Nb3Sn超导复合股线临界电流与纤维损伤耦合作用的研究

Nb3Sn superconducting composite strands composed of superconducting filaments and a multilayered metal matrix have excellent superconducting properties so have important applications in ITER magnetic system and other high magnetic field engineering. Because Nb3Sn is a typical brittle material, the damage of the superconducting filaments in strands are easily produced, and the critical current of strands is very sensitive to the strain and stress states of strands. Under the complicated thermal-electrical-magnetic-hydraulic service conditions, the critical current and filament damage of Nb3Sn strands possess a complicated electro-mechanical coupling mechanism. Thus, to predict the effective properties of strands, it is very important to accurately reveal and depict the coupling between the critical current and filament damage of Nb3Sn strands. In the proposed program, the initiation and evaluation model of filament damage under different strain modes for various routine produced Nb3Sn strands will be developed; on this basis, to different types of cracks and different crack distributions, the superconducting current redistribution at crack and the current transfer between filaments will be analyzed and simulated quantitatively; simultaneously, considering the change of the stress state of strands due to the degradation of the critical current, so its effect on filament damage evaluation will be analyzed; hence, the coupling between the critical current and filament damage of Nb3Sn strands will be described accurately and effectively. The validation of the model and method will be performed by the comparision with the previous experimental results.

Nb3Sn超导股线是超导纤维与多层金属基底构成的复合线材，具有优良的超导性能，在ITER 磁体系统及其它强磁场工程中有着重要应用。由于Nb3Sn是典型的脆性材料，股线中的超导纤维极易发生损伤，其临界电流对其所处的应力应变状态极为敏感，在复杂的热-电-磁-液压服役条件下，Nb3Sn超导股线临界电流和纤维损伤之间具有复杂的力-电耦合机制，正确揭示和刻画两者之间的耦合作用是预测股线有效性能的关键。本项目拟针对不同方法制备的Nb3Sn超导股线，建立不同应变模式下纤维损伤的形成和演化模型；在此基础上，对于不同类型的裂纹和裂纹分布，实现裂纹处超导电流重新分布以及纤维间电流重新传递机制的定量分析和模拟；同时，考虑临界电流退化导致的股线所处应力状态的改变，进而分析其对纤维损伤演化发展的影响；从而准确和有效刻画Nb3Sn超导股线临界电流与纤维损伤的耦合作用，并与已有实验结果对比以验证模型和分析方法的有效性。

Nb3Sn 超导复合股线不但具有良好的超导性能而且又能承受一定的荷载作用，在国际热核聚变实验堆（International Thermonuclear Experimental Reaction，简称ITER）、大型强子对撞机（Large Hadron Collider）的磁体系统和其它强磁场工程中有着重要的应用。项目组针对复杂非线性问题的理论建模和定量模拟这一科学问题和研究目标，围绕Nb3Sn超导复合股线临界电流密度与晶粒尺寸、纤维损伤以及临界电流密度与超导各向异性的关系，Nb3Sn超导复合股线的统计强度与纤维初始损伤和演化的关系，摩擦系数和弹簧刚度系数对CICC中Nb3Sn超导股线横向压缩行为的影响，裂纹对单丝Nb3Sn超导复合材料裂纹电流输运特性的影响，弹性梁在简谐移动荷载下振动临界速度以及弹性体振动的径向积分边界元法等相关课题进行了系列研究工作。研究表明纳米多晶Nb3Sn超导材料的有效临界电流密度不仅与晶粒尺寸有关，而且还强烈地依赖于透射率的大小；当透射率小于一定值时，存在临界电流密度的最佳晶粒尺寸。建立了描述层状各向异性超导体中由随机点缺陷导致的各向异性磁通钉扎和临界电流密度的解析模型，所建模型有助于更好地理解各向异性超导体相关实验中临界电流密度各向异性的标度行为。研究发现纤维初始损伤和Weibull模量对Nb3Sn复合股线拉伸统计平均强度和失效概率函数的分布有着显著影响；得到了CICC中的周期性弯曲的载荷因子，便于评估股线的周期性弯曲强度。研究指出CICC中股线间的摩擦系数和互相接触时的弹簧刚度系数将对CICC中电缆体横向压缩力学行为有着重要的影响。研究建立了单根纤维复合股线中全横向裂纹处超导电流重新分布的有限元模型，讨论了裂纹处电流重新分布特征。项目针对简谐移动荷载作用下Bernoulli-Euler和Timoshenko弹性梁在Winkler弹性地基上的振动临界速度进行了分析和讨论。研究还表明径向积分边界元法对于弹性动力问题非常有效和适用。通过本项目的开展，丰富和发展了相关科学问题的研究，为超导材料在工程中的运用提供了有益指导和参考。