Adhesive-adherend Interface Research Articles

Effects of laminate stacking sequence on the strength properties of aluminum alloy–carbon fiber-reinforced plastic dissimilar single-lap adhesive joints

Here, two single-lap adhesive joints (SLJs) of dissimilar materials were subjected to static and cyclic loading tests. A2017 aluminum alloy was used as an adherend for one, and carbon fiber reinforced plastic (CFRP) was used as the adherend for the other. Four types of orthogonal laminated CFRPs with different laminate stacking sequences were used as adherends to investigate the effect of adherend stiffness on the strength properties of the joints. Furthermore, the results of the finite element analysis of the dissimilar SLJs revealed that when the tensile load was applied to them, the out-of-plane deformation asymmetry increased with increasing difference in stiffness between both adherends. This asymmetry affected the peel and shear stress distributions. Furthermore, the experiments revealed that the static tensile strength of the SLJs increased with increasing stiffness of the CFRP adherend. Additionally, fracture simulation using cohesive-zone modeling (CZM) revealed that the SLJs with higher CFRP stiffness exhibited higher strength, qualitatively agreeing with the experimental results. CZM analysis and adhesion strain measurements during the tests indicated that failure occurred at the A2017 adherend–adhesive interface. In contrast, no differences were observed between the fatigue strengths of the different types of adherends in the short-life region, with a number of cycles to failure (Nf) being ≤ 2 × 105. However, in the long-life region, beyond Nf = 2 × 105, the SLJ bearing the unidirectional CFRP adherend exhibited lower fatigue strength than the others. The anodizing process on A2017 was found to improve fatigue strength by a factor of two or more.

Modeling of single-lap joints with auxetic adhesive utilizing homogeneous and heterogeneous bondline microstructures

Producing lightweight structures can be effectively achieved using adhesive bonding that incorporates a high-performance adhesive, eliminating the mechanical joints (rivet, bolts). Here, the proof-of-concept of tailoring the high-performance adhesive using microstructures exhibiting a negative Poisson's ratio (auxetic) is investigated using two single lap joint (SLJ) models developed in ABAQUS/Standard. The SLJ models consist of two rigid adherends that are bonded using either homogeneous or heterogeneous adhesive that produces auxetic response. In homogeneous adhesive, a negative value of Poisson's ratio is defined in the adhesive part of the models. In heterogeneous adhesive, the negative Poisson's ratio is obtained by explicitly building orthogonally-arranged elliptical voids in the adhesive part of the models. In addition, the adherend-adhesive interface is represented by cohesive elements with bi-linear traction-separation rule. The effects of using two different adhesive models and thickness on peel and shear stresses, as well as failure mechanism and joint strength, are evaluated. We found that the effect of auxetic adhesive on SLJ response is more profound in the model using heterogeneous adhesive rather than the homogeneous one, where the joint strength enhancement could achieve 45 % in reference to the baseline model. The heterogeneous adhesive embedded between adherends is able to activate ligaments, the mechanism that could not be obtained using merely homogeneous adhesive. Our proposed modeling strategy can be a starting point to further the modeling approach of bonded joints utilizing auxetic adhesive.

Thermal Stress Formation in a Functionally Graded Al2O3-Adhesive Single Lap Joint Subjected to a Uniform Temperature Field

