Single Lap Joints with Rounded Adherend Corners: Experimental Results and Strength Prediction

The objective of the present study was to better understand the effect of the change in the geometry of the adherend corners on the stress distribution in single lap joints and, therefore, on the joint strength. Various degrees of rounding were studied and two different types of adhesives were used: one very brittle and another which had a large plastic deformation. Experimental results on the strength of joints with different degrees of rounding are presented. For joints bonded with brittle adhesives, the effect of the rounded adherend corners is larger than that with ductile adhesives. The strength of joints with brittle adhesives with a large radius adherend corner increases by about 40% compared to that with a sharp adherend corner. It is shown that for joints bonded with brittle adhesives, crack propagation occurs for a short period before it grows into catastrophic failure. However, for ductile adhesives, there is large adhesive yielding and small crack propagation before final failure. Another important feature of joints bonded with ductile adhesives is that there may be more than one crack in the adhesive layer before failure. This makes strength predictions more difficult. The second part of the paper presents an approximate method for predicting the strength of joints bonded with brittle and ductile adhesives, with and without adherend corner rounding. The predictions, based on an average value around the singularity, compare well with the experimental results, especially for joints bonded with ductile adhesives.     

Investigating tensile behaviour of toughened epoxy paste adhesives using circumferentially notched cylindrical bulk specimens

Toughened epoxy adhesives are frequently used to bond metals and polymer-matrix composite materials in many structural applications. The mechanical properties of adhesives are often characterised by testing either bulk adhesive specimens or bonded joints (i.e. in-situ form). In this paper, cylindrical bulk specimens with circumferential notches were manufactured and tested to investigate the tensile behaviour of an epoxy paste adhesive toughened with hollow glass microspheres. Bulk specimens were manufactured from the paste adhesive using injection moulding. Tensile tests were conducted for strain-rate and stress triaxiality effects by varying displacement rates and notch radii, respectively. Fracture surfaces were examined using optical and scanning electron microscopy to identify failure mechanisms. The results obtained from the toughened paste adhesive indicate that strain-rate and stress triaxiality influence its tensile fracture behaviour.     

The Shear Stress–Strain Behaviour of Low-modulus Structural Adhesives

The shear stress–strain behaviour of two low-modulus structural adhesives has been measured using the butt-torsion test. The Nadai correction for non-linear shear behaviour is explained as it is necessary to understand how this correction can be applied to butt joints. The results for one adhesive were accurately used to predict the strength of a lap joint, and it was shown that the strength of such a joint can approach that of a conventional, modern, structural epoxy. Structural adhesives are usually reckoned to be those with a high strength (50 MPa and upwards) and (these days), a strain to failure of at least 10% in tension, and which usually have a tensile modulus of 2 GPa or so. However, adhesives which are significantly less stiff, less strong, but much more ductile are entering the ‘structural’ arena. In order to evaluate their effectiveness and use in design, it is necessary to be able to measure accurately their stress–strain behaviour. Two such materials are 3M 9245 Structural Bonding Tape (SBT) and 3M 7838 B/A.     

Comparison of fracture behavior between acrylic and epoxy adhesives

An experimental study was conducted on the strength of adhesively bonded steel joints, prepared epoxy and acrylic adhesives. At first, to obtain strength characteristics of these adhesives under uniform stress distributions in the adhesive layer, tensile tests for butt, scarf and torsional test for butt joints with thin-wall tube were conducted. Based on the above strength data, the fracture envelope in the normal stress-shear stress plane for the acrylic adhesive was compared with that for the epoxy adhesive. Furthermore, for the epoxy and acrylic adhesives, the effect of stress triaxiality parameter on the failure stress was also investigated. From those comparison, it was found that the effect of stress tri-axiality in the adhesive layer on the joint strength with the epoxy adhesive differed from that with the acrylic adhesive. Fracture toughness tests were then conducted under mode l loading using double cantilever beam (DCB) specimens with the epoxy and acrylic adhesives. The results of the fracture toughness tests revealed continuous crack propagation for the acrylic adhesive, whereas stick-slip type propagation for the epoxy one. Finally, lap shear tests were conducted using lap joints bonded by the epoxy and acrylic adhesives with several lap lengths. The results of the lap shear tests indicated that the shear strength with the epoxy adhesive rapidly decreases with increasing lap length, whereas the shear strength with the acrylic adhesive decreases gently with increasing the lap length.     

