Brittleness

“ A series of annoying accidents has demonstrated beyond doubt its exceeding brittleness. The ragged-edged splinters that result from such occasions suggest that the solid is amorphous, or without regular crystalline form. ”

“ Though all true cast iron is brittle, in the sense that it is not usefully malleable, i.e. that it cannot be hammered from one shape into another, yet its degree of brittleness differs as that of soapstone does from that of glass, so that there are the intensely hard and brittle cast irons, and the less brittle ones, softer and unhurt by a shock which would shiver the former. ”

“ But the invaluable and rather delicate art of tempering the hardened steel by a very careful and gentle reheating, which removes its extreme brittleness though leaving most of its precious hardness, needs such skilful handling that it can hardly have become known until very long after the art of hot-forging. ”

“ But just as the chemist will tell you that there are no substances which are absolutely insoluble, just so does cast iron appear to be extremely brittle only when compared with the iron alloys of considerably less brittleness. ”

“ If you attempt to bend the rod there is no slight separation of many particles, as in the bent wood, but a full and permanent separation in some one part of the rod. We call the property on which this [27] result depends brittleness. Brittle substances are generally hard. Glass, while the most brittle of all substances, is hard enough to scratch iron. Brittle substances also have much tenacity. A rod of glass can hold up a heavy weight, although a slight blow suddenly given would break it. ”

“ The story was intended to teach all workers in brass and other metals, that they must be very patient, especially in annealing. Annealing oxidizes and softens the metal. While hammering cold sheet brass, the oft repeated blows harden the metal and cause brittleness. ”

“ The change which takes place among the molecules of the metal in consequence of the change of temperature causes internal strains, and thereby puts portions in a state of unequal tension. This state renders the strained parts liable to yield when an additional strain is thrown upon them while the tool is in use; in other words, the brittleness of the steel increases with its hardness. Here again the proportion of carbon present comes into play, and it must be borne in mind that for equal degrees of hardness [10] the steel which contains the least carbon will be the most brittle. ”

“ When they are thus hardened, they are laid in an iron shovel on a fire more or less brisk in proportion to the thickness of the needles; taking care to move them from time to time. This serves to temper them, and take off their brittleness ”

“ But, on the other hand, when the cooling is effected gradually, time is given for the arrangement.
31. Tempering of Steel.—Steel suddenly hardened is too brittle for common use. A process called tempering is therefore resorted to for diminishing the brittleness. The steel is reheated after the hardening, and is then allowed to cool slowly. ”

“ Vast as the force must have been which could fold the solid rocks of the crust as one may crumple the leaves of a magazine in the fingers, it is only under certain conditions that it could have produced the results which we see. Rocks are brittle, and it is only when under a HEAVY LOAD and by GREAT PRESSURE SLOWLY APPLIED, that they can thus be folded and bent instead of being crushed to pieces. Under these conditions, experiments prove that not only metals such as steel, but also brittle rocks such as marble, can be deformed and molded and made to flow like plastic clay. ”

“ Substances are said to be more or less elastic, according to the facility with which they regain their bulk or volume when the pressure is removed; thus liquids resist compression on account of their elasticity, and in solids the resistance is much greater but variable, and the effort required to break a substance is a measure of the cohesive force exerted by its particles. In stone, iron, steel, and all brittle and hard substances, the cohesion of the particles is powerful but of small extent; in elastic bodies, on the contrary, its action is weak, but more extensive. ”

“ To burst is to break by pressure from within, as a bombshell, but it is used also for the result of violent force otherwise exerted; as, to burst in a door, where the door yields as if to an explosion. To crush is to break by pressure from without, as an egg-shell. To crack is to break without complete severance of parts; a cracked cup or mirror may still hold together. Fracture has a somewhat similar sense. In a fractured limb, the ends of the broken bone may be separated, tho both portions are still retained within the common muscular tissue. ”

“ Every one of these needles, in the making, helped to shorten a man’s life. The grinding of the points gave out a never-ceasing dust, composed of gritstone and steel particles, which infested the workmen’s eyes, nostrils, mouth, and lungs, so that no one of them lived to forty years of age. This is the peril which makes life so short among the Sheffield cutlers, and which renders the grinders of steel everywhere, whether for needles, or razors, or scissors, or skates, or sickles, a peculiar class of men. ”

“ Let us imagine a solid prism ABCD fastened into a wall at the end AB, and supporting a weight E at the other end; understand also that the wall is vertical and that the prism or cylinder is fastened at right angles to the wall. It is clear that, if the cylinder breaks, fracture will occur at the point B where the edge of the mortise acts as a fulcrum for the lever BC, to which the force is applied; the thickness of the solid BA is the other arm of the lever along which is located the resistance. ”

“ Imagine forces of some kind, acting laterally, to compress those layers. There will be corrugations, sometimes actual fractures, the two sides of which, sliding one against the other, will be either raised or depressed. Faults is the name which we give to the fractures that penetrate the terrestrial shell and separate two masses of rock, one of which slides over the plane of fracture. The fault, therefore, reveals an edge, a lower lip produced by the subsidence of the soil, and an upper lip produced by an elevation. ”

“ The stresses which occur in engineering practice are, or ought to be, in all cases within the limits of elasticity, and within these limits the change of cross-section caused by longitudinal pull or push is so small that it may be neglected in reckoning the intensity of stress.
Ultimate tensile strength and ultimate shearing strength are well defined, since these modes of stress (simple pull and simple shearing stress) lead to distinct fracture if the stress is sufficiently increased. ”

“ Burns, rough, ragged holes in the faces or on the corners, are inexcusable and should be rejected; the steel has been abused, or it is red-short; in either case the ragged breaks are good starting-points for final rupture. ”

