Axial and transverse stress–strain characterization of the EU dipole high current densityNb3Sn strand

Abstract

We have measured the critical current(Ic) of a highcurrent density Nb3Sn strand subjected to spatial periodic bending, periodic contact stress and uniaxial strain.The strand is destined for the cable-in-conduit conductors (CICC) of the European dipole(EDIPO) 12.5 T superconducting magnet test facility. The spatial periodic bending wasapplied on the strand, using the bending wavelengths from 5 to 10 mm with a peakbending strain of 1.5%, a periodic contact stress with a periodicity of 4.7 mm anda stress level exceeding 250 MPa. For the uniaxial strain characterization, thevoltage–current characteristics were measured with an applied axial strain from−0.9% to+0.3%, with a magnetic field from 6 to 14 T, temperature from 4.2 to 10 K and currents up toalmost 900 A. In addition the axial stiffness was determined by a tensile axial stress–straintest. The characterization of the strand is essential for understanding the behaviour of thestrand under mainly axial thermal stress variation during cool down and transverseelectromagnetic forces during charging, which is essential for the design of the CICC for thedipole magnet.The strand appears to be fully reversible in the compressive regime during the axialstrain testing, while in the tensile regime, the behaviour is already irreversiblydegraded when reaching the maximum in the critical current versus straincharacteristic. The degradation is accentuated by an immediate decrease of then value by a factor of 2. The parameters for the improved deviatoric strain description are derived fromthe Ic data, giving the accuracy of the scaling with a standard deviation of 4 A, which is by farwithin the expected deviation for the large scale strand production of such a highJc strand.The Ic versus the applied bending strain follows the low resistivity limit, indicative of fullinterfilament current transfer, while a strong decrease is observed at a peak bending strain of∼0.5%. For the periodic contact stress it appears that beyond 60 MPa, theIc becomes noticeablyirreversible. The n value, being a better indicator for the irreversible behaviour than theIc, seems to indicate a somewhat lower level of the transverse stress irreversibility. Since thestrand appears to be quite sensitive to the tensile strain, for the EDIPO CICC design,the tensile intrinsic axial strain in the filamentary region should stay well belowzero (only compressive) and the contact stress sufficiently below 50 MPa. Theserequirements are feasible, in particular for a CICC design with steel conduit which wouldprovide sufficient thermal compression, a sufficiently low void fraction and a cablingpattern that limits the peak bending strain during transverse load to about 0.3%.