In-plane crushing behavior and energy absorption design of composite honeycombs

Theoretical analysis and numerical simulation methods were used to study the in-plane crushing behavior of single-cell structures and regular and composite honeycombs. Square, hexagonal, and circular honeycombs were selected as honeycomb layers to establish composite honeycomb models in the form of composite structures and realize the complementary advantages of honeycombs with type I and type II structures. The effects of honeycomb layer arrangement, plastic collapse strength, relative density, and crushing velocity on the deformation mode, plateau stress, load uniformity, and energy absorption performance of the composite honeycombs were mainly considered. A semi-empirical formula for plateau stress and energy absorption rate per unit mass for the composite honeycombs was developed. The results showed that the arrangement mode of honeycomb layers is an important factor that affects their mechanical properties. Appropriately selecting the arrangement of honeycomb layers and the proportion of honeycomb layers with different structures in a composite honeycomb can effectively improve its load uniformity and control the magnitude of plateau stress and energy absorption capacity.     

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。     

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。     

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。     

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。     

具有恒定冲击载荷的梯度泡沫金属材料设计

多胞材料可通过大变形大量地吸收冲击能量,引入密度梯度可进一步提高其耐撞性。梯度多胞材料的宏观力学响应对材料密度分布极为敏感,不同类型的细观构型的影响也极为不同。已有的研究工作主要局限在对给定的密度梯度分析其动态响应,较少对耐撞性设计方法进行研究。本文针对梯度闭孔泡沫金属材料,基于非线性塑性冲击波模型发展了耐撞性反向设计方法,以维持冲击物受载恒定为目标,运用级数法获得了简化模型和渐近解。利用变胞元尺寸法构建了连续梯度变化的三维Voronoi细观有限元模型,并利用ABAQUS/Explicit有限元软件对理论设计进行数值验证。结果表明,反向设计理论简化模型的渐近解对于梯度闭孔泡沫金属材料的耐撞性设计是有效的,所提出的耐撞性设计方法在控制冲击吸能过程和冲击物受载方面具有指导意义。     

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     

Compressive response and failure of balsa wood

Balsa wood is a natural cellular material with excellent stiffness-to-weight and strength-to-weight ratios as well as superior energy absorption characteristics. These properties are derived from the microstructure, which consists of long slender cells (tracheids) with approximately hexagonal crosssections that are arranged axially. Parenchyma are a second type of cells that are radially arranged in groups that periodically penetrate the tracheids (rays). Under compression in the axial direction the material exhibits a linearly elastic regime that terminates by the initiation of failure in the form of localized kinking. Subsequently, under displacement-controlled compression, a stress plateau is traced associated with the gradual spreading of crushing of the cells through the material. The material is less stiff and weaker in the tangential and radial directions. Compression in these directions crushes the tracheids laterally but results in a monotonically increasing response typical of lateral crushing of elastic honeycombs. The elastic and inelastic properties in the three directions have been established experimentally as a function of the wood density. The microstructure and its deformation modes under compression have been characterized using scanning electron microscopy. In the axial direction it was observed that in the majority of the tests, failure initiated by kinking in the axial–tangential plane. The local misalignment of tracheids in zones penetrated by rays ranged from 4° to 10° and axial compression results in shear in these zones. Measurement of the shear response and the shear strength in the planes of interest enabled estimation of the kinking stress using the Argon–Budiansky kinking model. The material strength predicted in this manner has been found to provide a bounding estimate of the axial strength for a broad range of wood densities. The energy absorption characteristics of the wood have also been measured and the specific energy absorption was found to be comparable to that of metallic honeycombs of the same relative density.     

The effects of hierarchy on the in-plane elastic properties of honeycombs

Introducing hierarchy into structures has been credited with improving elastic properties and damage tolerance. Specifically, adding hierarchical sub-structures to honeycombs, which themselves have good-density specific elastic and energy-absorbing properties, has been proposed in the literature. An investigation of the elastic properties and structural hierarchy in honeycombs was undertaken, exploring the effects of adding hierarchy into a range of honeycombs, with hexagonal, triangular or square geometry super and sub-structure cells, via simulation using finite elements. Key parameters describing these geometries included the relative lengths of the sub- and super-structures, the fraction of mass shared between the sub- and super-structures, the co-ordination number of the honeycomb cells, the form and extent of functional grading, and the Poisson’s ratio of the sub-structure. The introduction of a hierarchical sub-structure into a honeycomb, in most cases, has a deleterious effect upon the in-plane density specific elastic modulus, typically a reduction of 40 to 50% vs a conventional non-hierarchical version. More complex sub-structures, e.g. graded density, can recover values of density specific elastic modulus. With careful design of functionally graded unit cells it is possible to exceed, by up to 75%, the density specific modulus of conventional versions. A negative Poisson’s ratio sub-structure also engenders substantial increases to the density modulus versus conventional honeycombs.     

