Fracture behavior of filament in Nb3Sn strands with crack-bridging model

• The crack-bridging model is used to study the fracture behavior of filaments.
• Two different fracture modes are characterized by the number of bridging bronzes.
• Short twist pitch has better mechanical stability for the tensile loadings.
• The widths of bridging bronze and filament have different effects for the central crack and two collinear cracks.

The Nb3Sn strands which have high critical field are used in cable-in-conduit conductors (CICCs). The superconducting strands are twisted multistage and experience complex thermal and electromagnetic loadings. Due to their brittleness, the cracking of the Nb3Sn filaments will occur under mechanical loading. In this paper, based on the linear elastic fracture theory, we study the effects of tension loading on the fracture behavior of central crack firstly. The strain energy release rates for different twist pitches and cabling stages are presented. As the triplet is subjected to the uniaxial strain, the cracking probability will increase with the twist pitch. The crack number increases with the applied strain, and wider filament or bronze can lead to smaller crack number under the same applied strain. In addition, multistage cabling has better mechanical stability. Next, the two collinear crack problem is considered. The variations of microcrack number show that the wider bronze can provide more resistance for the propagating of the large cracks. We can conclude that the bronze plays an important role in improving the stability and strength.