“ Wrought iron and mild steel may be made to show a short and crystalline fracture by a sudden application of stress, while if drawn asunder slowly they develop the silky, fibrous appearance. The men who design and work in metals have to take account of these vital differences and characteristics, and must be careful not to apply treatment suitable to one kind to another of a dissimilar character. ”

“ Sometimes only passive, it merely offers a resistance to the power which puts the bone in motion; thus, in a fall, the os femoris, being pressed between the ground which resists, and the weight of the body that bears on it, bends beyond the extent of its flexibility or pliancy, and finally gives way. At other times the influence of external bodies is actively and directly exerted in this accident: thus a stone, or a piece of timber, falling on the thigh, fractures the bone, in consequence of communicating to it a degree of motion greater than its power of resistance. ”

“ Also the state of the blunted edges, whether battered or fractured, will show what degree of hardness it is desirable to produce. From inattention to these matters, good steel is not unfrequently condemned as unsuitable. ”

“ This is the case with all bodies which are hard, strong, and brittle, which no force can extend or stretch in any perceptible degree, and which require a great force to break or tear them asunder. Such, for example, is cast iron, certain stones, and various other substances. In some bodies the cohesive force is weak, but the sphere of its action considerable. Bodies which are easily extended, without being broken or torn asunder, furnish examples of this. Such are Indian-rubber, or caoutchouc, several animal and vegetable products, and, in general, all solids of a soft and viscid kind. ”

“ When a hole is punched in a plate the material contiguous to the hole is severely distorted by shear, and is so much hardened in consequence that when a strip containing the punched hole is broken by tensile stress the hardened portion, being unable to extend so much as the rest, receives an undue proportion of the stress, and the strip breaks with a smaller load than it would have -borne had the stress been uniformly distri- buted. This bad effect of punching is especially noticeable in thick plates of mild steel. ”

“ All the malleable metals in the ingot or in their cast state are brittle, and exhibit a crystalline fracture. By hammering or rolling, they become more tenacious, and break with difficulty, exhibiting what is called a fibrous fracture. At the same time, they become stiffer and more elastic. ”

“ The huge instrument of war not only did not swerve an inch from its course, but, so great was its weight, that it passed, without so much as a bump, over the bodies of those who fell beneath it, scrunching bone and limb into shapeless and quivering pulp. ”

“ The holes are produced in the metal by forcing a steel punch through it; and this is accomplished without even heating the plate so as to soften the iron. It is needless to say that an intense force must be applied to the punch. ”

“ Turning now to the physical properties of the several classes of iron and steel mentioned above, it may be noted, first, that strength may be subdivided into strength against rupture by direct pull, or tensile strength ”

“ If the grain side of a belt is the outside when on the pulleys, and a crack should consequently start, the destruction of the belt proceeds rapidly, because the line of crack is the weakest part of the belt, and the belt has less elasticity as a continuous body, and more at the line of crack. ”

“ In other countries, and over considerable areas, the corresponding strata are raised up, bent, and contorted in different ways, having been subjected to dislocations through enormous thicknesses. Such lifts and foldings can not have taken place in solid masses without being accompanied by many and important fractures. The principal classes of such fractures, which are nearly vertical, are called faults. They crop out and cut the surface of the ground, sometimes for tens and hundreds of miles, and are of indefinite depth, or descend to below where it is possible for man to penetrate ”

“ In almost every case a slight fusion is seen on the sharp edges of fractures, which become smooth, lose their sharpness, and have the appearance of partly fused [Pg 95] glass. The hardness varies from 7-1/4 to 8, the crystals being very brittle, breaking with a fracture of great unevenness. ”

“ For instance, the line of gravity of the body is, by some cause, removed behind it; the anterior muscles contract themselves to bring it forward again; the extensors act on the rotula; it is broken, and a fall ensues. Again, the leg is suddenly thrown into a state of violent extension; the extensors act with great force; a fracture is the consequence, and the patient falls. A soldier once fractured his rotula in kicking at his serjeant; thus the olecranon, in like manner, has been broken by throwing a stone. ”

“ But this “critical value” of the force is found to depend in an unexpected manner upon the hardness of the steel; the critical value diminishes as the hardness becomes greater up to a certain point, corresponding to a yellow temper, after which it increases and with the hardest steel becomes very high. ”

“ It yields with the greatest difficulty to the hammer, but its edges are as brittle almost as glass, and fly into numerous minute splinters with sharp edges—fracture distinctly conchoidal, dull, and rather earthy. ”

“ The most curious object among these intimate things is a steel fore-arm and hand, with fingers of steel, made to move and counterfeit as far as possible the lost members of some unfortunate person who had lost his arm. ”

“ In reckoning ultimate strength, however, engineers take, not the actual intensity of stress at which rupture occurs, but the value which this intensity would have reached had rupture ensued without previous alteration of shape. Thus, if a bar whose original cross-section is 2 sq. in. breaks under a uniformly distributed pull of 60 tons, the ultimate tensile strength of the material is reckoned to be 30 tons per square inch, although the actual intensity of stress which produced rupture may have been much greater than this, owing to the contraction of the section previous to fracture. ”

“ It is not considered good practice to stress a material, especially a brittle material, repeatedly to a point half-way to its elastic limit. (The elastic limit is that stress that may be given to a body without producing a permanent set; below which it will return to its original shape upon the removal of the stress.) ”

“ The large spade shows a very brittle fracture. The iron is too brittle for gun-work; it would be liable to break. This iron, if REPEATEDLY heated and hammered, would become decarbonized, and would then possess the qualities found in the spear-head, which, after being curled up by being struck against a hard substance, was restored, by hammering, to its original form without injury. ”