572 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



1000 



6 7 8 9 10 11 



STRAIN IN PER CENT 



14 15 16 



Fig. 14 — Stress and birefringence strain characteristic of quenched poly- 

 ethylene. 



mercury line. This relative retardation R/i is related to the number of 

 fringes A^ and the wavelength X by the equation 



t 



(11) 



and hence for a 1 per cent strain there are 7.3 fringes for a 1-cm path 

 length in the optic direction. It will be observed that for both increasing 

 and decreasing strains, the birefringence is directly proportional to the 

 strain. Hence, by using polyethylene it appeared possible to measure 

 the strain even in the plastic region. 



The first experiment tried was to wrap a square metal rod with a 

 polyethylene ''wire" one-sixteenth inch in diameter with a wrapping 

 stress about half the yield stress. It was found, however, that the wire 

 sprang off the metal rod when the constant stress was released. This 

 is due to the fact that the polyethylene has considerably more re- 

 covery than the metal wire and the dissymmetry is not sufficient to 

 lock in the bending stress. This shows that one of the requirements of 

 the wire is that the recovery shall not be too large. 



If we are to use polyethylene as a photoplastic material, it is neces- 

 sary to simulate the unloading curve as well as the loading curve. This 

 can be done by heating up the polyethylene when it is wound under a 



