STRENGTH OF MATERIALS 239 



In figuring the ultimate elongation of a test piece broken 

 in the machine, it does not matter what the sectional area 

 of the piece is. All we need to know is the increase in length 

 over the original length. This increase should be divided 

 by the original length. The quotient will be the ultimate 

 elongation of the tested piece. 



287. The Stress of Compression. Compression is one 

 of the most common of all stresses and everywhere things 

 are seen undergoing compression. The foundation walls 

 of the shop, the legs of the table, the foundation of the lathe, 

 the shaper, the drill press, or the planer, the posts or columns 

 that support the shop roof are all in compression. 



If the length of a piece is not more than five times its 

 least transverse dimension, the laws of compression are simi- 

 lar to those of tension, and the strain is proportional to the 

 stress until the elastic limit has been reached. Upon reach- 

 ing the elastic limit, the strain increases more rapidly than 

 the stress, as in the case of tension. 



288. Testing Laws Applicable to Materials. Repeated 

 experiments with materials in testing machines and in prac- 

 tice have proved that there are certain laws which always 

 hold true. These laws may be enumerated as follows : 



I. When a body is subjected to a small stress a small 

 strain is produced. When the stress is removed the body 

 springs back to its original shape. 



II. Within certain limits the change of shape, or strain, 

 is directly proportional to the stress producing it. This is 

 the same as saying that when a tensile force is gradually 

 applied to a bar it elongates the bar and that up to a cer- 

 tain limit this elongation is proportional to the force. 



