Art. 28 



FATIGUE OF IRON AND STEEL. 



31 



raised ultimate strength, as explained in Art. 26. This is the 

 more remarkable because if a piece brought nearly to the point 

 of rupture in this way, is tested in the ordinary way, it will show 

 no loss of strength or ductility. This corresponds with practical 

 experience for it has been noticed that failures occur after many 

 years of service, yet when such metal is tested in the ordinary 

 way, it shows no loss of strength. What the nature of the 

 deterioration is, has not been satisfactorily explained, but it is 

 called fatigue. 1 There seems to be no alteration of structure 

 except just at the surface of fracture. 



' ' Fatigue ' ' experiments have been made by Woehler 2 , Spang- 

 enberg 3 , Bauschingei 4 , Baker 5 , and others, but further experi- 

 ments are necessary to satisfactorily settle some disputed points. 

 All these experiments show that the number of repetitions of a 

 variation of stress, which a piece of metal will endure before 

 rupture, depends upon the range of stress ; the range of stress is 

 the greater the further the maximum is from the ultimate 

 strength. If for bar iron, for example, the range of stress is 

 from to about 32000 Ibs per sq. in., it will stand from five to 

 ten million repetitions ; in order to stand as many repetitions 

 when the upper limit is 49000 Ibs. per sq. in., the range of stress 

 must not exceed about 19000 Ibs. per sq. in., that is, it will be from 

 30000 to 49000 Ibs. per sq. in. The range of stress for stresses 

 alternating between tension and compression is the sum of the 

 tit'o, and, in this case, for equal resistance, each stress is somewhat 

 more than half of that for stress of one kind with a lower limit 

 of zero. 



The experiments also show that for stresses within the elastic 

 limit or yield point (natural elastic limits for alternating 

 stresses), there is no "fatigue." The yield point should, there- 

 fore, be considered the limit of strength as stated in Art. 23. 

 Since it has been the custom to base working stresses on the 

 ultimate strength, it was the practice of some engineers to base 



iProf. Johnson, in his Material* of Construction. Chapter XXVII, 

 explains "fatigue" by saying that some of the millions of incipient de- 

 fects or "micro-flows'" gradually extend their weakening influence. 



2 See Unwin's The Testing of Materials of Construction and the 

 Transactions of the American Society of Civil Engineers, Vol. 41, p. 222. 



3 See Burr's Elasticity and Resistance of the Materials of Engi- 

 neering, p. 844. 



4 SeeUnwin's The Testing of Materials of Construction and Greene's 

 Structural Mechanics, p. 160. 



6 See Unwin's The Testing of Materials of Construction 



