Adhesion–Delamination Mechanics of a Prestressed Circular Film Adhered onto a Rigid Substrate

A thin circular film clamped at the periphery is adhered to the planar surface of a rigid cylindrical punch. An external tensile load is applied to the punch, causing the film to delaminate from the substrate and the circular contact edge to contract. The film spontaneously separates from the punch, or pulls off, when the contact radius reduces to a range between 0.1758 and 0.3651 of the film radius, depending on the magnitude of the residual membrane stress. The mechanical delamination process is derived by a thermodynamic energy balance based on a coupled interfacial adhesion and residual membrane stress. The theoretical model has significant implications in nanoforce measurement, microelectromechanical systems (MEMS) comprising active moveable films, and biological cell adhesion.     

Adhesion-Delamination Mechanics of a Prestressed Circular Film Adhered onto a Rigid Substrate

A thin circular film clamped at the periphery is adhered to the planar surface of a rigid cylindrical punch. An external tensile load is applied to the punch, causing the film to delaminate from the substrate and the circular contact edge to contract. The film spontaneously separates from the punch, or pulls off, when the contact radius reduces to a range between 0.1758 and 0.3651 of the film radius, depending on the magnitude of the residual membrane stress. The mechanical delamination process is derived by a thermodynamic energy balance based on a coupled interfacial adhesion and residual membrane stress. The theoretical model has significant implications in nanoforce measurement, microelectromechanical systems (MEMS) comprising active moveable films, and biological cell adhesion.     

A clamped punch-loaded blister test for adhesion: a lumped parameter model and the bending-to-stretching behavior

This paper presents a clamped punch-loaded blister test and the corresponding lumped parameter model for the adhesion between thin film and rigid substrate. In the test, circular thin film is adhered to rigid substrate and an external load is applied with a clamped punch to cause the deflection of thin film. The relations among load, deflection, residual stress and strain energy release rate are thoroughly investigated. Analytical solutions in linear and nonlinear behaviors are obtained and the bending and stretching effects are associated through a lumped parameter model. The result of lumped-parameter model shows that the blister deflection is directly proportional to the applied load in the linear bending dominant region and the blister deflection is in direct proportion to the cubic root of the applied load in the nonlinear stretching dominant region. This has been proved by the result of nonlinear finite element analysis and the experimental result as well.     

A Self-Delamination Method of Measuring the Surface Energy of Adhesion of Coatings

A self-delamination technique is presented to assess the surface energy of adhesion of coatings. By measuring the debond area resulting from a circular hole cut in a film adhered onto a substrate, the surface energy may be determined if the tensile modulus, Poisson's ratio and residual stress level of the film are known. This method is useful for coating thicknesses greater than a critical thickness for self-peeling. Using this technique, the surface energy for a polyimide film (cured up to 220°C) on a glass plate was determined to 3.2 Nm^-1.     

Scanning Kelvin Probe Blister test measurements of adhesive delamination – Bridging the gap between experiment and theory

The delamination of an epoxy-adhesive film from a zinc coated steel substrate was studied by means of the electrochemical Height Regulated Scanning Kelvin Probe Blister Test (HR-SKP-BT¹) under controlled atmospheric conditions, applied pressure and interfacial electrode potential. The experimental studies focused on the analysis of the critical environmental water activity that leads to a corrosive delamination process under applied mechanical load and the analysis of the corrosion and delamination mechanisms at the front of delamination. The influence of applied pressure and relative humidity on the increase in the maximum blister height and the delamination rate was measured under constant polarization of the defect. 90° peel-tests were performed in order to correlate the water activity with the resulting peel force. The corrosion products that formed across the delamination front were analyzed by Raman microscopy. Through these HR-SKP-BT studies, a critical value was found for relative humidity for the delamination process. A transition zone was detected in which electrochemical degradation precedes mechanical delamination. In addition to the experimental studies, the critical energy release rates of the blister were calculated in finite element (FE²) simulations so as to enable a better understanding of the delamination of adhesives on metal surfaces. The combined experimental and theoretical studies show that the delamination process is controlled by the interfacial electrochemical reactions at the delamination front and that a transition area of few hundred micrometers exists in which the adhesion strength is lowered by the cathodic oxygen reduction process to a value which can be overcome by the mechanical stress in this area.     