This study investigates the strain and stress states in an aluminum single lap joint bonded with a functionally graded Al2O3 micro particle reinforced adhesive layer subjected to a uniform temperature field. Navier equations of elasticity theory were designated by considering the spatial derivatives of Lamé constants and the coefficient of thermal expansion for local material composition. The set of partial differential equations and mechanical boundary conditions for a two-dimensional model was reduced to a set of linear equations by means of the central finite difference approximation at each grid point of a discretized joint. The through-thickness Al2O3-adhesive composition was tailored by the functional grading concept, and the mechanical and thermal properties of local adhesive composition were predicted by Mori–Tanaka’s homogenization approach. The adherend–adhesive interfaces exhibited sharp discontinuous thermal stresses, whereas the discontinuous nature of thermal strains along bi-material interfaces can be moderated by the gradient power index, which controls the through-thickness variation of particle amount in the local adhesive composition. The free edges of the adhesive layer were also critical due to the occurrence of high normal and shear strains and stresses. The gradient power index can influence the distribution and levels of strain and stress components only for a sufficiently high volume fraction of particles. The grading direction of particles in the adhesive layer was not influential because the temperature field is uniform; namely, it can only upturn the low and high strain and stress regions so that the neat adhesive–adherend interface and the particle-rich adhesive–adherend interface can be relocated.

Multi-layer continuum model for adhesive films with knitted carrier

Film adhesives are a form of composite consisting of two constituents i.e. fibrous carrier and matrix. The fracture surfaces are usually dominated by the carrier-matrix de-bonding, but it also co-exists with a minor failure related to resin closer to the adhesive–adherend interface. This investigation aims at a realistic representation of the failure modes. For this, a 3D multi-layer approach of a FM 300-2 adhesive is defined where a non-local Continuum Damage Model (CDM) is developed accounting for the proper matrix mechanism defined with lower fracture energies. The carrier-matrix de-bonding was implemented by an in-built Cohesive Zone Model (CZM) at the interfaces of the matrix-carrier layers. For the characterization and validation, the modelling was tested with the Mode I fracture response of the adhesive in joints (butt joint, BJ, and double cantilever beam specimen, DCB) and bulk form (tensile tests). Numerical results with CZM-CDM implementation agreed with the experimental data in terms of load response and representation of the failure mechanisms, when correct material constants are applied. The multi-layer damage model was able to define strength and fracture properties of the matrix phase by the prediction of failure initiation in the BJ test. Consequently, the mechanism was able to reproduce the local damage path that occurred close to the adhesive–adherend interface.

Study of effects of surface preparation on carbon-fiber-reinforced-polymer (CFRP) Single Lap Joints (SLJ) using positioning of gates in ultrasonic signals

ABSTRACT Ultrasonic inspection of adhesively bonded Single Lap Joint (SLJ) with composite material as adherend is quite challenging due to very close and overlapping reflections of ultrasonic waves from multiple interfaces across the joint. This study aims to precisely identify gate positions for these reflections in the pulse-echo technique. Further, investigation of surface roughness effects in the bonded region of SLJ is attempted using reflection coefficient R. For a real-valued A-scan signal in ultrasonic pulse-echo mode, a complex-valued analytic signal with amplitude and phase was obtained from the Hilbert transform. A sudden phase change of the analytic signal and a trough in the analytic signal’s envelope was observed at the reflection plane in A-scan. Analysis of spectra of reflected signals shows variation in magnitude at peak frequencies with the surface roughness of adherend. This paper provides a technique for identifying precise gate placement for the reflected signal and R-values evaluation for composite SLJ. For each surface roughness category, reflection coefficient R was experimentally evaluated at the adherend-adhesive interface of SLJ. These R values are correlated with the shear strength of the SLJ obtained from standard destructive tests.

Failure mechanism of single lap, adhesively bonded composite-titanium joints subjected to solid projectile impact

The failure mechanism of single lap, adhesively bonded composite-titanium joints subjected to solid projectile impact is addressed in this article. Three-dimensional finite element analysis was conducted in Abaqus/Explicit using two damage models. The first damage model accounts for debonding failure modes of adhesive–adherend interface using cohesive zone method. The second damage model accounts for in-plane failure modes of composite adherend using Hashin criterion and continuum damage mechanics. Mechanical tests were carried out by impacting the adhesively bonded joints with solid projectile using a one-stage gas gun. The recorded strain responses and damage patterns were found to be in good correlation with those of the finite element analysis. Results have shown that, for single lap, adhesively bonded composite-titanium joints under increasing impact velocities of solid projectile, the progression of failure modes is: local debonding in the adhesive–titanium adherend interface, matrix cracking in the composite adherend and global debonding in the adhesive–titanium adherend interface.

