Applique armour plates are used to prolong the service life of otherwise-obsolete vehicles, by increasing the armour protection against increasingly powerful anti-tank weapons. These armour plates are either welded or bolted onto the existing armour plates, which can be done both at factories and field workshops. For this reason, applique armour can be both official and improvised (the latter often being of value only to the morale of the tank crews, as high-quality steel will not usually be readily available to field workshops).
Applique armour does not offer the same protection as one armour plate of the same thickness and hardness as the combined thickness of the applique and original armour plate, either when placed close together and when separated. When placed close together, the nature of the displacement of material by the shot will depend on the area of the plate. While the middle of the armour plate will only displace material laterally through the plate, the outer surfaces will also displace material outwards. The outwards displacement requires less energy, as either side has free air surrounding it. Since two armour plates will have twice the number of outer surfaces, the shot can pass through more easily. (WAL 710/930-1)
This was demonstrated by a US test, which determined that two 38,1 mm (1,5 in) armour plates with a Brinell hardness of 270, with no space between them, were proof against a Projectile, A.P.C.-T., M82 (90 mm APCBC-HE-T) at an angle of 45 degrees up to a velocity of 604 m/s (1 980 ft/s) (WAL 710/930-1). This was compared to a single armour plate with a thickness of 76,2 mm (3 in), with the same Brinell hardness and angle, which was proof up to a velocity of 716 m/s (2 350 ft/s) (WAL 710/930-1). Comparing these figures with the firing table for the Projectile, A.P.C.-T., M82, this is equivilant to a difference in firing distance of approximately 1 600 m (1 700 yds) (Armor Piercing Ammunition for Gun, 90-mm, M3).
Spaced armour is when more than one armour plate is used, seperated by air. The use and effectiveness depends on both the type of shot fired at it, and the thickness and layout of the armour plates.
US tests made during World War II showed, that by placing a relatively thin armour plate (one-twelfth the diameter of the shot fired at it) at least one shot length from the main armour, it was possible to strip the cap from capped ammunition. The tests were done against both face-hardened and rolled homogenous armour, with similar results. As the shot was decapped, the penetration capability would be reduced in that the shot would be more likely to shatter upon impacting the main armour, than if the main armour thickness was increased with the same thickness as the spaced armour. (WAL 710/607-3)
One test demonstrated the result against a 7,62 cm (3 in) APCBC shot of adding 0,635 cm (0,25 in) of armour by increasing the main armour thickness and adding a decapping plate:
|Main armour||Decrease in maximum angle of penetration without shatter|
|0,635 cm (0,25 in) added to main armour||0,635 cm (0,25 in) added as decapping plate|
|6,35 cm (2,5 in)||3°||53° to 35°|
|7,62 cm (3 in)||3°||50° to 35°|
|8,89 cm (3,5 in)||3°||45° to 35°|
While a thin de-capping plate will have a positive effect against capped shots without any significant weight increase, increasing the thickness of the outer armour plate further while decreaseing the thickness of the inner (main) armour plate will have a negative effect.
The same US test as was referred to under applique armour demonstrated that two parallel 38,1 mm (1,5 in) armour plates with a Brinell hardness of 270 seperated by 152,4 mm (6 in) were proof against a Projectile, A.P.C.-T., M82 (90 mm APCBC-HE-T) at an angle of 45 degrees up to a velocity of 500 m/s (1 640 ft/s). This was compared to a single armour plate with a thickness of 76,2 mm (3 in), with the same Brinell hardness and angle, which was proof up to a velocity of 716 m/s (2 350 ft/s) (WAL 710/930-1). When again compared to the firing table for the Projectile, A.P.C.-T., M82, this is equivilant to a difference in firing distance of approximately 3 000 m (3 300 yds) (Armor Piercing Ammunition for Gun, 90-mm, M3).
The reason is that, as the shot hits the first plate at an angle of 45 degrees, the cap will reduce the angle. This will increase the penetration against the first armour plate, and when the shot exists the rear of the first armour plate, the angle against the second armour plate will be reduced. As two thin plates will not offer the same resistance as one thick plate, as demonstrated in the applique section, the second plate will be weaker against the shot, even though the cap has been stripped. (WAL 710/930-1)
Armour skirts effectiveness in defeating HEAT shots is something which is still debated. Proponents argue that the skirts will prematurely set off the HEAT shot, increasing the distance the jet has to travel and thereby decreasing the effectiveness. Opponents argue that, while it is true that the shot will be prematurely set off, the increased distance will allow the jet to form better, neutralising the effect or even increasing penetration capability.
US and British test results from World War II show that spaced armour did have an effect against HEAT shots. This effect could not be reproduced after World War II, however,
unless the skirting plates are placed at distances from the main armor which are impossible from a practical engineering viewpoint (WAL 710/930-1). In addition, the Russians reported to the US during World War II that German armour skirts did not protect against HEAT shots. (WAL 710/930-1)
Since the optimum stand-off distance of HEAT shots increases with the quality of the shot, one explanation for these conflicting reports could be, that the effectiveness of skirts against HEAT shots had more to do with the individual HEAT shot than the skirts.