Stress Systems in the Solderless Wrapped Connection and Their Permanence

Abstract

The solderless wrapped connection is initially held together by the hoop stress in the wire which enters the connection as a result of the tension put on the wire by the wrapping tool. Measurements made out to a time of 1.5 years at room temperature show that the tension has decreased to 70 per cent of the one day value (8000 lbs per square inch) in this period. Two methods of extrapolation are discussed, both of which indicate that at least half of the initial one day value will remain at the end of forty years at room temperature. Another set of stresses enters the connection as a function of time, namely the diffusion forces produced by diffusion of the tin plating into the brass terminal and copper wire. A number of experiments are discussed which show that the activation energy of diffusion is materially reduced by the shearing stresses in the connection. Measurements at two temperatures, which allow extrapolation to room temperature, indicate that at the end of two years the force required to strip the wire from the terminal has increased by 5 per cent over the initial value and that at the end of forty years the increase will be 20 per cent. Support for these conclusions is furnished by tests on actual connections that have been in the field for one year and ten months, which show an increase of 5 per cent in the stripping force even though the relaxed hoop stress is only 68 per cent of the initial value. The increase, which is due to diffusion forces, can be made higher by using zinc, cadmium or aluminum plating, and the fusion occurs in a shorter time.