自相似多级纳米蜂窝铝结构力学性能的分子动力学模拟

首次采用分子动力学方法预测了自相似多级纳米蜂窝铝面内和面外（轴向）的压缩力学性能（弹性模量和压缩强度）。重点研究了相对密度、层级数和长度比对自相似多级纳米蜂窝铝结构力学性能的影响。在Gibson模型中引入了表面效应因子,结果表明修正的Gibson-Ashby模型与分子动力学计算结果更加吻合。此外,通过比较一级、二级和三级纳米蜂窝铝结构的变形机制发现,二级和三级纳米蜂窝铝结构由于分别在单级蜂窝和二级蜂窝的角点处接入六边形,在压缩过程中,多级纳米蜂窝铝结构激发的位错远高于单级蜂窝铝结构。也就是说,在压缩载荷下,多级蜂窝铝结构可以更好地利用结构的承载能力,吸收更多的能量。但是,自相似纳米蜂窝铝结构的力学性能无法通过增加级数的方法来无限增强,在相对密度和长度比不变的情况下,当纳米蜂窝铝结构的级数达到二级时,其综合力学性能最佳。研究结果还表明,相对密度不变时,二级纳米蜂窝铝结构长度比分别在0.3和0.4附近时,二级蜂窝铝结构具有最佳的面内和面外力学性能。研究成果对自相似多级纳米蜂窝结构的优化设计具有重要的指导作用。      

六边形蜂窝芯材异面冲击性能的有限元研究

建立了六边形蜂窝单元阵列的有限元模型,利用有限元分析软件分析研究了六边形金属蜂窝的异面冲击性能。当六边形蜂窝铝芯的各结构参数固定时,在不同的冲击速度下,研究了壁厚边长比对其异面冲击动态峰应力的影响规律。根据有限元模拟计算结果,给出了所用六边形蜂窝铝芯样品异面动态峰应力的计算公式。     

凹六边形蜂窝纸板面内承载性能有限元仿真分析

目的 探究以凹六边形为芯层的蜂窝纸板的力学性能,为凹六边形蜂窝纸箱的运输包装设计提供理论基础.方法 运用有限元仿真,分析不同结构参数影响下的凹六边形蜂窝纸板的面内承载性能.结果 当水平胞壁的长度减小,其他结构参数保持不变,凹六边形蜂窝纸板面内方向上的平台应力增大,能量吸收平台阶段标准化应力增大,最佳吸能点也上升.结论 凹六边形蜂窝纸板结构参数的变化对其平台应力和能量吸收特性有深远影响,其性能研究促进了蜂窝纸箱的进一步发展.     

组合蜂窝材料面内冲击性能的研究

基于三角形和六边形蜂窝结构面内冲击性能的研究,该文探讨了面内冲击荷载作用下组合Kagome蜂窝结构的变形机制和能量吸收特性。首先,在保证蜂窝结构胞元厚度与边长尺寸比值不变的前提下,分析了不同形状胞元及其组合结构的动态冲击性能,给出了试件宏观及微观胞元结构的动态演化过程。在此基础上,探讨了冲击速度和相对密度一定情况下单位质量不同蜂窝结构的能量吸收特性。其结论将对蜂窝材料微拓扑结构的动力学优化设计提供指导。     

复合材料夹层结构自插式平接节点抗弯性能试验研究

采用复合材料连接型材的复合材料夹层结构连接节点具有轻质、防腐、无磁性等优点。该文试验研究复合材料夹层结构自插式平接胶结节点的受弯性能,重点考察连接型材的纤维铺设、节点搭接长度等因素对连接节点荷载-变形关系、应变发展规律和破坏模式的影响。试验结果表明,自插式平接节点的破坏形式主要表现为三类:节点连接区域的纤维面层与芯材的剥离破坏、搭接胶层剥离破坏、插入端变截面处纤维面层与芯材剥离破坏;连接型材纤维布的铺设方法与层数、胶结搭接长度以及节点区域的构造措施是影响节点承载力的主要因素。论文对平接节点的破坏机理进行了分析,研究成果对复合材料夹层板结构平接胶结节点的设计具有参考价值。     

