Delamination damage analyses of adhesively bonded lap shear joints in laminated FRP composites

Three-dimensional non-linear finite ele- ment analyses have been carried out to evaluate the out-of-plane stresses in the adhesive layer existing between the lap and the strap adherends of the Lap Shear Joint (LSJ) in laminated FRP composites for varied delamination lengths. The delaminations are presumed to be pre-embedded in the thin resin rich layer existing between the first and second plies of the strap adherend. Sublaminate technique has been used to model the LSJ with the delamination. Contact finite element analyses have been performed in order to avoid interpenetration of delaminated surfaces. The effects of varied delamination lengths on the peel and interlaminar shear stresses and the individual modes of Energy Release Rate (ERR) in the delamination zones are highlighted in this paper. It is seen that three-dimensional effects exist near the free edges of the overlap end of the joint. The delamination propagation significantly affects the stress distributions in the adhesive layer existing between the lap and the strap adherends of the LSJ. The variations of interlaminar stresses and ERRs on both the delamination fronts are found to be significantly different and thus, indicate that the propagation of delamination does not occur at same rate at the two delamination fronts. This may throw some light to the evaluation of structural integrity of the LSJ in the presence of pre-embedded delaminations.     

Elasto-plastic stress distributions in adhesively bonded double lap joints

Elasto-plastic stress analysis is an important solution for a used ductile adhesive in single or double lap solutions. After loading, the joint strength can be increased by residual stresses. In this study, an elasto-plastic stress analyses of the shear and peel stresses are carried out in a double-lap joint. The adhesive material is chosen as a ductile material which is DP460. Elasto-plastic solution of the shear stress is obtained in a simple and accurate form in one dimensional case. Then, the solution of the peel stress is performed by considering the shear stress in the elasto-plastic form. In this solution, the boundary conditions are satisfied by the Newton–Raphson method. A good agreement is obtained between these analytical and numerical (ANSYS 10) solutions. The values are calculated highest at the ends of the adhesive.     

Fatigue de-bond growth in adhesively bonded single lap joints

The fatigue de-bond growth studies have been conducted on adhesively bonded lap joint specimens between aluminium and aluminium with Redux-319A adhesive with a pre-defined crack of 3 mm at the bond end. The correlations between fracture parameters and the de-bond growth data are established using both numerical and experimental techniques. In the numerical method, geometrically non-linear finite element analyses were carried out on adhesively bonded joint specimen for various de-bond lengths measured from the lap end along the mid-bond line of the adhesive. The finite element results were post processed to estimate the SERR components G _I and G _II using the Modified Virtual Crack Closure Integral (MVCCI) procedure. In experimental work, specimens were fabricated and fatigue de-bond growth tests were conducted at a stress ratio R = − 1. The results obtained from both numerical analyses and testing have been used to generate de-bond growth curve and establish de-bond growth law in the Paris regime for such joints. The de-bond growth rate is primarily function of mode-I SERR component G _I since the rate of growth in shear mode is relatively small. The value of Paris exponent m is found to be 6.55. The high value of de-bond growth exponent in Paris regime is expected, since the adhesive is less ductile than conventional metallic materials. This study is important for estimating the life of adhesively bonded joints under both constant and variable amplitude fatigue loads.     

Parametric study of the creep failure of double lap adhesively bonded joints

In this paper the influence of adhesive thickness and adhesive fillet on the creep deformation and creep life time of the adhesively bonded double lap joint have been studied experimentally. Also finite element modeling was used to simulate creep behavior of bonded joints and the results are compared with those obtained from experimental tests. The adhesive used in this research was Araldite 2015 which is an epoxy based adhesive. Research procedure is carried out in two major stages. Firstly, uniaxial creep tests were conducted in 63 °C to obtain the creep characteristics and constitutive equation parameters of the adhesive at 63 °C. An empirical based rheological model based on Maxwell and Zener’s model is proposed to simulate the creep behavior of the adhesive and it is used to predict the creep behavior of the bonded joint using finite element method. Numerical results show good agreement with experimental data. It was observed that applying fillet increases creep life and decreases joint creep deformation, however increasing adhesive thickness has slight effect on the creep life time of the joint.     

