Stress Systems in the Solderless Wrapped Connection and Their Permanence

As discussed in a series of papers, 1 the solderless wrapped connection is an efficient and inexpensive method of connecting a wire to a terminal. All the tests made so far indicate that it is mechanically sound and sufficiently free from the effects of corrosion to have a trouble free life of at least forty years. Photoelastic and stress studies show that the 1 The Solderless Wrapped Connection, B. S. T. J., 32, May 1953. 1093 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 wrapping 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 eliminate 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 experiments 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 wire 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 surfaces aids diffusion.