= The tensile strength of plate iron also is reduced by annealing as shown by Mr. Kirkaldy's experiments on six kinds of Yorkshire iron,, and inch thick, which gave the loss 5.6 per cent. lengthways, and 5.2 per cent. crossways of the grain; Lowmoor giving 4.8 and 1.8; Bowling, 8.0 and 9.1 per cent. respectively. The effect of annealing chain is shown by Nos. 19, 20 in Table 21, to be 16.34 17.54 93, or 7 per cent. loss of strength (109). = CHAPTER II. ON RIVETED JOINTS. (15.) Riveting two plates of metal together may appear to be a very simple matter, but the fact is that there is more philosophy involved in it than is commonly supposed. The extreme importance of riveted joints, not only as applied to steam-boilers, but also to girders, railway bridges, and other structures will justify the most careful attention to the principles by which the strength is governed, and the proper proportions are fixed. The strength of a riveted joint is dependent, first, on the tensile strength of the plate, measured at its weakest place, namely, through the line of the rivet holes; second, on the shearing strength of the rivets; and third, on the friction of the plates against one another due to the pressure of the rivets. (16.) "Strength of Punched Plates."-It has been found by experiment that when wrought-iron plates are punched cold in the usual way, the strength of the plate is reduced not only by the removal of the metal punched out, but also by the damaging of the fibres of the metal that remains between the rivet-holes. Direct evidence of this is given by the experiments on Yorkshire plates by Mr. Kirkaldy in Table 4, which shows that the mean loss was 13 per cent. with the grain, and 17 26 per cent. across the grain, this being the result of eighteen experiments on six kinds of iron by different makers, with plates,, and inch thick. The plates were in all cases 8 inches wide, with four rivet-holes TABLE 4.-Of the Loss of STRENGTH by PUNCHING RIVET-HOLES in YORKSHIRE BOILER-PLATES. in a line, 85 inch diameter; hence the ratio of the solid part {8 — (·85 × 4} of the plate to the metal between rivet-holes was 8 - (85 × 4 8575 to 1.0. This proportion is about the same as that adopted in ordinary riveting, a fact which is important, for obviously, the damage to the fibres will be the greatest close to the rivet-holes, and will diminish with the distance: now when the holes are pretty close together as in ordinary riveting, we may suppose that the whole of the metal between them will be affected, but where the distance is very great, the metal at middistance may be wholly unaffected, and in that case the mean strength would be much greater than in others where the pitch of the rivets is small. Mr. Kirkaldy's experiments are the more conclusive because the strengths of the punched plates were compared with those of unpunched ones cut out of the same plate. Moreover, in order to avoid any possible loss of strength by shearing, which would probably be analogous to that due to punching, the plates were all cut out in a slotting machine. (17.) These experiments appear to be reliable, nevertheless there are some remarkable differences between them and the results obtained by other authorities. For example, Mr. Fairbairn's experiments in Table 1 give for Lowmoor iron 24.56 tons per square inch both lengthways and crossways, whereas Mr. Kirkaldy gives for plates not annealed 21.3 lengthways, and 20.3 tons crossways, and for annealed plates 20.1 and 19.2 tons respectively, these being the means for six kinds of Yorkshire iron, and they show a difference of 15, 22, 20, and 28 per cent. as compared with Fairbairn's results. Messrs. Napier and Son's experiments in Table 1 give 24.1 tons lengthways as the strength of Yorkshire plate iron; agreeing with Mr. Fairbairn. Another remarkable difference is that the loss due to punching, which as we have seen (16) was 13, and 17.26 per cent. by Kirkaldy, was as much as 24 per cent. in single riveted joints by Mr. Fairbairn, as shown by col. 4 of Table 5. Probably the fact that the strain is not central or symmetrical, as shown by the broken centre-line in A, Fig. 6, may account for the difference. (18.) "Strength of Drilled Plates."-When the rivet-holes are drilled, the loss of strength in the metal between the rivet-holes is practically nothing, the mean result of eighteen experiments on six kinds of Yorkshire iron (16) was 1.13 per cent. length ways, and 0.9 per cent. crossways. Notwithstanding the advan tage which is thus shown to accrue from drilling rivet-holes, it is hardly likely ever to be adopted extensively in practice; the extra cost of drilling would not be compensated by the extra strength obtained. (19.) "Shearing Strength of Rivets."