Accuracy of cohesive laws with different shape for the shear behaviour prediction of bonded joints

The use of adhesive bonding as a joining technique is increasingly being used in many industries because of its convenience and high efficiency. Cohesive Zone Models (CZM) are a powerful tool for the strength prediction of bonded joints, but they require an accurate estimation of the tensile and shear cohesive laws of the adhesive layer. This work evaluated the shear fracture toughness (J_IIC) and CZM laws of bonded joints for three adhesives with distinct ductility. The End-Notched Flexure (ENF) test geometry was used. The experimental work consisted of the shear fracture characterization of the bond by the J-integral. Additionally, by this technique, the precise shape of the cohesive law was defined. For the J-integral, digital image correlation was used for the evaluation of the adhesive layer shear displacement at the crack tip during the test, coupled to a Matlab sub-routine for extraction of this parameter automatically. Finite Element Method (FEM) simulations were carried out in Abaqus® to assess the accuracy of triangular, trapezoidal and linear-exponential CZM laws in predicting the experimental behaviour of the ENF tests. As output of this work, fracture data is provided in shear for the selected adhesives, allowing the subsequent strength prediction of bonded joints.     

Testing different cohesive law shapes to predict damage growth in bonded joints loaded in pure tension

ABSTRACT

Adhesive bonding is a widely used joining method because of specific advantages compared to the traditional fastening methods. Cohesive zone modelling (CZM) is currently the most widely used technique for strength prediction. CZM supposes the characterization of the CZM laws in tension and shear. This work evaluated the tensile fracture toughness (G_IC) and CZM laws of bonded joints with three adhesives by the double-cantilever beam (DCB) test. The experimental work consisted of the adhesives’ tensile fracture characterization by the J-integral technique. As the main novelty of this work, the precise shape of the cohesive law of adhesives ranging from brittle to highly ductile was defined by the direct method, using a digital image correlation method to evaluate the tensile relative displacement (δ_n) of the adhesive layer at the crack tip and adherends’ rotation at the crack tip (?_o). Moreover, finite element (FE) simulations permitted assessing the accuracy of triangular, trapezoidal and linear-exponential CZM laws in predicting the experimental behaviour of the DCB bonded joints with markedly distinct behaviours. As output of this work, fracture data and information regarding the applicability of these CZM laws to each type of adhesive is provided, allowing the subsequent strength prediction of bonded joints.     

单搭接胶接接头的拓扑优化

在有限元方法的基础上，利用变密度法对单搭接胶接接头搭接区域的被粘物形状进行了拓扑优化，通过曲线拟舍得到了较为合理的轮廓。拓扑优化的结果表明：在体积减少20％的情况下。胶接结构的强度不会降低；经拓扑优化后，胶层中剪切应力的峰值比优化以前增加不大，约1％。     

Comparison between the ENF and 4ENF fracture characterization tests to evaluate GIIC of bonded aluminium joints

ABSTRACT

Adhesively bonded joints have been increasingly used in structural applications over mechanical joints. Cohesive Zone Modelling (CZM) is the most widespread technique to predict the strength of these joints, and it uses the tensile fracture toughness (G_IC) and the shear fracture toughness (G_IIC). Different fracture characterization methods are available for shear loadings, among which the End-Notched Flexure (ENF) is undoubtedly the most popular. The 4-Point End-Notched Flexure (4ENF) is also available. This work consists of a detailed comparison between the ENF and 4ENF tests for the experimental estimation of G_IIC of bonded aluminium joints. Three adhesives were used: a strong and brittle (Araldite® AV138), a less strong but with intermediate ductility (Araldite® 2015) and a highly ductile (SikaForce®7752). Different data reduction methods were tested, and the comparison included the load-displacement (P-δ) curves, resistance curves (R-curves) and measured G_IIC. It was found that the ENF test presents a simpler setup and has a higher availability of reliable data reduction methods, one of these not requiring measuring the crack length (a) during its growth. For the 4ENF test, only one test method proved to be accurate, and the test geometry revealed to be highly affected by friction effects.     

