具有负泊松比效应蜂窝材料的面内冲击动力学性能

基于显式动力有限元ANSYS/LS-DYNA,研究了面内冲击作用下具有负泊松比效应蜂窝材料的 动态冲击性能。在保证胞元壁长和壁厚不变的前提下,通过改变胞元扩张角,建立了内凹六边形蜂窝模型。 具体讨论了胞元扩张角和冲击速度对蜂窝材料面内冲击变形和能量吸收能力的影响。研究发现,在冲击载荷 作用下,内凹蜂窝材料的面内冲击性能依赖于胞元扩张角。胞元扩张角的绝对值越大,冲击端的平台应力越 高。随着冲击速度的提高,蜂窝材料表现出更强的能量吸收能力。     

胞孔构型对金属蜂窝动态力学性能的影响机理

采用ANSYS/LS-DYNA有限元研究了具有不同胞孔构型和排列方式的金属蜂窝材料在面内冲击荷载下的力学性能。在蜂窝的相对密度和冲击速度保持恒定的情况下,比较了它们的变形模式、动态承载力和能量吸收性能。结果表明,不同的胞孔构型导致在蜂窝压垮过程中胞壁的受力状态不同,从而影响蜂窝的宏观力学性能。根据胞壁的应力状态,可将蜂窝分为膜力主导蜂窝和弯曲主导蜂窝2大类。研究结果显示,蜂窝吸收的能量绝大部分转化为变形所需的内能,并且膜力主导蜂窝的内能占总能量的百分比更大。胞壁的屈曲导致膜力主导蜂窝的应力应变曲线呈现较大的波动。膜力主导蜂窝在变形过程中其胞壁会耗散更多的内能,从而比弯曲主导蜂窝具有更高的动态承载力和能量吸收能力。     

胞元微拓扑结构对蜂窝材料面内冲击性能的影响

研究了面内冲击载荷作用下胞元微拓扑结构对蜂窝材料动态冲击性能的影响。首先,在胞元边长、厚度一致的条件下,讨论了不同形状胞元、以及胞元形状相同但排列方式不同的蜂窝材料的动态冲击性能,并给出了试件及其微结构的动态演化过程。在此基础上,讨论了胞元微观排列方式对蜂窝材料的能量吸收能力的影响。计算结果表明,除了胞元基本结构参数（边长、壁厚等）,胞元形状及排布方式也是影响蜂窝材料动态性能的重要因素。由于三角形单胞的稳定性,三角形填充蜂窝材料与四边形填充蜂窝材料相比,表现出更强的能量吸收能力。而交错排布则对应着更加均匀的变形和稳定的平台区。同时,局部拓扑结构的变化,交错排布的试件在冲击压缩的过程中表现出独特的颈缩现象。此结论将为蜂窝材料微结构的动力学优化设计提供指导和依据。     

含随机填充孔圆形蜂窝结构的面内冲击性能

研究多孔材料细观结构与宏观力学性能之间的关系, 建立具有固定相对密度的含随机固体填充孔的圆形蜂窝结构模型。在此模型的基础上具体讨论了不同孔洞填充比和冲击速度对圆形蜂窝结构变形模式、动态冲击平台应力以及能量吸收性能的影响。研究结果表明:填充孔在蜂窝变形过程中有局部牵制作用, 蜂窝材料变形模式仍为准静态模式、过渡模式、动态模式; 当变形模式为过渡模式或动态模式时, 结构的平台应力与速度的平方成线性关系, 存在明显的速度效应; 高速冲击下, 含固体填充孔的蜂窝结构单位质量吸收的能量高于规则蜂窝结构。研究结果可为蜂窝材料的研究和设计提供参考。     

随机缺陷对蜂窝结构动态行为影响的有限元分析

通过对胞壁随机移除的蜂窝结构动态变形过程的有限元模拟,分析了随机缺陷对蜂窝结构变形模式的影响,得到蜂窝结构在两个加载方向上的变形模式图及不同模式间转换的临界速度. 对含缺陷蜂窝结构平台应力的研究发现,当变形模式为过渡模式或动态模式时结构平台应力与冲击速度的平方成线性关系. 相同密度下,低缺陷蜂窝结构的平台应力在由过渡模式向动态模式转变的临界速度附近高于规则蜂窝结构,较高的随机缺陷则使蜂窝结构的平台应力在由准静态模式向过渡模式转变的临界速度附近显著下降.关键词:多孔材料,蜂窝,缺陷,平台应力,有限元分析      

