Effect of cyclic strains on electrical conductivity and work hardening of copper at 4.2K

Several decisions in the design of superconducting magnets depend upon the effects of strain cycling that will occur during the expected lifetime of the device. This report describes the effects of strain cycling in liquid helium on the electrical resistivity and the progressive work hardening in several grades of copper that may be used as the matrix in composite conductors or as the sole conductor in a normal magnet. It was found that the resistivity increase with number of constant strain cycles is relatively large after a few hundred cycles at strain amplitudes per cycle greater than 0.175%. The major parameters in determining the resistivity increase and the degree of work hardening in C101 grade copper are described. About one-half of the matrix copper resistivity induced by strain cycling can be annealed out by warming to ambient room temperatures, but work hardening is not reduced. Applications of the results to design considerations are discussed and the results are related to basic causes by comparing with other work where point defects are introduced in copper at cryogenic temperatures. (author)