The Effect of Lateral Constraint on the Strength of a Single Lap Joint

The mechanical performance of a single lap joint (SLJ) is mainly affected by the lateral normal tensile stresses acting at the edges of its interlaminar adhesive layer (IAL). Owing to these stresses, the delamination failure which initiates at the IAL edges and propagates inward, is predominantly of the peel type. The subject of this study is the effect of constraint of the lateral deflection of adhering edges applied by tightly binding them together.

Experimental results showed that the effect of this type of constraint is a reduction in the extent of peel and an overall increase in the joint tensile strength. This effect is more pronounced in the case of brittle than in the case of ductile adhesives.     

Strap repairs using embedded patches: numerical analysis and experimental results

Adhesively bonded repairs offer an attractive option for repair of aluminium structures, compared to more traditional methods such as fastening or welding. The single-strap (SS) and double-strap (DS) repairs are very straightforward to execute but stresses in the adhesive layer peak at the overlap ends. The DS repair requires both sides of the damaged structures to be reachable for repair, which is often not possible. In strap repairs, with the patches bonded at the outer surfaces, some limitations emerge such as the weight, aerodynamics and aesthetics. To minimize these effects, SS and DS repairs with embedded patches were evaluated in this work, such that the patches are flush with the adherends. For this purpose, in this work standard SS and DS repairs, and also with the patches embedded in the adherends, were tested under tension to allow the optimization of some repair variables such as the overlap length (L_O) and type of adhesive, thus allowing the maximization of the repair strength. The effect of embedding the patch/patches on the fracture modes and failure loads was compared with finite elements (FE) analysis. The FE analysis was performed in ABAQUS® and cohesive zone modelling was used for the simulation of damage onset and growth in the adhesive layer. The comparison with the test data revealed an accurate prediction for all kinds of joints and provided some principles regarding this technique.     

Increasing single-lap joint strength by adherend curvature-induced residual stresses

Single-lap joint (SLJ) geometry is the most widely used type of adhesive joint geometry. In this joint, peel stresses occur at the overlap ends due to load eccentricity and the presence of shear-free adhesive termination surfaces. These peel stresses, along with the transverse tensile stresses which occur along the overlap longitudinal axes, and adhesive shear stresses, ultimately cause joint failure. Obviously, reductions in these stresses should result in higher joint strength and increased load capacity. To this end, we exploited elastic spring-back capability of (steel) metal adherends by initially forming curved segments of varying arc lengths and radii at overlap ends. These adherends with curved-end sections were then bonded in single-lap configuration, simply by applying sufficient bonding pressure to elastically flatten the curved segments to result in typically flat overlap sections subsequent to adhesive cure and the removal of bonding pressure. Since the elastic adherend overlap ends tend to revert back to their initial curved form, they exert compressive residual stresses on the adhesive layer in the overlap end regions. We determined that the compressive residual stresses induced in this fashion considerably increased the load capacity of SLJs subjected to tension.     

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.     

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.     

胶层中间隙长度及位置对接头剪切强度的影响

研究了在单搭接接头上、胶缝中预留的不同长度间隙对接头剪切强度和剪切应力分布的影响。结果表明，随着间隙长度的增加，接头的承栽能力趋于减小，但接头的实际剪切强度却持续上升．当间隙长度再继续增加时，接头的实际强度趋于下降。研究中还发现间隙所处的位置对接头的剪切强度有较大的影响，胶层端部预留间隙使接头的承载能力和实际强度均显著下降。有限元数值分析的结果表明，间隙长度超过某特定值后，胶层中的应力集中系数会急剧上升，间隙位于端部时胶层中的应力集中程度明显高于位于中部处。     

A simple approach for characterizing the performance of metallic tubular adhesively-bonded joints under torsion loading

