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PROF. W. E. DALBY: RESEARCHES ON THE ELASTIC 
A diagram recording the elastic extension of mild steel is seen in fig. 3. 
This steel contains 0'156 per cent, of carbon. The extension scale of the diagram is 
defined by the distance between the two vertical lines seen in the diagram. This 
distance represents an extension of O'Ol inch. 
Proportionality between load and extension ceases at about 4'5 tons corresponding 
to a stress of 14'67 tons per sq. inch. Yield occurs at 6'7 tons which corresponds to 
Fig. 3 (mild steel). 
21 85 tons per sq. inch. The load drops away from yield to about 5'5 tons giving a 
stress of 18 tons per sq. inch. These stress are reckoned on the original area of the 
cross-section of the test piece. 
I he slope of the line from the origin to the limit of proportionality defines E, the 
modulus of elasticity. From the diagram its value is 13,300 tons per sq. inch. 
§ 4. Restoration of Perfect Elasticity after Overstrain. 
1 he term “overstrain” means that a metal has been loaded beyond its limit of 
proportionality. 
If the load is removed after a test piece has been strained beyond the limit of 
proportionality and then the piece is immediately re-tested, the record shows a curved 
line. 
Tt has no range of proportionality and no modulus of elasticity which can he 
identified with E. 
I he material still possesses elasticity because it shrinks as the load is removed, but 
the elasticity is imperfect in the sense that change of length is no longer proportional 
to change of load. 
But, if the metal is iron or mild steel, proportional elasticity is slowly recovered 
