Compound armour

Compound armour was a type of armour used on warships in the 1880s, developed in response to the emergence of armor-piercing shells and the continual need for reliable protection with the increasing size in naval ordnance. Compound armour was a non-alloyed attempt to combine the benefits of two different metals—the hardness of steel with the toughness of iron—that would stand up to intense and repeated punishment in battle. By the end of the decade it had been rendered obsolete by nickel-steel armour. However, the general principle of compound iron was used for case-hardened armour, which replaced nickel-steel in the mid-1890s and is still used today.

Prior to the 1880s, all naval armour plating was made from uniform homogeneous wrought iron plates on top of several inches of teak to absorb the shock of projectile impact. A typical installation consisted of four to five inches of iron backed by thirty-six inches of solid wood timbers.

Various experiments were carried out in order to improve the armour, which included breaking up the armour into a laminate of several thinner layers of iron with wood between them, as well as various experiments with cast vs. wrought iron. In all of these experiments, simple blocks of wrought iron consistently proved to provide the best protection.

There had been several attempts to improve on iron with the addition of harder steels on the face, but these all failed for the same reason as the earlier laminate experiments; the ability for the armour to spread sideways into its softer backing allowed it to be penetrated more easily. In the case of steel facing, the problem was that the steel would not adhere well to the underlying iron, allowing it to shift or separate entirely.

In 1876 a competition was held by the Italian Navy at Spezia to trial new armours. By that point conventional iron armours had to be 22 inches (560 mm) thick to stop contemporary naval artillery. The decisive winner was a new soft steel from the French firm of Schneider et Cie, but this proved to be prone to breakage when stressed, making it less useful in naval applications.

Compound armour was made from two different types of steel; a very hard but brittle high-carbon steel front plate backed by a more elastic low-carbon wrought iron plate. The front plate was intended to break up an incoming shell, whilst the rear plate would catch any splinters and hold the armour together if the brittle front plate shattered.

...
Wikipedia