Page:The American Cyclopædia (1879) Volume XV.djvu/434

 418 STRENGTH OF MATERIALS secured by the use of a large factor of safety in proportioning parts exposed to them, by care during construction in the selection of tough resilient materials, and in management by carefully adjusting all parts, and applying the load so as to avoid jarring action as far as possible. 17. If a weight, acting as a steady load, produces a given deflection or change of dimensions, it will require but half that weight suddenly applied to produce a similar effect, whether it be fracture or a stated alteration of form. The extension of ordinary wrought iron within its limit of elasticity is about -0001 per ton per square inch of section. The amount of extension before fracture by tension is given, with the finest quality of wrought iron, at 20 per cent., with medium quality 16 per cent., and it runs in some irons as low as 4 per cent. Cast iron of fair quality is elongated but a fraction of 1 per cent. 18. The extension of steel varies with the amount of carbon, and nearly inversely as its tenacity. The follow- ing table is taken in part from Trautwine's "Engineer's Pocket Book:" ULTIMATE TENSILE STRENGTH IN POUNDS PER SQ. IN., AND ELONGATION IN INCHES, BEFORE BREAKING. NOTE. The specimens tested were steel bars of different grades made from pure Swedish iron, and each bar was turned to a diameter of one inch for a length of 14 inches. SPECIMENS. Per cent, of carbon. Breaking weight. Elongation. Resilience. No 1 0-33 68,100 0-093 4,450 No. 2 0-43 76,160 0-039 4,970 No 8 0-48 84000 0'089 5,040 No. 4 No 5 0-58 0-53 95,200 92,960 0-080 0-058 5,080 8,600 No. 6 0-68 100,800 0-071 4,770 No. 7 No 8 0-74 0-84 101,920 123 200 0-050 0*080 8,400 6580 No 9 1-00 134400 0'071 6,300 No. 10 1-25 154,560 0-044 4,530 In the larger table, the ultimate resilience of metals is given as tested in the Stevens insti- tute of technology, Hoboken, N. J. Phosphor bronze considerably exceeds ordinary bronze in ductility and resilience. 19. Heating wrought iron within certain limits, and then cooling under stress, increases its strength by relieving internal strain. Cold rolling and wire-draw- ing increase it, in some cases, 100 per cent. Mr. Dean of Boston and Uchatius of Vienna have similarly increased the strength and elas- ticity of bronze. Overheating, annealing, and cold hammering decrease its strength. Cast iron of open structure and low density is increased in strength by successive remelt- ings, sometimes to the amount of 100 per cent., over pig metal. Casting under a head, or under considerable pressure, similarly bene- fits both cast iron and cast steel. Sir Joseph Whitworth produced a steel of extraordinary strength and toughness by casting under heavy pressure. The internal strain consequent upon sudden cooling, or upon cooling awkwardly shaped castings, seriously reduces their strength and sometimes produces actual fracture. The character of cast iron is largely determined by its density, 7'2 to 7'3 representing the best limits for ordinary practice. Cold wrought iron is more than twice as strong as red-hot. Strength, ductility, and resilience increase with diminishing temperatures, when the materials are of good quality. Cold-blast cast iron is usually stronger than hot-blast iron made from the same ores. Copper loses 25 per cent, of its tenacity at 550 F., 50 per cent, at 810, and 67 per cent, at 1,000, the diminution of tenacity varying nearly as the square root of the third power of the temperature. Metals in large masses have usually less density and strength than when worked into sheets, bars, or wire. Wrought iron is particularly liable to loss of strength in large forgings. Bars two inches in diameter being made of the same metal as other bars one inch in diame- ter, the latter are sometimes found to have 20 per cent, more strength. Steel exhibits even greater differences. 20. Indentation is resist- ed by wrought iron nearly in proportion to its thickness. Fairbairn found the force neces- sary to push a blunt point or a ball 3 in. in diameter through boiler plate, one quarter of an inch thick, to be 17,000 Ibs., and nearly equal to that required to drive the same in- strument through a three-inch oak plank. Re- sistance of armor plate to penetration by shot varies, if the plate be well backed, as tho square of thickness, within the limit of mod- erate thickness. The material should be strong and ductile. 21. Generally, in designing ma- chines or parts of machines, they should be so proportioned that all parts will have factors of safety of nearly equal value. Economy of material is thus secured, and also the very important advantage, where exposed to severe shock or sudden strains, of utilizing the resili- ence of the whole machine in resisting them. Forms of uniform strength should therefore be used wherever possible. Suspension rods of uniform strength must have a greater section at the point of support than at the point of attachment of the load, as the upper portions carry not only the load but the weight of the lower part of the rod. Pump rods and wire ropes for deep mines are for this reason made tapering, with the largest section at the top. Care should always be taken that the pieces connected and their fastenings are, when pos- sible, equally strong. Tall columns are slightly swollen at the middle portion in order that they may be equally liable to break at all points, and the Hodgkinson form of cast-iron beams, and the Fairbairn (I) form of section of wrought- iron beams, are given their peculiar shapes in order that no surplus material may exist in either top or bottom flange. Beams of uniform strength, when fixed at one end and loaded at the other, if of uniform depth, are triangular in plan. If uniformly loaded, they represent in plan a pair of parabolas whose vertices touch at the outer end. When of uniform breadth, their vertical sections are parabolic in the first