Remarks. Exp. 40. By the word countersunk is understood a conical recess on one side of the plate to receive the head of the rivet, in order that it might not project beyond the surface of the plate. Tore across the three rivet-holes. Exp. 41. In an unsuccessful experiment made before this upon plates precisely the same, and riveted in the same manner, they were torn across the rivet-holes in attempting to lay on 18,667 lbs. Plates tore across the rivet-holes. Exp. 42. All the rivets on one side were cut in two in the middle, and the plates left sound. When the comparative merits of plates and their riveted joints were under consideration, it appeared desirable to repeat several of the experiments, particularly those which seemed to throw light upon their relative powers of resistance. I considered these experiments to be of importance, as they increased our knowledge, as respects the strength of the material, and also its properties in combination. Remarks.-Exp. 43. Both side plates were torn across, and two of the rivets cut off. The sum of the thickness of the side plates was 24 inch, the middle plates being 22 inch thick. The middle plates were left sound. Exp. 44. Second experiment broken as before, the two outside plates torn off; all the rest sound. Remarks. Exp. 45. Middle plate torn straight across the rivet-holes. All the rivets and both plates left sound. Exp. 46. Both outside plates torn across at the three rivets. Exp. 47. Outer plate sound; torn across the two rivet-holes. Rivets sound; inner plate only torn. In ship-building these objects are of some value, as any reduction in the powers or parts of a vessel by imperfect construc tion, or misapplied material, might lead to serious error and even great risk to the safety of the ship. Since the first use of iron for these objects, it has been the practice to countersink the heads of the rivets in order to present a smooth surface for the passage of the vessel through the water. This practice is in general use at my works at Millwall, and I believe the same methods are pursued at the establishment of Messrs. John Laird and Co. and others in different parts of the country. The introduction of this system of riveting caused a further extension of the experiments, in order to elucidate the various forms of joints given in the preceding Tables, and further to investigate the strength of the joint with a plate riveted on each side, which appears to be the strongest and best calculated to resist a tensile strain. This description of joint is seldom used in ship-building, but in order to render the experiments as perfect as possible, it will be necessary to consider it in this paper with others of equal importance and probably of more general use. The system of countersinking the rivets is only used when smooth surfaces are required; under other circumstances their introduction would not be desirable, as they do not add to the strength of the joint, but to a certain extent reduce it. This reduction is not observable in the experiments, but the simple fact of sinking the head of the rivet into the plate and cutting out a greater portion of metal, must of necessity lessen its strength, and render it weaker than the plain joint with raised heads. This must appear evident from the fact of the sectional area of the plate being diminished, and the consequent reduction of the heads of the rivets, which in this state are less able to sustain the effects of an oblique or transverse strain. It is, however, satisfactory to observe that countersinking the heads of the rivets does not seriously injure the joint in its powers of resistance to a direct tensile force; but the rivets are liable to start when exposed to collisions or a strong impinging force, such as the sides of ships are frequently doomed to encounter. On referring to experiments (Table XI.), the same results as to strength are obtained with the countersunk rivets as those with rounded heads they are rather under the mean of the former experiments, but not more than is easily accounted for by the reduced section of the countersunk plates. The joint with plates, riveted on each side, is seldom used, a circumstance which probably arises from its greater complexity of form, and the danger which a treble thickness of plate would be subject to if used in boilers or vessels exposed to the action of intense heat. It is also inadmissible in ship-building, as the smooth surface requires to be maintained, and the greatest care observed in the formation of the outer sheathing, to lessen the resistance of every part of the hull immersed in the water. In other respects the double-riveted plate is a strong joint, and in every case, where great strength is required, it may be used with perfect safety. It will be unnecessary to go through a further comparison of the experiments, as sufficient data have already been furnished to enable us to calculate the force per square inch, and to resolve the whole into a general summary exhibiting the relative strengths-1st, of the plates; 2ndly, of the single- and doubleriveted joints; and lastly, the ratio of the strengths as deduced from the whole series of experiments. GENERAL SUMMARY OF RESULTS AS OBTAINED FROM THE From the above it will be seen that the single-riveted joints have lost one-fifth of the actual strength of the plates, whilst the double-riveted have retained their resisting powers unimpaired. These are important and convincing proofs of the superior value of the double joint; and in all cases where strength is required this description of joint should never be omitted. On referring to the experiments contained in the separate tables, there will be found a striking coincidence in the facts tending to establish the principle of double-riveting as superior in every respect to the general practice now in use of the single rivets. It appears, when plates are riveted in this manner, that the strength of the joints is to the strength of the plates of equal sections of metal as the numbers 1,000 In a former analysis it was 1,000: which gives us a mean of 1,000 1,021 and 791.* 933 and 731 which in practice we may safely assume as the correct value of each. Exclusive of this difference, we must however deduct 30 per cent. for the loss of metal actually punched out for the reception of the rivets; and the absolute strength of the plates will then be, to that of the riveted joints, as the numbers 100, 68, and 46. In some cases, where the rivets are wider apart, the loss sustained is, however, not so great; but in boilers and similar *The cause of the increase of strength in the double-riveted plates may be attributed to the riveted specimens being made from the best iron; whereas the mean strength of the plates is taken from all the irons experimented upon, some of inferior quality, which will account for the high value of the double-riveted joint. In ordinary cases, and in practice, it will therefore be safer to take the mean of the whole, viz.:— Strength of plates Strength of double-riveting And of single-riveting. 100 97 76 |