正方形自填充蜂窝结构异面平台应力的研究

目的 为了促进正方形自填充蜂窝的合理使用，研究其异面平台应力随冲击速度、壁厚边长比和自填充级数的变化规律。方法 利用ANSYS/LS-DYNA建立基于胞元阵列的正方形自填充蜂窝异面冲击分析的有限元模型。对自填充级数为0的正方形蜂窝进行异面压缩试验和相应的仿真分析，证明有限元模型的可靠性。基于简化的超折叠单元理论，建立蜂窝准静态平台应力的理论模型，并证明理论模型的可靠性。结果 正方形自填充蜂窝在大的壁厚边长比和冲击速度下拥有更高的动态平台应力;在自填充级数由0变为1时，动态平台应力增长率最大。结论 在其他因素不变的情况下，正方形自填充蜂窝的异面动态平台应力与冲击速度的平方呈线性关系，与壁厚边长比呈幂指函数关系，其增长率随自填充级数逐级递减。基于数值模拟结果，得到了不同自填充级数下正方形自填充蜂窝异面动态平台应力的经验公式。     

三角形蜂窝异面动态压缩仿真研究

目的研究三角形蜂窝材料结构参数和压缩速度对其异面压缩性能的影响。方法借助Ansys/LS-DYNA建立基于特征单元的三角形蜂窝异面动态压缩有限元分析模型,而后对三角形蜂窝在不同单元结构参数和压缩速度下进行异面压缩参数化仿真计算。结果对于三角形蜂窝,当所有结构参数保持不变时,异面动态峰应力与压缩速度呈二次曲线关系。在给定压缩速度下,扩展角固定的三角形蜂窝的异面动态峰应力与壁厚度边长比呈幂函数关系;壁厚边长比固定的三角形蜂窝的异面动态峰应力与拓展角呈二次曲线关系。结论当相对密度一致时,正三角形蜂窝比正六边形蜂窝异面比吸能值更大;异面动态峰应力与压缩速度、单元结构参数之间的相互关系,可用一定关系曲线进行拟合。     

复合材料蜂窝夹层结构T型接头拉伸性能研究

通过试验测量了复合材料蜂窝夹层结构T型整体接头的拉伸性能,得到其拉伸强度与破坏模式。建立了接头结构有限元模型,利用分类损伤判据、失效准则与刚度退化准则对结构的损伤情况进行模拟,研究了接头的拉伸破坏行为。有限元分析结果与试验结果吻合良好。研究结果表明,结构的薄弱点位于腹板内靠近蒙皮的位置。蜂窝在此处发生面外拉伸破坏,从而导致结构的最终破坏。腹板上的拉伸载荷主要通过过渡区填料传递给蒙皮,腹板与蒙皮间的搭接段对载荷传递的贡献较小。参数研究表明,对于复合材料蜂窝夹层结构T型接头,搭接段长度对结构的强度几乎没有影响,而增大蒙皮蜂窝的高度或采用低模量蜂窝可以提高结构强度。     

内含气体对蜂窝纸板异面动态冲击性能的影响

目的以六边形蜂窝纸板为对象,研究内含气体对其异面冲击性能的影响。方法通过动态冲击实验分析内含气体对接触力、最大接触力、最大位移、最大应变和吸收能的影响,得出不同孔隙率时,蜂窝纸板的接触力-时间曲线,最大接触力、最大位移、最大应变、吸收能与冲击能曲线和吸收能-孔隙率曲线。结果在给定冲击能的情况下,最大接触力与吸收能随着孔隙率的增大而减小,最大位移及最大应变随着孔隙率的增大而增大。在孔隙率一定时,最大接触力、最大位移、最大应变和吸收能随冲击能线性增大。此外,冲击能越大,接触力达到峰值的时间越短,接触持续时间越长。结论在动态冲击实验中,内含气体使蜂窝纸板吸收冲击能的能力明显增强,并且当冲击能一定时,孔隙率越大,蜂窝纸板越容易被压变形,吸收能越少。     

基于有限元的蜂窝纸芯屈曲分析

提出了蜂窝纸芯正六边形边长值对压缩性能的影响.分析了蜂窝纸芯的压缩机理,并将压缩过程分为4个阶段.建立蜂窝纸单芯分析模型,在有限元MARC进行屈曲分析,得到蜂窝纸压缩变形过程,比较不同边长的受压情况.为以后蜂窝纸进一步分析提供帮助.     

