STRESS SYSTEMS IN THE SOLDERLESS "VVI! APPED CONNECTION 1095 



terminal to twist through an angle of about 25° wIumi 100 (urns are 

 wrapped around the terminal. This twist is the result of a tor(iu(> wWn-h 

 is caused by the fact that the wire has a hoop stress and the wire does 

 not come back on itself but advances by the thickness of the wire for 

 each turn. As shown previously,^ the total torc^ue is equal to 



Torque = (W.F.) ( -^ ) L 

 .a + 6, 



(1) 



where (W.F.) is the wrapping force, 2a and 2h the thickness and width 

 of th(> tei-minal and L the total length of the wrapped section. Hence, by 

 calibrating the spring constant, the average hoop stress can be evaluated. 

 In this manner, Mallina^ has found that after transient creep (defined 



1.1 



Ul 



in 1.0 



UJ 



a: 



<n!o 0.9 



o^ 

 UJ t 

 xt^O.8 



_J _i 



UJ < « -. 



a p 0.7 



°-0.6 



< 0.5 



10'' 10° 



TIME IN SECONDS 



Fig. 1 — Relaxation of stress in copper solderless wrapped connection at room 

 temperature. 



in this paper as the loss in stress during one day) has occurred, the stress 

 level for the standard 24 gauge connection is about 8,000 pounds per 

 square inch. 



This same technique can be used to measure the loss in hoop stress as 

 a function of time for all one has to do is to put a pointer on the spring 

 and observe the rate the angle decreases. It will be noted that this 

 system reproduces all of the stress systems and strain hardening that 

 occur in the wrapped solderless connection, and hence a measurement 

 of the relaxed angle is the most representative measurement of the 

 relaxation of the solderless wrapped connection. Fig. 1 shows an average 

 of the results of four such springs from the time of their intial formation 

 out to a period of one year and one month. The rate of relaxation is 

 quite rapid during the first day (transient creep) but at the end of one 



2 R. F. Mallina, Solderless Wrapped Connections, Part I — Structure and 

 Tools, B. S. T. J., pp. 525-555, 32, May 1953. 



