Force-Thermal Coupling Response of Sapphire under Impact Loading Based on Molecular Dynamics Simulation
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摘要: 蓝宝石因其出色的强度、硬度和光学透明度，常被选为冲击波实验中的观测窗口。深入了解蓝宝石在冲击载荷下的力学和热力学响应机制以及内部损伤原因，对准确评估其性能和稳定性至关重要。利用分子动力学模拟，从原子层面探讨蓝宝石单晶在沿(0001)晶面（C面）冲击作用下的力热响应行为。模拟结果表明，蓝宝石C面冲击作用下激活的滑移系为基于R面{$ 0 \overline 1 12$}的菱形面滑移。冲击速度为1～3 km/s时未出现滑移现象，冲击速度为4 km/s时出现菱形面滑移，冲击速度为5～6 km/s时试样出现以不规则条带为主的非均匀形变。研究表明，蓝宝石滑移系的激活不仅依赖其晶格结构，还需分剪切应力达到临界值。温度场的分析结果表明，局域温升与滑移之间存在对应关系，剪切应变集中区域的温度较高。Abstract: Sapphire is often chosen as the observation window in shock wave experiments due to its excellent strength, hardness and optical transparency. A deep understanding of the mechanical and thermodynamic response mechanisms of sapphire under impact loading and the causes of internal damage is crucial for accurately evaluating its performance and stability. In this work, molecular dynamics simulations were performed to explore the mechanical and thermal response of a sapphire single crystal under shock loading along the C-plane. The results indicate that the activated slip system after the impact loading is the rhombic plane slip based on the R-plane {$0 \overline 1 12 $}. When the impact velocity is in the range of 1−3 km/s, no slip occurs; when the impact velocity reaches 4 km/s, slip occurs. When the impact velocity reaches to the range of 5−6 km/s, the sample shows inhomogeneous deformation, mainly composed of irregular stripes. Such results suggest that the activation of the slip system in sapphire depends not only on its lattice structure, but also on the partial shear stress (which needs to reach a critical value). The analysis of the temperature field indicates that there is an intrinsic relation between the local slip and temperature increase, i.e., the formation of intense shear localization is accompanied by the higher temperature.
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Key words:
- molecular dynamics /
- sapphire /
- impact response /
- damage type
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图 1 蓝宝石单晶扩胞示意图（为显示方便，a、b、c轴上的长度均在单胞长度的基础上扩大了1倍）
Figure 1. Schematic diagrams of an enlarged model of sapphire single crystal (For visualization reason,the unit cell is replicated once in the a, b, and c axes.)
图 3 冲击速度为1 km/s时试样中心位置处沿z轴的正应力和剪切应力随时间的变化
Figure 3. Temporal variations of the normal and shear stresses at the central position of the sample along z axis at the shock velocity of 1 km/s
图 4 (a) t =5.0 ps时不同加载速度下的压强剖面，(b) 对应的氧原子结构示意图
Figure 4. (a) Pressure profiles at t=5.0 ps under different loading velocities; (b) the corresponding schematic diagrams of the oxygen atom structure
图 5 t=20.0 ps时不同冲击速度下试样内剪切应变等高图
Figure 5. Contour maps of shear strain inside samples under different impact velocities at t=20.0 ps
图 7 冲击速度为4 km/s时试样内部滑移面的形成和演化过程
Figure 7. Initiation and development of the shear plane inside the sample under an impact velocity of 4 km/s
图 8 (a) 11.2 ps时垂直于滑移面截面上的剪切应变分布，(b) (a)中红框的放大（黑圈内2个氧原子的剪切应变大于0.18），(c) 20.0 ps时红框内的应变分布（黑圈内的原子对应(b)中的2个氧原子）
Figure 8. (a) Distribution of shear strain in the cross-section vertical to the later shear plane at t =11.2 ps; (b) zoomed-in view of the red box region in (a) (The shear strains of the two oxygen atoms highlighted by black circles are greater than 0.18.); (c) distributionof shear strain at t=20.0 ps (The two atoms highlighted by black circles correspond to that shown in (b).)
图 9 图8中放大区域的冲击压缩过程
Figure 9. Impact compression process of the enlarged region in Fig.8
图 12 垂直于冲击方向截面的剪切应变分布：(a)截面位置，(b) 11.2 ps时剪切应变的分布，(c) 20.0 ps时剪切应变的分布
Figure 12. Shear strain distributions in the cross-section plane perpendicular to the impact direction: (a) schematic diagram of the location of the target plane; (b) shear strain distribution at t=11.2 ps; (c) shear strain distribution at t=20.0 ps
图 13 (a) t=16.0 ps时的缺陷分析结果（粉色箭头为伯格斯矢量），(b) 位错分析结果（蓝线为位错线，灰色区域为缺陷）
Figure 13. (a) Defect analysis result at t=16.0 ps (Pink arrows denote the Burgers vectors.); (b) the dislocation analysis result (The blue line represents the dislocation line, and the gray area represents the defect.)
图 14 4 km/s冲击速度下5.6 ps时试样内剪切应变等高图以及剪切应力和温度沿z轴的分布
Figure 14. Contour of shear strain inside the sample and the distributions of shear stress and temperature along the z-axis at 5.6 ps under an impact velocity of 4 km/s
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