PROPERTIES AND THE PLASTIC EXTENSION OF METALS. 
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When the load is removed the steel shrinks unproportionally, for example from C 
to D, by an amount approximately equal to its previous proportional elastic extension 
and then stops at a dimension greater than its primitive dimension by an amount 
approximately equal to its plastic extension. 
The increase of size measured after the removal of the load is called the Permanent 
Set. OD is the permanent set produced by the first stretching of the test piece to C. 
From the diagram the permanent set measures 0'0052 inch and the unproportional 
elastic recovery measures O'Oll inch. 
When the load is re-applied the spot of light moves from D to E along a curved 
path. The extension is no longer proportional to the load. The metal is in a 
different elastic state. Stretching beyond the limit of proportionality has robbed 
the metal of its power of proportional extension and of perfect recovery after removal 
of load. It may be said to be in a state of imperfect elasticity , or alternatively it 
may be described as in a state of unproportional elasticity. 
The imperfect state is disclosed by the loop formed by the removal and re¬ 
application of the load. 
The diagram shows four loops. Each loop is slightly larger than the loop 
preceding it. 
The four loops shown were all recorded on a half plate inserted in the camera. 
A succession of plates was taken and the last plate is shown in fig. 6. 
Fig. 6 (overstrained nickel steel). 
This last plate shows that unproportional elastic shrinkage occurs right up to the 
load at which local contraction begins. The last loop is just seen on the plate. 
The last line seen curving up from the origin Q is the typical curve of overstrained 
material. The primitive gauge length of 5 inches had been stretched to 5’67 inches 
before the last plate, fig. 6, was taken. 
