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Wrinkling patterns are beneficial for a wide range of engineering applications including flexible electronics, tuned optics, sensoring, energy harvesting, surface wetting and biomedicine. Here we report on controllable multimode wrinkling patterns of metal (iron) films sputter deposited on flexible substrates with gradient modulus. It is found that disordered wrinkles (G1 wrinkling) form in the weakly crosslinking region at the early stage of film deposition. Disordered labyrinth wrinkles with larger wavelengths and ordered stripe wrinkles localized by cracks (both G2 wrinkling) appear in the strongly crosslinking region for the samples with small film thickness and large thickness, respectively. The coexistence of the labyrinth G1 wrinkling and labyrinth G2 wrinkling (or stripe G2 wrinkling) can be obtained by tuning the substrate modulus and film thickness. The morphologies, evolutions and mechanisms of such wrinkles are discussed in detail based on the stress theory. This study can promote better understanding of the substrate effect on the complex wrinkling patterns. [ABSTRACT FROM AUTHOR]

In the present work, austenitic stainless steel (ASS) 304 foils with a thickness of 50 µm were first annealed at temperatures ranging from 700 to 1100 ℃ for 1 h to obtain different microstructural characteristics. Then the effects of microstructural characteristics on the formability of ASS 304 foils and the quality of drawn cups using micro deep drawing (MDD) were studied, and the mechanism involved was discussed. The results show that the as-received ASS 304 foil has a poor formability and cannot be used to form a cup using MDD. Serious wrinkling problem occurs on the drawn cup, and the height profile distribution on the mouth and the symmetry of the drawn cup is quite non-uniform when the annealing temperature is 700 ℃. At annealing temperatures of 900 and 950 ℃, the drawn cups are both characterized with very few wrinkles, and the distribution of height profile, symmetry and mouth thickness are uniform on the mouths of the drawn cups. The wrinkling becomes increasingly significant with a further increase of annealing temperature from 950 to 1100 ℃. The optimal annealing temperatures obtained in this study are 900 and 950 ℃ for reducing the generation of wrinkling, and therefore improving the quality of drawn cups. With non-optimized microstructure, the distribution of the compressive stress in the circumferential direction of the drawn foils becomes inhomogeneous, which is thought to be the cause of the occurrence of localized deformation till wrinkling during MDD. [ABSTRACT FROM AUTHOR]

Deep drawing is one of the important sheet-forming processes. Several process parameters are responsible for producing defect-free products of deep drawing. Out of those parameters, blank holder force is one of the widely studied process parameters, which significantly influence deep drawing. In the present study, the effect of velocity of the punch on deformation behavior of aluminum alloys is investigated. FEM simulation is conducted using commercially available software ABAQUS. It is found through FEM simulation that effective stress increases by nearly 56% with an increment in punch velocity from 150 to 350 mm/s. Besides this, equivalent plastic strain increases by five times on increment in punch velocity from 150 to 350 mm/s. von Mises stress and equivalent plastic strain found to be maximum at flange radii region (die corner) at all velocities of punch. Wrinkling is found to be absent during deformation (loading step) at all punch velocities. Wrinkling is obtained in deep drawn cups after unloading of the punch at all punch velocities except the lowest velocity of 150 mm/s. The phenomenon of wrinkling was found to be pronounced with increment in velocity of the punch after unloading of the punch. For prevention of wrinkling tendency during deep drawing, the velocity of punch should be less than 200 mm/s. Besides this, punch force, effective stress, and equivalent plastic strain found to increase nonlinearly due to increment in punch stroke. It is gathered through FEM simulation that wrinkling phenomenon increases with increment in punch velocity due to unloading of the punch during deep drawing. [ABSTRACT FROM AUTHOR]

In this study, element-free Galerkin method (EFGM), a meshless method, is proposed for wrinkling analysis of pre-stressed rectangular membranes. The mathematical model for studying wrinkling of pre-stressed membranes is derived by considering the bending stiffness, though it is negligible. Moving least-square approximation for deflection is constructed by considering three degrees of freedom per node. Essential boundary conditions are imposed using scaled transformation matrix method. Initially, compression-induced buckling of a homogeneous thin plate without pre-stress is solved to validate the method and then a pre-stressed homogeneous membrane is analyzed for both compression-induced and shear-induced wrinkling. Capabilities of the proposed method for membrane analysis are compared with that of the finite element method (FEM). Comparative study on wrinkling analysis using EFGM and different FEM element types in a commercial FEM package shows that in lower modes both methods show satisfying consistency in eigenvalues with respect to the total of number of nodes, while at higher modes EFGM shows better consistency than FEM. Further, the study is extended to wrinkling of nonhomogeneous membranes subjected to linearly-varying in-plane load. The results obtained from EFGM analysis is compared and found to be matching well with those available in the literature. [ABSTRACT FROM AUTHOR]

The variable curvature local-induction-heating bending forming (VC-LIHBF) technology is commonly used in the automotive industry to lighten and stiffen the structure parts. This technology allows the simultaneously bending forming and hardening of ultrahigh strength steel tubes, and allows the tensile strength of B1500HS steel to reach 1500 MPa. By adjusting the bending curvature parameters of the experimental facility through the numerical control system, the production of the tubes with variable curvatures could be achieved without changing dies. However, the more major problem of wrinkling occurs when a small-radius bending is applied on thin-walled rectangular tubes. This paper focuses on improving of forming quality and forming limit of the thin-walled rectangular steel tube (TWRST) with mandrel supported. In this study, an analytical model of the mandrel is established and some reference formulas for the selection of the mandrel parameters are deduced. Based on the above analysis, a three-dimensional elastic-plastic finite element method (FEM) model of the LIHBF process of the B1500HS TWRST is developed using the dynamic explicit FEM code ABAQUS/Temp-disp/Explicit, and key technological problems are solved. Experiments are carried out to verify the accuracy of the analytical model and confirm the reliability of the FEM model. The influences of the mandrel on stress distribution during the local-induction-heating bending process are also investigated. The influence mechanism of the mandrel parameters on the minimum radius of the LIHBF without wrinkling and the forming quality is revealed. The appropriate process parameters for the B1500HS TWRST are obtained through experiments and simulations. [ABSTRACT FROM AUTHOR]