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      

冲击载荷下径向密度排布对泡沫金属力学性能影响的研究

参照层状密度梯度泡沫模型实现方法,利用3D-Voronoi技术设计了新型径向密度梯度泡沫模型,并用有限元软件,对它在不同冲击载荷下的力学行为进行数值模拟。研究冲击速度、密度梯度和平均相对密度对金属泡沫冲击端、支撑端应力和能量吸收能力的影响,发现:径向正梯度泡沫与层状正、负梯度泡沫相比,其两端的应力值均较小,可同时保护冲击端、支撑端物体;径向负梯度泡沫两端应力变化幅度较小,能够保证物体受力稳定;几种泡沫金属的能量吸收能力在不同冲击速度下发生交替变化。对于径向梯度泡沫,能量吸收能力对密度梯度大小不敏感,对梯度方向敏感,径向负梯度泡沫的能量吸收能力始终大于径向正梯度泡沫;平均相对密度越大,径向正、负梯度泡沫两端应力越大、吸能效果越好。     

梯度蜂窝面外动态压缩力学行为与吸能特性研究

蜂窝材料具有优异的抗冲击吸能特性。为进一步提高蜂窝材料的比吸能与压缩力效率,提出了一种几何参数或材料参数沿厚度方向梯度渐变的蜂窝材料模型,并针对六边形蜂窝构型研究了胞元壁厚和屈服强度梯度变化的蜂窝材料在面外动态压缩载荷下的力学行为与吸能特性。研究结果表明,通过调控梯度变化的指数,胞元壁厚或母体材料屈服强度的梯度设计均可有效降低初始峰值应力,并使蜂窝材料的比吸能和压缩力效率同时增大。研究结果可为蜂窝材料的防撞性优化设计提供新的思路。     

Role of cell regularity and relative density on elasto-plastic compression response of random honeycombs generated using Voronoi diagrams

The Voronoi tessellation technique and solid modeling methods are used in this work to create virtual random structures and link cell morphology with the mechanical behavior. Their compression responses are analyzed using the finite element method. First, the effect of loading direction is analyzed for structures with different levels of randomness characterized by a regularity parameter to assess the degree of scatter in the results. Subsequently, morphological characteristics such as arrangement of cells and randomness are analyzed separately. The effect of relative density on structures with different levels of randomness is also studied. Simulations suggest that at low relative densities the arrangement of cells has a negligible effect on the compression response of random honeycombs. On the contrary, the cellular randomness has significant influence on the elastic and plastic characteristics especially when fully random structures are compared with the regular counterparts.     

分层递变梯度蜂窝材料的面内冲击性能

提出了一种分层递变梯度蜂窝材料模型,以期控制蜂窝材料的能量动态吸收性能.此模型通过改变胞元的半径来改变蜂窝材料的面内特征参数,以实现蜂窝材料面内动力响应特性的多目标优化设计.计算结果表明,此模型可以在减小初始峰值应力水平的前提下,同时实现材料能量吸收过程的控制,并可以有效控制进入被保护结构的应力水平.此模型可为蜂窝材料...     

Propagation of compaction waves in metal foams exhibiting strain hardening

One-dimensional models for compaction of cellular materials exhibiting strain hardening are proposed for two different impact scenarios. The models reveal the characteristic features of deformation under the condition of decreasing velocity during the impact event. It was established that an unloading plastic wave of strong discontinuity propagates in the foam and it has a significant dynamic effect on the foam compaction and energy absorption. The proposed models are based on the actual experimentally derived stress strain curves. The compaction mechanism in three aluminium based foam materials, two of them with relatively low density – Alporas and Cymat with 9% and 9.3% relative density, respectively and a higher density Cymat foam with 21% relative density, is analysed. Numerical simulations were carried out to verify the proposed models.The predictions of the proposed models are compared with published analytical models of compaction of cellular materials which assume a predefined densification strain. It is shown that the approximation of a cellular material with significant strain hardening by the Rigid Perfectly-Plastic-Locking (RPPL) model can lead to an overestimation of the energy absorption capacity for the observed stroke due to the non-uniform strains along the compacted zone of the actual material in contrast to the predefined constant densification strain in the RPPL model. The assumption of a constant densification strain leads also to an overestimation of the maximum stress, which occurs under impact.     