The Shear Stress-Strain Behaviour of Low-modulus Structural Adhesives

The shear stress-strain behaviour of two low-modulus structural adhesives has been measured using the butt-torsion test. The Nadai correction for non-linear shear behaviour is explained as it is necessary to understand how this correction can be applied to butt joints. The results for one adhesive were accurately used to predict the strength of a lap joint, and it was shown that the strength of such a joint can approach that of a conventional, modern, structural epoxy. Structural adhesives are usually reckoned to be those with a high strength (50 MPa and upwards) and (these days), a strain to failure of at least 10% in tension, and which usually have a tensile modulus of 2 GPa or so. However, adhesives which are significantly less stiff, less strong, but much more ductile are entering the 'structural' arena. In order to evaluate their effectiveness and use in design, it is necessary to be able to measure accurately their stress-strain behaviour. Two such materials are 3M 9245 Structural Bonding Tape (SBT) and 3M 7838 B/A.     

Analysis of adhesively-bonded T-joints by experimentation and cohesive zone models

Within the scope of adhesively-bonded joints, one of the joint types having industrial application is the T-joint, for example, in marine applications (joining of panels to the hull and connecting the glass-fibre composite hull with anti-flood panels) and aeronautical applications (wing panels, fuselage sections). This work aims to experimentally and numerically study, by cohesive zone models (CZM), the behaviour of T-joints under peel loads. The experimentally evaluated adhesives are the Araldite^® AV138 (high ultimate strength but brittle) and Araldite^® 2015 (less stress to failure but ductile and more flexible). The joint strength is evaluated with different L-shaped adherends’ thickness (t_P2). With the numerical analysis, the stress distributions, damage evolution and strength are studied. Additionally, a purely numerical study compared joints with or without adhesive filling at the curvature of the L-shaped adherends, and an extremely ductile adhesive (Sikaforce^® 7752) was additionally evaluated. The experimental tests validated the numerical results and showed that CZM is an accurate technique for the study of T-joints. It was also shown that the geometry of the L-parts, the presence of filler adhesive and the type of adhesive have a direct influence on the joint strength. In fact, in this particular joint configuration, the ductile but with lower ultimate strength adhesive Sikaforce^® 7752 clearly outperforms the two adhesives with higher mechanical properties but less ductility.     

Strength of epoxy adhesive-bonded stainless-steel joints

The strength of stainless-steel joints bonded with two epoxy adhesives was investigated. The experimental programme included tests on single-lap and butt joints, as well as thick-adherend and napkin ring shear tests. Results suggested that the tensile and shear strengths of the epoxy adhesives were quite similar. However, finite element (FE) analyses raised doubts on the true adhesive strengths, due to the complex stress state in joint tests and pressure-dependent adhesive behaviour. In spite of some uncertainties, FE analyses showed that failure could be fairly well predicted by a maximum shear strain criterion.     

Modeling of cohesive failure processes in structural adhesive bonded joints

This paper presents an approach to predicting the strength of joints bonded by structural adhesives using a finite element method. The material properties of a commercial structural adhesive and the strength of single-lap joints and scarf joints of aluminum bonded by this adhesive were experimentally measured to provide input for and comparison with the finite element model. Criteria based on maximum strain and stress were used to characterize the cohesive failure within the adhesive and adherend failure observed in this study. In addition to its simplicity, the approach described in this paper is capable of analyzing the entire deformation and failure process of adhesive joints in which different fracture modes may dominate and both adhesive and adherends may undergo inelastic deformation. It was shown that the finite element predictions of the joint strength generally agreed well with the experimental measurements.     

The Effect of Stress Whitening on Moisture Diffusion in Thermosetting Polymers

The effects of stress whitening on the moisture diffusion rate and concentration in a polymer adhesive containing a secondary phase were investigated. This was accomplished by performing an absorption test on both stress whitened and virgin samples of the bulk adhesive and comparing the rate and amount of moisture diffusion in each. The presence of stress whitening in samples was not only observed visually, but also confirmed analytically using the “Bilinear RAMOD-2” equation. Experimental results reveal that visibly-present stress whitening resulting from fracture does indeed affect the rate and amount of moisture absorption in a polymer adhesive. Consequently, a diffusion model representing two different regions, stress-whitened and non-stress-whitened, is proposed for path of diffusion in polymer adhesives.     

The Effect of Stress Whitening on Moisture Diffusion in Thermosetting Polymers

The effects of stress whitening on the moisture diffusion rate and concentration in a polymer adhesive containing a secondary phase were investigated. This was accomplished by performing an absorption test on both stress whitened and virgin samples of the bulk adhesive and comparing the rate and amount of moisture diffusion in each. The presence of stress whitening in samples was not only observed visually, but also confirmed analytically using the “Bilinear RAMOD-2” equation. Experimental results reveal that visibly-present stress whitening resulting from fracture does indeed affect the rate and amount of moisture absorption in a polymer adhesive. Consequently, a diffusion model representing two different regions, stress-whitened and non-stress-whitened, is proposed for path of diffusion in polymer adhesives.     

