaped charge penetrators. The armour 'reacts' by detonating. Reactive panels may be fitted to existing armoured vehicles as an outer cover, often only applied to 'vital' parts.
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