蒙皮点阵结构参数化及力学性能优化

针对蒙皮点阵结构的拓扑参数设计困难的问题,提出了基于胞元拓扑结构的蒙皮点阵结构建模方法,构建了蒙皮点阵结构试件的参数化建模及力学性能协同优化系统。将5种胞元构成的试件的蒙皮厚度与胞元尺寸及支柱截面半径作为优化参数,试件质量、变形和应力等设定为优化目标,使试件在分别受到压缩、弯曲、扭转载荷时具有较好的力学性能,从而得到轻质、高强度的点阵结构材料。研究得到了在各种载荷下蒙皮点阵材料的设计与优化的分析方法及各种胞元结构试件力学性能的数据,并通过实例验证了该方法的正确性及可靠性。     

基于能量均匀化的高剪切强度周期性点阵结构拓扑优化

传统多孔结构构型型式单一、缺乏科学设计方法的指导,为了从拓扑构型角度设计抗剪切性能更优的周期性点阵结构,基于拓扑优化技术,以周期性单胞为研究对象,以其切变模量最大为目标,以结构的材料用量和力学控制方程为约束条件,利用能量均匀化方法建立基于宏观力学性能的细观点阵结构的优化模型,通过改进的优化算法求解模型,得到了一种宏观上的拓扑边界清晰的周期性点阵结构。然后根据优化结果,在考虑胞元非等壁厚和横向剪切变形影响条件下进行等效材料力学性能分析,得到剪切性能关于微结构胞元几何参数的表征,同时加工制造了优化得到的周期性点阵结构,并进行了剪切力学性能测试。从理论分析和性能试验两个方面与六边形蜂窝结构相应的切变模量进行对比,结果表明,经优化得到的周期性点阵结构切变模量有大幅提升、抗剪切性能更优越。验证了所提出方法能有效地应用于周期性点阵结构抗剪切性能的优化设计。     

点阵材料抛物面结构的参数化有限元建模方法及赋形分析

针对点阵材料抛物面结构进行有限元分析时的几何建模需求,提出了一种点阵材料抛物面结构的参数化有限元建模方法。首先,利用数学公式表征出抛物面结构在平面投影区域内的点阵材料胞元特征点坐标信息,生成点阵材料的平面几何模型。然后,采用映射法将平面模型映射到标准抛物面,并将抛物面上每一胞元特征点进行有序编号,基于该特征点序号,编译循环程序,根据需求划分网格生成相应的节点和单元。基于该方法,建立了手性六边形点阵材料抛物面结构的有限元模型,进行了初步的胞元几何参数优化,并通过分析该抛物面结构重构赋形时的变形能力和最大应力,初步验证文中方法的有效性,为手性六边形点阵材料抛物面结构的进一步优化分析奠定了基础。     

基于拓扑优化的泡沫填充点阵结构力学行为研究

采用拓扑优化方法设计刚度导向点阵结构,结合三维打印技术和泡沫填充技术,获得泡沫填充点阵结构,并结合材料试验机研究泡沫填充点阵结构在准静态压缩条件下的力学行为.研究结果表明,泡沫填充点阵结构相比未填充的点阵结构,初始压溃应力增大24％,能量吸收能力提高25％,增大和提高的幅值远大于点阵结构与单纯泡沫试件性能的叠加.通过研究确认,泡沫填充有改善点阵结构力学性能的作用.     

点阵夹芯圆筒扭转稳定性的参数化分析

针对扭转载荷作用的四面体点阵夹芯圆筒,以基于等质量圆壳结构的性能增强系数为评价指标,利用有限元数值方法,对点阵杆截面形状、圆筒蒙皮厚度、单胞相对密度、单胞数目及单胞构型对结构扭转稳定性的影响进行了参数化分析。结果表明:结构扭转稳定性能受单胞相对密度和蒙皮厚度变化显著影响,但不同截面形状的等截面积实心杆点阵结构的扭转屈曲性能基本相同,包含更多的点阵单胞有利于增强结构扭转稳定性,而较小相对密度时杆长短的点阵单胞夹芯圆筒扭转性能更具优势。计算结果对点阵夹芯圆筒的设计具有指导意义。     

面向增材制造的微桁架胞元几何与力学性能分析

针对BCC、FCC及其衍化型共6种微桁架胞元结构进行了参数化建模及几何特性分析,通过有限元法对不同构型的胞元分别进行了压缩载荷条件下的力学性能分析,提出了等效比刚度的概念以表征不同构型胞元结构的刚度特性.结果表明:胞元构型对比表面积影响不大,但对相对密度有较大影响,FCC和FCCZ型胞元具有较优良的几何特性及减重效果....     

