制作超导磁体一般须要选用具有较高上临界磁场的超导材料，比如低温超导材料中的Nb3Sn，高温超导材料中的YBCO。但是Nb3Sn硬而脆，临界电流对应变敏感；YBCO材料与基带结合强度低，受到特定载荷时非常容易破坏超导材料从而破坏超导性。故超导线中的应力情况及超导线临界电流-应变依赖关系受到了研究者的广泛关注。本文完成了Nb3Sn低温超导线的内应力分析，揭示了超导线中内应力的形成机制。在室温下对经过热处理的青铜线进行了X射线衍射（XRD）应力分析，发现其平均轴向残余应力为-135.7 MPa，应力振荡约300 MPa。我们利用弹性理论分析了热处理过程中Nb3Sn生成时体积收缩导致的应力失配，解释了轴向压缩应力的成因。电子探针显微分析（EPMA）显示样品中元素非比例部份分布非均匀，浓度失配导致的应力涨落与XRD观察到的应力涨落水平一致。该工作表明体积变化和化学成份分布对超导线的应力状态有重要影响，二者与热失配共同作用形成超导线中的应力状态。提出了离散载荷-环芯法，并将其与数字图像相关方法相结合对超导线表面的残余应力进行了精细的分析，确认了该残余应力的高度非均匀性。提出了一种用于分析YBCO超导带材界面强度的新方法，称为弯曲-剥离法，并对弯曲-剥离法的原理、误差与适用性进行了详细讨论。在室温以及液氮温度下，利用该方法对YBCO超导带材的界面强度进行了直接测量。实验结果表明，室温下YBCO超导带材的平均界面强度为2.6 MPa，液氮环境中其平均界面强度达26.0 MPa以上。通过对剥离界面的扫描电子显微成像形貌观察和能量色散X射线光谱分析，确定了界面开裂位置。为考虑应力、应变对超导线超导电性的影响，对Godeke临界电流-应变依赖偏量标度关系进行了细致的讨论，提出了考虑超导线工艺过程初始应变相关参数的估算方法；应用Nb3Sn工艺过程数据，通过数值模拟估算了残余应力对超导线应变-临界电流敏感性的影响；完成了液氮环境下YBCO超导带材的弯曲-剥离实验，测量了带材中的超导电流，发现在YBCO带材的剥离过程中，超导相完整时超导电流对横向应力不敏感。
Superconductor magnets generally need superconductor materials with high upper critical magnetic field, such as low temperature superconductor Nb3Sn and high temperature superconductor YBCO. Nb3Sn is fragile and its critical current is sensitive to the strain. YBCO has poor interface strength between itself and the matrix in the YBCO tape and its critical current is degenerative under certain load. Therefore, stress statuses and critical current dependence on strain in the superconductor draw widespread concern.This paper analyzed the internal stress of Nb3Sn made from bronze method and explained its formation mechanism. X-ray diffraction (XRD) stress analysis of the heat-treated bronze wire at room temperature showed that the average axial residual stress was –135.7 MPa with high fluctuation of 300 MPa. We used elastic theory to study how the volume changed during heat treatment when Nb3Sn was formed and explained the origin of the wire's axial compressive stress. Electron probing material analysis showed that the non-proportional chemical composition distribution was highly uneven, and would contribute to the stress fluctuation. The magnitude of XRD observed stress fluctuation could match the evaluation from the chemical distribution. This indicated that volume change, chemical distribution and thermal mismatch worked together to influent the stress distribution within a superconductor strand. In addition, we proposed a discrete loading-ring method combined with core-image correlation to perform a detailed analysis of the local residual stress on the superconducting wire surface, confirming the highly non-uniformity of the stress field.A new method to analyze the interface strength of YBCO tape was designed, named as bending-peeling method. The principle, error and applicability of the bending-peeling method was discussed in detail. The interface strength of YBCO tape was measured at room temperature and liquid nitrogen temperature by this method. The average interfacial strength at room temperature was only 2.6 MPa, but the interfacial strength in the liquid nitrogen environment was greatly enhanced with an average stress over 26.0 MPa. Peeled interfaces were observed under scanning electron microscopy and analyzed with energy disperse spectroscopy.To studty the critical current dependence on strain in the superconductor, Godeke's scaling relation for the critical current density in Nb3Sn was discussed, and a method to obtain the parameters related to the residual stress within the superconductor strand was put forward. In this method, Nb3Sn strand processing schedule was simulated with finite element model and the resulting data were used to further estimate the effect of residual stresses on the strain-critical current sensitivity of Nb3Sn superconducting wires. In liquid nitrogen bending-peeling experiment, a steady direct current below 80% of the critical current was applied to the YBCO tape, while the current and voltage in the tape was observed. Experiment showed that superconducting current in YBCO tape was not sensitive to transverse tension until the YBCO was damaged together with the superconductivity.