Delamination Mechanics of a Clamped Rectangular Membrane in the Presence of Long-Range Intersurface Forces: Transition from JKR to DMT Limits

A 1-dimensional rectangular freestanding membrane clamped at opposite ends adheres to the planar surface of a rectangular punch. A tensile load applied to the punch causes the membrane to deform and gradually delaminate from the substrate. At equilibrium, the applied load is balanced by the disjoining pressure at the membrane-punch interface with range, y, and magnitude, p. Applying the Dugdale-Barenblatt-Maugis cohesive zone approximation, the disjoining pressure is taken to be uniform and confined to a finite cohesive length at the contact edge. For a fixed adhesion energy, γ = p y, we investigate the following: (i) the Derjaguin-Muller-Toporov (DMT) limit where y → ∞ and p → 0, (ii) the Johnson-Kendall-Roberts (JKR) limit where y → 0 and p → ∞, and (iii) the general case for intermediate but finite y and p. Delamination continues until the contact area shrinks to a line prior to “pinch-off”. The results are compared with the 2-dimensional axisymmetric membrane counterpart.     

Interfacial delamination of thin-film PTFE (polytetrafluoroethylene) coatings

Interfacial adhesion is a major concern with respect to successful performance of thin polymer films in developing new thin-film processes. Micro-indentation was used to induce interfacial delamination of polytetrafluoroethylene (PTFE) films deposited on glass substrates using hot filament chemical vapour deposition (HFCVD). Film thickness (1, 2, 3, 5, 10 µm) and indentation load (0.5, 0.75, 1, 2, 3 N) effects on the delamination diameter were investigated. A three-dimensional finite element model using shear material failure criterion and cohesive zone model (CZM) was developed to simulate the delamination. A normalized load–delamination radius relationship was obtained to evaluate the interfacial fracture toughness. The experimental observations showed that the delamination diameter depends on film thickness and indentation load. The numerical simulation indicates the delamination diameter depends on film thickness, material properties, and indentation force. The predictions of interfacial fracture toughness for 5- and 10-µm PTFE films are much smaller than those values using Rosenfeld et al.’s equation, which excludes the energy spent during the penetration.     

Fracture Mechanics of a Shaft-loaded Blister Test - Transition from a Bending Plate to a Stretching Membrane

A linear elastic solution is proposed for a circular disc in transition from a plate-like (pure bending) to a membrane-like behavior (pure stretching) under a central point load. The strain energy release rate for film delamination is found to be G = χ(Pw₀/πa²) with χ a numerical constant varying from 1/2 for a plate-like disc to 1/4 for a thin flexible membrane.     

Adhesion and delamination of tantalum and chromium films on glass

The adhesion and delamination behavior of sputtered Ta and Cr films on barium boroaluminosilicate glass were investigated. The locus of delamination was determined by XPS analysis of peeled film strips to be at the film-glass interface. We found a strong correlation between the intensity and thickness of the metal suboxide interphase layer formed and resistance to film delamination. For Ta, we found that the presence of a sufficiently thick TaO_x layer at the glass-film interface effectively prevented debonding of the film from the glass substrate. The role of oxygen in metal-glass bonding chemistry was explored by sputter deposition of metal films with in situ oxygen or air dosing. SIMS sputter-depth composition profiles confirmed the presence of a Ta-oxide phase (20-90 nm, depending on the dosing conditions) at the film-glass interface. We found that the delamination mechanism was prevented by in situ oxygen doping of the glass surface during film deposition. The cleanliness of the glass surface prior to sputtering, the ability to provide surface oxygen, and the film deposition conditions were found to be critical in avoiding delamination of the films from the glass. Although other factors may be involved in the mechanism of adhesion of Ta, the ability of the glass surface to provide oxygen for bonding is the overriding one, as shown in this work. Thus, oxide formation and chemical bonding at the metal-glass interface are more important than surface topography in the bonding of oxygen-active transition metals, such as Ta and Cr, to glass.     