Gyroid结构力学性能及数值收敛性研究

为了更好地揭示三周期极小曲面（TPMS）结构设计与力学性能参数之间的对应关系,研究了螺旋二十四面体Gyroid结构（GCS）不同网格参数的数值收敛性和不同排列方式的力学性能。设计了体积分数和胞元尺寸固定的GCS试件;基于单元参数变化的体素化模型,使用有限元方法对GCS进行收敛性分析;通过试件拉伸试验验证了仿真的正确性;最后以拉伸和弯曲工况研究了GCS在不同排列形式时的力学性能差异。研究结果表明：仿真分析与试验之间的抗拉强度、极限载荷误差在1.5%以内;基于体素化方法,调整雅可比参数值可显著降低收敛时的单元数量。对变厚度GCS力学性能做了定量分析,拉伸工况中,针对4×4×4结构,其等效弹性模量沿厚度方向变化最大可达14.41%;弯曲工况中,针对20×4×4结构,等效弹性模量最大差距为21.25%。     

具有创新拓扑构型的周期性点阵结构抗剪切性能分析

以六边形为胞元的周期性点阵结构抗剪切能力较差,对六边形构型进行改造,并在内部嵌套菱形结构,提出了一种"Y型"周期性点阵结构;然后在考虑胞元非等壁厚的条件下,推导了周期性点阵结构的等效弹性常数计算公式;通过理论计算结果对比,"Y型"周期性点阵结构相比六边形结构剪切模量有较大提升,对六边形和本文提出的"Y型"周期性点阵结构进行数值模拟分析,表明了"Y型"结构承载性能的优越性。     

反Kagome点阵结构主参数分析及力学行为

为探寻力学性能更好的点阵结构,以点群理论为基础提出反Kagome点阵单胞模型,并与Kagome单胞模型进行力学性能对比分析.以结构压缩性能、剪切性能为目标,相对密度和承载能力为约束条件,运用应力应变理论确定反Kagome单胞关键参数-倾斜角度ω的取值范围.结合有限元分析结果,确定该模型在最佳力学性能下的ω取值.进行反K...     

SLM制备的Ti6Al4V轻质点阵结构多目标结构优化设计研究

体心立方(Body-centered cubic,BCC)点阵结构作为目前被广泛关注的点阵材料构型,其拓扑类型简单、SLM(Selective laser melting,SLM)成型可靠性好、压缩失效形式单一,但存在着承载能力相对较差的缺点。为探寻兼具轻质与高强性能的点阵构型,首先解除BCC点阵单胞各向尺寸相同的约束,提出体心四方(Body-centered tetragonal,BCT)点阵结构一般模型。然后,以BCT单胞构型尺寸为设计变量,以点阵材料尺寸与成型工艺为约束条件,以相对密度、初始刚度、塑性破坏强度为多目标评价函数,建立BCT点阵结构构型尺寸的多目标优化数学模型,采用理想点法求解得到BCT点阵单胞综合最优构型尺寸,并与BCC参照结构进行实例仿真对比分析,论证BCT优化结构相比BCC参照结构在性能上的优势。最后,采用Ti6Al4V材料通过SLM方法制备BCT优化结构与BCC参照结构的实验样件,并进行准静态单向压缩性能实验,验证理论分析结果的正确性,为轻质点阵结构材料的设计与研究提供了理论参考。     

Plastic continuum models for truss lattice materials with cubic symmetry

Analytical and numerical studies on continuum models for the elastic-plastic behavior of uniformly periodic lattice materials under multi-axial loading are presented in this paper. This study firstly investigates the basic topology of unit cell structures for three different lattice materials with cubic symmetry. By homogenizing the mechanical properties of these materials within the unit volume space, the equivalent continuum models are obtained with the internal variables which result in the mechanical and geometrical characteristics of discrete truss members at the micro-scale such as structural packing, axial stiffness, and material density. Therefore, in this study, the strain hardening was applied to the material model of individual truss members in a valuable effort to explain the plastic behavior of the homogenized lattice material. The expansion of pressure-dependent stress surface at the macro-scale level is estimated by analytical predictions, which are derived from the equivalent continuum models. Analytical predictions show good agreements with existing results obtained by finite element (FE) analyses.     