Deep-draw experiments are performed on both glass and carbon fabric both to better understand the origin of wrinkle growth during complex forming experiments and to assess the accuracy, robustness and computational cost of forming simulations of engineering fabrics. During experiments, the shape of the deformed blanks is digitised using two different non-contact measurement techniques (Structured Light Scanning and Photogrammetry). The resulting digital point clouds permit detailed analysis of wrinkle growth and shows that the simulations perform well in predicting the very different forming behaviours of the two fabrics. The sensitivity of wrinkle formation to initial conditions is demonstrated in both experiments and simulations and the direction of wrinkle growth during the forming process is shown to influence wrinkle draw-in; an observation that could potentially be used to mitigate wrinkle draw-in during actual forming processes. [ABSTRACT FROM AUTHOR]

Different approaches for the simulation of wrinkling during forming of textile reinforcements are presented. It is shown that 3D finite element modeling requires the consideration of an additional bending stiffness of the fibers. In shell-type modeling, the bending stiffness is important because it conditions the size of the wrinkles. Different methods to take into account the bending stiffness independently of the tensile stiffness are presented. The onset and development of wrinkles during forming is a global problem that concerns all deformation modes. It is shown in examples that the shear locking angle is not sufficient to conclude about the development of wrinkles. This article highlights the two points common to the different cases of wrinkling of continuous fiber textile reinforcements: the quasi-inextensibility of the fibers and the possible slippage between the fibers. It presents and compares different approaches to consider these two aspects. The simulation of the simultaneous forming of multilayered textile reinforcements makes it possible to see the influence of the orientation of different plies which is an important factor with regard to wrinkling. [ABSTRACT FROM AUTHOR]

Neural regeneration after lesions is still limited by several factors and new technologies are developed to address this issue. Here, we present and test in animal models a new regenerative nerve cuff electrode (RnCE). It is based on a novel low-cost fabrication strategy, called “Print and Shrink”, which combines the inkjet printing of a conducting polymer with a heat-shrinkable polymer substrate for the development of a bioelectronic interface. This method allows to produce miniaturized regenerative cuff electrodes without the use of cleanroom facilities and vacuum based deposition methods, thus highly reducing the production costs. To fully proof the electrodes performance in vivo we assessed functional recovery and adequacy to support axonal regeneration after section of rat sciatic nerves and repair with RnCE. We investigated the possibility to stimulate the nerve to activate different muscles, both in acute and chronic scenarios. Three months after implantation, RnCEs were able to stimulate regenerated motor axons and induce a muscular response. The capability to produce fully-transparent nerve interfaces provided with polymeric microelectrodes through a cost-effective manufacturing process is an unexplored approach in neuroprosthesis field. Our findings pave the way to the development of new and more usable technologies for nerve regeneration and neuromodulation.

Curved aluminium extrusions are applied in a wide range of industrial applications. Because extrusions are initially straight, an additional process is required to curve the product. Undesired wrinkling of the plate part at the inner radius is frequently observed during the curving process. Wrinkling has already been extensively studied for the rotary-draw bending process. This paper aims at predicting the conditions for which wrinkling of a hollow section can occur during the three-point-roll bending process. It is shown that the most important condition for wrinkling is that buckling of the compressed plate part at the inner radius occurs. An analytical prediction model for buckling is presented, which predicts the critical bending radius as a function of the plate slenderness. The analytical model is validated with a finite element model, which in turn is validated with an experiment. Both the finite element model and the experiment confirm that wrinkling does not occur if the applied radius exceeds the model predicted critical radius. [ABSTRACT FROM AUTHOR]

Drop morphology can be manipulated by designing localized solid/liquid interactions to create a favorable interfacial energy equilibrium. A topographical surface with hierarchical roughness can be harnessed to generate complex drop morphologies, enhance uniaxial and anisotropic spreading, in a designable fashion. Here, using an active surface is proposed with a responsive roughness (wrinkle patterns) under uniaxial compression/stretching, to morph droplet shape biaxially in a continuous and reversible manner. The keys to achieve biaxial drop shaping are the in‐plane confinement from lattice hole patterns and the programmable formation of roughness, to pin and guide contact line movement in both in plane directions. The complex interplay between wetting and the patterns is elucidated by both experiments and numerical analysis. The results enrich the current understanding of shaping droplets by managing the contact line pinning/movement on an engineered elastic substrate, and providing insights for emerging applications in the areas such as droplet emicrofluidics, liquid robotics, ink‐jet printing, 3D printing and healthcare. [ABSTRACT FROM AUTHOR]

