基于摄影测量法的薄膜褶皱实验研究

采用非接触式的测试方式摄影测量法对薄膜褶皱进行了实验研究.建立了剪切薄膜褶皱的分析模型,并基于摄影测量法对该模型的褶皱变形模式进行了实验研究.摄影测量法是通过对不同角度上相机拍摄到的同一靶点的图像信息进行处理,可以得到相应靶点的三维坐标,从而实现对微小变形的非接触式的测量.通过摄影测量法得到了剪切薄膜褶皱的变形模式.实验研究表明,在剪切距离增大的情况下薄膜褶皱的数量保持不变,但是褶皱面外变形幅度随着剪切距离的增加而增大.试验结果与数值分析结果具有相关性,摄影测量法适合于薄膜结构微小面外变形的非接触式测量.     

薄膜褶皱的非线性屈曲有限元分析

有限元模拟已经成为目前薄膜褶皱预测的重要工具。本文采用ANSYS SHELL63单元对薄膜受剪情况下的褶皱形变进行了非线性屈曲分析。通过本征屈曲分析得到的模态模拟了薄膜的初始缺陷。利用本文的非线性有限元模型分析得到了薄膜褶皱的波长和幅度,并与理论分析结果进行了比较,结果相近。     

空间薄膜结构的褶皱形变预测

空间充气结构技术是一项全新的空间结构构建技术,基于该技术的空间充气结构是未来空间任务的主要需求对象．该结构主要由薄膜构成,因此结构形面精度的保持是关键问题．褶皱是薄膜特有的现象,它的存在会严重影响结构形面精度,因此进行褶皱研究很有必要．根据屈曲理论联合薄膜褶皱的实际构型建立了薄膜结构褶皱的预测模型,分析了矩形平面薄膜受面内水平剪切情况下的褶皱,得到了褶皱幅度和褶皱波长以及薄膜结构产生褶皱时的临界压缩应力,通过与已有文献结果及本文实验结果的比较验证了该分析方法的有效性和结果的正确性．     

空间充气薄膜结构的褶皱分析

引入一个表征褶皱在纹理方向收缩特性的参数``收缩系数'来替代泊松比参与计算褶皱形变,并联合褶皱产生时的应力条件和褶皱特性,实现了对空间充气薄膜结构的褶皱分析,得到了褶皱存在时的充气薄膜结构的应力场和变形. 分析中重点讨论了``收缩系数'与泊松比的关系,并讨论了引入``收缩系数'约束变形后的变形协调条件. 将两维的褶皱分析方法拓展应用到三维充气结构的褶皱分析中,分析了充气管的弯皱变形,得到了临界皱曲和极限弯矩随结构及力学参数的变化规律. 通过简单的褶皱试验,定性地分析了褶皱的形变特性并与分析结果进行了比较验证,结果一致.      

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建立了薄膜褶皱分析的力学模型,并基于张力场理论和稳定性理论分析了褶皱区域的应 力应变关系. 在此基础上提出了褶皱薄膜动态分析的NBA-UM(nonlinear buckling analysis-update matrix)分析方法，分析了褶皱薄膜振动特性. 结 果表明，褶皱使薄膜产生``过度收缩' 现象，并改变了薄膜中的应力分布. 褶皱对薄膜的振动特性有很大的影响，大幅度的褶皱改 变了薄膜振动的固有频率和模态形式. 提出的NBA-UM分析方法可以有效地预报褶皱薄膜 的动态行为.     

对角受拉方膜褶皱变形幅值的理论预测及实验验证

柔性薄膜结构广泛应用于航天飞行器的关键部件.形面平整度是影响薄膜结构性能的主要因素之一.褶皱幅值是评价薄膜反射面天线形面平整度的重要指标.褶皱幅值的大小与垂直于褶皱方向的横向应变密切相关.本文基于薄板稳定性理论,针对对角受拉方形薄膜建立了一个能够准确预测其褶皱变形幅值的理论模型.该模型考虑了横向拉伸力对薄膜变形的影响,将垂直于褶皱方向的位移分解为由泊松效应造成的横向收缩位移、由面外变形造成的褶皱位移以及由横向拉伸力造成的拉伸位移三个部分,重新推导了褶皱幅值的理论公式. 基于数字图像相关技术,对受拉方形薄膜进行了散斑实验测试. 利用双目视觉三维测量系统,测量了方形薄膜的三维位移场, 获得了薄膜的三维变形形貌和褶皱波形图,研究了褶皱幅值与拉伸载荷之间的非线性关系. 与已有理论模型相比,该模型进一步提高了褶皱幅值的计算精度, 与实验结果吻合良好.本文呈现的理论研究可为精确数值模型的建立及算法实现提供有意义的指导.      

