Finite element simulations of elasto-plastic processes in Nb 3Sn strands

Abstract

The manufacturing of Nb 3Sn strands, with drawing and annealing of multifilamentary strands followed by a heat treatment at about 900 K to form the Nb 3Sn by reaction of tin and niobium, has the potential to create a complex internal stress system. The strain sensitivity of the Nb 3Sn superconducting properties makes prediction of the internal stresses a necessary step to understanding the performance of Nb 3Sn 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 Nb 3Sn 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 Nb 3Sn stresses.