Date of Award

Spring 1972

Document Type

Thesis

Degree Name

Master of Science in Mechanical Engineering - (M.S.)

Department

Mechanical Engineering

First Advisor

Harry Herman

Second Advisor

Michael Pappas

Third Advisor

Theodore A. Schneider

Abstract

Creep and stress-relaxation experiments were performed on silverless and silver bearing copper in the temperature range 73F to 250F. The creep experiments were conducted at constant load and temperature, the stress-relaxation tests at constant total strain and temperature for a total time period of 1000 hours. The effect of silver addition of up to 0.2 percent w/o (60oz per ton) on the creep and stress-relaxation behavior of spectrographically pure copper (99.999+ Cu) as well as on that of tough pitch and oxygen free coppers of lesser purity was studied. Spectrographically pure copper was found to be much more susceptible to creep deformation than commercial copper containing small amounts of impurities. The high purity copper appears to fail by separation of the grain boundaries with incipient microcrack formation. These microcracks act as stress concentrations, thus accelerating the creep rate. Oxide particles, present in tough pitch copper, act as stress raisers and cause this material to have a lower resistance to creep and stress relaxation than oxygen free copper containing about the same level of other impurities (with the exception of oxygen). The addition of silver to either of these two conductor materials raises the recrystallization temperature and therefore results in material having a much finer and more uniform grain structure. This results in improved resistance to creep and stress relaxation and inhibits the formation of microcracks by grain boundary separation. Creep and stress-relaxation data are presented at two stress levels and three temperatures for each of the materials studied, that will permit designers to formulate appropriate time, temperature, stress-strain relations.

Silver addition in excess of 25oz per ton (0.09 w/o%) produces further improvement in resistance to creep and stress relaxation. Sufficient strengthening is achieved, however, with the 25oz per ton addition in the temperature range of interest (73F to 250F) to justify the use of this material as a suitable and economical replacement for tough pitch copper in miniaturized applications where long time dimensional stability is important.

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