PROPERTIES AND THE PLASTIC EXTENSION OF METALS. 
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centre dot in which the points rest, elongates as stretching proceeds and the points 
slip. 
The modulus of elasticity, E, found from flanged test pieces agrees with the values 
found from plain bars. The flanges therefore have negligible influence on the elastic 
extension of the gauge length. They restrict the plastic extension slightly. In mild 
steel the total extension is about 3 per cent, less when found from a flanged test piece 
than it would be if found from a plain bar. 
Dr. Coker has kindly examined the distribution of stress produced by a flange in a 
xylonite test piece made to the dimensions of fig. 2. When the xylonite test piece is 
stretched the colours show that there is no stress in the flange itself and there is a 
slight but symmetrical modification of the stress distribution at its root. This means 
that as stretching proceeds the flange is not distorted, and therefore the distance 
between the flanges is a true measure of the extension of the primitive gauge length 
which they define. 
§ 3. The Elasticity of Materials and, a Typical Load Elastic Extension Diagram 
of Mild Steel. 
The elasticity of a material means in a general sense its power of returning to its 
primitive form after loading has been applied and removed. 
The recovery may be partial or complete. 
The power of complete recovery is lost when the stress produced by the loading has 
once passed beyond a certain limiting value peculiar to the material. 
Below this limiting stress the extension of a steel test piece is proportional to the 
load. 
Above this limiting stress the extension increases at a greater rate than the load. 
The limit is therefore called the limit of proportionality. 
The power of recovery may thus be distinguished into the power of complete 
recovery possessed and retained only so long as the stress in the steel has never once 
exceeded the limit of proportionality: and the power of partial recovery peculiar to 
the state into which the metal passes directly it has once been loaded beyond the 
limit of proportionality. 
Provided that the material has never been loaded beyond its limit of proportionality 
the material may be said to be in a state of perfect elasticity, because it possesses the 
power of complete recovery of form after removal of load; alternative^ it may be 
said to be in a state of proportional elasticity because its extension is found to be 
proportional to the load. 
The one term includes the other. If it is found to extend proportionally to the 
load its recovery is perfect after removal of load. 
No metal is, however, quite perfect in its recovery, but the term perfect used in 
the sense defined above is convenient and substantially expresses the experimental 
results. 
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