Viscoelastic Adhesive Layer Research Articles

Constrained thickness-shear vibration-based piezoelectric transducers for generating unidirectional-propagation SH0 wave

Excitation of a pure guided wave with a controllable wavefield is essential in structural health monitoring (SHM). For example, a unidirectional-propagation guided wave can significantly reduce the complexity of signal interpretations by avoiding unwanted reflections. However, few transducers are currently capable of exciting a pure unidirectional-propagation guided wave, which cannot satisfy the emerging demands from the field of SHM. In this work, the thickness-shear vibration characteristics of the piezoelectric PZT wafer bonded on a waveguide are investigated by theoretical modeling and numerical simulations. It is found that there is a phase difference between the electric-excitation signal applied on the PZT wafer and the mechanical response signal of the bottom surface of the viscoelastic adhesive layer that connects the PZT wafer and waveguide. Moreover, such a phase difference can be adjusted by changing the equivalent width of the PZT wafer. Based on this finding, two piezoelectric transducers with different shape configurations are proposed to excite the unidirectional-propagation SH0 wave (the fundamental shear horizontal wave). Finite element simulations and experiments are conducted to verify the performances of the two unidirectional transducers. Results show that the two transducers can excite a pure SH0 wave and enhance the wave energy along a single direction. No time delay is required to excite the proposed transducers. Due to their simple configurations, the developed unidirectional SH0 wave transducers will have great potential applications in SHM.

The Role of Denture Adhesives on the Oral Health of Partial Denture Wearers: A Numerical Study

AbstractRemovable partial dentures (RPDs) significantly influence the mechanical stress characteristics of the entire dental arch. Unlike normal teeth, they are not anchored firmly in the jaws and hence are prone to denture slippage. The aim of this study is to examine numerically using finite element (FE) method, the role of denture adhesive creams in the stress response of the dental structures, and to understand its impact on the oral health of denture wearers. For this purpose, computer tomography data of the jawbone and RPDs are utilized to develop corresponding 3D models, which are further used for FE simulations. The partial denture system is bonded onto the surface of the oral mucosa with the help of a viscoelastic adhesive layer and is also supported by three abutment teeth. The application of bite forces on the denture generates varying contact mechanical response to the stimulus, across the partial denture, which are compared with the clinical pressure pain threshold (PPT) value for soft tissue, which potentially lowers the risk of pain. The model with the denture adhesive shows better retention and are within the PPT and hence can potentially lower the risks associated with denture slippage.

Acrylate pressure-sensitive adhesives tape as cover membrane for preventing ultrasound probes from cross-infections

Ultrasonic diagnosis is convenient and effective. However, insufficient and improper cleaning or disinfection will not only cause cross-infection, but also damage the outermost acoustic lens which is the most critical acoustic part of the medical ultrasonic imaging system. Herein, acrylate pressure-sensitive adhesive (PSA) tapes, intended as coverings for protecting ultrasound probes to prevent cross-infections, were prepared by the transfer coating method using acrylate copolymer as the adhesive and polyethylene film as the substrate. Ultrasound testing using PSA tape as a barrier for the probe was carried out on a skin-like silicone puncture phantom and the distal forearm of a volunteer. The clarity of ultrasonic images analyzed by Image J Software based on gray value and judged by the clinician was compared. Effect of acrylate copolymers with different molecular weights synthesized through free radical polymerization by adjusting the amount of solvent (ethyl acetate) and crosslinker (aluminum acetylacetonate) on adhesive properties and clarity of ultrasonic images were investigated in detail. Results showed that the shear strength of PSA tapes improved with increasing molecular weight of acrylate copolymer, however, tack and 180° peel strength showed a downward trend. The formation of a cross-linked network among polymer chains also greatly improved the shear strength of PSA tapes to meet the requirements of ultrasound probe covers, such as ‘easy to adhere and peel from the ultrasound probe’, ‘without adhesive residue on the surface of the ultrasound probe’ and ‘less impact on the clarity of ultrasonic images’. Viscoelastic adhesive layer of tape replaces the coupling gel between ultrasound probe and PSA tape. The ease of preparation, convenience of use, and effectiveness of acrylate PSA tapes indicate that they have broad application prospects for sonographic examinations.

