基于水平土拱效应的桩间挡土板土压力计算研究

基于``平板开缝-装配-焊接'工艺制备了以高聚物为基体的Kagome等蜂窝结构,并开展 了Kagome, 正三角形和菱形蜂窝结构的面内准静态压缩力学行为实验研究,实验过程中应用 CCD图像采集系统和图像相关法对试件进行了全场位移监测. 另外对比传统正六边形蜂窝, 采用数值分析技术,模拟了低速冲击下不同蜂窝结构坍塌行为. 实验结果和数值模拟均 揭示了在材料用量和结构尺寸完全相同的情况下,Kagome蜂窝结构的面内能量吸收性能优于 其它3种蜂窝结构,并发现了Kagome蜂窝压缩变形时所特有的局部蜂窝旋转变形. 研究结果 表明改变蜂窝形状和周期性排布会对蜂窝结构整体的变形模式以及能量吸收性能产生较大的 影响.     

四边手性蜂窝动态压溃行为的数值模拟

建立了四边手性蜂窝的有限元模型,采用数值模拟方法研究了四边手性蜂窝在不同冲击速度下的变形模式和能量吸收等动力学响应特性,并同普通六边形蜂窝的冲击行为进行了对比。计算得到了这2种蜂窝的变形模式图、动力响应曲线和能量吸收曲线。模拟结果表明:低速冲击下,四边形手性蜂窝的变形模式为“Z”字形;高速冲击下,四边手性蜂窝的变形模式与普通蜂窝的“I”字形模式类似;在适中速度的冲击下,四边手性蜂窝表现出兼具高速冲击和低速冲击特征的一种过渡态变形模式;随着冲击速度的提高,局部变形带由固定端向冲击端移动,并且能量吸收能力也随之提高;在中、低速度的冲击下,能够观察到拉胀材料压缩时特有的“缩颈”现象。     

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...     

爆炸载荷作用下铝蜂窝夹芯板动力响应研究

采用自行设计的冲击摆实验系统对铝蜂窝夹芯板在爆炸载荷作用下的动力响应进行了系统实验研究,给出了面板和铝蜂窝不同区域的不同变形模态,得到了不同炸药当量对铝蜂窝夹芯板动态响应的影响规律,证实了铝蜂窝夹芯板产生较大塑性变形时,比一般的结构具有更好的能量吸收特性.并利用LS-DYNA对其动力响应进行了数值仿真,考察了炸药起爆、接触界面及上表面接触力对夹芯板变形影响的全过程,得到了板中心的最终变形和芯层的变形模式,与实验结果吻合较好.     

低速冲击下蜂窝铝板的表面变形检测及其吸能特性研究

铝质蜂窝结构由于其优良的物理和机械性能被广泛地应用于工程结构,因而对其力学性能的研究也成为越来越重要的课题。本文通过摆锤冲击实验,研究了蜂窝铝的动态压缩性能及低速冲击吸能特性。结果表明,在球形摆锤冲击下,蜂窝铝结构具有很好的冲击吸能特性,其变形特征与准静态变形类似。研究中通过高速摄像系统和加速度传感器记录摆锤的加速度衰减过程,通过投影光载波技术获得受冲击蜂窝铝板表面的压缩行为和三维变形形貌。最后利用有限元方法进一步分析了蜂窝铝结构中内部损伤形式等实验中无法观测的现象,并对蜂窝铝结构参数对其吸能行为的影响进行了详细的讨论。     

蜂窝夹芯板与Whipple结构对撞击能量吸收与耗散的特性比较

通过数值仿真模拟弹丸高速撞击蜂窝夹芯板和Whipple结构,研究蜂窝芯对弹丸碎片云形态的影响;并研究了弹丸、蜂窝夹芯板、Whipple结构的能量吸收与耗散。结果表明:弹丸撞击蜂窝夹芯板后碎片云形态呈近似椭球体,且长半轴明显较长,而弹丸撞击Whipple结构的碎片云形态呈近似球体;蜂窝芯吸收的能量随弹丸的破碎程度的增强而增加;弹丸能量衰减主要发生在撞击蜂窝夹芯板的前后面板和Whipple结构的两层板,蜂窝芯的吸能作用使得Whipple结构吸收的能量高于蜂窝夹心板面板吸收的能量。     

