Thin Adhesive Layer Research Articles

Numerical study of delamination process of the CFRP composite

A quasi-static approach for an interface damage model, incorporating Rayleigh damping for viscoelastic carbon fiber reinforced polymer composites, is introduced. The interface traction-relative displacement behavior is modeled using a thin adhesive layer, similar to cohesive zone models. The problem is solved numerically with a semi-implicit time-stepping method. The cohesive interface model, integrated into the Finite Element Method, was applied to determine the critical force away from the crack tip. Results showed that the numerical delamination under mode I conditions aligned well with experimental data for the cohesive zone formulations studied. The numerical example demonstrates the effectiveness of the proposed model.

TiO2/Se heterojunction photovoltaic device made from Se/Te/Se stacked precursor

This paper presents an innovative approach for forming crystallized selenium (c-Se) using a Se/Te/Se stacked precursor. The aim is to enhance the adhesion between the TiO2 and Se layers to improve the power conversion efficiency of TiO2/Se heterojunction photovoltaic devices. Our previous study revealed that the accumulation of Te near the TiO2/Se interface resulted in insufficient shunt resistance, leading to increased dark leakage current. This issue arises because Te possesses a high carrier density (∼1018 cm−3) and narrow bandgap energy (0.26 eV). However, a Te additive layer remains necessary to passivate the unbonded TiO2/Se interface and to maximize the adhesion between the absorber and the substrate. Previous research on Se-based photovoltaic devices demonstrated that an extremely thin (∼0.5 nm) and homogeneous Te adhesion layer could be formed beneath the Se photo-absorbing layer and subsequently crystallized through air-annealing post-deposition. However, achieving precise and homogeneous sub-nanometer Te films using a compact resistance heating evaporation system in a laboratory setting proved challenging so far. To address these limitations, we propose a novel technique for forming the Se absorber layer by post-annealing a stacked 1st-Se/Te/2nd-Se precursor on a TiO2 window layer. Through experimentation, we achieved the highest power conversion efficiency of 4.49 % without antireflective coating, accompanied by Voc 0.795V, JSC 11.13 mA/cm2, and FF 0.507 when the 1st-Se thickness was 5 nm. Furthermore, the impact of decreasing Te/(Te + Se) near the TiO2 window layer on photovoltaic performance was theoretically predicted using modelling by SCAPS-1D. Overall, this work introduces a promising method for enhancing the formation of c-Se, improving adhesion between TiO2 and Se layers, and enhancing the power conversion efficiency of TiO2/Se heterojunction photovoltaic devices.

Concentration-induced spontaneous polymerization of protic ionic liquids for efficient in situ adhesion

The advancement of contemporary adhesives is often limited by the balancing act between cohesion and interfacial adhesion strength. This study explores an approach to overcome this trade-off by utilizing the spontaneous polymerization of a protic ionic liquid-based monomer obtained through the neutralization of 2-acrylamide-2-methyl propane sulfonic acid and hydroxylamine. The initiator-free polymerization process is carried out through a gradual increase in monomer concentration in aqueous solutions caused by solvent evaporation upon heating, which results in the in-situ formation of a tough and thin adhesive layer with a highly entangled polymeric network and an intimate interface contact between the adhesive and substrate. The abundance of internal and external non-covalent interactions also contributes to both cohesion and interfacial adhesion. Consequently, the produced protic poly(ionic liquid)s exhibit considerable adhesion strength on a variety of substrates. This method also allows for the creation of advanced adhesive composites with electrical conductivity or visualized sensing functionality by incorporating commercially available fillers into the ionic liquid adhesive. This study provides a strategy for creating high-performance ionic liquid-based adhesives and highlights the importance of in-situ polymerization for constructing adhesive composites.

Influence of Varied Silane Commercial Brands and Adhesive Application on Bond Strength and Stability to Lithium Disilicate Glass Ceramic.