Finite Element Evaluation of the Effect of Adhesive Creams on the Stress State of Dentures and Oral Mucosa.

The base fit between a removable partial denture (RPD) and the underlying soft tissue plays a significant role in its performance. The application of a denture adhesive is hypothesized to result in better retention of RPDs and, as a result, contribute to lower stress on the oral mucosa. The objectives of this study were to observe and compare the distribution of simulated bite forces applied to the RPD through the abutments and soft tissue for models with and without the use of a denture adhesive. Furthermore, we evaluated the possible benefit of using a denture adhesive in lowering stresses on the oral mucosa. The RPD, mandible, oral mucosa, abutment teeth supporting the RPD, and the corresponding abutment periodontal ligaments (PDLs) were modelled as 3D volumes based on computer tomography (CT) datasets. A viscoelastic adhesive layer between the RPD and oral mucosa was incorporated into this base model using Prony series approximation. The layer was developed as a volume extract using the denture surface. Finite element (FE) simulations were performed for the bite force on one of the RPD segments, with the resulting force and moments experienced by the dental structures and oral mucosa compared between the model with the adhesive layer and the base model without. As a result, the contact pressure on the oral mucosa for the model with the denture adhesive decreased to 0.15 MPa as compared to 0.25 MPa for the model without the adhesive. The potential role of denture adhesives in leading to a better fit between the RPD and oral mucosa as well as lowering contact pressures could be used to improve comfort in patients wearing RPDs.

Vibration and damping characteristics of the masonry wall strengthened with bonded fibre composite patch with viscoelastic adhesive layer

The present paper treats the free vibration problem of the masonry wall strengthened with thin composite plate by viscoelastic adhesive layer. For this goal two steps are considered in the analytical solution. In the first one, an efficient homogenisation procedure is given to provide the anisotropic properties of the masonry wall. The second one is dedicated to purpose simplified mathematical models related to both in-plane and out-of-plane vibration problems. In these models, the higher order shear theories (HSDT's) are employed for a more rigours description of the shear deformation trough the masonry wall and the composite sheet. Ritz's method is deployed as solution strategy in order to get the natural frequencies and their corresponding loss factors. The obtained results are validated with the finite element method (FEM) and then, a parametric study is undertaken for different kinds of masonry walls strengthened with composite sheets.

Dynamics of Two Linearly Elastic Bodies Connected by a Heavy Thin Soft Viscoelastic Layer

In this paper, we extend the asymptotic analysis in (Licht et al. in J. Math. Pures Appl. 99:685–703, 2013) performed, in the framework of small strains, on a structure consisting of two linearly elastic bodies connected by a thin soft nonlinear Kelvin–Voigt viscoelastic adhesive layer to the case in which the total mass of the layer remains strictly positive as its thickness tends to zero. We obtain convergence results by means of a nonlinear version of Trotter’s theory of approximation of semigroups acting on variable Hilbert spaces. Differently from the limit models derived in (Licht et al. in J. Math. Pures Appl. 99:685–703, 2013), in the present analysis the dynamic effects on the surface to which the layer shrinks do not disappear. Thus, the limiting behavior of the remaining bodies is described not only in terms of their displacements on the contact surface, but also by an additional variable that keeps track of the dynamics in the adhesive layer.

Free vibration analysis and optimal design of adhesively bonded double-strap joints by using artificial neural networks

Abstract This study investigates the free vibration characteristics of an adhesively bonded double-strap joint with viscoelastic adhesive layer. To simplify the spatial finite element mesh generation and efficiently model the adhesively bonded joint, a layerwise plate finite element was extended to accommodate to the modeling of the joint, where the joint structure is treated as a special sandwich laminate. The proposed method was validated by three-dimensional finite element analysis and then applied to generate sampling points for training artificial neural networks (ANNs). The effects of the adhesive material properties and joint geometrical parameters on the joint dynamic characteristics were investigated in detail using the trained ANNs. The optimum design problem is defined as a multi-objective optimization problem considering maximizing the first natural frequency and corresponding loss factor while minimizing the total structural weight. The nondominated sorting genetic algorithm combined with the ANNs were employed to tackle the problem. The proposed method provides a computationally efficient alternative for analyzing and optimizing the adhesive double-strap joints.

