1098 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1954 



40 

 35 

 30 

 25 

 20 

 15 

 10 



01 2345678 



HOOP STRESS IN THOUSANDS OF POUNDS PER SQ IN. 



Fig. 3 — Activation energj' for stress relaxation as a function of hoop stress. 



Measurements of the rate of stress relaxation for tinned copper wire 

 at 200°C, 175°C, 150°C, and 100°C are shown by Fig. 2, and the activa- 

 tion energies plotted against average hoop stress are shown b}^ Fig. 3. 

 Values are given using 150°C to 200°C as the temperature range and 

 100° to 175° also. Both ranges give the same activation energies within 

 the experimental errors and show that down to about 0.4 relaxation the 

 activation energies satisfy an equation of the type 



H'= H - ^a 



(8) 



The curvature exhibited by the relaxation versus log t shown for all 

 temperatures indicates that the activation energy must increase faster 

 than (8) for low values of relaxation and the dotted line shows a hj'po- 

 thetical curve ending up at the self diffusion activation energy for cop- 

 per, 57 kilocalories per mole. 



To apply this method in general, one has to take account of anj^ trans- 

 formation such as recrystallization in the temperature range of measure- 

 ment. For example, Fig. 4, shows similar curves for aluminum wire. 

 Recrystallization in aluminum is known to occur at temperatures above 

 150°C and this change is shown in the relaxation measurements by the 

 lower values of relaxation that occur for long times. If one takes values 

 of time above and below the recrystallization temperature, the activa- 