多孔材料吸能行为对相对密度和冲击速度的依赖性

多孔材料是一种优异的吸能缓冲材料,但由于其变形模式的非单一性以及动态应力应变曲线的难获取性,其吸能行为对相对密度和冲击速度的依赖性关系还并不完全明朗。本文基于不需要提前作本构假定的波传播法,开展了多孔材料的吸能行为研究。采用多孔材料的细观有限元模型进行Taylor冲击虚拟实验,获取全场质点速度时程曲线,结合Lagrange分析法得到多孔材料的局部应力应变信息,进而探讨了动态吸能性能对材料相对密度和冲击速度的依赖性。研究结果表明多孔材料的吸能行为可依据变形模式分为三个阶段。在冲击模式下,多孔材料单位体积吸能与相对密度成线性增加关系,此时惯性起主导作用;在过渡模式下,惯性的主导作用减弱,单位体积吸能量的增加速率随相对密度的增加而减弱;在准静态模式下,多孔材料只能发生微小的变形,其吸能很少。本文进一步获得了区别于多孔材料准静态应力-应变曲线的动态应力-应变状态曲线,并考察了其与相对密度之间的关系。结果表明：随着相对密度的增加,多孔材料的动态压实应变将变小,而动态塑性平台应力将提高。     

负梯度闭孔泡沫金属的力学性能分析

运用三维Voronoi技术生成闭孔梯度泡沫模型,结合有限元分析方法模拟负梯度闭孔泡沫金属在不同冲击速度下的力学行为。结果表明,随着冲击速度的提高,得到了与均匀泡沫一样的三种变形模式：准静态模式,过渡模式和冲击模式。通过对名义应力应变曲线和变形模式的研究,提出了一种新的定义局部密实化应变的方法,并研究了相对密度和密度梯度对它的影响。分别建立了相对密度和密度梯度与冲击速度的变形模式图。通过引入密实化因子,确定了三种变形模式对应的临界冲击速度。最后讨论了不同冲击速度下,密度梯度大小对泡沫材料能量吸收能力的影响。结果表明,在高速冲击的变形初期,密度梯度的绝对值越大,泡沫材料的能量吸收能力越强。     

金属多级类蜂窝的压溃行为研究

通过实验和数值模拟方法系统研究了单胞壁开孔金属多级类蜂窝与双胞壁开孔金属多级类蜂窝的压溃行为. 重点分析了试件尺寸、开孔位置、孔偏距和孔梯度等因素对多级类蜂窝力学性能的影响. 结果表明,多级类蜂窝的压溃过程可分为3个阶段:弹性变形、屈曲变形以及密实;单胞壁开孔多级类蜂窝的压溃过程趋向于渐近内凹压溃,而双胞壁开孔多级类蜂窝趋向于轴向压溃;试件尺寸对多级类蜂窝的力学行为有明显的影响,当胞元数达到一定数目时,其力学性能几乎与蜂窝胞元数无关. 单胞壁开孔多级类蜂窝的峰值应力大于双胞壁开孔多级类蜂窝的峰值应力,但其平均压溃应力小于双胞壁开孔多级类蜂窝的平均压溃应力;与传统蜂窝相比,蜂窝胞壁开孔设计降低了蜂窝材料的比吸能;孔偏距的存在导致单胞壁开孔多级类蜂窝的峰值应力降低,但随着孔偏距的增加其平均压溃应力呈先减低后增加趋势;多梯度孔设计对多级类蜂窝材料的力学性能有重要影响,与均匀孔多级类蜂窝相比,正梯度孔分布设计降低了多级类蜂窝峰值应力,但提高了其平均压溃应力;多梯度孔分布设计对多级类蜂窝的峰值应力和平均压溃应力影响不大.      

Crashworthiness design of density-graded cellular metals

Crashworthiness of cellular metals with a linear density gradient was analyzed by using cell-based finite element models and shock models. Mechanisms of energy absorption and deformation of graded cellular metals were explored by shock wave propagation analysis. Results show that a positive density gradient is a good choice for protecting the impacting object because it can meet the crashworthiness requirements of high energy absorption, stable impact resistance and low peak stress.