Behaviour Of Adhesive Joints Research Articles

Engineering solutions for complex composite material behaviour spanning time and temperature scales

The issue of time and temperature dependencies is considered in the behaviour of advanced fibre-reinforced polymer composite materials. Currently, for the vast majority of analyses of composite structures, time and temperature are considered invariant. In an effort to further improve the design of composite structures, more advanced analyses are now being developed to accurately capture the behaviour under a range of conditions. Various events and load cases in which time and temperature are critical are described in general terms. The specific cases of viscoelastic distortion under mechanical and thermal loading, the behaviour of adhesive joints and the structural response of composites to fire are discussed in detail. The key material response, characterisation methods and analysis approaches developed are described. It is observed that key challenges in the development of improved predictive models are measurement of time- and temperature-dependent material properties and the implementation of efficient multidisciplinary analysis methods.

Modified DCB and CLS specimens for mixed-mode fatigue testing of adhesively bonded thin sheets

The fatigue behavior of adhesive joints made with commercially coil-coated thin aluminum sheets has been measured using modified double cantilever beam (DCB) and cracked lap shear (CLS) specimens that avoided yielding. The thin sheets were bonded to a stiffener using a reinforcing adhesive layer. Experimental and finite element analyses demonstrated that the reinforcing adhesive layer had an insignificant effect on the stress state at the crack tip and the resulting fatigue behavior. The modified DCB test geometry was then used to measure the effect of the loading phase angle and test environment on the fatigue performance of the pretreated aluminum sheet. It was observed that the phase angle affected the threshold strain energy release rate, G th , and the crack growth rates. The presence of moisture in the test environment greatly reduced the fatigue threshold. It was also observed that the G th was sensitive to the rolling line orientation on the surface of the sheet.

Determination of cohesive parameters for a mixed-mode cohesive zone model

The mechanical behaviour and failure properties of adhesive joints are influenced not only by adhesive properties but also by interface characteristics. Various modelling methods have been proposed to predict the failure behaviour of adhesive joints. Among those methods, cohesive zone modelling, which consists of introducing a cohesive law in the numerical models to reproduce the adhesive layer, is widely used to simulate the behaviour of adhesive joints. The cohesive parameters of the traction–separation laws in each pure mode should be determined for application of this technique. It is known that the cohesive parameters are affected by both adhesive properties and interface characteristics so, in most cases, these parameters are iteratively obtained by matching numerical results to experimental results. In this paper, a systematic procedure for the determination of the cohesive parameters is proposed by introducing an optimization technique (design of experiment and the kriging metamodel). The first step is to identify design variables affecting a response of a system. In this study, the cohesive parameters are selected as the design variables. Once the design variables are identified, design ranges or sampling ranges of the design variables are determined in order to result in a near optimal response with the determined sampling ranges. If the design variables and the sampling ranges are obtained then the design of experiment is performed to determine appropriate sampling points in the sampling ranges and a minimum number of simulations. Then, the load difference between numerical and experimental load–displacement results at several values of displacements are defined as an error, and the kriging metamodel, which surrogates the error, is constructed based on the sampling points. Cohesive parameters are determined by applying the numerical nonlinear optimization algorithm to minimize the error. The proposed procedure is applied to a co-cured Single Leg Bending (SLB) joint under mode I and mode II-dominant modes and the cohesive parameters are determined. From these parameters, the mixed-mode cohesive zone model is constructed and applied to the simulation of the co-cured SLB joint under varying mode-mixities. Simulation results are compared with the experimental results. It is shown that the failure behaviour of the SLB joint is well-described by using the mixed-mode cohesive zone model with the determined parameters.

