Multi-scale study on the dynamic behavior of two-dimensional materials

二维材料具有强度高、柔韧性好、性能可调等优点，在各个领域都发挥着重要的作用。与此同时，二维材料优异的冲击能量耗散能力，使其在冲击防护领域有重要的潜在应用价值。本文主要围绕先进二维材料的冲击动力学行为与高性能设计方法开展相关研究，通过建立微尺度冲击动力学实验技术，发展多尺度数值模拟方法，揭示二维材料的冲击防护性能、能量耗散的宏微观机制，提出具有高防护性能的结构设计方案。主要研究进展包括： 1.针对微米尺度薄膜材料的冲击动力学性能表征，发展了强激光驱动微颗粒高速冲击（Laser-induced particle impact tests, LIPIT）实验技术，建立了真空环境以及高低温（-150oC~800oC）的实验环境，实现了极端环境下的微尺度冲击加载方法。同时，通过量纲分析得到了影响颗粒冲击速度的主控无量纲参数，结合有限元模拟（Finite element method, FEM）得到了主控无量纲参数的影响规律。 2.表征了二维材料的基本力学性能，针对石墨炔（Graphdiyne, GDY）这种新型二维材料，首次通过AFM实验获得了其弹性模量（218.50GPa），并分析了实验参数如纳米探针的加载速度和直径对测量结果的影响。实验测得的弹性模量约为分子动力学（Molecular dynamics, MD）模拟计算结果的一半。通过在MD模型中引入不同数量的缺陷和层数，很好地解释了造成两者结果差异的原因。通过MD计算，在原子水平上得到了GDY薄膜的破坏行为以及断键重组的规律，揭示了GDY薄膜优异的柔韧性。 3.通过LIPIT实验测试了GDY及石墨烯（Graphene, GR）薄膜的微尺度冲击动力学行为。发现随着厚度的增加，材料的比吸能快速减小，并观察到了卷曲和多裂尖的失效模式。通过MD对GDY及GR的冲击动力学行为进行了模拟，得到了材料的弹道极限速度，发现超快的弹性波速、锥形波速和较大的变形为材料提供了优异的耗能能力。另外，计算表明单层石墨炔（Single-layer graphdiyne, SLGDY）与单层石墨烯（Single-layer graphene, SLGR）的比吸能接近，表明SLGDY在冲击防护领域的潜在应用价值。同时，发现GDY与GR的失效模式均与冲击速度相关。 4.测量得到了不同厚度碳纳米管（Carbon nanotube, CNT）薄膜的冲击防护性能，发现其比吸能可达到约1.3MJ/kg，比聚合物薄膜和金属薄膜高1~3倍；建立了CNT薄膜冲击的粗粒化分子动力学（Coarse-grained molecular dynamics, CGMD）模型，揭示了CNT在冲击过程中存在断裂、摩擦、振动等多种耗能机制。 5.发展了高性能薄膜材料设计方案。针对GDY与GR，提出了GR/GDY复合薄膜设计思想，充分利用GR高强度和GDY高柔韧性的特点，通过二者的耦合增强耗散效应，提高了材料的冲击防护性能。针对CNT薄膜，提出了交联的方法来加强CNT之间的相互作用、调控材料的吸能模式，实现冲击能量的非局域化耗散，从而实现高防护性能设计，并通过CGMD模拟，给出了优化的结构设计参数。</p

一种真空及高低温加载的微颗粒高速冲击实验装置

本发明涉及微颗粒高速冲击实验装置技术领域,提供了一种真空及高低温加载的微颗粒高速冲击实验装置包括：真空箱,真空箱内设置有用于测试并且可调节温度的试验机构,试验机构上设置有样品,真空箱外设置有光件机构,真空箱和试验机构以及光件机构均电性连接至控制系统；光件机构结合位于真空箱内的试验机构,通过控制系统的操控实现了高低温控制及微尺度冲击加载,对微纳尺度材料的动态力学行为及能量耗散机制进行表征,为材料在极端环境下的应用提供关键技术支撑与理论依据；显著提升了冲击加载手段实验效率,测量数据精准,并且使用过程中不易发生磨损,降低维修费用

Dynamic Mechanical Properties of Several High-Performance Single Fibers

High-performance fiber-reinforced composites (FRCs) are widely used in bulletproof structures, in which the mechanical properties of the single fibers play a crucial role in ballistic resistance. In this paper, the quasi-static and dynamic mechanical properties of three commonly used fibers, single aramid III, polyimide (PI), and poly-p-phenylenebenzobisoxazole (PBO) fibers are measured by a small-scale tensile testing machine and mini-split Hopkinson tension bar (mini-SHTB), respectively. The results show that the PBO fiber is superior to the other two fibers in terms of strength and elongation. Both the PBO and aramid III fibers exhibit an obvious strain-rate strengthening effect, while the tensile strength of the PI fiber increases initially, then decreases with the increase in strain rate. In addition, the PBO and aramid III fibers show ductile-to-brittle transition with increasing strain rate, and the PI fiber possesses plasticity in the employed strain rate range. Under a high strain rate, a noticeable radial splitting and fibrillation is observed for the PBO fiber, which can explain the strain-rate strengthening effect. Moreover, the large dispersion of the strength at the same strain rate is observed for all the single fibers, and it increases with increasing strain rate, which can be ascribed to the defects in the fibers. Considering the effect of strain rate, only the PBO fiber follows the Weibull distribution, suggesting that the hypothesis of Weibull distribution for single fibers needs to be revisited