Adhesive bonding of joints is one of the most commonly and widely used joining methods in piping systems. This work is concerned with the investigation of the influence of the non-linear behavior of the adhesive used in such bonded joints on their performance. The parametric analysis module of ABAQUS was used to model the joint. The model facilitated the analysis of different geometric, loading and material characteristics of the system, in particular the adhesive nonlinearity, which is of prime interest in this work. By using the Ramberg–Osgood plasticity model, the failure threshold of the adhesive for various joint lengths (hereafter referred to overlap length) was characterized. The plasticity model used in this study was fine-tuned using only a limited number of known parameters, through comparison with the results of the finite element (FE) simulation. The results obtained from the FE analysis were verified by experimental results. The FE strategy is demonstrated to be an effective means for predicting the capacity of such joints, where conducting a pure shear test is either impossible or difficult to accomplish. Contrary to the findings based on the elastic finite element analysis, the plasticity analysis revealed that the overlap length affects the ultimate strength of the joint.     

间隙长度与间隙位置对单搭接接头强度的影响

研究了间隙长度和间隙位置对不同单搭接接头强度的影响。结果表明,随着间隙长度的增加,接头的承载能力趋于下降,但在适当的条件下,间隙连接接头并没有大幅度地减小其强度。研究中发现间隙所处的位置对接头的拉剪强度有较大的影响,间隙位置对接头实际剪切强度的影响大于间隙长度的影响。文中还论述了同一长度间隙位于不同位置时对强度的影响。     

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.     

Thermal residual stresses in an adhesively-bonded functionally graded single-lap joint

This study investigates three-dimensional thermal residual stresses occurring in an adhesively-bonded functionally graded single-lap joint subjected to a uniform cooling. The adherends are composed of a through-the-thickness functionally graded region between Al₂O₃ ceramic and Ni metal layers. Their mechanical properties were calculated using a power law for the volume fraction of the metal phase and a 3D layered finite element was implemented. In a free single-lap joint the normal stress σ_xx was dominant through the overlap region of the upper and lower adherends and along the adhesive free edges, whereas the transverse shear stress σ_xy concentrations appeared only along the free edges. The peel stress σ_yy and the transverse shear stress σ_xy became dominant along the free edges of the adhesive layer. In addition, the von Mises stress decreased uniformly through the adherend thickness from compressive in the top ceramic-rich layer to tensile in the bottom metal-rich layer. In addition, the layer number had only a minor effect on the through-the-thickness stress profiles after a layer number of 50, except for the peak stress values in the ceramic layer. In a single-lap joint fixed at two edges both adherends underwent considerable normal stress σ_xx concentrations varying from compressive in the top ceramic-rich layer to tensile in the bottom metal-rich layer along the free edges of both adherend–adhesive interfaces, whereas the peel stress σ_yy and transverse shear stress σ_xy reached peak levels along the left and right free edges of the adhesive layer. The layer number and the compositional gradient exponent had only minor effects on the through-the-thickness von Mises stress profiles but considerably affected the peak stress levels. The free edges of adhesive–adherend interfaces and the corresponding adherend regions are the most critical regions, and the adherend edge conditions play more important role in the critical adherend and adhesive stresses. Therefore, the first initiation of the joint failure can be expected along the left and right free edges of the upper and lower adherend–adhesive interfaces.     

Elastic stresses in an adhesively-bonded functionally-graded tubular single-lap joint in tension

