The most common substances encountered by projectiles are arranged in the following series, in the order of their resistance to penetration: air, water, sand, wood, lead, copper, wrought-iron, soft steel, cast-iron, chilled-iron, hardened steel, etc. All other substances may be arranged between these or in continuation of the series. Air opposes the motion of a projectile by its inertia, elastic force, and the pressure due to its weight. The projectile compresses the air in its front and disperses it laterally, while the rear of the projectile is relieved by its motion of the normal pressure of air. A small amount of resistance is also met with in the shape of friction. In the case of water these resistances are increased by the greater density and weight of this substance, and there is also a slight additional resistance due to the cohesion among the particles. Sand, being a solid, or at least made up of solid elements, presents the additional resistance of "crushing strength." It cannot be penetrated at a high velocity without crushing some of the grains, and the higher the velocity the greater the amount of work expended in this manner. This resistance to crushing implies a continuation of the elastic force beyond the elastic limits, and involves indirectly tensile strength, since a solid in being crushed must enlarge laterally and finally yield to a strain of tension. In penetrating wood, lead, or any of the other materials, "tensile strength" forms the chief element of the resistance, while inertia and friction become of minor importance.
The office of elasticity in all these cases is to transmit the effect of the projectile from those particles first acted upon to those more remote, and thus calling into play their inertia or tensile strength, as the case may be: and were it not for this property, the statical resistance of a plate of any material to perforation would be entirely independent of the thickness of the plate; a thick plate would offer no greater resistance than a thin one, since each layer or unit of thickness would be perforated without receiving any assistance from its neighbors. The work of penetration would then vary directly with the distance penetrated, or the thickness of the plate; elasticity, however, has its maximum point of usefulness in resisting penetration, and beyond this it becomes a great disadvantage. While increasing the number of fibers or elementary portions of the material broken at once, thereby increasing the statical resistance, it diminishes the time during which this resistance opposes the motion of the projectile in like ratio; and the amount of motion destroyed or generated increases with the time as well as with the force or resistance. For this reason hardened steel and chilled iron are less efficient in stopping projectiles than soft iron, although they offer a much greater statical resistance to penetration. There are many reasons for believing that a general formula for the penetration of projectiles in all materials may be deduced, when experiments have been sufficiently extended.
The respective motions of a flat and pointed headed projectile on oblique impact are explained as follows: It is asserted that the flat-headed projectile, on striking, cuts out a portion of the face of the plate, which it carries along in front, thus increasing the thickness to be penetrated; and, remaining nearly parallel to its original direction, it has to pass through the plate obliquely. While, if the projectile has a pointed head, the point enters at first more deeply into the plate than the flat head, and the center of gravity moving forward, the projectile turns around more readily than with the latter, so that its axis becomes perpendicular, or nearly so, to the face of the plate, having then only the least thickness to penetrate.
It is difficult to obtain for comparison the results of practice with the flat and pointed headed projectiles of the same material fired at targets inclined to the line of the range; the former having been so little used, as its form is so objectionable, both as regards accuracy and velocity. On the whole, it may be said that in the case when the projectile ought to be capable of piercing the plate or target, there is little difference between the effect of a flat head and a hemispherical head; but when the target is beyond the power of the projectile, the hemispherical head makes the deepest indent.
The impact of a projectile, in addition to indenting or penetrating a target, produces more or less bending, tearing, and other damage at a distance from the point of impact; which effects may be classed under the term "Concussion." The effect of concussion is transmitted from the point of impact in all directions, in the same manner as sound-waves, and increases with the elasticity of the material. Whatever tends to diminish the elasticity of the structure, as dividing it into many pieces, or using soft ductile material to receive the projectile, will diminish the effect of concussion. This effect is expended in two ways—First, in giving motion to the structure or in developing inertia; and, Second, in overcoming the tenacity of the material, either in bending or tearing those portions first acted upon from those more remote. Both of these components increase with the whole amount of work expended by the projectile, other conditions being equal.
Generally speaking, the penetrative effect depends on the shape and material of the projectile, on its energy and diameter, and the direction with which it strikes the target. It is quite impossible to accurately determine the coefficients of resistance for the different materials of projectiles and plates; but practically the amount of penetration, whether for iron or steel plates, or masonry, or earth, maybe determined by experiment. Various empirical laws suffice to give approximate results; but they do not stand the test of any general application. In consequence of the varying qualities of resistance both in projectiles and targets, the variation in shape of the projectile on impact, the possibility of the projectile breaking up, and the amount of heat developed on impact, strictly analytical investigations cannot be made.
Farrow, Edward S. American Small Arms; a Veritable Encyclopedia of Knowledge for Sportsmen and Military Men. New York: Bradford, 1904. Print.
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