Plastic armour, called plastic protection in the
United States, was a type of vehicle armouroriginally developed for merchant ships by the British Admiraltyin 1940. The original composition was described as 50% clean graniteof half-inch size, 43% of limestonemineral, and 7% of bitumen. It was typically applied in a layer two inches thick and backed by half an inch of steel.
Plastic armour was highly effective at stopping
armour piercing bullets because the hard granite particles would turn the bullet which would then lodge between plastic armour and the steel backing plate. Plastic armour could be applied by pouring it into a cavity formed by the steel backing plate and a temporary wooden form.
At the start of
World War II, British production of steel plate was inadequate to equip the merchant navy with armour that would stop the armour piercing bullets from German war planes. The main requirement was to protect the personnel within the bridges and gun emplacements. At the outbreak of war, a number of ship bridges had been encased in concrete. However, it was soon found that this shattered on impact, and created multiple secondary fragments. In many cases, concrete armour was worse than no armour.
Plastic armour was developed by a small team in the Admiralty in August 1940, and first fitted to merchant ships in October of the same year. This was done despite resistance from the
Director of Naval Constructionwho felt that the Admiralty should not be developing armour. The armour was cheap and easy to install on ships, and the skills and equipment for installation came from the under-utilized road building industry.
Once installed on ships, plastic armour proved highly effective, when applied in sufficient thickness. Many anti aircraft guns such as the Oerlikon were fitted with only very thin plastic shields, which served mainly to improve the morale of the gunner. By some measures, it was as good as plate steel, and was widely adopted by allied ships.Fact|date=February 2007 In the United States, some 3,000 merchant ships and 1,000 other ships were equipped with it, and in Britain and the Commonwealth some 7,000 ships were fitted.Fact|date=February 2007
In August 1943, American experiments on the general problem of protection against
shaped charges were initiated, and by October of that year a plastic armour much lighter than the steel armour required for the same amount of protection was found. This armour, made by the Flintkote Company, was improved through a series of tests, and a modified armour of pure quartz gravel in a mastic of pitch and wood flour was designated HCR2. Tests were also conducted to test plastic armour's ability to protect ships from torpedoes with shaped charge warheads, but this project was abandoned due to the low probability of these weapons becoming a serious threat, and protection of armoured fighting vehicles and concrete fortifications became the priorities.
The original plan for tank protection with plastic armour was to produce HCR2-filled steel panels, small in size to reduce the area damaged by a single projectile, that could be fastened to an
M4 Shermanin an emergency. To protect against the largest Panzerfaust, eight to twelve tons of plastic protection were required for an M4, while an M26 Pershing's greater base armour mean it required only 7.1 tons of additional protection to equal an M4 with 11.7 tons of plastic protection. This was a 34% increase in weight for an M4, but only a 16% increase for an M26, and the panel for the M26's turret was only 10 3/4 inches thick compared to 13 3/4 inches for the M4. New panels made of welded steel armour, half an inch thick on the sides and three-quarters of an inch thick on the faces, were designed, but their construction was incomplete at the end of World War II. As a result of increasing tank losses to shaped charge weapons, another type of panel that could enter production in only a few weeks was designed. This new type of panel used 1 1/2-inch mild steel instead of armour steel, and had a 2-inch plate of 21ST aluminiumalloy backing the face plate for reinforcement. One set of this armour was completed and tested just after the end of World War II with the result of being considered quite satisfactory, although less so than the panels made of armour steel.
* Terrell, Edward: "Admiralty brief: The story of inventions that contributed to victory in the Battle of the Atlantic", Harrap 1958
* White, Merit P.: "Effects of Impact and Explosion", 1946
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