122 STRESSES AND STRAINS 



that produces an alteration in the form of a body, and this 

 alteration of form is called the strain. 



In accordance with the direction in which the forces act 

 with reference to a body, the stress produced may be either 

 tensile, compressive, or shearing. 



Tensile stress is the effect produced when the external 

 forces act in such a direction that they tend to stretch a body. 



Compressive stress is the effect produced when the tendency 

 of the forces is to compress the body. 



Shearing stress is when the forces act so as to produce a 

 tendency for the particles in one section of a body to slide 

 over the particles of the adjacent section. 



When a beam is loaded in such a manner that there is in it 

 a tendency to bend, it is subjected to a bending stress. In 

 this case, there is a combination of the three stresses already 

 mentioned (tension, compression, and shear) in different 

 parts of the beam. 



The unit stress (called, also, the intensity of stress) is the 

 name given to the stress per unit of area; or, it is the total 

 stress in a tie-rod, column, or the like divided by the area 

 of the cross-section. 



Let P represent the total stress, in pounds; A, the area of 

 cross-section, in square inches; and s, the unit stress, in 

 pounds per square inch. Then, 



P 



5= -, or P = As 

 A. 



STRAIN 



When a body is stretched, shortened, or in any way 

 deformed through the action of a force, the deformation is 

 called a strain. Thus, if a rod were elongated rV in. by a 

 load of 1,000 lb., the strain would be rV in. Within certain 

 limits, to be given hereafter, strains are proportional to the 

 stresses producing them. 



Unit strain is the strain per unit of length or of area, but 

 is usually taken per unit of length and called, for tension, the 

 elongation per unit of length. If the unit of length is taken 

 as 1 in., the unit strain is equal to the total strain divided by 

 the length of the body in inches. 