Loading-rate effect on tensile and bending strength of 3D-printed polylactic acid adhesively bonded joints

Additive manufacturing provides the production of many machine parts and components with complex geometries. The adhesive bonding technique can be alternative method for joining parts produced with additive manufacturing. This experimental study investigates the applicability of the adhesive bonding technique for PLA (polylactic acid) adherends produced with additive manufacturing and especially the effects of loading rate on the strength of 3D-printed PLA adhesive single-lap joints under tensile, three-point bending (with shear) and four-point bending (no shear effect) loadings. Both PLA and adhesive tensile test specimens exhibited a better strength but lower failure strain with increasing loading rate. PLA had better mechanical behaviour in the raster orientation than those in the layer-build direction. The strength of adhesive single-lap joints improved slightly with increasing loading rate for the tensile and three-point bending tests whilst a decrease of strength and an improvement of bending stiffness were observed for the four-point bending test. Failure initiated at the free edge of the top adherend-adhesive interface for all tests, and propagated along this interface for both bending tests whilst a sudden through-the-thickness failure of top adherend occurred for tensile load after a small interfacial damage propagation. The failure propagation appeared in a wavy form for the three-point bending test whilst it was along the top adherend-adhesive interface for the four-point bending test. Digital Image Correlation (DIC) method for tensile tests showed that the peeling and shear strains were more critical and concentrated around both free edges of adherend-adhesive interfaces; thus, at the right free edge of the top adherend-adhesive interface and at the left free edge of the bottom adherend-adhesive interface.

Investigating the Effect of Interface Morphology in Adhesively Bonded Composite Wavy-Lap Joints

Joints and interfaces are one of the key aspects of the design and production of composite structures. This paper investigates the effect of adhesive–adherend interface morphology on the mechanical behavior of wavy-lap joints with the aim to improve the mechanical performance. Intentional deviation from a flat joint plane was introduced in different bond angles (0°, 60°, 90° and 120°) and the joints were subjected to a quasi-static tensile load. Comparisons were made regarding the mechanical behavior of the conventional flat joint and the wavy joints. The visible failure modes that occurred within each of the joint configurations was also highlighted and explained. Load vs. displacement graphs were produced and compared, as well as the failure modes discussed both visually and qualitatively. It was observed that distinct interface morphologies result in variation in the load–displacement curve and damage types. The wavy-lap joints experience a considerably higher displacement due to the additional bending in the joint area, and the initial damage starts occurring at a higher displacement. However, the load level had its maximum value for the single-lap joints. Our findings provide insight for the development of different interface morphology angle variation to optimize the joints behavior, which is widely observed in some biological systems to improve their performance.

The effect of joint configuration on the strength and stress distributions of dissimilar adhesively bonded joints

The recent increase in the use of adhesively bonded joints (ABJs) made from dissimilar adherends demands the acquisition of a better understanding of the strength and behaviour of these joints, including their failure mechanisms. Several studies have reported on such joints individually, however few have compared the performances of dissimilar ABJs with varying configurations and design parameters, in order to determine the optimal design configuration for hybrid structures. In this work, a comparative study using experimental methods and finite element analysis was conducted, focusing on four joint configurations (scarf joints, stepped-lap joints, half-lap splice joints and single-lap joints), with the aim of evaluating the ways in which their performances differ. In addition, the effects of overlap length (L0) and the mechanical properties of the adherends on the overall success of each joint were particularly closely analysed and compared. The results showed that the scarf joint provided the best performance of all the designs discussed, and it was found that increasing the overlap length was only significantly beneficial for certain joint configurations and adherend combinations. When the overlap length was increased from 12.5 mm to 25 mm, the failure load increased by 47.50% and 21.25% for the scarf and the stepped-lap joints, respectively. In comparison, the percentage increases for the half-lap splice and single-lap joints under the same conditions were less than 10%. Moreover, the mechanical properties of the adherends considerably affected the failure mechanisms of the dissimilar joints, and for all four joint configurations, the failure was initiated by a crack at the adherend-adhesive interface adjacent to the adherend with a lower modulus.

