Simulation of Dynamic Crack Propagation in Superconducting Nb3Sn at Extreme Low Temperature
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摘要: 研究Nb3Sn超导体的损伤断裂行为对于揭示超导临界性能弱化背后的力学机制具有重要的意义。采用分子动力学模拟方法，研究了极低温下不含裂纹和含中心裂纹的Nb3Sn单晶在力学拉伸变形作用下的断裂机制和裂纹扩展行为，同时分析了应变率效应对Nb3Sn单晶断裂机制与裂纹扩展行为的影响。结果表明：不含裂纹的Nb3Sn单晶在结构受力后出现滑移，滑移带上位错塞积导致应力集中，应力集中使原子键断裂从而萌生裂纹致使Nb3Sn单晶断裂；而含中心裂纹的Nb3Sn单晶则由于裂纹尖端应力集中使得原子键断裂形成微裂纹，裂纹扩展致使Nb3Sn单晶断裂。Nb3Sn单晶在不同的应变率下表现出不同的断裂机制，在低应变率下表现为脆性断裂，而在高应变率下表现为韧性断裂。Abstract: The study on damage and fracture in superconducting Nb3Sn is an indispensible part of understanding the origin of strain sensitivity in Nb3Sn. In this paper, by using molecular dynamic simulations, the fracture and crack propagation behavior of single crystal Nb3Sn with ideal lattice and with central crack at extreme low temperature are studied, respectively. The strain rate effects on the crack initiation and growth in both Nb3Sn sample cases are also carefully analyzed. The results show that for stressed Nb3Sn single crystal with ideal lattice, it slips with the dislocations plugging emerging on the slip band, which contributes to stress concentration and atomic bond breaking, resulting in the failure of Nb3Sn. While for Nb3Sn single crystal with central crack, the atomic bonds break due to the stress concentration concurring at the crack tip, microcracks form and propagate to induce the Nb3Sn fracture. The analysis on the damage fracture and failure mechanism of single crystal Nb3Sn at different strain rates reveals that it shows brittle fracture at low strain rate and ductile fracture at high strain rate.
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Key words:
- Nb3Sn single crystal /
- molecular dynamic simulation /
- crack propagation /
- fracture
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图 2 Nb3Sn单晶的分子动力学计算模型
Figure 2. Molecular dynamics simulation model of Nb3Sn single crystal
图 3 300 K下不含裂纹和含中心裂纹的Nb3Sn单晶拉伸应力-应变曲线
Figure 3. Tensile stress-strain curves of Nb3Sn single crystal without crack and with central crack at 300 K
图 4 4.2 K下Nb3Sn单晶的拉伸应力-应变曲线
Figure 4. Tensile stress-strain curves of Nb3Sn single crystal at 4.2 K
图 5 4.2 K下不含裂纹的Nb3Sn单晶的原子结构演化
Figure 5. Atomic structure evolution of Nb3Sn single crystal without crack at 4.2 K
图 6 4.2 K下含中心裂纹的Nb3Sn单晶的原子结构演化
Figure 6. Atomic structure evolution of Nb3Sn single crystal with central crack at 4.2 K
图 7 4.2 K下不含裂纹的Nb3Sn单晶在拉伸方向的原子应力云图
Figure 7. Atomic stress distribution of Nb3Sn single crystal without crack at 4.2 K
图 8 4.2 K下含中心裂纹的Nb3Sn单晶在拉伸方向的原子应力云图
Figure 8. Atomic stress distribution of Nb3Sn single crystal with central crack at 4.2 K
图 9 4.2 K不同应变率下Nb3Sn单晶的拉伸应力-应变曲线
Figure 9. Tensile stress-strain curves of Nb3Sn single crystal under different strain rates at 4.2 K
图 10 应变率为5×109 s−1时不含裂纹的Nb3Sn单晶的原子结构演化
Figure 10. Atomic structure evolution in Nb3Sn single crystal without crack at strain rate of 5×109 s−1
图 11 应变率为1×1010 s−1时不含裂纹的Nb3Sn单晶的原子结构演化
Figure 11. Atomic structure evolution in Nb3Sn single crystal without crack at strain rate of 1×1010 s−1
图 12 应变率为5×109 s−1时含中心裂纹的Nb3Sn单晶的原子结构演化
Figure 12. Atomic structure evolution in Nb3Sn single crystal with central crack at strain rate of 5×109 s−1
图 13 应变率为1×1010 s−1时含中心裂纹的Nb3Sn单晶的原子结构演化
Figure 13. Atomic structure evolution in Nb3Sn single crystal with central crack at strain rate of 1×1010 s−1
图 14 不同应变率下应力峰值处的原子应力云图
Figure 14. Atomic stress distribution at stress peak under different strain rates
表 1 Nb3Sn单晶的弹性常数和晶格常数
Table 1. Elastic constants and lattice constant of Nb3Sn single crystal
Method C11/GPa C12/GPa C44/GPa a/Å This work 284.10 95.84 53.76 5.21 First principle 284.32 107.70 67.07 5.32 
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