On the Novel Structure and Metallization of Hydrogen under High Pressure
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摘要: 在极端压缩状态下,氢呈现出丰富的物理及化学变化,其结构与相图揭示了凝聚态物质高压行为的典型特征,在天体物理和新材料研究中有重要应用。本文简要回顾了金属氢概念的提出,以及直至最近几年的研究进展,分析总结了高密度氢研究中的一些核心问题和发展态势。利用密度泛函理论计算和状态方程模型分析,综合探讨了氢在高压下复杂的原子结构、分子氢离解区域附近的复杂行为、金属氢的亚稳定性和可回收性,以及“DAC+冲击”加载方法在金属氢研究中的优势与不足等问题。结果表明:通过快速或缓慢的压力释放回收金属氢的高压相到常压是几乎不可能的;高压下氢的复杂行为给实验和理论研究带来了巨大挑战,特别是离解区域附近理论与理论、实验与实验、以及理论与实验之间的结果都存在巨大差异,暗示当前通用的实验测试方法和常用的多电子理论计算方法还存在很大的改进空间。Abstract: Hydrogen under extremely compressed conditions exhibits extraordinary richness in physics and chemistry.Its structural transitions and phase diagram are fundamental for understanding the high pressure behavior of condensed matters, and also for important applications in astrophysics and material science.In this paper, we reviewed the research progress on metallic hydrogen since it was proposed until to very recent years.Several key issues and possible future progress about dense hydrogen are analyzed and summarized.With density functional theory calculations and a model analysis using an equation of states, we explored and demonstrated:(1)the complex and novel atomic structures of dense hydrogen; (2)the novel behaviors of hydrogen molecules near the dissociation region; (3)the stability of metallic hydrogen under pressure and the possibility of recovery to zero pressure; (4)the advantages and disadvantages of the "DAC+shock" experimental method to achieve metallic hydrogen.Our results showed that it is impossible to quench the high pressure phases of metallic hydrogen to ambient conditions.The complex behaviors of dense hydrogen present as a great challenge for both experimental and theoretical studies, especially near the dissociation of hydrogen molecules, where sharp discrepancies were unveiled, implying that much effort is still required to improve the state-of-the-art experimental and many-body theoretical methods.
