2 RESISTANCE OF MATERIALS 



Taking any plane section of a body under Strain, the stress act- 

 ing on tliis plane section may in general be resolved, like any force, 

 into two components, one perpendicular to the plane and the other 

 lying in the plane. The perpendicular, or normal, component is 

 called direct stress and is either tension or compression. The 

 tangential component, or that lying in the plane of the cross 

 section, is called shear. In what follows, the letter p will always 

 be used to denote normal, or direct, stress, and q to denote tan- 

 gential stress, or shear. 



Compression 



FIG. 1 



The effect of a normal stress is to produce extension or com- 

 pression, that is, a lengthening or shortening of the fibers, thereby 

 changing the dimensions of the body ; whereas shear tends to slide 

 any given cross section over the one adjacent to it, thus producing 

 angular deformation, or change in shape, of the body without 

 altering its dimensions. 



2. Varieties of strain. The nature of the deformation produced 

 by external forces acting on an elastic body depends on where and 

 how these forces are applied. Although only two kinds of stress 

 can occur, namely, normal stress (tension or compression) and 



