Axial and transverse stress–strain characterization of the EU dipole high current density Nb3Sn strand
Axial and transverse stress–strain characterization of the EU dipole high current density Nb3Sn strand
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DOI：
10.1088/0953-2048/21/6/065001
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发表时间：
2008-06
影响因子：
3.6
通讯作者：
A. Nijhuis;Y. Ilyin;W. Abbas
中科院分区：
文献类型：
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作者：
A. Nijhuis;Y. Ilyin;W. Abbas
本文测量了高电流密度Nb_3Sn超导线材在空间周期弯曲、周期接触应力和单轴应变作用下的临界电流Ic。该股线将用于欧洲偶极（EDIPO）12.5 T超导磁体测试设施的电缆导管导体（CICC）。在股线上施加空间周期性弯曲，使用5至10 mm的弯曲波长，峰值弯曲应变为1.5%，周期性接触应力为4.7 mm，应力水平超过250 MPa。对于单轴应变表征，电压-电流特性是在施加的轴向应变为-0.9%至+0.3%，磁场为6至14 T，温度为4.2至10 K，电流高达近900 A的情况下测量的。此外，通过拉伸轴向应力-应变测试确定轴向刚度。股线的表征对于理解股线在冷却期间的主要轴向热应力变化和充电期间的横向电磁力下的行为是必不可少的，这对于偶极磁体的CICC的设计是必不可少的。该股似乎是完全可逆的压缩制度在轴向应变测试过程中，而在拉伸制度，行为已经不可逆地退化时，达到最大值的临界电流与应变特性。通过将n值立即降低2倍来加剧退化。用于改进的偏应变描述的参数是从Ic数据导出的，给出了标度的准确性，标准偏差为4 A，这远远在这种高Jc股线的大规模股线生产的预期偏差内。Ic与所施加的弯曲应变的关系遵循低电阻率极限，表明完全丝间电流转移，而在峰值弯曲应变为1.05%时观察到强烈的降低。对于周期性接触应力，似乎超过60 MPa，Ic变得明显不可逆。n值是比Ic更好的不可逆行为的指标，似乎表明横向应力不可逆性的水平略低。由于钢绞线似乎对拉伸应变非常敏感，因此对于EDIPO CICC设计，在摩擦区域中的拉伸固有轴向应变应保持远低于零（仅压缩），接触应力应充分低于50 MPa。这些要求是可行的，特别是对于具有钢导管的CICC设计，其将提供足够的热压缩、足够低的空隙率和将横向载荷期间的峰值弯曲应变限制到约0.3%的布线图案。
We have measured the critical current (Ic) 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 Ic 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 Jc strand. The Ic 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 ∼0.5%. For the periodic contact stress it appears that beyond 60 MPa, the Ic becomes noticeably irreversible. The n value, being a better indicator for the irreversible behaviour than the Ic, 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%.