Overall elastic domain of thin polysilicon films

We present a numerical study aimed to define the overall elastic domain of thin polysilicon films subjected to in-plane loadings. Homogenized properties are obtained for digital polycrystalline microstructures, generated in a representative volume element (RVE) through Voronoi tessellations. To locate the elastic limit, three micromechanical sources of dissipation are allowed for: (i) trans-granular cracking, as due to tensile stresses attaining the local strength inside silicon grains; (ii) inter-granular failure, as due to coupled normal-shear tractions attaining a local effective strength along grain boundaries; (iii) trans-granular phase transformation, as due to compressive stresses attaining a critical threshold inside silicon grains.Results of the homogenization procedure show that Rankine-type overall domains are too crude approximations to the polysilicon elastic envelope, since corners arise because of switching among the three dissipative modes allowed for. Outcomes of the analysis allow also to estimate the size of the RVE required to get objective overall polysilicon properties: according to data already available in the literature, it is shown that the RVE has to gather at least a few hundreds of grains.     

Representative volume elements for discontinuous carbon fibre composites – Part 1: Boundary conditions

In part 1 of this 2 part paper a method is presented for generating RVE analyses of discontinuous fibre planar distributions of random discontinuous carbon fibre bundles, with no limitation to the fibre volume fraction. An embedded element approach is used to simplify mesh generation for the matrix phase. This approach is shown to yield errors of 1% compared with a more conventional unstructured mesh, whilst offering significant reductions in model size. Periodic boundary conditions are adopted, applying Saint-Venant’s principle to provide an approximation of the effective material properties within an inner RVE region, some distance away from the outer boundary of the model. The critical decay length of prescribing such boundary conditions is shown to be approximately two times the fibre length in all cases, when a heterogeneous material (rather than homogeneous material) is used to model the outer region.     

Biaxial buckling analysis of double-orthotropic microplate-systems including in-plane magnetic field based on strain gradient theory

Background/purposeThis paper deals with analysis of biaxial buckling behavior of double-orthotropic microplate system including in-plane magnetic field, using strain gradient theory.MethodsTwo Kirchhoff microplates are coupled by an internal elastic medium and also are limited to the external Pasternak elastic foundation. Utilizing the principle of total potential energy, the equilibrium equations of motion for three cases (out-of-phase buckling, in-phase buckling and buckling with a plate) are acquired. In this study, we assumed boundary conditions of all the edges are simply supported. In order to get exact solution for buckling load of system, Navier approach which satisfies the simply supported boundary conditions is applied.ResultsVariations of the buckling load of double-microplate system subjected to biaxial compression corresponding to various values of the thickness, length scale parameter, magnetic field, stiffness of internal and external elastic medium, aspect ratio, shear stiffness of the Pasternak foundation and biaxial compression ratio are investigated. Furthermore, influence of higher modes on buckling load is shown. By comparing the numerical results, it is found that dimensionless buckling load ratio for in-phase mode is more than those of out of phase and one microplate fixed. Also it is shown that the value of buckling load ratio reduces, when non-dimensional length scale parameter increases.ConclusionHowever, we found when properties of plate are orthotropic the buckling load ratio is more than isotropic state. Also, by considering the effect of magnetic field, non-dimensional buckling load ratio reduces.     

Identification of material parameters of micropolar theory for composites by homogenization method

The objective of the present work is to develop an analytical homogenization method to derive higher-order material parameters of micropolar theory. With help of Airy’s stress function, the constitutive equations for a homogenized micropolar medium are analytically established by considering a cylindrical representative volume element (RVE) subjected to quadratic and cubic boundary displacement conditions. Both porous and composite materials are considered, an analytical relation between the intrinsic length and the microstructural parameters is given.     

Buckling analysis of rectangular composite plates with rectangular cutout subjected to linearly varying in-plane loading using fem

A numerical study is carried out using finite element method, to examine the effects of square and rectangular cutout on the buckling behavior of a sixteen ply quasi-isotropic graphite/epoxy symmetrically laminated rectangular composite plate [0°/+45°/-45°/90°]_2s, subjected to various linearly varying in-plane compressive loads. Further, this paper addresses the effects of size of square/rectangular cutout, orientation of square/rectangular cutout, plate aspect ratio(a/b), plate length/thickness ratio(a/t), boundary conditions on the buckling bahaviour of symmetrically laminated rectangular composite plates subjected to various linearly varying in-plane compressive loading. It is observed that the various linearly varying in-plane loads and boundary conditions have a substantial influence on buckling strength of rectangular composite plate with square/rectangular cutout.     

