Thermal stress analysis and lifetime prediction of solder joints for superconducting tapes using the Coffin–Manson model

Since their invention, high-temperature superconductors (HTS) have been widely applied as current leads for superconducting coils because of the advantages they offer in terms of the operating temperature and cost, performance in the presence of a magnetic field, and thermal leakage prevention. Unfortunately, due to the existence of soldered parts, the mechanical stability of a HTS current lead cannot be the same as that of a conventional current lead manufactured from a non-superconducting metal. Attempts to overcome this problem have prompted research on the analysis of past experiences and the causes of accidents involving operating current leads. However, the effect of thermal stress on operating current leads is insufficiently understood and fundamental tools for the prediction and prevention of failure are not readily available. In this study, we examined whether the Coffin–Manson model could be applied to assess the mechanical fatigue behavior of the solder layer in an HTS current-lead-like structure. An HTS current-lead-like specimen containing In–3Ag solder joints was subjected to repeated thermal cycling between 77 and 273 K. A crack detectable by naked-eye inspection was first observed after 304 thermal cycles near the edge of the solder layer. The measured temperature profile was then applied to a three-dimensional elastoplastic finite-element model, which predicted a maximum accumulated equivalent plastic strain of approximately 0.0348 during the first complete thermal cycle in the same region. Using this strain value, the modified Coffin–Manson model estimated a solder-joint fatigue cycle number of 273 cycles. The FEM–Coffin–Manson estimate and the first crack detected by naked-eye inspection were of the same order of magnitude under the applied thermal-cycling condition.