118 DEPARTMENT BULLETIN 1216, U. S. DEPT. OF AGRICULTURE 
Figure 56. The initial rate of deformation is given by the ratio mn:Om. 
nq=Q.5mn, mq=1.5nm, and the slope of Oq represents a rate of deformation 
50 per cent greater than the initial rate. O'q' is drawn parallel to Oq and 
tangent to the stress-strain diagram. The point of tangency J locates the elastic 
limit. In using method 2 the same precautions should be observed in attaching 
the strainometer as for the determination of the elastic limit by method 1. 
Method 2. in tension tests of metals, gives lower values for the elastic limit 
than does method 1. It is recommended that in using method 2 for tension 
tests the strainometer be of such sensitiveness that it will indicate a change 
of strain of 0.0001 inch per inch of gauge length (0.0001 millimeter per milli- 
meter of gauge length). 
Methods of high precision. — When it is desired to determine the elastic limit 
with a high degree of precision it is suggested that the experimenter devise his 
own procedure, and in reporting his results describe the procedure in detail, 
including a statement of the limits of sensitiveness of the apparatus used and 
of the procedure used in plotting the stress-strain diagram from which the 
determination of elastic limit is made. It is to be remembered that the accu- 
rate determination of the elastic limit requires the use of accurate and sensitive 
instruments and the use of accurate methods of plotting test data. 
21. The term "yield point" is defined as follows: 
Yield point. — The stress in a material at which there occurs a marked increase 
in strain without an increase in stress. 
Two methods are in use for determining the yield point: (1) The "drop of 
the beam" method, and (2) the method by use of dividers: 
Method 1. — In method 1, load is applied to the specimen at a steady rate of 
increase and the operator keeps the beam in balance by running out the poise 
at a steady rate. At the yield point the increase of load stops (and for some 
metals there is an actual falling off of load), but the operator, running out the 
poise at a steady rate, runs it a trifle beyond the balance position, and the beam 
of the machine drops for a brief but appreciable interval of time. In a ma- 
chine fitted with a self-indicating load-measuring device there is a sudden halt 
of the load-indicating pointer, corresponding to the drop of the beam. The load 
at the "halt" or the "drop" is recorded, and the corresponding stress is taken 
as the yield point. This method of determining the yield point requires only 
one man to conduct the test. 
Method 2. — In method 2 an observer with a pair of dividers watches for 
visible elongation between two section marks on the specimens. When visible 
stretch is observed, the load at that instant is noted, and the stress correspond- 
ing to the load is taken as the yield point. When a strainometer M is used in 
place of dividers a deformation of 0.01 inch over a gauge length of 2 inches 
shall be regarded as indicating the yield point. 
Some metals do not have a well-defined yield point. 
22. The term " tensile strength " is defined as follows : 
Tensile strength. — The maximum tensile stress which a material is capable of 
developing. 
Note. — In practice, it is considered to be the maximum stress developed by a specimen 
representing the material in a tension test carried to rupture, under definite prescribed 
conditions. Tensile strength is computed from the maximum load carried during a ten- 
sion test and the original cross-sectional area of the specimen. 
The conditions that must be prescribed in a tension test of metallic materials 
are the form of specimen, the method of gripping the specimen, and the rate of 
application of load. These are covered in foregoing sections. No further de- 
tailed discussion of methods of determining tensile strength is necessary, as 
there is involved merely the observing and the recording of the maximum load 
carried by the specimen during the test. 
THE MEASUREMENT, AFTER FRACTURE, OF THE ELONGATION 
AND THE REDUCTION OF CROSS SECTION OF TENSION-TEST 
SPECIMENS 
23. The measurement of the elongation after fracture of tension-test speci- 
mens can be made with sufficient accuracy by means of a pair of dividers and 
60 The determination of yield point with a strainometer is sometimes made in tests on 
nonferrous metals. 
