Stress Analysis of Tubular Adhesive Joints with Delaminated Adherend

The use of adhesive joints is becoming increasingly important in aerospace, automotive and other industries where the use of traditional fasteners is discouraged. When using composite adherends, the use of adhesively bonded joints is preferable rather than the traditional bolts and other types of fasteners, because they do not require holes, thereby removing the problems of stress concentrations around the holes. However, when using an adhesively bonded joint, there will be concentrations of the distributions of shear and peel stresses within the adhesive layer which should be controlled effectively. Therefore, the investigation of such stress variation has attracted many researchers. The aforementioned stress distributions become more complicated if the composite adherend contains a pre-existing delamination. Delamination is one of the most common failure modes in laminated composite materials; it can occur due to sudden impact by an external object, during the manufacturing process (e.g., during the filament winding process), or as a result of excessive stresses due to an applied load. It is clear that the existence of a delamination in any composite structure causes a reduction in its stiffness and in some critical situations, it may cause complete failure. This paper investigates the effect of delamination on the structural response of an adhesively bonded tubular joint with composite and aluminum adherends. The finite element method, using the commercial package ABAQUS, is used to conduct a parametric investigation. The effects of the delamination's spatial location, length, width, and the applied loading are studied. Results provide interesting insight (not necessarily intuitive) into the effect of an interlayer delamination on the stress distribution within the adhesive.     

Through-the-width delamination damage propagation characteristics in single-lap laminated FRP composite joints

Three-dimensional non-linear finite element analyses have been carried out to study the effects of through-the-width delaminations on delamination damage propagation characteristics in adhesively bonded single-lap laminated FRP composite joints. The delaminations have been presumed either to pre-exist or to get evolved due to coupled stress failure criteria in the laminated FRP composite adherends near the overlap ends beneath the ply adjacent to the overlap region. The out-of-plane stresses in the adhesive layer, the interlaminar stress distributions along the delamination fronts and the strain energy release rates (SERRs) corresponding to the three individual modes have been evaluated for varying positions of the delaminations pre-embedded in either of the adherends. A good matching between the present 3D results and experimental and analytical solution of the literature has been established for the undamaged and a damaged model. A significant difference in the interlaminar stresses and the SERR values has been observed and is largely dependent on the adherends (bottom or top) possessing the through-the-width delamination damages. Also, the interlaminar stresses and SERR values along the two corresponding delamination fronts are different. Accordingly, it can be concluded that the positions of the through-the-width delaminations significantly influence the delamination damage propagation behaviour vis-a-vis the performance of the composite joint.     

Analysis of thermoelastic interaction of manufacturing stresses along the interface on interlaminar delamination progression in multi-ply composite laminates

This paper deals with the study of interaction of manufacturing thermal residual stresses and mechanical loading in penny-shaped delaminations embedded between dissimilar, anisotropic fiber composite layers by conducting two sets of three-dimensional thermoelastic finite element analyses with and without residual stress effects. Modified crack closure integral (MCCI) techniques based on the concepts of linear elastic fracture mechanics (LEFM) have been used to calculate the distribution of individual modes of strain energy release rates (SERR) to investigate the interlaminar delamination initiation and propagation characteristics. Asymmetric variations of strain energy release rates obtained along the delamination front are caused by the overlapping stress fields due to the coupling effect of thermal and mechanical loadings. It is found that parameters such as ply sequence and orientation, thermoelastic anisotropy and material heterogeneity, and ply properties of the delaminated interface dictate the interlaminar fracture behavior of multi-ply laminated FRP composites.     

Polypropylene matrix composites reinforced with pre-stressed glass fibers

Thermoplastic matrix composites have recently emerged as promising engineering materials because of their desirable properties such as high service temperatures, high impact resistance, and processing advantages. However, residual stresses in composites introduced during fabrication are cited as one of the most significant problems in the processing of composites. In some instances these stresses have been shown to significantly degrade the strength of the material, resulting in matrix cracking, debonding, reduced fracture toughness, and delamination. In this work, studies have been carried out on glass fiber reinforced polypropylene composites formed by compression molding process from co-mingled fabrics. The fibers were pre-stressed during the process to produce high performance composite products with low residual microstresses, which are harmful to the properties of the composite. Mechanical tests showed that pre-stress can increase the tensile, flexural and interlaminar shear properties of the composites, and there exists an optimum pre-stress level to gain best properties for each external loading condition.     

风电叶片复合材料层间剪切破坏声发射监测

采用声发射技术对含分层缺陷风电叶片多轴向复合材料的层间剪切破坏实验进行实时监测,研究分层缺陷对复合材料层间力学性能的影响规律及其损伤破坏过程的声发射响应特征.结果表明,具有不同分层面积的两类复合材料试样破坏载荷相近,当分层缺陷位于剪切面中间位置时,分层缺陷大小对界面承载能力影响不大,损伤演化主要集中在剪切面上偏离中心两...     

