Evolution Of Wrinkles Research Articles

Numerical analysis of the wrinkling behavior of thin membranes

Membranes are widely applied in the large-span buildings and spatial deployable structures. They are prone to wrinkle under compression due to their small bending stiffness. The wrinkling deformation may affect the surface precision of a membrane structure and its static and dynamic behaviors. Research on the wrinkling behavior of a membrane and its variation with the wrinkle-influencing factors would shed light on the evolution of wrinkles and contribute significantly to the effective control over the wrinkling deformation. In this paper, a rectangular membrane under shear is numerically studied based on the stability theory of plates and shells to explore the wrinkle-influencing factors, such as boundary conditions, pre-stress, thickness and material constants, and their effects on the characteristic parameters of wrinkles. Besides, some of the results are also compared with those derived from the membrane element previously proposed by the authors for a further exploration. Some new and interesting findings are obtained.

Mechanical behavior of thin films due to in-plane tensile and fatigue loadings

The mechanical properties, wrinkling phenomenon and fatigue response of polymeric polyethylene rectangular thin films due to both static tensile and cyclic tests were investigated. From the tensile tests we obtained the stress-strain curves and observed the formation of wrinkles for original films. The evolution of wrinkles resulted from in-plane to out-of-plane deformation and deflection. Also, the tensile tests for notched films were performed such as with a central hole, two vertical holes and two horizontal holes. From the fatigue tests the total strain vs. life curve was achieved. It was composed of two straight lines of both elastic and plastic parts.

Dynamic Interpenetrating Polymer Network (IPN) Strategy for Multiresponsive Hierarchical Pattern of Reversible Wrinkle.

Dynamic micro-/nanowrinkle patterns with response to multienvironmental stimuli can offer a facile method for on-demand regulation of surface properties, thus allowing for generation of a smart surface. Here a practical yet robust strategy is described to fabricate redox, light and thermal responsive wrinkle by building dynamic double interpenetrating polymer network (IPN) as the top layer for a typical bilayer system. IPNs were constructed through the photochemical reaction of a mixture comprised of light-sensitive anthracene-containing polymer (PAN) and redox-sensitive disulfide-containing diacrylate monomer (DSDA). Thanks to the dynamic covalent reversible C-C bond in PAN and S-S bond in DSDA, the morphology of wrinkled surface not only can be reversibly and precisely (micrometer scale) tailored to all kinds of complicated hierarchical pattern permanently, but also can be controlled temporarily by irradiation of near-infrared light (NIR). A sine wave model is proposed to investigate the dynamics of real-time reversible wrinkle evolution. This general approach based on IPN allows independent multistimuli control over wettability and optical properties on the wrinkled surface,thus, presents a considerable alternative to implement a smart surface.

Evolution of wrinkles with spiral crack on stiff film/compliant substrate under controllable micro-probe loading

Global wrinkling can be widely observed in nature and engineering materials, however, the dynamic evolution of localized wrinkles is seldom observed in experiments. In this paper, a controllable micro-probe is applied to an ultrathin Au film deposited onto a thick PDMS substrate to demonstrate the co-evolution of the localized wrinkles and crack. The experiment shows that the film subjected to the increasing indentation will produce two stress relief mechanisms: the tensile stress is relieved by the formation of spiral crack and the compressive stress is relieved by the formation of radial wrinkles. The crack forms near the edge of the probe tip and then propagates along right- or left-handed spiral path. Meanwhile, the high compressive stress is generated and contained in a narrow strip between consecutive passes of the spiral crack, which causes the propagation of the crack to be always accompanied by wrinkles. As the indentation increases, circular crack tends to be formed in the film and the crack propagation is ceased. The localized wrinkling and cracking patterns may have potential applications in some fields.

Investigating the influence of relative humidity on expression microwrinkles

The quest for efficient anti-wrinkle treatments has mainly focused on biochemical approaches aiming to mitigate or slow down the effects of both intrinsic and extrinsic ageing. However, the biophysical principles that govern the formation and evolution of wrinkles remain to be elucidated. Georges Limbert shares the findings of a study his computational biophysics group conducted with US researchers

Evolution of local wrinkles near defects on stiff film/compliant substrate.