Effect of hole drilling at the overlap on the strength of single-lap joints

Bonded unions are gaining importance in many fields of manufacturing owing to a significant number of advantages to the traditional fastening, riveting, bolting and welding techniques. Between the available bonding configurations, the single-lap joint is the most commonly used and studied by the scientific community due to its simplicity, although it endures significant bending due to the non-collinear load path, which negatively affects its load bearing capabilities. The use of material or geometric changes in single-lap joints is widely documented in the literature to reduce this handicap, acting by reduction of peel and shear peak stresses at the damage initiation sites in structures or alterations of the failure mechanism emerging from local modifications. In this work, the effect of hole drilling at the overlap on the strength of single-lap joints was analyzed experimentally with two main purposes: (1) to check whether or not the anchorage effect of the adhesive within the holes is more preponderant than the stress concentrations near the holes, arising from the sharp edges, and modification of the joints straining behaviour (strength improvement or reduction, respectively) and (2) picturing a real scenario on which the components to be bonded are modified by some external factor (e.g. retrofitting of decaying/old-fashioned fastened unions). Tests were made with two adhesives (a brittle and a ductile one) varying the adherend thickness and the number, layout and diameter of the holes. Experimental testing showed that the joints strength never increases from the un-modified condition, showing a varying degree of weakening, depending on the selected adhesive and hole drilling configuration.     

Cyclic creep response of adhesively bonded steel lap joints

The viscoelastic nature of polymeric adhesives means that the effect of fatigue frequency has to be treated cautiously. However, this subject has received limited attention and very few studies can be found. Therefore, this work aims at investigating the cyclic creep response of adhesively bonded steel lap joints. Load-controlled fatigue tests were performed with shear stresses of 9.1, 7.4, and 6.3 MPa, which are typically low cycle fatigue stresses. Only during the last 20% of fatigue life can we observe an increase in the cycle hysteresis area due to the decrease of the shear stiffness caused by the failure mechanisms. Under fatigue load, the maximum/minimum strain curves exhibit a shape being similar to that of the steady creep curves, in which occurs a second stage with nearly constant strain rate, independently of the number of cycles and increasing with the load range. A linear relationship between the log cyclic creep rate and the log of the number of cycles to failure was observed, indicating that fatigue behaviour is strictly related to cyclic creep.     

Experimental investigation on the enhancement of Mode I fracture toughness of adhesive bonded joints by electrospun nanofibers

ABSTRACT

The use of an electrospun nylon nanofibrous mat at the interface between adjacent plies of a composite laminate is a promising mean to improve the delamination strength, as the nanomat acts a reinforcing web enabling a ply-to-ply bridging. This kind of reinforcement can be potentially used in other applications, such as adhesive bonding, where it may also work as adhesive carrier. The present work is therefore aimed at analysing the potential of an electrospun polymeric nanomat as adhesive carrier and reinforcing web in adhesive bonding. The adhesive is used to pre-impregnate a nylon nanofibrous mat that is then placed at the interface between two metal pieces and cured. The effectiveness of this procedure is evaluated by comparing of the mode-I fracture toughness measured 2024-T3 aluminum alloy DCB (Double Cantilever Beam) specimen bonded using a two-part epoxy resin with and without the nanomat.     

Fatigue life and backface strain predictions in adhesively bonded joints

Fatigue is a very important factor in any adhesively bonded structure subject to service loads. Prediction of fatigue life using finite element analysis (FEA) techniques is very complicated due to the complex nature of fatigue damage. This paper presents experimental data obtained by testing single lap joints (SLJs) in constant amplitude fatigue at a range of load levels and associated fatigue damage modelling. Six strain gauges (SGs) placed along the overlap were used to monitor fatigue initiation and propagation within the adhesive layer. An elasto-plastic damage model was developed that was capable of predicting the experimentally observed backface strain patterns and fatigue life at different fatigue loads. It was implemented in the finite element code ABAQUS and used a user defined subroutine to calculate the damage, and the resultant degradation in adhesive Young's modulus and yield stress.     

