Fracture behavior under fatigue loading at room temperature and its influence on critical current of Nb–Ti/Cu composite wire

Abstract

Fracture behavior under fatigue loading at room temperature and its influence on critical superconducting current at 4.2 K were investigated for Nb–Ti/Cu multifilamentary composite wire with a copper ratio of 7.83 and an overall diameter of 0.504 mm in which 24 Nb–Ti filaments were embedded. There was a knee in the relation between the applied maximum stress and the number of cycles to failure ( S– N relations). The fracture mechanism below and above this knee was different. When the maximum stress in the fatigue test was high, extensive multiple necking of the Nb–Ti filament occurred, and this was responsible for the final fracture of the composite wire. On the other hand, when the maximum stress in the fatigue tests was low, the fatigue crack nucleated in the copper, which grew and caused the fracture of the Nb–Ti filaments. The critical current was insensitive to fatigue loading when the maximum stress was low. When the maximum stress was high, the critical current after fatigue loading was lower than that after static loading. This was due to severe multiple necking during fatigue loading. Extracted Nb–Ti filaments were also fatigued using a specially designed fatigue testing machine. The S– N curve of filaments agreed well with the stress component of the filament calculated from the S– N relation of the composite wire.