TORSIONAL STRENGTH OF CAST IRON. The only direct experiments recorded, worth notice, on the torsional resistance of cast iron, are those of Mr. Dunlop at Glasgow, in 1819.1 They were made to ascertain the torsional strength of shafts as usually cast Two old bars of cast iron, about 5 feet long each, in Glasgow at the time. one of them 3 inches and the other 4 inches square, were turned down in the lathe at five different places, to ten different diameters, of from 2 to 44 inches. The load was applied at the end of a lever 14 feet 2 inches long. Particulars of the experiments are given in table No. 185; the values of h, the shearing resistance, calculated by the general formula (3), page 535, are added. Table No. 185.-TORSIONAL STRENGTH OF CAST IRON. (Reduced from Mr. Dunlop's data.) 1819. 1 Ratio of the Breaking Weights. Shearing Stress per square inch. Average,. 8.375 It seems that the ultimate torsional strength increased very nearly as the cube of the diameter, and that the average torsional resistance per square Assuming, as explained at page 561, inch of section was 8.375 tons. that the shearing resistance of cast iron is equal to its direct tensile resistance, the general formulas for torsional strength ( 1 ), page 534, and (6), page 536, become, by substitution, For cast-iron round shafts, W-278 d3s R For cast-iron square shafts, W = ·40 b3 s W = the force, in tons. R R = the radius of the force, in inches. d3 s (12) 3.6 R WR = the moment of the force, in statical inch-tons. d = the diameter of the round shaft, in inches. b = the side of the square shaft, in inches. the ultimate tensile strength, in tons per square inch. 1Annals of Philosophy, vol. xiii. 1819. If the tensile strength, s, be taken at 7.2 tons, for iron of average quality, then, by substitution and reduction: Ultimate Torsional Strength and Sizes of Cast-Iron Shafts of average quality. TORSIONAL DEFLECTION OF CAST-IRON BARS. In the absence of direct data for the torsional deflection of cast-iron bars, it is assumed that it is 123 times that of wrought-iron shafts-the same proportion as that of the transverse deflections of cast-iron and wrought-iron shafts, as indicated by a comparison of the formulas (8), page 564, and (5), page 590. Multiply, therefore, the second member of the formula (14), page 592, by 123; the coefficient 1070, or exactly 1073, becomes (1073 x 3/5) 644 Torsional Deflection of Round Cast-Iron Bars. STRENGTH OF WROUGHT IRON. TENSILE STRENGTH. Mr. Telford deduced from his experiments, an average tensile strength of 29.25 tons per square inch for wrought-iron bars. Mr. Barlow deduced from the results of eight bars of wrought ironSwedish, Russian, and Welsh, from 14 inches square to 2 inches in diameter an average tensile strength of 25 tons per square inch. Mr. Barlow also deduced from experiments on bars of from 1 inch in diameter to 2 inches square, that the elastic tensile strength of good medium wrought iron was 10 tons per square inch; and that the extension was at the rate of 1/10,000 th part of the length per ton per square inch; and that, therefore, the elasticity was fully excited when the bar was stretched 1/10th part of its length. Sir William Fairbairn published, in 1861, results of experiments on the tensile strength of wrought iron, which are rendered, slightly adapted, in tables Nos. 186 and 187: Table No. 186.-TENSILE STRENGTH OF WROUGHT IRON. 1861. Table No. 187.-TENSILE STRENGTH OF IRON AND STEEL PLATES THAT HAD BEEN SUBJECTED TO EXPERIMENT WITH ORDNANCE AT SHOEBURYNESS. 1861. Elongations before rupture of specimens, in part of the length From the first table, No. 186, it appears that the strength of iron plates, in the direction of the fibre, varied from 28.66 tons for Lowmoor iron, to 20.10 tons for Staffordshire charcoal-iron; and that there is no tensile strength in the direction of the fibre so low as 20 tons per square inch. Also, that the averages of nine irons show, for the breaking weight,— .23.68 tons per square inch. 21.59 With the fibre.... Across the fibre... Difference..... 2.09 tons, or about 9 per cent. From the second table, No. 187, it appears that the thicker plates have less tensile strength than the thinner plates; whilst the elongation before rupture is greater; thus:— stress. 24.06 elongation 23.7 18.8 27.03 27.3 Sir William Fairbairn tested the resistance of iron plates to a bulging He stretched two 4-inch plates and two 1⁄2-inch plates over a cast-iron frame 12 inches square inside, as in Fig. 191, and subjected them to pressure from an iron bolt 3 inches in diameter, with a hemispherical end, which was applied to the plate. Fig. 191.-Specimen Plate to resist Bulging Stress. Fig. 192.-Effects of Bulging Stress. The 14-inch plates were indented 4 inch and 1⁄2 inch respectively, when they commenced to fail by cracking on the convex side, under a pressure of 4.7 tons applied at the centre. Under 7 tons of pressure they were cracked through. The 1⁄2 inch plates were indented 33 inch when they commenced to crack, under a pressure of 9 tons; and they were cracked through under a pressure of 17 tons. See Fig. 192. The resistance to bulging in these experiments was in proportion to the thickness of the plates. To test the effect of cold-rolling on iron bars, Sir William Fairbairn tested three bars, and obtained the following results:— showing that the tensile strength was increased by one-half; but that the elongation was reduced to less than a half. With respect to the influence of temperature, Sir William Fairbairn, in 1857, found that the strength of ordinary Staffordshire iron plates, either with or across the grain, remained the same for temperatures varying from 0° F. to 400° F. At higher temperatures the strength declined, until, at a red heat, it fell from an ordinary average of 20 tons to 154 tons per square inch. Mr. Thomas Lloyd tested the tensile strength of Staffordshire S. C. Crown bars, 13% inches in diameter, of one kind. The same bars were broken four times in succession, and the successive breaking weights were for the |