Axial/Bending Coupled Analysis for FRP Adhesive Lap Joints

In this paper a finite element analysis of the behavior of adhesive joints between FRP (fiber-reinforced polymers) adherents is presented. In particular, double-lap joints, in the case of both normal and shear/flexure stresses, are considered. The problem is nonlinear due to the mathematical formulation of the cohesive laws introduced to model the interfacial interactions. On the contrary, the adherents are supposed to be linear elastic up to failure. The common approach of analyzing shear/flexure behavior separately from extensional behavior, as well as disregarding the mutual effects between the normal and tangential stresses acting at the joint interfaces, has been updated. The paper takes into account the coupling effects between shear/flexure and extensional equilibrium problems. As highlighted in literature, only a coupled analysis can be considered correct when the mechanical characteristics of the adherents are quite different, as usually occurs in practice. Furthermore it also has been assumed that the adherents are shear deformable. The numerical results obtained by such an approach are presented, discussed and compared with others available in literature.

Fatigue threshold behavior of adhesive joints

The fatigue threshold and slow crack growth rate behavior of a highly-toughened epoxy adhesive was studied as a function of the starting condition (fatigue precrack and fillet), testing approach and interfacial bond strength. For cohesive failure, the threshold behavior was very similar when starting from a fatigue precrack or an uncracked fillet; however, the threshold was lower when interfacial cracks grew from a fatigue precrack. When compared to the time required for a crack to reach a significant length in a practical joint, the crack initiation from a fillet at fatigue threshold loads occurred relatively quickly. For loading and displacement ratios of 0.1 and 0.5, fatigue testing under force control proved to be equivalent to the faster and more convenient testing under displacement control. Mixed-mode fatigue crack growth at slow rates near the threshold became progressively interfacial as the cyclic crack speed decreased, revealing differences in bond strength that were unapparent in fracture tests and in fatigue tests at higher crack speeds. Slow crack growth fatigue tests were thus found to be exceptionally sensitive to differences in interfacial bond strength caused by changes in adhesive batch, adherend roughness and surface pretreatment.

Micro–macro characterisation of DGEBA-based epoxies as a preliminary to polymer interphase modelling

Reactive polymer adhesives in contact to substrates are known to form so-called interphases, a notion comprising the domain within which the polymer, compared to its bulk, exhibits structural inhomogeneities and gradients in material properties. Induced by the interface between substrate and polymer the formation of such interphases is usually ascribed to processes like segregation or phase separation of polymer components, selective adsorption, steric hindrance, orientation effects or curing shrinkage. Quantitative information on mechanical interphase properties is obtainable only by considerable efforts since interphases belong to the class of buried layers, i.e. they are located between bulk polymer and substrate, which impedes a majority of experimental techniques.Within this contribution, a two-component epoxy-based model polymer (DGEBA/DETA) is examined by methods on different scales and with respect to the effects that both the resin/hardener mixing ratio and the chemical structure of the hardener exert on the mechanical bulk properties. By means of these variations the above mentioned processes disturbing the polymer network formation in the vicinity of the substrate are emulated within the bulk. Macroscopic tension tests, nanoindentation and calorimetric methods (DSC) are applied to obtain relations between structural variations and material behaviour. Inversely identifying the governing parameters of suitable constitutive laws from experimental data will later conclude the first step towards a quantitative interphase model.It is demonstrated that modifications of the resin/hardener mixing ratio and the hardener formulation lead to variations in mechanical bulk properties which are quantitatively determinable by methods on different scales and do lie in ranges similar to those of property profiles that have been observed within interphases.In future work, the local mechanical behaviour of adhesive joints under load will then be investigated by a microscale videoextensometry. The resulting data will be compared to the structure–property relations from step one to conclude on the local polymer structure within the interphase.

Comportamiento a fatiga de uniones a solape simple con adhesivo epoxi de acero y acero prepintado

The mechanical properties of adhesive joints depend not only on the nature and properties of the adhesives; there are many parameters which influence the behaviour of the adhesive joints. Some of more significant parameters are: the surface conditions of materials, area and thickness of adhesive layer, compatibility of the design, applying sequence, chemical properties of surface and the surface treatments of substrates prior to be bonded. The mechanical resistance of adhesive joints is interrelated to the fracture process depended on cohesion-adhesion mechanism. The aim of this paper is analyse the influence of two different surfaces on the fatigue behaviour of epoxy adhesive joints. The results show that chemical compatibility of adhesive and paint improve adhesion of joints and the mechanical resistance against static and dynamic loads. The fatigue limited has been obtained for joints with both surfaces.

