Simulation of Fatigue in High-Temperature Superconductor using Findley criterion

Fatigue is the weakening of a material caused by cyclic loading that results in progressive and localized structural damage and the growth of cracks. Once a fatigue crack has been initiated, each loading cycle will grow the crack a small amount. To predict the fatigue life of a component, fatigue tests are carried out to measure the rate of crack growth by applying constant amplitude cyclic loading and averaging the measured growth of a crack over thousands of cycles.In the case of High-temperature superconducting (HTS) tapes such as repeated thermal cycles, periodic electromagnetic force, etc., affect the performance of superconductors. This may degrade the superconductor properties and in turn affects the performance of superconducting magnets and power systems. Therefore, it is important to understand the mechanical and electrical fatigue strengths or limits while designing superconducting devices.

High-cycle uniaxial fatigue tests are difficult and time-intensive, but these challenges must be faced to determine the electrical and mechanical fatigue limits at cryogenic temperatures. The fatigue tests will be terminated till there are any mechanical failures. In a practical fatigue loading test, it is difficult to test each sample with different maximum stress values of cables and their critical current variation with the number of cycles (minimum 1x105 cycles). This study focuses on developing Stress based models of fatigue (Findley criterion) using FEA software. The experiment results from the open literature support the predicted fatigue strength of the HTS tape. The fatigue effect is modelled for various stress ratios and applied forces. It is found that with increasing stress ratio, fatigue strength also increases. The results also showed that the Findley criterion is a more appropriate approach than the Matake criterion from the literature, especially when considering the variation in stress ratio. For REBCO tape, the endurance limit is 207.12 MPa. The findings of this study will help in assessing the rate of crack growth in HTS under different conditions for thousands of fatigue cycles which experimentally otherwise would be cumbersome and time taking. The data generated would help in designing HTS based superconducting cables and wires for the future.