PROPERTIES OF IRON AND STEEL CONSTRUCTIONS. 165 



t: 3,520, 3,803, 3,978,4,186; Bauschinger tore apart a piece of iron seven 

 times, and the resistance increased from 3,200 to 4,400. 



Paget found that iron chains after stretching bore a greater.dead weight 

 but had less resistance to shock, Fairbairn thought all these phenomena 

 could be explained bj the hypothesis that the resistance of all the parts 

 was not at first called into action, but, like ropes, they became gradually 

 strained in common under sufficient load. With this accords the fact that 

 Bauschinger observed that increase of resistance, especially in rolled iron, 

 was notably regular when the stress was in the direction of the fibres. The 

 analogy holds further; for a rope, when tense is more easily broken by 

 shock. And this explains why a rod under sudden increase of stress breaks 

 more readily than in case of gradually increasing pull. 



When the limit of elasticity is passed, this limit is again raised. Tresca, 

 in tests of rails, succeeded in pushing the limit of elasticit}' to near the limit 

 of rupture, so that it was less by about one-tenth. The practice hitherto 

 has been to assume as permissible stress (b) a fraction of the elastic limit. 

 In this case b increases with the number of loads. But the material be- 

 comes more brittle, and less resistant to shock, and local passages beyond 

 elastic limits are not excluded. So that we need not assent to the often-ad- 

 vocated opinion that a test of material beyond the elastic limit would be of 

 advantage. It is worth mention that the increase ot resistance with the 

 passage beyond each limit cannot go on indefinitely; but a diminution must 

 occur at some time, unless we assume that with very gradual increase of 

 stresses and longer intervals, the original resistance becomes greater than 

 the initial ultimate strength. 



Now, if passage beyond the elastic limit can work unfavorably, it should 

 not be permitted. But it is enough to know that, according to the numer- 

 ous experiments of Styfi'e and others upon all sorts of iron and steel, the 

 ratio of elastic limit to ultimate strength generally lies between /^ and ^\, 

 and under the most favorable circumstances seldom roaches -|. 



Wertheim and Styffe have attempted to establish more precise definitions 

 of the elastic limit, but as they are not better, either theoretically or prac- 

 tically, than others, it would be snjDerfluous to consider them. It is since 

 the time of Hodgkinson and Clard that an empirical importance has at- 

 tached to this limit; and it is still very narrow in its scope, because the 

 limit, as above defined, is of no avail in case of sudden change of strain 

 and of repeated stresses. 



Yicat made experiments to determine the effect of lapse of time upon a 

 dead load. He kept wires loaded up to three-fourths the tensile resistance, 

 during thirty-three months. The one withjieaviest load broke. Yicat in- 

 ferred from this, and because the extension seemed to be proportional to the 

 time, that every load beyond the elastic limit would, after lapse of time, 

 cause rupture. Considering that very small loads cause permanent changes 

 in form, it would be more correct to infer that any load, if given time 

 enough, will cause rupture. Fairbairn thought he could prove this by tests 



