单晶冰动态力学行为的分子动力学模拟

本文通过分子动力学方法研究了高应变率均匀压缩加载情况,单晶冰在不同初始温度条件下的动态力学响应行为。研究表明,对于单晶冰而言,其强度随初始温度的降低而升高。值得注意的是,在不同温度条件下,单晶冰的形变机制发生了变化。温度相对较高时,主要发生固-液相变,即压缩过程中出现局部融化,而后固液界面向内部推移;然而,当降低初始温度至一定值后,则相变机制由固-液相变转变为固-固相变,即压缩过程中,出现了不同固态相之间的转化。对于高温条件下,发生相变后,应力随应变的增加而缓慢增加;低温条件下,相变阶段应力幅值会有急剧下降的过程。这两种不同的形变机制可以对吴先前等人(Xianqian Wu et al. Cold Reg Sci Technol)的实验结果给出一定解释。他们发现冰的强度在不同温度条件下会出现阶段线性的变化:当温度高于-125oC,峰值应力(强度)随温度的降低而升高;当继续降低温度至-173oC时,则峰值应力基本维持不变。其原因可能是相对较高温度时,主要出现局部裂纹扩展、局部融化导致破坏,冰的强度与传统材料一致,随温度的降低而增加。当温度降低至一定值后,则压缩过程中可能会发生冰的固相之间的转化,则峰值应力不仅与温度相关,还受相变行为的影响,从而导致冰的峰值应力在一定温度范围内基本维持不变

Multi-scale Studies on Dynamic Behavior of High Performance Energy Absorption Materials

材料与结构的爆炸与冲击响应一直以来都是冲击动力学领域重要的研究方向。在过去的几十年中，相关研究人员在这个领域开展了大量理论、实验以及计算等方面的研究工作，其研究对象主要针对于传统的多孔泡沫材料、硬质陶瓷材料或者颗粒材料。随着防护技术的发展，传统的吸能材料或结构面临着新的挑战，急需寻求一些新的高性能吸能材料或吸能结构，来面对日益增长的国防军事及民用需求。本文主要围绕两种高性能先进吸能材料：剪切增稠流体（Shear Thickening Fluid, STF）与镍钛形状记忆合金（NiTi Shape Memory Alloys, NiTi SMAs），通过系统实验表征结合微观数值模拟的多尺度研究方法，对两种材料的动态力学行为开展了系统的研究。主要研究进展包括： 1. 获得了STF在较高压力条件下的冲击波衰减与能量耗散行为，得到了高压条件下STF能量耗散的机制。发展了针对流体材料的激光冲击加载实验方法，基于PDV速度测量，获得了铝板-STF铝板组合结构背表面质点速度。通过行波分析，获得了STF的状态方程，以及冲击波的传播与衰减规律、能量耗散规律。进一步，通过改变加载半径大小、激光功率密度、环境温度的方式，获得了压力状态、压力幅值以及温度对于STF增稠机制的影响规律，观测到STF在高压条件下出现颗粒破碎这一新的能量耗散机制。在实验基础上，通过MD模拟，获得了STF的颗粒破碎的微观作用机制。 2. 获得了STF填充轻质点阵夹层板的动力学行为与能量耦合耗散机制。通过改进的SHPB实验，获得了STF填充轻质点阵夹层板的吸能性能，发现由于STF与夹芯金字塔胞元结构的耦合作用，导致点阵夹层板的屈曲行为发生了改变。在无填充材料的情况下，点阵夹层板的胞元发生非对称屈曲行为，而填充STF的金字塔点阵夹层板发生对称屈曲行为。由于屈曲行为的改变，导致组合结构的吸能性能表现出“1+1>2”的效果。 3. 阐释了NiTi的相变与塑性变形行为的竞争机制，给出了NiTi应变率与温度相关的相图。通过MD建立了NiTi形状记忆合金在高应变率均匀压缩条件下的微观物理模型。系统性地研究了温度、应变率对于NiTi合金相变行为的影响规律。发现在变形过程中，存在相变行为与塑性变形行为的竞争机制。同时，给出了NiTi合金温度与应变率相关的相图。计算结果很好地解释目前对于NiTi合金在高应变加载条件下相变行为的争议。 4. 获得了NiTi中冲击波传播与衰减规律及其与材料微观结构演化之间的关联。基于高应变率均匀压缩的计算结果，建立了考虑绝热温升效应的冲击加载模型。获得了温度、压力耦合作用下，NiTi合金微结构的演化规律。发现低温下，主要发生孪晶塑性变形，而高温下则主要发生位错滑移。与此同时，获得了微结构演化与应力波相互作用规律，得到了NiTi中应力波的传播与衰减特性。由于不同相结构中应力波波速不同，导致应力波在传播过程中出现多峰结构

