Modelling Environmental Degradation in EA9321-Bonded Joints using a Progressive Damage Failure Model

A progressive cohesive failure model has been proposed to predict the residual strength of adhesively bonded joints using a moisture-dependent critical equivalent plastic strain for the adhesive. Joints bonded with a ductile adhesive (EA9321) were studied for a range of environmental degradations. A single, moisture-dependent failure parameter, the critical strain, was calibrated using an aged, mixed-mode flexure (MMF) test. The mesh dependence of this parameter was also investigated. The parameter was then used without further modification to model failure in aluminum and composite single-lap joints (SLJ) bonded with the same adhesive. The FEA package ABAQUS was used to implement the coupled mechanical-diffusion analyses required. The elastic-plastic response of the adhesive and the substrates, both obtained from the bulk tensile tests, were incorporated. Both two-dimensional and three-dimensional modelling was undertaken and the results compared. The predicted joint residual strengths agreed well with the corresponding experimental data, and the damage propagation pattern in the adhesive was also predicted correctly. This cohesive failure model provides a simple but reliable method to model environmental degradation in ductile adhesive bonded joints, where failure is predominantly within the adhesive layer.     

Modelling Environmental Degradation in EA9321-Bonded Joints using a Progressive Damage Failure Model

A progressive cohesive failure model has been proposed to predict the residual strength of adhesively bonded joints using a moisture-dependent critical equivalent plastic strain for the adhesive. Joints bonded with a ductile adhesive (EA9321) were studied for a range of environmental degradations. A single, moisture-dependent failure parameter, the critical strain, was calibrated using an aged, mixed-mode flexure (MMF) test. The mesh dependence of this parameter was also investigated. The parameter was then used without further modification to model failure in aluminum and composite single-lap joints (SLJ) bonded with the same adhesive. The FEA package ABAQUS was used to implement the coupled mechanical-diffusion analyses required. The elastic–plastic response of the adhesive and the substrates, both obtained from the bulk tensile tests, were incorporated. Both two-dimensional and three-dimensional modelling was undertaken and the results compared. The predicted joint residual strengths agreed well with the corresponding experimental data, and the damage propagation pattern in the adhesive was also predicted correctly. This cohesive failure model provides a simple but reliable method to model environmental degradation in ductile adhesive bonded joints, where failure is predominantly within the adhesive layer.     

Stress analysis on a non-flat zigzag interface bonded joint

This article aims to explain the wide range of joint strengths observed depending on the nature of substrate surface geometry by linking the stress distributions in the adhesive layer to the overall strength of the adhesively bonded single lap joints (SLJs). The key factors to which the adhesion strength of non-flat interfaces is attributed are investigated numerically and experimentally. In an attempt to clarify the effect of mechanical interlock on overall adhesion strength of the adhesive joint, zigzag patterns were fabricated on the bonding surfaces of aluminum substrates. It was found that the local compressive and tensile stresses in the vicinity of the interface are the major cause of the practical enhancement or reduction of the joint strength. A comparison between the experimental results of optimum non-flat and standard SLJs showed a strength improvement of up to 40% compared to the conventional SLJ. According to the results of parametric studies, higher wave heights, lower wavelengths, and thinner bond lines improved the efficiency of the method.     

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.     

Evaluation of Fatigue Damage in Adhesive Bonding: Part 2: Single Lap Joint

The damage parameters for crack initiation in a single lap joint (SLJ) are determined by combining continuous damage mechanics, finite element analysis (FEA) and experimental fatigue data. Even though a SLJ has a simple configuration, the stresses in the adhesive region are quite complex and exhibit multi-axial states. Such a condition leads to the need to introduce a general value for the triaxiality function in the damage evolution law rather than using a triaxiality function which equals unity, as in the case of a uni-axial stress state, e.g., the bulk adhesive test specimen presented in Part 1 of this paper. The effect of stress singularity, due to the presence of corners at edges, also contributes to the complex state of stress and to the variability of the triaxiality function along the adhesive layer in a SLJ. The damage parameters A and β determined in Part 1 for bulk adhesive are now extended to take into account the multi-axial stress state in the adhesive layer, as calculated from FEA.     

