RESISTANCE OF MATERIALS 



SECTION I 



STRESS AND DEFORMATION 



1. Elastic resistance, or stress. The effect of an external force 

 acting upon an elastic body is to produce deformation, or change of 

 shape. For example, if a bar is placed in a testing machine and a 

 tensile load applied, it will be found that the length of the bar is 

 increased and the area of its cross section diminished. Similarly, if 

 a compressive load is applied, the length of the bar is diminished 

 and the area of its cross section increased. 



All solid bodies off er^ more or less resistance to the deformation, 

 or change of shape, produced by external force. This internal resist- 

 ance, when expressed in definite units, is called stress. A body under 

 the action of stress is said to be strained. 



In general the stress is not the same at all points of a body, but 

 varies from point to point. The intensity of the stress at any par- 

 ticular point is therefore expressed as the force in pounds which 

 would be exerted if the stress were uniform and acted over an area 

 one square inch in extent. That is to say, whatever the actual extent 

 of the area considered, whether finite or infinitesimal, the stress is 

 expressed in pounds per square inch (abbreviated into lb./in. 2 ). 



For example, suppose that a wire ^ in. in diameter is pulled 

 with a force of 50 Ib. Then for equilibrium the total stress acting 

 on any cross section of the wire must also be 50 Ib. But since the 

 area of the cross section is only .049 in. 2 , the intensity of the stress 



is -- - , or 1000 lb./in. 2 In other words, if the wire were 1 sq. in. 



in cross section, the strain under a load of 1000 Ib. would be 

 the same as that produced by a load of 50 Ib. on a wire in. 

 in diameter. 



1 



