Fund Project:Project supported by the YUMIN Foundation, China (Grant No. TCYM1820-02) and the Science Challenge Project, China (Grant Nos. TZ2016001, TZ2018001)
Received Date:17 July 2020
Accepted Date:07 December 2020
Available Online:15 April 2021
Published Online:05 May 2021
Abstract:After high pressure shock, the shock wave in the metal is unloaded at the metal-gas interface, and micro spallation occurs when the metal melts. When the micro spallation develops to a certain extent, the high pressure gas penetrates the zero pressure vacuum gap between the metal melt droplets. In this paper, the phenomenon of gas penetrating metal micro spallation zone is analyzed theoretically. Based on the regular hexahedron periodic arrangement of metal droplets, the calculation formulas of the maximum penetration depth, the sealing time of the penetration channel and the maximum mass of the gas penetrating the metal micro spallation zone are given through theoretical analysis under the quasi-static and semi-dynamic conditions. The quasi-static process is considered to be the gas penetration process that can be approximated as the escape process of gas into the vacuum, and the gap in the metal micro spallation zone will be filled with gas. The semi-dynamic analysis is based on two basic assumptions: one is the equal droplet size and spacing in the micro spallation zone and the other is the critical sealing condition of gas penetration. In the process of semi-dynamic analysis it is demonstrated that the initial critical sealing distance is independent of the shape factor of the droplet single control volume. The semi-dynamic analysis can give various critical sealing information when the gas stops penetrating the metal micro spallation zone. The results of quasi-static analysis can be used as the upper limit of gas penetration, and the semi-dynamic analysis results can be used as the lower limit of gas penetration. From the sensitivity analysis, it can be seen that the change law of physical phenomena given by theoretical analysis accords with the basic physical understanding of the problem. Through this study, the upper and lower limit of the mixed state of gas penetrating the metal micro spallation zone can be estimated, which can provide more accurate initial metal-gas mixed state for subsequent research of the evolution of mixed state. The theoretical analyses given in this paper are based on a lot of uncertain assumptions, and the in-depth study of this phenomenon is still needed based on the law summary and mutual confirmation of experiment and simulation. Keywords:metal micro spallation/ gas penetration/ penetration depth/ penetration quality
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--> --> --> 1.引 言金属受冲击后在金属外界面反射稀疏波, 会由外界面向内形成层裂、微层裂现象. 若冲击压力较低、金属处于固相则会发生层断裂现象, 层裂区头部向内会形成具有一定厚度的层裂片(见图1(a)), 国内外****已在此方面开展了大量实验、理论、模拟等研究工作[1-4]. 若冲击压力较高, 金属发生(冲击或卸载)熔化现象, 经历一定时间演化后会在外界面向内形成由熔化液滴组成的低密度金属微层裂区(参见图1(b)), 即为金属微层裂现象[5-14]. 这里需要指出的是, 金属微层裂现象与受冲击后金属表面向外形成的物质喷射、微喷射现象[8, 15, 16]是不同的, 金属微层裂可理解为熔化金属受拉伸后由外表面向内的层裂现象. 图 1 质子照相获取的不同冲击压力下锡外界面演化[5] (a)层裂; (b)微层裂 Figure1. Evolution of tin under different impact pressures by proton radiography[5]: (a) Spallation; (b) micro spallation.
金属的微层裂现象最早由Andriot等[6]于1983年发现. 2003年Holtkamp等[5,7]开展了爆轰驱动铅、锡等低熔点材料的层裂、微层裂实验, 并利用质子照相技术获取了金属层裂区、微层裂区内部状态. 2007年De Resseguier等[8]通过激光加载手段开展了此类低熔点金属的微层裂实验, 并对微层裂区的金属液滴进行了回收(见图2). 近年陈永涛等[9-11]也开展了爆轰驱动金属微层裂相关实验研究, 清晰地获取了金属微层裂区X光图像. 目前, 金属微层裂现象的模拟研究中多采用分子动力学方法[12-14], 但由于模拟尺度过小并不能展现宏观金属微层裂现象全貌, 其分析结果仅能作为定性认识. 图 2 回收护盾收集到的微层裂区球形液滴形态金属锡[8] Figure2. Spherical tin droplet in micro spallation zone collected by recovery shield[8].
2.准静态情况下气体渗入的理论分析图3给出了金属受冲击后产生的微层裂区经过一定时间演化后, 与气体接触面上的金属微层裂液滴间不再相互贯通, 形成了具有真空间隙的独立小液滴形态. 认为该状态是本文研究气体渗入情况的初始状态. 初始状态下金属微层裂区与气体的运动初速度基本保持一致, 即初始相对速度为0. 图 3 气体渗入金属微层裂区的初始状态示意图 Figure3. Initial state of gas permeation into metal micro spallation zone.
在金属微层裂区孔隙度较大(真空间隙较大)并且熔化液滴受气体加速作用可忽略情况下, 可认为气体渗入微层裂区为准静态过程, 气体渗入过程即为气体向真空的逃逸过程. 显然, 准静态渗透情况下, 气体向微层裂区的渗入层宽度、渗透质量与气体压力、渗透时间相关. 准静态渗透情况下气体向金属微层裂区的渗透示意见图4. 图 4 准静态情况下, 气体向金属微层裂区的渗透 Figure4. Gas permeation into metal micro spallation zone under quasi-static condition.
3.半动态情况下气体渗入的理论分析本小节忽略气体在熔化的金属液滴附近绕流减速及对周围及后方气体流场的影响, 仅考虑金属熔化液滴在气体流场中的加速, 以半动态的形式对气体渗入微层裂区现象进行理论分析, 气体渗透过程见图5. 由于半动态情况下金属液滴运动速度前高后低, 所以本节重点需要讨论的是, 液滴运动后封闭气体渗入通道使气体停止渗入的问题. 图 5 考虑金属液滴运动情况下的气体渗透过程 Figure5. Gas permeation process considering the movement of metal droplets.