Adhesion of a rigid punch to a confined elastic layer revisited

The adhesion of a punch to a linear elastic, confined layer is investigated. Numerical analysis is performed to determine the equivalent elastic modulus in terms of layer confinement. The size of the layer relative to the punch radius and its Poisson’s ratio are found to affect the layer stiffness. The results reveal that the equivalent modulus of a highly confined layer depends on its Poisson’s ratio, whereas, in contrast, an unconfined layer is only sensitive to the extent of the elastic film. The solutions of the equivalent modulus obtained from the simulations are fitted by an analytical function that, subsequently, is utilized to deduce the energy release rate for detachment of the punch via linear elastic fracture mechanics. The energy release rate strongly varies with layer confinement. Regimes for stable and unstable crack growth can be identified that, in turn, are correlated to interfacial stress distributions to distinguish between different detachment mechanisms.     

Electric-Field Induced Morphological Transitions in Elastic Contact Instability of Soft Solid Films

A soft elastic film, when placed in adhesive proximity with a contactor in a crack-like geometry, spontaneously undergoes a surface instability to form finger patterns with a characteristic wavelength of approximately 4h, where h is the film thickness. We study the morphological evolution and control of this elastic contact instability under the influence of an external electric field. The distinct electric field induced morphological changes, leading to the formation of two-dimensional hexagonally arranged pillars, large-amplitude fingers, and straightening of contact edge, which are studied comprehensively. The conditions for the evolution of morphologically distinct patterns are governed by the film parameters, such as its shear modulus and thickness. A theoretical model and its stability analysis provide an approximate estimate of the critical voltage required for the onset of changes and its scaling with the film parameters (thickness and shear modulus). Further, three-dimensional simulations based on energy minimization are presented to provide important clues regarding the physics of pattern evolution on soft elastic interfaces.     

Electric-Field Induced Morphological Transitions in Elastic Contact Instability of Soft Solid Films

A soft elastic film, when placed in adhesive proximity with a contactor in a crack-like geometry, spontaneously undergoes a surface instability to form finger patterns with a characteristic wavelength of approximately 4h, where h is the film thickness. We study the morphological evolution and control of this elastic contact instability under the influence of an external electric field. The distinct electric field induced morphological changes, leading to the formation of two-dimensional hexagonally arranged pillars, large-amplitude fingers, and straightening of contact edge, which are studied comprehensively. The conditions for the evolution of morphologically distinct patterns are governed by the film parameters, such as its shear modulus and thickness. A theoretical model and its stability analysis provide an approximate estimate of the critical voltage required for the onset of changes and its scaling with the film parameters (thickness and shear modulus). Further, three-dimensional simulations based on energy minimization are presented to provide important clues regarding the physics of pattern evolution on soft elastic interfaces.     

Effects of Crystalline Anisotropy and Indenter Size on Nanoindentation by Multiscale Simulation

Nanoindentation processes in single crystal Ag thin film under different crystallographic orientations and various indenter widths are simulated by the quasicontinuum method. The nanoindentation deformation processes under influences of crystalline anisotropy and indenter size are investigated about hardness, load distribution, critical load for first dislocation emission and strain energy under the indenter. The simulation results are compared with previous experimental results and Rice-Thomson (R-T) dislocation model solution. It is shown that entirely different dislocation activities are presented under the effect of crystalline anisotropy during nanoindentation. The sharp load drops in the load–displacement curves are caused by the different dislocation activities. Both crystalline anisotropy and indenter size are found to have distinct effect on hardness, contact stress distribution, critical load for first dislocation emission and strain energy under the indenter. The above quantities are decreased at the indenter into Ag thin film along the crystal orientation with more favorable slip directions that easy trigger slip systems; whereas those will increase at the indenter into Ag thin film along the crystal orientation with less or without favorable slip directions that hard trigger slip systems. The results are shown to be in good agreement with experimental results and R-T dislocation model solution.     