Disordered wrinkles are widely observed in stiff film deposited onto a thermally expanded polymer when compressive stress exceeds the critical wrinkling stress of the film. Highly ordered wrinkles can be fabricated by introducing regularly arranged patterns on the polymer before deposition. However, the study on the morphological evolution of localized wrinkling patterns near defects on the stiff film/compliant substrate is neglected. In this paper, we show two morphological transitions of the local wrinkles induced by defects on an Au film/PDMS substrate. The observation shows that the straight wrinkles form perpendicularly to the line defects and the radial wrinkles form near spot-like defects. We observe that the extended radial wrinkles tend to split and evolve into branching patterns, this limits the deviation of the local wrinkle wavelength from the equilibrium wrinkle wavelength and causes the wrinkle wavelength to be always maintained in a narrow interval. Because the herringbone patterns have the minimum energy state, the straight and radial wrinkles evolve into herringbone wrinkles spontaneously. The morphological characteristic and evolution mechanism of the local wrinkles are described in detail. The observation may provide some clues to the formation and evolution of some localized wrinkling patterns in nature and multilayer materials.

Wrinkling of thin films on a microstructured substrate

ABSTRACTSurface 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.

Direct numerical simulation of the turbulent premixed flame propagation with radiation effects

This paper investigates the effects of turbulence on radiative heat transfer in premixed flame propagation using a combination of direct numerical simulation (DNS) and the discrete ordinates method (DOM). The DNS code has been validated and established in previous research, and the newly written DOM code is validated by solving a sample radiative heat transfer problem. The DOM code is explicitly combined with the DNS code using the 3rd-order Runge–Kutta scheme. In the numerical experiments, an initially flat laminar premixed flame interacts with imposed turbulent fluctuations and evolves over time. A remarkable increase in the temperature self-correlation factor <T4>/<T>4 is observed in the middle of the reaction. Higher u′ conditions induced faster and more intensive flame wrinkle evolution. However, flow turbulence reduces the radiative heat loss in planar premixed flame due to the flame curvature effects.

Post-wrinkling analysis of a torsionally sheared annular thin film by using a compound series method

This paper investigates the torsional wrinkling behavior of an annular thin film. Non-dimensional nonlinear von Karman buckling equations are established, which are solved by introducing a compound series method to acquire the post-wrinkling characteristics. The proposed theoretical model can accurately predict the critical wrinkling behavior and post-wrinkling characteristics of the annular thin film, which are verified by the experimental measurement based on the digital image correlation (DIC) technique. The theoretical results show that the post-wrinkling stress is intimately associated with the wrinkle configuration in the post-wrinkling stage. The hoop post-wrinkling stress along the wrinkle texture direction of the annular thin film dictates the wrinkle evolution. The wrinkle number remains constant in the elastic regime, which is determined by the critical buckling load factor. The results provide good guides to tune or control the wrinkles in the thin film.

Tuning and Erasing Surface Wrinkles by Reversible Visible‐Light‐Induced Photoisomerization

AbstractPeriodic wrinkling across different scales has received considerable attention because it not only represents structure failure but also finds wide applications. How to prevent wrinkling or create desired wrinkling patterns is non‐trivial because the dynamic evolution of wrinkles is a highly nonlinear problem. Herein, we report a simple yet powerful method to dynamically tune and/or erase wrinkling patterns with visible light. The light‐induced photoisomerization of azobenzene units in azopolymer films leads to stress release and consequently to the erasure of the wrinkles. The wrinkles in unexposed regions are also affected and oriented perpendicular to the exposed boundary during the stress reorganization. Theoretical models were developed to understand the dynamics of the reversible photoisomerization‐induced wrinkle evolution. This method can be applied for designing functional materials/devices, for example, for the reversible optical writing/erasure of information as demonstrated here.

Tuning and Erasing Surface Wrinkles by Reversible Visible-Light-Induced Photoisomerization.

Periodic wrinkling across different scales has received considerable attention because it not only represents structure failure but also finds wide applications. How to prevent wrinkling or create desired wrinkling patterns is non-trivial because the dynamic evolution of wrinkles is a highly nonlinear problem. Herein, we report a simple yet powerful method to dynamically tune and/or erase wrinkling patterns with visible light. The light-induced photoisomerization of azobenzene units in azopolymer films leads to stress release and consequently to the erasure of the wrinkles. The wrinkles in unexposed regions are also affected and oriented perpendicular to the exposed boundary during the stress reorganization. Theoretical models were developed to understand the dynamics of the reversible photoisomerization-induced wrinkle evolution. This method can be applied for designing functional materials/devices, for example, for the reversible optical writing/erasure of information as demonstrated here.

