Wrinkling of nonlinear membranes

 Membranes are stiff under tension but switch over to wrinkling when compression occurs. Roddeman proposed a kinematic model to handle this phenomenon under finite deformation conditions. The wrinkling conditions of Roddeman are transformed into the reference configuration. This results in a more simple nonlinear formulation. For application in a finite element code a consistent linearization was carried out. Numerical examples for linear and nonlinear orthotropic constitutive equations are discussed. Received 21 December 2001 / Accepted 10 April 2002     

Wrinkling on irreversibly deforming foundations

Wrinkling is a mechanical instability occurring in multi-layer structures comprising a thin and stiff layer resting on a supporting underlying compliant layer. The present study is motivated by experimental observations of wrinkling leading to irreversible deformation and to the initiation of defects. An incremental Spectral Method is employed to solve the governing differential equations. This approach enables the analysis of wrinkling processes non-linear foundations and under cyclic loading. Numerical simulation results are compared to a set of analytical solutions describing wrinkling events on nonlinear foundations. Wrinkling is analyzed for cases of elastic, elastic damageable, and elastic plastic foundations. The behavior of homogeneous foundation properties is contrasted to situations where local defects are present.     

Wrinkling of corrugated skin sandwich panels

Compression wrinkling of composite sandwich panels with corrugated skins was investigated numerically, analytically and experimentally. Semi-circular and sine-wave shaped corrugations were studied. The corrugations significantly increased wrinkling strength when compared with equal mass flat panels. Semi-circular corrugations proved to be highly preferable to sine-wave shaped corrugations due to localized buckling in the latter. Over 40 fiberglass and foam core sandwich specimens were manufactured with semi-circular skin corrugations. These specimens were tested to failure, providing confirmation of the numerical and analytical results.     

Wrinkling of thin films on a microstructured substrate

Surface wrinkling of thin films on substrates offers an effective strategy to create controllable surface patterns of wide application. In this article, both theoretical analysis and numerical simulations are performed to study the surface wrinkling of thin films bonded on a microstructured soft substrate under compression. We consider two typical kinds of substrates that have different mechanical properties. One possesses a periodic array of micropillars, and the other has a period of alternating unbonded—bonded regions at the film—substrate interface. The characteristics of surface wrinkling and postbuckling evolution of films are revealed. It is found that the interfacial microstructures modulate the normal mechanical properties of the substrate and, thus, tailor the buckling behavior of the thin film atop it. Under lateral compression, the film on the substrate shows periodic arrays of micropillars exhibiting sinusoidal wrinkling first, and then with further compression, the wrinkles give way to a deep fold accompanied by Euler buckling of the micropillars and the decay of neighboring wrinkles. For the system with periodic unbonded—bonded regions at the film—substrate interface, wrinkling of the film is characterized by a relative shift in the normal direction. This study offers potential applications in the fabrication of tunable surface morphologies.     

Curvature‐Controlled Wrinkling Surfaces for Friction

Topographical patterns endow material surfaces with unique and intriguing physical and chemical properties. Spontaneously formed wrinkling has been harnessed to generate surface topography for various functionalities. Despite promising applications in biomedical devices and robot engineering, the friction behavior of wrinkling on curved surfaces remains unclear. Herein, wrinkled surfaces are induced by sputtering metals on soft polymer microspheres. The wrinkle morphologies and length scales can be controlled precisely by tailoring the microsphere radius (substrate curvature) and film thickness. The wrinkled surfaces exhibit controlled friction property, depending on the wrinkling patterns and length scales. An increase in friction force with increasing surface roughness is generally found for dimple patterns and labyrinth patterns. The dimple patterns show the lowest friction due to strong curvature constraint. The herringbone patterns exhibit apparent friction anisotropy with respect to topographic orientation. These results will guide future design of wrinkled surfaces for friction by harnessing substrate curvature.     

Anisotropic Nanoscale Wrinkling in Solid‐State Substrates

Pattern formation induced by wrinkling is a very common phenomenon exhibited in soft‐matter substrates. In all these systems, wrinkles develop in the presence of compressively stressed thin films lying on compliant substrates. Here, the controlled growth of self‐organized nanopatterns exploiting a wrinkling instability on a solid‐state substrate is demonstrated. Soda‐lime glasses are modified in the surface layers by a defocused ion beam, which triggers the formation of a compressively stressed surface layer deprived of alkali ions. When the substrate is heated up near its glass transition temperature, the wrinkling instability boosts the growth rate of the pattern by about two orders of magnitude. High‐aspect‐ratio anisotropic ripples bound by faceted ridges are thus formed, which represent an optimal template for guiding the growth of large‐area arrays of functional nanostructures. The engineering over large square centimeter areas of quasi‐1D arrays of Au nanostripe dimers endowed with tunable plasmonic response, strong optical dichroism, and high electrical conductivity is demonstrated. These peculiar functionalities allow these large‐area substrates to be exploited as active metamaterials in nanophotonics, biosensing, and optoelectronics.     

