Evaluation of different modelling conditions in the cohesive zone analysis of single-lap bonded joints

Cohesive Zone Models (CZM) are widely used for the strength prediction of adhesive joints. Different simulation conditions, such as damage initiation and growth criteria, are available for use in CZM analyses to provide the mixed-mode behaviour. Thus, it is highly relevant to understand in detail their influence on the simulations’ outcome. This work studies the influence of different conditions used in CZM simulations to model a thin adhesive layer in single-lap joints (SLJ) under a tensile loading, for an estimation of their influence on the strength prediction under diverse geometrical and material conditions. Validation with experimental data is considered. Adhesives ranging from brittle to highly ductile and overlap lengths (L_O) between 12.5 and 50 mm were considered. Different studies were considered: Variation of the elastic stiffness of the cohesive laws, different mesh refinements, study of the element type, and evaluation of several damage initiation and growth criteria. The analysis carried out in this work confirmed the known suitability of CZM for static strength prediction of bonded joints and pointed out the best set of numerical conditions for this purpose. Inaccurate results can be obtained if the choice of the modelling conditions is not the most suitable for the problem.     

On the effect of pulsed laser ablation on shear strength and mode I fracture toughness of Al/epoxy adhesive joints

The present work describes an experimental study about the shear strength and the mode I fracture toughness of adhesive joints with substrates pre-treated by pulsed laser ablation. An ytterbium-doped pulsed fiber laser was employed to perform laser irradiation on AA6082-T4 alloy. Morphological and chemical modifications were evaluated by means of surface profilometry, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Thick adherend shear tests were carried out in order to assess the shear strength while the mode I fracture toughness was determined using the double cantilever beam. For comparison, control samples were prepared using classical surface degreasing. The results indicated that laser ablation has a favorable effect on the mechanical behavior of epoxy bonded joints; however, while a + 20% increase was recorded for shear strength, a remarkable threefold enhancement of fracture toughness was observed with respect to control samples. XPS analyses of treated substrates and SEM observations of the fracture surfaces indicated that laser pre-treatment promoted chemical and morphological modifications able to sustain energy dissipation through mechanical interlocking. As a result cohesive failure within the adhesive bond-line was enabled under predominant peel loading.     

Experimental study of the Mode I adhesive fracture energy in DCB specimens bonded with a polyurethane adhesive

The Mode I fracture energy of a polyurethane adhesive with low Young’s modulus was investigated. Metal adherends in standardized double cantilever beam (DCB) tests are typically too stiff for soft adhesives, making it difficult to measure the fracture energy accurately. However, soft adhesives, such as a single-component polyurethane adhesive tested in this paper, are in high demand in the automobile industry. Thus, accurate measurement techniques must be established. Flexible substrates composed of spring steel were used for the DCB tests to accommodate the deformation of the adhesive layer. First, the applicability of the flexible substrates was discussed using specimens bonded with an epoxy adhesive. For soft adhesives, however, the deformation of the adhesive layer must be considered in the calculation methods of the fracture energy. Although the deformation effect on the DCB tests has been discussed with Winkler’s elastic foundation, the crack length must be measured along with the load and displacement. To overcome the difficulty of measuring the crack length, a calculation method based on Winkler’s elastic foundation was introduced applying the compliance-based beam method (CBBM). Finally, the fracture energy of the polyurethane adhesive was discussed by comparing the calculation methods with and without measuring the crack length.     

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.     

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.     

Mode I fracture toughness of adhesively bonded joints as a function of temperature: Experimental and numerical study

Adhesives used in structural high temperature aerospace applications must operate in extreme environments. They need to exhibit high-temperature capabilities in order to maintain their mechanical properties and their structural integrity at the intended service temperature. One of the main problems caused by high temperature conditions is the fact that the adhesives have different mechanical properties with temperature. As is known, adhesive strength generally shows temperature dependence. Similarly, the fracture toughness is expected to show temperature dependence.In this work, the Double Cantilever Beam (DCB) test is analysed in order to evaluate the effect of the temperature on the adhesive mode I fracture toughness of a high temperature epoxy adhesive. Cohesive zone models, in which the failure behaviour is expressed by a bilinear traction–separation law, have been used to define the adhesive behaviour and to predict the adhesive P–δ curves as a function of temperature. The simulation response for various temperatures matched the experimental results very well. The sensitivity of the various cohesive zone parameters in predicting the overall mechanical response as a function of temperature was examined as well for a deeper understanding of this predictive method. Also, issues of mesh sensitivity were explored to ensure that the results obtained were mesh independent.     