The surface wrinkling of metal films on spherical polydimethylsiloxane (PDMS) substrates was induced by thermal contraction. A novel scaling law among wrinkle wavelength, film thickness, and substrate radius was discovered experimentally and rationalized theoretically, with scaling exponent being constant and coefficient being material-dependent. The theoretical predictions agree well with the existing experimental data of other spherical shell/core systems, with shell/core modulus ratio spanning several orders of magnitude, indicative of the universality and robustness. The results shed new light on the wrinkling phenomenon of spherical surfaces and demonstrate a predictive pathway to create wrinkling patterns on demand. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The good formability of textile composite materials over complex mold geometries is one of the reasons to make their use expanding in various modern industries. However, different defects in these reinforcements could have occurred during the forming step in the manufacturing process. The defects are arising for many reasons; some are related to the fabric itself and others related to the draping parameters. Understanding the textile structure mechanics and draping behavior is essential to choose the proper reinforcement as well as to attain better simulation. Fabric wrinkles and local out-of-plane bucking of yarns were the fundamental defects in focus. The main objective of this part of the project was to experimentally investigate and compare the draping behavior of six commercially available glass fabrics from the same category of warp-knitted non-crimp fabrics (WKNCFs). The tested fabrics included two stitching patterns: tricot and chain. Also, they were relatively heavy with approximate mass per square meter. A double-dome punching test was performed to implement draping for each fabric; then, the defects were detected and characterized. Punching load-displacement curves were also recorded. In addition, a defect code was designated for the main defects to characterize forming defects at the meso-macroscopic scale. The structure and the number of fabric axes, stacking sequence, and stitching pattern all contribute to defect formation during draping. The studied configurations in this paper can help in studying the simulation of deformed technical fabric and provide a method to minimize and even eliminate the draping defects. [ABSTRACT FROM AUTHOR]

A variety of structures encountered in nature only arise in materials under highly nonequilibrium conditions, suggesting to us that the scope for creating new functional block copolymer (BCP) structures might be significantly enlarged by embracing complex processing histories that allow for the fabrication of structures quite unlike those created under "near-equilibrium" conditions. The present work examines the creation of polymer film structures in which highly nonequilibrium processing conditions allow for the creation of entirely new types of transient BCP morphologies achieved by transitioning between different ordered states. Most previous studies of BCP materials have emphasized ordering them from their disordered state obtained from a solution film casting process, followed by a slow thermal annealing (TA) process at elevated temperatures normally well above room temperature. We have previously shown that achieving the equilibrium TA state can be accelerated by a direct solvent immersion annealing (DIA) preordering step that creates nascent ordered microstructures, followed by TA. In the present work, we examine the reverse nonequilibrium sequential processing in which we first thermally anneal the BCP film to different levels of partial (lamellar) order and then subject it to DIA to swell the lamellae. This sequential processing rapidly leads to a swelling-induced wrinkle pattern that initially grows with immersion time and can be quenched by solvent evaporation into its corresponding glassy state morphology. The article demonstrates the formation of wrinkling "defect" patterns in entangled BCP films by this sequential annealing that does not form under ordinary TA conditions. At long DIA times, these highly "defective" film structures evolve in favor of the equilibrium morphology of parallel lamellae observed with DIA alone. In conjunction with our previous study of sequential DIA + TA, the present TA + DIA study demonstrates that switching the order of these processing methods for block copolymer films gives the same final state morphology in the limit of long time as any one method alone, but with drastically different intermediate transient state morphologies. These transient morphologies could have many applications.

Stretchable electronic devices must conform to curved surfaces and display highly reproducible and predictable performance over a range of mechanical deformations. Mechanical resilience in stretchable devices arises from the inherent robustness and stretchability of each component, as well as from good adhesive contact between functional and structural components. In this work, we combine bench-top thin film structuring with solvent assisted lift-off transfer to produce flexible and stretchable multi-material thin film devices. Patterned wrinkled thin films made of gold (Au), silicon dioxide (SiO2), or indium tin oxide (ITO) were produced through thermal shrinking of pre-stressed polystyrene (PS) substrates. The wrinkled films were then transferred from the PS to poly(dimethylsiloxane) (PDMS) substrates through covalent bonding and solvent-assisted dissolution of the PS. Using this approach, different materials and hybrid structures could be lifted off simultaneously from the PS, simplifying the fabrication of multi-material stretchable thin film devices. As proof-of-concept, we used this structuring and transfer method to fabricate flexible and stretchable thin film heaters. Their characterization at a variety of applied voltages and under cyclic tensile strain showed highly reproducible heating performance. We anticipate this fabrication method can aid in the development of flexible and stretchable electronic devices.

Kagome graphene is a carbon allotrope similar to graphene, with a single-atom thickness and a co-planar atomic structure. Despite interesting electronic properties, its mechanical behavior is still elusive. We have investigated the tensile properties of Kagome graphene under various strain rates and finite temperatures using molecular dynamics simulations. The Young’s modulus, ultimate tensile strength, fracture strain, and fracture toughness of the unsupported bulk material were measured as 96 GPa, 43 GPa, 0.05, and 1.9 J m−3, respectively, at room temperature and a strain rate of 109 s−1. Two deformation-stages were observed under tensile loading: normal and wrinkled. Initially, the Kagome graphene system stays in a co-planar structure without wrinkling until the tensile strain reaches 0.04, where it starts to wrinkle, unlike graphene. The wrinkle wavelength and magnitude suggest a very low bending rigidity, and wrinkle formation does not follow a rate predicted by continuum mechanics. Furthermore, the fracture mechanism of wrinkled Kagome graphene is briefly discussed.

A simple equation modelling an inextensible elastic lining of an inner-lined tube subject to an imposed pressure difference is derived from a consideration of the idealised elastic properties of the lining and the pressure and soft-substrate forces. Two cases are considered in detail, one with prominent wrinkling and a second one in which wrinkling is absent and only buckling remains. Bifurcation diagrams are computed via numerical continuation for both cases. Wrinkling, buckling, folding, and mixed-mode solutions are found and organised according to system-response measures including tension, in-plane compression, maximum curvature and energy. Approximate wrinkle solutions are constructed using weakly nonlinear theory, in excellent agreement with numerics. Our approach explains how the wavelength of the wrinkles is selected as a function of the parameters in compressed wrinkling systems and shows how localised folds and mixed-mode states form in secondary bifurcations from wrinkled states. Our model aims to capture the wrinkling response of arterial endothelium to blood pressure changes but applies much more broadly.

