Abstract :
The manufacturing of Nb3Sn strands, with drawing and annealing of multifilamentary strands followed by a heat treatment at about 900 K to form the Nb3Sn by reaction of tin and niobium, has the potential to create a complex internal stress system. The strain sensitivity of the Nb3Sn superconducting properties makes prediction of the internal stresses a necessary step to understanding the performance of Nb3Sn conductors under the magnetic load conditions experienced in a coil. An elasto-plastic one dimensional finite element model, including temperature dependent stress–strain curves, annealing and manufacturing process stresses, is used to derive the internal stresses of Nb3Sn strands. The model is benchmarked against a range of experimental data, including stress–strain tensile tests, superconducting critical current–strain tests, and length changes through heat treatment and through a 4 K thermal cycle. The model can predict all the experimental features and shows a number of unexpected conclusions regarding the origin of the Nb3Sn stresses.
