Dynamic wrinkling in sandwich beams

Facings of sandwich structures employed in typical applications are often subject to parametric periodic loading. Such loading can cause local dynamic instability of the facings, i.e. large-amplitude small wavelength lateral vibrations. This phenomenon, called in the paper dynamic wrinkling, may result in fatigue damage or immediate failure. The problem of dynamic wrinkling of the facings is analyzed in the present paper for sandwich beams and for large aspect ratio wide panels that vibrate forming a cylindrical surface. The solution is obtained for the case of a relatively thick or compliant core where the Winkler elastic foundation model of the core is applicable. In addition, the problem is formulated as an extension of the Plantema core model that may be preferable for thinner and stiffer cores. In addition, a new simplified elasticity model is introduced in the paper that is based on the assumption that both facings experience simultaneous and interactive dynamic wrinkling instability. Numerical results shown for the elastic foundation model include the criterion for the onset of dynamic wrinkling and the critical value of the damping coefficient of the facing that is sufficient to prevent such wrinkling. As follows from these results, dynamic wrinkling is unlikely in most engineering applications, except for the case in which the maximum stresses in the facing approach the static wrinkling value.     

印刷起皱的几种情况分析与处理

印刷起皱故障是日常生产过程中的常见故障现象。印刷起皱的原因是多方面的,有纸张含水量异常之故,也有印刷版面结构弊病之源;有包衬工艺不当上的原因,亦有印刷压力不适所致;有咬纸牙调整不良或叼口量过小引起的,还有纸张敲勒方法不当造成的,等等。认真分析各种起皱故障现象,正确把握好印刷操作和工艺技术关,是防止印刷起皱保证产品印刷质量的有效措施。一、印刷起皱故障若干现象的分析和预防措施     

Wax morphology in bitumen

Wax crystallisation and melting in bitumen is usually considered detrimental to bitumen quality and asphalt performance. The objectives of this paper are to study wax morphology in bitumen and to investigate effects of time, temperature, and thermal cycling on wax crystallisation. Various samples were selected, including eight waxy bitumens of different sources and three laboratory blends prepared by adding a slack wax and two isolated bitumen waxes to the non-waxy bitumen. Test methods used were differential scanning calorimetry (DSC), polarised light microscopy (PLM), confocal laser scanning microscopy (CLSM), and freeze etching (fracture) in combination with transmission electron microscopy (FF-TEM). The DSC results indicated that the selected bitumen samples differ widely in wax content and wax crystallisation starting and melting out temperatures. It was found that non-waxy bitumen displayed no structure or crystals neither in PLM, CLSM or FF-TEM, while waxy bitumens from different crude origins showed a large variation of structures. The morphology of wax crystals was highly dependent on crystallisation temperature as well as temperature history. The wax which has been isolated from waxy bitumen and mixed into non-waxy bitumen displayed similar morphology as the wax in the original bitumen. It was also found that bitumen wax usually melted at temperatures lower than 60°C although in one case a temperature of 80°C was needed until complete melting of the wax.     

Nested self-similar wrinkling patterns in skins

Stiff thin films on soft substrates are both ancient and commonplace in nature; for instance, animal skin comprises a stiff epidermis attached to a soft dermis. Although more recent and rare, artificial skins are increasingly used in a broad range of applications, including flexible electronics, tunable diffraction gratings, force spectroscopy in cells, modern metrology methods, and other devices. Here we show that model elastomeric artificial skins wrinkle in a hierarchical pattern consisting of self-similar buckles extending over five orders of magnitude in length scale, ranging from a few nanometres to a few millimetres. We provide a mechanism for the formation of this hierarchical wrinkling pattern, and quantify our experimental findings with both computations and a simple scaling theory. This allows us to harness the substrates for applications. In particular, we show how to use the multigeneration-wrinkled substrate for separating particles based on their size, while simultaneously forming linear chains of monodisperse particles.     

Investigation of wrinkling behaviour in the creased thin-film laminates

International Journal of Mechanics and Materials in Design - The use of metal-polymer membrane laminates for gossamer space structures emerge in recent times. Wrinkle formation, fold-line effect,...     

Viscoelastic wrinkling in compression-stressed metal film-polymer sublayer system

The laws of corrugation (wrinkling) that takes place in thin aluminum films on silicon substrates with styrene sublayers under the conditions of thermal treatment have been studied using atomic force microscopy techniques. Measurements of the amplitude and period (wavelength) of wrinkles revealed stages in the viscoelastic corrugation process at various temperatures. It is established that the evolution of wrinkles in the course of annealing is controlled by the periodic distribution of normal and tangential stresses at the film-sublayer interface.     