Wrinkling Phenomenon during Uniaxial Extension of a Woven Thermoplastic Composite

In the current research, wrinkling behaviour of a pre-consolidated woven self-reinforced polypropylene (SRPP) composite is investigated. Specimens possessing different aspect ratios were uniaxially extended at room temperature. Principal strains were recorded by a 3D optical photogrammetric system. Continuous measurement of out-of-plane displacements and induced in-plane strains elucidated the effect of specimens’ aspect ratios on wrinkling behaviour of SRPP composite. Investigating the evolution of meridian strains offered a valuable insight into development of principal strains before and after wrinkling onset. Plotting major and minor strains in a 2D strain space clarified the role of different deformation modes in the evolution of wrinkles. It was shown that common wrinkling predictors used on metals cannot effectively predict wrinkling initiation in this class of composites and a more reliable measure, based on evolution of local strains, should be employed. The current study investigates the wrinkling behaviour of a woven thermoplastic composite for the first time through a modified Yoshida buckling experiment. The outcomes illustrated effective parameters on the wrinkling evolution in SRPP specimens.

A comparative analysis of numerical approaches to the mechanics of elastic sheets

Numerically simulating deformations in thin elastic sheets is a challenging problem in computational mechanics due to destabilizing compressive stresses that result in wrinkling. Determining the location, structure, and evolution of wrinkles in these problems has important implications in design and is an area of increasing interest in the fields of physics and engineering. In this work, several numerical approaches previously proposed to model equilibrium deformations in thin elastic sheets are compared. These include standard finite element-based static post-buckling approaches as well as a recently proposed method based on dynamic relaxation, which are applied to the problem of an annular sheet with opposed tractions where wrinkling is a key feature. Numerical solutions are compared to analytic predictions of the ground state, enabling a quantitative evaluation of the predictive power of the various methods. Results indicate that static finite element approaches produce local minima that are highly sensitive to initial imperfections, relying on a priori knowledge of the equilibrium wrinkling pattern to generate optimal results. In contrast, dynamic relaxation is much less sensitive to initial imperfections and can generate low-energy solutions for a wide variety of loading conditions without requiring knowledge of the equilibrium solution beforehand.

Swelling/deswelling-induced reversible surface wrinkling on layer-by-layer multilayers.

Layer-by-layer (LbL) multilayer film is incorporated in the fabrication of a film/substrate system for the investigation of swelling/deswelling-induced wrinkle evolution for the first time. As one typical example, hydrogen-bonded (PAA/PEG)n (PAA, poly(acrylic acid); PEG, poly(ethylene glycol)) is deposited on a poly(dimethylsiloxane) (PDMS) substrate via the LbL technique. Heating treatment causes the covalent cross-linking reaction to occur in the H-bonded multilayers with simultaneously spontaneous formation of labyrinth wrinkles. Subsequent water immersion leads to the evolution of a series of the swelling-sensitive wrinkles in the thermally cross-linked (PAA/PEG)n/PDMS bilayer, ranging from initial labyrinth wrinkles (a) to an intermediate smooth wrinkle-free state (b), hexagonally arranged dimples (c), and the later-segmented labyrinth patterns (d). Upon deswelling by reheating of the swollen bilayer, the reverse wrinkle evolution happens via the process of d → b, or d → b → a, or c → b, or c → b → a, which is dependent on the reheating temperature and the swelling-induced pattern. We investigate the influences of experimental conditions on the swelling kinetics and the resulting wrinkle evolution, which include the thickness of (PAA/PEG)n, the additionally deposited outermost layer (e.g., Pt and polystyrene), and the swelling solution pH. The involved mechanism has been discussed from the viewpoint of the relation between the wrinkling behavior and the swelling/deswelling-induced stress state. The results indicate that the combined strategy of LbL assembly with the introduction of additional layers endows us with considerable freedom to fabricate multifunctional film/substrate systems and to tune the instability-driven patterns for advanced properties and extended applications.