Flexible roll forming is an advanced sheet-metal-forming process that allows the production of parts with variable cross-sections. Double-layered blanks can have enhanced properties such as a high stiffness-to-weight ratio and high corrosion resistivity when combined with suitable materials. However, in the flexible-roll-forming process of double-layered blanks, additional shape defects and interface delamination can occur owing to their inhomogeneous mechanical properties compared with those of single-layered blanks. In this study, investigations on shape defects generated during the flexible-roll-forming process of steel/aluminum double-layered blanks were performed. Shape defects such as web-warping, wrinkling, and delamination at the interface were investigated on three different blanks with trapezoidal, convex, and concave shapes. The results show that the process-induced longitudinal strains significantly affect the occurrence of shape defects. Moreover, the longitudinal strains strongly depend on the blank shapes, their stacking order, and mechanical properties of the constituent layers. [ABSTRACT FROM AUTHOR]

Structural integrity, fuel efficiency, and reduced emission of greenhouse gases are the topmost priorities of the modern-day aerospace and automobile industries. Fiber-metal laminates (FMLs) can reduce most of these concerns by reducing a significant amount of weight without compromising structural reliability. Although this material was invented a few decades ago, large scale production, especially the forming process of small and complex-shaped products, has not matured yet. The necessity of manual processing makes it more cost-intensive and time-consuming. The forming difficulty comes with the limited elongation of the fiber layers compared to the metallic layers. As a result, the conventional approaches to form FMLs parts are not very suitable. In this paper, an investigation has been conducted to improve the drawability of FMLs sheets in deep drawing of cylindrical cups, made of semi-cured Glare material. A variable blank holder force (VBHF) method was proposed, which means the variation of the friction force of the holder and the laminate on contact surfaces, as a function of the position of outer flange edge. Fracture and wrinkling limits have been determined, and a BHF control trajectory has been presented to improve the forming quality, increase the drawing depth, and eliminate defects. Both experimental and numerical results from the VBHF were compared with constant BHF results. It is evident that the proposed VBHF scheme helps to improve the drawing quality compared to the conventional constant BHF. As a result, this method can enhance the production rate as well. [ABSTRACT FROM AUTHOR]

The significance of the stochastic nature of a multi-point dieless forming (MDF) process for final product accuracy has been extensively recognized. It is an under-research area in terms of effective techniques and computational tools for easy industrial applications and implementation. It is also recognized that wrinkling and dimpling are the most MDF product defects. Deterministic optimization using the finite element (FE) analysis have been used to improve the formability of the MDF process; however, not considering the noises of the system could affect the variation of the final product quality. In this study, the uncertainty parameters are considered to avoid the random variation of the final product accuracy while using the same control process parameters. In addition, to avoiding computationally expensive FE analysis, the approximation model has been used for both product defects. A six-sigma robust optimization is applied to obtain both reliable and robust MDF products. Finally a numerical simulation with one experimental result of a given target shape shows that the wrinkling and dimpling defect is controlled using the obtained optimal process parameter setup. [ABSTRACT FROM AUTHOR]

Finite element analysis method is used to study the wrinkling of the inner wall of the "日"-shaped profile during the rotary draw bending process in this paper. Based on Abaqus/explicit platform, we establish finite element analysis (FEA) models of rotary draw bending process with flexible core device and rigid core device and discuss the improvement methods of two types of core and the causes of defects in profile forming. Comparing with the forming effect of two kinds of core devices in the rotary draw bending process through FEA, it is proved that adopting a rigid core device can reduce wrinkling obviously. It provides a theoretical basis for the reasonable design of the mold for the "日"-shaped section and has certain reference value for the actual production. [ABSTRACT FROM AUTHOR]

In this communication, we report on an innovative solvent‐free method that allows for the design of nano‐/micropatterns with tuneable dimensions. Our approach is based on the spontaneous wrinkling phenomenon taking place in a bilayer system formed by a mechanically responsive bottom plasma polymer layer and a top aluminum thin film. The dimensions of the wrinkles can be adjusted in a wide range (i.e., from nanometer to micrometer range) by modulating the cross‐linking density as well as the thickness of the plasma polymer layer. Finally, it is demonstrated that these wrinkled surfaces could efficiently be used as flexible electrodes. The whole set of our data unambiguously reveals the attractiveness of our method for the fabrication of the micro‐/nanopattern with dimensions on demand. [ABSTRACT FROM AUTHOR]