Micron-Thick Interlocked Carbon Nanotube Films with Excellent Impact Resistance via Micro-Ballistic Impact

The highest specific energy absorption (SEA) of interlocked micron-thickness carbon nanotube (IMCNT) films subjected to micro-ballistic impact is reported in this paper. The SEA of the IMCNT films ranges from 0.8 to 1.6 MJ kg(-1), the greatest value for micron-thickness films to date. The multiple deformation-induced dissipation channels at the nanoscale involving disorder-to-order transition, frictional sliding, and entanglement of CNT fibrils contribute to the ultra-high SEA of the IMCNT. Furthermore, an anomalous thickness dependency of the SEA is observed, that is, the SEA increases with increasing thickness, which should be ascribed to the exponential growth in nano-interface that further boosts the energy dissipation efficiency as the film thickness increases. The results indicate that the developed IMCNT overcomes the size-dependent impact resistance of traditional materials and demonstrates great potential as a bulletproof material for high-performance flexible armor

Nanoparticles produced by nanosecond pulse laser ablation of a metallic glass in water

In this paper, we perform a single nanosecond pulse laser ablation of a Zr-based metallic glass (Vitreloy 1) target in water. The violent ejection of high-temperature ablation matter is observed from the target by means of explosive boiling, which is accompanied by a formation of cavitation bubble. These ablation products entered the water include a rich variety of nanoparticles that can be classified into three different types: full amorphous, amorphous-crystalline composite, and polycrystalline. The amorphous nanoparticles have relatively smaller sizes and a uniform elemental distribution. The latter two types of nanoparticles exhibit a unique core-shell feature with an obvious compositional segregation. It is proposed that the diversity of the nanoparticles closely depends on the different pathways that they enter the water: directly before the bubble formation or via the bubble, in which the cooling rate and the glass-forming ability of ablation products are two competing factors

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]

Load sharing and accumulated bond fracture in ion-irradiated carbon mat for energy dissipation

Multiwalled carbon nanotube (MWNT) aerogel mats were irradiated with carbon ions to explore the effect of irradiation-induced sp(3) bonds and sp(2) bond defects on ultrahigh strain rate mechanical properties. Energy dissipation was measured using a microprojectile impact test. Specific penetration energy Ep* increased strongly with irradiation with a maximum E-p* of similar to 26 megajoules per kilogram, over 200% higher than the previous best energy-absorbing material of pristine MWNT mats and at least an order of magnitude higher than any other material tested at the microscale. Perforation morphologies observed by electron microscopy show that a much larger network region is deformed due to sp(3) bond enhanced load sharing within and between tubes, while defects introduced by the radiation induce more bond, shell, and tube damage leading to strongly enhanced energy dissipation

Anomalous size effect of impact resistance in carbon nanotube film

Dynamic mechanical behavior and size-related impact resistance of CNT films are studied by employing laserinduced projectile impact test (LIPIT) and coarse-grained molecular dynamics (CGMD) simulation. The energy dissipation mechanisms of the CNT films are investigated via CGMD simulations. An evident anomalous thickness-dependent effect is directly observed in the experiment, consistent with simulation phenomena. The mechanisms underlying this anomalous thickness-dependent effect are investigated at the atomic scale. The disparities between experiments and simulations are discussed. Our analysis of energy dissipation modes, deformation behaviors during impact, and impact area reveals that kinetic energy change predominantly governs the deformation mode. Meanwhile, a plugging failure mode near the exit face of CNT film is identified at high impact velocity (similar to 160 m/s), leading to a deterioration in impact resistance and a corresponding reduction in SEA with increasing CNT film thickness. These findings provide a feasible strategy for the protection design of CNT film in broaden protective application scenarios

Nanoindentation of thin graphdiyne films: Experiments and molecular dynamics simulation

Graphdiyne possesses not only high strength but also excellent ductility, making it possible to be used in future high-performance protective structures. In this paper, the mechanical properties of graphdiyne were firstly measured by AFM experiments, and the failure behavior during low velocity perforation was also investigated by molecular dynamics (MD) simulations. Firstly, the elastic modulus was measured to be about 218.5 GPa by AFM experiments, which is about half of its ideal value due to various defects and the layer numbers of the synthesized graphdiyne film. Then, the nanoindentation processes of graphdiyne films were investigated by MD simulations, and the elastic modulus and strength were simulated to be about 489.04 GPa and 33.95 GPa, respectively. The failure behavior of the graphdiyne film was also studied in atomic level. Sequential broken of C C, C=C and C-C bonds and recombination of the broken bonds were observed to form a unique lathy crack. Furthermore, the effects of loading speed and indenter radius on the mechanical response of graphdiyne were investigated. A revised formula was developed for analyzing the mechanical properties of films in AFM experiments under various loading conditions. (C) 2018 Elsevier Ltd. All rights reserved

Residual stress analysis of thin film photovoltaic cells subjected to massive micro-particle impact

Residual stresses play a crucial role in both light-electricity conversion performances and the lifespan of photovoltaic (PV) cells. In this paper, the residual stress of triple junction cells (i.e. GaInP/GaInAs/Ge) induced by laser-driven massive micro-particle impact is analyzed with a novel method based on backscattering Raman spectroscopy. The impact process, which induces damage to the PV cells and brings the residual stress, is also investigated by optical microscopy (OM) and Scanning Electron Microscopy (SEM). The results show that the PV cells would exhibit various damage patterns. At the same time, strong residual stresses up to hundreds of MPa introduced in the damaged PV cells after impact have been analysis, providing an effective perspective to better understand the damage behavior and residual stress features of PV cells during their service life