562 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



2200 



2000 



1800 



1600 



1400 



1200 



1000 



800 



600 



400 



200 



1 2 3 4 5 6 7 8 9 10 11 12 13 14 



DISTANCE ACROSS PICTURE IN CENTIMETERS 



Fig. 5 — Stresses along X and Y directions for a square terminal. 



or 71 per cent of the winding stress applied to the wire. The rest of the 

 stress is lost in the contraction that occurs when the wire causes plastic 

 flow to occur at the corners of the specimen. This plastic flow causes 

 the terminal (both bakelite and metal) to flow around the wire and pro- 

 vides an air tight joint which is an essential requirement for a good 

 solderless wrapped connection. That the stress in the terminal is high 

 enough to do this can be seen directly from the photoelastic picture, 

 Fig. 3. Counting the lines as far as they can be distinguished by a mi- 

 croscope, there are 72 lines from the eyes of the picture (which are iso- 

 tropic points) up to 0.0525 inches from the geometrical corner lines. 

 Using the stress constant of photoelastic bakelite which is 88 pounds 

 per square inch per fringe per inch length along the optic path, this cor- 

 responds to a stress per unit area of 



!r = 



72 X 88 

 .95 



= 6700 pounds per sq in. 



(3) 



Since the total force put on by the wire is supported by successively 

 smaller cross sections, as one approaches the corners the yield stress of 

 bakehte of 15,000 pounds per square inch will be attained at a radius of 



