Cross- Head of 
Testing , Machine 
Spherical Sear 
122 DEPARTMENT BULLETIN 1216, U. S. DEPT. OF AGRICULTURE 
4. Figure 59 shows the proportions of the three classes of compression speci- 
mens. Suggested dimensions for compression specimens for general use are : 
Short specimens: d=iy 8 inches±0.01 inch (area of cross section is approxi- 
mately 1 square inch), 7i=l inch; 
Medium-length specimens : #=0.798 inch ±0.01 inch, h=2% inches, or 
d=l inch ±0.01 inch, h—S inches, or 
d=iy s inches ±0.01 inch, 7i=3% inches. 
Long specimens: d—l^A inches±0.01 inch, 7i=12 1 /> inches. 
5. In all compression tests of metallic materials, the actual dimensions of 
cross section of the test specimens shall be measured with a micrometer reading 
to one-two hundredth of the dimension measured, and stresses shall be com- 
puted on the basis of the measured cross section. It should never be assumed 
that the dimensions of the cross section of the specimen are identical with the 
nominal dimensions. 
6. Specimens shall be finished so that the surfaces are smooth and free from 
nicks and tools marks. All ragged edges shall be smoothed. 
BEARING BLOCKS FOR COMPRESSION-TEST SPECIMENS 
7. Both ends of a compression-test specimen shall bear on a carefully 
machined plane surface. The bearing blocks shall either be made of hardened 
steel or shall be faced with hardened steel. 
8. One end of a compression 
test specimen shall bear on a 
spherical-seated compression 
block. 63 Figure 60 shows a sat- 
isfactory arrangement of com- 
pression specimen and bearing 
blocks. It is desirable that 
the spherical-seated bearing 
block be at the upper end of 
the test specimen (for speci- 
mens tested with the axis ver- 
tical). It is important that the 
center of the spherical surface 
of the spherical-seated bearing 
block should be in the flat face 
which bears on the specimen ; 
otherwise lack of parallelism 
between faces of the specimen 
sets up a bending moment in the 
specimen (14)- 
9. It should be borne in mind 
that the object of a spherical- 
seated bearing block is to give 
the specimen as even a distribu- 
tion of initial load as is possible. Owing to friction, tbe spherical-seated bearing 
can not be relied on to adjust itself to bending action which may occur during 
the test. 
10. In making compression tests, care should be taken to preserve the align- 
ment of the heads of the testing machine, and to center the specimen in the 
testing machine. From time to time the heads of the testing machine shall be 
tested for parallelism. Deviations from parallelism should not exceed 0.01 inch. 
The movable head of the machine should be free from sidewise motion that is 
visible to the unaided eye as the machine is run at its fastest speed. 
SPEED OF OPERATION OF TESTING MACHINE 
11. In making compression tests of specimens of metallic materials the speed 
of the machine, by which is meant the speed of the cross head when the machine 
is running idle, shall be such that the load can be accurately weighed, but in no 
case shall the speed be greater than the values given in Table 11. 
Spec/men 
Center of 
Spherical Surface 
Jr-i ,-- Bearing Blotk 
7/7777P 7 
Weighing Table of 
Testing Machine. 1 
Fig. GO. — A satisfactory arrangement of compres- 
sion specimen and bearing block 
03 The device shown in Figure 61 has been successfully used in compression tests of 
metallic specimens whose ends were machined with extreme care. It will be noted that 
this device does not involve a spherical-seated compression block bearing directly on the 
test specimen. This device or one similar has been successfully used in the British 
National Phvsical Laboratory and in the laboratories of the United States Air Service at 
McCook Field, Dayton, Ohio. 
