PAPERS READ. ON THE EFFECT OF PROLONGED STRESS UPON THE STRENGTH AND ELASTICITY OF PINE TIMBER. By ROBERT H. THURSTON, of Hoboken, N. J. IN papers read before the American Society of Civil Engineers at various dates, the writer has given the results of investigations made to determine the behavior of metals under loads of varying magnitude and under intermitted stresses, and to ascertain in what cases and under what conditions the variation, with period of stress, of the normal line of elastic limits, discovered and announced by him in the year 1873, occurs in practice. Experiments made by Mr. Herman Haupt, forty years ago, revealed a fact not even now generally understood and appreciated -that timber may be injured by a prolonged stress far within that which leaves the material uninjured when the test is made in the usual way and occupies a few minutes only. Thus, using pieces 60X3X1 inches (152.4X7.62×2.54 cm.) set as cantilevers with a breaking moment, due the load, of Pl=48 inch-pounds (.55 kilogrammetres) he obtained for the value of 6w the following figures: R= wl bd2 An extended series of experiments made intermittently in the mechanical laboratory of the Stevens Institute of Technology, Department of Engineering, during some years past, had included an examination of this subject and the result has confirmed Haupt's earlier work and has given a tolerably good idea of the effect of prolonged stress in modifying the primitive relation of stress and strain where the wood is good southern yellow pine. A selected yellow pine plank was obtained for test, the history of which was known. The stick was cut at Jacksonville, Fla., in October, 1879, was received early in the following year and was piled in the yard, air-seasoning, until taken for test in the spring of 1880. The plank measured 4 in. X12 in. X24 ft. (10.16×30. 48X731.52 cm.) When tested, it had been seasoning six months, the latter part of the time indoors. From the middle of this plank a stick was first cut 3 in. ×3 in. X24 ft. (7.62×7.62×731.5 cm.) and from this was cut a set of ten pieces from 40 to 54 in. long (101.6 to 137.2 cm.) and from 1 to 3 in. square in cross-section (3.16 to 7.62 cm.) square. These latter pieces were tested under various conditions, as then reported, to determine the values of their moduli of elasticity and rupture. Pl b d2 The moduli of rupture were usually 11,000 to 12,000 for the expression R= (in metric measure, 773.3 to 843.6) and the moduli of elasticity ranged from two and a quarter millions (in metric measure, 106 X1406 to 158175+10). In specific gravity the wood ranged from 0.75 to 1.00, usually about 0.85. When kiln-dried to a moderate extent, the density was but little altered, if at all, but the modulus of elasticity rose to two and a half millions (17375 Xby 103) and the modulus of rupture was increased about 20 per cent. From the previously unused part of the plank a set of three test pieces was cut about one inch (2.54 cm.) square in section and tested on supports 40 inches (101.6 cm.) apart, to determine their breaking loads. The result is shown in detail in the appended table. In these specimens the annual rings were in the cross-section of each piece, indicated by lines making angles of 45° with the edges. These pieces broke at 345, 380 and 410 pounds respectively. The weakest piece broke by splintering, and had it been as sound as the others would probably also have sustained a somewhat heavier load. As will be seen by comparison with the other and with subsequent tests, the deflection of the strongest piece in the set is exceptionally small and the piece probably exceptionally strong and stiff. We may, therefore, take 375 pounds (170 kilog.) or a trifle over, as a good average for loads breaking pieces of this size. Nine other pieces were cut and dressed to the same size and were mounted on supports 40 in. apart, in a frame arranged for the purpose in the workshop of the Institute, in three sets of three each. These sets were loaded thus: Or to about 60, 80 and 95 per cent. of their probable maximum strength, as indicated by ordinary test of the companion lot above described. Their deflections were measured when set, and at intervals subsequently, by means of an accurate micrometer reading to ten-thousandths of an inch. The whole set of bars, loaded most heavily as above, broke within two days; one bar yielding, as shown in Table 2, at the end of a period included between observations taken at 4 and 13 hours from the beginning, the second breaking at some time between 27 and 30 hours and the third giving way at the end of 43 hours. A load of 87 per cent., the maximum obtained by usual methods of test, is thus shown to be capable of breaking the piece under the conditions here described, and an apparent "factor of safety" of 14 is evidently not a factor of safety at all when time is given for the piece to yield. The second set, loaded with 0.75 the maximum momentary weight, all broke, as is shown by Table 3, one at the end of about 3 days, another after 5 days, and the third at the end of a little more than a month. It is probable that these differences of time are due to differences of strength more than to variations of the effect of time of stress. A "factor of safety" of 1 is evidently not a real factor of safety for wood in such cases as this. The behavior of the third and last set of test pieces is shown in Table 4. These pieces were loaded with 60 per cent. of the average breaking weight under ordinary test. Left under this load, the deflection, in every instance, slowly and steadily increased from about one inch (2.54cm.) to some considerably larger amount |