in actual practice. The latter is variously stated at from a third to a tenth. It will be observed that whilst much discussion has arisen as to the amount of working strain, or the ratio the load should bear to that of the breaking strain, the important circumstance of the quality of the iron, as influencing the working strain, has been overlooked. The Board of Trade limits the strain to 5 tons, or 11,200 lbs. per square inch. It must be abundantly evident, from the facts which have been produced, that the breaking strain, when taken alone, gives a false impression of, instead of indicating, the real quality of the iron, as the experiments which have been instituted reveal the somewhat startling fact, that frequently the inferior kinds of iron actually yield a higher result than the superior. The reason of this difference was shown to be due to the fact that, whilst the one quality retained its original area, only very slightly decreased by the strain, the other was reduced to less than one-half. Now, surely this variation, hitherto unaccountably completely overlooked, is of importance as indicating the relative hardness or softness of the material, and thus, it is submitted, forms an essential element in considering the safe load that can be practically applied in various structures. It must be borne in mind that although the softness of the material has the effect of lessening the amount of the breaking strain, it has the very opposite effect as regards the working strain. This holds good for two reasons: first, the softer the iron the less liable it is to snap; and second, fine or soft iron, being more uniform in quality, can be more depended upon in practice. Hence the load which this description of iron can suspend with safety may approach much more nearly the limit of its breaking strain than can be attempted with the harder or coarser sorts, where a greater margin must necessarily be left. Special attention is now solicited to the practical use that may be made of the new mode of comparison introduced by the writer, viz., the breaking strain per square inch of the fractured area of the specimen, instead of the breaking strain per square inch of the original area. As a necessary corollary to what he has just endeavored to establish, the writer now submits, in addition, that the working strain should be in proportion to the breaking strain per square inch of fractured area, and not to the breaking strain per square inch of original area, as heretofore. He does not presume to say what that ratio should be, but he fully maintains that some kinds of iron experimented on by him will sustain with safety more than double the load that others can suspend, especially in circumstances where the load is unsteady, and the structure exposed to concussions, as in a ship, or to vibratory action, as in a railway bridge. 369. Resistance to Compressive Strain. But few experiments have been published on the resistance of this material to compression. Rondelet states that it commences to yield under a pressure of about 70,800 lbs. per square inch, and that when the altitude of the specimen tried is greater than three times the diameter of the base it yields by bending. Mr. Hodgkinson states that the circumstances of its rupture from crushing indicate a law similar to what obtains in cast iron. The same rule for proportioning the working strain to the crushing strain is followed in wrought iron subjected to compression as in cast iron. Resistance to a Transverse Strain. The following tables exhibit the circumstances of deflection from a transverse strain on bars laid on horizontal supports; the weight being applied at the middle of the bar. The table I. gives the results on bars 2 inches square, laid on supports 33 inches asunder; table II. the results on bars 2 inches deep, 1.9 in. broad, bearing as in table I. Weight in tons. inches for each Weight in tons. inches for each half ton. Deflections in half ton. The above experiments were made by Professor Barlow, and published in his report already cited. He remarks on the results in Table II., that the elasticity was injured by 2.50 tons and destroyed by 3.00 tons. 370. Trials were made to ascertain mechanically the position of the neutral axis on the cross section. Professor Barlow remarks on these trials, that "the measurements obtained in these experiments being tension 1.6, compression 0.4, giv ing exactly the ratio of 1 to 4 in rectangular bars. These results seem the most positive of any hitherto obtained; still there can be little doubt this ratio varies in iron of different qualities; but looking to the preceding experiments, it is probably always from 1 to 3, to 1 to 5." 371. Effects of Time on the Elongation of Wrought Iron from a Constant Strain of Extension. M. Vicat has given, in the Annales de Chimie et de Physique, vol. 54, some experiments on this point, made on iron wires which had not been annealed, by subjecting four wires, respectively, to strains amounting to the 1, the, the, and of their tensile strength, during a period of 33 months. From the results of these experiments it appears, that each wire, immediately upon the application of the strain to which it was subjected, received a certain amount of extension. The first wire, which was subjected to a strain of 4th its tensile strength, was found at the end of the time in question not to have acquired any increase of extension. The second, submitted to 3d its tensile strength, was elongated 0.027 in. per foot, independently of the elongation it at first received. The third, subjected under like circumstances to a strain of 1th its tensile strength, was elongated 0.40 in. per foot, besides its first elongation. The fourth, similarly subjected to ths the tensile strength, was elongated 0.061, besides its first elongation. From observations made during the experiments, it was found that, reckoning from the time when the first elongations took place, the rapidity of the subsequent elongations was nearly proportional to the times; and that the elongations from strains greater than 4th the tensile strength are, after equal times, nearly proportional to the strains. M. Vicat remarks in substance, upon the results of these experiments, that iron wire, when not annealed, commences to exhibit a permanent set when subjected to a strain between the and of its tensile strength, and that therefore it is rendered probable that the wire ropes of a suspension bridge, which should be subjected to a like strain, would, when the vibratory motion to which such structures are liable is considered, yield constantly from year to year, until they entirely gave way. M. Vicat further remarks, in substance, that the measure of the resistance offered by materials to strains exerted only some minutes, or hours, is entirely relative to the duration of the experiments. To ascertain the absolute measure of this resistance, which should serve as a guide to the engineer, the materials ought to be subjected for some months to strains; while observations should be made during this period, with accurate instruments, upon the manner in which they yield under these strains. The following tables, on the comparative strength of iron, steel and hemp rope are taken from Stoney's work on the Theory of Strains, Vol. II. The weights are given in English units. Tearing strain, tons. |