鼓泡法试验薄膜力学行为的研究

本文采用弹性理论,研究了鼓泡法试验过程中,外力(P)与薄膜中心挠度(w_0)之间的本构关系,计算出P与w_0的数学表达式.该分析模型考虑了薄膜内延展变形及薄膜中残余拉应力对本构关系的影响,并在较大挠曲区间范围内(挠度为10~(-2)～10~2倍薄膜厚度)分析本构关系特征.分析结果表明,外力与薄膜中心挠度之间本构关系可表示为:P∝w_0~n,n为指数.随着薄膜中心挠度的增加,薄膜的主要力学行为由弯曲过渡到延展,本构关系由线性过渡到立方关系,本构关系方程指数n由1增加到3.利用本构方程进行数值计算结果表明,薄膜挠曲所需外力随着薄膜半径增加而减小,随着残余拉应力的增加而增加.相对于以往不考虑延展变形的小挠度模型,本文所提出的力学模型更适合于大挠度条件下鼓泡法试验.     

对角受拉方膜褶皱变形幅值的理论预测及实验验证

柔性薄膜结构广泛应用于航天飞行器的关键部件.形面平整度是影响薄膜结构性能的主要因素之一.褶皱幅值是评价薄膜反射面天线形面平整度的重要指标.褶皱幅值的大小与垂直于褶皱方向的横向应变密切相关.本文基于薄板稳定性理论,针对对角受拉方形薄膜建立了一个能够准确预测其褶皱变形幅值的理论模型.该模型考虑了横向拉伸力对薄膜变形的影响,将垂直于褶皱方向的位移分解为由泊松效应造成的横向收缩位移、由面外变形造成的褶皱位移以及由横向拉伸力造成的拉伸位移三个部分,重新推导了褶皱幅值的理论公式.基于数字图像相关技术,对受拉方形薄膜进行了散斑实验测试.利用双目视觉三维测量系统,测量了方形薄膜的三维位移场,获得了薄膜的三维变形形貌和褶皱波形图,研究了褶皱幅值与拉伸载荷之间的非线性关系.与已有理论模型相比,该模型进一步提高了褶皱幅值的计算精度,与实验结果吻合良好.本文呈现的理论研究可为精确数值模型的建立及算法实现提供有意义的指导.     

准静态条件下金属材料的临界断裂准则研究

本文针对9种金属材料完成了具有不同约束程度的10类试样的延性断裂试验, 获得了发生拉、压、扭和裂尖断裂等破坏形式构型试样的载荷-位移试验关系; 基于圆棒漏斗试样拉伸试验所得直至破坏的载荷-位移曲线, 采用有限元辅助试验(finite-element-analysis aided testing, FAT)方法得到了9种材料直至破坏的全程等效应力-应变曲线, 以此作为材料本构关系通过有限元分析获得了各类试样直至临界破坏的载荷-位移关系模拟. 载荷-位移关系模拟结果与试验结果有较好的一致性, 表明用于解决试样颈缩问题的FAT方法所获得的全程材料本构关系针对各向同性材料具有真实性和普适性. 对应9种材料、10类试样的36 个载荷-位移临界断裂点, 通过有限元分析获得了对应的材料临界断裂应力、应变与临界应力三轴度, 研究表明, 第一主应力在延性变形过程中为主控断裂的主导参量; 通过研究光滑、缺口、裂纹等构型试样的断裂状态, 提出了$-1$至3范围的应力三轴度下由第一主应力主控的统一塑性临界断裂准则.      

