Method for determining the irreversible strain limit of Nb3Sn wires

We define a rigorous and reliable method for determining the irreversible strain limit of Nb3Sn wires. The critical current (I-c) is measured as a function of applied longitudinal strain (epsilon), I-c(epsilon), at one magnetic field and a temperature of 4.0 K. The sample is loaded and partially unloaded at progressively higher strain levels to determine the irreversible strain limit, epsilon(irr), which is defined as the maximum loaded strain where I-c is still reversible. Our method uses a polynomial fit of the loaded I-c(epsilon) to derive the I-c residuals for the loaded and unloaded points that are analyzed to determine the limit of irreversibility. The effect of varying the amount of strain unloading is also studied. The possibility and problems of using the strain dependent n-value (which indicates the steepness of the electric field-current, E-I, curve) to determine epsilon(irr) are discussed. The method presented here to determine epsilon(irr) has proven to be repeatable for many types of commercial Nb3Sn wires. This method can also be more generally used to determine epsilon(irr) for any brittle low-temperature or high-temperature superconducting material.