Non-destructive characterization of bondlines in composite adhesive joints

Aerospace structures use polymeric composite materials extensively. These composite materials are normally bonded together by adhesives to form structural parts. The existence of any kind of defects or discontinuities in the bonds is completely undesirable for such applications. Ultrasonic imaging (UI) is a widely used technique for non-destructive evaluation (NDE) and can be adopted to evaluate the integrity of such adhesively-bonded joints. However, characterization of adhesive bonds in composite materials using UI has deficiencies due to problems such as high acoustic attenuation and high signal-to-noise ratio. These problems can be attributed to the inhomogeneity in composite structures. The present study addresses the problems of detection of disbonds and porosity in adhesively bonded carbon fiber reinforced composite panels. Five sets of adhesively-joined carbon/epoxy composites with different adherend surface preparations were fabricated and subjected to UI. The panels contained known defects in the bondline of the samples. UI results are interpreted to identify various existing defects such as voids, cracks and disbonds in the joints. Attenuation coefficient values for all types of composites are utilized to ascertain the validity of the image analysis.     

Photoelastic thermal stress measurements in scarf adhesive joints under uniform temperature changes

This study deals with the investigation of thermal stresses and delamination growth in scarf joints under a uniform temperature change by photoelastic measurements and a two-dimensional finite element analysis. The adherends were fabricated from aluminum plates, and an adhesive layer was modeled and fabricated from an epoxide resin plate. The adherends and the epoxide resin plate were bonded using a heat-setting and one-component-type adhesive. The adhesive was cured at 85 °C and cooled down to room temperature. The thermal stress was then generated in the scarf joint under a temperature change and measured by photoelasticity. After the scarf joints were cooled in a stepwise manner, the delamination growth, which initiates from the edge of the interface, was measured. It was found that the delamination initiates from the edge of the interface with the acute angle side and it never initiates from the edge with the obtuse angle side. When the scarf angle is 90°, i.e. in adhesive butt joints, the resistance against the delamination is minimal. The thermal stresses in the scarf joints with a thin adhesive layer were also analyzed. It was found that the thermal strength increases as the adhesive thickness decreases. The stress singularity near the edge of the interface was calculated from the stress distributions in the joints with different scarf angles. As a result, it was found that the stress singularity in the scarf joints under thermal loads is quite different from that under static tensile loads.     

Fracture mechanics approach to forecast delamination lifetime

Interlaminar fracture (delamination) is one of the major concerns in the design of laminated composite structures, adhesive joints, coatings, sealants and other multilayered material systems. Service lifetime of a laminated structure is limited by the time an interlaminar flaw propagates to a size perceived critical to the stiffness and/or the strength of the structure. The time required to cause certain magnitude of delamination, under stresses below the initiation stress, could be forecasted if the constitutive equation for the rate of delamination is known. This paper describes an approach to develop the constitutive equation for delamination under mode I conditions. The approach rests on principles of linear elastic fracture mechanics (LEFM) and uses elevated temperature to accelerate interlaminar fracture at constant loads. The experiments used double cantilever beam test specimens fabricated as a model system from poly(methyl methacrylate) (PMMA) beams and epoxy adhesive whose stiffness was equivalent to that of a typical carbon/epoxy laminated composite. Mechanistic observations indicated that the fracture front displayed similar mechanism at all test conditions. A modified form of Paris power law is suggested to forecast service lifetime in terms of temperature, service load and the initial flaw size.     

Effect of imbibed moisture on monotonic and low-velocity impact behavior of injection over-molded short/continuous fiber reinforced polypropylene composites

Design of automotive components with over-molded short/continuous fiber reinforced thermoplastic composites necessitates understanding of their behavior under extreme outdoor conditions. The short, quasi-isotropic and over-molded short/continuous glass fiber reinforced polypropylene (PP) composite specimens were prepared as per standard and immersed in water until equilibration to study their relative moisture absorption characteristics and consequent mechanical behavior. As the absorbed moisture mostly occupied the interface between fiber and matrix in laminated composite inserts and moisture absorption of short fiber composite core is insignificant, the moisture absorption of over-molded composites is just above 50% of that of laminated composites. The flexural, interlaminar shear and impact behavior of equilibrated composites is primarily governed by the quantum of imbibed moisture of composite materials. Optical analysis of failed moisture equilibrated over-molded specimens showed a marginal delamination between plies of the inserts without any perceptible damage within the short fiber composite similar to dry as molded specimens.     