This study aimed to evaluate the impact of various commercial silane brands with varied chemical compositions with or without the application of an adhesive layer on the microshear bond strength and durability of a resin luting agent to lithium disilicate glass ceramic. Lithium disilicate glass ceramic discs (EMX, IPS e.max Press, Ivoclar Vivadent) measuring 10 mm in diameter and 3 mm in thickness were fabricated (n=240). Surfaces were etched using 5% hydrofluoric acid and randomly assigned to 10 groups based on the commercial brand of silane used (n=24): [RP] RelyX Ceramic Primer (3M ESPE); [PS] Prosil (FGM); [SA] Silano (Angelus); [SM] Silano (Maquira); [SU] Silane (Ultradent); [GL] GLUMA Ceramic Primer (Kulzer); [CB] Ceramic Bond (VOCO); [MB] Monobond N (Ivoclar Vivadent); [CP] Clearfil Ceramic Primer (Kuraray); and [DE] 2-step silane (Dentsply Sirona). Half of the EMXs (n=12) received a thin adhesive layer (+) after the silane and prior to resin luting agent, while the other half (n=12) did not receive an adhesive layer (-). For the microshear bond strength test (μSBS), four light-cured resin luting agent cylinders (1 mm in diameter) were created on each EMX surface. Half of these specimens were tested after 24 hours, while the other half were stored in deionized water for 6 months. The μSBS test was conducted using a universal testing machine (DL 500, EMIC) at a crosshead speed of 1 mm/min until failure. The obtained data underwent statistical analysis using analysis of variance (ANOVA) and the Tukey test (α=0.05). There was significant influence of the silane commercial brand on bond strength. Notably, "universal primers" yielded lower bond strength results compared to "pure" silane solutions. Water storage had a detrimental effect on microshear bond strength for certain silane commercial brands. Additionally, the application of an adhesive layer negatively impacted bond strength results for all silanes. This study confirms the importance of both silane commercial brand and chemical composition in relation to bond strength of resin luting agents to lithium disilicate glass ceramic. Furthermore, the application of an adhesive layer may have an adverse effect on bond stability over time.

A new crack-tip element for the logarithmic stress-singularity of Mode-III cracks in spring interfaces

A new crack-tip finite element able to improve the accuracy of Finite Element Method (FEM) solutions for cracks growing along the Winkler-type spring interfaces between linear elastic adherents is proposed. The spring model for interface fracture, sometimes called Linear-Elastic (perfectly) Brittle Interface Model (LEBIM), can be used, e.g., to analyse fracture of adhesive joints with a thin adhesive layer. Recently an analytical expression for the asymptotic elastic solution with logarithmic stress-singularity at the interface crack tip considering spring-like interface behaviour under fracture Mode III was deduced by some of the authors. Based on this asymptotic solution, a special 5-node triangular crack-tip finite element is developed. The generated special singular shape functions reproduce the radial behaviour of the first main term and shadow terms of the asymptotic solution. This special element implemented in a FEM code written in Matlab has successfully passed various patch tests with spring boundary conditions. The new element allows to model cracks in spring interfaces without the need of using excessively refined FEM meshes, which is one of the current disadvantages in the use of LEBIM when stiff spring interfaces are considered. Numerical tests carried out by h-refinement of uniform meshes show that the new singular element consistently provides significantly more accurate results than the standard finite elements, especially for stiff interfaces, which could be relevant for practical applications minimizing computational costs. The new element can also be used to solve other problems with logarithmic stress-singularities.

Efficiency assessment of vortex tubular enameled coalescing devices for oil emulsion destabilization

The greatest intensity of the process of dispersion and subsequent coalescence of oil droplets in emulsified wastewater of oilfields takes place in the near-wall areas of tubular coalescing devices. Strengthening of the wall effect is possible by covering the surface of the flowing part of the coalescing device with silicate enamel, which provides the effect of sliding of the turbulent flow of waste water (emulsion) on a thin adhesive layer that appears directly on the surface of the enamel. Experimental studies to determine the effectiveness of treatment of oily wastewater in tubular vortex-type coalescing devices were carried out on an experimental unit consisting of three coalescing devices connected in series. Each coalescing device was a tubular shaft coaxially connected to the inlet and outlet chambers. Vortex tubular coalescing devices made of stainless steel (not coated with enamel) and vortex coalescing devices of similar design coated with silicate enamel were used in the experiments. The wastewater treated in the unit was sedimented, and the concentrations of oil and suspended particles were determined in the clarified water. Analysis of the data obtained showed that at the time of wastewater treatment from 1.1 to 6.3 s sedimentation effect on oil particles after coalescing devices coated with enamel on average by 1.5 to 1.8% was higher than the sedimentation effect obtained after the treatment of oil emulsion in coalescing devices not coated with enamel. The sedimentation effect on suspended solids in the second series of experiments was greater than the sedimentation effect observed in the first series by an average of 1.1-1.4 %.