Ageing in modified bitumen using FTIR spectroscopy

Modified binders exhibit complex rheological behaviour due to the interaction of the modifiers with the base binder and the manner in which they age during field applications. Quantification of the interaction and the ageing of such modified binders is currently a necessity. In this investigation, three modified binders (elastomer, plastomer, and crumb rubber) and the base bitumen were subjected to different ageing conditions and the evolution of the chemical functionalities during ageing were tracked using FTIR spectroscopy. Analysis of the spectra of modified binders immediately after the production process showed that while the elastomer modified binders exhibited physical interaction, the plastomer and crumb rubber modified binders exhibited physical and chemical interactions. During ageing, there was no subsequent evolution of the interactions and one could only see chemical functionalities related to oxidation. Analysis of the spectra showed that the carbonyl and sulphoxide exhibit identical trends whereas aliphaticity and aromaticity deviate drastically.     

Determination of polymer content in modified bitumen

Polymer-modified bitumen (PMB) has been increasingly used to enhance pavement performance. Two styrenebutadiene-styrene (SBS) copolymers were mixed with two bitumens by weight of the blend. This paper aims at developing the procedure to determine the proper polymer content to be mixed with bitumen. Tests including storage stability test, dynamic shear rheometer and scanning electron microscopy (SEM) were conducted to investigate the viscoelastic properties and microstructures of PMB. The addition of polymers increased the viscosity, softening point, toughness and complex modulus of bitumens. SEM results indicated that, as the polymer content increased, SBS gradually became the dominant phase that resulted in an increase in PMB's mechanic properties. Good compatibility produced an elastic network into the PMB up to 6% polymer concentration. The optimum polymer content was determined based on the rheological properties and the formation of the critical network. Adding higher polymer contents could lead to the separation of polymer and bitumen. The softening point temperature difference between top and bottom samples should be controlled within 2°C to monitor PMB's stability.     

Localized wrinkling instabilities in radially stretched annular thin films

Summary We investigate a pre-stressed annular thin film subjected to a uniform displacement field along its inner boundary. This loading scenario leads to a variable stress distribution characterized by an orthoradial component that may change sign along a concentric circle within the annular domain. When the intensity of the applied field is strong enough, elastic buckling occurs circumferentially, leading to a localized wrinkling pattern near the inner edge. Using a linear non-homogeneous pre-bifurcation state, the eigenvalue problem describing this instability is cast as a singularly-perturbed fourth-order linear differential equation with variable coefficients. The dependence of the lowest eigenvalue on various non-dimensional quantities is numerically investigated using the compound matrix method. These results are complemented by a WKB analysis which suggests that the qualitative and quantitative features of the full model can be described by a simplified second-order eigenvalue problem which takes into account the finite stiffness of the system.     

Laboratory studies of diffusion in bitumen using markers

Performance of recycled asphalt pavements depends, among other things, on the degree of mixing of old and new binders. One of the factors contributing to the mixing is diffusion, which increases the homogeneity of recycled binders. In this study, FTIR-ATR (Fourier Transform Infrared Spectroscopy with Attenuated Total Reflectance) has been applied to investigate influence of temperature, viscosity of the diffusion medium, and diffusant size and polarity on diffusion characteristics. Several substances, referred to as markers, have been monitored when diffusing through different binders. By careful selection of markers, the importance of diffusant size and polarity on rate of diffusion has been demonstrated. Diffusion has been modelled using Stoke-Einstein's equation and compared with experimental data obtained by FTIR-ATR. The equation has shown useful in explaining the effects of temperature, viscosity and molecular size on diffusion.     

Some applications of the WKB method to the wrinkling of bi-annular plates in tension

An application of WKB methods is proposed here for a stretched annular thin plate with piecewise-constant mechanical properties (also known as a bi-annular plate). Unlike the classical scenario involving only a simple annular such plate, in certain cases the neutral stability curve fails to be convex and the critical eigenmodes behave rather differently as the plate becomes progressively thinner (equivalent to ${\mu \to \infty}$ in our notations). On one side of this curve, the corresponding eigenmodes are localised near the inner rim of the annulus, while in the remaining part these functions are concentrated along the interface separating the two annular sub-regions. By using the asymptotic reduction technique proposed by Coman and Haughton in (Acta Mech 185:179–200, 2006), the original fourth-order three-point boundary-value problem is formally reduced to a pair of second-order differential equations coupled through a set of matching conditions at the interface. It is shown that for ${\mu \gg 1}$ the critical eigenvalues for both cases mentioned above can be approximated by solving a couple of simple transcendental equations and that the results predicted compare well with the direct numerical simulations of the original problem.     