褶皱与晶界耦合作用对石墨烯断裂行为的影响

由CVD方法制备的石墨烯含有大量的晶界,通常还带有许多褶皱,本文通过分子动力学方法研究了具有褶皱和晶界的石墨烯 平面拉伸断裂行为,结果显示,在垂直晶界方向,褶皱能够显著提高小角度双晶石墨烯的断裂应力,断裂应力增幅最 大约为50%,褶皱对断裂应力的影响随晶界角的增大减弱,导致双晶石墨烯断裂应力对晶界角不敏感,只略低于单晶石墨 烯,和实验结果完全吻合;在沿晶界方向,褶皱对双晶石墨烯断裂应力影响不明显. 另外,褶皱可以显著提高双晶石墨烯的断 裂应变,增幅最大约为100%. 增强机制归纳主要如下:通过面外变形,褶皱可以部分释放晶界5-7环中C---C键的预拉伸变形, 提高双晶石墨烯的断裂应力;褶皱可以降低相邻5-7环之间相互作用,导致断裂应力对晶界角不敏感;在拉伸作用下,褶皱被部分 拉平,这可以显著降低C---C键面内拉伸变形,导致断裂应变显著增大. 本研究为准确理解多晶石墨烯断裂行为提供重要帮助.      

Controlled wrinkling analysis of thin films on gradient substrates

The paper investigates continuously changing wrinkle patterns of thin films bonded to a gradient substrate. Three types of gradient substrates including exponential,power-law, and symmetry models are considered. The Galerkin method is used to discretize the governing equation of film bonded to gradient substrates. The wavelength and the normalized amplitude of the wrinkles for substrates of various material gradients are obtained. The numerical simulation based on the finite element method(FEM) is used to evolve the wrinkle patterns. The result agrees well with that of the analytical model.It is concluded that localization of wrinkle patterns strongly depends on the material gradient. The critical membrane force depends on both the minimum value of wrinkle stiffness and the gradient of wrinkle stiffness when the wrinkle stiffness is at its minimum.This work provides a better understanding for local wrinkle formation caused by gradient substrates.     

粘性基底上溅射薄膜褶皱的边界效应研究

复杂的表面微结构在自然界中普遍存在,它们赋予生物组织优异的物化功能.受此启发,各类仿生结构在工程领域(如微机电系统、能源材料、生物传感等)得到了广泛应用.论文利用磁控溅射技术在粘性的胶体基底上沉积金属铁薄膜,研究了薄膜中自发形成的褶皱结构,重点分析了约束边界和膜厚梯度边界对褶皱形貌的调控效应.实验发现,随着膜厚的增加,...     

Kinetic wrinkling of an elastic film on a viscoelastic substrate

A compressed elastic film on a compliant substrate can form wrinkles. On an elastic substrate, equilibrium and energetics set the critical condition and select the wrinkle wavelength and amplitude. On a viscous substrate, wrinkle grows over time and the kinetics selects the fastest growing wavelength. More generally, on a viscoelastic substrate, both energetics and kinetics play important roles in determining the critical condition, the growth rate, and the wavelength. This paper studies the wrinkling process of an elastic film on a viscoelastic layer, which in turn lies on a rigid substrate. The film is elastic and modeled by the nonlinear von Karman plate theory. The substrate is linear viscoelastic with a relaxation modulus typical of a cross-linked polymer. Beyond a critical stress, the film wrinkles by the out-of-plane displacement but remains bonded to the substrate. This study considers plane strain wrinkling and neglects the in-plane displacement. A classification of the wrinkling behavior is made based on the critical conditions at the elastic limits, the glassy and rubbery states of the viscoelastic substrate. Linear perturbation analyses are conducted to reveal the kinetics of wrinkling in films subjected to intermediate and large compressive stresses. It is shown that, depending on the stress level, the growth of wrinkles at the initial stage can be exponential, accelerating, linear, or decelerating. In all cases, the wrinkle amplitude saturates at an equilibrium state after a long time. Subsequently, both amplitude and wavelength of the wrinkle evolve, but the process is kinetically constrained and slow compared to the initial growth.     