In this study, the elastic stress analysis of an adhesively-bonded tubular lap joint with functionally-graded Ni-Al₂O₃ adherends in tension was carried out using a 3D 8-node isoparametric multilayered finite element with 3 degrees-of-freedom at each node. Stress concentrations were observed along the edges of both outer and inner tubes in the overlap region. Thus, the outer tube region near the free edge of the inner tube and the inner tube region near the free edge of the outer tube experienced considerable stress concentrations. Normal σ_zz and shear σ_rz stresses were dominant among the stress components. In addition, both edges of the adhesive layer experience stress concentrations, and the von Mises σ _eqv stress decreases uniformly across the adhesive thickness at the free edge of the outer tube, whereas it increases at the free edge of the inner tube. However, different compositional gradients had only a small effect on the through-the-thickness normal and shear stress profiles of both outer and inner tubes, and the peak von Mises σ _eqv stresses occurred inside the tube walls. As the ceramic phase in the material composition of the outer and inner tubes was increased, peak von Mises σ _eqv stress appeared in the ceramic layer. However, its magnitude was increased 1.75-fold in both tubes. In addition, the peak adhesive stresses appeared at the edge of the outer tube–adhesive interface near the free edge of the inner tube and at the edge of the inner tube–adhesive interface near the free edge of the outer tube. Increasing the ceramic phase in the material composition caused 1.22–1.67-times higher von Mises stresses along the free edges of the adhesivetube interfaces. In addition, with increasing number of layers across the inner and outer tubes the profiles of the normal σ_zz , shear σ_r and von Mises σ _eqv stresses across the tube walls and adhesive layer become similar. Increasing the ceramic phase in the material composition of the tubes causes also evident increases in the normal σ_zz and von Mises stresses while it does not affect their through-the-thickness profiles. However, it affects only shear σ_r and von Mises stresses across the adhesive layer. Finally, the layer number and the compositional gradient do not affect considerably through-the-thickness normal and shear stress profiles but levels in a functionally graded plate subjected to structural loads.     

Non-linear stress and failure analyses of adhesively-bonded joints subjected to a bending moment

In this paper, the mechanical behavior of the Single-Lap Joints (SLJs) bonded with two different adhesives (FM 73 and SBT 9244) under a bending moment was analyzed, both experimentally and numerically. Four-point bending experiments for the joints with different overlap lengths were carried out and fracture surfaces of the SLJs were examined with a Scanning Electron Microscope (SEM). After the stress analysis in the SLJs was performed via a finite element method by considering the material non-linearities of the adhesives and adherend (AA2024-T3), the Finite Element Analysis (FEA) results were compared with experimental results. Finally, the stress analyses and experimental results show that the failure in the SLJs subjected to a bending moment probably initiates from the overlap region on the adhesive–upper adherend interface in tension and propagates towards the centre of the overlap. Also, in the joint subjected to a bending moment, it is seen that the load carried by the SLJ with SBT 9244 adhesive with increasing overlap length is more than that of the SLJ with FM 73 adhesive, although in the bulk form FM 73 adhesive is about three times stronger than SBT 9244 adhesive.     

Effects of Different Curvature Patches on the Strength of Double-Strap Adhesive Joints

In general, the damage in adhesively bonded joints initiates from and propagates through the ends of the overlap area due to high stress concentration in that area. The reduction of these stress concentrations results in an increase in the strength of the joints. For this reason, the rounding of the overlap region before bonding and then applying compression during the bonding process will exert compressive residual stresses on the adhesive layer in the overlap end regions. It is known that the residual stresses formed in this process increase the failure strength of the joint and hence delay the initiation of the damage.

In this study, the effects of overlap length (L = 50,75, and 100 mm), patch thickness (h = 1.6, 3.2, and 5 mm) and patch materials (AA2024 aluminum alloy, AISI 304 steel, AISI 1040 steel) on bond strength were experimentally investigated for adhesively bonded double-strap joint (DSJ) and curvature double-strap joint (CDSJ) subjected totensile loading. The experimental study showed that the overlap length, patch thickness and patch materials have considerable influence on the failure strength and displacement capacity of the joints.     

Mechanical characterization of a high elongation and high toughness epoxy adhesive

In this paper, a new epoxy adhesive has been mechanically characterized. The adhesive combines the properties of an epoxy adhesive and typical polyurethane (PU) adhesive, such as high elongation and high toughness. Experimental tests were performed to measure the tensile properties, shear properties, thermal properties and fracture properties. The tensile test shows high tensile strength and high elongation. The single lap joint (SLJ) test shows that the failure load is proportional to the overlap length for hard steel adherends. For the SLJs with mild steel adherends, the failure occurred due to adherend yielding. Impact tests were conducted using SLJ specimens and the results are consistent with the SLJ tested under static conditions. The Tg was obtained using a Dynamic Mechanical Analysis (DMA) type of test. The toughness in mode I was determined using the Double Cantilever Beam (DCB) test and the toughness in mode II using End Notched Flexure (ENF) test.     

