Axial and transverse stress-strain characterization of the EU dipole high current density Nb3Sn strand

摘要

We have measured the critical current (I-c) of a high current 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 was applied on the strand, using the bending wavelengths from 5 to 10 mm with a peak bending strain of 1.5%, a periodic contact stress with a periodicity of 4.7 mm and a stress level exceeding 250 MPa. For the uniaxial strain characterization, the voltage-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 to almost 900 A. In addition the axial stiffness was determined by a tensile axial stress-strain test. The characterization of the strand is essential for understanding the behaviour of the strand under mainly axial thermal stress variation during cool down and transverse electromagnetic forces during charging, which is essential for the design of the CICC for the dipole magnet. The strand appears to be fully reversible in the compressive regime during the axial strain testing, while in the tensile regime, the behaviour is already irreversibly degraded when reaching the maximum in the critical current versus strain characteristic. The degradation is accentuated by an immediate decrease of the n value by a factor of 2. The parameters for the improved deviatoric strain description are derived from the I-c data, giving the accuracy of the scaling with a standard deviation of 4 A, which is by far within the expected deviation for the large scale strand production of such a high J(c) strand. The I-c versus the applied bending strain follows the low resistivity limit, indicative of full interfilament current transfer, while a strong decrease is observed at a peak bending strain of similar to 0.5%. For the periodic contact stress it appears that beyond 60 MPa, the I-c becomes noticeably irreversible. The n value, being a better indicator for the irreversible behaviour than the I-c, seems to indicate a somewhat lower level of the transverse stress irreversibility. Since the strand 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 below zero (only compressive) and the contact stress sufficiently below 50 MPa. These requirements are feasible, in particular for a CICC design with steel conduit which would provide sufficient thermal compression, a sufficiently low void fraction and a cabling pattern that limits the peak bending strain during transverse load to about 0.3%.