Out-of-plane fiber waviness, also referred to as wrinkling, is considered one of the most significant effects that occur in composite materials. It significantly affects mechanical properties, such as stiffness, strength and fatigue and; therefore, dramatically reduces the load-carrying capacity of the material. Fiber waviness is inherent to various manufacturing processes of fiber-reinforced composite parts. They cannot be completely avoided and thus have to be tolerated and considered as an integral part of the structure. Because of this influenceable but in many cases unavoidable nature of fiber waviness, it might be more appropriate to consider fiber waviness as effects or features rather than defects. Hence, it is important to understand the impact of different process parameters on the formation of fiber waviness in order to reduce or, in the best case, completely avoid them as early as possible in the product and process development phases. Mostly depending on the chosen geometry of the part and the specific manufacturing process used, different types of fiber waviness result. In this study, various types of waviness are investigated and a classification scheme is developed for categorization purposes. Numerous mechanisms of wrinkling were analyzed, leading to several recommendations to prevent wrinkle formation, not only during composite processing, but also at an earlier design stage, where generally several influence factors are defined. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulations of the titanium-graphene nanocomposites (TiGNCs) under uniaxial tension are carried out to investigate the mechanical properties and reinforcement mechanism of graphene in composites. It is found that introduction of mechanically robust graphene limits the strain-induced dislocation and amorphization and thereby highly improves the mechanical properties of metallic titanium that are greatly affected by the crystal stacking orientation of graphene and titanium layers. The thickness of titanium layers, interface interaction and working temperature play an important role in the mechanical strength and elastic moduli of composites. The results show the mechanical properties of TiGNCs are monotonically enhanced with reduction of the titanium layer thickness and working temperature, and the Young's modulus obtained by MD simulation are higher than that predicted by the rule of mixture (ROM) due to consideration of interfacial interaction in computational calculation. In addition, once the critical thickness of titanium layer is reached, graphene wrinkles are induced in composites because of Poisson's effect induced large lateral compression stress in the interface region. This study provides helpful insights into fundamental understanding reinforcing mechanism of graphene and ultimately contribute to the optimal design and performance of mechanically robust graphene-based metallic composites. [ABSTRACT FROM AUTHOR]

When a polymer is extruded freely from a rectangular die of large cross‐sectional aspect ratio, wrinkles are observed. While not present in extruded Newtonian materials, such wrinkles develop in extruded viscoelastic sheets and are understood as an elastic stress‐driven instability. The present study is devoted in developing a transient finite element method, which combines the matrix‐logarithm‐based formulation of the conformation tensor and the single‐phase level set method, for simulating wrinkles that form during sheet extrusion of viscoelastic fluids. Numerical analyses of sheet extrusion were conducted over a wide range of flow rate and width‐to‐thickness ratio of the die exit cross section, χ, to determine critical conditions for the onset of wrinkling of extruded sheets. For large aspect ratios, that is, χ >> 1, wrinkles develop at moderate extrusion flow rate, corresponding to a Weissenberg number of about 29. Calculations based on Rayleigh's energy method show that the critical compressive stress, σc, for the onset of wrinkling of an elastic sheet scales like σc~1/χ2, with a significant drop for χ >> 1. As next to the die exit lip, compressive normal stresses are induced in the extruded sheet, wrinkling will take place for large χ (σc being small), in accordance with numerical predictions. [ABSTRACT FROM AUTHOR]

Although fiber metal laminates (FMLs) were invented a few decades ago, large-scale manufacturing, especially the forming process of small and complex-shaped products, has not been matured yet. The forming difficulty comes with the limited strain rate of the fiber layers compared to the metallic layers. As a result, the conventional approaches to form FML parts are not very suitable. Understanding the material behavior during the forming process is critical to find a new technique for relatively intricate and smaller FML parts. The blank holder gap (BHG) is one of the effective parameters to control the material flow in the deep drawing process, but in the case of the FMLs, the situation is completely different due to the laminate constituent. The compression of the fiberglass and thermo-plastique resin to the blank holder force (BHF) is not the same as the metals. The resin acts like a rubber, which decreases the thickness of the laminate and affects the friction between the laminate layers, especially when forming semi-cured laminates, which makes it hard to define an ideal BHF. This paper presents the results of numerical and experimental investigation of the effect of the BHG on the hydromechanical deep drawing (HDD) of a cylindrical cup made with 2/1 Glare sheets. It is found that the optimized BHF and cavity pressure (CP) with a BHG of 1.1 mm, smaller than the laminate's initial thickness of 1.2 mm, resulted in a good part with a significant depth improvement of 35 mm. Results also exhibited that FML parts manufactured by considering the BHG can enhance its applications and lead to a reduction of time and effort spent in mass production. [ABSTRACT FROM AUTHOR]

Buckling destabilization and wrinkling of an ethylene-propylene-diene monomer automobile weather-strip seal in assemblage and its structural optimization were studied in this paper. First, an innovative approach that traces buckling bifurcation paths was developed based on an arc-length method, and algorithmic parameters of the method were defined. A finite element analysis model of the automobile weather-strip seal in assemblage was then developed and analyzed using the arc-length method. The maximum buckling load, the deformation of the seal, and the thickness decrease of the lower tube wall in the critical region where it was prone to wrinkle were obtained by this finite element analysis method. Finally, an optimization seal structure was proposed and analyzed, and the deformations and the thickness decrease of the original and optimal structures in the critical regions were compared. The analysis conclusion implies that the optimal structure is more stable. The proposed analysis and optimization method can shorten the product design cycle, improve the structural stability, and decrease the design and trial-product cost considerably. [ABSTRACT FROM AUTHOR]

The paper deals with the numerical computation of difficult problems requiring many steps and many degrees of freedom such as the finite element analysis of wrinkling of film--substrate systems. The asymptotic numerical method (ANM) is well adapted to such computations but with a progressive loss of accuracy during step chaining. Thus, correction phases are necessary, which are rarely carried out within ANM. A convergence acceleration algorithm and a step-length adaptation have been included to limit the growth of computation time and to strengthen the reliability of the procedure. This modified version of the ANM is assessed by simulating the appearance and evolution of sinusoidal wrinkles under uniaxial compression. [ABSTRACT FROM AUTHOR]

