By F.R. BRAINARD, U.S.N.
The building of a navy, which has been actively going on for the past
few years, has drawn public attention to naval subjects, and recent
important experiments with armor plates have attracted large
attention, hence it may not be amiss to give a description of the
manufacture and testing of armor. It would be interesting to wade
through the history of armor, studying each little step in its
development, but we shall simply take a hasty glance at the past, and
then devote our attention to modern armor and its immediate future.
Modern armor has arrived at its present state of development through a
long series of experiments. These experiments have been conducted with
great care and skill, and have been varied from time to time as the
improvements in the manufacture of materials have developed, and as
the physical laws connected with the subject have been better
understood. There has been very little war experience to draw from,
and hence about all that is now known has been acquired in peaceful
experiments.
The fundamental object to be obtained by the use of armor is to keep
out the enemy's shot, and thus protect from destruction the vulnerable
things that may be behind it. The first serious effort to do this
dates with the introduction of iron armor. With this form of armor we
have had a small amount of war experience. The combat of the Monitor
and Merrimac, in Hampton Roads, in May, 1862, not only marked an epoch
in the development of models of fighting ships, but also marked one in
the use of armor. The Monitor's turret was composed of nine one-inch
plates of wrought iron, bolted together. Plates built in this manner
form what is known as laminated armor. (See Fig. 1.) The side armor of
the hull was composed of four one-inch plates. The Merrimac's casemate
was composed of four one-inch plates or two two-inch plates backed by
oak. The later monitors had laminated armor composed of one-inch
plates. The foregoing, with the Albemarle and Tennessee rams under the
Confederate flag, are about the sum of our practical experience in the
use of armor.
[Illustration: Fig. 1.]
European nations took up the subject of armor and energetically
conducted experiments which have cost large sums of money, but have
given much valuable data. For a long time wrought iron was the only
material used for armor, and the resisting power depending on the
thickness; and the caliber and penetration of guns rapidly increasing,
it was not long before a point was reached where the requisite
thickness made the load of armor so great that it was impracticable
for a ship to carry it. The question then arose as to what were the
most important parts of a ship to protect. The attempted solutions of
this question brought out various systems of distributions.
Armored ships were formerly of two classes; in one the guns were
mounted in broadside, in the other in turrets. Every part of the ship
was protected with iron to a greater or less thickness. In more modern
ships the guns are mounted in an armored citadel, in armored barbettes
or turrets, the engines, boilers and waterline being the only other
parts protected. There may be said to be three systems of armor
distribution. The belt system consists in protecting the whole
waterline by an armored belt, the armor being thickest abreast of the
engines and boilers. The guns are protected by breastworks, turrets or
barbettes, the other parts of the ship being unprotected. The French
use the belt system, and our own monitors may be classed under it. The
central citadel system consists in armoring that part of the waterline
which is abreast of the engines and boilers. Forward and aft the
waterline is unprotected, but a protective deck extends from the
citadel in each direction, preventing the projectiles from entering
the compartments below. The hull is divided into numerous compartments
by water-tight bulkheads, and, having a reserve of flotation, the
stability of the ship is not lost, even though the parts above the
protective deck, forward and aft, be destroyed or filled with water.
The guns are protected by turrets or barbettes. The deflective system
consists in inclining the armor, or in so placing it that it will be
difficult or impossible to make a projectile strike normal to the face
of the plate. A plate that is inclined to the path of a projectile
will, of course, offer greater resistance to penetration than one
which is perpendicular; hence, when there is no other condition to
outweigh this one, the armor is placed in such a manner as to be at
the smallest possible angle with the probable path of the projectile.
This system is designed to cause the projectile to glance or deflect
on impact. Deflective armor should be at such an angle that the
projectiles fired at it cannot bite, and hence the angle will vary
according to the projectile most likely to be used. In the usual form
of deflective deck the armor is at such a small inclination with the
horizon that it becomes very effective. Turret and barbette armor may
be considered as deflective armor. The term inclined armor denotes
deflective armor that is inclined to the vertical. The kinds of armor
that are in use may be designated as rolled iron, chilled cast iron,
compound, forged and tempered steel, and nickel steel. Iron armor
consists of wrought iron plates, rolled or forged, and of cast iron or
chilled cast iron, as in the Gruson armor. Compound armor consists of
a forged combination of a steel plate and an iron plate. Steel armor
consists of wrought steel plates. Nickel-steel armor consists of
plates made from an alloy of nickel and steel.
