Solderless wrapped connections: Part II — Necessary conditions for obtaining a permanent connection

In order to study the stresses and strains occurring in a solderless wrapped connection, a photoelastic technique using photoelastic bakelite and a photoplastic technique using polyethylene have been used. Polyethylene has a stress strain curve similar to a metal and can be used to investigate strains in the plastic region. Using these techniques, it is shown that the connection is held together by the hoop stress in the wrapping wire. In order to lock this in, a dissymmetry from a circular form has to occur. This may be in the direction of an oval shape or a square or rectangular shape. Sharp corners are preferred since a more definite contact area results. A number of rules are derived for constructing the most satisfactory solderless wrapped connection. It is shown that the connection between the wire and terminal is intimate enough to permit solid stale diffusion, but the strains are not high enough to cause cold welding of the connection. The life of the joint depends on the twin processes of stress relaxation and self diffusion. Stress relaxation occurs at a rate such that half the hoop stress is relaxed in 2500 years at room temperature. This loss of stress is compensated by the diffusion of one part of the joint into the other. Since the activation energies for stress relaxation and self diffusion are approximately equal for most metals, the two effects complement each other and produce a connection which should remain unchanged for limes in excess of forty years under any likely ambient conditions.