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THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



Fig. 16 — Photoelastic picture of a polyethylene "wire" wrapped at 97°C 

 around a rectangular terminal and cooled off under the applied load. 



jewelers saw with a good deal of set to the teeth, no strains were intro- 

 duced in the sawing process. Using a jig with two parallel guides a 

 sample 0.040 inches thick was cut through the polyethylene wire. Tak- 

 ing one in the center of the wrap, a photoelastic picture was taken with 

 the result shown by Fig. 16 with an enlargement of one corner shown 

 by Fig. 17. 



The easiest parts to analyze are the strains in the two legs of the 

 sample since the strains are similar to those for a bent section. The long 

 leg which was twice as long as the short leg has its zero order fringe 

 along the inside edge. There are twelve lines to the outer edge which 

 corresponds to a tensile strain of 16 per cent with an average tensile 

 strain of about 8 per cent which is about the strain caused by the load- 

 ing weight. In the short leg the zero order fringe is the inside oval and 

 the strain is about 11 per cent compressive at the inner edge of the 

 segment and about 38 per cent tensile at the outer edge. These values 

 are consistent with the radius of curvature that the wire is bent around 

 for it can be shown that the strain in a wire of diameter d bent around 

 a cylinder of diameter D without tension is equal to 



S = 



2p 



D + d 



(12) 



where p is the radial distance measured from the center of the wire 

 outward, D the diameter of the cylinder and d the diameter of the wire. 

 If p is positive or the point is outside the center line, the strain is posi- 