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作为液体和气体长距离运输的一种重要方式,管道运输在国家经济发展和国民生活中发挥着重要作用。然而,随着城镇化进程的加速,城市管网系统密集分布,爆炸作用引起的管道安全问题受到国内外广泛关注[1-3]。都的箭等[4]通过实验研究发现,正对爆心管段背面受到很大的轴向拉应力作用,且管道受爆炸载荷的影响主要与爆心距有关。Ji等[5]研究了X70钢管在局部爆炸载荷下的动力响应,发现管道的挠度和损伤程度随炸药量和接触面积的增大而增大,且壁厚对管道损伤和失效后的运动有重要作用。数值模拟是研究爆炸问题的一种重要方法,只要方法得当,模拟效果可与实际情况相吻合[6-7]。为此,梁政等[8]利用数值模拟方法研究了管道埋深、药量和管道壁厚因素对爆炸载荷下的埋地管道动力响应的影响。房冲[9]通过模拟研究发现,在爆炸载荷下充水管道的变形量、位移和峰值压强都比内空管道小。余洋等[10]采用野外实验与数值计算相结合的方法研究了初始条件对钢质方管在侧向局部爆炸载荷作用下损伤破坏效应的影响。
迄今为止,对爆炸载荷作用下焊缝区附近埋地钢管的动力响应的相关研究鲜有报道。基于此,以两种含Y型焊缝(坡口有2 mm余高焊缝和坡口无余高焊缝)的埋地X70钢管为例,采用有限元软件ANSYS/LS-DYNA,数值模拟研究爆炸载荷作用下焊缝区附近埋地X70钢管的动力响应规律,以期为埋地管线附近的爆破施工设计和埋地管线的安全防护提供一定的理论参考。
1. 有限元模型
1.1 计算模型
采用cm-g-μs单位制,建立由TNT炸药、黄土和焊接管道组成的计算模型,如图1所示。模型纵向长38.4 cm,管道中心到模型侧面的宽度为130.0 cm,模型整体高271.6 cm,其中:TNT炸药为边长14.0 cm的正方体,采用中心起爆方式;焊接管道为外径1 016.0 mm、壁厚14.6 mm的X70钢管。焊缝选取两种尺寸[11],分别为Y型坡口有余高(H = 2.0 mm)焊缝和Y型坡口无余高(H = 0)焊缝,如图2所示。为了提高计算收敛速度,将焊缝尺寸设计图进行适当的简化,简化模型如图3所示。两种焊缝均不考虑分层焊接工艺的影响,焊缝与管道采用共节点方式连接。
考虑到计算模型的对称性,取1/2模型建模。炸药、黄土、管道及焊缝选用SOLID164六面体实体单元,用扫掠方式划分网格,并对焊缝位置进行网格细化处理。炸药和黄土采用欧拉网格,焊接管道和焊缝采用拉格朗日网格,运用任意拉格朗日-欧拉算法及管土间流固耦合算法模拟爆炸载荷作用下埋地焊接管道的动力响应。在土体外侧和底面设置透射边界条件,模型对称面施加对称约束。
1.2 计算工况
为了初步揭示爆炸载荷作用下两种焊缝形式的埋地焊接管道的动力响应规律,选取药包尺寸为14.0 cm × 14.0 cm × 14.0 cm的TNT炸药,对埋深为1.5 m的焊缝有余高(H = 2.0 mm)管道(管道A)和焊缝无余高(H = 0)管道(管道B),在炸高分别为60.0、85.0和110.0 cm的3种条件下的6种工况进行模拟计算,如表1所示,其中,hB为炸高。
表 1 计算工况Table 1. Calculation conditionsWeld type Buried depth of pipeline/m Size of TNT/(cm × cm × cm) hB/cm No weld reinforcement (H = 0) 1.5 14.0 × 14.0 × 14.0 60.0, 85.0, 110.0 Weld reinforcement (H = 2.0 mm) 1.5 14.0 × 14.0 × 14.0 60.0, 85.0, 110.0 1.3 材料参数
TNT炸药选用高能炸药模型(Mat_High_Explosive_Burn)和JWL状态方程定义。JWL状态方程表达式为
pz=A(1−ωR1ν)e−R1ν+B(1−ωR2ν)e−R2ν+ωEν (1) 式中:pz为爆炸产物的压力,A、B、R1、R2、ω为TNT材料常数,v为爆炸产物的相对比容,E为炸药初始内能。炸药密度ρz、爆速D以及JWL状态方程参数见表2[12]。
黄土选用泡沫模型(Mat_Soil_and_Foam)描述。该材料模型的应力屈服常数f为
f=Sijδij/2−(a0+a1pt+a2pt2) (2) 式中:Sij为土体材料的Cauchy偏应力张量,δij为土体材料的Kronecker系数,a0、a1、a2分别为土体摩擦角、土体黏聚力和土体爆炸动载效应的影响系数,pt为土体压力。a0、a1、a2由土工实验测得的内摩擦角和土壤黏聚力参数确定,土体密度ρt、剪切模量G、体积模量K等参数见表3[13-14]。
X70钢管道和焊缝均采用双线性随动材料模型(Mat_Plastic_Kinematic)描述,遵循von Mises屈服准则,其表达式为