Elastic analysis and engineering design formulae for bonded joints

The general elastic plane strain problem of adhesively bonded structures which consist of two different adherends is considered. To facilitate a truly general approach the adhesive joint is modelled as an adherend-adhesive sandwich with any combination of tensile, shear and moment loading being applied at the ends of both adherends. A full elastic analysis is presented which calculates the adhesive shear and tensile stresses in the overlap region, this analysis has been validated for a range of load cases using a finite element program. Basic design approaches are outlined and explicit expressions are developed which enable the simple evaluation of the stress distributions in the adhesive overlap. Simplified two parameter design formulae are also produced which accurately describe the peak stresses at the ends of the adhesive overlap in both the transverse and longitudinal shear directions. In all of the analyses the adherends are assumed to behave as linear elastic cylindrically bent plates with the adhesive forming an elastic interlayer between them. In the simplified analyses only one component of adhesive stress is considered, while in the full elastic analysis two components of stress are considered with a consequent increase in the complexity of the required solution method, but also an increase in accuracy over the simplified analyses for a wider range of joint configurations.     

Experimental and numerical analysis of single lap bonded joints: Epoxy and castor oil PU-glass fibre composites

ABSTRACT

Currently, there is a growing concern for the environment. Several studies of new materials to reduce environmental impact have been carried out by different research groups, and many companies have replaced parts made of fossil sources by renewable materials. The use of polyurethane (PU) derived from castor oil as a matrix for composite materials and adhesives is one example. Hence, the present work aims to compare the numerical and experimental analyses of castor oil PU and epoxy resin not only as a matrix of composite materials, but also as an adhesive of bonded joints. The joint coupons were manufactured by using castor oil PU-glass fibre and epoxy-glass fibre as adherents, which were bonded by epoxy or castor oil PU. Thus, four combinations of adherents and adhesives were investigated. Specimens with identical geometry were used in all tests, which were based on guidelines for single lap bonded joints. Computational simulations via Finite Element Method were performed for predictions of the adhesive layer stresses and strength. In addition, a material model is proposed to predict the failure of the adhesive layer. The experimental and numerical results showed that PU derived from castor oil has good mechanical performance, making this material a feasible alternative for bonded joints, mostly nowadays when environment is a major concern.     

The use of expandable graphite as a disbonding agent in structural adhesive joints

ABSTRACT

Expandable graphite (EG) offers the potential to be used as a controllable adhesive disbonding mechanism in an adhesively bonded joint. The graphite particles can be incorporated, in small quantities, into the adhesive layer. Heat can be used to trigger the expansion and thereby facilitate disassembly of structurally bonded assemblies. An experimental programme was developed to investigate the engineering capabilities of different types of bonded joints, made with an epoxy and a polyurethane (PU) adhesive, containing different amounts of EG. The short- and long-term mechanical properties of lap shear and wedge cleavage joints were evaluated. In contrast to the incorporation of more conventional physical and chemical foaming agents into bondlines, typically in excess of a 10% addition level, it was found that the addition of just a few per cent of graphite was sufficient to provide a reliable disbonding mechanism. Moreover, the usual adverse side effects on joint performance, associated with incorporating functional additives, were far more limited.     

Validation of pure tensile and shear cohesive laws obtained by the direct method with single-lap joints

Joining with structural adhesives in the aeronautical industry dates back to some decades, although only more recently this technique has been implemented to load bearing parts in other industries. This technique enables joining steel with aluminium or fibre-reinforced composites, with a major weight advantage. Cohesive Zone Models (CZM) are an accurate design method for bonded structures but, depending on the adhesive type and specimen's geometry, the accuracy of the strength predictions may be highly compromised by the choice of the cohesive laws. This work presents a validation of tensile and shear CZM laws of three adhesives obtained by the direct method applied to Double-Cantilever Beam (DCB) and End-Notched Flexure (ENF) tests, respectively. The validation is carried out by considering a mixed-mode bonded geometry (the single-lap joint) with different overlap lengths (L_O) and adhesives of distinct ductility. Initially, the precise shape of the cohesive law in tension and shear of the adhesives is estimated, followed by their simplification to parameterized triangular, trapezoidal and linear-exponential CZM laws. Validation of the CZM laws was accomplished by direct comparison of the load-displacement (P-δ) curves and maximum load (P_m) of the single-lap joints as a function of the tested L_O values. The strength predictions were accurate for a CZM law shape consistent with the adhesive type, although the differences between CZM shapes were not too significant.     