Viscoelastic analysis of stress distribution in balanced and unbalanced adhesively bonded single-lap joints with functionally graded adherends under the Reddy model

In this study, shear and peel stress distributions in the viscoelastic adhesive layer of a single-lap joint (SLJ) with functionally graded (FG) adherends are investigated. The study focuses on the effect of different adherend profiles and material composition on the time-dependent stress concentration/distribution in balanced and unbalanced SLJs. For this purpose, the Reddy model is applied to the FG adherends and a three-parameter solid viscoelastic model is used to simulate the adhesive layer behavior. Using the first-order shear deformation theory for the FG adherends, the governing differential equations are derived and then transformed into the Laplace domain. A finite element model of the joint was also developed to further backup the numerical solution. The numerical inverse Laplace transform method was used to extract the desired results that were then compared with those of finite element method (FEM) findings. Very good agreements were observed between the results of both methods. Results show that the geometric and mechanical properties of the FG adherends have an essential role in reducing the shear and peel stress concentrations as well as the uniformity of shear stress distribution in the overlap region. Results also show that either method (finite element or the proposed semi-analytical method) can be utilized with confidence for prediction of stress relaxation in the adhesively bonded SLJs with FG adherends.

Bandgap Structures of SH-Wave in a One-Dimensional Phononic Crystal with Viscoelastic Interfaces

Phononic crystal is an artificial periodic structure with the ability to regulate and control the wave propagation of particular frequencies and has been widely used in many applications. The adhesive layer bonding different constituents in the periodic structure of phononic crystals is usually a viscoelastic material, which has frequency-dependent material properties. In this paper, an analytical model based on the transfer matrix method is developed to study the bandgap structures of SH-wave (a shear wave with the propagation direction normal to the motion plane) in a one-dimensional phononic crystal consisting of two different elastic constituents bonded by the viscoelastic adhesive layer. The results show that the viscosity of the adhesive layer has a significant influence on the bandgap structure at the region of high frequency. The effects of various material parameters of the viscoelastic adhesive layer such as the relaxation time, the final-state modulus and the initial-state modulus are systematically studied. These results are very helpful in the practical design of phononic crystals involving the viscoelastic adhesive layers.

The effect of the viscoelastic film and metallic skin on the dynamic properties of thin sandwich structures

In this work, the influence of design parameters on the dynamic properties of thin sandwich structures composed of micron-size viscoelastic adhesive layers is analysed. Sandwiches composed of different types of viscoelastic film and metallic skins are characterised in the frequency bandwidth of 0–1kHz. The numerical modelling of thin sandwich structures is presented and validated with experimental results. A four-parameter fractional derivative model is used to describe the dynamic behaviour of the viscoelastic films, and a three-layer beam element is used to model the sandwich structures. The analysis of experimental and numerical results allows the establishment of design considerations to obtain thin sandwich structures with high stiffness and damping. The viscoelastic adhesive film should have a high shear modulus and loss factor, and the metallic skin should have a high storage modulus.

Localized contacts, stress concentrations and transient states in bent-lamination with viscoelastic adhesion. An analytical study

An analytical study is presented for the bent-lamination of curved layered beams, a process consisting in gluing the constituent plies together after they have been elastically bent against a constraining negative mould. Possible applications range from glued laminated timber manufacturing, to cold-lamination-bending of structural glass. After removal of the constraint, the shear coupling through the glue maintains the curvature only partially, because the laminate suffers an initial spring-back followed by a long-term relaxation. The model problem considered here is that of two Euler–Bernoulli beams coupled by a thin viscoelastic adhesive layer. Within a variational approach, we analytically describe the relationship between the mould shape and the shape of the curved beam, which is time-dependent due to the viscosity of the adhesive layer. Localized contacts with the mould and stress concentrations may occur, depending upon the type of profile that is initially imposed. Comparison of the cases of instantaneous or gradual release of the contact with the mould, evidences a remarkable reduction of the transient state of stress in the second case.