面外压力下蜂窝结构弹性屈曲临界载荷

1 引言蜂窝结构越来越广泛地用于船舶、车辆、飞机、航天器、大型空间站,并被用作能量吸收器件.例如,航天器登陆舱上,用蜂窝结构吸收冲击能量.蜂窝结构在面内及面外(面外即图1(a)的z 方向)载荷作用下的强度和稳定性分析,在工程实践中有重要意义.面内载荷作用下蜂窝结构的强度分析、弹塑性屈曲和断裂特性的研究,近年来发展较快.面外压力下蜂窝结构的弹性屈曲分析,目前还没有理论分析模型,只有半经验     

粘接多胞管三点弯曲实验与数值研究

基于实验和数值模拟方法,本文研究了一种易制备粘接多胞薄壁结构的弯曲性能,分析了粘接多胞管在横向三点弯曲加载下的变形和能量吸收性能。三点弯曲准静态实验表明：由于粘接的作用,通常情况下粘接多胞管的能量吸收性能高于其基本构成单胞管能量吸收的总和,但在某些情况下粘接可以带来70%的性能提升。借助于LSDYNA,我们计算模拟了三点弯曲实验,计算得到的粘接多胞管变形模式和力-位移曲线与实验结果吻合良好。此外,采用计算模拟方法,我还对三种不同接触条件下的结构响应进行了对比分析,结果表明：如果未出现明显的粘接脱开,则粘接多胞管的吸能特性与完整的多胞结构相当,否则其能量吸收性能会被严重削弱。     

冲击载荷下板杆组合结构的动态响应

结构受冲击载荷作用可能会变形甚至失效.合理的结构形式有助于增强结构的抗冲击性能.本文对冲击载荷作用下板杆组合结构的动力学行为进行了分析,采用有限元方法模拟冲击载荷下结构的大挠度变形模式和能量吸收特性,并研究了杆件长度对抗冲击性能的影响.     

纳米流控能量吸收耗散系统

基于纳米流控行为设计的新一代能量吸收耗散系统(nanofluidic energy absorption system,NEAS)将会比传统吸能材料具有更高的能量吸收密度,而且还可以重复使用,特别是在小体积应用环境下具有显著的优势.本文从实验和计算模拟两方面综述了目前关于NEAS能量吸收耗散行为的最新研究进展,其中实验研究主要包括准静态压缩和动态压缩测试,计算模拟研究主要是采用基于经验势的分子动力学模拟方法.通过准静态压缩实验,可以测量NEAS模型的载荷-位移关系曲线,从而获得NEAS模型的临界渗透压强,了解卸载后系统是否能够恢复到加载前的状态(即是否可以重复使用),并通过载荷-位移关系曲线下面积估算NEAS模型的吸能密度;通过动态压缩实验可以测量NEAS模型对脉冲载荷的缓冲保护作用,主要体现为降低脉冲载荷幅值和扩展脉冲宽度.计算模型研究可以明确给出NEAS对外载荷的微观响应,从而可以准确了解NEAS的能量吸收耗散机制以及吸能密度的主要影响因素.本研究可以帮助我们全面了解NEAS的研究进展,为NEAS的设计与优化提供重要指导.     

The damage tolerance of elastic-brittle, two-dimensional isotropic lattices

The fracture toughness of elastic-brittle 2D lattices is determined by the finite element method for three isotropic periodic topologies: the regular hexagonal honeycomb, the Kagome lattice and the regular triangular honeycomb. The dependence of mode I and mode II fracture toughness upon relative density is determined for each lattice, and the fracture envelope is obtained in combined mode I-mode II stress intensity factor space. Analytical estimates are also made for the dependence of mode I and mode II toughness upon relative density. The high nodal connectivity of the triangular grid ensures that it deforms predominantly by stretching of the constituent bars, while the hexagonal honeycomb deforms by bar bending. The Kagome microstructure deforms by bar stretching remote from the crack tip, and by a combination of bar bending and bar stretching within a characteristic elastic deformation zone near the crack tip. This elastic zone reduces the stress concentration at the crack tip in the Kagome lattice and leads to an elevated macroscopic toughness.Predictions are given for the tensile and shear strengths of a centre-cracked panel with microstructure given explicitly by each of the three topologies. The hexagonal and triangular honeycombs are flaw-sensitive, with a strength adequately predicted by linear elastic fracture mechanics (LEFM) for cracks spanning more than a few cells. In contrast, the Kagome microstructure is damage tolerant, and for cracks shorter than a transition length its tensile strength and shear strength are independent of crack length but are somewhat below the unnotched strength. At crack lengths exceeding the transition value, the strength decreases with increasing crack length in accordance with the LEFM estimate. This transition crack length scales with the parameter of bar length divided by relative density of the Kagome grid, and can be an order of magnitude greater than the cell size at low relative densities. Finally, the presence of a boundary layer is noted at the free edge of a crack-free Kagome grid loaded in tension and in shear. Deformation within this boundary layer is by a combination of bar bending and stretching whereas remote from the free edge the Kagome grid deforms by bar stretching (with a negligible contribution from bar bending). The edge boundary layer degrades both the macroscopic stiffness and strength of the Kagome plate. No such boundary layer is evident for the hexagonal and triangular honeycombs.     

