Invariant temperature and field strain functions for Nb3Sn composite superconductors

The observed strain dependence of Nb₃Sn over an extended range of applied strain requires revision of the functions used previously to represent that strain dependence. The concept and various definitions of the strain function for the upper critical field and the critical temperature are reviewed. An invariant strain function for the critical temperature is derived from the strain energy potential of the general invariant strain analysis. The invariant strain function is applied to a study of uniaxial longitudinal applied strain on wire and tape conductors. Uniaxial forms of the general strain function are derived, and parameter values of the strain function are determined from a fit of the uniaxial forms to published uniaxial strain data for the critical temperature. The concept of precompression is examined in the context of the strain function. The invariant strain function, as an analytic function, is compared with numerical calculations of the general invariant strain analysis for longitudinal and transverse applied strain. The correspondence between the strain dependence of the critical temperature and the upper critical field is discussed, and a form of the invariant strain function for the upper critical field is presented.