I have spoken above of laminated armor. To secure the full benefit of
this kind, the plates must be neatly fitted to each other; the
surfaces must make close contact. This requires accurate machining,
and hence is expensive. To overcome this point sandwiched armor was
suggested. This consists in placing a layer of wood between the
laminations, as shown in Fig. 2. It was found that laminated and
sandwiched armor gave very much less resisting power than solid rolled
plates of the same thickness. Wrought iron armor is made under the
hammer or under the rolls, in the ordinary manner of making plates,
and has been exhaustively studied and experimented with--more so than
any other form of armor.
[Illustration: Fig. 2.]
Chilled cast iron armor is manufactured by Gruson, in Germany, and is
used in sea coast defense forts of Europe.
In 1867 several compound plates were made by Chas. Cammell & Co., of
Sheffield, England, and were tested at Shoeburyness, in England, and
at Tegel, in Russia. These plates were made by welding slabs of steel
to iron; but the difficulties were so great that the idea was
abandoned for the time.
[Illustration: Fig. 3.]
[Illustration: FIG. 4.]
Compound armor, as now manufactured, is of two types: Wilson's patent,
a backing of rolled iron, faced with Bessemer steel; Ellis' patent, a
backing of rolled iron, faced with a plate of hard rolled steel,
cemented with a layer of Bessemer steel. Both these kinds are
manufactured in England and France in sizes up to fifty tons weight.
The Wilson process is used at the works of Messrs. Cammell & Co., of
Sheffield, England, and the Ellis process at the Atlas Works of Sir
John Brown & Co., of the same place. These are the two leading
manufacturers of compound plate.
[Illustration: Fig. 5.]
The method employed by Wilson in making compound plate is to first
make a good wrought iron plate. To the surface of this and along each
side of the length of the plate are fixed two small channel irons, as
shown in Fig. 5. The plate is then raised to a welding heat in a gas
furnace, and transferred to an iron flask or mould. Wedges are driven
in between the back of the plate and the side of the mould, thus
forcing the channel irons up snug against the opposite side of the
mould. Moulding sand is then packed around the back and sides of the
plate (see Fig. 6). The mould is lowered in a vertical position into a
pit. Molten steel, manufactured by either the Siemens-Martin or
Bessemer process, is then poured in through a trough that forms
several streams, and forms the hard face of the plate. The molten
steel as it runs down cleans the face of the wrought iron plate,
scoring it in places, and, being of much higher temperature, the
excessive heat carbonates the iron to a depth of one-eighth to
three-sixteenths of an inch, forming a zone of mild steel between the
hard steel and soft iron. The mould is placed in a vertical position
to insure closeness of structure and the forcing of gases out of the
steel. After solidifying, the whole plate is pressed, and passed
through the rolls to obtain thorough welding. It is then bent, planed,
fitted, tempered, and annealed to remove internal strains.
[Illustration: Fig. 6.]
In 1887, Wilson took out a patent for improvements in his process of
making compound plates. In this method of manufacture he takes a
wrought iron, fibrous plate, fifteen inches thick, built up from a
number of thin plates. While hot from the forging press, he places
this plate in an iron mould (see Fig. 7) about 28 inches deep, and
upon it runs "ingot iron" or very mild steel to a depth of thirteen
inches. In this form of mould the plate rests on brickwork, and is
held in place by two grooved side clamps or strips which are caused to
grip the plate by means of screws which extend through the sides of
the mould. After solidifying, the plate, which is twenty-eight inches
thick, is reheated and rolled down to eighteen inches. This is the
iron backing of the finished plate, and it is again put in the iron
mould and heated, when a layer of hard steel is run on the exposed
surface of the original wrought iron plate to a depth of eight inches.
This makes a plate about twenty-eight inches thick. It is taken from
the mould, reheated, rolled, hammered or pressed down to twenty
inches. After cooling, it is bent, planed, and fitted as desired, then
tempered and annealed to relieve internal strains.
[Illustration: Fig. 7.]
The method employed by Ellis in making compound plates is to take two
separate plates, one of good wrought iron and one of hard forged
steel, placing the forged steel plate on the wrought iron plate,
keeping them separate by a wedge frame or berm of steel around three
sides, and placing small blocks of steel at various points near the
middle of the plates (see Fig. 8). These blocks are called distance
blocks. After covering all the exposed steel surfaces with ganister,
the plates are put in a gas furnace and heated to a welding heat. They
are then lowered into a vertical iron pit with the open side
uppermost. The plates are held in position by hydraulic rams, which
also prevent bulging. Molten steel of medium softness is then poured
into the space between the plates, by means of a distributing trough
having holes in the bottom, and after this has solidified, the whole
plate is placed under the hydraulic press and reduced about twenty per
cent. in thickness. The plate is then passed through the rolls, bent,
planed, fitted, tempered, and annealed to reduce internal strains.