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.     

Strength Improvement of Adhesively-Bonded Joints Using a Reverse-Bent Geometry

Adhesive bonding of components has become more efficient in recent years due to the developments in adhesive technology, which has resulted in higher peel and shear strengths, and also in allowable ductility up to failure. As a result, fastening and riveting methods are being progressively replaced by adhesive bonding, allowing a big step towards stronger and lighter unions. However, single-lap bonded joints still generate substantial peel and shear stress concentrations at the overlap edges that can be harmful to the structure, especially when using brittle adhesives that do not allow plasticization in these regions. In this work, a numerical and experimental study is performed to evaluate the feasibility of bending the adherends at the ends of the overlap for the strength improvement of single-lap aluminium joints bonded with a brittle and a ductile adhesive. Different combinations of joint eccentricity were tested, including absence of eccentricity, allowing the optimization of the joint. A Finite Element stress and failure analysis in ABAQUS^® was also carried out to provide a better understanding of the bent configuration. Results showed a major advantage of using the proposed modification for the brittle adhesive, but the joints with the ductile adhesive were not much affected by the bending technique.     

Optimization of adhesively-bonded single lap joints by adherend notching

The effect of adherend notching on the strength and deformation behavior of single lap joints was investigated. First, a parametric study was conducted using finite element analysis (FEA). This initial part of the research into the effect of notches on joint behavior involved determination of the optimum notch location and notch dimensions. This was done by using FEA in a series of models with different notch positions and geometries. The results of this parametric study were used to select the most promising lap geometries for further study. Next, more detailed FEA were conducted on the selected lap geometries. These data were compared with the experimental single-lap shear test results to assess the applicability of different failure criteria. Three different model adhesives were used: a rubber toughened film epoxy with nylon carrier, a styrene-butadiene-styrene block copolymer based deformable 'gel' adhesive, and a two-part, metal filled brittle epoxy adhesive. The FEA for single lap joints containing 'top notches' on the unbonded, top side of the adherends, at locations corresponding to the overlap ends, and bonded with the two-part metal filled epoxy provided the best agreement with the experimental results. The experimental results showed a 29% increase in joint strength with the introduction of the notches, which matched very well with the 27% decrease in the peak peel stress observed by the FEA results. For this brittle adhesive, the peel stress is almost certainly the governing failure stress. This was confirmed by matching of the FEA peak peel stress ratios with the experimental load ratios, for both the notched and unnotched specimens.     

Theoretical and experimental studies for the failure criterion of adhesively bonded joints

The objective of this work was to develop a criterion for predicting the failure strength of joints bonded by ductile adhesives. To obtain the criterion, first, fracture tests were carried out on T-peel joints and single-lap joints with various joint geometries, adhesives, and adherend materials. Then using the fracture loads obtained in the tests, a finite element analysis was performed by which the stresses in the adhesive joints were calculated. It is concluded that the failure of an adhesively bonded joint occurs when the maximum of the ratio of the mean to effective stresses exceeds a certain value, which can be considered a new material constant of a ductile adhesive.     

An investigation of fatigue failure prediction of adhesively bonded metal/metal joints

A fracture mechanics-based model for fatigue failure prediction of adhesive joints has been applied in this work. The model is based on the integration of the kinetic law of evolution of defects originated at stress concentrations within the joint. Final failure can be either brittle (fracture toughness-driven) or ductile (tensile/shear strength-driven) depending on the adhesive. The model has been validated against experiments conducted on single-lap shear joints bonded with a structural adhesive. Three different kinds of adhesives, namely a modified methacrylate, a one-part epoxy and a two-part epoxy supplied by Henkel, have been considered and three different overlap lengths have been tested. Fracture toughness and fatigue crack growth properties of the adhesives have been determined with mode I tests. The number of cycles to failure has been successfully predicted in several cases. It is interesting to notice that in the case of joints loaded at the same average shear stress, the shorter the joint, the longer the duration. This fact is also captured by the model.     

Tensile creep of a structural epoxy adhesive: Experimental and analytical characterization

Epoxy adhesives are nowadays being extensively used in Civil Engineering applications, mostly in the scope of the rehabilitation of reinforced concrete (RC) structures. In this context, epoxy adhesives are used to provide adequate stress transference from fibre reinforced polymers (FRP) to the surrounding concrete substrate. Most recently, the possibility of using prestressed FRPs bonded with these epoxy adhesives is also being explored in order to maximize the potentialities of this strengthening approach. In this context, the understanding of the long term behaviour of the involved materials becomes essential. Even when non-prestressed FRPs are used a certain amount of stress is permanently applied on the adhesive interface during the serviceability conditions of the strengthened structure, and the creep of the adhesive may cause a continuous variation in the deformational response of the element. In this context, this paper presents a study aiming to experimentally characterize the tensile creep behaviour of an epoxy-based adhesive currently used in the strengthening of concrete structures with carbon FRP (CFRP) systems. To analytically describe the tensile creep behaviour, the modified Burgers model was fitted to the experimental creep curves, and the obtained results revealed that this model is capable of predicting with very good accuracy the long term behaviour of this material up to a sustained stress level of 60% of the adhesive׳s tensile strength.     

