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Micromechanical modelling on the elastoplastic damage and irreversible critical current degradation of the twisted multi-filamentary Nb3Sn superconducting strand

多芯绞扭Nb3Sn超导股线弹塑性损伤和临界电流不可逆退化的细观力学建模

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Abstract

Nb3Sn is widely accepted as the enabling technology for high field superconducting magnets. However, it is brittle and with strain-sensitive superconducting properties. In high field applications, Nb3Sn strand experiences significant elastoplastic strain or even damage which causes degradation in its current carrying capacity. In this work, a 3D mean-field homogenization model based on the incremental micromechanics scheme is developed to investigate the elastoplastic damage and irreversible degradation of the twisted multifilamentary Nb3Sn strand. The effective stress-strain curves and strain distribution in the Nb3Sn filaments are calculated for the strand under monotonic and cyclic loads. The invariant strain scaling law supplemented with the damage-induced reduction is adopted to characterize the irreversible degradation of the critical current. It is found that twisting plays an important role in elastoplastic damage and strain-induced critical current degradation. With the increasing of twist pitch, the strand becomes stiffer and the strain limit surpasses which the filaments start to damage sharply decreases. Both the accumulated residual strain and damage of the filaments contribute to the irreversible degradation of the critical current. The experimentally observed “strain irreversibility cliff” is the result of damage to the Nb3Sn filaments. From a mechanical point of view, a short twist pitch will be a good choice to alleviate the strain-induced irreversible degradation of the Nb3Sn strands.

摘要

Nb3Sn被广泛接受是实现高场超导磁体的关键材料. 然而, Nb3Sn是脆性材料且其超导性能具有应变敏感性. 在强磁场应用中, Nb3Sn股线会受到显著的弹塑性应变甚至损伤, 导致其载流能力下降. 本文基于细观力学方法建立了三维平均场均匀化模型, 研究多芯绞扭Nb3Sn超导股线的弹塑性损伤行为和临界电流的不可逆退化. 计算股线在单调和循环载荷作用下的等效应力-应变曲线以及Nb3Sn 内部的应变分布. 采用辅以损伤引起退化的应变不变量标度律来表征临界电流的不可逆退化. 研究发现, 绞扭对股线弹塑性损伤行为和应变导致的临界电流退化起着重要作用. 随着绞扭节距的增大, 复合股线的刚度增大, 应变极限(超过该极限时芯丝开始损伤)急剧减小. 累积残余应变和芯丝的损伤共同导致超导股线临界电流的不可逆退化. 实验观察到的“断崖式不可逆退化”是由于Nb3Sn芯丝的损伤造成的. 从力学的角度, 短绞节距是缓解应变引起超导股线临界电流不可逆退化的良好选择.

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Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant Nos. 11602185, 11972271, and 12322208), the Young Elite Scientists Sponsorship Program by CAST (Grant No. 2020QNRC001), and the Fundamental Research Funds for the Central Universities.

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Authors and Affiliations

  1. Institute of Extreme Mechanics and School of Aeronautics, Northwestern Polytechnical University, Xi’an, 710072, China

    Ze Jing  (景泽) & Yu Zhang  (张雨)

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  1. Ze Jing  (景泽)
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  2. Yu Zhang  (张雨)
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Author contributions Ze Jing designed the model and the computational framework of this study, made numerical simulations, and wrote and edited the manuscript. Yu Zhang collected experimental data and aided in preparing the figures and tables.

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Correspondence to Ze Jing  (景泽).

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Jing, Z., Zhang, Y. Micromechanical modelling on the elastoplastic damage and irreversible critical current degradation of the twisted multi-filamentary Nb3Sn superconducting strand. Acta Mech. Sin. 40, 723661 (2024). https://doi.org/10.1007/s10409-024-23611-x

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