STRENGTH OF MATERIALS 235 



Tension, or pulling stress, is the force that overcomes external 

 forces that tend to stretch a body. A rope or wire sup- 

 porting a load is an example of tensile stress. The rope 

 or wire is subjected to a tensile stress of the weight of 

 the load. 



Compression, or crushing stress, is produced when external 

 forms act so as to compress a volume or any supporting body. 

 When an engine rests upon rails, the rails are subjected to 

 the compressive stress of the weight of the engine. 



Shearing, or cutting stress, is produced when forces tend 

 to cause the particles of one section to slide over the section of 

 an adjacent body. When a bolt is in tension the head of the 

 bolt is subjected to a shearing stress tending to strip the 

 head from the shank of the bolt. 



Torsion, or twisting stress, is produced when forces tend 

 to twist. A rotating shaft is obliged to resist a twisting force. 



Flexure, or bending stress, is produced when forces tend to 

 bend. A floor timber in a house has to resist the bending 

 force that tends to break it. 



283. The Effect of Strains. Since a strain is the length- 

 ening due to the action of a stress it is measured in fractions 

 or decimals of an inch. 



To illustrate: If a bar of steel, such as a piston rod, has been 

 stretched or lengthened ^ in. by the stress caused by the weight 

 of the driving box, the strain in the steel rod is -fa in. 



If a weight is hung from a beam resting on two supports A and B 

 as in Fig. 100, the beam is a lever of the second class. If we consider 

 the pressure on the support A as the power and the pressure on the 

 support B as the fulcrum, we can easily find the power if we know 

 the weight. Then knowing the power, we can find the pressure on 

 support B, provided we know the distance of the weight from 