台阶盒形件起皱和开裂改善

目的 改善台阶盒形件拉深成形时凸缘区起皱和圆角区开裂的缺陷。方法 理论分析了起皱和开裂产生的原因,利用有限元模拟分析了压边力大小、凹模运行方式和板料形状对台阶盒形件拉深成形的影响,采用实验验证了有限元模拟结果的准确性。结果 长方形板料拉深成形时,4个角部相比直边部位流动阻力更大,直边部位材料过度向模具型腔内流动,造成凸缘区周向压应力过大,进而引起起皱,当零件拉深深度较大时,圆角部位材料变形剧烈且材料流动不均匀,极易产生开裂;采用20 kN的压边力、梯形的凹模向下运行方式和类椭圆形板料的工艺参数可以控制材料流动,使板料变形均匀并改善凸缘区起皱和圆角区开裂缺陷。结论 有限元模拟可为冲裁工艺参数的选取提供理论指导。     

Moisture‐Responsive Wrinkling Surfaces with Tunable Dynamics

The wrinkle dynamics (such as reversibility and stability) of human skin are affected by the external stimuli, as well as the skin's structure and mechanical properties. Inspired by these tunable responses, three types of moisture‐responsive wrinkle dynamics are achieved, for the first time, through a single film–substrate system. These dynamics include: (1) completely reversible wrinkles formation; (2) irreversible wrinkles formation I: the initially formed wrinkles can be permanently erased and never reappear; and (3) irreversible wrinkles formation II: once the wrinkles form, they can no longer be erased. The key to success is to control the stiffness and thickness ratios of the film and the substrate, and tailor the crosslink degree/gradient of the film to allow for moisture‐dependent changes of modulus and swelling degree. These unique responsive dynamics motivate the invention of a series of optical devices triggered by moisture, including anticounterfeit tabs, encryption devices, water indicators, light diffusors, and antiglare films. This study also paves the road for further understanding of the skin wrinkling dynamics and manipulation.     

Cover Picture: Fabricating Microlens Arrays by Surface Wrinkling (Adv. Mater. 24/2006)

The unique lateral confinement of a swelling polymer network leads to surface wrinkles that define a multitude of surface structures, including microlens arrays (background). The versatility of this approach, which is reported on p. 3238 by Crosby and Chan, allows for the realization of a variety of functional devices on both planar and nonplanar surfaces, such as synthetic compound lens structures (see inset).     

矩形截面件开式液压成形变形规律及起皱行为

目的 开式筒壳液压成形采用具有搭接结构的开式筒壳作为坯料,具有提高成形能力、改善壁厚分布、降低成形压力等优势。旨在以矩形截面件为对象,研究内压作用下开式筒壳的圆角充填行为。方法 采用数值模拟手段,分析了矩形截面件开式液压成形过程中的应力应变变化及成形缺陷。利用内高压成形机和自主研制的自密封装置,开展了搭接区中点分别位于圆角区、过渡区和直边区3种搭接位置条件下的矩形截面件开式液压成形实验研究。结果 模拟和实验结果表明,开式筒壳搭接区的外层为易起皱区。当搭接量为80 mm且搭接区中点位于圆角区时,模具型腔的外压作用有效抑制了起皱的形成。4个圆角中距离对应搭接位置最近的圆角最先完成充填。搭接区域所在位置的壁厚减薄率显著低于其他区域,3种条件下成形件中间截面的最大减薄率分别为10.4%、9.8%和10.2%,搭接区中点位于过渡区时最大减薄率较低。结论 采用一定搭接量的开式筒壳比封闭截面管坯更有利于圆角的充填。选取适宜的搭接位置可避免成形过程中起皱缺陷的产生。此外,搭接位置对开式筒壳圆角充填的应力应变分布、圆角半径以及壁厚分布起到重要作用。     

Electrochemical Oxidation to Fabricate Micro-Nano-Scale Surface Wrinkling of Liquid Metals

Constructing wrinkled structures on the surface of materials to obtain new functions has broad application prospects. Here a generalized method is reported to fabricate multi-scale and diverse-dimensional oxide wrinkles on liquid metal surfaces by an electrochemical anodization method. The oxide film on the surface of the liquid metal is successfully thickened to hundreds of nanometers by electrochemical anodization, and then the micro-wrinkles with height differences of several hundred nanometers are obtained by the growth stress. It is succeeded in altering the distribution of growth stress by changing the substrate geometry to induce different wrinkle morphologies, such as one-dimensional striped wrinkles and two-dimensional labyrinth wrinkles. Further, radial wrinkles are obtained under the hoop stress induced by the difference in surface tensions. These hierarchical wrinkles of different scales can exist on the liquid metal surface simultaneously. Surface wrinkles of liquid metal may have potential applications in the future for flexible electronics, sensors, displays, and so on.     

Wrinkling analysis of membranes with elastic–plastic material behavior

This paper concerns membrane wrinkling in conjunction with inelastic effects. A method will be presented, able to treat wrinkling in membranes with elastic–plastic material behavior. Some effort is spent to obtain a suitable initial guess for the internal wrinkling algorithm iteration, which improves the reliability of the method. The wrinkling algorithm is implemented into a FE–program and applied to structural analysis. Numerical and experimental results are discussed.