Effect of the interface stiffness and skin–core adhesion efficiency on the interfacial stress distribution of sandwich structures

This paper investigates the load distribution along the skin–core interface length of FRP-metal laminates subjected to in-plane tension, using an improved shear lag model. The model elucidates the significance of the interface stiffness concept on the skin–core interface load distribution in FRP-metal laminate beams and introduces a basic hypothesis according which, the interface stiffness depends on the difference in shear moduli of the constituent materials as well as on the degree of adhesion which, in turn, depends on the abrupt jump in shear moduli at the skin–core boundary. Details of an experimental procedure carried out and of experimental results obtained are also presented. From the combined experimental and theoretical investigation it can be concluded that for a given FRP-metal laminate structure with known core and facing properties, it is possible to evaluate the skin–core degree of adhesion as well as the interface stiffness and predict the interfacial stress distribution by simply conducting a single tensile test. Thus, the proposed method can be considered as a tool for the structural performance and reliability control of FRP-metal laminate structures.     

Stress-controlled weakly periodic boundary conditions: Axial stress under varying orientations

The accuracy of multiscale modeling approaches for the analysis of heterogeneous materials hinges on the representativeness of the micromodel. One of the issues that affects this representativeness is the application of appropriate boundary conditions. Periodic boundary conditions are the most common choice. However, when localization takes place, periodic boundary conditions tend to overconstrain the microscopic problem. Weakly periodic boundary conditions have been proposed to overcome this effect. In this study, the effectiveness of weakly periodic boundary conditions in restoring transverse isotropy of representative volume elements (RVE) for a fiber-reinforced composite with elastoplastic matrix is investigated. The formulation of weakly periodic boundary conditions is extended to allow for force-controlled simulations where a uniaxial stress can be applied. A series of simulations is performed where the orientation of applied stress is gradually varied and the influence of this orientation on the averaged response is examined. An original method is presented to test the correlation between the ultimate principal stress and average localization angle of shear bands within an RVE. It is concluded that weakly periodic boundary conditions alleviate anisotropy in the RVE response but do not remove it.     

Localization aligned weakly periodic boundary conditions

When computing the homogenized response of a representative volume element (RVE), a popular choice is to impose periodic boundary conditions on the RVE. Despite their popularity, it is well known that standard periodic boundary conditions lead to inaccurate results if cracks or localization bands in the RVE are not aligned with the periodicity directions. A previously proposed remedy is to use modified strong periodic boundary conditions that are aligned with the dominating localization direction in the RVE. In the present work, we show that alignment of periodic boundary conditions can also conveniently be performed on weak form. Starting from a previously proposed format for weak micro‐periodicity that does not require a periodic mesh, we show that aligned weakly periodic boundary conditions may be constructed by only modifying the mapping (mirror function) between the associated parts of the RVE boundary. In particular, we propose a modified mirror function that allows alignment with an identified localization direction. This modified mirror function corresponds to a shifted stacking of RVEs, and thereby ensures compatibility of the dominating discontinuity over the RVE boundaries. The proposed method leads to more accurate results compared to using unaligned periodic boundary conditions, as demonstrated by the numerical examples. Copyright © 2016 John Wiley & Sons, Ltd.     

短切碳纤维/乙烯基酯树脂片状模塑料拉伸性能的有限元模拟

采用Python语言对ABAQUS软件进行二次开发,生成了短切碳纤维在乙烯基酯树脂中随机分布的二维平面应变模型,建立代表性体积单元（RVE）,通过Hypermesh对模型进行网格划分,为保证RVE应力场的均匀协调性,在周期性边界条件下,对短切碳纤维/乙烯基树脂进行拉伸性能数值模拟计算分析,并结合实验研究了短切碳纤维长度对片状模塑料拉伸性能的影响。结果验证了对短切碳纤维/乙烯基树脂片状模塑料拉伸模量随纤维长度的增大出现先增大后变缓的趋势,表明利用Python-ABAQUS建立二维平面应变RVE模型的合理性。本研究为短切碳纤维/乙烯基树脂片状模塑料在汽车轻量化行业的应用提供了理论及实验依据。