Elastic and dissipative moduli of pressure-supported cellular substances

A unified approach is presented for modeling the elastic and dissipative moduli of substances composed of pressurized cells. The substances examined include liquid foams, physiological tissues such as lung and udder, and plant tissues such as vegetables and leaves. All are characterized by macroscopic elastic moduli linearly proportional to the inflating pressure. An isotropic dodecahedron is chosen to represent the basic microunit of such a structure. Variational statements of nonlinear structural mechanics, applied to the dodecahedron, bring out the specific macromechanical properties of the cellular material under static and dynamic loading, in terms of the mechanical attributes of its microunits.     

Experimental study on the high-damping properties of metallic lattice structures obtained from SLM

Modern additive manufacturing technologies allow the creation of parts characterized by complex geometries that cannot be created using conventional production techniques. Among them the Selective Laser Melting (SLM) technique is very promising. By using SLM it is possible to create lightweight lattice structures that may fill void regions or partially replace bulk regions of a given mechanical component. As a consequence, the overall mechanical properties of the final component can be greatly enhanced, such as the resistance to weight ratio and its damping capacity against undesired vibrations and acoustic noise. Nevertheless, only a few research works focused on the characterization of the dynamic behavior of lattice structures, that were mainly investigated in the low frequency range or directly tested on some specific applications. In this work the dynamic behavior of lattice structures in the medium-high frequency range was experimentally investigated and then modelled. For this purpose, different types of lattice structures made of AlSi10Mg and AISI 316L were measured. Experimental modal analysis was performed on the obtained specimens in order to assess the influence of lattice material and unit cell geometry on their global dynamic behavior. Experimental results revealed that lattice structures have superior damping characteristics compared to solid materials having an equivalent static stiffness. Eventually, the classic Rayleigh model was found to be adequate - with some approximation - to explain the damping behavior of a generic lattice structure.     

Stiffness of bi-modulus hexagonal and diamond honeycombs

Cellular materials are widely used in various applications because of their low density and high strength. The mechanical behavior of cellular materials under various loading conditions has been investigated. Nevertheless, many of these previous studies assume that the Young’s modulus of constituting struts is the same in tension and compression. The present work first derives analytical expressions for the effective Young’s moduli of hexagonal and diamond lattices composed of struts with different tension and compression moduli under the assumption of small strain deformation. It also uses the finite element method to further investigate the mechanical responses of these lattices. The macroscopic Young’s moduli under both compressive and tensile loads are reported as a function of the ratio of compression and tension moduli of constituting struts. The numerical finite element models are implemented by a user defined material subroutine in ABAQUS. Results reveal that the effective Young’s moduli of periodic hexagonal and diamond lattices significantly decrease with decreasing ratio of compression and tension moduli of the struts. Furthermore, the mechanical behavior of hexagonal lattices composed of struts with different tension-compression moduli is dependent on the loading direction and whether they are compressed or stretched. The unique mechanical properties of bi-modulus cellular materials could find important applications in the automotive and construction industries.

     

Modulus characterization of cells with submicron colloidal probes by atomic force microscope

Colloidal probes have been increasingly demanded for the characterization of cellular modulus in atomic force microscope because of their well-defined geometry and large contact area with cell. In this work, submicron colloidal probes are prepared by scanning electron microscope/focused ion beam and compared with sharp tip and micron colloidal probe, in conjunction with loading velocity and indentation depth on the apparent elastic modulus. NIM and cartilage cells are used as specimens. The results show that modulus value measured by sharp tip changes significantly with loading velocity while remains almost stable by colloidal probes. Also, submicron colloidal probe is superior in characterizing the modulus with increasing indentation depth, which could help reveal the mechanical details of cellular membrane and the modulus of the whole cell. To test the submicron colloidal probe further, the modulus distribution map of cell is scanned with submicron colloidal probe of 50 nm radius during small and large indentation depths with high spatial resolution. The outcome of this work will provide the effective submicron colloidal probe according to the effect of loading velocity and indentation depth, characterizing the mechanical properties of the cells.     