Delamination propagation analyses of spar wingskin joints made with curved laminated FRP composite panels

Initiation and propagation of inter-laminar delamination in adhesive bonded spar wingskin joint (SWJ) made with laminated fibre-reinforced plastic (FRP) composite curved panels have been studied employing three-dimensional finite element analyses. In-plane and out-of-plane normal and shear stress distributions are seen to be highly three-dimensional in nature. Tsai-Wu coupled stress failure criteria have been employed to identify critical locations of onset of delamination-induced damage. This occurs underneath the toe-end of the spar overlap and at the inter-laminar surface between the first and second plies of the curved FRP wingskin panel. Significant edge effects on the joint strength have been observed due to the curvature geometry of the composite wingskin panels. Non-linear finite element analyses have been carried out for study of delamination propagation using contact and multi point constraint (MPC) elements. The use of contact elements prevents inter-penetration of delaminated surfaces. Whereas, sequential release of MPC elements facilitates computation of opening, sliding and cross-sliding modes of delamination-induced strain energy release rates (SERR) by using virtual crack closure technique. Variation in delamination lengths significantly effects the variation of peel and inter-laminar shear stresses and different modes of SERRs. Variations on the two delamination fronts are seen to be quite different indicating dis-similar propagation rates. The Mode I SERR (G_I) predominantly governs the delamination propagation in the SWJ.     

Contact-Induced Failure of Prestressed Glass Plates

An indentation fracture technique was used to determine critical contact conditions under which prestressed brittle surfaces are subject to catastrophic failure. A theoretical model based on the growth of a well-developed, contact-induced half-penny crack leads to a simple inverse-cube power relation between indentation load and tensile prestress. The analysis is developed in terms of fracture parameters which are readily calibrated in routine indentation/strength tests. Experiments on glass disks loaded simultaneously in biaxial flexure and Vickers indentation confirm the essential failure predictions of the theory; toughness is the key material parameter controlling resistance to failure. The results emphasize the danger of spurious tensile stresses in ceramic systems exposed to severe contact events.     

The effect of glass-resin interface strength on the impact strength of fiber reinforced plastics

The effect of glass-resin interface strength on the impact energy of glass fabric (style 181) reinforced epoxy and polyester laminates has been determined. The interface strength was altered by surface treatment of the fabrics with silane coupling agents and with a silicone fluid mold release and the interlaminar shear strength was determined as a means to evaluate the interface strength. An instrumented Charpy impact test was used on unnotehed specimens and thus both initiation and propagation energies could be determined as well as dynamic strength. It was found that the initiation energy for both polyester and epoxy laminates increased with increasing interlaminar shear strength, The propagation energy and thus the total energy for polyester laminates displays a minimum at a critical value of interlaminar shear strength (ILSS). Below this critical value, the total impact energy increases with decreasing shear strength and the dominant energy absorption mode appears to be delamination. Above the critical value, the impact energy increases with increasing values of ILSS and the fracture mode is predominantly one of fiber failure. In all cases, even with mold release applied, the shear strength of epoxy laminates was above this critical value and-thus the total impact energy increases with Increasing values of ILSS. The maximum energy absorbed for the epoxy laminate and the polyester laminate is nearly identical. However, the maximum for the epoxy laminate occurs when the shear strength is maximized while for the polyester laminate the shear strength must be minimized. For the polyester laminate when delamination is predominant, it was found that the glass surface treatment affects the amount of delamination as opposed to the specific value of delamination fracture work.     