The main defects due to flexible roll forming (FRF) processes include longitudinal bow and wrinkling. In this study, experimental and numerical analyses were performed using three different blank shapes to characterize the effects of the process parameters on defects in parts fabricated by FRF with and without leveling roll. Owing to the complexity of the FRF process, two algorithms were combined for its optimization. Artificial neural network-based Non-dominated Sorting Genetic Algorithm II (NSGA-II) was used to optimize the effective parameters of the FRF process, such as the sheet thickness, yield strength, and blank shape, with respect to the target bend angle to minimize the longitudinal bow and wrinkling of the product. The back-propagation neural network (BPNN) was used to identify two objective functions, while non-sorting multi-objective algorithm simulation was used to optimize the input parameters to minimize the objective functions. The results showed that the sheet thickness had the greatest effect on the minimization of the two objective functions, followed by the yield strength and blank shape, respectively. [ABSTRACT FROM AUTHOR]

This paper examines the behavior of wrinkling instability of a thermoelectric thin film bonded to substrate. The critical temperature differences for wrinkling occurrence and buckling initiation are obtained. Damage growth following wrinkling is also determined. These critical temperatures can provide guidelines for the design of thermoelectric thin film devices. Numerical results show that the stability of thermoelectric thin film is affected by the electric current. The critical temperature differences become smaller when the electric current density in thermoelectric thin film is higher. Effect of the wavelength of wrinkling on the critical temperature differences of wrinkling occurrence is also identified. [ABSTRACT FROM AUTHOR]

Tube hydroforming is an advanced metal forming processes, which is widely used to form various tubular parts. Axial feeding is usually used to avoid excessive thinning in hydroforming of a variable-diameter part. However, wrinkling defects are susceptible to occur easily under the axial loading if the wall thickness of the tube is small. A new method was proposed to enhance the expansion ratio and improve the thickness distribution for hydroforming of thin-walled spherical parts using overlapping tubular blanks. A special loading tool was created and AISI 304 stainless steel blanks were used for the experimental research. The effects of blank shapes and normal constraints were studied on wrinkling defects of the overlapping blanks. The results show that wrinkling defects at the inner layer of the overlap are prevented by using curved-edge blanks. Wrinkling defects at the outer layer of the overlap is eliminated by using normal constraints. Finally, a sound thin-walled spherical part was obtained by using an overlapping tubular blank. The maximum expansion ratio is 60% and increased by 30.2% compared with that of conventional hydroforming using a closed cross-sections tube. The maximum thinning ratio was 32.4%, which was decreased by 29.3%. In general, it is feasible to use an overlapping blank to form a variable diameter part. The maximum expansion enhances significantly and the thickness distribution improves apparently. [ABSTRACT FROM AUTHOR]

Pipe is the main transportation way for oil and natural gas. Fault movement mainly caused by earthquake, which will induce pipe bending, tension and compression. Then oil or gas leakage appear. Based on the moving mechanism of strike‐slip fault and reverse fault, a numerical simulation model was employed to study the buckling evolution mechanism of the buried steel pipe under fault movements. The evolution processes of buried pipe under the fault moving action were analyzed, and the effects of pipe internal pressure, fault displacement, and pipe diameter‐to‐thickness ratio on the pipe buckling were discussed. The results demonstrate that there are three mechanical evolution stages on the pipe in the process of fault movement. High stress appears on the bending regions of pipe wall, and axial strain always fluctuates along the axial length. When the fault displacement is large, pipe collapsing and wrinkling patterns occur, which can be reflected by a sharp fluctuation of axial strain. The high‐pressure pipe under the action of reverse fault is prone to failure than the low‐pressure pipe. The pipe with a large D/t in the hanging wall is easier to be buckled than that with a small D/t in the footwall. The results obtained can be used for the design and evaluation of buried oil and gas pipes. [ABSTRACT FROM AUTHOR]

The good formability of textile composite materials over complex mold geometries is one of the reasons to make their use expanding in various modern industries. However, different defects in these reinforcements could have occurred during the forming step in the manufacturing process. The defects are arising for many reasons; some are related to the fabric itself and others related to the draping parameters. Understanding the textile structure mechanics and draping behavior is essential to choose the proper reinforcement as well as to attain better simulation. Fabric wrinkles and local out-of-plane bucking of yarns were the fundamental defects in focus. The main objective of this part of the project was to experimentally investigate and compare the draping behavior of six commercially available glass fabrics from the same category of warp-knitted non-crimp fabrics (WKNCFs). The tested fabrics included two stitching patterns: tricot and chain. Also, they were relatively heavy with approximate mass per square meter. A double-dome punching test was performed to implement draping for each fabric; then, the defects were detected and characterized. Punching load-displacement curves were also recorded. In addition, a defect code was designated for the main defects to characterize forming defects at the meso-macroscopic scale. The structure and the number of fabric axes, stacking sequence, and stitching pattern all contribute to defect formation during draping. The studied configurations in this paper can help in studying the simulation of deformed technical fabric and provide a method to minimize and even eliminate the draping defects. [ABSTRACT FROM AUTHOR]

This paper newly proposes a strategy for constructing the multi-stage forming procedures to minimize the wrinkling formation with GPa-grade steels in the crash forming. Although the optimum design variables were adopted in the conventional crash forming, it is not possible to control the wrinkling formation in severe cases. In the multi-stage crash forming, the safe forming window against the wrinkling formation for the pre-stage forming has been constructed with numerical simulations representing the effects of design variables such as die radius, die angle, step height variations, and flange lengths. It does not only substantially reduce the area fraction of the wrinkling formation, but also recommend the other optimum design variables by interpreting the safe forming window. Experimental validations were carried out by applying the design variables suggested by the safe forming window for the multi-stage forming, which is compared with the conventional crash forming with TRIP1180 steel sheet. [ABSTRACT FROM AUTHOR]

