116 DEPARTMENT BULLETIN 1216, U. S. DEPT. OF AGRICULTURE 
GRIPPING DEVICES FOR TENSION-TEST SPECIMENS 
<-To Fit 
Gripping Device 
on Testing 
Machine 
12. For lone bars of ductile metal and for the flat-plate test specimen shown 
in Figure 48, wedge grips, bearing directly on the end of the test specimen, 
generally furnish a satisfactory means of gripping the specimens. 
13. For short specimens and for specimens of brittle material, wedge grips are 
not a satisfactory means of holding test specimens ; it is necessary to use 
machined specimens and to use special means for insuring that the specimen, 
when under load, shall be as nearly as possible in pure axial tension. Figure 
45 shows a device for gripping tension specimens with threaded ends and i 
a device for gripping specimens with shouldered ends. Both these gripping 
devices are attached to the heads of the testing machin through spherical- 
seated bearings The distance between spherical bearings should be as great 
as is feasible. For brittle materials, even the spherical-seated bearings shown 
in Figure 45 are not always effective in avoiding bending stress on the speci- 
men. The gripping device shown in Figure 53 has proved satisfactory for 
testing such specimens, and is an excel- 
lent gripping device for any machined 
tension test specimen. 
14. For specimens cut from thin plate 
material and from sheet material, ordi- 
nary wedge grips are liable to cut the 
specimens near the edges, giving a tear- 
ing stress rather than an axial tension. 
The self-adjusting grips shown in Figure 
54 have proved satisfactory for tests of 
thin sheet copper and aluminum. 
15. For tests of specimens of wire, 
ordinary wedge grips are liable to cut 
the wire at the edges of the grips. This 
cutting has been found to be largely 
eliminated by the use of grips of the 
form shown in Figure 55. 
16. In making tension tests, care shall 
be taken to see that the axis of the 
test specimen is located in the center 
line of the heads of the testing machine 
and that the liners used behind wedge 
grips are of proper thickness. From 
time to time the openings in the heads 
of the testing machine shall be lined up 
and the heads tested for parallelism. 
Deviations from correct alignment and 
from parallelism should not exceed 0.01 
inch for any position of the movable 
head within the range of movement 
during test. The heads 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. 
Serrated 
Face on Grip 
~~T 
Rounded 
Corner 
Pig. 55. 
^--Specimen 
-Grip for use with wire 
SPEED OF OPERATION OF TESTING MACHINE 
17. The pulling speed has a marked influence on the tensile properties shown 
by materials rested, an increase in speed increasing the values found for yield 
point and tensile strength. In testing specimens of metallic materials in gauge 
lengths of 2 and 8 inches, the speed of the machine, by which is meant the speed 
of the crosshead when the machine is running idle, shall conform to the 
following requirements : 
The speed of head of the testing machine 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 10, which values represent the extreme of good laboratory 
practice for steel. 69 
r>9 Tbe speeds given here are probably too high for satisfactory use in testing ferrous 
metals. 
