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PROF. W. E. DALBY: RESEARCHES ON THE ELASTIC 
(2) the yield load ; 
(3) the value of E : this is given by the slope of the elastic line; 
(4) the work lost per loop ; 
(5) the rate of increase of the work lost per loop. 
The diagram from normal material satisfying known conditions of composition and 
manufacture may be used as a diagram of comparison. The form of the curve, the 
loop area and the rate of increase are sensitive to changes in the kind of material and 
to changes in the inner state of materials. 
The diagram is specially useful in showing the load at the limit of proportionality, 
for this load bears neither a constant relation to the yield point, when there is one, 
nor to the ultimate load. Consequently factors of safety reckoned against either the 
yield load or the ultimate load are ambiguous. 
This is specially important in gun design. The whole theory rests upon the elastic 
property of the material, and the theory ceases to apply after the limit of proportionality 
is passed. 
Considerable research in many directions is necessary before a full interpretation can 
be given to the looped diagrams, and for the present I will reserve further discussion. 
§11. Correlation of Diverse Tests by the Load Extension Diagram. 
Load extension diagrams of the kind shown in this and former papers are likely to 
be useful to the engineer and metallurgist in the correlation of the many different 
tests now made to ascertain the quality of metals. 
For example, fig. 25 was taken from a test piece of material giving a low impact 
number. Its shape differs markedly from the shape of a normal diagram. It 
corresponds in fact with the shape of the curve found from overstrained material. 
Fig. 25 (overstrained mild steel). 