Dynamic energy absorption behavior of lattice material filled with shear thickening fluid

In the present research the dynamic energy absorption behavior of lattice material filled with shear thickening fluid (STF) is studied by the modified split Hopkinson pressure bar (SHPB) apparatus. The nominal engineering stresses versus strain curves are measured to analyze the dynamic energy absorption behavior of the combined material. The dynamic behavior of lattice material is also measured as a comparison. The results show the dynamic strength of the lattice material is about 3 MPa, which is a little higher than the static strength of about 2.3 MPa because of the inertial effect during impact. However, when filled with STF, the combined material shows extraordinary energy absorption behavior when compared to the empty lattice material. The stress of the lattice material filled with STF increase almost linearly to about 9 MPa and then increases slowly with the increase of strain. The excellent energy absorption behavior of the lattice material filled with STF could be explained by the interaction between the lattice cores and the STF during compression. For the lattice material, while some of cores buckling, it will lose the load capacity. While filled with STF, if the initial buckling of some of the lattice cores happens during impact, the strong lateral drag force is generated by the ambient STF due to the fast lateral velocities of the cores, leading to the great increase of its dynamic energy absorption behavior. (C) 2017 The Authors. Published by Elsevier Ltd

Temperature-dependent phase transformation of ice-1h under ultrafast uniaxial compression: A molecular dynamics simulation

The temperature and strain rate-dependent compressive behaviour of ice-1h was investigated through molecular dynamics simulations. The ice strength increased in response to a decrease in the initial temperature and an increase in the strain rate. Various deformation mechanisms depended on the ice's initial temperature. Solid-liquid phase transformation was more likely to at a relatively high temperature. However, solid-solid phase transformation and dislocation-like glide were observed at relatively low temperatures. Experimental observations on the strength versus strain rate relationship of ice could be interpreted based on a prior study by Wu and Prakash [1]

Experimental study on dynamic compressive behaviour of sandwich panel with shear thickening fluid filled pyramidal lattice truss core

The dynamic compressive behaviour of sandwich panels with shear thickening fluid (STF) filled pyramidal lattice truss cores at high strain rates is studied and compared with that of pure STF as well as the sandwich panels with empty and water filled pyramidal lattice truss cores by modified split Hopkinson pressure bar (SHPB) apparatus. The dynamic compressive strengths of the sandwich panels while filled with STF increase significantly when compared to the strengths of the sandwich panels with empty pyramidal lattice truss cores. It is interesting to note that the sandwich panel with the STF filled pyramidal lattice truss core shows "1 + 1 >> 2" dynamic energy absorption behaviour. The excellent energy absorption behaviour of the sandwich panel with STF filled truss core is interpreted by the transformation of deformation modes of core beams from non-symmetry to symmetry after filled with the STF through strong interaction between the buckling behaviour of pyramidal lattice truss core and the shear thickening behaviour of the filled STF material. (C) 2018 Elsevier Ltd. All rights reserved

几种特种材料动态力学性能研究

对NiTi形状记忆合金、冰以及剪切增稠流体(STF)这3种特种材料的动态力学性能开展了研究工作.首先,对NiTi.形状记忆合金的率相关相变行为进行了研究.其次,对冰的动态力学性能进行了测量.最后,研究了强激光冲击作用下STF的动态力学响应

Atomistic study on the anomalous temperature-dependent dynamic tensile strength of ice under shock loading

