Long-term creep of copper at cryogenic temperatures

Abstract

The equipment designed and used in this study basically consists of a constant load lever arm creep machine, a vacuum thermally isolated cryostat that is cooled by a low rate of cryogen flow, and a testing to transmit the load onto the specimen. Briefly, Figure i shows the tensile testing insert and the cryostat schematically. The tensile testing insert consists of the pull rod, the compression tube, and the specimen grips. The temperature of the specimen can be controlled with an accuracy of +l°K. The strain of the specimen was measured directly using a strain gage technique, which is capable of resolving +1 uc on the specimen. Typical creep results at 77K and 34.5 MPa initial applied stress are given in Figure 2. There is no indication of the exhaustion of creep strain when the test was terminated at i00 hours. Indeed, a steady-state creep behavior rather than a logarithmic creep claimed in another study of copper (8) was observed. The derivative of the creep curve, i.e., strain rate versus time, is also plotted in Figure 2. The onset of the steady-state region, as shown by the minimum creep rate, was determined to occur at 75 hours. The steady-state creep rate was 4 x 10 -11 sec -I. From these results, it can be concluded that long-term creep testing (about 100 hours) is necessary to study the creep behavior of materials at very low temperatures. With the current ability of conducting long-term cryogenic creep tests, more creep data will be generated to systematically investigate the long-term low temperature creep behavior of OFHC copper and other materials at the 77 K range and at 4.2K. Acknowledgments The authors wish to thank the International Copper Research Association (INCRA) and Dr. L.M. Schetky, the Technical Director, for supporting this program. We also are grate