σ={Esεε⩽εeσy+Et(ε−εe)ε>εe (3) 式中:σ为应力;σy为屈服应力;Es为弹性模量;Et为切线模量,0 < Et < Es;ε为应变;εe为弹性极限应变。管道和焊缝的具体材料参数见表4[15-18],其中,μ为泊松比。
2. 结果与讨论
2.1 管道应力分析
图4为边长14.0 cm的正方体TNT炸药爆炸时,炸高hB为60.0 cm,埋深为1.5 m的两种X70管道焊缝附近的von-Mises应力云图。由图4可以看出:当传播时间为1 440 μs时,爆炸应力波阵面已经接触管道;当传播时间为1 600 μs时,焊缝有2.0 mm余高的管道A和焊缝无余高的管道B的最大应力增幅分别为81.4 MPa和43.0 MPa;当传播时间为1 920 μs时,管道A和管道B的最大应力均大于焊缝与管道的材料屈服应力,且应力沿迎爆面正对爆心位置向外扩展,其中管道A的应力呈“十”字形扩展,而管道B的应力以椭圆形向四周扩展;在3 520 μs时,管道应力集中主要沿裂缝位置发展,管道A和管道B的应力最大值分别为601.2 MPa和591.0 MPa;在6 080 μs时,管道A和管道B继续变形但应力减小,应力最大值分别减小到581.8 MPa和565.8 MPa;在9 120 μs时,管道A和管道B的应力集中基本消失。
图5和图6分别为两种管道外表面上正对爆心位置的焊缝与管道分界面处焊缝单元与管道单元的应力时程曲线。在管道受爆炸载荷作用阶段,两种管道的应力在大约480 μs内呈跳跃式上升。其主要原因是管道为瞬时受力,一部分爆炸能量使管道变形并向管道四周传递,导致焊缝与管道分界面处两个典型单元的应力呈降低趋势,此现象与图4的应力云图吻合。根据应力集中系数和余高关系的经验公式[19]可得:管道A和管道B的应力集中系数分别为1.016和1.008,即随着余高增大,应力集中系数逐渐增大。对比图5和图6可知,管道A的焊缝单元应力峰值较高,应力下降趋势相对较陡。这也说明焊缝余高的存在使得焊缝与管道分界面的截面尺寸突变增大,从而导致焊缝有余高的焊接管道受应力集中的影响较大。在1 912 μs时,图5(管道A)和图6(管道B)的焊缝单元应力最大,分别约为560.0 MPa(焊缝的屈服强度为550 MPa)和545.6 MPa。同时,管道A的焊缝处首先达到管道屈服强度(480 MPa),按照von Mises屈服准则,管道A开始进入局部塑性变形阶段,此时管道B的应力尚未达到材料的屈服强度。
2.2 管道位移分析
埋地X70管道的迎爆面和背爆面的最大位移如表5所示。从表5可知,由于爆炸冲击波的一部分能量在土中被耗散,且随着爆炸冲击波在土中传播距离的增大,两种焊缝形式管道的迎爆面和背爆面的最大位移均呈现减小的趋势。当炸高hB从60.0 cm增加到85.0 cm以及从85.0 cm增加到110.0 cm时,管道A和管道B迎爆面的最大位移减小量分别为2.303 cm、0.715 cm和2.300 cm、0.572 cm,而管道A和管道B背爆面的最大位移减小量分别为0.391 cm、0.235 cm和0.373 cm、0.280 cm。两种焊缝形式管道迎爆面的最大位移减小量大于背爆面,这是由于爆炸冲击载荷在土中传播后直接作用于管道迎爆面,对管道迎爆面产生的影响较大,土体对管道背爆面具有一定的支撑作用,从而减小了管道背爆面位移。在相同炸高下管道A比管道B的最大位移大,且在炸高为60.0、85.0和110.0 cm时,两种焊缝形式管道迎爆面的最大位移差值分别为0.270、0.267和0.124 cm,即随着炸高的增大,两种焊缝形式的埋地焊接管道最大位移的差值逐渐减小,也说明当炸高较小时,管道A整体抵抗变形的能力弱于管道B。然而,随着炸高的增大,作用于管道的能量减小[20],管道塑性变形较小,使得这种现象逐渐模糊。
表 5 埋地X70管道的迎爆面和背爆面的最大位移Table 5. Maximum displacement of explosion-front and explosion-back surfaces of buried X70 pipelineTypes of weld hB/cm Maximum displacement/cm Explosion-front surface Explosion-back surface Weld reinforcement (H = 2.0 mm) 60.0 5.482 0.846 85.0 3.179 0.455 110.0 2.464 0.220 No weld reinforcement (H = 0) 60.0 5.212 0.943 85.0 2.912 0.570 110.0 2.340 0.290 2.3 管道等效应变分析