Energy release rate and mode-mixity of adhesive joint specimens

Fracture behaviour of adhesive joints under mixed mode loading is analysed by using the beam/adhesive-layer (b/a) model, in which, the adherends are beamlike and the adhesive is constrained to a thin flexible layer between the adherends. The adhesive layer deforms in peel (mode I), in shear (mode II) or in a combination of peel and shear (mixed mode). Macroscopically, the ends of the bonded part of the joints can be considered as crack tips. The energy release rate of a single-layer adhesive joint is then formulated as a function of the crack tip deformation and the mode-mixity is defined by the shear portion of the total energy release rate. The effects of transversal forces and the flexibility of the adhesive layer are included in the b/a-model, which can be applied to joints with short crack length as well as short bonding length. The commonly used end-loaded unsymmetric semi-infinite joints are examined and closed-form solutions are given. In comparison to the singular-field model in the context of linear elastic fracture mechanics, the b/a-model replaces the singularity at the crack tip with a stress concentration zone. It is shown that the b/a-model and the singular-field model yield fundamentally different mode-mixities for unsymmetric systems. The presented closed-form b/a-model solutions facilitates parametric studies of the influence of unbalance in loading, unsymmetry of the adherends, as well as the flexibility of the adhesive layer, on the mode mixity of an adhesive joint.

Damage evolution in adhesive joints subjected to impact fatigue

There is increasing interest in the effects of low-velocity impacts produced in components and structures by vibrating loads. This type of loading is known as impact-fatigue. The main aim of this paper is to investigate the behaviour of adhesive joints exposed to low-velocity impacting, to study the impact-fatigue life and to compare this loading regime with standard fatigue (i.e. non-impacting, constant amplitude, sinusoidal fatigue). To this effect, bonded aluminium single lap joints have been subjected to multiple impacting tensile loads and it has been shown that this is an extremely damaging load regime compared to standard fatigue. Two modifications of the accumulated time-stress model have been proposed to characterise the impact-fatigue results presented in this paper. The first model has been termed the modified load-time model and relates the total cumulative loading time of the primary tensile load wave to the mean maximum force. The second model attempts to characterise sample damage under impact-fatigue by relating the maximum force normalised with respect to initial maximum force to the accumulated loading time normalised with respect to the total accumulated loading time. This model has been termed the normalised load-time model. It is shown that both models provide a suitable characterisation of impact-fatigue in bonded joints.

Failure Modes in Adhesively Bonded Carton Boards

Carton board packages are often adhesively bonded. The adhesive joint may fail due to cohesive fracture in the adhesive, interfacial fracture between the adhesive and one of the carton board surfaces, or cohesive fracture in the carton board. The failure may also be a combination of these failure modes. From previous studies, it is well known that the failure mechanism greatly impacts the integrity and mechanical behaviour of adhesive joints. To explore these matters, detailed experiments on adhesively bonded carton boards were performed using the Y-peel setup. By monitoring the joint at high magnification with a digital video camera during progressive loading, it was possible to link the mechanical behaviour of the adhesive joint to the fracture mechanisms involved in each case. It was found that the adhesive joint failures could be categorised into four main failure modes. The two (modes M1 and M2) failure modes with low toughness, i.e., low dissipative energy, failed by interfacial fracture with small permanent deformation in the adhesive and in the carton board. High dissipative energy modes (modes M3 and M4), however, involved multiple failures and final failure by delamination or tearing of the outermost carton board ply. It was found that the Y-peel equipment could be used as a tool to develop carton boards and hot melt adhesives in order to optimise the adhesive joint for certain package applications. From the force–elongation curve characteristics, it is possible to perceive when and how the adhesive joint may fail in a real package application.