[Illustration: Fig. 8.]
In heating the compound plates for rolling, the plate is placed in the
furnace with the steel face down, so that the iron part gets well
heated and the steel does not become too hot. Great care must be taken
not to overheat the plate, and in working, many passes are given the
plate with small closings of the rolls. The steel part of a compound
plate is usually about one third of the full thickness of the plate.
Forged steel armor, tempered in oil, is fabricated at Le Creusot,
France, by Schneider & Co., using open-hearth steel, and forging under
the 100 ton hammer. The ingots are cast, with twenty-five per cent.
sinking head and are cubical in form. The porter bar is attached to a
lug on one side of the ingot. By means of a crane with a curved jib
which gives springiness under the hammer, the ingot is thrust into the
heating furnace. On arriving at a good forging heat it is swung around
to the 100 ton hammer, under which it is worked down to the required
shape. A seventy-five ton ingot requires about eight reheatings before
being reduced to shape. Having been reduced to shape, the plate is
carefully annealed, then raised to a high tempering heat, and the face
tempered in oil. It is reannealed to take out the internal strains,
care being taken not to reduce the face hardness more than necessary.
The Schneider process of tempering is based upon the utilization of
the absorption of heat caused by the fusing or melting of a solid
substance, and of the fact that so long as a solid is melting or
dissolving in a liquid substance, the liquid cannot get appreciably
hotter, except locally around the heating surface. The body to be
hardened is plunged at the requisite temperature into a bath
containing the solid melting body, or is kept under pressure in the
solid material of low melting point until the required extraction of
heat has taken place, more solid material being added if necessary as
that originally present melts and dissolves.
Nickel steel armor is made in a similar manner to the steel plates,
the material used in casting the ingot being an alloy of nickel and
steel containing between three and four per cent. of nickel.
The Harvey process of making armor consists in taking an all-steel
plate and carbonizing the face. This carbonizing process is very
similar to the cementation process of producing steel, and by it the
face of the plate is made high in carbon and very hard.
The system invented by Sir Joseph Whitworth, of Manchester, England,
consists in what might be called scale armor. A section of a sample of
the armor represents four plates. The outer layer, one inch thick, is
composed of steel of a tensile strength of 80 tons per square inch;
the second layer, one inch thick, of steel whose tensile strength is
40 tons per square inch; the third and fourth layers, each one-half
inch thickness, of mild steel. The outer layer is in small squares of
about ten inches on a side, and is fastened to the second layer by
bolts at the corners and one in the middle of each square. The surface
is flush. (See Fig. 9.) The end sought by the above system is to break
up the shot by the hard steel face and to restrict any starring or
cracking of the metal to the limit of the squares or scales struck.
The bolts are of high carbon and are extremely hard steel.
[Illustration: Fig. 9.]
Armor plates must often be bent or curved to single or double
curvature and sometimes to a warped surface to fit the form of the
ship. There are several methods of bending plates. One method employs
a cast iron slab of the required form, which is placed on the piston
of a hydraulic press. The armor plate is placed face down on this
slab, and on top of the plate are laid packing blocks of cast iron, of
such sizes and shapes as to conform to the required curve. These
blocks take against the upper table of the press, when the piston is
forced up, and the hot plate is thus dished to the proper form.
In the French method of bending, an anvil or bed plate of the required
curve is used, and the armor plate is forced to take the curve by
being hammered all over its upper surface with a specially designed
steam hammer.
The edges of the plate are trimmed by large, powerful slotting
machines or circular saws; the latter, however, operate in exactly the
same manner as a slotter, except that there is no return motion to the
tool. Each tooth of the saw is but a slotting tool, and these teeth
are, by screws, rendered capable of being nicely adjusted in the
circumference of the saw.
The plates are fastened to the hulls and backing by heavy bolts,
varying in size according to the weight of the individual plate. For
the 6,000 ton armored ships, these bolts are from 2.75 to 3.1 inches
in diameter and from 18.45 to 23 inches in length. They are tapped two
or three inches into the armor and do not go through the plate. They
pass through wrought iron tubes in the backing and set up with cups,
washers and nuts against the inner skin of the ship.
At steel works where plates for our new navy are being manufactured,
there are inspectors who look after the government's interests.
Officers of the navy are detailed for this work, and their duty is to
watch the manufacture of plates through each part of the process and
to see that the conditions of the specifications and contract are
complied with.