The structural performance of the periodic truss

Matrix methods of linear algebra are used to analyse the structural mechanics of the periodic pin-jointed truss by application of Bloch's theorem. Periodic collapse mechanisms and periodic states of self-stress are deduced from the four fundamental subspaces of the kinematic and equilibrium matrix for the periodic structure. The methodology developed is then applied to the Kagome lattice and the triangular-triangular (T-T) lattice. Both periodic collapse mechanisms and collapse mechanisms associated with uniform macroscopic straining are determined. It is found that the T-T lattice possesses only macroscopic strain-producing mechanisms, while the Kagome lattice possesses only periodic mechanisms which do not generate macroscopic strain. Consequently, the Kagome lattice can support all macroscopic stress states. The macroscopic stiffness of the Kagome and T-T trusses is obtained from energy considerations. The paper concludes with a classification of collapse mechanisms for periodic lattices.     

PLASTIC ZONE OF SEMI-INFINITE CRACK IN PLANAR KAGOME AND TRIANGULAR LATTICES

The fracture investigations of the planar lattices made of ductile cell walls are currently limited to bending-dominated hexagonal honeycomb. In this paper, the plastic zones of stretching-dominated lattices, including Kagome and triangular lattices, are estimated by analyzing their effective yield loci. The normalized in-plane yield loci of these two lattices are almost identical convex curves enclosed by 4 straight lines, which is almost independent of the relative density but is highly sensitive to the principal stress directions. Therefore, the plastic zones around the crack tip of Kagome and triangular are estimated to be quite different to those of the continuum solid and also hexagonal lattice. The plastic zones predictions by convex yield surfaces of both lattices are validated by FE calculations, although the shear lag region caused by non-local bending effect in the Kagome lattice enlarges the plastic zone in cases of small ratio of rp/l.     

Mechanical Properties of two novel planar lattice structures

Two novel statically indeterminate planar lattice materials are designed: a new Kagome cell (N-Kagome) and a statically indeterminate square cell (SI-square). Their in-plane mechanical properties, such as stiffness, yielding, buckling and collapse mechanisms are investigated by analytical methods. The analytical stiffness is also verified by means of finite element (FE) simulations. In the case of uniaxial loading, effective modulus, yield strength, buckling strength and critical relative density are compared for various lattice structures. At a critical relative density, the collapse mode will change from buckling to yielding. Elastic buckling under macroscopic shear loading is found to have significant influence on failure of lattice structures, especially at low relative densities. Comparison of the analytical bulk and shear moduli with the Hashin–Shtrikman bounds indicates that the mechanical properties of the SI-square honeycomb are relatively close to being optimal. It is found that compared with the other existing stretching-dominated 2D lattice structures, the N-Kagome cell possesses the largest continuous cavities for fixed relative densities and wall thicknesses, which is convenient for oil storage, disposal of heat exchanger, battery deploying and for other functions. And the initial yield strength of the N-Kagome cell is slightly lower than that of the Kagome cell. The SI-square cell has similar high stiffness and strength as the mixed cell while its buckling resistance is about twice than that of the mixed cell.     

Kagome平面格栅结构的屈服面及裂尖塑性区分析

分析了Kagome格栅的等效刚度和屈服面. 其屈服面奇异,由4段直线围成. 利用该屈服面,估算了Kagome具有I型、II型半无限大裂纹的裂尖塑性区,有限元计算验证了解析预测的准确性. 与奇异屈服面相比,由Mises光滑屈服面给出的塑性区误差较大. 因此只有弹性情况,可以将Kagome等效为各向同性;若材料塑性,或应力场奇异性较强,Kagome的强度依赖于主应力方向,不能用各向同性模型来描述.      