AFM observation of sea-island structure formed by second generation acrylic adhesive

ABSTRACT Although adhesive-adherend interfaces play an important role in improving adhesive joint strength, their observation is a challenging task. If the adhesive and adherend can be clearly separated, the interface can be observed directly. Because this was fortunately achieved with the combination of a second-generation acrylic adhesive and several plastics, the differences in the formation of sea-island structures at interfaces was verified as related to adhesive strength. Phase mapping, stiffness measurements, and thermal analysis were conducted with atomic force microscopy (AFM) to investigate the physical properties of the sea-island structures. The internal adhesive structure observed by AFM was in good agreement with the transmission electron microscopy analyses. The structure of the adhesive-adherend interface that showed a good bonding strength was similar to the internal structure, forming a large acrylic-rich island and rubber-rich sea. In the case of plastics with low surface energies, however, the interface formed a distinctive structure with small acrylic islands in a rubber sea. The difference in the formation of the sea-island structure is considered to be related to the wettability of the adhesive to the adherends, and it was revealed that a stronger interface corresponds to a structure more closely resembling that of the inside.

Optimization of laminate’s single lap joints by FSDT theory

The focus of this paper was to optimize the interconnection mapping of the adhesive single-lap joints that are bonding composite laminates by utilizing the adherent’s layers’ arrangement method. In this work, as the first step, shear and peeling stress distributions in the adhesive layer of a single lap joint under tensional loading were determined by a 2-D first-order shear deformation theory (FSDT). Subsequently, the stacking sequence of the laminated adherents was optimized by the bees algorithm (BA) in order to reduce the maximum stresses in the adhesive layer. The adhesive layer was assumed to be isotropic while the laminated adherends were assumed to be made of orthotropic laminae. All stresses were determined by an improved closed-form solution based on the deduced displacement fields. The results of the analytical solution were compared with those of finite element solution using ANSYS software, version 13.0. Good agreements were observed between the results of both methods. The results of BA optimization technique were also provided. Results indicated that a decrease in orientation of fiber angles in the laminated adherends would decrease the maximum shear stress in the adhesive layer (and vice versa). Additionally, to decrease the maximum peeling stress, it is crucial that the outer laminae in each adherend layer have a low fiber angle. However, an increase in the fiber angles will increase the maximum peeling stress in the adhesive, causing possible damage to the joint. The presence of high fiber angles in the outer laminae of each adherend layer (away from adhesive-adherend interface) will reduce the peeling stress significantly. However, the presence of such laminae adjacent to the adhesive-adherend interface produces lower shear stress in the adhesive layer.

An improved four-parameter model on stress analysis of adhesive layer in plated beam

The prediction of stresses in an adhesive layer is helpful in revealing the mechanism of debonding failure in plated beams. This study proposes an improved analytical model for the stress analysis of an adhesive layer in a plated beam. The beam and the soffit plate are individually modelled as a single Timoshenko sub-beam with separate rotations, while the adhesive layer is modelled as a two-dimensional elastic continuum in plane stress, which considers different adherend-adhesive interface stresses. The internal forces of the adhesive layer are assumed to satisfy the Timoshenko beam theory, and the shear deformation and bending moment of the adhesive layer can be considered. The internal forces and displacements of the adhesive layer are fully considered in the displacement compatibility equations, and deformable interfaces are assembled so that the effect of interface stresses on local deformation is captured. Based on equilibrium equations and displacement continuity, the governing differential equations of beam forces are derived, and then the analytical solutions of interface stresses and stresses along the thickness of the adhesive layer are obtained. Comparisons of the results of the finite-element analysis and the existing four-parameter model solutions show that the present model is reasonable. The influence of adhesive thickness on stress distributions in adhesive layers is also investigated.