沉痛悼念许兵先生

通过对胞壁随机移除的蜂窝结构动态变形过程的有限元模拟,分析了随机缺陷对蜂窝 结构变形模式的影响,得到蜂窝结构在两个加载方向上的变形模式图及不同模式间转换的临 界速度. 对含缺陷蜂窝结构平台应力的研究发现,当变形模式为过渡模式或动态模式时结构 平台应力与冲击速度的平方成线性关系. 相同密度下,低缺陷蜂窝结构的平台应力在由过渡 模式向动态模式转变的临界速度附近高于规则蜂窝结构,较高的随机缺陷则使蜂窝结构的平 台应力在由准静态模式向过渡模式转变的临界速度附近显著下降. 关键词：多孔材料,蜂窝,缺陷,平台应力,有限元分析     

负泊松比蜂窝面内动态压缩行为与吸能特性研究

利用有限元软件ANSYS-LSDYNA研究了负泊松比蜂窝结构面内冲击动力学特性。在壁长和相对密度不变的前提下,建立了负泊松比蜂窝模型;通过改变胞元扩展角,讨论了冲击速度对蜂窝材料面内冲击变形模式和能量吸收能力的影响。数值研究发现,冲击载荷作用下负泊松比蜂窝结构的面内冲击性能更多依赖于冲击速度。提高冲击速度,冲击端的峰值应力、平台应力、试件的比吸能均增高;但在相同冲击速度下,冲击端和支撑端的峰值应力、平台应力、试件的比吸能均随胞元扩展角的增大而降低。     

负泊松比蜂窝材料的动力学响应及能量吸收特性

针对传统正方形蜂窝,通过用更小的双向内凹结构胞元替代原蜂窝材料的结构节点,得到了一种具有负泊松比特性的节点层级蜂窝材料模型。利用显式动力有限元方法,研究了冲击荷载作用下该负泊松比蜂窝结构的动力学响应及能量吸收特性。研究结果表明,除了冲击速度和相对密度,负泊松比蜂窝材料的动力学性能亦取决于胞元微结构。与正方形蜂窝相比,该负泊松比层级蜂窝材料的动态承载能力和能量吸收能力明显增强。在中低速冲击下,试件表现为拉胀材料明显的"颈缩"现象,并展示出负泊松比材料独特的平台应力增强效应。基于能量吸收效率方法和一维冲击波理论,给出了负泊松比蜂窝材料的密实应变和动态平台应力的经验公式,以预测该蜂窝材料的动态承载能力。本文的研究将为负泊松比多胞材料冲击动力学性能的多目标优化设计提供新的设计思路。     

部分充液多胞元结构的面内动态力学特性研究

为探究部分充液多胞元结构的抗冲击防护性能,结合充液内凹胞元的落锤冲击试验,建立了充液内凹胞元、部分充液内凹多胞元结构的冲击动态特性二维FEM数值分析,计算得到了部分充液内凹多胞元结构的变形破坏模式,讨论了不同冲击速度下部分充液内凹多胞元结构的动力学响应特性。结果表明:在充液胞元破损后,水介质会流入相邻未充液胞元,形成二次鼓胀吸能效应,从而有效提高结构壁面的变形吸能水平;结构中的充液区域和未充液区域的变形破坏模式分别为鼓胀拉伸和屈曲弯折;随着冲击速度的提高,结构的单位体积应变能以及对初始冲击载荷的削弱作用均得到增强。横向充液方式可以等效为变刚度弹簧的串联布置,该方式仅影响结构的局部刚度,纵向充液方式可以等效为多层变刚度弹簧的并联布置,该方式会影响结构的整体刚度;充液区域与未充液区域的等效刚度呈动态变化,结构变形模式由各区域实时的等效刚度决定。当载荷冲击速度较高时,横向和纵向部分充液内凹多胞元结构对初始冲击载荷的削弱能力均优于未充液内凹多胞元结构。     

负泊松比蜂窝材料抗爆炸特性及优化设计研究

为了深入研究车辆底部防护组件爆炸冲击下的结构响应,提高防护型车辆的抗爆炸冲击性能,建立了某车辆底部防护组件在爆炸冲击下的有限元模型,并进行爆炸冲击台架试验验证了有限元模拟的可靠性;将内凹六边形负泊松比蜂窝材料作为防护组件的夹芯部分,分析负泊松比蜂窝材料在爆炸冲击下的变形模式,并对比了同等质量的其他3种防护组件的抗爆炸冲击性能。结果表明,含有负泊松比蜂窝夹芯的防护组件具有更优的抗爆性能。建立了以内凹六边形负泊松比蜂窝胞元尺寸参数为设计变量的多目标优化问题的数学模型,采用多目标遗传算法获得胞元几何参数的最优方案,有效降低了防护组件基板的最大挠度和最大动能。     