The inspection and testing of armor plates consists in examining them
for pits, scales, laminations, forging cracks, etc., in determining
the chemical analysis of specimens taken from different parts, in
determining the physical qualities of specimens taken longitudinally
and transversely, and the ballistic test. Specifications for these
different tests are constantly undergoing change, and it would be
impossible to state, with exactness, what the requirements are or will
be in the near future. The ballistic test is the important one, and is
made by taking one plate of a group and subjecting it to the fire of a
suitable gun. The other tests are simply to insure, as far as
practicable, that all the other plates of the group are similar to and
are capable of standing as severe a ballistic test as the test plate.
The following will give an idea of the ballistic test as prescribed by
the Bureau of Ordnance, Navy Department. The test plate, irrespective
of its thickness, is to be backed by thirty-six inches of oak or other
substantial wood. Near the middle region of the plate an equilateral
triangle will be marked, each side of which will be three and one-half
calibers long. The lower side of the triangle will be horizontal.
Three shots will be fired, the points of impact being as near as
possible the extremities of the triangle. The velocity of the shot
will be such as to give the projectile sufficient energy to just pass
through a wrought iron plate of equal thickness to the test plate, and
through its wood backing. The velocity is calculated by the Gavre
formula:
a
V² = --- { 3507 E² × 2265464 e^{1.4} }
w
[TEX: V^2 = \frac{a}{w} \{ 3507 \ E^2 \times 2265464 \ e^{1.4} \}]
V = the velocity of the projectile in feet per second.
a = the diameter of the projectile in inches.
w = the weight of the projectile in pounds.
E = the thickness of the backing in inches.
e = the thickness of the plate in inches.
Using the above formula we can make out a table as follows:
-------+-------+-------------+-------+-------+------+---------+
Plate. |Backi'g| Gun, service| w, | a, | V. | Energy, |
Inches.|Inches.| shot. |Pounds.|Inches.| f. 8.| Impact. |
| | | | | | f. tons.|
-------+-------+-------------+-------+-------+------+---------+
6 | 36 | 6" B.L.R. | 100 | 5.96 | 1389 | 1337 |
7 | 36 | 6" " | 100 | 5.96 | 1528 | 1619 |
8 | 36 | 8" " | 250 | 7.96 | 1213 | 2550 |
9 | 36 | 8" " | 250 | 7.96 | 1308 | 2966 |
10 | 36 | 8" " | 250 | 7.96 | 1399 | 3390 |
11 | 36 | 8" " | 250 | 7.96 | 1489 | 3839 |
12 | 36 | 10" " | 500 | 9.96 | 1247 | 5386 |
13 | 36 | 10" " | 500 | 9.96 | 1315 | 5987 |
14 | 36 | 10" " | 500 | 9.96 | 1381 | 6608 |
15 | 36 | 12" " | 850 | 11.96 | 1215 | 8699 |
16 | 36 | 12" " | 850 | 11.96 | 1269 | 9710 |
17 | 36 | 12" " | 850 | 11.96 | 1332 | 10454 |
18 | 36 | 12" " | 850 | 11.96 | 1374 | 11124 |
19 | 36 | 12" " | 850 | 11.96 | 1425 | 11965 |
20 | 36 | 12" " | 850 | 11.96 | 1476 | 12837 |
-------+-------+-------------+-------+-------+------+---------+
No projectile or fragment of the plate or projectile must get wholly
through the plate and backing. The plate must not break up or give
such cracks as to expose the backing, previous to the third shot.
The penetration of projectiles of different forms into various styles
of armor has been very thoroughly studied and many attempts have been
made to bring the subject down to mathematical formulæ. These formulæ
are based on several suppositions, and agree very closely with results
obtained in actual experiments, but there are so many varying
conditions that it is extremely doubtful if any formulæ will ever be
written that will properly express the penetration.
Many different forms have been given to the heads of projectiles, as
flat, ogival, hemispherical, conoidal, parabolic, blunt trifaced, etc.
The flat headed projectile has the shape of a right cylinder, and acts
like a punch, driving the material of the armor plate in front of it.
These projectiles are especially valuable when firing at oblique
armor, for they will bite or cut into the armor when striking at an
angle of thirty degrees.
The ogival head acts more as a wedge, pushing the metal aside, and
generally will give more penetration in thick solid plates than the
flat headed projectile. The ogival head is usually designed by using a
radius of two calibers.
The hemispherical, conoidal, parabolic and blunt trifaced all give
more or less of the wedging effect. The blunt trifaced has all the
good qualities of the ogival of two calibers. It bites at a slightly
less angle, and the three faces start cracks radiating from the point
of impact.
Forged steel is the best material for armor-piercing projectiles, but
many are made of chilled cast iron, on account of its great hardness
and cheapness.