COMPRESSION TESTS. 



157 



In a compression test the main object is to determine 

 the load at which the elastic limit is reached, because after 

 this point has been passed very little further information 



can be obtained from the test. In a tension test the case 

 is different. Here many facts relating to the nature and 

 properties of the material are revealed when the specimen 

 is tested to destruction, and measurements of the final 

 dimensions are made. The curve on Fig. 74 shows 

 distinctly that a very well marked plastic deformation 

 takes place after a certain load had been reached. Beyond 

 this point the curve is very regular and uniform. Before 

 this is reached the curve is almost flat, but it begins to fall 

 away after the elastic limit has be on passed. 



The appearance of three typical compression specimens 

 after sustaining a high load are shown in Fig. 71. Here 

 A represents the appearance of a plastic substance, such as 

 mild steel or copper, after compression ; B indicates the 

 appearance of a piece of a metal which has a tendency 