Asymmetric toughening in the lap shear of metamaterial structural adhesives.

Metamaterial structural adhesives (MSAs), whose properties primarily rely on structural design, offer promising advantages over traditional adhesives, including asymmetric, switchable, and programmable adhesion. However, the effects of thick backing structures on the adhesion properties remain largely underexplored. Herein, we investigate a series of MSAs featuring a thin adhesive layer and an asymmetric thick beam structure terminated with a film. We conduct lap shear tests on the MSAs with varying terminated film thickness (t) and beam tilting angle (θ) while maintaining an identical adhesive layer. For MSAs with a thick terminated film (t = 2 mm), the effective adhesion energy is double that of solid samples without compromising shear strength, consistent with the theoretical predictions based on the crack trapping mechanism. Conversely, for MSAs with a thin terminated film (t = 0.5 mm), the maximum shear strength and effective adhesion energy are ∼2.8 times and ∼18.6 times those of solid samples, respectively, deviating significantly from the theoretical predictions due to new crack initiations. We further explore adhesion asymmetry by tuning the beam tilting angle (θ). For MSAs with highly tilted beams (θ = 70.3°), we achieve a maximum adhesion strength asymmetry factor of τ2/τ1 ∼ 2.2 for a thick terminated film (t = 2 mm), and a maximum adhesion energy asymmetry factor of Γ1/Γ2 ∼ 5.3 for a thin terminated film (t = 0.5 mm). Our work provides useful insights for designing metamaterial structural adhesives suitable for robotic grippers, wall-climbing robots, and wearable devices, particularly those requiring asymmetric, switchable, and stimuli-responsive adhesion, and adhesives on rough surfaces or in underwater conditions.

Healthcare Wearable Sensors Adhesion to Human Fingernails and Toenails.

A novel adhesion method of a sensor to a fingernail is described. Wearable sensors can provide health insights to humans for a wide variety of benefits, such as continuous wellness monitoring and disease monitoring throughout a patient's daily life. While there are many locations to place these wearable sensors on the body, we will focus on the fingertip, one significant way that people interact with the world. Like artificial fingernails used for aesthetics, wearable healthcare sensors can be attached to the fingernail for short or long time periods with minimal irritation and disruption to daily life. In this study the structure and methods of healthcare sensors' attachment and removal have been explored to support (1) the sensor functional requirements, (2) biological and environmentally compatible solutions and (3) ease of attachment and removal for short- and long-term user applications. Initial fingernail sensors were attached using a thin adhesive layer of commonly available cosmetic nail glue. While this approach allowed for easy application and strong adhesion to the nail, the removal could expose the fingernail and finger to a commercially available cosmetic nail removal (acetone-based chemical) for extended times measured in minutes. Therefore, a novel structure and method were developed for rapid healthcare sensor attachment and removal in seconds, which supported both the sensor functional objectives and the biologically and environmentally safe use objectives.

Исследование предела упругости тонкого адгезионного слоя композита при его нагружении нормальным отрывом

Исследуется деформирование под действием нормального отрыва композитной пластины, состоящей из двух консолей, связанных адгезионным слоем в состоянии плоской деформации. Из общей вариационной постановки с учетом теории Миндлина—Рейсснера получена постановка в дифференциальном виде. Исследуется аналитическое решение, полученное при комплексных корнях характеристического уравнения системы дифференциальных уравнений второго порядка. Используя критерий Треска—Сен-Венана, найдено значение внешней нагрузки, соответствующее пределу упругости и началу пластических деформаций в адгезионном слое. Deformation under the action of normal separation of a composite plate consisting of two consoles connected by an adhesive layer in a state of flat deformation is investigated. From the general variational formulation, taking into account the Mindlin—Reissner theory, a differential formulation is obtained. An analytical solution obtained with complex roots of the characteristic equation of a system of second-order differential equations is investigated. Using the Tresca—SaintVenant criterion, the value of the external load corresponding to the elastic limit and the onset of plastic deformations in the adhesive layer was found.

