542 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



8500 PSI 



175° C 



4000 PS 



y\ 



2 3 4 5 6 7 



IME IN DAYS AT 20 DEGREES C 



8 



0.5 1.0 1.5 2.0 2.5 3.0 



TIME IN HOURS AT 175 DEGREES C 



Fig. 19 — Stress relaxation in wire. 



concentration is in the center of the contacting area. From this point 

 to the periphery there is a pressure gradient which is similar to that of 

 a circular compressed thin film of viscous material. At the boundary line 

 the pressure is zero. The average pressure within the contact area is 

 about 29,000 P.S.I. but the maximum stress in the center of the contact 

 area may be as high as 100,000 P.S.I. The relaxation, that takes place 

 in eight days at room temperature (Fig. 13), is assumed to be due to 

 the very high initial stress in the center of the contact area which seeks 

 equalization. 



1.20 



1.00 



S 0.80 



u 



a. 



uu 



a 



^ 0.60 



111 



a. 0.40 



u 



0.20 



14 16 18 20 

 TIME IN MONTHS 



Z2 24 26 28 30 32 34 



Fig. 20 — Creep curves of annealed copper for various stresses. (Courtesy of 

 Chase Brass and Copper Co., Waterbury, Conn.). 



I 



