Micrographic studies on adhesively bonded scarf repairs to thick composite aircraft structure

The hard-patch approach to scarf repairs involves adhesively bonding a pre-formed patch into the scarf cavity. This approach has several potential advantages compared with the conventional soft-patch approach, which involves forming the patch from pre-preg and co-bonding it with the adhesive during cure of the patch directly in the repair cavity.Two methods for producing the hard-patch were investigated. The first was the moulded approach where the patch was laid up in a mould and cured prior to bonding in the repair cavity. The development and implementation of the moulded hard-patch repair technique on an F/A-18 horizontal stabiliser is described. The second approach involves machining the patch from a composite panel using digitised data obtained from the use of surface profiling equipment to capture the scarf cavity surface. Micrographic techniques were used to assess critical features of the bond-line produced from the different techniques. The results are compared with microscopic studies from a second F/A-18 horizontal stabiliser that was repaired much earlier using the soft-patch approach. Each repair is assessed in terms of the consolidation of plies along the bond-line and the conformity of the patch to the repair cavity as well as adhesive uniformity and porosity.     

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.     

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.     

Fatigue crack growth simulation in adhesively bonded composite joints

A numerical model has been developed for simulating fatigue crack growth (debonding) in adhesively bonded composite joints subjected to mode‐I, mode‐II, and mixed‐mode I + II loading conditions. The model employs a cohesive zone model described by a modified bilinear traction‐separation law. Fatigue damage in the composite adherends is not considered in the model. To account for crack divergence and reduce sensitivity of numerical results on mesh density, a crack front detection algorithm based on the effective element's length was employed. The model is implemented as a user‐defined subroutine (UMAT) in the commercial FE code LS‐DYNA. The model's input parameters, in the form of a modified Paris law, and the validation data were obtained from experimental tests conducted by the authors. It was found that the model is able to successfully simulate crack growth in the regime of the experimental data.     

复合材料夹芯管胶接连接结构力学特性分析

针对在航空结构中广泛应用的复合材料蜂窝夹芯圆管中的接头这一最脆弱的部分,发展了一种分析复合材料蜂窝夹芯圆管胶粘接头力学特性的解析模型.该模型根据Gibson修正公式得到了蜂窝芯子的等效弹性参数,再运用经典的复合材料壳理论和线弹性理论得到管接头的控制方程,并通过状态空间法进行求解.运用本文模型,计算了管接头在扭矩和弯矩作用下胶层内的剪应力和剥离应力;同时采用有限元法对模型进行了数值模拟,并将模拟结果与模型计算结果进行了对比,最后分析了搭接长度对胶层内应力的影响.     

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.     

An analytical solution for the analysis of symmetric composite adhesively bonded joints

Analytical solutions for adhesively bonded balanced composite and metallic joints are presented in this paper. The classical laminate plate theory and adhesive interface constitutive model are employed for this deduction. Both theoretical and numerical (finite element analysis) studies of the balanced joints are conducted to reveal the adhesive peel and shear stresses. The methodology can be extended to the application of various joint configurations, such as single-lap and single-strap joints to name a few. The methodology was used to evaluate stresses in several balanced adhesively bonded metallic and composite joints subjected to the tensile, moment and transverse shear loadings. The results showed good agreements with those obtained through FEM.     

Effect of debonding on adhesively bonded composite to metal joints in compression

This paper examines the effects of delamination damage in composite to metal joints. Experiments on debonded double lap joints of graphite/epoxy to aluminium were conducted. Various debonded lengths were considered. Finite element analysis was also carried out in order to study the effect of debond length on various lap joints. The experimental and analytical results were found to be in good agreement. It is also shown that as the size of the damage is increased a stage is reached after which a significant further increase in damage does not result in a significant decrease in residual compressive strength.     

End geometry and pin-hole effects on axially loaded adhesively bonded composite joints

An experimental investigation was performed to analyze the potential impacts of varying joint region geometries and adhesive filled pin holes on adhesively bonded composite structures. Tapers, especially half-length ones are observed to provide an anticipated progress in single lap joints. Besides, scarf joints with aligned adherends in the same plane exhibited enhanced stiffness and strength in consideration of single lap joints. In terms of the stiffness and strength, thickening of adherends was also found to be impressively efficient on composite single lap joints as well as scarf joints. Contrary to the expectation of that the hardened adhesive previously filled into the holes during adhesion would create a pin effect in load bearing, holey specimens exhibited poor performance and induced degradation in joint quality.     

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.     

