STRESS SYSTEMS IN THE SOLDERLESS WHAPPED CONNECTION 1099 



tion energy will appear higher for this range than for a temperature 

 range below the recrystallization temperatuiv. The agreement of the 

 activation energy for copper for the tAvo temjierature ranges shown by 

 Fig. 3, sliows that no transformation occurs from 25°C to 200°C and 

 hence we can extrapolate the relaxation to room temperature taken as 

 25°C, with the result shown by the solid line lal)elled 25°C/. ""Jliis agrees 

 well with the measured values and indicates a stress at the end of forty 

 3'ears equal to 5,200 pounds per square in. 



DIFFUSION STRESSES IN SOLDERLESS WRAPPED CONNECTIONS 



In addition to the hoop stresses, another set of stresses develops as a 

 function of time, namely, the diffusion stresses caused by the diffusion 

 of one part of the connection into the other. The first experiment that 

 showed the presence of these stresses was the stripping force tests of 

 Fig. 23 of the previous paper referred to in Footnote 3. These measure- 

 ments were carried out on connections which had been held at 175°C 

 for lengths of time up to ten days and it was found that the stripping 

 forces did not decrease with time. A more careful set with twenty con- 

 nections for each point have recently been run with the results shown 

 by Fig. 5, solid line labelled 175°C. From this curve, it is seen that the 

 stripping force decreases to 88 per cent of its initial value of 15.5 pounds 

 average for six turns of 24 gauge tinned wire and then increases to 120 

 per cent of the initial value at the end of ten days. Similar increases are 

 shown at 100°C over a longer period of time and recent tests of solder- 

 less wrapped connections that have been in the field for one year and 

 ten months show that the stripping force is about 5 per cent higher on 

 the average than it was when the connection was foraied. 



10-^ 10^ 10= 10° 



TIME IN SECONDS 



Fig. 4 — Relaxation curves for aluminum wire in solderless wrapped connec- 

 tions. 