Theoretical modeling of low-attenuation lamb wave modes generation in three-layer adhesive joints using angle beam transducer

IN THIS PAPER, AT FIRST THE ATTENUATION OF LAMB WAVES IN THREE-LAYER ADHESIVE JOINTS, INCLUDING TWO ELASTIC PLATES BONDED TOGETHER BY A VISCOELASTIC ADHESIVE LAYER, IS INVESTIGATED USING GLOBAL MATRIX METHOD AND THEN SUITABLE INCIDENCE ANGLE IS THEORETICALLY CALCULATED TO GENERATE LOW-ATTENUATION LAMB WAVES USING ANGLE BEAM TRANSDUCER. THEORETICAL BOUNDARY VALUE PROBLEM IN THREE-LAYER ADHESIVE JOINTS WITH PERFECT BOND AND TRACTION-FREE BOUNDARY CONDITIONS ON THEIR OUTER SURFACES IS SOLVED TO FIND A COMBINATION OF FREQUENCIES AND MODES WITH LOWEST ATTENUATION. CHARACTERISTIC EQUATION IS DERIVED BY APPLYING CONTINUITY AND BOUNDARY CONDITIONS IN THREE-LAYER JOINTS USING GLOBAL MATRIX METHOD. PHASE VELOCITY DISPERSION CURVES AND ATTENUATION INTENSITY PLOT IN HIGH AND LOW FREQUENCIES ARE OBTAINED WITH NUMERICAL SOLUTION OF THIS EQUATION BY A COMPUTER CODE FOR A THREE-LAYER JOINT, INCLUDING AN ALUMINUM REPAIR PATCH BONDED TO THE AIRCRAFT ALUMINUM SKIN BY A LAYER OF VISCOELASTIC EPOXY ADHESIVE. TO VALIDATE THE NUMERICAL SOLUTION RESULTS OF CHARACTERISTIC EQUATION, WAVE STRUCTURE CURVES ARE PLOTTED FOR A SPECIAL MODE IN TWO DIFFERENT FREQUENCIES IN THE ADHESIVE JOINT. ALSO, TRANSDUCER INCIDENCE ANGLE IS CALCULATED IN TERMS OF FREQUENCY FOR DIFFERENT MODES USING THEORETICAL METHOD TO GENERATE LAMB WAVE MODES WITH LOW ATTENUATION LEVEL BY ANGLE BEAM TRANSDUCER. THESE MODES ARE RECOGNIZABLE BY TRANSDUCERS IN INSPECTIONS WITH LAMB WAVES BECAUSE OF LOW ATTENUATION LEVEL.

Experimental study of cohesive strain prior to crack initiation in constant force single cantilever beam test

Retarded deformation at the bond line edge of a thick, viscoelastic adhesive layer is observed using digital image correlation. Such information is required for proposing physical crack onset/propagation parameters which take into account viscoelastic and viscous mechanisms in the evaluation of dissipated energy. A complex stress/strain redistribution is observed in the early stage after specimen loading. The results illustrate that viscoelasticity dissipates a considerable amount of energy prior to crack nucleation and the lack of predictive models to account for that phenomenon.

