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ENGINEERING ON THE FARM 



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On every unit of surface there is a certain stress, and this 

 stress multiplied by the total number of units in the cross 

 section is equal to the total force at the end of the rod. The 

 amount of stress on each unit of surface is called 

 the unit of stress and is commonly measured in 

 pounds per square inch. If the rod has a 

 cross-sectional area of i^ square inches, the 

 unit stress is 1,333 pounds per square inch. 

 Figure 89 shows typical fractures caused by 

 tension. The first is a brittle material, and 

 the second is one of a more plastic nature 

 which draws out and necks in before it breaks. 

 Compression. Figure 90 illustrates another 

 kind of simple stress. This is called com- 

 pression. As the figure shows, it is a stress 

 which resists a tendency to crush the material. 

 Fig. 89. Change In this case, also, the stress is distributed uni- 

 caused by formly over the whole cross section of the body 

 tension an( ^ ma y j^ measured in pounds per square 

 inch. Figure 91 shows a typical fracture, under compression, 

 of a brittle material such as brick or stone, of a less brittle 

 material such as wood, and of a material such as lead, which 

 is so plastic that it will not 

 actually rupture at all when 

 subjected to compression, 

 but will flow out. 



Shear. Figure 92 illus- 

 trates a third kind of simple 

 stress, called shear. It is a 

 stress which resists a tend- 

 ency to slip two adjoining ^w - sm^ 

 planes within the body 

 past each other. It derives 

 its name from the fact that FlG - *>°- Compression 



the forces producing it act in a manner similar to the 

 blades of a pair of shears. As in the cases of tension and 



To ro/ /oa</ on pier 

 /6000 /bs. 



Unit stress 

 on sec f/ on 

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