Mechanical behavior of interlocking multi-stepped double scarf adhesive joints including void and disbond effects

Surface topographical effects on the mechanical behavior of interlocking multi-stepped double scarf adhesive joints under tensile load were studied. For this purpose, finite element analysis (FEA) of the joint geometry at 10 different step angles was carried out. In the second stage, the effects of substrate voids and adhesive delaminations on the interfacial strength were studied for the scarf angle of 32.2° by FEA simulation as well as experimentally. For the cases of the missing steps (voids) and delamination (absence of bonding induced by release agent) the ratios of maximum stresses (principal, von Mises, normal, shear and transverse) between the completely bonded and altered (void or delaminated) joints were compared with the failure load ratios for the same joints to interpret the mechanism of failure. The results revealed that except for the normal stress, the maximum stress ratios reach a maximum value and then decrease with increasing scarf angle. FEA analysis with the voids showed that the strength of the joint not only depends on their size, but also on their location in the joint. When the experimental results were compared with the FEA using the stress ratio between the unmodified (completely bonded) and modified (void or disbond) cases, the results indicated that the normal stress dominates the failure behavior of the 32.2° scarf angle joint. Comparison of the experimental results for the void, and disbond cases revealed that the disbond cases can possess higher joint strength in comparison to the void cases. This finding could not be predicted by FEA, and was attributed to the presence of friction at the interface subsequent to delamination.     

Adhesion failure propagation in adhesively-bonded single-lap laminated FRP composite joints

This paper deals with three-dimensional non-linear finite element analyses to study the behaviour of embedded adhesion failure propagation in adhesively-bonded single-lap laminated FRP composite joints clamped at one end and subjected to uniform extension at the other end. Because of loading eccentricity and joint material heterogeneity, the embedded adhesion failure is likely to initiate from the stress singularity points and will propagate from either end of the adhesive layer along the adherend–adhesive interfaces. The effects of interaction of such failures and their propagations along the interfaces of the adherends and adhesive are the main concerns of this paper. The peel and shear stresses have been computed along the mid-surface of the adhesive layer for varying adhesion failure lengths to find out the influence of adhesion failure length on the strength of the joint being analyzed. The concept of fracture mechanics has been used to calculate the strain energy release rate (SERR) as the adhesion failure propagates using the virtual crack closure technique (VCCT). It is seen that mode-II SERR is predominant in the propagation of such adhesion failures. The SERR values computed with respect to the adhesion failure lengths being propagated from the two ends of the adhesive layer are seen to be different.     

Experimental investigation on environmental degradation of automotive mixed-adhesive joints

In this paper, environmental strength degradation of 180 different adhesive single lap joints (SLJ), including mono-adhesive Araldite 2015, mono-adhesive Araldite AV138, and a mixed-adhesive of Araldite 2015 and Araldite AV138 subjected to moist conditions are experimentally studied. Four different moist conditions, i.e. dry, 75.3, 84.2 RH% and immersion in tap water, have been taken into consideration and the specimens are tested after exposing to these environments at room temperature for 0, 35, 80 and 270 days. The specimens have been tested in two different strain rate, i.e. 1 mm/min and 100 mm/min. The results reveal that although, in a dry environment, mixed-adhesive joints have higher failure loads in comparison to mono-adhesive SLJs, in a moist environment, they have the highest reduction in static failure load with regard to the mono-adhesive ones. Moreover, despite the finally brittle trend in failure load, mixed-adhesives manifest a behavior very similar to ductile mono-adhesives regarding elongation. Analytical predictions of failure load are also consistent with the experimental observations in dry condition.     

Single and dual adhesive bond strength analysis of single lap joint between dissimilar adherends

The emerging trends for joining of aircraft structural parts made up of different materials are essential for structural optimization. Adhesively bonded joints are widely used in the aircraft structural constructions for joining of the similar and dissimilar materials. The bond strength mainly depends on the type of adhesive and its properties. Dual adhesive bonded single lap joint concept is preferred where there is large difference in properties of the two dissimilar adherends and demanding environmental conditions. In this work, Araldite-2015 ductile and AV138 brittle adhesives have been used separately between the dissimilar adherends such as, CFRP and aluminium adherends. In the dual adhesive case, the ductile adhesive Araldite-2015 has been used at the ends of the overlap because of high shear and peel strength, whereas in the middle of the bonded region the brittle adhesive AV138 has been used at different dimensions. The bond strength and corresponding failure patterns have been evaluated. The Digital Image Correlation (DIC) method has been used to monitor the relative displacements between the dissimilar adherends. Finite element analysis (FEA) has been carried-out using ABAQUS software. The variation of peel and shear stresses along the single and dual adhesive bond length have been captured. Comparison of experimental and numerical studies have been carried-out and the results of numerical values are closely matching with the experimental values. From the studies it is found that, the use of dual adhesive helps in increasing the bond strength.     

