Mode-I,mode-II and mixed-mode I+II fracture behavior of composite bonded joints: Experimental characterization and numerical simulation

The fracture behavior of composite bonded joints subjected to mode-I, mode-II and mixed-mode I + II loading conditions was characterized by mechanical testing and numerical simulation. The composite adherents were bonded using two different epoxy adhesives; namely, the EA 9695 film adhesive and the mixed EA 9395-EA 9396 paste adhesive. The fracture toughness of the joints was evaluated in terms of the critical energy release rate. Mode-I tests were conducted using the double-cantilever beam specimen, mode-II tests using the end-notch flexure specimen and mixed-mode tests (three mixity ratios) using a combination of the two aforementioned specimens. The fracture behavior of the bonded joints was also simulated using the cohesive zone modeling method aiming to evaluate the method and point out its strengths and weaknesses. The simulations were performed using the explicit FE code LS-DYNA. The experimental results show a considerable scatter which is common for fracture toughness tests. The joints attained with the film adhesive have much larger fracture toughness (by 30–60%) than the joints with the paste adhesive, which exhibited a rather brittle behavior. The simulation results revealed that the cohesive zone modeling method performs well for mode-I load-cases while for mode-II and mixed-mode load-cases, modifications of the input parameters and the traction-separation law are needed in order for the method to effectively simulate the fracture behavior of the joints.     

Fracture toughness and shear behavior of composite bonded joints based on a novel aerospace adhesive

In this paper, the mechanical performance of the newly developed LMB aerospace structural adhesive has been characterized experimentally. To this end, a comparison has been performed with the Epibond 1590 A/B adhesive on the basis of the effects of thermal aging, wet aging and adhesive thickness on the fracture toughness and shear behavior (static and fatigue) of bonded joints between CFRP laminates. Mode-I and -II fracture toughness was measured through tensile and 3-point bending tests on double-cantilever beam specimen, respectively, while the shear behavior of the joints was assessed through static and fatigue double-lap shear tests. A fractographic analysis of the fracture surfaces was performed in order to detect the failure mode of the bonded joint. The experimental results show a higher fracture toughness and improved fatigue behavior for the LMB adhesive.     

Fracture and yield behavior of adhesively bonded joints under triaxial stress conditions

Most of adhesively bonded joints are under complicatedly distributed triaxial stress in the adhesive layer. For the estimating of the strength of adhesively bonded joints, it is crucial to clarify behavior of yield and failure of the adhesives layer under triaxial stress conditions. Two types of the adhesively bonded joints were used in this study: One is the scarf joint which is under considerably uniform normal and shear stresses in the adhesive layer, where their combination ratio can be varied with scarf angle. The other is the butt joint with thin wall tube in which considerably uniform pure shear can be realized in the adhesive layer under torsional load conditions. These joints can cover the stress triaxiality in adhesive layers of most joints in industrial application. The effect of stress triaxiality on the yield and fracture stresses in the adhesive layer were investigated using the joints bonded by three kinds of adhesives in heterogeneous and homogeneous systems. The results showed that both the yield and failure criterion depend on the stress triaxiality and that the fracture mechanism of the homogeneous adhesive is different from that of the heterogeneous one. From these experimental results, a method of estimating the yield and failure stresses was proposed in terms of a stress triaxiality parameter.     

Out-of-plane behavior of unreinforced masonry walls strengthened with FRP strips

The out-of-plane behavior of unreinforced masonry walls strengthened with externally bonded fiber reinforced polymer (FRP) strips is analytically studied. The analytical model uses variational principles, equilibrium requirements, and compatibility conditions between the structural components (masonry units, mortar joints, FRP strips, and adhesive layers) and assumes one-way flexural action of the strengthened wall. The masonry units and the mortar joints are modeled as Timoshenko’s beams. The FRP strips are modeled using the lamination and the first-order shear deformation theories, and the adhesive layers are modeled as 2D linear elastic continua. The model accounts for cracking of the mortar joints and for the development of debonding zones near the cracked joints. Numerical and parametric studies that reveal the capabilities of the model, throw light on the interaction between the variables, and quantitatively explain some aspects of the behavior of the strengthened wall are also presented.     

