Many of the materials used to protect a vehicle’s occupants are “single-use”. After a serious incident, 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. 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 penetrationor 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. Both can produce "spalling", which is when small fragments on the inside of the armour break away because of an impact on the other side. Because softer steels bent more readily, they can produce spalling from low-energy fragmentation impacts. Harder steels tend not to produce spalling unless the impact has sufficient force to penetrate, when spalling may be far more extensive than with softer steels.
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