Comparative Study of the Results of Various Experimental Tests Used for the Analysis of the Mechanical Behaviour of an Adhesive in a Bonded Joint

The use of adhesively bonded joints is often limited by a lack of reliable models able to accurately predict their behaviour in industrial applications, in which the stress distribution is often complex. The mechanical behaviour of an adhesive in a bonded joint is often heavily dependent on its stress state (i.e., the tensile–shear combinations). Thus, a large experimental database is required to accurately represent the complex behaviour of an adhesive in a bonded joint. On the one hand, the initial yield surface (initial elastic limit) often has to be described taking into account the two stress invariants, hydrostatic stress and von Mises equivalent stress, and on the other hand the non-linear behaviour of the adhesive is also quite complex to model. However, the mechanical response of adhesively bonded joints often presents quite large stress concentrations; thus, the analysis of experimental tests is made particularly difficult. Obtaining reliable experimental results makes it possible to contribute to optimization of an adhesive in a bonded joint. This paper presents comparisons between results of different experimental tests (with bulk and bonded joints), some of them are designed to greatly limit the edge effects. Results are presented for two adhesives under proportional monotonic loadings. The two adhesives have very different behaviours (a ductile adhesive and a brittle adhesive) and two different surface preparations of aluminium substrates (a mechanical preparation and a chemical preparation recommended by the adhesive manufacturer) were studied.     

G／Epoxy复合材料胶接强度研究

使用G／Epoxy作为底材研究了垫板、结构胶黏剂厚度和底材表面处理对拉伸剪切强度的影响。使用光学显微镜观察了断口形貌。结果表明加垫板能减小试验过程中由于加载偏心引起剥离应力,测试结果较大;结构胶黏剂的厚度和底材表面处理对拉伸剪切强度影响十分明显,随着厚度的增大而减小,经打磨表面裸露出纤维的试样拉伸剪切强度很低。结构胶黏剂厚度较小时以内聚破坏为主,随着厚度的增加破坏模式转变为粘接破坏。     

Experimental Analysis of the Bondline Stress Concentrations to Characterize the Influence of Adhesive Ductility on the Composite Single Lap Joint Strength

Adhesively bonded composite single lap joints were experimentally investigated to analyze the bondline stress concentrations and characterize the influence of adhesive ductility on the joint strength. Two epoxy paste adhesives—one with high tensile strength and low ductility, and the other with relatively low tensile strength and high ductility—were used to manufacture composite single lap joints. Quasi-static tensile tests were conducted on the single lap joints to failure at room temperature. High magnification two-dimensional digital image correlation was used to analyze strain distributions near the adhesive fillet regions. The failure mechanisms were examined using scanning electron microscopy to understand the effect of adhesive ductility on the joint strength. For a given surface treatment and laminate type, the results show that adhesive ductility significantly increases the joint strength by positively influencing stress distribution and failure mechanism near the overlap edges. Moreover, it is shown that high magnification two-dimensional digital image correlation can successfully be used to study the damage initiation phase in composite bonded joints.     

Strengthening of the adhesive bond using a mixture of adhesives between dissimilar adherends in a single lap joint

Abstract

Adhesive bonding is the best alternative to riveting in aircraft structures but the strength of the adhesive bonded joint is low and is limited by strength of adhesive. Strengthening of adhesive bonding is an important requirement. In this work, an attempt has been made to strengthen the adhesive bonding by mixing different quantities of brittle adhesive in the ductile adhesive and vice-versa. Two different adhesives, one brittle (AV138) and another ductile (Araldite-2015) adhesive have been considered. Initially single lap joint has been constructed between the CFRP and aluminium with individual adhesives, then the mixture of adhesives have been used in the bonded region in varied proportions. The X-ray radiography and ultrasonic testing have been performed to check the quality of bonding. Uniaxial tensile tests have been conducted on the lap joints along with Digital Image Correlations (DIC) to obtain the individual and mixed adhesive bond strength. The failure patterns have been identified using optical and scanning electron microscope. These studies indicate that strengthening of the adhesive bonding achieved by mixing of two adhesives and highest bond strength obtained when the mixture of AV138 and Araldite-2015 adhesives are used in equal proportions.