Nonlinear analysis of adhesively bonded tubular single-lap joints for composites in torsion

The nonlinear analysis of tubular single-lap adhesive joints with composite adherends was performed by incorporating the nonlinear behavior of the adhesive into the analysis. For this purpose, the stress and strain in the laminated composite tube were first calculated under a general loading scheme, and then the iterative solution of the joints was derived by including the nonlinear properties of the adhesive. The stress distributions in the adhesive were investigated for different types of composite adherends and compared with the results of the linear analysis. The effect of the bonding length on torque transmission capability was also taken into consideration. The results indicate that the nonlinear analysis relieves the stress concentration at the edge of the joint, resulting in a more accurate prediction of joint strength.     

Optimum thickness of joints made of GFPR pultruded adherends and polyurethane adhesive

This paper presents results of experimental and numerical investigations on double-lap joints composed of pultruded GFRP profiles and polyurethane adhesive subjected to quasi-static axial tensile loading. The objective was to investigate the effect of the joint geometry on the structural response of adhesively-bonded joints and, in particular, to seek for experimental evidence of an optimum adhesive layer thickness. The influence of the adhesive thickness (0.3–10.0 mm) and the overlap length (50–200 mm) on the joint behavior was investigated. It was found that there is an optimum adhesive thickness of approximately 1.0 mm and joint strength consistently increases with the overlap length.     

Strength prediction of tubular composite adhesive joints under torsion

This paper addresses prediction of the strength of tubular adhesive joints with composite adherends by combining thermal and mechanical analyses. A finite element analysis was used to calculate the residual thermal stresses generated by cooling down from the adhesive cure temperature, and a nonlinear analysis incorporating the nonlinear adhesive behavior was performed to accurately estimate the mechanical stresses in the adhesive. Joint failure was estimated by three failure criteria: interfacial failure, adhesive bulk failure, and adherend failure. The distributions of residual thermal stresses were investigated for various stacking angles. The effect of residual thermal stresses on joint strength was also taken into consideration. The results indicate that the residual thermal stresses, depending on the stacking angle, have a significant influence on the failure mode and strength of adhesive joints when a subsequent mechanical load is applied. Good agreement is also obtained between the predicted joint strength and the available experimental data.     

Effect of crack-growth mechanism on the prediction of fracture load of adhesive joints

This paper presents observations regarding the cracking behavior of tensile-loaded structural adhesive joints. Experiments showed that fracture occurred by the development and propagation of a damage zone, rather than a single, sharp crack, and that the presence of the adhesive spew fillet did not affect the fracture load of the adhesive joints studied. For joints bonded with the mineral-filled epoxy Cybond 4523GB (American Cyanamid), there was approximately 5 mm of subcritical crack propagation prior to final fracture. Fracture-load predictions based on the initial uncracked geometry made in previous papers were unaffected by this small change in geometry. For joints bonded with the rubber-toughened epoxy Permabond ESP 310, approximately 50 mm of subcritical crack propagation was observed. It was again found that predictions made in previous papers on the basis of the initial geometry gave a good estimate of the final fracture load even though this subcritical crack propagation significantly altered the geometry, and thus the applied energy release rates. The effect of shear deformations of the adherends was also investigated, and it was found that shear deformations could be neglected in engineering calculations for joints subject to remote tensile loading.     

Crack path selection in the fracture of fresh and degraded epoxy adhesive joints

The crack paths and fracture surfaces of aluminum–epoxy adhesive joints were characterized as a function of the mode ratio of loading and the amount of degradation that had been generated using the open-faced aging technique. A finite element (FE) model was used to predict the extent of the plastic zone at different crack growth lengths and mode ratios, and a close relationship was found between the evolution of the plastic zone and the previously reported R-curve behavior of these joints. The micro-topography of the fracture surfaces, measured using an optical profilometer, showed that a ductile–brittle transition occurred in the fracture behavior of the joints as degradation progressed. The crack path in the (brittle) degraded specimens was normal to the first principal stress, but could not be predicted in the undegraded joints because of its highly three-dimensional nature. Based on the distribution of the maximum von Mises stress in the adhesive layer ahead of the crack tip, a crack growth mechanism was proposed that is consistent with these experimental observations and explains the highly three-dimensional nature of fracture in these highly constrained joints.     