The effects of overlap length and adherend thickness on the strength of adhesively bonded joints subjected to bending moment

In this work, elasto-plastic stress analysis of a Single Lap Joint (SLJ) subjected to bending moment was investigated using 2D non-linear Finite Element Analysis (FEA). The SLJs, consisting of hardened steel as the adherend bonded by two adhesives, one stiff and one flexible, with very different mechanical behaviors were analyzed. In order to determine the effect of geometrical parameters on the performance of the SLJs, four different adherend thicknesses and overlap lengths for each adhesive were used. For verification of the analysis, the FEA results were compared with experimental results. It was observed that there was a significant effect of adherend thickness on the strength of the joint with both adhesives. However, the load carried by the SLJ with the flexible adhesive increased with increasing overlap length.     

Synergistic Toughening of Epoxy–Copper Interface Using a Thiol-Based Coupling Layer

A novel concept of tuning the fracture properties of the interface through the treatment process of the coupling layer according to the cohesive critical strain energy release rate of the epoxy is proposed for optimizing the joint strength between epoxy and copper substrate. In most coupling agent application recipes, the treatment condition design has omitted the influence of the fracture properties of the corresponding adhesive. Conceivably, excessive strengthening of the adhesive–substrate interface may not lead to optimal interfacial strength. Synergistic toughening of the interface takes place when there is simultaneous interfacial debonding and failure of adhesive under a comparable critical stress state. Under critical applied load, energy is concurrently dissipated through the fracture of the interface, the fracture in the adhesive, and possible non-reversible failure processes such as shear yielding or micro-cracking of the adhesive. These combined energy dissipation processes result in extensive energy absorption around the crack tip. The adhesive joint, therefore, becomes more crack resistant. In this study, the interfacial adhesion promotion concept with synergistic toughening was demonstrated using three different epoxy systems bonded to copper substrates modified by a thiol-based coupling layer. The coupling layer was formed by treating the copper substrate with a thiol-based coupling agent. Critical strain energy release rate of the treated tapered double cantilever beam samples in different treatment conditions was measured for each of the epoxy systems. From the failure path analysis, mixed interfacial and cohesive failure was observed. This observation indicated that extensive energy dissipation occurs around the crack tip that results in synergistic toughening of the interface. This work shows the significance of matching the fracture property of the coupling layer with the adhesive. Up to 2.3 times improvement in the critical strain energy release rate was achieved with optimized thiol treatment compared to non-optimized treatment.     

The use of flexible interface theory for fully coupled nonlinear analysis of balanced adhesive single-lap joint

A theoretical model is presented for determining the edge moment factors, the transverse deflections and the interfacial stresses of the balanced adhesive single-lap joint (SLJ). Based on the flexible interface theory, the improved one-dimensional beam model incorporates simultaneously the effects of interfacial compliances, the overlap geometric nonlinearity and the transverse shear deformations for the adherends. On the basis of normal and tangential displacement compatibility condition for the flexible interface, two sets of fully coupled governing equations concerning rotation of transverse normal and longitudinal displacement of adherends are constructed, from which the improved solutions for the edge moment factors, the transverse deflections, and the interfacial stresses can be obtained. The applicability and accuracy of the improved one-dimensional beam model are validated by comparing the present solutions with the results of the classical model, non-linear finite element analysis, and experimental results. Finally, the effects of the interface compliances on the adhesive stresses distributions of the balanced SLJ are studied.     

Numerical analysis of the strength and interfacial properties of adhesive joints with graded adherends

The strength and interfacial behavior of single lap joints with graded adherends subjected to uniaxial tensile loading are investigated in the present paper. A bilinear cohesive zone model coupled with the finite element method is adopted to describe the damage and failure process of the adhesive layer. The peak loading, the rotation angle between the overlap of the joint and the horizontal direction, as well as the failure energy are investigated comprehensively. It is interesting to find that adopting different variation law in the graded adherends may result in varying strength of adhesive joints. By means of choosing proper material and geometry parameters of adhesive joints, the peak loading, the rotation angle and the failure energy of joints can be greatly improved. What is more, the strength of the SLJ is found to depend much more on the property of the soft part near the adhesive layer. The results should be helpful to guide the design of novel structures of adhesive joints in present and potential applications.     