• Distinct buckling modes of BLGSs on a foundation are studied. • New morphology evolutions of the post-buckling deformation are reported. • Novel finite element for analyzing atomic van der Waals interactions is developed. The buckling and the post-buckling of bilayer graphene sheets (BLGSs) on an elastic foundation subjected to an axial compressive load in plane-strain condition are studied. The solutions for the buckling strain and the characteristic wavelength at the onset of the buckling are obtained by using proposed continuum models describing deformations of BLGSs. The buckling analysis reveals that the buckling of BLGSs on an elastic foundation could be categorized into four. When the foundation is very soft, the Euler beam kinematics prevails in the entire BLGSs. On the stiffer foundation, the shear deformation on the cross-section of BLGSs occurs, and the characteristic wavelength changes sensitively as the stiffness of the foundation varies. With the foundation whose stiffness is almost the same as the shear modulus of the graphene interlayer, the buckling strain and the characteristic wavelength are nearly linear versus the elastic modulus of the foundation in logarithmic scale. When the foundation is much stiffer than the rigidities of the graphene interlayer, detachment between graphene layers could occur. Finally, we conduct the post-buckling analysis by using the proposed finite element model and obtain various morphology evolutions which contain detachment between graphene layers, period doubling, localization, period switching, and folding. Also, we present a diagram includes distinct deformation patterns according to the stiffness of the foundation and the magnitude of the applied compressive strain. [ABSTRACT FROM AUTHOR]

The use of laminates allows an optimization of the material properties, these hybrid materials can unravel some problems in the industrial sector, particularly in aerospace, and advanced automotive industry. Fiber Metal Laminates (FMLs) can be produced both by laminating and by forming. Flat laminates are easier to produce by the lay-up process, for small and relatively complex shapes with low thickness there are significant challenges in the forming process even for small drawing ratios, In this investigation, a cylindrical GLARE cup was chosen, this shape with sharp curves and vertical geometrical walls, still face numerous difficulties in the forming procedure. Numerical simulations have been used and compared with the experimental results in the stamp forming to achieve good forming quality with higher depth. An extensive investigation of the effect of process variables has been done such as blank-holding force, and blank holder gap and the curing condition. Also, their roles in wrinkles formation, tearing and thinning, as well as formability have been performed. The results show a good agreement between the experiments and the numerical simulation and show clearly that the application of blank holder force and blank holder gap within specified limits has a positive effect on the quality of the formed cup and leads to higher depths. Understanding these parameters and the GLARE forming conduct and having a decent choice of these parameters can give the advantage to achieve smaller and more complex shapes with higher depth, especially for large scale manufacturing. Finally, this investigation can expand the modern application regions of the FMLs and GLARE parts. [ABSTRACT FROM AUTHOR]

• Novel conic programming approach for modeling wrinkling in hyperelastic membranes. • Linearized volumetric potential incorporated for pneumatic membranes. • Primal-dual interior point method solves potential energy minimization problem. • Robust and stable method, even with highly wrinkled membranes. • Approach applicable to nonlinear problems beyond membrane mechanics. This paper presents a novel conic programming approach for modeling the wrinkling of isotropic hyperelastic membranes subject to nonlinear loading and finite strains, employing convex potentials derived from tension field theory. The incompressible neo-Hookean strain energy is recast as a minimization problem over a set of cones, with a semidefinite constraint on the deformed surface metric. To address pneumatic membranes, a linearized volumetric potential is introduced, reestablishing convexity for follower forces, and Boyle's law is expressed as a minimization problem over the exponential cone. The primal–dual interior point method solves the total potential energy minimization problem, with convergence verified using the analytical solution of a sheared planar membrane. The proposed model, applied to examples of increasing complexity, reveals intriguing membrane behaviors rarely discussed in existing literature. The method is shown to be robust, even when handling highly wrinkled membranes, which are challenging to address using direct nonlinear equilibrium analysis or nonlinear interior point methods. Implementation using high-level automatic code generation tools (FEniCS) results in concise, extensible code. The findings point to several avenues for future research, such as exploring complex material models, mesh refinement, dynamics, and parametric design. Additionally, the linearization process implies potential applicability of the methods to nonlinear problems beyond membrane mechanics. [ABSTRACT FROM AUTHOR]

• Cold-formed steel sandwich panel with a polyurethane foam core for structural floor. • Using digital image correlation technique to characterise construction materials. • Stiffness and peak load of sandwich panels subjected to edgewise compression. • Numerical modelling of cold-formed steel, polyurethane foam, composite structures. • Analytical prediction of wrinkling and global buckling loads of sandwich panels. With the objective of leading to innovative lightweight structural components in the construction sector using sandwich panels with polyurethane foam and steel face sheets, different studies were conducted on such structures, including experimental, numerical, and analytical campaigns. The experimental campaign includes the mechanical characterisation of the constituent materials and the edgewise compressive test of small-scale sandwich panels. The test protocols are detailed, and their influence on the results is assessed. Through the use of digital image correlation recordings, deformation patterns and failure mechanisms in the constituent materials and the composite structure are identified. Material constitutive models are calibrated based on the experimentally obtained results. The accuracy of such models is first evaluated against the stress–strain curves recorded during the experiments. In the second stage, the response of the sandwich panels subjected to edgewise compressive loading is simulated. A parametric analysis on the sensitivity of the numerical models to the initial geometrical imperfections is shown to be fundamental to capturing the behaviour of the sandwich panels. Furthermore, the experimental and numerical results are compared to analytical solutions of the problem of the edgewise compression of sandwich panels. Finally, a thorough discussion of the obtained experimental, numerical and analytical results is presented, along with comparisons to results available in the literature. [ABSTRACT FROM AUTHOR]