Mechanical behavior of hexagonal honeycombs under low-velocity impact – theory and simulations

Based on the cells’ collapse mechanisms of the hexagonal honeycombs revealed from the numerical simulations under the low-velocity impact, an analytical model is established to deduce the crushing strength of the honeycomb and the stress at the supporting end both as functions of impact velocity, cell size, cell-wall angle, and the mechanical properties of the base material. The results show that the honeycomb’s crushing strength increases with the impact velocity, while the supporting stress decreases with the increase of the impact velocity. Combining with the dynamic predictions under the high-velocity impact in our previous work (Hu and Yu, 2010), the crushing strength of the honeycombs can be analytically predicted over wide range of crushing velocities. The analytical expression of the critical velocity is also obtained, which offers the boundary for the application of the functions of the honeycomb’s crushing strength under the low-velocity and the high-velocity impacts. All of the analytical predictions are in good agreement with the numerical simulation results.     

A numerical study on the rate sensitivity of cellular metals

Metallic foams have non-linear deformation behavior, which make them attractive in many applications. Many experimental researches on the dynamic behavior and rate sensitivity of cellular metals have been reported in the literature, but they contain conflicting, and sometimes confusing, conclusions on the strain-rate and inertia effect of cellular metals. In this paper, the dynamic crushing behavior of 2D Voronoi honeycomb is studied by finite element method. The influences of inertia, strain hardening and strain-rate hardening of metal matrix on the deformation mode and plateau stress of the honeycomb are investigated. Three deformation modes are found in different velocity ranges. According to the numerical results, it is found that the plateau stress increases significantly with the increase of impact velocity due to non-uniform deformation induced by inertia. The strain-hardening effect is slight in our numerical tests and the rate effect of the honeycomb is obviously weaker than that of the cell wall material.     

Inertia effects on the progressive crushing of aluminium honeycombs under impact loading

This paper presents the test results under quasi-static and impact loadings for a series of aluminum honeycombs (3003 and 5052 alloys) of different cell sizes, showing significantly different enhancements of the crushing pressure between 3003 honeycombs and the 5052 ones. A comprehensive numerical investigation with rate insensitive constitutive laws is also performed to model the experimental results for different cell size/wall thickness/base material, which suggests that honeycomb crushing pressure enhancement under impact loading is mostly due to a structural effect.Such simulated tests provide detailed local information such as stress and strain fields (in the cell wall) during the whole crushing process of honeycombs. A larger strain (in the cell wall) under impact loading than for the quasi-static case before each successive folding of honeycombs is observed, because of the lateral inertia effect. Thus, differences of the ratios of the stress increase due to strain hardening over the yield stress between 3003 and 5052 alloys lead to the different enhancements of crushing pressure. This result illustrates that the lateral inertia effect in the successive folding of honeycombs is the main factor responsible for the enhancement of the crushing pressure under impact loading.     

Impact behavior of honeycombs under combined shear-compression. Part II: Analysis

In this paper, a numerical virtual model of honeycomb specimen as a small structure is used to simulate its combined shear-compression behavior under impact loading. With ABAQUS/Explicit code, the response of such a structure made of shell elements is calculated under prescribed velocities as those measured in the combined shear-compression tests presented in Part I of this study.The simulated results agree well with the experimental ones in terms of overall pressure/crush curves and deformation modes. It allows for the determination of the separated normal behavior and shear behavior of honeycomb specimen under dynamic combined shear-compression. It is found that the normal strength of honeycombs decreases with increasing shearing load. Quasi-static calculations were also performed and a significant dynamic strength enhancement found in experiments was validated again in the numerical work. A crushing envelope in normal strength vs. shear strength plane was obtained on the basis of these simulations.     

Dynamic crushing and energy absorption of regular,irregular and functionally graded cellular structures

The in-plane dynamic crushing of two dimensional honeycombs with both regular hexagonal and irregular arrangements was investigated using detailed finite element models. The energy absorption of honeycombs made of a linear elastic-perfectly plastic material with constant and functionally graded density were estimated up to large crushing strains. Our numerical simulations showed three distinct crushing modes for honeycombs with a constant relative density: quasi-static, transition and dynamic. Moreover, irregular cellular structures showed to have energy absorption similar to their counterpart regular honeycombs of same relative density and mass. To study the dynamic crushing of functionally graded cellular structures, a density gradient in the direction of crushing was introduced in the computational models by a gradual change of the cell wall thickness. Decreasing the relative density in the direction of crushing was shown to enhance the energy absorption of honeycombs at early stages of crushing. The study provides new insight into the behavior of engineered and biological cellular materials, and could be used to develop novel energy absorbent structures.