166 PROPERTIES OF IRON AND STEEL CONSTRUCTIONS. 



on cast-iron girders. But we do not find that the results of his experiments 

 warrant his conclusion. But the fact that under stress beyond the elastic 

 limit the ultimate strength increases, leads to the the conclusion that se- 

 curity against dead-load increases with time. But if it is objected that a 

 decrease may follow an increase of ultimate strength, it must be admitted, 

 in view of all that has been said, that the influence of duration of dead-load 

 has not been clearly determined. That each load requires a certain time to 

 cause its correspondent permanent change has been known since the time 

 of Hodgkinson and Wertheim, and also accords with Fairbairn's comparison 

 with ropes; and, again, it has been observed by Bauschinger, This also 

 holds true for further changes in form; and if a rod stretched again when 

 released, does not at once return to its previous condition, a so-called sec- 

 ondary action takes place. This was observed in Kupffer's experiments. 

 Thurston thinks that in this he has discovered a new phenomenon; that 

 ultimate strength and elastic limit increase after a strain greater than the 

 latter, continued for twenty-four hours. But there is nothing new in it. 

 That the tensile resistance of iron and steel is greater under the action of 

 an electric current, and that the ductility is effected, now one way, now an- 

 other, by dipping the metal in acid, seem to be shown by detached experi- 

 ments, but this needs confirmation. 



INFLUENCE OP TEMPERATURE. 



The influence of different temperatures upon the strength of steel and 

 iron is not satisfactorily explained. With respect to ultimate resistance 

 only, because of numerous experiments, has their been a growing accord of 

 views. For most kinds of metal, especially for iron, the ultimate strength 

 appears to increase with the decrease of temperature below zero, but also 

 to reach a maximum at a little above 100° C. Within a certain interval 

 near 16° ihe resistance is quite constant; the beginning and the rapidity of 

 the increase and the position of the maximum are dependent upon the con- 

 ditions already considered. 



Fairbairn, in tension experiments with bar iron, found, in one case, the 

 resistance at 0° equal to, in another, 1 per cent, higher than at 60°. Thurs- 

 ton found in torsion experiments a decided increase of strength to — 12°. 

 Spence, in experiments in bending cast-iron, found at — 18°, a strength 

 greater by about 3.5 per cent, than at + 15°. At higher temperatures, 

 Fairbairn found for bolt iron the maximum of ultimate tensile strength at 

 163° 41 per cent, greater than at 18°; later experiments with bar iron put 

 the maximum at 213°. A commission of the Franklin Institute, at Phila- 

 delphia, found the maximum strength 15 per cent, greater than its ordinary 

 value at about 288°. Styffe has published the results of numerous experi- 

 ments. See his Table VII. 



Beyond the maximum the ultimate resistance decreases at first slowly, 

 but very rapidly at red-heat. In this respect, too, the different kinds of 

 metal behave very differently, and the diminution may possibly be the 



