UNIFICATION OF METHODS IN TESTING MATERIALS, XVII. 
area should be calculated up to the limit corresponding to rupture. 
Where such diagrams are not used, it is recommended that as 
many observations as possible should be made during the test 
from which diagrams may be plotted. In specifications intended 
to govern the supply of steel for works of construction, it is usual 
to specify an upper and a lower limit of tensile strength, and to 
State also the minimum percentage of elongation. It generally 
happens with ductile materials that high tensile strengths are 
accompanied with smaller elongations, and that the lower tensile 
strengths give correspondingly greater elongations. 
In order to express the value of a material under these circum- 
stances, Prof. Tetmaier has proposed a coefficient of quality which 
is based upon the following considerations :— 
Plate t. fig. 10, represents an autographic diagram such as would 
be produced in testing a piece of mild steel, but somewhat distorted 
from 0 to e and ¢ to y in order to show more clearly the charac- 
teristic points in the diagram. The loads are represented as 
ordinates and the elongations by abscisse. The point ¢ indicates 
the elastic limit, and the line is straight from 0 toe; from e to y 
the line curves slightly from the straight portion and then takes 
horizontal direction; ‘indicates the yield point or the com- 
mencement of the permanent elongations, 1.¢., the commencement 
of the plastic state of the material. The test piece elongates con- 
siderably and the point m denotes the maximum load supported, 
local contraction of area then occurs as shown by the line mr, 
after which rupture takes place. 
Let Le denote the load sustained at the elastic limit 
» Ly a fr a » yield point 
as LM a ie . » maximum load 
» Lr a fe a » load at rupture 
» Al denote the elongation at the elastic limit 
2 ae a » maximum load 
sot, rupture 
” ” 
