152 



COMPRESSION TESTS. 



take place by crushing alone ; if, on the other 

 hand, the length is great relatively, failure will take 

 place by bending or buckling ; a length midway between 



FIG. 71. 



these will result in the specimen collapsing, partly by 

 reason of its material being crushed and partly by 

 bending. In a compression test, unless it is otherwise 

 specified, it is supposed that failure only takes place by 

 simple crushing, and therefore it is necessary that the 

 specimen for this purpose shall be short. Metal com- 

 pression specimens are generally turned in a lathe, and 

 should be not more than two or three times their 

 diameter in length. Such a specimen is shown on 

 Fig. 70. The ends of short compression specimens must 

 be truly at right angles to their axes and quite plane. This 

 is most important, and it is better to have the ends very 

 slightly holloAv than rounded. The plates of the testing 

 machine, between which the specimen is to be crushed, 

 should be perfectly flat and free from lumps or un- 

 evennesses : the surfaces of these plates must be at right 

 angles to the direction of the thrust. The reason of all 

 these points is obvious. A tension specimen will tend to 

 straighten and accommodate itself to the direction of 

 the pull if this is initially a little out of centre. In a com- 

 pression test the case is different. If there is a slight 

 bending or want of truth at the beginning, the effect of 

 the load will be to increase and accentuate this instead of 

 diminishing it ; so that great care must be taken that the 

 load shall be transmitted truly along the axis of the bar ; 

 otherwise the result of the test will be greatly modified. 



