COMPKESSION TESTS. 



153 



In the case of some of the softer materials it is usual 

 and necessary to insert a bedding substance between the 

 specimen and the compression plates, so as to distribute 

 the load uniformly, just as mortar is used in masonry con- 

 struction. But this is not necessary in the case of the 

 metals, and it is sufficient to place the specimen between a 

 pair of hard, true plates. 



A reference to Fig. 25, page 67, will make clear the 

 arrangement which is made use of for carrying out com- 

 pression tests on a vertical Buckton-Wicksteed machine. 

 Here the lower compression plate B is slung from the 

 upper tension shackle A, and the specimen to be crushed 

 is placed between this plate and the crosshead C, which is 

 attached to the ram. A somewhat similar arrangement is 

 made use of in most of the other machines. 



A form of compression gear, adapted 

 for machines which are primarily 

 arranged for testing in tension only, is 

 shown in the accompanying figure.* 

 This form of shackle has been de- 

 signed and used by Professor Unwin, 

 and consists, as will be seen, of a pair 

 of stirrups fitting into one another, 

 and which can be pulled apart by 

 being attached to the tension shackles 

 of the machine. The specimen is 

 marked S. The top seating against 

 which the specimen rests is made 

 with a spherical joint, so that the 

 pressure is applied directly in the line 

 of pull. 



Reference should also be made to 

 Figs. 26, 27, 31, 34, 35, and 38. The 

 four-pillar machine of Fig. 27 pos- 

 sesses advantages over other machines 

 of the vertical type, for compression, 

 from the fact that it is provided with 

 means whereby the thrust blocks can 

 be truly guided along the four pillar slides. 



The strains of a short compression specimen are not so 

 easy to determine as in the case of a longer tension 

 specimen. Where the modulus of elasticity in compression 

 is required, it is better to make use of a specimen of rather 

 greater length than ten inches, and to use an ordinary 



*Unwm's "Testing," p. 185. 



FIG. 72. 