복합재료 접착체를 가지는 튜브형 접합부의 토크전달능력 예측

Since the performance of joints usually determines the structural efficiency of composite structures, an extensive knowledge of the behavior of adhesive joints and the related effect on joint strength is essential for design purposes. In this study, the torque capacity of adhesive joints was predicted using the combined thermal and mechanical analyses when the adherend was a composite tube. A finite element analysis was performed to evaluate residual thermal stresses developed in the joint, and mechanical stresses in the adhesive were calculated including both the nonlinear adhesive behavior and the behavior of composite tubes. Three different joint failure modes were considered to predict joint failure: interfacial failure, adhesive bulk failure, and adherend failure. The influence of the composite adherend stacking angle on the residual thermal stresses was investigated, and how the residual thermal stresses affect the joint strength was also discussed. Finally, the predicted results were compared with experimental results available in literature.

Comportamiento mecánico de uniones estructurales con adhesivo. Influencia de los parámetros operacionales

The influence of operational conditions on the mechanic behaviour of adhesive joints is one of the most controversial subjects and one of the main causes of the disparity of the results obtained in the tests. The surface condition of the materials being joined and the thickness of the adhesive layer in the joint, are two of the fundamental parameters for the design of the structural joint. Cyanocrylate and polyurethane adhesives applied to galvanized, aluminized and coated metallic materials show important differences in the mechanic resistance of the joints, depending on the operational parameters, which will be analyzed in this job. The joint mechanical resistance is interrelated to the fracture mechanism (ductile-brittle) and, on the other hand, to the cohesion-adhesion mechanism, both included in this study.

Characterizing Dynamic Fracture Behavior of Adhesive Joints under Quasi-Static and Impact Loading

Abstract An experimental evaluation of the dynamic fracture properties of an automotive epoxy is presented. Pronounced stick-slip behavior was observed in both quasi-static and impact tests of aluminum and composite adherends bonded with this adhesive. An experimental technique for conducting low speed impact of adhesively bonded automotive composite joints is presented. Based on the use of a modified drop tower, mode I, II, and mixed mode values for critical energy release rate were determined to create a fracture envelope for the composite/epoxy system. Because load measurements are erratic and unreliable at higher test rates, displacement-based relationships were used to quantify these energy release rates. Displacement data were collected with an imaging system that utilizes edge detection to determine displacement profiles, end displacements, and opening displacements where applicable. Because of the resolution of the image-based approach being used, determining crack length experimentally is difficult. As a result, numerical methods based on edge detection algorithms were developed to objectively determine the crack length based on the available experimental data in mode I, II, and mixed mode I/II configurations.

Study of anisotropic conductive adhesive joint behavior under 3-point bending

Flip chip interconnections using anisotropic conductive film (ACF) are now a very attractive technique for electronic packaging assembly. Although ACF is environmentally friendly, many factors may influence the reliability of the final ACF joint. External mechanical loading is one of these factors. Finite element analysis (FEA) was carried out to understand the effect of mechanical loading on the ACF joint. A 3-dimensional model of adhesively bonded flip chip assembly was built and simulations were performed for the 3-point bending test. The results show that the stress at its highest value at the corners, where the chip and ACF were connected together. The ACF thickness was increased at these corner regions. It was found that higher mechanical loading results in higher stress that causes a greater gap between the chip and the substrate at the corner position. Experimental work was also carried out to study the electrical reliability of the ACF joint with the applied bending load. As per the prediction from FEA, it was found that at first the corner joint failed. Successive open joints from the corner towards the middle were also noticed with the increase of the applied load.

A simple continuum damage model for adhesively bonded butt joints

The present work is concerned with the study of the damage behavior of adhesive joints consisting of an epoxy adhesive layer bonding aluminium alloy substrates. A model for butt joints, developed within the framework of Continuum Damage Mechanics, that accounts for the effect of the thickness of the adhesive layer on the strength of the system is proposed and analyzed. The predicted values of rupture stress for different values of the thickness of the adhesive layer are compared with experimental data, showing a good agreement.