Theoretical model of adhesively bonded single lap joints with functionally graded adherends

Adhesively bonded joints with functionally graded (FG) adherends are of practical significance since tailoring material composition through the adherend thickness can lead to more uniform shear or peeling stress distributions in the adherends and the adhesive layer near the edges of the joint. Stresses at the free edges of the adhesive layer have been found to be critical to the integrity of the joint. To this end, an analytical model is proposed for an adhesively bonded single lap joint with FG adherends. In this model, the adhesive layer is modeled as a three parameter, elastic foundation, allowing for different peel stress values at the two adherend-adhesive interfaces. Closed-form expressions for interface stresses and internal forces in the adherends are obtained. The model is validated by its agreement with finite element analysis simulations. This model shows that the peel stresses are critical at the left edge of the upper adherend-adhesive interface and at the right edge of the lower adherend–adhesive interface, suggesting that the joint is vulnerable to delaminations along the upper adherend-adhesive interface at the left edge and along the lower adherend-adhesive interface at the right edge. Parametric studies reveal the effects of adhesive thickness, adhesive stiffness, and FGM configuration on the stresses within the single lap joint. Results show that stress concentrations can be reduced near the edges of the joint by increasing the thickness of the adhesive layer, reducing the Young’s modulus of the adhesive layer, and/or configuring the FG adherends so that the stiffer material is nearest the adhesive layer.

Analysis of composite single lap joints using numerical and experimental approach

Adhesives are widely used to execute the assembly of aerospace and automotive structures due to their ability to join dissimilar materials, reduced stress concentration, and improved fatigue resistance. The mechanical behavior of adhesive joints can be studied either using analytical models or by conducting mechanical tests. However, the complexity owing to multiple interfaces, layers with different properties, material and geometric nonlinearity and its three-dimensional nature combine to increase the difficulty in obtaining an overall system of governing equations to predict the joint behavior. On the other hand, experiments are often time consuming and expensive due to a number of parameters involved. Finite element analysis (FEA) is profoundly used in recent years to overcome these limitations. The work presented in this paper involves the finite element modeling and analysis of a composite single lap joint where the adhesive–adherend interface region was modeled using connector elements. The computed stresses were compared with the experimental stresses obtained using digital image correlation technique. The results showed an agreement. Further, the failure load predicted using FEA was found to be closer to the actual failure load obtained by mechanical tests.

Adhesively bonded lap joints with extreme interface geometry

The role of adhesive–adherend interface morphology (through intentional deviation from a flat joint plane) on the mechanical behavior of adhesively bonded lap joints is studied. Two mirror-image types of joints with a zigzag interface containing ‘positive and negative’ interlocking teeth were fabricated and their tensile behavior was measured and compared to the response of a standard flat joint. Numerical simulations were used to explore the role of tooth height and width on the stress distribution in the adhesive, and on crack propagation and arrest after initial fracture. The data suggest that stress distribution along the bond line – and thus, the initial fracture load of the joint – is altered considerably by the positive and negative interlocking teeth. The tendency of a crack to either propagate along the bond or to arrest also depends strongly on morphological details. When crack arrests, the bonded joint can sustain a higher load and thus benefits from some of the intrinsic properties of the adherends (e.g. the plasticity of metal adherends) to enhance energy absorption and toughness. Our findings provide insight for the development of robust multi-material and multi-component structural systems with tailorable properties, and for understanding the role of interface morphology in some biological systems.

