CHAPTER IV 



MECHANICS OF MATERIALS 



" Mechanics of materials" is the science which deals with 

 changes occurring in the size and shape of bodies when they 

 are subjected to external forces. The external forces are 

 applied in various ways and produce different changes, 

 known as deformations, which conform to definite laws. 

 The forces are generally applied slowly, and the changes in 

 size and shape take place accordingly. 



STRESS 



Definition. Stress is an internal force, set up within the 

 material composing a body, which resists the tendency to 

 change in size and shape due to the action of an external 

 force. As long as the. external forces are not so great as to 

 cause any break in the material, 

 the stresses are equal to the 

 external forces causing them, 

 and the two hold each other in 

 equilibrium. Both sets of forces 

 are measured in pounds per 

 square inch. 



Simple stresses. Simple 

 stresses are as follows: 



Tension. When a weight is 

 hung on the end of a rod (Fig. 

 88) it produces a stress within 

 the rod which is called tension. 

 This stress tends to hold two 

 adjoining planes within the body 

 from being pulled apart. The 

 total internal stress is equal to the total weight and is dis- 

 tributed uniformly over the whole cross section of the rod. 



63 



Area of section, 

 /£ Jouore inches 



Unit stress /333 

 lbs. per so. in. 







£000 /bs. 



Fig. 88. Tension 