Finite element analysis of torsional free vibration of adhesively bonded single-lap joints

Adhesively bonding is a high-speed fastening technique which is suitable for joining advanced lightweight sheet materials that are dissimilar, coated and hard to weld. In this paper, the free torsional vibration characteristics of adhesively bonded single-lap joints are investigated in detail using finite element method. The effectiveness of finite element analysis technique used in the study is validated by experimental tests. The focus of the analysis is to reveal the influence on the torsional natural frequencies and mode shapes of these joints caused by variations in the material properties of adhesives. It is shown that the torsional natural frequencies and the torsional natural frequency ratios of the adhesively bonded single-lap joints increases significantly as the Young′s modulus of the adhesives increase, but only slight changes are encountered for variations of Poisson's ratio. The mode shapes analysis show that the adhesive stiffness has a significant effect on the torsional mode shapes. When the adhesive is relatively soft, the torsional mode shapes at the lap joint are slightly distorted. But when the adhesive is relatively very stiff, the torsional mode shapes at the lap joint are fairly smooth and there is a relatively higher local stiffening effect. The consequence of this is that higher stresses will be developed in the stiffer adhesive than in the softer adhesive.     

Application of the design of experiments procedure to the behaviour of adhesively bonded joints with plastically deformable adherends to enable further understanding of strain rate sensitivity

The study presented in this paper was carried out to investigate further the effects of strain rate on the strength of adhesively bonded single lap shear joints. Tests were carried out on two different configurations of adhesively bonded joints that were designed to exhibit different behaviours. In one configuration both adherends were made from a relatively low strength grade of aluminium such that both would exhibit significant plastic deformation prior to adhesive failure. The other configuration used one adherend that was significantly stronger such that only elastic deformation was exhibited prior to failure of the adhesive. The joint specimens were tested at several different strain rates using a servo-hydraulic test machine and the results analysed using statistical methods. To further understand the results Finite Element models of the joints were created using a Cohesive Zone Model to predict damage development and failure in the adhesive. The Design of Experiments procedure was used to study the effects of material parameters relating to both the adherends and the adhesive in the Finite Element models. The results of the testing suggested that the strength of joints formed from two adherends that exhibited plastic deformation prior to failure did not show statistically significant sensitivity to strain rate. Interpretation of the results of the Finite Element analyses suggested that the adherend yield was the main factor influencing failure load in the adhesive for joints of this type.     

Failure analysis of adhesive-patch-repaired edge-notched composite plates

In this study, failure behaviors of edge-notched composite plates which were repaired with using patch and adhesive were investigated experimentally and numerically. The composite plates had a (0°/90°)₃ anti-symmetric stacking sequence; the patches were obtained from the same plates and were bonded using an epoxy-based adhesive. The effects of the geometry of the notch were investigated using U-, V-, and square-shaped notches. We varied the notch geometry, investigated single-lap and double-lap repairs, and varied the patch fiber-stacking sequence. Simulations were carried out to analyze the three-dimensional failure progression using ANSYS. The failure loads of the repaired plates increased by 170–304% for single-lap repairs and 240–476% for double-lap repairs compared with the notched plates. The simulated and measured failure loads were in agreement within 0.2–23.6%.     

