残余应力对YBCO超导薄膜基底结构中裂纹扩展的影响

The influence of residual stress on the fracture mechanical properties under extreme conditions, such as low temperature and high magnetic field have become an important subject for the study in strong current application of YBCO superconducting film tapes. The effect of electromagnetic force induced by flux pinning on fracture mechanics of bulk superconductor and the measurement and calculation of residual stress have been studied. But the effect of residual stress on crack growth in superconducting films has not been studied. Therefore, the project firstly plans to test and analyze the distribution of residual stress in the buffer layer using nanoindentation technique and a new designed stress-applying device. The lattice constant in the superconducting layer is obtained by skew symmetric diffraction method, and then the residual stress is calculated. Secondly, the residual stress field is introduced into the crack model by MPC and isoparametric inverse mapping method. The redistribution behavior of residual stress field in the process of surface crack growth is determined, and the influence of residual stress field on growth is considered in different excitation processes. Finally, the residual stress is introduced into the calculation model of inter-facial fracture toughness, and the influence of residual stress on the interfacial crack growth behavior and the energy release rate of inter-facial crack growth is considered in detail. The purpose of this project is to reveal the effect of residual stress on crack growth in the superconducting film, and to provide a reference for improving the fracture mechanical properties and optimizing the fabrication process for YBCO superconducting tapes.

低温强磁场等极端条件下残余应力对YBCO超导薄膜基底结构断裂力学性能的影响成为研究高温超导带材强电应用的重要课题。已有工作研究了磁通钉扎诱导的电磁力对超导块材断裂力学的影响和残余应力本身的测量和计算，但尚未涉及超导薄膜中残余应力对裂纹扩展的影响。因此本项目首先拟结合纳米压痕技术及新型等双轴残余应力施加装置测试并分析缓冲层中残余应力的分布，然后采用斜对称衍射方法获得超导层面内晶格常数并计算出残余应力；其次，采用MPC及等参逆映射方法将残余应力场导入裂纹模型中，明确残余应力场在表面裂纹扩展过程中的重分布行为，考虑不同励磁过程中残余应力场对表面裂纹扩展的影响；最后，将残余应力引入界面断裂韧性的计算模型中，重点考虑残余应力对界面裂纹扩展行为和界面裂纹扩展能量释放率的影响；本项目的实施旨在揭示残余应力对超导薄膜基底结构中裂纹扩展的影响，为提高YBCO超导带材断裂力学性能、优化/简化制备工艺提供参考。

超导磁体科学发源于超导物理、超导材料和低温技术，是基于非理想II型超导体形成的块材、带材和薄膜在诸如低温、强磁场及微重力等极端环境条件下研究强磁场的产生、能量转换及应用的交叉学科。由于高温超导体多为脆性陶瓷材料且在制备过程中不可避免会存在裂纹、孔洞、位错等缺陷，这使得超导材料在残余应力等作用下产生的应力集中更容易引起裂纹扩展并导致构件失效。为了对高温超导磁体的服役性能有更清晰的认识，设计出性能更优的超导材料，有必要深入研究超导材料与结构的力学行为和断裂特性。高承载电流能力是制约YBCO超导带材实用化的瓶颈难题。针对低温强磁场条件下提高超导薄膜电流承载能力的要求，需要增加薄膜厚度。然而，对于各种方法生长的YBCO薄膜基本上都有一个趋势：随着薄膜厚度的增加，不但临界电流密度会逐渐降低，而且由于薄膜与基底在微观结构、晶格常数及热力学参数上的失配从而导致薄膜在制备过程中不可避免产生残余应力，其中残余张应力能够导致薄膜表面出现孔洞和裂纹，而残余压应力则使得薄膜与基底的界面分层，严重影响其力学性能和使用寿命。因此低温强磁场极端条件下残余应力对YBCO薄膜力学性能的影响成为该研究领域中一个重要课题。.针对高温超导强电应用中容易导致YBCO超导薄膜断裂失效的残余应力，本项目分别对缓冲层和超导层的残余应力进行测量来准确评估其中的初始残余应力，结合实验数据和理论模型，分别将残余应力场导入界/表面裂纹模型中，分析并验证残余应力对界/表面裂纹扩展行为的影响规律，根据不同励磁过程中磁致伸缩-裂纹扩展的相互影响，将其从块材推广并应用于超导薄膜，初步揭示残余应力对超导薄膜基底结构中界/表面裂纹的影响，为增加对低温强磁场条件下超导薄膜断裂力学行为的认识提供素材，并为在大规模生产实践中提高YBCO超导带材断裂力学性能、优化/简化制备工艺过程提供参考。