Application of adhesively bonded single lap joints for fracture assessment of adhesive materials

In this study, a method has been proposed to obtain the failure envelope of brittle adhesives using the experimental failure loads of precracked single lap joints (SLJs). The proposed technique is based on the principles of linear elastic fracture mechanics (LEFM), on J-integral relations, and on results of a numerical analysis. Compared to the previous approaches, the introduced experimental method has some advantages such as low manufacturing costs and simpler test procedure. The proposed method can also provide a wide range of mode mix ratios without the need of an additional apparatus. The fracture envelope obtained from the proposed method was then verified by performing some fracture tests including double cantilever beam (DCB), end-notched flexure (ENF), and single leg bending (SLB) specimens. Good correlation was seen between the fracture envelopes of the proposed method and the ones obtained from the fracture mechanics experiments.     

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 high loading rate and low temperature on mode I fracture toughness of ductile polyurethane adhesive

The mode I fracture toughness of an adhesive at low temperatures under high loading rates are studied experimentally. Typical R-curves of the polyurethane adhesive under different loading rates (0.5?mm/min, 50?mm/min, 500?mm/min) at different temperatures (room temperature, ?20?°C, ?40?°C) respectively are obtained. From the experimental results, the mode I fracture toughness of this adhesive is extremely sensitive to the high loading rates and low temperatures. With the increase of the loading rate and decrease of temperature, the mode I fracture toughness of this adhesive decreases significantly. Under the loading rate of 500?mm/min at ?40?°C, the mode I fracture toughness of adhesive is 15% of the value at room temperature (RT) under quasi-static conditions. Through the experiment, the relationship between mode I fracture toughness of this adhesive, nominal strain rate and temperature is obtained.     

Lowering curing temperature in adhesive bonding of aluminum AA6061-T4

ABSTRACT

To minimize the part distortion and investment in the E-coat oven in adhesive bonding of metals for automotive applications, lowering the curing temperature of adhesive without apparent loss of the joint strength is desirable. The key to lower the curing temperature of adhesive bonding of lightweight materials is to accelerate the curing process of structural adhesives. In this study, curing agent (i.e., aliphatic polyamine) and curing accelerator (i.e., acetylacetone salt) were added into commercial Henkel 5089 adhesive and the effect of these curing additives on the curing temperature of Henkel 5089 in adhesive bonding of aluminum AA6061-T4 was studied. The test results showed that the addition of a curing agent and accelerator in Henkel 5089 lowered the curing temperature from 177°C to 130°C without sacrificing the strength of the adhesive-bonded aluminum AA6061-T4.     

Effect of the mean stress on the fatigue behaviour of single lap joints

Steel is the most important construction material for the mass production of engineered structures, especially in the transport industry. On the other hand, adhesive joints are typically used to join load-bearing components. Therefore, this work intends to investigate the stress ratio effects on the fatigue behaviour of adhesively bonded steel lap joints. S–N diagrams of fatigue tests, under constant amplitude loading, were obtained for stress ratios ranging between 0.05 and 0.7. It was observed that the fatigue life of the adhesive joints has very little dependence on the stress amplitude, indicating that only the maximum stress is important. The combination of a linear equation with a quadratic equation seems to be the best formulation to fit the experimental results. Finally, the Palmgren–Miner’s Law is accurate enough to predict the fatigue design for sequential block loadings.     

