Ordnance, Strains Upon

The exterior form of cannon is determined by the variable thickne of the metal which surrounds the bore at different points of its length. In general terms, the thickne is greatest at the seat of the charge, and least at or near the muzzle. This arrangement is made on account of the variable action of the powder and projectile along the bore, and the nece ity of disposing the metal in the safest and most economical manner. The pre ure at different points may be approximately determined by calculation, or, more accurately, by experiment. In the latter method, the plan generally employed consists in boring a series of small holes through the side of a gun at right angles to its axis at known distances apart. A steel ball is projected from each hole in succe ion into a target, or ballistic pendulum, by the force of the charge acting through it, and the pre ure at the various points is deduced from the velocities communicated to these balls. This method was adopted by Col. Bomford. Instead of the projectile a steel punch may be employed, which is pre ed by the force of the charge into a piece of soft copper. (See Pre ure-gauge .) The weight nece ary to make an equal indentation in the same piece is then ascertained by a testing machine. The strains to which all fire-arms are subjected may be cla ified as follows: (1) The tangential strain which tends to split the piece open longitudinally, and is similar in its action to the force which bursts the hoops of a barrel. (2) The longitudinal strain which acts to pull the piece apart in the direction of its length. Its action is greatest at or near the bottom of the bore, and least at the muzzle, where it is nothing; these two strains increase the volume of the metal to which they are applied. (3) A strain of compre ion which acts from the axis outward to crush the truncated wedges of which a unit of length of the piece may be supposed to consist; this strain compre es the metal and enlarges the bore. (4) A transverse strain which acts to break transversely by bending outward the staves of which the piece may be supposed to consist. This strain compre es the metal on the inner and extends it on the outer surface. It is known that rupture will take place due to the tangential strain alone, when three times the pre ure upon a unit of surface of the bore is greater than twice the tensile strength. Due to the longitudinal strain alone, rupture will take place in the direction of the length, when the pre ure is greater than twice the tensile strength; and if the transverse strain alone is considered, rupture will take place when twice the pre ure is greater than three times the tensile strength. It therefore appears that the tendency to rupture is greater from the action of the tangential force than from any other, and for lengths above two, or perhaps three calibers, the tangential resistance may be said to act alone, as the aid derived from the transverse resistance will be but trifling for greater lengths of bore; but for lengths of bore le than two calibers, this resistance will be aided by both the transverse and the longitudinal resistance. Every piece should therefore have sufficient thickne of breech to prevent splitting through the latter; after this point has been attained, any additional thickne of breech adds nothing to the strength of the piece. It therefore appears that a fire-arm is strongest at or near the bottom of the bore, and that its strength is diminished rapidly as the length of the bore increases to a certain point (probably not more than three calibers from the bottom); after which, for equal thickne of metal, its strength becomes sensibly uniform. The metals of which cannon are made being crystalline in structure, the size and arrangement of the crystals have an important influence on its strength to resist a particular force; and a metal will have the greatest strength with reference to a particular force when its crystals are small, and the principal faces are parallel to the straining force, if it be one of extension, and perpendicular to it, if it be one of compre ion. The position of the principal crystalline faces of a cooling solid is found to be perpendicular to the cooling surface; the result of this arrangement of crystals is to create planes of weakne where the different systems of crystals intersect. The effect of this law upon cannons, it has been discovered, is to render radial specimens more tenacious than those cut tangentially from the same gun. The manner and rapidity of cooling have also a great effect upon the ability of cannon to resist strains, and as all solid bodies contract their size in the operation of cooling, it follows that if the different parts of a cannon cool unequally, it will change its form, provided it be not restrained by the presence of a superior force. If it be so restrained, the contractile force will diminish the adhesion of the parts by an amount which depends on the rate of cooling of the different parts, and the contractibility of the metal. This is an important consideration in estimating the strength and endurance of cannon, particularly those made of cast iron. All such cannon cooled from the exterior (see Ordnance, Construction of ) are affected by two straining forces; the outer portion of the metal being compre ed, and the interior extended, in proportion to their distances from the neutral axis or line composed of particles which are neither extended nor compre ed by the cooling proce . The effect of this unequal contraction may be so great as to crack the interior metal of cast iron even before it has been subjected to the force of gunpowder. gunpowder is not distributed equally over the thickne of metal, but it varies inversely as the square of the distance from the centre; it therefore follows that the sides of a cannon are not rent asunder as by a simple tensile force, but they are torn apart like a piece of cloth, commencing at the surface of the bore. Hence it is that the effect of ordinary cooling is to diminish the strength and hardne of the metal of cannon at or near a point where the greatest strength and hardne are required, i.e. , at the surface of the bore. increase with the diameter of the casting and the irregularity of its form. This explains the great difficulty found in making large cast-iron cannon proportionally as strong as small ones, and also how projections like bands, moldings, etc., injure the strength of cannon. It also explains why cannon made of “light” cast iron, or cast iron made more tenacious by partial decarbonization, are not so strong as cannon made of weaker iron; for it is well known that such iron contracts more than the latter in cooling, and therefore produces a greater strain of extension on the surface of the bore. Capt. Rodman of the U. S. Ordnance Department has proposed a plan for cooling cannon from the interior (see Ordnance, Construction of ), thereby reversing the making them contribute to the endurance rather than to the injury of the piece. It is likely, however, that the strains produced by unequal cooling are modified by time, which probably allows the particles to accommodate themselves to a certain extent to their constrained position. In confirmation of this, great endurance has been frequently found in old solid cast guns, as in the old 42-pounders tested about the beginning of the war, 1861-65.