METHODS OF SAMPLING AND TESTING HIGHWAY MATERIALS 117 
Table 10. — Limit of speed of testing head 
Specified minimum tensile strength of material, pounds per square inch 
Maximum cross- 
head speed, 
inches per min- 
ute 
Tensile 
strength 
80,000 or under. 
Over 80,000 
2.0 
fi.0 
1.0 
2.0 
18. In determining the elastic limit in accordance with method 1, section 20, 
the crosshead speed for the 2-inch gauge length shall not exceed 0.125 inch per 
minnte. 
19. In determining the elastic limit by method 2, section 20, the crosshead 
speed shall not exceed 0.025 inch per inch of gauge length per minute. 
DETERMINATION OF ELASTIC LIMIT, YIELD POINT, AND TENSILE 
STRENGTH 
20. The term "elastic limit" is defined as follows: 
Elastic limit. — The greatest stress which a material is capable of developing 
without a permanent deformation remaining upon complete release of the stress. 
The determination of the elastic limit as thus defined would logically involve 
the application and release of a succession of increasing loads on a test speci- 
men until there was observed deformation after release of a load. This pro- 
cedure is very slow, and since for many 
materials experience does not indicate any 
appreciable difference between the elastic 
limit and the limit of proportionality of 
stress to strain (sometimes called the pro- 
portional limit), the determination of the 
stress at the limit of proportionality of stress 
to strain is regarded as an accurate determi- 
nation of the elastic limit. 
It is obvious that the values obtained in 
tests for determining the elastic limit will 
depend on the delicacy of methods and in- 
struments used. It becomes necessary, there- 
fore, that in any test the method used in 
obtaining the elastic limit be clearly stated. 
The following methods are in common use 
for determining a value designated as the 
elastic limit : 
Method 1, proposed by committee A-l 
on steel, and embodied in a number of 
specifications for steel. (For tension tests only.) The elastic limit shall be 
determined by an extensometer reading to 0.0002 inch. The extensometer shall 
be attached To the specimen at the gauge marks and not to the shoulders of the 
specimen nor to any part of the testing machine. When the specimen is in 
place and the extensometer attached, the testing machine shall be operated so 
as to increase the load on the specimen at a uniform rate. The observer shall 
watch the elongation of the specimen as shown by the extensometer and shall 
note, for this determination, the load at which the rate of elongation shows a 
sudden increase. The extensometer may then be removed from the specimen 
and the test continued to determine the tensile strength. 
Method 2 (proposed by the late J. B. Johnson) is a somewhat more delicate 
method for locating the elastic limit than is method 1. In method 2 the elastic 
limit shall be taken as that stress at which the rate of deformation is 50 per 
cent greater than the initial rate of deformation. A stress-strain diagram is 
necessary for determining the elastic limit by this method. It is illustrated in 
Fig. 
Strain or Strainometer Reading 
56. — Illustrating method 2 for 
determining elastic limit 
