Fracture Strength Distribution of Individual Nb3Sn Filaments

Brittle fracture of Nb3Sn filaments is one mechanism by which the current-carrying capacity of composite Nb3Sn wires is degraded. However, there are relatively little data in the literature on the intrinsic material fracture properties of Nb3Sn filaments, because the complex composite structure (matrix, secondary phases, and defects such as voids) acts as an integrated mechanical unit. In this study, we extracted individual Nb3Sn filaments from a fusion-style Nb3Sn composite wire and conducted tensile testing to determine the fracture strength distribution of the isolated filaments. The distribution is modeled using a Weibull function. The relative fracture propensity of fully reacted filaments versus those with unreacted Nb cores is compared. The presence of a Nb core reduces, on average, the fracture strength of a filament by 38% and the strain to failure by 29%. Understanding the fracture probability of Nb3Sn as a function of both stress and volume will allow strand and conductor modeling efforts to more accurately represent the relative contributions of fracture and other effects (such as plasticity) to irreversible current density degradation, and may assist wire manufacturers in assessing the mechanical impact of wire design changes.