Pressure-sensitive adhesion in the blends of poly(N-vinyl pyrrolidone) and poly(ethylene glycol) of disparate chain lengths

Adhesive behavior in blends of high molecular weight poly(N-vinyl pyrrolidone (PVP) with a short-chain, liquid poly(ethylene glycol) (PEG) has been studied using a 180° peel test as a function of PVP-PEG composition and water vapor sorption. Hydrophilic pressure-sensitive adhesives are keenly needed in various fields of contemporary industry and medicine, and the PVP-PEG blends, pressure-sensitive adhesion has been established to appear within a narrow composition range, in the vicinity of 36 wt% PEG, and it is affected by the blend hydration. Both plasticizers, PEG and water, behave as tackifiers (enhancers of adhesion) in the blends with glassy PVP. However, PEP alone is shown to account for the occurrence of adhesion, and the tackifying effect of PEG is appreciably stronger than that of sorbed water. Blend hydration enhances adhesion for the systems that exhibit an apparently adhesive type of debonding from a standard substrate (at PEG content less than 36 wt%), but the same amounts of sorbed water are also capable of depressign adhesion in the PEG-overloaded blends, where a cohesive mechanism of adhesive joint failure is typical. The PVP-PEG blend with 36% PEG couples both the adhesive and cohesive mechanisms of bond rupture (i.e., the fibrillation of adhesive polymer under debonding force and predominantly adhesive locus of failure). Blend hydration effect on adhesion has been found to be reversible. The micromechanics of adhesive joint failure for PVP-PEG hydrogels involves the fibrillation of adhesive polymer, followed by fibrils stretching and fracturing as their elongation attains 1000-1500%. Peel force to rupture the adhesive bond of PVP-PEG blends increases with increasing size of the tensile deformation zone, increasing cohesive strength of the material, and increasing tensile compliance of the material, obeying the well-known Kaelble equation, derived originally for conventional rubbery pressure-sensitive adhesives. The major deformation mode upon peeling the PVP-PEG adhesive from a standard substrate is extension, and direct correlations have been established between the composition behaviour of peel strength and that of the total work of viscoelastic strain to break the PVP-PEG films under uniaxial drawing. As a result of strong interfacial interaction with the PET backing film, the PVP-PEG adhesive has a heterogeneous two-layer structure, where different layers demonstrate dissimilar adhesive characteristics.     

Effect of Adherend Recessing on the Tensile Strength of Single Lap Joints

Bonded joints are gaining importance in many fields of manufacturing owing to a significant number of advantages to the traditional methods. The single lap joint (SLJ) is the most commonly used method. The use of material or geometric changes in SLJ reduces peel and shear peak stresses at the damage initiation sites. In this work, the effect of adherend recessing at the overlap edges on the tensile strength of SLJ, bonded with a brittle adhesive, was experimentally and numerically studied. The recess dimensions (length and depth) were optimized for different values of overlap length (L _O), thus allowing the maximization of the joint's strength by the reduction of peak stresses at the overlap edges. The effect of recessing was also investigated by a finite element (FE) analysis and cohesive zone modelling (CZM), which allowed characterizing the entire fracture process and provided joint strength predictions. For this purpose, a static FE analysis was performed in ABAQUS® considering geometric nonlinearities. In the end, the experimental and FE results revealed the accuracy of the FE analysis in predicting the strength and also provided some design principles for the strength improvement of SLJ using a relatively simple and straightforward technique.     

Influence of the degree of crosslinking on the stringiness of crosslinked polyacrylic pressure‐sensitive adhesives

The stringiness of crosslinked polyacrylic pressure‐sensitive adhesive (PSA) was observed during 90° peeling under the constant peel load. The random copolymer of butyl acrylate with 5 wt % acrylic acid crosslinked by N,N,N′,N′‐tetraglycidyl‐m‐xylenediamine was used as PSA. All observed stringiness upon peeling was sawtooth‐shaped, but it could be classified into three types dependent on the degree of crosslinking. The typical sawtooth‐shaped stringiness with interfacial failure was observed at the relatively higher crosslinker content ranging from 0.008 to 0.016 chemical equivalents (Eq.), where the PSA has high cohesive strength and low interfacial adhesion. The frame formed at the front end of stringiness at the content ranging from 0.002 to 0.004 Eq. Sufficient interfacial adhesion and deformability generate large internal deformation of the PSA layer. Internal deformation occurred preferentially over peeling as a result of front frame formation. The mode of peeling was changed from cohesive failure to interfacial failure in this range of crosslinker content. The sawtooth‐shaped with cohesive failure was observed at the lower content ranging from 0 to 0.001 Eq. The PSA has high interfacial adhesion and low cohesive strength, and thus exhibited cohesive failure. The PSA after peeling remained in the shape of belts. It was found that the shape of stringiness is strongly dependent on the balance between the interfacial adhesion and the cohesive strength of PSA. When the sawtooth‐shaped stringiness with frame formed, the peeling rate was lowest. This means the peel strength should be the maximum in this shape of stringiness. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40336.     

