COMPRESSION TESTS. 155 



end. But in the compression tests of many of the metals, 

 it is possible to go on increasing the loads almost indefi- 

 nitely. When carrying out a compression test, the 

 measurement of the strains, beyond the elastic limit, can be 

 made in one of two ways. The most accurate plan is to 

 release the load after each increment has been imposed, 

 take the bar out of the machine, and measure its length 

 with a pair of vernier calipers. After this the bar is put 

 back and the test proceeded with. 



Another and quicker way is to measure the distance 

 between the compression blocks after each increment of 

 load, and assume this distance to be the same as the length 

 of the specimen. If the plates are true, and the measure- 

 ments are always taken in the same place, this method is 

 sufficiently near for ordinary purposes. In taking the 

 measurements a pair of inside calipers and a rule may be 

 used. Better than this is an inside micrometer caliper. 



The behaviour of short compression specimens of the 

 various metals is very different. Annealed copper is one 

 of those which display properties of almost perfect plas- 

 ticity. As the load is increased the specimen diminishes 

 in length and increases in its lateral dimensions ; for a 

 time the true cylindrical form is maintained, but later on 

 the specimen assumes a more barrel-shaped form, the 

 greatest increase in diameter taking place in the middle 

 and diminishing towards the two ends. This is probably 

 assisted by the friction between the specimen and the 

 compression blocks. Mild steel behaves in very much the 

 same way, though if the load is carried beyond a certain 

 point, the edges of the disc will begin to show signs of 

 cracking. Wrought iron also can be compressed almost 

 indefinitely, but cracks begin to show at lower loads than 

 in the case of mild steel, and this is more especially true of 

 the poorer brands. Brass and gun metal show less plasticity 

 than those mentioned above. In some cases they may be 

 compressed until they finally crumble into small pieces ; 

 in others they fail at an earlier load by violent and sudden 

 rupture, fracture taking place by shearing across a plane 

 forming an angle of about 45 deg. with the axis. Of course, 

 it will readily be understood that there is a great difference 

 between brass and gun-metal specimens made from different 

 mixtures. Cast iron always fails by sudden rupture across 

 a plane forming an angle of 45 deg. with the axis of the 

 specimen. The rupture is accompanied by a loud report, 

 and the several pieces of the specimen fly outwards with 