Non-destructive testing of structural composites and adhesively bonded composite joints: pulsed thermography

Structural adhesive bonding is an enabling technology for the implementation of composite assemblies in automotive applications. Therefore, the quality and reliability of the composite bond must be assured. An advanced thermal non-destructive test (NDT) method, pulsed thermography, was evaluated for its capability to assess joint quality in an adhesively bonded composite pickup truck box. Pulsed thermography, used under in-plant, pre-production conditions as would exist during process start-up and optimization trials, was shown effective in determining both the quality of the structural adhesive bonds and the quality of the composite itself. With one exception, NDT showed that bonding was performed correctly, i.e. the bond was continuous and properly placed. The exception was a ‘starved’ bond-line that we believe exists due to poor fixturing at that location. Pulsed thermography illustrated the effects of environmental and mechanical exposure on the bonded joints. Finally, the NDT method was able to show mechanical damage to the composite itself, identifying impact damage not visible to the unaided eye.     

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.     

Failure analysis of adhesively bonded composite joints under transverse impact and different temperatures

In this study, load-carrying capacity of a single-lap joint bonded by an adhesive was found by experimental method. Glass fiber–epoxy composites were used as adherends. They were manufactured by using vacuum assisted resin infusion method (VARIM). Loctite 9466 A&B2 was used as an adhesive material. In this experimental study, two different surfaces (rough and smooth surface), four different temperatures (−20, 23, 50, and 80 °C) and four different impact energies (5, 10, 15, and 20 J) were considered. The result shows that when the temperature increases or decreases, adhesive joint losses its adhesion and rough surfaces provide high strength according to smooth surfaces. Load-carrying capacity decrease at 5, 10 and 15 J; however, it increases for 20 J.     

Buckling and debond growth of partial debonds in adhesively bonded composite splice joints

The strains at which buckling and debond growth occur in adhesively bonded composite flanges containing an initial debond were experimentally measured. Test parameters including initial debond geometry, flange material stiffness, and the adhesive critical strain energy release rate (G_c) were investigated. Debond growth was found to be strongly dependent on initial debond length but weakly dependent on flange width; i.e., debonding resistance did not increase in direct proportion with the bonded overlap dimension. Flanges having higher bending stiffness exhibited significantly lower debonding strain. Finally, the effect of G_c was evaluated at three levels by controlling the adhesive cure temperature and bondline thickness. Lower values of G_c (207 and 552 J/m²) allowed debond growth to occur while at the highest value of G_c (1500 J/m²), alternate failure modes occurred prior to debond growth. Ultrasonic C-scans revealed that debond growth occurred along a curved front, as dictated by the post-buckling deformation of the flanges.     

Ultimate behaviour of adhesively bonded FRP lap joints

This paper deals with a numerical investigation on double-lap and symmetrical single-lap joints subjected to shear/bending moment and axial force. The analysis has been developed using the theoretical model proposed by the author in [Ascione F. Mechanical behaviour of FRP adhesive joints: a theoretical model; 2007].The mechanical behavior of the adhesive is modeled through two sets of independent interfacial springs capable of characterizing the normal and transversal interactions, respectively. The adherents are modeled following the hypotheses of the beam technical theory. The mathematical model is based on two fundamental hypotheses: the possibility to separate the shear-flexure problem from the extensional one; the total fracture energy is additionally broken down in a term relative to mode I of fracture (opening) and in a term relative to mode II of fracture (sliding).Five dimensionless parameters which influence the design problem of the joints are identified. Several examples of the ultimate domains of the interface between the adherents are also presented as well as comparisons with some results reported in literature.     

Modelling of adhesively bonded joints by an asymptotic method

To utilize the potential of adhesive bonding, there is an increasing need for effective and accurate computational methods. The geometry and behaviour of an adhesive joint is, however, not so simple to model effectively by regular finite elements. The main reason is that the very thin adhesive layer with a low Young’s modulus must be modelled by a large number of finite elements in the thickness direction to achieve sufficiently accurate calculations. To overcome this difficulty, a material surface treatment of the adhesive and the joined parts can be attempted. This paper concerns the derivation of such a model by introducing scalings on the geometry and on the material properties in terms of a perturbation parameter. Within the framework of three-dimensional elasticity, together with an asymptotic expansion method, a family of limit models are obtained through a systematic procedure. In such a derivation no a priori assumptions on the displacements or stress fields are needed. The final result is a variational equation posed over a single reference surface. In regions near the boundary of the joint a boundary layer phenomena occurs. This indicates that the asymptotic series needs to be complemented by additional terms, in order to satisfy all boundary conditions. A structural model including shear- and peel deformation is finally proposed which improves the solution close to the boundary.     

Analysis of adhesive bonded composite lap joints with transverse stitching

The effect of transverse stitching on the stresses in the adhesive is investigated using an adhesive sandwich model with nonlinear adhesive properties and a transverse stitching model for adhesive bonded composite single-lap and double-lap joints. Numerical results indicate that, among all stitching parameters, thread pretension and stitch density have significant effect on the peel stresses in the adhesive; increase in the thread pretension and the stitch density leads to a decrease in peel stress in the adhesive, while an increase in other parameters generally results in a negligible reduction in peel stress. The effect of stitching was found to be negligible on the shear stresses in the adhesive. Thus it is concluded that stitching is effective for the joints where peel stresses are critical and ineffective for those where shear stresses are critical.