. The Bell System technical journal . e distance, D the diffusion constant and t the time,this observation shows that nucleation starts at a given point and pro-ceeds for a certain fraction of the contacting surface before fusionstrengths are observed. After the induction period, the fusion force— the force required to pull wire and block apart ■— increased at arapid rate for the cases of copper, tinned copper, and zinc wire on anickel silver block. In order to account for the effect of fusion due to the increase of con-tact area, the ratio of the fusion force to the contact area was determine
. The Bell System technical journal . e distance, D the diffusion constant and t the time,this observation shows that nucleation starts at a given point and pro-ceeds for a certain fraction of the contacting surface before fusionstrengths are observed. After the induction period, the fusion force— the force required to pull wire and block apart ■— increased at arapid rate for the cases of copper, tinned copper, and zinc wire on anickel silver block. In order to account for the effect of fusion due to the increase of con-tact area, the ratio of the fusion force to the contact area was determined,yielding a shearing stress. The area of contact could be easily measuredwith a microscope since a bright surface was produced hy the shearingprocess. The shearing stress, Fig. 8, for the case of tinned copper wireon nickel silver is shown to approach a limit of about 3,000 psi which isapproximate^ the limiting shearing stress for tin. Hence, it appearsthat tin diffuses into the copper wire and the nickel silver base. Further-. TIME IN SECONDS Fig. 7 — Diffusion forces for tinned and bare copper and zinc wire on nickelsilver base. STRESS SYSTEMS IX THE SOLDERLESS WRAPPKI) (OX \ KCTK )\ IKK^ IT) 9^5-< z<o - DoJ 2 go:I- D ± 10 in cc (O UJlU Q.(£ t- lo lO Q . is ^ } / r / ( 10 10= TIME IN SECONDS l06 Fig. 8 — Diffusion shearing strength for tinned copper wire on nickel silver at17o°C as a function of time. more, it appears that the fused bond will just withstand a shearing stressequal to the limiting shearing stress of tin. Metallurgical evidence forthis diffusion was presented in Fig. 19 of the paper referred to in Foot-note 4. In this figure, a tin plate layer between the copper and nickelsilver was diffused in the two metals in a time of about 400 hours at180°C. The new evidence indicates that this layer of tin has the shearstrength of bulk tin. The previously described data constitutes measurements of fusionforces separate from the complications of frictio
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