109G THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1954 



year and one month the average hoop stress is 5,900 pounds per square 

 inch and it is decreasing at the rate of 240 pounds per square inch be- 

 tween the first and second year. In the previous paper, since the tran- 

 sient creep occurring during the first day is somewhat variable, the angle 

 of twist reached 24 hours after the connection had been made, was taken 

 as the initial 100 per cent value and the relaxation has been plotted 

 with respect to this. Fig. 2 shows this plot with data from Mason and 

 Osmer and a set of eight relaxing springs measured by MalUna and 

 McKettrick all plotted on the curve. At the end of 1.5 years, the ob- 

 served stress is 70 per cent of 8,000 pounds per square inch or 5,600 

 pounds per square inch on the average. 



The measurements out to 1.5 years by this method appear to be the 

 longest measurements of stress relaxation in annealed copper wire in the 



10= 10° 



TIME IN SECONDS 



Fig. 2 — ■ Relaxation of stress in tinned copper wire as a function of time and 

 temperature. 



literature. Since the wrapped solderless connection is expected to last 

 at least forty years, it is desirable to be able to extrapolate the stress 

 value out to forty years time. Several methods are possible. According 

 to theoretical treatment, stress relaxation proceeds as follows: 



o-o — 0- = — - loge (1 + i/h) 



(2) 



3 W. P. Mason and T. F. Osmer, Solderless Wrapped Connection, Part II — 

 Necessary Conditions for Obtaining a Permanent Connection, B. S. T. J., 32, 

 pp. 557-590, May 1953. 



■• See Progress in Metal Phjjsics, Volume IV, Pergamon Press, Limited, 1953 

 Chapter V, Theory of Dislocations by A. H. Cottrell, pp. 233 and 251 to 260 

 Kuhlmann, D. and Z. Physik, 24, p. 43, 1952; and Proc. Phys. Soc, 64, p. 64, 1951 

 and A. H. Cottrell and V. J. Ayetekin, Inst. Metals, 77, p. 389, 1952. The last 

 reference gives data for stress relaxation in zinc which shows that it obeys equa- 

 tions (2), (.3) and (4). 



