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THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



point of compression develops just below the point of contact between 

 the wire and terminal. The distribution of compressive and tensile 

 strains in the wire is shown pictorially by Fig. 18. The distribution of 

 strains in a metal wire can be considered to be quite similar except that 

 the tensile strain due to winding should be only 2 to 3 per cent as seen 

 from Fig. 4, while the strains due to bending may be even higher, for 

 as seen from Fig. 9 the ratio of B/d may be in the order of 1 and the 

 bending strains may be as high as 50 per cent. 



In order to specify the properties that the wire must have in making 

 a good solderless connection, experiments have been made on how much 

 recovery can be tolerated in the wire. Since it is difficult to get a series 

 of metal wdres having different amounts of recovery, the technique was 

 resorted to of heating polyethylene to a definite temperature, winding 

 under a load equal to half the yield stress at the winding temperature, 

 and cooUng to room temperature under the load. As shown by Fig. 15, 

 known recoveries can be obtained in this way. It was found that the 

 largest amount of recovery that could be tolerated to make a joint at 

 all was 20 per cent while a reasonable hoop stress was not obtained until 

 the recovery was less than 10 per cent. 



In summary, the necessary conditions that the wire should fulfill are: 

 1. Since strains of 50 per cent may be encountered in bending wires 

 around sharp corners, wires should be used which have a large difference 

 between the yield strain and the breaking strain. Copper, aluminum and 

 soft iron are materials of this class while phosphor bronze and music 

 wire are not as satisfactory. 



WIRE 



- TENSION 



- COMPRESSION 



Fig. 18 — Distribution of compressional and tensile stress in the wire of a 

 wrapped solderless connection. 



