SOLDERLESS WRAPPED CONNECTIONS — PART II 



579 



V = vT/D, 



(13) 



where D is the thickness of the layer, 77 the coefficient of viscosity and 

 T the shearing stress. Hence no matter how small the shearing stress, 

 one grain will move with respect to the other in a finite time. The amount 

 that the grains can move is limited by the necessity of making the grain 

 boundaries fit. According to Zener, the situation is analogous to the 

 case of a jigsaw puzzle in which the overall configuration possesses 

 rigidity in spite of the fact that no shearing stress exists between adja- 

 cent pieces. Zener has calculated that the ratio of the relaxed stress to 



1.2 



,o^.o 



0.8 



< 



I- 

 5 0.4 



0.2 



0.06 0.1 0.2 0.4 



1.0 2 4 6 10 20 40 60 100 200 400 

 TIME IN SECONDS (REFERRING TO 200° C ) 



1000 2000 4000 10,000 



Fig. 20 — Stress relaxation in aluminum at three temperatures for strains less 

 than 10-4 (after Ke). 



the initial stress is equal to 



i(7 



5(7) 



7+ (T 



5(r2 



(14) 



where a is the value of Poisson's ratio. For values of Poisson ratio of 

 from 0.25 to 0.5 this ratio Ues betw^een 0.595 and 0.76. 



Fig. 20 shows measurements of stress relaxation plotted against time 

 for aluminum for three different temperatures. These were obtained^ by 

 twisting an alimiinum wire through a definite angle and observing the 

 force required to hold it at this angle as a function of time and the 

 temperature of the wire. All the curves can be made to coincide by 

 multiplying the times by different factors. If we define the relaxation 

 time r as the time required to relax half of the variable component of 

 stress, i.e. H(l — 0.67) of the stress, this relaxation time fits an equa- 

 tion of the form 



r = Ke^^«^ (15) 



where K is a constant, H an activation energy, T the absolute tempera- 