Recent Advances in Synthesis and Characterization of Adhesives

Recent Advances in Synthesis and Characterization of Adhesives

Static and fatigue failure analysis of adhesively bonded thick composite single lap joints

Static and fatigue behavior of thick composite laminate single lap joints with thick adhesive bondline is experimentally and numerically investigated. Stress based analysis is used to understand the uniaxial tensile behavior of thick glass fiber reinforced epoxy laminates bonded with relatively thick epoxy adhesive. The effect of adhesive thickness on relatively thick adherend on the fatigue initiation life was modeled with generalized stress singularity approach. It was observed that the failure onset always occurred at adherend–adhesive interface. Subsequent crack propagation mostly resulted in interlaminar failure of adjacent first angle ply near to this interface. Both static and fatigue strength values decrease with increase of bondline thickness. Evaluation of generalized stress intensity factors of the corner geometry (square edge joint) provided a good correlation between predictions and experimental measurements.

Shear stress distribution in adhesive layers of a double-lap joint with void or bond separation

In this work, stress distribution in adhesive layers of a double-lap joint subjected to tension and suffering from a void or a partial debond at the adhesive–adherend interface is examined. For symmetric voids, the deduced equilibrium equations are decoupled for better application of boundary conditions at the extreme ends of each adhesive layer. The proposed method of solution has resulted in better estimates on peak shear stress developed in the adhesive layers. The results based on analytical solution are compared with those of finite element findings. Very good agreement is observed between the two. The major difference between stresses stemming from debonds and voids occurs at the edge of the large size defects. For small central defects, it is hardly discernible by the stresses to distinguish the type of defect. Moreover, there appears to be an optimum length to thickness ratio for each adhesive layer which produces minimum peak interfacial shear stress. This value seems to be a function of defect size and location. A double-lap joint shows to experience smaller interfacial shear stresses due to a single void or debond in comparison with a single-lap joint with a similar defect. The peak interfacial shear stress in a double-lap joint suffering from symmetric voids or debonds is still lower than that of a single-lap joint with a single defect of the same size and location.

Microscopic analysis of the wood bond line using liquefied wood as adhesive

The bonding of beech (Fagus sylvatica L.) with liquefied wood (LW) causes deterioration of the wood surface, resulting in a high percentage of wood failure at a relatively low bond shear strength. Light microscopy, scanning electron microscopy, FT-IR micro-spectroscopy and elemental carbon, nitrogen and sulphur (CNS) analysis techniques were used to investigate the formation of such bonds. It was assumed that the degradation of lignin, hemicelluloses and parts of the cellulose occurred in the cells of the wood surface where the LW had been applied. At the elevated temperatures occurring during the bonding process, the deteriorated cells were carbonised to some extent. The weak boundary layer of the bond was determined to be a layer of delignified cells located between the zone of partly carbonised cells on the one side and the cells of the undamaged wood of the adherend on the other side. The bonds which formed during the bonding of wood with LW were found to be very untypical compared to bonds formed by synthetic wood adhesives. No adhesive film was formed, the adhesive-adherend interface was not clear and the cells of the adherend subsurface were damaged.

An Analytical Study on Peeling of an Adhesively Bonded Joint Based on the Timoshenko Beam Theory

Peeling of a thin film adhesively bonded to a rigid substrate is analytically studied using the Timoshenko beam theory and a trapezoidal cohesive zone model. The film (adherend) is modeled as a Timoshenko beam on an elastic foundation, and the interaction between the adhesive and the adherend is described by a trapezoidal cohesive zone model. Three cases are considered: (1) the normal stress acting alone (mode I loading), (2) only the shear stress present (mode II loading), and (3) the normal and shear stresses co-existing (mixed mode loading) on the adhesive-adherend interface. The governing equations are derived in terms of the displacements and rotation angle of the adherend centerline in each case, which are subsequently solved to obtain closed-form solutions. These governing equations reduce to those provided in an existing model based on the Bernoulli-Euler beam theory when the normality assumption is reinstated. By applying the newly derived solutions directly, sample cases are analyzed to show the trends of the displacements and rotation angle for selected geometrical and loading conditions. The numerical results are also compared with those predicted by the existing model.