10 ' STRENGTH OF MATERIALS 



gradually increases; and when the load is removed the ivturn -f 

 the body to its original configuration is also gradual. This j.lirn..m- 

 enon of the deformation lagging behind the stress which produc . 

 is called hysteresis. 



The gradual increase in the deformation under constant stress 

 also called the flow of the material ; and the gradual return of 

 body to its original shape upon removal of the stress is known as 

 elastic afterwork. Flow of the material is characteristic of such metals 

 as soft steel and wrought iron, and becomes especially noticeable- ju>t 

 before rupture. 



14. Fatigue of metals. If a stress lies within the elastic limit, it 

 can be removed and repeated as often as desired will unit causing 

 rupture. If, however, a metal is stressed beyond the elastic limit, 

 and this stress is removed and repeated, or alternates bct\\ cm t, : 

 and compression, a sufficient number of times, it will eventually cause 

 rupture. This phenomenon is known as the fatigue of metals, and has 

 been made the subject of laborious experiment by \Vohlcr. liau.schin- 

 ger, and others. The results of their experiments show that tin- 

 the range of variation of stress, the greater the number of re; 

 or reversals of stress necessary to produce rupture. Among <: 

 results Bauschinger found that for cast iron with an ultin 

 sile strength of 64,100 lb./in. 2 , the maximum tensile stress \vi 

 could be removed and repeated indefinitely without causing MIJ 

 was 35,300 lb./in. 2 ; and that the maximum stress which cnul 

 alternated indefinitely between tension and compression .f c.jual 

 amounts without causing rupture was 29,100 lb./in. 2 . For other 

 kinds of iron and steel Bauschinger obtained similar results, tin- 

 limit of reversible stress in each case agreeing closely with the el 

 limit. From this we conclude that the elastic limit of a material is 

 much more important than its ultimate strength in determining 

 stability of an engineering structure of which it forms a part 



The fatigue of metals indicates that dislocation of matter begins 

 to be produced as soon as the elastic limit is passed, and continues 

 under the action of relatively small forces. This is confirmed by 

 well-known fact that if, as the result of a blow, a fissure or crack is 

 started in a piece of glass or cast iron, tin's fissure will spread \\ 

 out any apparent cause until the piece breaks in two, the only 