Investigation of Lamb waves attenuation in elastic–viscoelastic three-layer adhesive joints in low and high frequencies: Theoretical modeling

In this paper the attenuation of Lamb waves in three-layer joints, including two elastic plates bonded together by a viscoelastic adhesive layer, is investigated in high and low frequencies using global matrix method. Theoretical boundary value problem in three-layer adhesive joints with perfect bond and traction-free boundary conditions on their outer surfaces is solved to find a combination of frequencies and modes with lowest attenuation. Characteristic equation is derived by applying continuity and boundary conditions in three-layer joints using global matrix method. Attenuation and phase velocity dispersion curves in high and low frequencies are obtained with numerical solution of this equation by a computer code for a three-layer joint, including an aluminum repair patch bonded to the aircraft aluminum skin by a layer of viscoelastic epoxy adhesive. To validate the numerical solution results of characteristic equation, wave structure curves are plotted for a special mode in two different frequencies in the adhesive joint. Also, attenuated displacement amplitude curves in a round-trip of Lamb wave along the adhesive joint are plotted to investigate the reduction of the signal amplitude generated by a single transducer. The purpose of present paper is to find a combination of frequencies and modes with minimum attenuation in high and low frequencies. These frequencies and modes are recognizable by transducers in inspections with Lamb waves because of low attenuation level.

Dynamics of elastic bodies connected by a thin soft viscoelastic layer

A dynamic study was performed on a structure consisting of two three-dimensional linearly elastic bodies connected by a thin soft nonlinear Kelvin–Voigt viscoelastic adhesive layer. The adhesive is assumed to be viscoelastic of Kelvin–Voigt generalized type, which makes it possible to deal with a relatively wide range of physical behavior by choosing suitable dissipation potentials. In the static and purely elastic case, convergence results when geometrical and mechanical parameters tend to zero have already been obtained using variational convergence methods. To obtain convergence results in the dynamic case, the main tool, as in the quasistatic case, is a nonlinear version of Trotterʼs theory of approximation of semigroups acting on variable Hilbert spaces. The limit problem involves a mechanical constraint imposed along the surface to which the layer shrinks. The meaning of this limit with respect to the relative behavior of the parameters is discussed. The problem applies in particular to wave phenomena in bonded domains.

Interface stress redistribution in FRP-strengthened reinforced concrete beams using a three-parameter viscoelastic foundation model

External bonding of FRP plates or sheets has become a popular method for strengthening reinforced concrete structures. Stresses along the FRP–concrete interface are critical to the effectiveness of this technique because high stress concentration along the FRP–concrete interface can lead to the FRP debonding from the concrete beam. In this study, a novel analytical solution has been developed to predict the interface stress redistribution of FRP-strengthened reinforced concrete beams induced by the viscoelastic adhesive layer. Both the FRP plate and the RC beam are modeled as Timoshenko’s beams, connected through the adhesive layer. The adhesive layer is modeled as a three-parameter viscoelastic foundation (3PVF) using Standard Linear Solid model. The 3PVF model satisfies the equilibrium equation of the adhesive layer and the zero shear-stress boundary condition at the free edge. Closed-form expressions of the time-dependent interface stresses and deflection of the beam are obtained using Laplace transform. Finite element analysis is also conducted to verify the analytical solution using a subroutine UMAT based on the Standard Linear Solid model. Numerical results suggest that the stress concentrations within the FRP–concrete interface relax with time. The axial force in the FRP plate also reduces with time due to the creep of the adhesive layer. However, this relaxation is limited to a small zone close to the end of the FPR plate.

The interfacial fracture behavior of foam core composite sandwich structures by a viscoelastic cohesive model

A sandwich beam model consisting of two face sheets and a foam core bonded by a viscoelastic adhesive layer is considered in order to investigate interfacial fracture behavior. Firstly, a cohesive zone model in conjunction with a Maxwell element in parallel, or with a Kelvin element in series, respectively, is employed to describe the characteristics of viscoelasticity for the adhesive layer. The models can be implemented into the implicit finite element code. Next, the parametric study shows that the influences of loading rates on the cohesive zone energy and strength are quite different for different models. Finally, a sandwich double cantilever beam model is adopted to simulate the interface crack growth between the face sheet and core. Numerical examples are presented for various loading rates to demonstrate the efficacy of the rate-dependent cohesive models.