Effect of fibre wrinkling to the spring-in behaviour of L-shaped composite materials

To determine the amount of deformation resulting from fibre wrinkling at corner regions, a set of experiments have been conducted. As known in the conventional lay-up method, the prepregs are laid sequentially layer by layer on the mould surface. At the corner region of a female tool the radius decreases at the inner surface and the amount of wrinkles increase towards the top layer as the layers are laid up. In order to determine how much these wrinkles influence the dimensional stability of the manufactured parts, an alternative lay-up method is used. The amount of the wrinkles can be increased for the parts of same geometry by first stacking prepregs on a flat plate and then bending the whole stack to conform to the surface of the L-shaped mould. In this method, more wrinkling occurs on the inner surface of the corner regions as compared to the conventional lay-up procedure. It was found that fibre wrinkling decreases the spring-in values. The mechanism behind that observation is discussed with the help of a heuristic Finite Element Analysis (FEA). The conformation of the stacked prepregs on the mould was simulated by using FEA.     

On the applicability of tension field theory to a wrinkling instability problem

Summary The wrinkling instability of a pre-stressed annular membrane loaded uniformly along its inner boundary is investigated with the help of a tension field theory. The theoretical solution is shown to capture the essential qualitative features involved in axisymmetric wrinkling, but displays several limitations regarding the quantitative aspects of this issue. The relationship with some related experimental work reported recently in the literature is also discussed.     

Quantifying the stress relaxation modulus of polymer thin films via thermal wrinkling

The viscoelastic properties of polymer thin films can have a significant impact on the performance in many small-scale devices. In this work, we use a phenomenon based on a thermally induced instability, termed thermal wrinkling, to measure viscoelastic properties of polystyrene films as a function of geometric confinement via changes in film thickness. With application of the appropriate buckling mechanics model for incompressible and geometrically confined films, we estimate the stress-relaxation modulus of polystyrene films by measuring the time-evolved wrinkle wavelength at fixed annealing temperatures. Specifically, we use time-temperature superposition to shift the stress relaxation curves and generate a modulus master curve for polystyrene films investigated here. On the basis of this master curve, we are able to identify the rubbery plateau, terminal relaxation time, and viscous flow region as a function of annealing time and temperatures that are well-above its glass transition. Our measurement technique and analysis provide an alternative means to measure viscoelastic properties and relaxation behavior of geometrically confined polymer films.     

Chemical reactivity of mineral aggregates in aqueous solution: relationship with bitumen emulsion breaking

This paper reports a study on chemical reactivity of gneiss, diorite and limestone aggregates in aqueous solution. The originality of this study is that it extended to very short times (less than 1440 min). Rise in pH tests was implemented and dissolution kinetics was analysed. The results showed that calcium was the major element released by the aggregates. It has also been found that dissolution had an influence on the final morphology of aggregates. Polyamine emulsifier adsorption onto aggregates has been assessed using electrophoresis. Finally, the rise in pH and electrophoretic tests were compared to the breaking test traditionally performed to characterise bitumen emulsions. It was found that breaking values were controlled by both the surface area and the surface charge of the particles. Results may be correlated to polyamine adsorption on aggregates. Adsorption seemed to be efficient for gneiss and diorite: at pH 2, their charge turned from slightly negative to highly positive. At this pH value, limestone particles were dissolved and polyamine adsorption must be less efficient than with gneiss and diorite, for which the emulsion breaking was facilitated by the high attraction of particles for the emulsifier, due to their negative surface charge.     

The effects of in-plane core stiffness on the wrinkling behavior of thick sandwiches

Summary This contribution presents a refined analytical solution for the wrinkling of sandwich plates with isotropic face layers and thick orthotropic cores, taking into accountin-plane deformations of the core. A single explicit equation for the critical wrinkling load in an asymptotic sense in derived. The results have been verified extensively by a numerical model [1] and show that, when dealing with highly orthotropic cores (e.g., honeycombs), the wrinkling loads and deformation patterns can strongly depend on the in-plane stiffness of the core. This new theoretical finding which is of considerable practical importance is the main motivation for this paper. Classical wrinkling formulae [2], [3], [4] can lead to significant errors when used in connection with highly orthotropic cores.