Electro-mechanical properties of PIT Nb₃Sn wires under transverse stress: experimental results and FEM analysis

Abstract

Nowadays there is a great deal of interest in the scientific community in developing nextgeneration accelerator magnets based on high-J c Nb 3 Sn Rutherford cables. Inside a cable the wires are subjected to the combined effect of axial and transverse load. Since Nb 3 Sn is a strain sensitive material, electromechanical characterization of cables is essential for magnet design. Testing a full-size Rutherford cable is an extremely complex and involved task. For this reason special Walters springs have been developed at the University of Geneva to test single wires under longitudinal and transverse load. In this work we analyze three PIT wires under transverse compressive load. To better understand the experimental results, a finite element model was developed. This model enabled better understanding of the mechanical behavior of the three samples and investigation of the mechanisms that determine wire performance degradation upon loading.

Determination of the electromechanical limits of high-performance Nb3Sn Rutherford cables under transverse stress from a single-wire experiment

“…The Ic reduction has a reversible component, which is fully recovered when removing the load and is associated to the reversible reduction under stress of the upper critical field [18], and an irreversible and thus permanent component. Two phenomena work together to determine the irreversible reduction of the wire performance: cracks and plastic deformations [19]. First, Nb3Sn is a brittle intermetallic compound characterized by a strong propensity to fracture.…”

Design Optimization of Round Strands Made by Twisted Stacks of HTS Tapes

“…distribution of the von Mises stress in order to have a more obvious interpretation and comparison of the model's results (see [14] for the full details). The main idea of the approach is to build a cumulative area function (CAF) for the s-domain (9).…”

Transverse pressure effect on superconducting Nb3Sn Rutherford and ReBCO Roebel cables for accelerator magnets

“…More recently, at CERN [222] and at the University of Geneva [109] more detailed 2D FE modelling of a similar transverse loading situation essentially has led to the same qualitative conclusion, with taking into account the internal strand architecture. However, these detailed simulations also yield the strain distribution among the filaments and can be combined with the current-strain relation presented in Figure 3.1 to predict the reversible critical current reduction of the transversely stressed impregnated strands quantitively.…”

“…Several facilities were developed worldwide to study the effect of transverse pressure on the critical current of superconductors, for example the drag-free stress test apparatus at the University of Colorado [105], Walters springs at the University of Geneva [106][107][108][109] and Durham University [110], the Short Sample Test Facilities (SSTF) at Fermi National Accelerator Laboratory (FNAL) and Lawrence Berkeley National Laboratory (LBNL) [111][112][113], the straight conductor test set-up at the Sophia University [114][115][116][117][118][119], and the two-component strain apparatus at the University of Twente [120][121][122].…”

“…In the 2-wall version, non-impregnated wires, with cross sections of up to 4 mm 2 , are fixed in a one-turn Walters spring and pressed by two parallel stainless-steel walls [106][107][108]. The 4-wall version makes it possible to measure the critical current of a wire either with or without impregnation under transverse compressive stress [109] as shown in Figure 2.2 as well. The wire samples are constrained in a U-shaped groove at the bottom and pressed by an upper anvil that fits the groove.…”