Quasi-static and dynamic fracture behavior of composite sandwich beams with a viscoelastic interface crack

Fracture analysis of sandwich beams with a viscoelastic interface crack under quasi-static and dynamic loading has been studied. Firstly, a three-parameter standard solid material model was employed to describe the viscoelasticity of the adhesive layer. And a novel interfacial fracture analysis model called three material media model was established, in which an interface crack was inserted in the viscoelastic layer. Secondly, a finite element procedure based on Rice J-integral and Kishimoto J-integral theories was used to analyze quasi-static and dynamic interface fracture behavior of the sandwich beam, respectively. Finally, the influence of viscoelastic adhesive layer on the quasi-static J-integral was discussed. In addition, comparison of quasi-static Rice J-integral with Kishimoto J-integral under various loading rates was carried out. The numerical results show that the oscillating characteristic of dynamic J-integral is more evident with shorter loading rise time.

Dynamic response of a repaired composite beam with an adhesively bonded patch under a harmonic peeling load

The dynamic response of repaired composite beams under a harmonic peeling load was studied theoretically and experimentally. The repair method was based on removal of the damaged region and bonding a composite patch into the gap with adhesive. In the theoretical part, the equations of motion in the axial and transverse directions were derived assuming that the viscoelastic adhesive layer resists both peeling and shear stresses and both the patch and parent materials behave as Euler–Bernouli beams. The validity of the theoretical model for evaluating the dynamic response of repaired composite beams was examined with the results of the finite element model. The finite element results indicated that the deformation mechanism of the repaired composite beam depends on the adhesive elastic modulus. For low values of the adhesive elastic modulus, shear deformation in the adhesive layer is the dominant deformation mechanism and the proposed theoretical model replicated the results of the computational analysis. In the experimental part, the response of unidirectional fiberglass-reinforced epoxy composite specimens with various repaired patch lengths, thickness, and material properties were measured by hammer test technique using a non-contact laser vibrometer. The patch section was either the fiberglass-reinforced epoxy composite or E-glass fiber reinforced composites with various stacking sequences. The repairing patches were bonded to the composite beam with an epoxy. The experimental results were compared to those of the theoretical model and finite element analyses. The experimental results were related to the adhesive material properties, its loss factor and to the patch material properties.

Experimental and Analytical Investigations of the Dynamic Response of Adhesively Bonded Single Lap Joints

Dynamic response of single lap joints, subjected to a harmonic peeling load is studied theoretically and experimentally. In the theoretical part, dynamic response of a single lap joint clamped at one end and subjected to a harmonic peeling load at the other end is investigated. Adherents are modeled as Euler-Bernouli beams joined in the lap area by a viscoelastic adhesive layer. Both axial and transverse deformations of adherents are considered in deriving the equations of motion. The effects of adhesive layer thickness, mechanical properties and its loss factor on the dynamic response of the joint are investigated. Furthermore, effects of defects such as a void in the lap area on the dynamic response of the joints are studied. The results showed that frequencies where peak amplitudes occurred were little dependent on the adhesive loss factor. However, peak amplitudes reduced for joints with a higher adhesive loss factor. Furthermore, the results indicated that for the joint geometries and properties investigated the system resonant frequencies were not affected by the presence of a central void covering up to 80% of the overlap length. In the experimental part, single lap joints were fabricated using 6061-T6 Aluminum. Adherents were joined together using Hysol EA 9689 adhesive film. Joints with various central voids were manufactured by removing adhesive film from the desired area of lap joints prior to bonding adherents. Dynamic responses of the joints were investigated using the hammer test technique. The system response was measured using both an accelerometer and a noncontact laser vibrometer. The natural frequencies of the joints obtained by using the laser vibrometer were very close to those obtained theoretically. However, natural frequencies obtained by using an accelerometer depended on the accelerometer location in the system, which was attributed to its mass contribution to the over- all system mass. A central void covering less than 80% of the overlap length had little effect on the system resonance frequencies. This was in agreement with the theoretical results. In contrast total system-damping ratios were a function of the void size. Joints without a void exhibited higher damping.