Effects of high order deformations on the strength of planar lattice materials

Lattice materials have been attractive over the last decade for use as load-carrying structures, energy absorbing elements and heat exchanging structures because of their excellent mechanical properties and multifunctional characters. However, the quantitative analysis accounting for high order deformations upon the collapse of lattice materials, which is important for their applications, has not been reported. An analytical investigation of yield surfaces with respect to the high order deformations was carried out for two typical planar lattice materials： triangular and Kagome lattices separately. The analytical results were validated by the finite element method （FEM） simulations. It was found that the effect of high order deformation on the yield strength increases with the relative density. The bending effect of the Kagome lattice is more obvious than that of the triangular one with the same relative density and stress state. The yield strength of the Kagome lattice calculated by neglecting the bending effect overestimates the result by more than 10% when the relative density is higher than about 11.1%, which may not be ignored in engineering applications. The yielding surfaces of the two lattice materials demonstrated in the paper also confirm the analytical results.     

Effective thermal conductivity of wire-woven bulk Kagome sandwich panels

Thermal transport in a highly porous metallic wire-woven bulk Kagome (WBK) is numerically and analytically modeled. Based on topology similarity and upon introducing an elongation parameter in thermal tortuosity, an idealized Kagome with non-twisted struts is employed. Special focus is placed upon quantifying the effect of topological anisotropy of WBK upon its effective conductivity. It is demonstrated that the effective conductivity reduces linearly as the porosity increases, and the extent of the reduction is significantly dependent on the orientation of WBK. The governing physical mechanism of anisotropic thermal transport in WBK is found to be the anisotropic thermal tortuosity caused by the intrinsic anisotropic topology of WBK.     

Kagome plate structures for actuation

A class of planar, pin-jointed truss structures based on the ancient Kagome basket weave pattern with exceptional characteristics for actuation has been identified. Its in-plane stiffness is isotropic and has optimal weight among planar trusses for specified stiffness or strength. The version with welded joints resists plastic yielding and buckling, while storing minimal energy upon truss bending during actuation. Two plate structures are considered which employ the planar Kagome truss as the actuation plane. It is shown that these plates can be actuated with minimal internal resistance to achieve a wide range of shapes, while also sustaining large loads through their isotropic bending/stretching stiffness, and their excellent resistance to yielding/buckling.     

Actuation of the Kagome Double-Layer Grid. Part 1: Prediction of performance of the perfect structure

The Kagome Double-Layer Grid (KDLG) is a sandwich-like structure, based on the planar Kagome pattern, which has properties that make it attractive for application as a morphing material. In order to understand the passive and active properties of the KDLG with rigid joints, an analysis is made of the determinacy of the pin-jointed version. The number of internal mechanisms and states of self-stress of the finite pin-jointed structure are calculated as a function of the size of the structure. A statically and kinematically determinate version is obtained by relocating the internal nodes and by prescribing a set of patch bars around the periphery. The actuation performance of the rigid-jointed version is then explored theoretically by replacing a single bar in the structure by an actuator. The resistance to actuation is determined in terms of the stiffness and the allowable actuation strain as dictated by yield and buckling. The paper concludes with the optimal design of a double-layer grid to maximise actuation performance.     

The reinforcement and defect interaction of two-dimensional lattice materials with imperfections

The defect interaction and reinforcement of imperfect two-dimensional lattice materials are studied by theoretical investigations and finite element (FE) simulations. An analytical model is proposed to predict the interaction of two defects in lattice materials based on a single defect model. An interaction coefficient is introduced to characterize the degree of interaction. The effects of defect type and defect distance on interaction coefficients are studied. The critical interaction distance of defects, beyond which the interaction of two defects can be neglected, is derived. FE calculations are performed to validate the theoretical model. The simulated results indicate that increasing the number of defects can reduce the stress concentration rather than weakening the strength of the residual parts in certain circumstances. Subsequently, several reinforcement methods are proposed to reduce the stress concentration in the triangular and Kagome lattice for the single-bar-missing defect and single-joint-missing defect. An analytical model is developed for the reinforced lattices, and the predicted stress concentration factors are in good agreement with those of FE simulations. By theoretical studies and FE simulations, optimal reinforcement methods are derived for the triangular and Kagome lattice under planar loading conditions.