【作者】 赵丰鹏；

【导师】 吴恒安； 罗胜年；

【作者基本信息】 中国科学技术大学 ， 固体力学， 2014， 博士

【摘要】 多孔材料的冲击响应不仅对于冲击物理有着显著的科学意义,而且对于材料合成和工程具有重要的应用价值。本文运用分子动力学模拟方法对纳米多孔金属材料和纳米蜂窝金属材料的冲击响应进行了模拟。研究内容包括弹塑性变形、Hugoniot状态、冲击引起的熔化、部分和完全孔洞塌陷、热点的形成、纳米射流以及气化,讨论了微结构效应、各向异性和孔隙度的影响。本文建立了具有圆柱形孔洞的高孔隙度的铜纳米多孔材料的模型,研究了冲击过程在压缩和反射不同阶段下的速度、温度、密度、应力等物理性质。观察到不同冲击强度下冲击波的弹塑性双波结构和超越。提出一个修正的幂律P-α模型来描述Hugoniot状态,验证了Gruneisen状态方程对于多孔材料的有效性。从冲击诱导的熔化来看,没有整体预熔化和过热的明显迹象。孔洞塌陷的发生是通过孔洞的塑性流动成核实现的,其实际过程依赖于冲击强度。随着冲击强度的增加,孔洞塌陷从几何模式(孔洞塌陷由晶体学和孔洞几何主导,并且可能彼此不同)向流体动力学模式(各个孔洞塌陷彼此相似)转变。塌陷可能由以下几种方式主导产生：沿{111}滑移面的塑性流动：压缩和拉伸区域交替分布；横向流动方式：前向和横向流动混合方式；以及前向纳米射流。内部射流会引起明显的波阵面粗糙化,导致内部热点的形成和原子级光滑自由面上的高速射流。本文研究了微结构效应对纳米多孔金属铜冲击响应的影响,包括孔洞的形状、大小和排列分布以及晶界。孔洞的塌陷、射流和气化都不同程度地受到微结构的影响。孔洞的排列分布和长细比发挥了重要作用,而晶界和孔洞大小的影响较小。数值模拟结果表明Hugoniot P-V状态对微结构不敏感。孔洞塌陷在低速冲击时以横向塑性流动为主,高速时以内部射流为主。孔洞塌陷时的射流归因于速度梯度和应力梯度张量,是前进、发散和会聚流动不同程度作用的结果。自由面射流可能会包含颈缩和空穴化现象。具有大长细比并且中心排列成直线的椭圆孔洞易于形成高速射流和气化。本文建立了孔隙度为0.1-0.9的铜六边形纳米蜂窝的分子动力学模型,并进行了侧限压缩和冲击模拟。比较了不同冲击速度下纳米蜂窝的弹性波和塑性波的波速、应力、密度和温度等物理量和孔洞塌陷及塑性变形；考察了三个冲击方向的异同：还对不同相对密度的结果进行了对比。结果发现,相对密度(或孔隙度)和冲击速度是决定纳米蜂窝材料冲击响应的主要因素。更多还原

【Abstract】 Shock response of porous materials can be of crucial significance for shock physics and bears many practical applications in materials synthesis and engineering. Molecular dynamics simulations are carried out to investigate shock response of nanoporous metal materials and metal nano-honeycomb materials, including elastic-plastic deformation, Hugoniot states, shock-induced melting, partial or complete void collapse, hotspot formation, nanojetting, and vaporization. The influences of microstructure effects, anisotropy and porosity are discussed.A model nanoporous Cu with cylindrical voids and a high porosity under shocking is established to investigate such physical properties as velocity, temperature, density, stress and von Mises stress at different stages of compression and release. The elastic-plastic and overtaking shocks are observed at different shock strengths. A modified power-law P-a model is proposed to describe the Hugoniot states. The Gruneisen equation of state is validated. Shock-induced melting shows no clear signs of bulk premelting or superheating. Void collapse via plastic flow nucleated from voids, and the exact processes are shock strength dependent. With increasing shock strengths, void collapse transits from the "geometrical" mode (collapse of a void is dominated by crystallography and void geometry and can be different from that of one another) to "hydrodynamic" mode (collapse of a void is similar to one another). The collapse may be achieved predominantly by flow along the{111} slip planes, by way of alternating compression and tension zones, by means of transverse flows, via forward and transverse flows, or through forward nano-jetting. The internal jetting induces pronounced shock front roughening, leading to internal hotspot formation and sizable high speed jets on atomically flat free surfaces.Microstructure effects on shock response of nanoporous Cu are investigated, including pore shape, size and arrangement, as well as grain boundaries. The void collapse jetting and vaporization are dependent on the microstructure, although to some different extents. The void arrangement and aspect ratio play an important role. The effects of grain boundaries and void size are less pronounced. The high pressure Hugoniot states are not sensitive to microstructure. Jetting during void collapse is due to tensorial velocity gradients (direction and amplitude), and a combined result of forward, divergent and convergent flows with varying contributions. Free surface jetting involves necking and cavitation. Elliptical voids with large aspect ratios, and with their centers aligned linearly with the shock direction, are particularly efficient in inducing high speed jetting and vaporization.A serial of models of hexagonal nano-honeycombs Cu with porosities varying from0.1to0.9are established to simulate their shock response with molecular dynamic method. The shock velocities of elastic and plastic waves, stress, density and temperature at different shock velocities are compared. Similarities and differences for three shock directions are also considered. The shock results of nano-honeycombs with various relative densities are also compared. It shows that the relative density and impact velocity play primary roles in shock response of Cu nano-honeycombs.更多还原