Page:The American Cyclopædia (1879) Volume IX.djvu/389

 IRON 875 to 418 F., from which he deduces the follow- ing conclusions: "The absolute strength of iron and steel is not diminished by cold, but even at the lowest temperature which ever oc- curs in Sweden, it is at least as great as at the ordinary temperature (about 60 F.). At tem- peratures between 212 and 392 F., the abso- lute strength of steel is nearly the same as at the ordinary temperature ; but in soft iron it is always greater. In neither steel nor iron is the extensibility less in severe cold than at the ordinary temperature ; but from 266 to 320 F. it is generally diminished, not to any great extent in steel, indeed, but considerably in iron. The limit of elasticity in both steel and iron lies higher in severe cold ; but at about 284 F. it is lower, at least in iron, than at the ordina- ry temperature." In the experiments on which these conclusions are based, the strength was determined by a gradually increasing strain. The result is quite different if the strain is ap- plied suddenly, that is, if the iron or steel is submitted to shock, as is shown in the follow- ing experiments made by 0. P. Sandberg'(ap- pendix to the work of Knut Styffe) : Height of Fall of Ball (weighing 9 cwt.) required to break each Bail (Iron) at different temperatures. Distance between supports 4 ft. ; length of rail 10 ft. 5 in. WORKS WHERE RAILS WERE MAI >F. TEMPERATUEE. 84 F., ft. 35 F., ft. 10 F., ft. 45 66 85-5 45-8 66 56 35 45 85 85 45 35 22 80 49-6 89 26 89 26 26 11 5 is 5 5 26 11 IB 11 5 9 4 9-8 14-2 6-5 11 ti it It H u M It II 11 II 11 11 II II It II Average of 7 English rails " 2 BeMan " .... " 8 English, 5 French, and 2 Belgian rails... Mr. Sandberg concludes from these experi- ments that for such iron as is usually employed for rails in Wales, France, and Belgium, the breaking strain, as tested by sudden blows or shocks, is considerably influenced by cold ; such iron exhibiting at 10 F. only from one third to one fourth of the strength which it possess- es at 80. The ductility and flexibility of such iron he finds also much affected by cold ; rails broken at 10 showing on an average a perma- nent deflection of less than one inch, while the other halves of the same rails, broken at 84, showed a set of more than four inches before fracture. He says that at summer heat the strength of Aberdare rails was 20 per cent, greater than that of Creusot rails ; but that in winter the latter were 30 per cent, stronger than the former. The confusion in the state- ments regarding the strength of iron and steel at different temperatures has arisen from the fact that in the experimental determinations the difference between the effect of a gradually increased and a suddenly applied strain has been overlooked. The experiments of Mr. Sandberg are conclusive on this point, and con- firm the universal experience that iron and steel tools and utensils are much more liable to break in cold than in warm weather. The breaking of rails in winter has also been re- ferred to the hardness and rigidity of the road bed ; no experiments have yet been made that confirm this view. A very thorough investiga- tion of the strength of wrought iron at differ- ent temperatures was made by a committee of the Franklin institute of the state of Pennsyl- vania, consisting of Prof. W. R. Johnson, Prof. A. D. Bache, and Benjamin Reeves, from 1832 to 1837. These experiments were 73 in num- ber, at temperatures from 212 to 1317 F. A remarkable anomaly was discovered in the be- havior of iron at a temperature between 500 and 600. About 572 was found to be the temperature of the maximum strength of iron ; and the best qualities then showed a tenacity 15-17 per cent, over that possessed by the same iron at ordinary temperatures. Sir William Fairbairn made a similar observation on South Staffordshire iron. It showed from 60 to 325 a regular increase of tenacity from 62,- 186 Ibs. to 84,046 Ibs. per square inch, or 30 per cent. Mr. Clay has determined the effect of repeated workings on the tensile strength of wrought iron as follows : 1. Puddled iron.. 2. Eeheated Lbs. per K'I. in. 43,904 .ViSfit 59.635 59,585 57,844 61,824 7. Eeheated. 9. 10. " 11. 12. " Lbi. per tq. In. 57,844 57,344 54,104 51,968 43,904 The increase of strength is doubtless due to the increase of homogeneity, and the subse- quent decrease to an oxidation of the iron. A. Wohler (Ueber die Festigkeitsversuche von Eisen und Stahf) has investigated the effect of repeated strains on iron and steel, and has shown that the rupture of a material may be effected by frequently applied strains, none of which exceed the limit of rupture; that the destruction of cohesion depends on the differ- ences of tensions which form the limits of the oscillations of the strain ; and that the absolute amount of the extreme tensions is only of im- portance in so far as the differences of strain which effect rupture decrease with the increas- ing tension. When a fibre passes from a state of tension to a state of compression, or vice versa, we should consider the tensional strain as positive, and the compressive strain as neg- ative, so that the variation will be equal to the sum of the tensional and compressive strains. This condition, often called the "fatigue" of metals, is shown in the following table :