A comparative study on effects of thermal fatigue caused by thermal-oil cycling on tensile properties of single lap composite joints bonded with different kinds of adhesives

In this study, thermal-oil cycling process was applied to adhesively bonded glass fibre reinforced single lap composite joints (SLJs) in order to investigate the effects of thermal fatigue on their tensile properties. Joints were subjected to thermal cycling to achieve the thermal fatigue effect. Initially, separated specimens were heated on a magnetic stirrer from 25°C to 80°C, 25°C to 100°C and 25°C to 120°C and kept at these temperatures for 10 min. Right after that specimens were immersed into heat transfer oil cooled with dry ice at ?10°C for 10 min. and placed back into the magnetic stirrer to reach intended temperatures again. Thus, one cycle is completed. This thermal cycling process was repeated for 1, 10, 20, 30, 40 and 50 times for different groups of specimens. Single lap shear (SLS) tests have been performed for the determination of mechanical properties. As a result of this study, it is found that the load carrying capacities of specimens generally decrease as the thermal cycling count is increased. However, it is observed that specimens subjected to 30 times thermal cycling have the maximum load carrying capacity.     

Evaluation of FRP/wood adhesively bonded epoxy joints on environmental exposures

This article describes the evaluation of the durability of joints composed of wood adherends with a bonded layer of fibre-reinforced polymer (FRP) fabric. Carbon and glass fibres in an epoxy matrix were studied. The main purpose of FRP usage with timber in the construction industry is generally to improve the stiffness/strength of reinforced members without any influence on their service-life or any environmental impact. From the perspective of the timber reinforcement process, optimal dimensional stability during moisture changes in wood should be one of the most important criteria for such joints. Therefore, FRP/wood joints were evaluated with regard to the influence of real external environmental conditions on the bondline over a period of 40?months. During exposure to these conditions, specimen failures and defects were continuously visually evaluated. The decisive factor in this evaluation was bond integrity, verified by the tensile shear strength of the FRP/wood joint. After the experimental study, it was noted that the first 20?months have a significant effect on bondline failure occurrences, which involve decreases in tensile shear strength. In the next 20?months, the FRP/wood bondlines resist other severe hygrothermal stresses without significant strength decreases. An additional observed parameter was the percentage of wood failure in the bonded area of single lap joints, which characterises the mode of failure of the bonded joint. To determine the influence of ageing on adhesive due to ultraviolet radiation and varying temperature, infrared absorption spectroscopy analysis was performed to reveal changes in the macromolecular structure of the epoxy adhesive. Findings showed that UV radiation had a significant influence on the degradation of the adhesive structure.     

Experimental and numerical investigations of crack face adhesive bonding effect on the mixed-mode fracture strength of PMMA

Experimental and numerical investigations have been conducted to evaluate the effect of adhesive bonding of crack surfaces on the mixed-mode (I and II) fracture strength and effective stress intensity geometry/loading factor of a plate with an edge crack. The experimental tests were carried out on five batches of simple edge crack and specimens in which adhesive bonding was used on crack faces at different distances from the crack tip. The cracked specimens made from poly methyl-methacrylate rectangular plates. The specimens’ fracture strength was obtained by employing a tensile testing machine at different loading angles using a modified Arcan fixture. In the numerical part, finite element simulations were used to model the test specimens and thereby establishing their stress intensity geometry/loading factors. The results show that the adhesive bonding of the crack surfaces has a significant effect on reducing the equivalent mixed-mode stress intensity factor for all loading angles. The bonded specimens show considerable fracture force enhancement compared to the simple edge crack specimens.     

Influence of the engagement ratio on the shear strength of an epoxy adhesive by push-out tests on pin-and-collar joints: Part I: Campaign at room temperature

This paper focuses on an epoxy adhesive (LOCTITE 9466), which is particularly suitable for applications involving different materials and where a clearance is present between the adherents. The investigated subject is concerned with the effect of the Engagement Ratio (ER, coupling length over coupling diameter) on the shear strength of LOCTITE 9466 at room temperature. Motivations arise from the increasing interest in epoxy-adhesive joints in lightweight structures and from the consequent need for design data. Decoupling tests have been performed on pin-and-collar samples manufactured according to current Standards. The height has been adjusted in order to explore a sufficiently wide ER range at four different levels. The results have been processed by the tools of the Analysis of Variance and of the Fisher test to investigate the significance or the not significance of ER on the joint shear strength. The final outcome was that ER significantly affects resistance at a very high confidence level. This result has then been refined by the tool of orthogonality, in order to allocate the differences among the four levels of ER. This further analysis has shown that the joint strength is significantly enhanced, when ER exceeds 1 and assumes values around 1.3 or higher.     