In this research, a new flexible multi-point incremental sheet forming process with multi-layer sheets (F-MPIF-MLS) was proposed. The novelty of this new forming process is demonstrated by two specific aspects of contribution. The first is the development of a new flexible multi-point die system to replace the conventional multi-point die, in which the multi-point pins are in contact with and provide support to the blank sheet from the start to the end of the forming operation. The second is the use of multi-layer sheets, which allows the target blank sheet to be deformed without clamping constraints. The new F-MPIF-MLS process clearly shows the benefits of overcoming the limitations of excessive sheet thinning and poor surface finish of the existing conventional multi-point incremental sheet forming (MPIF) process. A detailed comparative investigation into the new F-MPIF-MLS and the conventional MPIF processes is conducted through experimental testing and finite element (FE) simulation. The results show a noticeable reduction of wrinkling by using the new flexible multi-point die over the conventional multi-point die system with multi-layer sheets to form a dome shape. Moreover, in using the new F-MPIF-MLS process, the maximum thickness reduction is significantly reduced from 35% to 5% as compared to the conventional MPIF process to achieve a more uniform thickness distribution of the formed part. • A new flexible multi-point incremental sheet forming process with multi-layer sheets (F-MPIF-MLS). • Experimental validation and FE simulation of the new F-MPIF-MLS process as compared to conventional MPIF process. • Demonstration of significantly reduced sheet thinning and occurrence of wrinkling [ABSTRACT FROM AUTHOR]

Conventional roll forming is a recognised room-temperature forming process to produce long sheet metal profiles with a continuous cross-section for automotive and structural applications. More complex cross-sections can be achieved with Flexible Roll Forming (FRF). However, the level of the part shape complexity that can be FRF from Advanced High-Strength Steels (AHSS) is limited due to flange wrinkling. Incremental Shape Rolling (ISR) was recently introduced and applied to manufacture long U-channel components of variable height over the length. This has reduced wrinkling severity compared to FRF but still resulted in wrinkling issues when forming Ultra-High Strength Steels (UHSS). In this study, the ISR concept is expanded to form a variable depth component with a top hat channel profile to further reduce wrinkling severity in the flange. Experimental forming trials and numerical analysis suggest that the forming of the top hat flange leads to an increased and more uniform distribution of transverse tensile strain in the sidewall compared to the U-channel case. This eliminates wrinkling in the flange but results in an end flare shape defect that increases with material strength. The end flare is due to process-related residual stresses that develop when the top hat flange is formed. Overall, this study suggests that ISR represents a promising alternative to conventional forming methods for the production of complex automotive profiles from UHSS. [Display omitted] • Incremental Shape Rolling (ISR) enables the forming of automotive parts from UHSS. • ISR of hat-profiles creates a high tensile strain compared to that in the U-channels. • The high tensile strain that forms in the ISR hat-profiles eliminates wrinkling. • The shear stresses that develop when forming the hat-flange lead to end flare. [ABSTRACT FROM AUTHOR]

Flexible webs such as those used in roll-to-roll process machines are prone to various detrimental forms of elastic instabilities. This study revisits recent numerical work pertaining to the divergence buckling of non-uniformly tensioned axially moving thin webs. We show that the eigen-deformations experienced by these configurations are characterised by localised short-wavelength spatial patterns (typically referred to as 'partial wrinkling'). The broader asymptotic structure of the associated bifurcation problem is also highlighted, while our theoretical predictions are further substantiated by additional assessments through direct numerical simulations. • The divergence-buckling instabilities of an axially moving, non-uniformly tensioned web are cast as an eigenproblem. • The nature of the critical axial mode number is elucidated by using perturbation methods. • Both weak and moderate non-homogeneities of the external tension are considered. • Predictions for the critical eigenvalue are proposed in a number of distinct scenarios. [ABSTRACT FROM AUTHOR]

Hydroformed aluminum hollow components with various sections have been widely used in the field of automotive lightweight engineering. The main manufacturing process of the component is that the tube is pre-formed by CNC bending, and then hydro-formed into the desired shape. The springback, cross section distortion and wrinkling defects caused by bending will affect subsequent hydro-forming process. Theoretical model of tube plastic bending and model of material were established, and the theoretical springback value of any bending angle was obtained by the combination of equilibrium equation and total strain theory, the undercut defect was avoided effectively during hydroforming. The cross section un-roundness was decreased from 7.55% to 1.43% by the improvement of the core shaft, and the fracture of bend tube during hydroforming can be significantly avoided. The wrinkle defects formed in tube bending cannot be eliminated by hydroforming. The 6063 aluminum components with various sections were developed by the technology experiment, and the parts without any defects were obtained. The dimensional precision of 47 hydroformed components were measured, and the maximum dimensional deviation is 1.08mm (1.63%), and the dimensional precision meets the requirements.

Surface instability via wrinkle formation is a common feature in thin films attached to a compliant substrate. Wrinkled thin-film structures have been increasingly exploited to enhance device performance. In this study, a numerical technique utilizing embedded imperfections is employed for direct simulations of wrinkle formation, extending from a single-film structure to composite films involving two or more layers. The incorporation of material elements, bearing different elastic properties at the film-substrate interface, assists in triggering buckling instability when the compressive strain reaches a critical value. The wrinkle wavelength and amplitude obtained from the numerical modeling show excellent agreements with available theoretical solutions involving bi-layer composite films, over the entire span of volume ratios of the constituent layers. A valid range of imperfection distribution, resulting in uniform wrinkle formation, is identified. The current numerical approach is robust and easy to implement and yields great promises in generating reliable wrinkling patterns. It can be readily applied to cases where realistic features cannot be captured by theories, such as the generalized plane strain deformation, indirect compression, and multilayer composite films.