Solderless Wrapped Connections: Part II - Necessary Conditions for Obtaining a Permanent Connection

558 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 connections are properly made. Not all methods of wrapping or all types of terminals are equally satisfactory and it is the purpose of this paper to describe investigations that have been made to determine the necessary conditions for the best wrapped terminal. These investigations include a photoelastic investigation of the stresses in the terminal and a photoplastic investigation of the strains in the outside wrapping wire. A new photoplastic material, polyethylene, has been used which has a stress strain curve similar to a metal and a birefringence proportional to the strain. The use of this material makes possible the evaluation of strains in the plastic region and may find applications in other plastic flow problems such as the extrusion of metals. Even after such terminals have been satisfactorily made, there remains the question of whether they will have sufficient life to satisfy the requirements of the telephone plant. A design objective for most relays and other switching apparatus of the telephone plant is an uninterrupted trouble-free life of forty years. Hence, unless the connections are to be the limiting factor in the maintenance of the equipment, they also should have a minimum life of forty years under the conditions for which the apparatus is designed. In order to investigate the probable length of life of such connections, theoretical and experimental work on stress relaxation in metals has served as the basis for calculations and tests.