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THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



one can count the number of fringes from the "eye" (which is an iso- 

 tropic point) to the center of the specimen which in this case is eighteen 

 fringes. Using the stress constant, 88 pounds per square inch per fringe 

 per inch along the optic path, the compressive stress normal to the mid 

 line is 



18 X 88 

 0.95 



1670 pounds/sq in. 



and the hoop residual stress per wire is 

 1670 X 0.4 X 0.95 



2 X 19 



= 16.6 pounds 



(5) 



(6) 



which is 60 per cent of the winding stress. Hence, the change in shape 

 has not made any appreciable difference in the residual hoop stress. A 

 specimen 0.4" x 1.6" was also tried and this had a residual stress of 56 

 per cent of the winding stress. 



In order to determine the number of turns required to make a satis- 

 factory joint, measurements were made of the residual stresses as a 

 function of the number of turns with the results shown by Fig. 7. All of 

 these experiments were made with the same weight, 28 pounds, which 

 results in a stress of 14,300 pounds/square inch. Down to five turns, 

 about 50 per cent of the winding stress is maintained. The results are 

 consistent with assuming that the wire unwinds to the extent of two 

 corners on each end while 60 per cent of the winding stress is main- 

 tained in all the other turns. As seen from Fig. 8, this is what one might 

 expect, for when the constant tension is released, recovery will cause the 



14 



16 



18 



2 4 6 8 10 12 



NUMBER OF TURNS 



Fig. 7 — Residual hoop stress as a function of number of turns. 