Failure analysis of crack-prone joints with Finite Fracture Mechanics using an advanced modeling approach for the adhesive layer

When adhesive joints are subjected to critical loads, the components can separate at the interface between the adhesive layer and one of the adherends. Due to the strongly differing stiffness properties of the adherends and the adhesive, which cause stress peaks to arise locally at the edge of the joint, a crack grows into the structure there once the critical load is reached. The concept of Finite Fracture Mechanics allows reliable failure assessment and determination of this critical load. For this purpose, a coupled criterion consisting of an energy and a strength criterion is evaluated, both of which must be satisfied simultaneously in case of failure. In this work, Finite Fracture Mechanics is evaluated analytically and using finite element data and compared and discussed with experimental results. In the analytical approach, the focus is on extended applicability to joints with both thin and thick adhesive layers, for which an advanced modeling approach of the adhesive layer is used.

TRANSFER OF LOADS FROM THREE HETEROGENEOUS ELASTIC STRINGERS TO AN INFINITE SHEET THROUGH ADHESIVE LAYERS

This paper considers the problem for an elastic infinite plate (sheet), which on parallel finite parts of its upper surface is strengthened by three finite stringers, two of which are located on the same line, having different elastic properties. The stringers are deformed under the action of horizontal forces. The interaction between infinite sheet and stringers takes place through thin elastic adhesive layers having other physical-mechanical properties and geometric configuration. The problem of determining unknown shear stresses acting between the infinite sheet and stringers is reduced to a system of Fredholm integral equations of second kind with respect to unknown functions, which are specified on three finite intervals. It is shown that in the certain domain of the change of the characteristic parameters of the problem this system of integral equations can be solved by the method of successive approximations. Particular cases are considered, the character and behaviour of unknown shear stresses are investigated. Further, for various values of changing characteristic parameters of the problem the multiple numerical results and its analysis are presented.

Limit States of Adhesive Layers under Combined Loading

The state of pre-fracture of a thin adhesive layer of finite thickness in the vicinity of a crack-like defect is considered. It is proposed to take into account the hydrostatic pressure, which forms the volume deformation energy, in order to find the critical state. The critical value of the J-integral for the I+II loading mode is assumed to be dependent on the product of the volume strain energy and the layer thickness at the end face of the adhesive. The limiting value of the product of the volume energy and the layer thickness under loading in mode I, as well as the critical values of the J-integral for loading modes I and II, determine the value of the loosening parameter of a particular adhesive in the proposed failure criterion.

Fatigue behaviour of hybrid grouted joints under axial loading

AbstractSteel constructions made of circular hollow sections are commonly joined by welding or ring‐flange bolting. An additional, novel joining method is the so‐called hybrid grouted joint. The hybrid grouted joint is characterised by its multi‐layered composition. In contrast to state‐of‐the‐art grouted joints, hybrid grouted joints include thin adhesive layers, which are applied on the steel surfaces prior to grouting. Sand grains are embedded in the yet uncured adhesive. After the curing of the adhesive, the surface has a high degree of roughness. This allows a high‐performance interlock between fine grain grout and the adhesively bonded sand granules across the contact surfaces in the overlap area. The ability to transfer loads across a wide area with low stress concentrations makes the hybrid grouted joint a promising joint alternative especially regarding the fatigue performance. This paper summarises the main findings concerning fatigue behaviour of the hybrid grouted joint under axial loading. Wöhler diagrams were determined in numerous fatigue tests for two combinations of adhesive and grout.

Numerical Investigation on High-Performance Cu-Based Surface Plasmon Resonance Sensor for Biosensing Application.