Computational analysis of geometric nonlinear effects in adhesively bonded single lap composite joints

It is well known that geometric nonlinear effects have to be taken into account when the ultimate strength of single lap composite joints are studied. In the present paper we investigate for which level of loads or prescribed end displacements nonlinear effects become significant and how they appear. These aspects are studied by comparing finite element results obtained from geometric nonlinear models with the results from the linear ones. The well-known software package ANSYS is applied in the numerical analysis together with a self-implemented module in the C++ library Diffpack. Some of the results are also compared with classical analytical theories of idealized joints showing significant differences.The joints examined are made of cross-ply laminates having 0 or 90° surface layers. A combined cross-ply/steel joint and an isotropic joint made of steel are also studied. All the models except the all-steel one are assembled with adhesives, while the latter is welded.Through the investigation a considerable departure from linear behavior has been detected for a large regime of prescribed end displacements or external loads. Geometric nonlinear effects begin to develop for external loads that produces stresses which are far below ultimate strength limits and for average longitudinal strains that are less than 0.5%. It has also been detected that the distribution of materials within the joint has some influence on the nonlinear behavior. Thus, geometric nonlinear methods should always be applied when single lap (or other non-symmetric) composite joints are analyzed.     

The analysis of elastic adhesive stresses in bonded lap joints in FRP structures

Numerous publications provide the closed form solution for adhesive stress distribution in bonded lap joints. However, in order to render these solutions tractable the authors invariably impose simplifications, one of the most restrictive being that the adherends are identical. This is unrealistic for bonded joints in most practical structures where a variety of dissimilar components may be assembled.This paper presents two analytical techniques, namely the classical and the finite element theories, which are used to determine the elastic adhesive stress distribution for the single, double and tubular lap configurations. The generality for dissimilar adherends is included. The governing differential equations obtained when using the classical approach necessitate a numerical solution and in this paper a procedure is described which uses the finite difference method. Example solutions are presented as case studies.     

The analysis of elastic-plastic adhesive stress in bonded lap joints in FRP structures

The majority of researchers who investigate the theoretical distribution of adhesive stresses in bonded lap joints assume that the adhesive behaves as a linear elastic material. While this assumption does provide useful information, for example, the intensities of stress concentrations and their locations, the results do not reflect the true stress distribution or behaviour at appreciable levels of loading. Practical joints using typical structural adhesives will incur considerable adhesive yielding as loading is increased to failure. The analysis must, therefore, include plastic yielding of the adhesive.This paper presents two analytical techniques, namely, the classical and finite element theories, which will be used to determine elastic-plastic adhesive stress distribution in bonded lap joints. The theoretical work will consider the single, double and tubular lap configurations having both similar and dissimilar adherends. Case studies will be presented to illustrate the development of adhesive yielding and to compare the different analytical techniques.The classical approach was found to be intractable to a closed form solution and consequently a numerical method was employed.     

Fatigue performance of adhesively bonded single lap joints with non‐flat sinusoid interfaces

The role of a sinusoid interface shape on the load bearing capacity and fatigue life of adhesively bonded single lap joints (SLJs) was investigated numerically and experimentally. Aluminium adherends with wavy interfaces were tested under both quasi‐static and fatigue loading conditions. The experimental results showed that the non‐flat SLJs were indeed much stronger than the conventional flat joints. In order to fully demonstrate the advantage of the non‐flat sinusoid SLJ over the conventional SLJ, comparative fatigue tests with different loading levels were performed to determine the durability performance of the SLJs with non‐flat interfaces. The experimental results revealed that the non‐flat SLJ had considerably higher fatigue life than the conventional lap joint.     

Damage modelling of adhesively bonded joints

A cohesive zone model (CZM) has been used in conjunction with both elastic and elasto– plastic continuum behaviour to predict the response of a mixed mode flexure and three different lap shear joints, all manufactured with the same adhesive. It was found that, for a specific dissipated CZM energy (Γ₀) there was a range of CZM tripping tractions (σ_u) that gave a fairly constant failure load. A value of σ_u below this range gave rise to global damage throughout the bonded region before any crack propagation initiated. A value above this range gave rise to a discontinuous process zone, which resulted in failure loads that were strongly dependent on σ_u. A discontinuous process zone gives rise to mesh dependent results. The CZM parameters used in the predictions were determined from the experimental fracture mechanics specimen test data. When damage initiated, a deviation from the linear load–displacement curve was observed. The value for σ _uwas determined by identifying the magnitude that gave rise to the experimentally observed deviation. The CZM energy (Γ ₀) was then obtained by correlating the simulated load-crack length response with corresponding experimental data. The R-curve behaviour seen with increasing crack length was successfully simulated when adhesive plasticity was included in the constitutive model of the adhesive layer. This was also seen to enhance the prediction of the lap shear specimens. Excellent correlation was found between the experimental and predicted joint strengths.     