Research on Wrinkling Behavior in Tube Hydroforming with Axial Feeding

Tube hydroforming (THF) is one of metal forming technologies which has been widely used to manufacture complex hollow workpeices. In THF, a variety of failures may occur and one of them is wrinkling. But recent researches show that wrinkling may be used as a preforming process to improve the formability of tubes. In this paper, a new geometry-based wrinkling indicator is proposed to evaluate the wrinkling level in THF and the wrinkle evolution diagram (WED) based on the shape change of the wrinkles is presented to display the four-stage evolution of the useful wrinkles. The wrinkling levels in THF with axial feeding under various loading paths are predicted respectively via finite element simulation, the influence of loading paths on the wrinkling behavior is investigated, and the evolving stages of the useful wrinkles is revealed via the proposed WED. The results indicate that the proposed wrinkle indicator can distinctly evaluate the wrinkling level, the wrinkling level under pulsating loading path is higher than that under polygonal linear one and four-stage evolution of the useful wrinkles could be evidently demonstrated via the WED. Notation

Experimental Research on Influence of Temperature and Compression on Wrinkle Evolution of Thin Films on Compliant Substrates

Wrinkle evolution under temperature and compression was experimentally explored in a metal film deposited on a compliant polymer. After wrinkle formation, the samples undergo thermal annealing in different temperature circumstances. In the range of 75-150°C, the wavelength and amplitude of wrinkles became both smaller as the temperature increased, indicating that annealing effectively releases residual stress and promote material modification. In contrast, compression only changed wrinkle rearrangement, and did not alter the wavelength and amplitude. These results suggest that external stimulation can transform wrinkled surfaces. This work provides the guidance for service conditions of surface wrinkling.

Wrinkling of the metal–polymer bilayer: the effect of periodical distribution of stresses and strains

The viscoelastic wrinkling of aluminium films formed on silicon substrates with polystyrene interlayers (aluminium–polystyrene bilayers) is studied under thermal annealing. Compressive thermal stresses induced by the difference in thermal expansions of the bilayer and the silicon substrate are shown to lead to wrinkling instability of the Al film. The effects of annealing conditions and film thickness on the wrinkling regularities are investigated. The evolution of wrinkles, which has a three-stage character, is found to be controlled by the sign and value of in-plane stresses and those normal to the metal–polymer interface. On increasing temperature and duration of annealing, lateral extension of the aluminum–polystyrene bilayer inhibits wrinkle growth and results in smoothening of the film surface.

Elasticity and rigidity percolation in flexible carbon nanotube films on PDMS substrates

The evolution of wrinkles and folds in compressed thin films of type-purified single-wall carbon nanotubes (SWCNTs) on polydimethylsiloxane (PDMS) substrates is used to study the mechanical response of pristine nanotube networks. While the low-strain moduli are consistent with the exceptional mechanical properties of individual nanotubes, the films are remarkably fragile, exhibiting small yield strains that decrease with increasing thickness. We find significant differences in the mechanical response of semiconducting as compared to metallic SWCNT networks, and we use simple scaling arguments to relate these differences to previously determined Hamaker constants associated with each electronic type. A comparison with conductivity measurements performed on identical films suggests more than a two-fold variation in the onset of rigidity vs. connectivity percolation, and we discuss the potential implications of this for both rigid-rod networks and the use of type-purified SWCNTs in flexible electronics.

Nonperturbative model for wrinkling in highly bendable sheets

The wrinkled geometry of thin films is known to vary appreciably as the applied stresses exceed their buckling threshold. Here we derive and analyze a minimal, nonperturbative set of equations that captures the continuous evolution of radial wrinkles in the simplest axisymmetric geometry from threshold to the far-from-threshold limit, where the compressive stress collapses. This description of the growth of wrinkles is different from the traditional post-buckling approach and is expected to be valid for highly bendable sheets. Numerical analysis of our model predicts two surprising results. First, the number of wrinkles scales anomalously with the thickness of the sheet and the exerted load, in apparent contradiction with previous predictions. Second, there exists an invariant quantity that characterizes the mutual variation of the amplitude and number of wrinkles from threshold to the far-from-threshold regime.

Analysis of crease-wrinkle interaction for thin sheets

In this paper, geometrically non-linear post-buckling analyses were performed to study the effect of sheet thickness, deployment angle, and load ratio on the crease-wrinkle interaction. A square sheet configuration with a single transverse crease was modeled using thin shell elements. The analysis proceeded by initially providing a realistic deployed state of a creased membrane sheet. Then an uneven corner loading was applied to introduce wrinkling. The effects of the induced anisotropy from the crease on the fine-scale detail of the wrinkle evolution, as a function of sheet thickness, loading, and crease deployment angle were systematically investigated. Significant differences were found in sheet compliance and crease-wrinkle interaction as these parameters were varied.