This numerical research presents a simple hybrid structure comprised of TiO2-Cu-BaTiO3 for a modified Kretschmann configuration that exhibits high sensitivity and high resolution for biosensing applications through an angular interrogation method. Recently, copper (Cu) emerged as an exceptional choice as a plasmonic metal for developing surface plasmon sensors (SPR) with high resolution as it yields finer, thinner SPR curves than Ag and Au. As copper is prone to oxidation, especially in ambient conditions, the proposed structure involves the utilization of barium titanate (BaTiO3) film as a protection layer that not only preserves Cu film from oxidizing but enhances the performance of the sensor to a great extent. Numerical results also show that the utilization of a thin adhesive layer of titanium dioxide (TiO2) between the prism base and Cu film not only induces strong interaction between them but also enhances the performance of the sensor. Such a configuration, upon suitable optimization of the thickness of each layer, is found to enhance sensitivity as high as 552°/RIU with a figure of merit (FOM) of 136.97 RIU-1. This suggested biosensor design with enhanced sensitivity is expected to enable long-term detection with greater accuracy and sensitivity even when using Cu as a plasmonic metal.

Investigation of the suitability of adhesive tapes for sealing building structures of various surfaces

Adhesive sealing tapes play an important role in ensuring the air tightness of the building, affecting the indoor microclimate and efficient energy use. However, there is still a lack of objective indicators that would demonstrate the ability of the sealing tape to ensure the long-term air tightness of the sealing joints of various building structures. Therefore, the purpose of this work is to investigate the indicators demonstrating the suitability of adhesive sealing tapes for sealing building structures of different surfaces and deformation, to provide detailed analyses of the results, and to propose practical recommendations. The investigated self-adhesive tapes differed in backing (nonwoven material, plastic reinforced film, paper, metal foil) and adhesives (acrylic and butyl). The materials of the sealed structural joints were plywood, OSB, plasterboard, cement sawdust board, plastered board, and plastic. The thickness, deformability, peel and shear adhesion of the self-adhesive tapes were used to determine its suitability for sealing of different structural joints. It was determined that tapes with higher deformability are more versatile, as they are suitable for sealing joints subjected to small and large deformations. The higher peel adhesion is typical for tapes of an air-permeable non-woven material backing and a thin adhesive layer. Tapes with a thicker adhesive layer are most suitable for sealing complex-shaped joints. Tapes with acrylic adhesive have a higher peeling force from smooth and moderately smooth surfaces, whereas butyl adhesive creates better contact with uneven surfaces in highly deformable joints.

A mixed-mode dependent interface and phase-field damage model for solids with inhomogeneities

The developed computational approach is capable of initiating and propagating cracks inside materials and along material interfaces of general multi-domain structures under quasi-static conditions. Special attention is paid to particular situation of a solid with inhomogeneities. Description of the fracture processes are based on the theory of material damage. It introduces two independent damage parameters to distinguish between interface and internal cracks. The parameter responsible for interface cracks is defined in a thin adhesive layer of the interface and renders relation between stress and strain quantities in fashion of cohesive zone models. The second parameter is defined inside material domains and it is founded on the theory of phase-field fracture guaranteeing the material damage to occur in a thin material strip introducing a regularised model of internal cracks. Additional property of both interface and phase-field damage is their capability to distinguish between fracture modes which is useful if the structures is subjected to combined loading. The solution methodology is based on a variational approach which allows implementation of non-linear programming optimisation into standard methods of finite-element discretisation and time stepping method. Computational implementation is prepared in MATLAB whose numerical data validate developed formulation for analysis of problems of fracture in multi-domain elements of structures.

Micro-Transfer Printing for Heterogeneous Integration of GaN and GaAs HEMTs

Here, we use the micro-transfer printing technique to demonstrate the device-level heterogeneous integration of two solid-state RF device technologies on the same interposer: GaN and GaAs high-electron-mobility transistors. The devices are released from their growth substrate using an epitaxial sacrificial layer while a thin polymer adhesion layer facilitates a strong bond between the target substrate and the compound semiconductor devices, allowing for post-transfer microfabrication processing. Transmission electron microscopy reveals no voids at the device/interposer interface and a polymer adhesion layer thickness of 5 ± 2 nm. No significant degradation in dc electrical characteristics is observed after device transfer for either device technology. Improvement in thermal performance of GaN devices was demonstrated when transferred to a diamond substrate, even with the thin polymer adhesion layer at the device/interposer interface, illustrating a pathway for enhanced thermal management for GaN and other high-output-power density semiconductor technologies. The ability to combine various solid-state technologies at the device level with high density provides an approach to meet next-generation demands for RF and mixed-signal circuits.

