07974The effect of temperature on the stress relaxation characteristics of a number of metals is reviewed. Stress-relaxation behavior is shown to be much more sensitive to change~ in temperature than to changes in the applied stress. It is shown that heat treatment will minimize stress relaxation by either reducing residual stress or by inducing strengthening mechanisms such as age hardening. In some alloy systems stress relaxation may proceed simultaneously with metallurgical processes which may affect the experimental results.

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The ability of pressure-type electrical connections to remain stable and reliable over long periods of time depends in large measure on the stress-relaxation characteristics of the conductor and the connector material. For applications such as the miniaturized solderless wrapped connection, traditionally made with copper No. C110 (ETP copper), the relaxation of the conductor is far greater than that which occurs in the terminal; hence, for this type of termination the behavior of the conductor material governs the stability of the termination. Miniaturization of telephone equipment and the associated increase in ambient operating temperatures requires improved conductor materials which, in addition to having adequate tensile strength, ductility, and high electrical conductivity, are able to resist stress relaxation at these higher temperatures. Based on these considerations the tensile stress-relaxation behavior of several 0.255-mm diameter (30 AWG) high-strength, high-conductivity copper and copper alloy conductors were studied in the temperature range 23°C to 149°C (73°F to 300°F) for a time period extending to 10,000 h. Stress-temperature-time relationships are shown to follow Arrhenius behavior, thus permitting a choice of realistic accelerated aging test requirements for connections made with these copper alloy conductors. Since high-speed production of these conductor materials possibly may require in-line strand annealing procedures, a comparison of the stress-relaxation behavior of commercially processed strand annealed samples with that of laboratory processed batch annealed material, as well as a comparison of the effects of gage size on stress relaxation [0.511-mm (24-AWG) diameter versus 0.255-mm (30-AWG) diameter wires] was included in this study. Since no significant size effects were observed, available stress-relaxation data on a number of 0.511-mm (24-AWG) diameter high-strength, high-conductivity copper and copper alloy conductor materials are also included for convenient reference.

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