胶粘剂在粘接接头内热分解动力学的计算

为了研究粘接接头内胶粘剂的耐热性能,采用X射线能谱分析确定了不同条件下粘接接头内胶粘剂的元素组成及其变化行为,利用X射线能谱计算了胶粘剂的热失重率,进而计算出聚酰亚胺薄膜粘接接头内胶粘剂的热分解动力学,并与空气环境下胶粘剂热分解活化能进行了比较.计算结果表明,粘接接头内胶粘剂的热分解速率低于空气环境下胶粘剂热分解速率,这种分析测试方法为原位表征粘接接头内胶粘剂耐热性能提供了一种新的分析方法.     

缺陷对单搭胶接接头力学性能的影响

通过单向拉伸试验对比分析了两种接头的破坏模式及载荷-位移曲线, 研究了T700/TDE85复合材料单向层合板单搭接胶层内缺陷对接头破坏行为的影响。试验结果表明, 接头破坏的主导模式为界面破坏, 胶层中微小缺陷对接头强度的影响不大。为研究接头的失效机制, 采用有限元方法对两种接头失效进行数值分析, 模拟了接头搭接区界面剥离应力及剪切应力分布情况, 并分析了缺陷位置变化及面积变化对接头强度的影响。结果表明, 随着缺陷位置距接头搭接区自由端部越近, 接头强度越小, 且缺陷位置距接头搭接区自由端部2.5 mm以内, 缺陷对接头强度影响较大; 接头强度随缺陷面积的增大而减小, 并且缺陷面积占搭接区面积的比率在4.4%以内, 缺陷对接头强度的影响较小; 数值计算结果与实验结果吻合较好。     

A nonlinear viscoelastic–viscoplastic constitutive model for adhesives under creep

Mechanics of Time-Dependent Materials - In this study, we consider the nonlinear viscoelastic–viscoplastic behavior of adhesive films in scarf joints. We develop a three-dimensional nonlinear...     

Fatigue life estimation of adhesive bonded shaft joints

Fatigue tests and analytical investigation of adhesive bonded shaft joints were conducted to propose the estimation method of fatigue strength. Two kinds of adhesive bonded joints were studied: one, shaft joints connected with adhesive coupling, the other, adhesive joints of thin wall tubes to obtain standard fatigue strength. Both pulsating tensile and torsional fatigue tests were conducted with each adhesive joint. Furthermore, the stress distributions under tensile and torsional load conditions were analyzed by finite element method. Based on the analytically computed maximum normal shear stress in the adhesive layer, fatigue strength of the shaft joints was tandardized and compared with that of adhesive joints of thin wall tubes. As a result, it is confirmed that the maximum normal and shear stresses are key parameters for estimating fatigue strength under pulsating tensile and forsional load conditions, respectively. Furthermore, this study indicates an improved method of estimating fatigue strength by using tapered coupling order to reduce the stress concentration at the end of the adhesive layer.     

Structural behavior of double-lap shear adhesive joints with metal substrates under humid conditions

Structural adhesive bonding is very often used joining method in aerospace and automotive industry, but in civil engineering, especially in façade applications, semi-flexible or semi-rigid adhesives are still rarely used. The article is focused on experimental analyses of structural adhesive joints intended for façade applications (e.g. bonding of façade cladding elements to the supporting substructure). The experimental study contains a comparison of the structural behavior of two different adhesives in joints with aluminum or zinc-electroplated steel substrates with various surface pre-treatments. The main goal of the study is a comparison of the mechanical properties of joints exposed and unexposed to laboratory ageing conditions; immersion on demineralized water according to ETAG 002 (Guideline for European Technical Approval for Structural Sealant Glazing Kits). Water content in adhesive layer can change significantly its mechanical properties and adhesion of glue to the substrate. Ageing resistance of joint can be improved by durability increasing of the substrate. For this reason, two different substrate materials with various surface treatments (mechanical roughening, smooth surface, anodizing) were tested. Different adhesive resistance against humid conditions was observed depending on the substrate material and pre-treatment. STP polymer joints showed strength reduction by 30% after immersion for almost all substrates, while acrylate adhesive proved 20% strength reduction for roughened aluminum substrate and 60% strength reduction for zinc-electroplated steel substrate with a roughened surface. The zinc-electroplated steel substrate showed problematic adhesion in case of the acrylate adhesive both reference set of specimens and specimens exposed to laboratory ageing. The positive effect of roughening on adhesion and ageing resistance was clearly observed in the specimens bonded by the acrylate adhesive.