表6为两种不同类型焊缝的埋地焊接管道在不同炸高下的最大等效应变统计。从表6可知,管道A和管道B的最大等效应变均随炸高的增大而减小。当炸高从60.0 cm增大到85.0 cm时,管道A和管道B的最大等效应变分别减小约58.12%和61.13%;当炸高从85.0 cm增大到110.0 cm时,管道A和管道B的最大等效应变分别减小约45.92%和38.05%,在炸高相同时,管道A的最大等效应变大于管道B,且管道A的最大等效应变位于焊缝余高表面,而管道B的最大等效应变在焊缝与管道处一定范围内沿纵向分布。这在一定程度上说明管道B能更好地协调焊缝与管道分界处的应变,有利于保障焊缝与管道的局部协同变形性能。
表 6 不同炸高时埋地X70管道的最大等效应变Table 6. Maximum effective strain of buried X70 pipeline with different blasting heightsTypes of weld hB/cm Peak effective strain/10–3 Weld reinforcement
(H = 2.0 mm)60.0 9.937 85.0 4.162 110.0 2.251 No weld reinforcement
(H = 0)60.0 6.877 85.0 2.673 110.0 1.656 2.4 管道振速分析
表7为不同炸高下两种焊缝形式管道的迎爆面和背爆面处焊缝位置中心单元X方向的最大振动速度。从表7可以看出,两种焊缝形式管道的迎爆面和背爆面的最大振动速度均随着炸高增大而减小,且迎爆面的最大振速均大于背爆面。这说明迎爆面受爆炸地震波的影响较大。当炸高hB为60.0、85.0和110.0 cm时,管道B的迎爆面的最大振动速度较管道A大,迎爆面差值分别为1.600、0.539和0.329 m/s,而背爆面差值在0.200 m/s以内。管道峰值速度随着管壁厚度的增大而减小[12],由于管道A增加了管道在焊缝位置的径向厚度,可将其视为管道焊缝位置的壁厚增大导致管道A的峰值振速减小。这说明管道A抵抗爆炸振动的性能优于管道B,且在炸高为60.0 cm时,管道A抵抗振动性能的优势较为明显。
表 7 埋地X70管道的迎爆面和背爆面最大振速Table 7. Maximum vibration velocity of explosion-front and explosion-back surfaces of buried X70 pipelineTypes of weld hB/cm Maximum vibration velocity/(m·s–1) Explosion-front surface Explosion-back surface Weld reinforcement (H = 2.0 mm) 60.0 22.748 4.431 85.0 9.316 2.817 110.0 4.503 1.693 No weld reinforcement (H = 0) 60.0 24.348 4.294 85.0 9.855 2.867 110.0 4.832 1.746 图7为不同炸高时两种焊缝形式的管道典型单元的速度时程曲线。当炸高hB分别为60.0、85.0和110.0 cm时,管道A和管道B达到最大振速的时间分别为2 560 μs和2 560 μs、4 500 μs和4 600 μs、7 200 μs和7 200 μs,两种焊缝形式的管道达到最大振速的时间差值均在100 μs以内。这说明两种焊缝形式的管道达到最大振速的时间主要受炸高的影响,受焊缝形式的影响较小。
3. 结 论
(1)当炸高为60.0 cm时,两种焊缝形式的埋地X70焊接管道在爆炸载荷作用下焊缝位置均出现应力集中,但焊缝有余高的管道受应力集中影响较大,且会先于焊缝无余高管道进入屈服阶段。
(2)当炸高为60.0~110.0 cm时,由于爆炸载荷直接作用于迎爆面,且管土间的相互作用对管道背爆面具有一定的支撑作用,两种焊缝形式管道迎爆面的最大位移均大于背爆面的最大位移。当炸高为60.0、85.0 cm时,焊缝有余高的管道整体抵抗变形的能力明显弱于焊缝无余高的管道。
(3)焊缝无余高管道较焊缝有余高管道在焊缝与管道分界处的应变更为协调,能更好地保障焊缝与管道的局部协同变形性能。
(4)在相同的爆炸载荷下,焊缝有余高管道抵抗振动的性能优于焊缝无余高管道。药量相同条件下,相对于焊缝形式,炸高对含焊缝区管道的最大振速起主要作用。
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