Free vibration analysis of adhesively bonded single lap joints with wide and narrow functionally graded plates

This study investigates the three-dimensional free vibration behaviour of an adhesively bonded functionally graded single lap joint. The functionally graded plates of the adhesive joint are composed of ceramic (Al₂O₃) and metal (Ni) phases varying through the plate thickness. The effects of geometrical parameters, such as plate width, plate thickness and overlap length, especially the effect of the similar and dissimilar material composition variations through-the-thicknesses of both upper and lower plates on the natural frequencies and corresponding mode shapes of the adhesive joint were also investigated using both the finite element method and the back-propagation artificial neural network (ANN) method. A series of the free vibration analyses were carried out for various random values of the geometrical parameters and the through-the-thickness material composition so that a suitable ANN model could be trained successfully. The proposed ANN models indicated that increasing plate thickness and compositional gradient exponent resulted in increases in the first 10 frequencies whereas the overlap length has negligible effect. In contrast, the natural frequencies decrease suddenly with increasing the plate width. For the plate width >50 mm, the natural frequencies become very low and the effect of the other design parameters on the natural frequencies becomes minor. In case the upper and lower plates have similar or dissimilar material composition variations the mode shapes were affected considerably, but the natural frequencies.     

Influence of disbond on notch crack behaviour in single bonded lap joints

The behaviour of the adhesive bonded joints due to the imposed eccentric loading generates a very complex distribution of the stress in the structure. Good adhesion between substrate and adhesive ensures a successful and lasting assembly. In this study the finite element method is used to analyze the behaviour of a bonded lap joint of dissimilar materials. The effects of the mechanical properties of the joints on the shear stress variation with and without presence of a circular notch are investigated. The results show that the maximum shear stresses are located at a distance of about 18% that of the lap length whatever the type of material used. In addition, the stress intensity factor is amplified by the presence of the negative effect of disband whose increase is linearly proportional to the square of the stress intensity factor. It reached its maximum value for a crack length equal to two-fifths of the notch radius.     

Inelastic fracture of adhesively bonded overlap joints

Adhesive bonding offers a simple and efficient way of joining structural components without weakening them by holes or welding.This article develops a new model to predict the fracture load of bonded overlap joints using a fracture mechanics approach. The bondline fracture resistance and effects of the nonlinear inelastic behaviour of structural adhesives are accounted for separately. For bonded single overlap joint configurations the model is expressed as simple explicit formulas.An experimental programme is presented where the design parameters that a designer can adjust to obtain the desired joint capacity are systematically varied. Comparison of test results with the predictions by current strength-of-materials capacity models highlights disparities between the theoretical predictions and experimental evidence. In contrast, the new model shows good agreement with the experimental results.It should be noted that the simple new formulas apply to a well-defined range of bonded overlap joint configurations and do not purport to apply in general to every other joint configuration.     

高温老化对玄武岩纤维增强树脂复合材料-铝合金单搭接接头失效的影响

为了研究持续高温环境对车用新材料粘接结构力学性能的影响,加工了铝合金-铝合金（Al-Al）和玄武岩纤维增强树脂复合材料-铝合金（BFRP-Al）单搭接接头,在高温（80℃）环境下进行了0天（未老化）、5天、10天、15天的老化实验,并对胶粘剂和BFRP复合材料进行了DSC和FTIR测试,分析高温老化后胶粘剂、BFRP复合材料的玻璃化转变温度（T_g）和化学成分变化,通过准静态拉伸测试获得老化后接头的失效载荷,并对其失效模式进行分析。研究结果表明:高温环境下,胶粘剂会发生后固化及氧化反应,BFRP复合材料发生热分解及氧化反应;Al-Al接头的失效载荷随老化周期的增加而不断增大,老化前后的失效模式均为内聚失效,其性能变化主要由胶粘剂决定;BFRP-Al接头的失效载荷先增加后减小,不同老化周期的接头均发生内聚和撕裂的混合失效,其性能变化由胶粘剂和BFRP复合材料共同作用决定,且随着老化周期的增加,BFRP复合材料撕裂面积不断增大,BFRP-Al接头的失效模式越来越倾向于玄武岩纤维/树脂界面的破坏,BFRP复合材料老化对接头失效载荷的影响越来越显著。      