Crack tip solution for Mode III cracks in spring interfaces

Considering an infinite linear elastic isotropic solid in antiplane shear, the Mode III crack-tip solution for a semi-infinite crack located in a straight spring interface is systematically studied for the first time. A new analytic expression for this crack-tip solution is given in the form of a double asymptotic series of the main and the so-called associated shadow terms. It is shown that the series of the shadow terms associated with a main term is infinite, and all shadow terms include logarithmic terms. Thus, although the interface tractions are bounded, the linear elastic solution at this crack-tip has a logarithmic stress singularity which is comprehensively analysed. Noteworthy, the character of this stress singularity is very different from the well-known square root singularity at the crack tip in the classical fracture mechanics. A key advantage of the present approach is its simplicity, as only elementary mathematical tools are employed, and also its easy implementation in a computer algebra software. The latter fact is very relevant because the expressions of higher-order shadow terms become increasingly complicated, so their generation by a computer code becomes crucial. The present results allow the implementation of new enriched or singular crack-tip finite elements for such cracks, and the automatic generation of analytic solutions for benchmark problems for testing the finite-element codes using these special elements. Such codes can be applied to efficient numerical modelling of interface cracks, e.g., in adhesively bonded joints with a thin adhesive layer.

Ultrasonic monitoring of adhesive curing process between adherends

An ultrasonic method to monitor the curing of a thin adhesive layer was developed theoretically and verified experimentally. The method was based on a model in which an interface spring represented an adhesive layer sandwiched between adherends of the same material. In this method, the interfacial stiffness in the spring model, which included the ultrasonic longitudinal wave velocity and the adhesive's density and thickness, was obtained from the measured reflection or transmission coefficient for the wave in the adhesive layer. The ranges of interfacial stiffness for valid application of the method were theoretically clarified in terms of the stiffness's sensitivity to the reflection and transmission coefficients and the stiffness error with respect to errors in the measured coefficients. The applicable ranges were verified by experimentally evaluating the curing process of an epoxy resin with adhesive several micrometers thick when joining aluminum alloy adherends. Specifically, the ranges were obtained as 3.1ρC<K/f<62ρC[Pa⋅s/m] for the reflection coefficient and K/f<6.3ρC[Pa⋅s/m] for the transmission coefficient, where ρC, K, and f are the acoustic impedance of the adherends, the interfacial stiffness of the adhesive, and the ultrasonic frequency. The measurable ranges of the adhesive's interfacial stiffness were also determined for typical adherend materials such as alloys, polymers, composites, and ceramics.

A geometrical parameter study of the analytical stress calculation in adhesive joints with thick bonding layers

AbstractStrength and failure analyses are critical to the safe and sustainable design of any structure. In the case of adhesively bonded joints, particular attention must be paid to the adhesive layer, which is comparably compliant and subject to high stresses. Determining the stresses in the adhesive layer is essential to identify critical areas in the joint and prevent the formation of cracks. In most applications, bonded joints have thin adhesive layers that can already be accounted for with highly simplified analytical models. However, for the consideration of thicker adhesive layers, advanced models are required that take the adhesive layer into account in more detail.Therefore, in this work, an analytical model with a higher‐order displacement approach is presented to account for more complex deformation behavior in the adhesive layer. The applied advanced displacement approach is particularly suitable for joints with thicker adhesive layers. In addition, the model enables the calculation of stresses and strains in bonded joints for a wide range of joint configurations and applications. A parameter study is carried out to evaluate the effect of different thicknesses of the adhesive layer and to show the advantages of the current model compared to other existing ones. For verification, all analytical results were compared with corresponding Finite Element Analyses.