-It is shown in (123) that the resistance to shearing is equal to the tensile strength of the iron, and Mr. E. Clark's experiments gave 22.1 tons per square inch, Mr. Fairbairn's experiments gave 22.04 tons in a singleriveted joint, where the result might possibly be complicated by friction (20), but with such a joint friction would be eliminated at the point of rupture, the surfaces separating by the unsymmetrical strain (17). We may therefore take 22 tons, or 49,280 lbs. per square inch as the mean shearing strength of wrought-iron rivets. (20.) "Friction from Grip of Rivets."-Mr. E. Clark made some experiments on the friction in riveted joints, and obtained some remarkable results, for he found that the friction increased considerably with the length of the rivet. With rivets inch diameter, riveted hot in the usual manner, and 11, 17, and 27 inches long, the friction was 4, 5, and 8 tons respectively. The experiments were made in the following manner: three plates were riveted together with one rivet; the central plate, having an oblong hole, was then drawn between the other two, the frictional resistance to which was 5 tons. Two-inch washers were then added, making the length 23, when the strain due to friction became 8 tons. Two plates and two washers gave a length of 14 inch, when friction became 4 tons. This last experiment approximates nearly to the conditions of ordinary riveted joints. (21.) "Principles of Riveting.”—We may now investigate the phenomena which occur with riveted joints, and to do that satisfactorily it will be well to take an experimental case, the reasoning can then be checked by practice. Fig. 7 is a joint of best Staffordshire plate, experimented upon by Mr. Brunel: the main plates were inch thick, and the joint was formed with a front and back plate each inch thick, and twenty rivets inch diameter. This joint failed with 164 tons, by the -inch plate tearing through the outer line of rivet-holes B, B: the rivets were not broken in this case, but evidently they must have been on the point of rupture, for in another and similar experiment the whole of the ten rivets in one half of the joint were sheared with a lower strain, namely 153 tons. We may therefore assume that 164 tons would or should rupture the plate and shear the rivets simultaneously. It was also found that a solid or unpunched plate of the same iron broke with a mean strain of 20.6 tons per square inch. We have no experimental evidence of the damaging effect of punching on Staffordshire plates (16), but with Lowmoor iron, Table 4 shows a mean loss of 19.5 per cent. when strained crossways of the grain, and 17 per cent. lengthways; taking 18 per с cent. as a mean for Staffordshire plates, the metal left between rivet-holes will be reduced to 100 18 82 per cent. of the strength of a solid plate; hence in our case, we have 20·6 × ·82 16.9 tons per square inch. The area through the line B, B = is {20 - (1 × 5} = 8·28 square inches, hence 8-28×16·9 = 140 tons, the breaking weight of the plate. To this has to be added the friction due to the grip of the five rivets in that row; by Mr. E. Clark's experiments (20) this may be taken at 43 tons per rivet, or in our case 43 × 5 = 24 tons, making with that due to the plate 140 + 24 which happens to be precisely as per experiment. = 164 tons, (22.) "Real and Apparent Strength."-The difference between real and apparent strength will now be manifest; the apparent strength or that of the whole combination is 164 tons, borne by 8.28 square inches, or 1648.28 19.8 tons per square inch, but the real strain on the metal between rivet-holes as we have seen (21) is 16.9 tons; the strain on the solid part of the plate at C, C is only 164 10 16.4 tons, whereas the breaking weight 20.6 tons per square inch. = = = Thus the normal strength of the solid plate, or 20.6 tons, is reduced by punching to 16.9 tons per square inch, which again is increased by friction to 19.8 tons, being restored within 20.6 19.8 0.8 ton of the normal strength. "Rivets.”—The ten rivets were each 1-inch diameter =3712 square inch area, and being subjected to a double shear give 3712 × 10 × 2 = 7.424 square inches shearing area; then, their apparent strength is 1647.424 = 22.09 tons per square inch, which is almost exactly the resistance given for double shear by Mr. E. Clark's experiments (123). With certain proportions of double-riveted joints the apparent strength of the metal between rivet-holes per square inch may exceed that of the solid plate, a result that seems anomalous, but may be easily explained. Thus, let Fig. 8 be a joint with seven 7-inch rivets in the outer row B, B; then the area of the central plate on that line will be {20 - (3 × 7} × 1 = 6.94 square inches, giving 16.9 x 6.94 117.28 tons. |