Influence of the Spew Fillet and other Parameters on the Stress Distribution in the Single Lap Joint

Even the most recent closed form analyses of single lap joints assume that the adhesive terminates in a square end. In practice a fillet of adhesive (hereafter called the spew) usually forms at the overlap ends. This spew can considerably reduce peak adhesive stresses and so strengthen the joint. An investigation has been made into the role of the spew for a wide range of joint parameters. The stress distribution across the adhesive thickness was also considered, and was found to be essentially uniform over a large part of the overlap length. However, near the overlap end, the stress variation across the thickness can be high, resulting in higher stresses and so lower strengths than would be expected considering average stress levels in the joint, but even after including the effect of this variation the maximum adhesive stresses have usually been found to be considerably lower than corresponding peak values predicted by closed form analysis of square ended joints.     

Progressive failure and experimental study of adhesively bonded composite single-lap joints subjected to axial tensile loads

Experimental tests and finite element method (FEM) simulation were implemented to investigate T700/TDE86 composite laminate single-lap joints with different adhesive overlap areas and adherend laminate thickness. Three-dimensional finite element models of the joints having various overlap experimental parameters have been established. The damage initiation and progressive evolution of the laminates were predicted based on Hashin criterion and continuum damage mechanics. The delamination of the laminates and the failure of the adhesive were simulated by cohesive zone model. The simulation results agree well with the experimental results, proving the applicability of FEM. Damage contours and stress distribution analysis of the joints show that the failure modes of single-lap joints are related to various adhesive areas and adherend thickness. The minimum strength of the lap with defective adhesive layer was obtained, but the influence of the adhesive with defect zone on lap strength was not decisive. Moreover, the adhesive with spew-fillets can enhance the lap strength of joint. The shear and normal stress concentrations are severe at the ends of single-lap joints, and are the initiation of the failure. Analysis of the stress distribution of SL-2-0.2-P/D/S joints indicates that the maximum normal and shear stresses of the adhesive layer emerge on the overlap ends along the adhesive length. However, for the SL-2-0.2-D joint, the maximum normal stress emerges at the adjacent middle position of the defect zone along the adhesive width; for the SL-2-0.2-S joint, the maximum normal stress and shear stress emerge on both edges along the adhesive width.     

Effect of hygrothermal conditioning on the energy release rate and failure mechanism of metal only adherend-glass fibre prepreg co-cured single lap joints

Abstract

Fibre-reinforced composite materials are extensively used in repair and rehabilitation of oil and gas metal infrastructures which are largely exposed to water and hydrocarbon. An important aspect to this is applying adequate surface preparation to the metal to ensure a durable bond between the composite and metal substrate. In this paper, mild steel surface was prepared using grit blasting and single lap joint (SLJ) test specimens were manufactured and tested to investigate the adhesion in terms of total energy release rate (G_T) of the interface between mild steel adherend and glass fibre prepreg. An out-of-water usable epoxy resin primer was incorporated to join mild steel adherend with glass fibre prepreg by curing at a temperature of 55 °C for 48 h. Upon durability testing of the SLJ specimens using hygrothermal conditioning at a temperature of 55 °C for 1000 h, the experimental G_T values were seen to reduce significantly. Comparatively lower amount of cohesive failure and increased amount of swelling or delamination of the adhesive was observed for conditioned SLJ specimens when compared to controlled SLJ specimens. Furthermore, the experimental G_T values were found to correlate well with an analytical adhesive interface model.     

