7 o 



ENGINEERING ON THE FARM 



materials, such as steel, become fatigued if forces are applied 

 to them repeatedly. This fatigue causes a loss of strength 

 and elasticity. An effect somewhat similar to fatigue is 



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11 



Square ends 



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Gound ends 



Combination of 

 round andsouore 



Fig. 97. Action of columns supporting a load 



produced when forces act on a body so as to produce a 

 repeated variation of stress, such as tension to compression 

 and back again. 



Columns. When the length of a body under compression 

 is more than eight or ten times its smallest lateral dimension, 

 the body has a tendency to bend or buckle. Such a body is 

 called a column. Figure 97 represents the nature of the 

 stresses in column sections. There are compression and 

 shear present in all cases, no matter what the load may be ; 



