120 DEPARTMENT BULLETIN 1216, U. S. DEPT. OF AGRICULTURE 
Stress-strain diagrams are in some cases drawn directly by an autographic 
attachment to the testing machine. A more usual method of procedure consists 
in taking a series of load readings (from the balanced scale beam of the testing 
machine) with corresponding readings of the strain-indicating apparatus. The 
term " strainometer reading " will be used to denote the reading of the strain- 
measuring instrument. From these readings, or from values computed from 
them, there is plotted a diagram with stress-indicating values (load or stress) 
as ordinates. and strain-indicating values (elongation or strain) as abscissas. 
In planning such a test it is necessary to decide on the increment of load or the 
increment of reading of strainometer to be used between successive readings. 
In Figure 58 are shown three typical stress-strain diagrams. The material 
for all three tests is the same, and the test specimens are all of the same 
size, so that the diagrams should be the same. The diagram shown in 
Figure 58 (a) is plotted from points determined by taking increments of 
load (8). In this diagram it is seen that, owing to the shape of the curve, 
data for locating points between M and N are lacking. That particular portion 
of the curve (the "knee" of the curve) is the part for which it is especially 
desirable to locate several points on the diagram. 
In Figure 58 (&) is shown a diagram plotted from points determined by 
taking increments of strainometer reading (e). It will be noted that for this 
,e x p c r^ 00m 
1 i /: ; i ! ! 
-o 
i y[ : j i j ; 
Id! ; i : : ; 
«u 
o 
./' ' 1 1 ' 1 1 
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Strainometer 
Strainometer 
Strainometer 
Reading 
Reading 
Reading 
{a) 
(&) 
(c) 
Fig. 58. — Illustrating three methods of plotting load-deformation curves. 
i«) With equal inerempnts of stress; (&) with equal increments of strain; 
(c) with two values of increments of stress 
diagram there are located several points near the knee of the curve, and the 
shape of the diagram in this important region is much more definitely deter- 
mined than for the curve shown in Figure 58 (a). 
The custom of choosing increments of load rather than increments of strain- 
ometer reading is quite common in tests of materials and is followed because, 
in general, it is easier to compute increments of load than it is to compute 
increments of strainometer reading. An estimate of the load necessary to 
stress the specimen up to the knee of the curve is made, and some fraction 
(frequently one-tenth) of this value is taken as an increment. The computa- 
tion of the corresponding increment of strainometer reading is more complicated. 
Sometimes the practice is followed of applying a few increments of load 
as determined above, and then applying load in much smaller increments (&') 
until the knee of the curve is passed. Figure 58 (c) shows a diagram obtained 
in this manner. This method involves a marked increase in the number of 
readings necessary for a test, and with unknown material there is always some 
danger that the knee of the curve will be reached before the use of small incre- 
ments of load is begun. 
The following method of choosing increments for a test is suggested : Estimate 
the load corresponding to the knee of the stress-strain diagram and choose a 
value for increment of load about one-tenth of this value. Apply this incre- 
ment of load (8) once and note the corresponding change of reading for 
the strainometer (e). Then for the remainder of the test use for the increment 
of strainometer reading a value which corresponds to some convenient interval 
on the scale of the strainometer and which is approximately equal to (e). 