Investigation of Damage Growth in Single Lap Joints of Composite Laminates

In this paper, growths of different types of failures including adhesion, cohesion and delamination for a single lap joint (SLJ) of composite laminates were investigated using three-dimensional geometrically nonlinear finite element analysis and by adopting a suitable modeling technique. A unique damage modeling method called sub-laminate modeling was employed for the modeling of damages of different failure modes so as to avoid the oscillatory stress and displacement fields around the damage front. The strain energy release rate (SERR) parameter was used for studying the damage growth and the individual and total components of the SERR along the various damage fronts are evaluated using the virtual crack closure technique (VCCT) based on the linear elastic fracture mechanics (LEFM) approach. This study reveals: that the opening mode is the dominant mode of the propagation for the adhesion and delamination damages, while the sliding mode is dominant for the cohesion failure; that the cohesion failure grows at a faster rate than the adhesion failure; and that the delamination front entrapped within the overlap region in the top adherend of the SLJ grows faster when the delamination damages are present simultaneously in both the adherends. This is particularly true when the delamination centers are exactly aligned with the overlap ends of the joint.     

Analysis of the Notched Coating Adhesion Test

An analysis of the notched coating adhesion (NCA) test is presented. This simple adhesion test method is appropriate for measuring the interfacial fracture toughness of some classes of coatings and open-faced adhesive bonds. The NCA specimen consists of a single substrate coated with a thin layer of adhesive. The coating is notched to sever the coating and induce sharp interfacial debonds, and the specimen is then loaded in tension. The substrate strain at which coating debonding occurs is recorded and used to determine the critical strain energy release rate. Yielding of the substrate is permitted, and does not significantly affect the calculation of the strain energy release rate. Analytical and finite element analysis are used to quantify the available strain energy release rate for both steady state and laterally-constrained cases. The available strain energy release rate is shown to be quite insensitive to the initial debond length. The specimen geometry results in a mode mix which causes the adhesive to debond along the interface.     

Comprehensive dynamic failure mechanism of thermal barrier coatings based on a novel crack propagation and TGO growth coupling model

Comprehensive understanding of failure mechanism of thermal barrier coatings (TBCs) is essential to develop the next generation advanced TBCs with longer lifetime. In this study, a novel numerical model coupling crack propagation and thermally grown oxide (TGO) growth is developed. The residual stresses induced in the top coat (TC) and in the TGO are calculated during thermal cycling. The stresses in the TC are used to calculate strain energy release rates (SERRs) for in-plane cracking above the valley of undulation. The overall dynamic failure process, including successive crack propagation, coalescence and spalling, is examined using extended finite element method (XFEM). The results show that the tensile stress in the TC increases continuously with an increase in an undulation amplitude. The SERRs for TC cracks accumulate with cycling, resulting in the propagation of crack toward the TC/TGO interface. The TGO cracks nucleate at the peak of the TGO/bond coat (BC) interface and propagate toward the flank region of the TC/TGO interface. Both TC cracks and TGO cracks successively propagate and finally linkup leading to coating spallation. The propagation and coalescence behavior of cracks predicted by this model are in accordance with the experiment observations. Therefore, this study proposed coating optimization methods towards advanced TBCs with prolonged thermal cyclic lifetime.     

Through-the-width delamination damage propagation characteristics in single-lap laminated FRP composite joints

Three-dimensional non-linear finite element analyses have been carried out to study the effects of through-the-width delaminations on delamination damage propagation characteristics in adhesively bonded single-lap laminated FRP composite joints. The delaminations have been presumed either to pre-exist or to get evolved due to coupled stress failure criteria in the laminated FRP composite adherends near the overlap ends beneath the ply adjacent to the overlap region. The out-of-plane stresses in the adhesive layer, the interlaminar stress distributions along the delamination fronts and the strain energy release rates (SERRs) corresponding to the three individual modes have been evaluated for varying positions of the delaminations pre-embedded in either of the adherends. A good matching between the present 3D results and experimental and analytical solution of the literature has been established for the undamaged and a damaged model. A significant difference in the interlaminar stresses and the SERR values has been observed and is largely dependent on the adherends (bottom or top) possessing the through-the-width delamination damages. Also, the interlaminar stresses and SERR values along the two corresponding delamination fronts are different. Accordingly, it can be concluded that the positions of the through-the-width delaminations significantly influence the delamination damage propagation behaviour vis-a-vis the performance of the composite joint.