Repository: UIHistories Project: Board of Trustees Minutes - 1886 [PAGE 247]

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239 Since in the tests of wool made the length of fibre used was 20 millimetres, if the results for stretch be multiplied by 5 we obtain expression in per cents, of length which is more convenient for comparison with other materials. If the figures thus obtained and those just given are compared with corresponding values for wrought iron, cast iron and steel made by the United States Testing Board, published in Thurston's Materials of Engineering, Vol. II, pp., 351/35& and 398, the following conclusions are reached: 1. The tensile strain for wool is about one-half that required to produce the same per cent, of total stretch in a wrought iron bar of equal cross seetion. 2. A permanent set commences in wool at about 59 per cent, of the amount of strain required to originate a set in a wrought iron bar, or at about 37 per cent, of the ultimate tenacity of wrought iron of good quality. 3. For steel the corresponding value is 34 per cent. 4. The ultimate average tenacity of wool appears to be nearly double that of average cast iron of equal cross section, about fourfifths that of good wrought iron and a little more than one third that of good steel. 5. The maximum stretch of wool is much greater than that of either metal, being 1.75 times that of wrought iron, 12.8 that of cast iron and 4.5 times that of steel. 6. The permanent stretch or set of wool appears to commence only when the total stretch equals nearly 5 per cent, of the original length of the fibres, which is at least ten times greater than the corresponding value for either metal. 7. Wool has more than twice the strength of toughest wood; 1\ times that of bone; 4 times that of white pine; 2.7 times that of whalebone; and nearly twice as much as soft brass wire, phosphor bronze, annealed iron wire or steel wire rope. The comparative values of wool may farther be expressed, and very conveniently, in the moduli of elasticity which may readily be determined from the data above given. The term modulus of elasticity, much employed in the discussion of the resistance of materials,, may be defined in either of two ways: a. It is the ratio between the elongation of a bar of any material (whose section is a square unit and its length a linear unit of similar denomination) and the tensile strain producing that elongation. Its numerical value equalling the quotient of the strain by the elongation. The length of the bar is usually one inch, its section a square inch, and the strain is taken in pounds, b. It is the tensile strain in pounds which would theoretically stretch a bar of one square inch section to just twice its original length, neglecting the reduction of section which occurs. The definition first given is that most frequently employed and is the one here intended.