     

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.     

Effects of low and high temperatures on tensile behavior of adhesively-bonded GFRP joints

The tensile behavior of adhesively-bonded double-lap joints composed of pultruded glass fiber-reinforced adherends and an epoxy adhesive was investigated under temperatures ranging between −35 °C and 60 °C. The load–elongation response was influenced primarily by the thermomechanical behavior of the adhesive and much less so by that of the adherends. For temperatures above the adhesive glass transition temperature, strength and stiffness decreased with the former being less affected than the latter. The failure mechanism changed with increasing temperature from fiber-tear to adhesive failure. The crack initiation loads were unaffected as long as the temperature remained below the adhesive glass transition temperature. However, the crack propagation rate was higher at low temperatures. Critical strain energy release rates for crack initiation and propagation consistently rose as temperature increased. Modeling results obtained using existing empirical models and FEA compared well to the experimental data in the examined temperature range.     

Strength prediction of adhesive joints after cyclic moisture conditioning using a cohesive zone model

This paper presents a methodology to predict the strength of adhesive joints under variable moisture conditions. The moisture uptake in adhesive joints was determined using a history dependent moisture prediction methodology where diffusion coefficients were based on experimental cyclic moisture uptake of bulk adhesive samples. The predicted moisture concentrations and moisture diffusion history were used in a structural analysis with a cohesive zone model to predict damage and failure of the joints. A moisture concentration and moisture history dependent bilinear cohesive zone law was used. The methodology was used to determine the damage and failure in aluminium alloy – epoxy adhesive single lap joints, conditioned at 50 °C and good predictions of failure load were observed. The damage in the adhesive joints decreased the load carrying capacity before reaching the failure load and a nonlinear relationship between the load and displacement was observed. Changes in crack initiation and crack propagation were also observed between different types of joints. The presented methodology is generic and may be applied to different types of adhesive joint and adhesive.     

基于循环载荷的单搭胶接接头残余强度分析

为研究循环载荷下单搭胶接接头的残余强度及失效机理,以5052铝合金单搭胶接接头为研究对象,先后对其进行静强度测试、疲劳强度测试和残余强度测试,引入威布尔分布对试验数据进行分析,检验其有效性,并采用超声扫描显微镜和扫描电子显微镜对失效胶层进行失效机理分析。结果表明,在疲劳循环载荷作用下,接头刚度基本稳定,而残余强度随着疲劳循环载荷周次的增加,呈现出先增大后减小的变化趋势;疲劳裂纹从接头搭接端部的界面端点处开始萌生,并快速向中间扩展,当疲劳循环达到一定次数时,胶层瞬间断裂,裂纹萌生阶段几乎占据了其全部疲劳寿命,失效后的胶层会出现"凹台"状微观结构。     

The Effect of Composite Patches on the Failure of Adhesively-Bonded Joints Under Bending Moment

In this study, it was aimed to compare mechanical behavior of double-strap joints with aluminum (AA2024-T3) or 16-ply laminate of carbon/epoxy composite (T300/934) patches of different orientation angles at their overlap area subjected to bending moment. For this purpose, AA2024-T3 aluminum was used as adherend, while the adhesive was a two-part paste (DP 460). Six different types of joint samples were subjected to bending moment. The effect of patch material on failure load and stress distribution was examined experimentally and numerically. In the numerical analysis, the composite patches were assumed to behave linearly elastic, while adherend and adhesive layers were assumed to be nonlinear. It was found that the data obtained from 3-D finite element analysis were coherent with experimental results. Meanwhile, experiments showed that fiber orientation angles of the patches markedly affected the failure load of joints, failure mode and stress distributions appeared in adhesive and composite.     

Strength prediction of adhesive joints using the modified damage zone theory

The composite joint has become an important research area because the structural efficiency of a structure with a joint is determined by its joints rather than by its basic structure since the joints are often the weakest areas in composite structures. In this paper, the strengths of adhesive joints consisting of metal and composites were predicted and tested by the modified damage zone theory. Nonlinear finite element analyses of adhesive joints considering the material nonlinearity of the adhesive layer were performed. From the tests and analyses, the strengths of the adhesive joints could be predicted to within 20.1% using the modified damage zone theory.