A review on the environmental degradation effects on fatigue behaviour of adhesively bonded joints

The structural applications of adhesively bonded joints on transportation industries have been increasing, and it is expected that this rising trend persists in the future. The appropriate design of these joints should address two main issues: fatigue behaviour and environmental effects. Environmental effects consist of the degradation of the bonded joints by means of harmful influence of temperature, moisture or both simultaneously. These effects can have an impact on the fatigue behaviour of bonded joints because they influence the quality of the bonding. The combination of environmental effects and fatigue lead to synergetic consequences resulting in premature and unpredictable rupture, which transforms these issues into relevant and actual research topics. The present paper describes the most recent works addressing the referred subjects. Experimental works and analytical/numerical approaches are also described aiming to give a picture of the real state‐of‐the‐art. Actual limitations and perspectives of future evolution are also discussed.     

Evaluation of fatigue strength of adhesively bonded single and single step double lap joints based on stress singularity parameters

Most adhesively bonded joints have stress singularity points at the corners of the adhesive/adherend interface. Recently, stress singularity parameters, i.e. the intensity of stress singularity, K, and the order of stress singularity, λ, have been used to evaluate the strength of adhesively bonded joints. However, in many cases, stress singularity fields of adhesive joints cannot be formulated strictly by using constant values of K and λ. To apply these parameters to evaluate the strength of an adhesive joint, it is necessary to determine a key stress component and characteristic range for calculating the apparent singularity parameters K_app and λ_app. In this study, the endurance limits of adhesively bonded single lap, cracked single lap and single step double lap joints are evaluated using the stress singularity parameters. The results indicate that fatigue failure criterion for these joints can be obtained by using the apparent singularity parameters K_app and λ_app which are calculated by the least square method for the maximum principal stress distributions in the range from 0.05 to 0.5 mm from the singularity point.     

A numerical study of adhesively bonded composite panel-flange joints

A geometrical non-linear numerical analysis for two-dimensional models of adhesively bonded composite panel-flange joints is presented to investigate the peel and shear stress redistribution in the joints when the panels buckle. The maximum stress failure criterion is used to predict failure loads and the associated failure modes induced by the buckled panels. Parametric studies for a variety of geometric configurations are carried out to show the effect of the relative stiffness and length ratios of the panel and flange on the redistribution of the peel and shear stresses as well as the failure loads and the associated modes. It is also shown that flexible joints provide higher joint efficiency.     

Environmental durability of adhesively bonded FRP/steel joints in civil engineering applications: State of the art

Over the past three decades, the strengthening and repair of existing civil engineering structures using FRP laminates has attracted a great deal of attention. With the advances in polymer science, adhesive bonding has become a common joining technology in these applications. Despite numerous studies that address the short-term behaviour of adhesively bonded FRP/steel joints, uncertainty with respect to long-term performance still remains. This knowledge gap is regarded as a critical barrier, hindering the widespread application of FRPs to strengthen and retrofit steel structures. This paper presents the state of the art in terms of the durability of FRP/steel joints used in civil engineering applications. Important influential factors relating to the durability of adhesively bonded joints are reviewed and different damage mechanisms are discussed. Moreover, related investigations of the combined environmental durability of these joints are critically reviewed and the findings are presented. The paper concludes with a discussion to motivate future research topics, while it is emphasised that the generalisation of the available results is questionable.     

Failure analysis of adhesively bonded joints in composite materials

The present paper is concerned with a phenomenological model to perform the failure analysis of composite adhesive single lap joints with arbitrary glued area. The theory is conceived for joints composed by highly resistant elastic adherends bonded with brittle–elastic adhesives. It is shown that, under certain conditions, the rupture forces (in the case of monotonic loading) and lifetimes (in the case of cyclic loading) of two joints with different glued areas can be correlated using a shape factor. Results from experimental static and fatigue testing of joints with carbon/epoxy laminates bonded with epoxy adhesive and different bonding areas are compared with model prediction showing a good agreement.