Effect of fiber orientation on Mode I fracture toughness of CFRP

The effect of fiber orientations on fracture toughness of carbon fiber reinforced plastics (CFRP) in Mode I loading was investigated using double cantilever beam (DCB) specimens, based on mesoscopic mechanics. Mesoscopic interlaminar fracture toughness of 0//0 interphase of CFRP was evaluated with mesoscopic finite element models using experimental data. The fracture surface roughness was observed by confocal laser scanning microscopy. Then the mesoscopic interlaminar fracture toughness of CFRP was correlated with the fracture surface roughness. Additionally, the change of the Mode I macroscopic fracture toughness of CFRP was experimentally measured with changing the numbers of 0 and ±θ layers of DCB specimens. The correlation between the fracture toughness of 0//0 and θ//?θ interphases was discussed and a novel procedure was proposed to predict the macroscopic fracture toughness of θ//?θ interphase using finite element method (FEM). The fracture toughness of θ//?θ interphase analyzed by FEM was finally compared with the experimental results to verify the proposed prediction procedure. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

The effect of a multicomponent silane primer on the interphase structure of aluminum/epoxy adhesive joints

The structure of films formed by a multicomponent silane primer applied to an aluminum adherend and the interactions of this primer with an amine-cured epoxy adhesive were studied using X-ray photoelectron spectroscopy, reflection-absorption infrared spectroscopy, and attenuated total reflectance infrared spectroscopy. The failure in joints prepared from primed adherends occurred extremely close to the adherend surface in a region that contained much interpenetrated primer and epoxy. IR spectra showed evidence of oxidation in the primer. Fracture occurred in a region of interpenetrated primer and adhesive with higher than normal crosslink density. The primer films have a stratified structure that is retained even after curing of the adhesive.     

Experimental investigation of nanoparticles effects on cohesive model and bridging laws of mode I fracture in the adhesive joints

In this paper, an experimental study is conducted on bridging and cohesive mechanism of adhesive bonded joints including Nano-composite and Nano-adhesive for mode I fracture. Nano-composite adherents with glass fibers and alumina nanoparticle have been fabricated and underwent DCB test. The concentration of this study is on the comparison of three different types (ECM, CBT, and MCC methods) of traction-separation laws and the effect of adding nanoparticles on them. A modified trapezoidal model (P1) is presented. The obtained bridging laws and cohesive mechanism (traction-separation laws) for different nanoparticles wt% (in adhesive and adherent) can be used in the finite element method for numerical simulation. Values obtained for critical displacement by different methods have a good consistency with each other with a relatively similar value, except for the sample by 0.5% wt Nano-particle in adhesive. In addition, critical displacement increases with increasing nanoparticle content in the adhesive, and then decreases. In that, the greatest critical displacement was observed in adhesive samples with nanoparticle content of 0.5%. Although, critical stress obtained from CBT to MCC methods were consistent, they were different from ECM results. The critical stress decreased with further addition of nanoparticles to the adhesive, and then started increasing. In that, the lowest critical stress was observed in sample adhesives with nanoparticle content of 0.5%.     

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.     

A detailed experimental study of the effects of pre-bond contamination with a hydraulic fluid,thermal degradation,and poor curing on fracture toughness of composite-bonded joints

Adhesive bonding is applied by the aircraft industry both for assembling composite structural parts and implementing composite patch repairs in damaged structural parts. In both applications, there exist several scenarios, related to surface contamination and processing, that could affect bonding quality and thus, degrade bond strength. In this paper, the detailed effects of pre-bond contamination with a hydraulic fluid, thermal degradation of the composite substrate, as well as poor curing (lower curing temperature) on strength of composite-bonded joints were studied experimentally by conducting mode I fracture toughness tests on double-cantilever beam specimens. These three application scenarios are possible to appear in the implementation of a composite patch repair in a damaged composite structural part. The experimental results showed a contradictory effect as the presence of the hydraulic fluid and poor curing degrades the fracture toughness whereas thermal degradation enhances fracture toughness of the composite-bonded joints. These findings are explained by means of extended non-destructive inspection, surface analysis, and evaluation of fracture surfaces.