光固化3D打印离型膜在树脂作用下的变形分析

在光固化光敏树脂成型过程中,聚四氟乙烯离型膜会在树脂液位的波动影响下发生变形,进而影响光固化制件的精度,因此需要减小离型膜在制件过程的变形量,确保其相对稳定。首先分析树脂液位变化,通过液位变化量,等效出树脂液波动带给离型膜的载荷,建立离型膜受力模型。然后基于离型薄膜的挠度变形理论,应用Ansys软件进行模拟研究,对不施加预应力和施加不同预应力作用下的离型膜变形结果分析,结果表明施加预应力有利于提高离型膜稳定性,且施加预应力为2. 645M Pa时离型膜受到树脂波动影响最小,位置精度变化在0. 01 mm。     

Effect of the aspect ratio of the pre-existing rectangular adhesion failure on the structural integrity of the adhesively bonded single lap joint

Non-linear three dimensional (3-D) finite element analyses (FEA) of the single lap joints (SLJs) having pre-existing rectangular adhesion failure in the interface of the strap adherend and the adhesive have been carried out. The effect of the size, the shape and the aspect ratio of the pre-existing rectangular adhesion failure on (i) the strength, (ii) the interfacial stresses and (iii) the strain energy release rates (SERRs) in the vicinity of the adhesion failure front have been presented in this research work. The SLJ is subjected to uniformly applied tensile load. The adherends are made with very high strength steels and the adhesive is a commercially available AV119. The analyses of the adhesion failure propagation have been carried out by sequentially releasing the constraints of the nodes ahead of the pre-existing adhesion failure front in finite element model. The SERR values in the vicinity of the adhesion failure fronts are computed using the virtual crack closure technique (VCCT) for assessment of the structural integrity of the SLJ. The strength of the SLJ, the interfacial stresses, and the three modes of strain energy release rates (SERRs) have been found to be significantly affected by the shape and size of adhesion failures. The SERRs and interfacial stresses along the rectangular adhesion failure front are compared with the corresponding values around the circular adhesion failure front of same area, pre-existing in the SLJ. It is observed that the circular and rectangular adhesion failures of the same area will have dissimilar growth rate and the mode II is the dominant failure mode. The total strain energy release rate and the failure strength, computed from the 3-D FEA of the SLJ is in good agreement with the experimental fracture toughness of the AV119 adhesive and the experimentally obtained failure loads, respectively.     

A numerical study on warm deep drawing of polypropylene

Warm deep drawing of polypropylene, a semi-crystalline thermoplastic polymer, is studied using finite element analysis. In this process, a circular polypropylene blank is preheated to a temperature much below its melting temperature and deep drawn into the shape of a flat-bottom cylindrical cup using a punch-die combination, both initially at 25°C. The material model used for the analysis considers the effects of varying temperature and strain rate during the deep drawing process on the depth of draw. The effects of blank holder force, initial blank temperature, blank diameter, and die and punch corner radii on the depth of draw are determined. Thickness, temperature, and strain variations in the drawn cups, punch forces, and failure modes are also determined.     

Evaluation of sub-micrometer parylene C films as an insulation layer using electrochemical impedance spectroscopy

We investigated the insulation performance of sub-micrometer parylene C films over time using electrochemical impedance spectroscopy (EIS). For this, interdigitated electrodes were fabricated and completely encapsulated with parylene C in thicknesses of 50, 100, 200, and 500 nm. The EIS was measured in phosphate buffered saline (PBS) solution under an accelerated aging condition at 90 °C over 45 days. To analyze the EIS data, the equivalent circuit models of coating at different stages of coating degradation were used and the lumped circuit parameters of the best fitted equivalent circuit model were extracted by curve fitting. The analysis of impedance using the equivalent circuit model and the FTIR measurements suggest that sub-micrometer parylene C coatings exhibited delamination resulting from water diffusion from the top surface as soon as being immersed in PBS solution, although the degree of delamination varied depending on the film thickness. The penetration of water through sub-micrometers thick parylene C films can occur as quickly as the film is in contact with solution, unlike for thicker coatings in several micrometers where water diffusion would be saturated before water reaches the bottom surface of the coating.