"FATIGUE STRESS" 61 



greatest practical value has sprung out of this 

 purely theoretical knowledge. This relates to the 

 failure of metals under what is called " fatigue" 

 and to the methods available for preventing these 

 failures. Suppose a round bar of iron to be fixed 

 horizontally firmly held at one end and projecting 

 freely at the other. Now let a load be hung to the 

 free end; if this load is steadily increased, the bar 

 will at first bend and ultimately it will break. 

 Now instead of increasing the load to the point 

 where fracture occurs, let us stop at the stage when 

 the bar has just begun to bend a very little. In 

 this state, the bar would continue to support the 

 same load, without appreciable further bending, 

 for an indefinite time. If, however, we were to 

 remove the load and then to re-apply it in quick 

 succession, the bar would break after some thousands 

 of repeated applications. The experiment is usually 

 made by leaving the load hanging from the bar 

 but rotating the bar; in that way opposite sides 

 of the bar come into play in turn as the bar revolves 

 and the effect is the same as that of leaving the 

 bar stationary and reversing the load repeatedly. 

 Such an alternating or intermittent load is known 

 as a " fatigue stress" and frequently occurs in the 

 moving parts of machinery, where the direction 

 of loading is sometimes reversed many hundred 

 times per minute. Pieces of metal subjected to 

 such conditions frequently fail under loads which 

 they could bear indefinitely if left at rest. Now 