Although the compressive strength of ice under both quasi-static [M. Arakawa and N. Maeno, Mechanical strength of polycrystalline ice under uniaxial compression. Cold Reg. Sci. Tech 26 (1997), pp. 215-229.] and dynamic [X. Wu and V. Prakash, Dynamic compressive behavior of ice at cryogenic temperatures. Cold Reg. Sci. Tech 118 (2015), pp. 1-13.] loadings shows an anomalous temperature effect that the compression strength is insensitive to temperature in a specific temperature range below -100(o)C, it is still unclear whether the anomalous temperature exists for the tensile strength of ice at cryogenic temperatures. In this paper, the temperature-dependent dynamic tensile strength of ice 1 h under shock loading is investigated by molecular dynamics simulations. It is intriguing to see that the dynamic tensile strength of the ice exhibits a similar anomalous temperature effect, i.e. it is almost insensitive to temperature in the range 117 similar to 163 K, which could be interpreted by the competitive mechanism between shock-induced pulverisation and melting. The evolution of the pentagonal-heptagonal defects and the ductile-to-brittle transformation are also observed with decreasing temperature, leading to the unique dynamic tensile behaviour of ice under shock

Atomistic study on shock behaviour of NiTi shape memory alloy

The shock behaviour of NiTi shape memory alloy is investigated by using molecular dynamics simulation. The nano-pillar samples of the alloy are subjected to the impact of a piston with a velocity of 350 m/s at initial environment temperatures of 325 and 500 K. At 325 K, we observe two different pathways of the formation of BCO phase, the gradient twins, and the detwinning phenomena, strongly depending on the local stress and the deformation state. As the initial temperature increases to 500 K, the plasticity is dominated by the dislocation movements rather than the twinning at 325 K. The phase transformation and plasticity result in stress attenuation when the stress wave propagates through the nano-pillar. Furthermore, it is interesting to note that multiple stress peaks occur due to the formation of local complex atomic structures with various wave speeds, leading to the catch up and overlap of the stress waves.</p

Dynamical Performance of Graphene Aerogel with Ductile and Brittle Characteristics

Current research regarding the efficiency of ultra-light graphene aerogel (GA) energy dissipation is limited to quasi-static tests and simulations. The lack of direct dynamical experiments has impeded its utilization in fields of energy dissipation. Therefore, in this study, the high dynamic energy dissipation capability of GA with ultra-low density is obtained directly from the experiment. It is found that the porous and anisotropic properties of GA render the projectile deflected hierarchically and further induce gradually cascaded failure with asymmetry expansion in the GA. This feature, taking advantage of ductile materials, facilitates energy dissipation capability. Failure morphologies of rippled graphene flakes involve brittle features such as micron-size cracks and local broken flakes. In addition, these coarse-grained molecular dynamics (CGMD) simulation results imply kinetic energy changes due to movement, and fluctuations of graphene flakes are effective ways to dissipate energy. Moreover, the stiffness increase of graphene flakes plays a weakened role in energy dissipation because reduced contact area impedes the effectiveness of stress wave and thermal transfer while also increasing the brittle characteristics of GA. Combining the failure characteristics of brittle materials with the benefits of ductile network materials, GA shows great promise in impact protection applications. The ultra-low-density with high energy dissipation capability of graphene aerogel (GA) is studied in this work. The hierarchical deflection of the projectile causes the gradually cascaded failure characteristic of the GA with an asymmetric expansion. Besides, failure morphologies of graphene flakes involve brittle failure characteristics like micron cracks and local broken flakes. imag

Dynamic response of shear thickening fluid under laser induced shock

The dynamic response of the 57 vol./vol. % dense spherical silica particle-polyethylene glycol suspension at&nbsp;high pressure&nbsp;was investigated through short pulsed laser induced shock experiments. The measured back&nbsp;free surface&nbsp;velocities by a photonic Doppler velocimetry showed that the shock and the&nbsp;particle velocities&nbsp;decreased while the&nbsp;shock wave&nbsp;transmitted in the&nbsp;shear thickening&nbsp;fluid (STF), from which an equation of state for the STF was obtained. In addition, the peak stress decreased and the absorbed energy increased rapidly with increasing the thickness for a thin layer of the STF, which should be attributed to the impact-jammed behavior through compression of particle matrix, the&nbsp;deformation&nbsp;or&nbsp;crack&nbsp;of the hard-sphere particles, and the volume compression of the particles and the polyethylene glycol.</span