The Governing Relationships of the Strength Behaviour of Adhesive Joints Based on Weak Solvents in Unplasticised PVC Pipes

The Governing Relationships of the Strength Behaviour of Adhesive Joints Based on Weak Solvents in Unplasticised PVC Pipes

Substrate constraint and adhesive thickness effects on fracture toughness of adhesive joints

The adhesive thickness effect on fracture behaviour of adhesive joints has been studied using the boundary effect model recently developed for specimen size effect on fracture properties of concrete, and the essential work of fracture model for ligament (uncracked region) effect on largescale yield of bulk metals and polymers. The leading common mechanism responsible for the nonlinear elastic fracture mechanics behaviours, such as adhesive thickness effect of adhesive joints, specimen size effect of brittle heterogeneous materials and notch dependence of deeply notched metal and polymer specimens, is discussed. These two fracture mechanics models show that the height variation of a fracture process zone (FPZ) or a plastic zone is directly responsible for any change in fracture energy measurements such as the specific fracture energy G f and the critical strain energy release rate G c. Both models show that G f is rapidly reduced when the crack-tip approaches the back-face boundary of a specimen because only a limited FPZ or plastic zone height h FPZ can be developed in the boundary region. In the case of a thin adhesive joint, the development of a plastic zone height is limited by the thickness of the adhesive sandwiched between the upper and lower adherends or substrates. Consequently, a linear relationship between the adhesive joint toughness and adhesive thickness is established. Test results on adhesive joints from the literature are analysed and compared with the new adhesive joint failure model based on the two well-established fracture mechanics models developed for other material systems.

Effects of substrate materials on fracture toughness measurement in adhesive joints

To understand the effects of substrate materials on the fracture behavior of adhesive joints, experimental studies and finite element analyses have both been conducted for double-cantilever-beams (DCB) with aluminum and steel substrates at different bond thickness ( h). Numerical results show that the region dominated by the crack singularity is much smaller than by the bond thickness. Very small plastic deformation may hence violate the requirements for small-scale yielding where the crack-tip field can be characterized uniquely by the stress intensity factor. Both critical strain energy release rate and J-integral for the joints with steel substrate are lower than those for the joints with aluminum substrate. Compared to the critical strain energy release rate, the critical J-integral is less sensitive to the substrate material if small plastic deformation occurs before cohesive failure takes place through the adhesive layer. For the joints with aluminum substrate, the fracture toughness initially increases and then decreases with bond thickness. Elastic–plastic crack-tip analysis indicates that at the same level of loading, a higher opening stress is observed in the joint with a smaller bond thickness. A self-similar stress field can be obtained by the normalised loading parameter, J/ hσ 0.

Effect of Bond Thickness on Fracture Behaviour in Adhesive Joints

To study the effects of bond thickness on the fracture behaviour of adhesive joints, experimental investigation and finite element analysis have been carried out for compact tension (CT) and double-cantilever-beam (DCB) specimens with different bond thickness. Fractography and fracture toughness exhibited apparent variations with bond thickness. Numerical results indicate that the crack tip stress fields are affected by bond thickness due to the restriction of plastic deformation by the adherends. At the same J level, a higher opening stress was observed in the joint with a smaller bond thickness (h). Beyond the crack tip region, a self-similar stress field can be described by the normalized loading parameter, J/hσ0. The relationship between J and crack tip opening displacement, δ, is dependent on the bond thickness. The strong dependence of toughness upon bond thickness is a result of the competition between two different fracture mechanisms. For small bond thickness, toughness is linearly proportional to bond thickness due to the high constraint. After reaching a critical bond thickness, the toughness decreases with further increase of bond thickness due to the rapid opening (blunting) of the crack tip with loading. A simple model has been proposed to predict the variation of toughness with bond thickness.

The role of the interphase in the environmental failure of adhesive joints

The cyclic-fatigue behaviour of adhesive joints, which consisted of an aerospace-grade epoxy-adhesive bonding aluminium-alloy substrates, has been investigated. Fracture-mechanics tests were used to obtain the relationship between the rate of fatigue crack growth per cycle, da/dN, and the maximum strain-energy release rate, Gmax, applied during the fatigue cycle. These cyclic-fatigue tests were conducted in both a “dry” environment of 23±1°C and 55% relative humidity and a “wet” environment of immersion in distilled water at 28±1°C. In particular, the effect of using various surface pretreatments for the aluminium-alloy substrates, prior to forming the adhesive joint, has been investigated. X-ray photoelectron spectroscopy and electron microscopy techniques have been used to identify the locus of joint failure and the mechanisms of environmental attack.