Stretch-induced stress patterns and wrinkles in hyperelastic thin sheets

Wrinkles are commonly observed in stretched thin sheets and membranes. This paper presents a numerical study on stretch-induced wrinkling of hyperelastic thin sheets based on nonlinear finite element analyses. The model problem is set up for uniaxial stretching of a rectangular sheet with two clamped ends and two free edges. A two-dimensional stress analysis is performed first under the plane-stress condition to determine stretch-induced stress distribution patterns in the elastic sheets, assuming no wrinkles. As a prerequisite for wrinkling, development of compressive stresses in the transverse direction is found to depend on both the length-to-width aspect ratio of the sheet and the applied tensile strain in the longitudinal direction. A phase diagram is constructed with four different distribution patterns of the stretch-induced compressive stresses, spanning a wide range of aspect ratio and tensile strain. Next, an eigenvalue analysis is performed to find the potential buckling modes of the elastic sheet under the prescribed boundary conditions. Finally, a nonlinear post-buckling analysis is performed to show evolution of stretch-induced wrinkles. In addition to the aspect ratio and tensile strain, it is found that the critical condition for wrinkling and the post-buckling behavior both depend sensitively on the sheet thickness. In general, wrinkles form only when both the magnitude and the distribution area of the compressive stresses are sufficiently large. The wrinkle wavelength decreases with increasing strain, in good agreement with the prediction by a scaling analysis. However, as the tensile strain increases, the wrinkle amplitude first increases and then decreases, eventually flattened beyond a moderately large critical strain, in contrast to the scaling analysis.     

空间薄膜阵面结构褶皱的实验研究

阵面薄膜上的褶皱会显著影响航天器性能,褶皱控制也是薄膜航天器研发的难点。本文采用白光扫描技术,测试了一个正方形薄膜阵面结构在纯剪切作用下的褶皱特性。测试结果显示,随着剪切力的增大,结构膜面褶皱数增加,褶皱幅度增大,膜面平整度降低。随着膜面预应力的增大,膜面上的褶皱数略有增大,褶皱幅度有所减小,膜面平整度有所提高。测试结果还显示,膜面上粘结缝的存在、以及粘结缝与剪切力间的方位关系对膜面褶皱有复杂的影响。论文所得结论对薄膜阵面结构设计研发具有参考价值。     

Wrinkling of Orthotropic Membranes: An Analysis by the Polar Method

In the present paper, a simple membrane model based on the wrinkle strain approach is revisited with the aim of examining how the material elastic constants affect the static response of anisotropic membranes when wrinkling is taken into account. Employing the polar method, we analyze the role played by the polar moduli, which enable expressing the elasticity matrix components of an anisotropic material in terms of its invariant quantities. With reference to orthotropic materials, we first address the issue of membrane susceptibility to wrinkling by investigating the influence of the three polar parameters characterizing the anisotropic part of the constitutive law. The stress and strain states at any given point in a wrinkled membrane are analyzed by searching for explicit expressions for the principal values of stress and wrinkle strain. Finally, a comparison between our results and those obtained by a numerical solution available in the literature is made in the basic case of a membrane subjected to a pure shear strain state.     

Wrinkle-crease interaction behavior simulation of a rectangular membrane under shearing

Wrinkling analysis of a rectangular membrane with a single crease under shearing is performed to understand the wrinkle-crease interaction behaviors.The crease is considered by introducing the residual stresses from creasing and the effective modulus into the baseline configuration with assumed circular cross-sectional crease geometry.The wrinkling analysis of the creased membrane is then performed by using the direct perturb-force(DP) simulation technique which is based on our modified displacement components(MDC) method.Results reveal that the crease may influence the stress transfer path in the membrane and further change the wrinkling direction.The crease appears to improve the bending stiffness of the membrane which has an effective resistance on the wrinkling evolution.The effects of the crease orientation on wrinkle-crease interaction are studied toward the end of this paper.The results show that the wrinkling amplitude,wavelength,and direction increase as the crease orientation increases,and the wrinkling number decreases with the increasing crease orientation.These re-sults will be of great benefit to the analysis and the control of the wrinkles in the membrane structures.