同轴单搭接胶接接头力学性能的研究

利用有限元方法研究了偏轴单搭接胶接接头和同轴单搭接胶接接头的力学性能,结果表明：同轴单搭接胶接接头可以使接头趾部的弯矩为零,进而有效地降低剥离应力的峰值和减轻剪切应力的集中程度,是一种极为合理的胶接接头形式。     

Analysis of bonded double containment cantilever joints

An important part of adhesive development is the formulation of analyses for predicting the stresses and strains of bonded joints. An analysis of a double containment cantilever joint was made using the finite element method. A double containment joint comprising steel adherends bonded by a two-part epoxy resin adhesive was constructed and the strains in the adhesive layer were measured by a two axes co-ordinate table. The experimental results were found to be in good agreement with those of the theoretical analysis.     

Experiments on the combined use of a double-sided pressure-sensitive tape and an epoxy adhesive to reduce handling time

In many industrial assemblies with structural adhesives, the handling time, i.e. the time to safely move adhesively bonded parts, may slow down manufacturing process and result into additional costs. For that reason, in some cases, the traditional mechanical connections (rivets, bolts, welds) are still preferable when the structure must be completed in a short time. In this work a new type of hybrid joint, characterized by the combined use of an epoxy adhesive and a double-sided pressure-sensitive tape, is proposed and characterized by experimental tests on single-lap joints with a large overlap. The results showed that these hybrid joints allow for reducing the handling time of the joint, preserving approximately the same strength. The proposed solution exploits the non-uniform stress distribution in the bondline of a single-lap joint, resulting in a low stress area at the centre of the overlap. The presence of an adhesive tape in this region produced either a negligible or minor drop (~20%) in strength depending on the epoxy used, while provided the joint with sufficient strength for being handled before the full cure of the epoxy.     

Simulation of adhesive joints using the superimposed finite element method and a cohesive zone model

Adhesive joints have been widely used in various fields because they are lighter than mechanical joints and show a more uniform stress distribution if compared with traditional joining techniques. Also they are appropriate to be used with composite materials. Therefore, several studies were performed for the simulation of the bonded joints mechanical behavior. In general for adhesive joints, there is a scale difference between the adhesive and the substrate in geometry. Thus, mesh generation for an analysis is difficult and a manual mesh technique is needed. This task is not efficient and sometimes some errors can be introduced. Also, element quality gets worse.In this paper, the superimposed finite element method is introduced to overcome this problem. The superimposed finite element method is one of the local mesh refinement methods. In this method, a fine mesh is generated by overlaying the patch of the local mesh on the existing mesh called the global mesh. Thus, re-meshing is not required.Elements in the substrate are generated. Then, the local refinement using the superimposed finite element method is performed near the interface between the substrate and the adhesive layer considering the shape of the element, the element size of the adhesive layer and the quality of the generated elements. After performing the local refinement, cohesive elements are generated automatically using the interface nodes. Consequently, a manual meshing process is not required and a fine mesh is generated in the adhesive layer without the need for any re-meshing process. Thus, the total mesh generation time is reduced and the element quality is improved. The proposed method is applied to several examples.     

Effect of adhesive thickness and surface roughness on the shear strength of aluminium one-component polyurethane adhesive single-lap joints for automotive applications

Adhesively bonded technology is now widely accepted as a valuable tool in mechanical design, allowing the production of connections with a very good strength‐to‐weight ratio. The bonding may be made between metal–metal, metal–composite or composite–composite. In the automotive industry, elastomeric adhesives such as polyurethanes are used in structural applications such as windshield bonding because they present important advantages in terms of damping, impact, fatigue and safety, which are critical factors. For efficient designs of adhesively bonded structures, the knowledge of the relationship between substrates and the adhesive layer is essential. The aim of this work, via an experimental study, is to carry out and quantify the various variables affecting the strength of single-lap joints (SLJs), especially the effect of the surface preparation and adhesive thickness. Aluminium SLJs were fabricated and tested to assess the adhesive performance in a joint. The effect of the bondline thickness on the lap-shear strength of the adhesives was studied. A decrease in surface roughness was found to increase the shear strength of the SLJs. Experimental results showed that rougher surfaces have less wettability which is coherent with shear strength tests. However, increasing the adhesive thickness decreased the shear strength of SLJs. Indeed, a numerical model was developed to search the impact of increasing adhesive thickness on the interface of the adherend.