On the von Mises elastic stress evaluations in the bi-adhesive single-lap joint: a numerical and analytical study

Bi-adhesive joints are an alternative stress-reduction technique for adhesively bonded joints. The joints have two types of adhesives in the overlap region. The stiff adhesive should be located in the middle and the flexible adhesive at the ends. This study is the extension of our previous paper to the von Mises stress evaluation and discusses the values and importance of the von Mises stresses in the bi-adhesive single-lap joint. Both analytical and numerical analyses were performed using three different bi-adhesive bondline configurations. The Zhao’s closed form (analytic) solution used includes the bending moment effect. In the finite element models, overlap surfaces of the adherends and the adhesives were modeled using surface-to-surface contact elements. The contribution levels of the peel and shear stresses for producing a peak von Mises stress are also studied. It is concluded that the contribution level of the shear stress at where von Mises stress becomes peak is more than that of the peel stress. Joint strength analyses were performed based on the peak elastic von Mises stresses. It is seen that joint strength can be increased using bi-adhesive bondline. The analytical and numerical results show that the appropriate bond-length ratio must be used to obtain high joint strength.     

Geometrically non-linear analysis of an adhesively bonded modified double containment corner joint — I

In this study, the geometrically non-linear analysis of an adhesively modified double containment corner joint was carried out using the incremental finite element method based on the small strain-large displacement (SSLD) theory. The plates, support, and adhesive layers were assumed to have linear elastic properties. The joint was analysed for two different loading conditions: one normal loading to the horizontal plate plane P _y and one horizontal loading at the horizontal plate free edge P _x . In addition, the small strain-small displacement (SSSD) analysis of this adhesive joint was also carried out in order to compare the capability of the two theories in predicting the effect of large displacements on the stress and deformation states of the joint members. Both analyses showed that stress and strain concentrations occurred around the adhesive free ends, corresponding to the vertical and horizontal slot free ends, and along the outer fibres of the horizontal and vertical plates. The peak stresses appeared at the slot corners inside the adhesive fillets and at the horizontal and vertical plate outer fibres corresponding to the two slot free ends. The variations of the Von Mises stresses at these critical adhesive and plate locations were evaluated versus increasing loads. The SSLD theory predicted an evident non-linear effect, as a result of the large displacements, on the stress variations for the loading P _x , whereas this non-linear effect disappeared on the stresses for the loading P _y ; thus, the stresses presented very close variations to those obtained by the SSSD theory. However, the SSSD theory predicted a lower stress variation proportional to the increasing load for both loading conditions. In the case of the loading P _y , the right vertical adhesive fillet and both plates appeared as the most critical joint regions, whereas the lower horizontal fillet and both plates were determined as the most critical regions for the loading P _x . The behaviour of all joint members towards the applied load is strictly dependent on the boundary and loading conditions. Finally, the SSSD theory may be misleading in the prediction of the stress and deformations, but the SSLD theory includes the non-linear effect of the large displacements and rotations and gives more realistic results, although it requires more computational effort. In addition, it was observed that the geometrical parameters, such as the support length, vertical support length, and vertical slot depth, had a considerable effect on the peak adhesive and plate stresses, depending on the loading condition.     

Optimization study of hybrid spot-welded/bonded single-lap joints

Joining of components with structural adhesives is currently one of the most widespread techniques for advanced structures (e.g., aerospace or aeronautical). Adhesive bonding does not involve drilling operations and it distributes the load over a larger area than mechanical joints. However, peak stresses tend to develop near the overlap edges because of differential straining of the adherends and load asymmetry. As a result, premature failures can be expected, especially for brittle adhesives. Moreover, bonded joints are very sensitive to the surface treatment of the material, service temperature, humidity and ageing. To surpass these limitations, the combination of adhesive bonding with spot-welding is a choice to be considered, adding a few advantages like superior static strength and stiffness, higher peeling and fatigue strength and easier fabrication, as fixtures during the adhesive curing are not needed. The experimental and numerical study presented here evaluates hybrid spot-welded/bonded single-lap joints in comparison with the purely spot-welded and bonded equivalents. A parametric study on the overlap length (L_O) allowed achieving different strength advantages, up to 58% compared to spot-welded joints and 24% over bonded joints. The Finite Element Method (FEM) and Cohesive Zone Models (CZM) for damage growth were also tested in Abaqus^® to evaluate this technique for strength prediction, showing accurate estimations for all kinds of joints.