1094 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1954 



connection is initially held together by the hoop stress in the wire which 

 enters the joint as a result of the tension put on the wire by the wrap- 

 ping tool. As time goes on, another system of stresses are generated, 

 namely, the diffusion stresses caused by the diffusion of one part of the 

 connection into the other which eventually ehminate the surface between 

 the wire and the terminal and in effect join the two together. 



It is the principal purpose of this paper to describe a number of ex- 

 periments which show how these stresses develop, how large they are 

 and how they can be increased by substitution of a different type of 

 plating between the terminal and the wire. A comparison of the strength 

 of a joint formed by a tin plated copper ware and a bare copper wire 

 both wound on nickel silver or brass terminals shows that the diffusion 

 forces develop more quickly when the wire is tin plated than when it is 

 bare. Measurements made at different temperatures show that the 

 activation energy of diffusion is decreased in proportion to the hoop 

 stress in the wire indicating that the shearing stress at the contact sur- 

 faces aids diffusion. This activation energy is considerably less for tin 

 than for copper. The diffusion joint has a strength per unit area equal 

 to the limiting shearing stress of the tin platmg. This is in the order of 

 3,000 pounds per square inch for tin but is considerably higher for other 

 types of plating such as zinc, aluminum or cadmium. ^Measurements of 

 the stripping force of connections made with bare and tinned copper 

 wire on zinc, aluminum or cadmium plated terminals show that the 

 stripping force increases by a factor of two as a function of time, and 

 the time required for the diffusion forces to operate is considerably less 

 with these types of plating. 



The combination of relaxation and diffusion stresses that are discussed 

 later show that as the mechanical strength due to the hoop stress de- 

 creases, the strength due to diffusion increases and at the end of forty 

 j^ears the standard tin plated wire on nickel silver or brass terminals 

 will be at least 20 per cent stronger than it is initially. The extensive 

 corrosion tests described in Footnote 1 , taken together with the mechan- 

 ical strength tests described here, show that the standard connection 

 should not fail in the forty year period under consideration. 



RELAXATION OF HOOP STRESS AS A FUNCTION OF TIME 



The rate of relaxation of the hoop stress in the wrapping wire is an 

 important quantity for the stability of the connections. This has been 

 studied by wrapping 24 gauge wire with a constant tension of three 

 pounds around a spring steel terminal 0.0124 inches thick and 0.062 

 inches wide. As shown by Figure 1 1 of Mallina's paper,^ this causes the 