Application of an Eddy-current method for the assessment of stored plastic deformation and residual mechanical properties after cyclic loading of an annealed medium-carbon steel

The effect of cyclic loading of annealed steel 45 during low-cycle fatigue on changes in its electrical characteristics is studied. In the region of both small and mediate strains, an eddy-current transducer signal is found to be highly sensitive to the plastic deformation stored during cyclic loading. Correlations between readings of an eddy-current instrument and the residual mechanical properties of the material after cyclic loading are obtained. The possibility of assessing the residual mechanical properties of the material during its cyclic loading is considered.     

A multi-field approach to modeling the dynamic response of cellular materials

A multi-field approach is developed for simulating the continuum-scale mechanical response of cellular materials. This approach departs from traditional methods used to model cellular materials, which focus almost exclusively on the mechanical response of the cellular solid, while essentially ignoring the fluids permeating these material systems. In the present work, conservation equations are derived in multi-field form, producing a coupled set of governing equations with source terms depending on gradients in the cellular solid stress, but also on gradients in the permeating fluid pressure and momentum exchange resulting from relative motion between the cellular solid and permeating fluid fields. The multi-field equations of motion are implemented in a standard finite-volume computational test bed and used to study the dynamic response of cellular material systems. The influence of various permeating fluids, along with the effects of aperture size, loading rate, and boundary conditions, also are examined. By incorporating an advanced constitutive model for cellular solids into a multi-field response formulation, a promising new approach for simulating the finite-strain dynamic response of cellular materials is offered. Results demonstrate that the permeating fluid can play a major role in the general response of cellular material systems, contributing to the overall load-carrying capacity of the materials and affecting rate dependence and signal propagation speeds. Furthermore, the results point to the usefulness of the multi-field formulation and provide evidence to suggest that any modeling approach developed for cellular materials gives a proper accounting of the pressure evolution and flow behavior of the fluids present in these material systems.     

高性能表面层制造:基于可控表面完整性的精密制造

高性能表面层制造是具有特殊功能性表面层结构零件的精密制造,体现了高性能零件性能与几何参数一体化制造的特点。依据功能性表面层结构零件的性能要求所设计的几何参数和材料特性,选择表面层加工制造方法,确定加工工艺载荷的物质与能量输入条件,通过减控加工工艺的几何、结构、物理、化学等多源耦合约束,构建主动协调的材料加工载荷的应力场、温度场和化学位场等(多)场环境,相应地揭示零件表面完整性变化关系内禀的加工过程印记,利用可控的表面完整性与高性能零件性能的关联模型,实现具有特殊功能性表面层的精密制造。高性能表面层加工制造原理的核心是表面完整性的形成机制、评价方法和调控作用,所提出的高性能表面层精密制造的体系框架,以基于知识方法取代实验迭代的试错法,可解决高性能制造的加工制造反问题。     

Measuring biomaterials mechanics with atomic force microscopy. 1. Influence of the loading rate and applied force (pyramidal tips)

Atomic force microscopy (AFM) is today an established tool in imaging and determination of mechanical properties of biomaterials. Due to their complex organization, those materials show intricate properties such as viscoelasticity. Therefore, one has to consider that the loading rate at which the sample is probed will lead to different mechanical response (properties). In this work, we studied the dependence of the mechanical properties of endothelial cells on the loading rate using AFM in force spectroscopy mode. We employed a sharp, four‐sided pyramidal indenter and loading rates ranging from 0.5 to 20 μm/s. In addition, by variation of the load (applied forces from 100 to 10,000 pN), the dependence of the cell properties on indentation depth could be determined. We then showed that the mechanical response of endothelial cells depends nonlinearly on the loading rate and follows a weak power‐law. In addition, regions of different viscous response at varying indentation depth could be determined. Based on the results we obtained, a general route map for AFM users for design of cell mechanics experiments was described.