3D FE delamination induced damage analyses of adhesive bonded lap shear joints made with curved laminated FRP composite panels

This paper deals with the evaluation of inter-laminar stresses in the adhesive layer existing between the lap and the strap adherends of lap shear joints (LSJ) made with curved laminated fibre reinforced plastic (FRP) composite panels for varied embedded delaminations between the first and second plies of the strap adherend. Non-linear finite element analyses have been carried out using contact and multi point constraint (MPC) elements. The use of contact elements ensures avoidance of inter-penetration of delaminated surfaces. Sequential release of MPC elements facilitates computation of individual modes of Strain Energy Release Rates (SERR). The effects of varied delamination lengths on variations of peel and inter-laminar shear stresses and different modes of SERR are seen to be very significant. Their variations on both the delamination fronts, for each size of the delamination, are found to be much different from each other indicating different propagation rates at the two delamination fronts. The structural integrity of the LSJ in the presence of delaminations, thus, can be predicted with adaptive finite element (FE) simulations. It is further seen that the peak stress magnitudes and SERRs are higher in the LSJs made with curved FRP composite panels as compared to the flat laminates. This may be due to the stiffening effects induced by the curvature geometry of the curved composite panels.     

Impact-Induced delamination in [05, 905, 05] carbon fiber/polyetheretherketone composite laminates

This paper reports both experimental and numerical investigations on delamination mechanisms in [0₅, 90₅, 0₅] carbon fiber(CF)/poly(etheretherketone) (PEEK) laminate subjected to low-velocity impact. It was found that the CF/PEEK composite exhibits the same damage mechanisms as epoxy-based composites, but superior delamination resistance. For the crossply laminate, the impact delamination results from a Mode II interlaminar fracture process, and a close association exists between the interlaminar shear stress field and the delamination growth. The prediction of impact-induced delamination sizes is discussed.     

Mechanical behavior of interlocking multi-stepped double scarf adhesive joints including void and disbond effects

Surface topographical effects on the mechanical behavior of interlocking multi-stepped double scarf adhesive joints under tensile load were studied. For this purpose, finite element analysis (FEA) of the joint geometry at 10 different step angles was carried out. In the second stage, the effects of substrate voids and adhesive delaminations on the interfacial strength were studied for the scarf angle of 32.2° by FEA simulation as well as experimentally. For the cases of the missing steps (voids) and delamination (absence of bonding induced by release agent) the ratios of maximum stresses (principal, von Mises, normal, shear and transverse) between the completely bonded and altered (void or delaminated) joints were compared with the failure load ratios for the same joints to interpret the mechanism of failure. The results revealed that except for the normal stress, the maximum stress ratios reach a maximum value and then decrease with increasing scarf angle. FEA analysis with the voids showed that the strength of the joint not only depends on their size, but also on their location in the joint. When the experimental results were compared with the FEA using the stress ratio between the unmodified (completely bonded) and modified (void or disbond) cases, the results indicated that the normal stress dominates the failure behavior of the 32.2° scarf angle joint. Comparison of the experimental results for the void, and disbond cases revealed that the disbond cases can possess higher joint strength in comparison to the void cases. This finding could not be predicted by FEA, and was attributed to the presence of friction at the interface subsequent to delamination.     

Controlled energy dissipation in fibrous composites I. Controlled delamination

Delamination mechanisms in continuous fiber reinforced composites were investigated. The concept of controlled interlaminar bonding (CIB) is proposed as a guideline for preparing fiber-epoxy composite laminates with enhanced fracture toughness without significant degradation in strength properties. The interlaminar bonding was manipulated by several specialized techniques including insertion of delamination promotors and surface modification of laminae. Results indicated that the plane-strain fracture toughness of E-glass-epoxy laminates could be improved by inserting perforated interlaminar films of aluminum, paper, polyester and polyimide, and fabrics. Such interlayers were used to promote delamination which dissipate strain energy by blunting and diverting a propagating crack. The fracture resistance of a laminate was found to be dependent on the degree of delamination. The competition between the growth of delamination cracks and the propagation of a main crack is controlled by the relative magnitude of the interlaminar bonding strength and the lamina cohesive strength. The interlaminar bonding is controlled by the degree of interlayer perforation and the adhesion between interlayer and lamina. The loading direction was found to be very important in dictating the failure processes. Experimental results from several composite systems are presented and discussed along with post-failure analysis data.     

A simple approach for characterizing the performance of metallic tubular adhesively-bonded joints under torsion loading

Adhesive bonding of joints is one of the most commonly and widely used joining methods in piping systems. This work is concerned with the investigation of the influence of the non-linear behavior of the adhesive used in such bonded joints on their performance. The parametric analysis module of ABAQUS was used to model the joint. The model facilitated the analysis of different geometric, loading and material characteristics of the system, in particular the adhesive nonlinearity, which is of prime interest in this work. By using the Ramberg–Osgood plasticity model, the failure threshold of the adhesive for various joint lengths (hereafter referred to overlap length) was characterized. The plasticity model used in this study was fine-tuned using only a limited number of known parameters, through comparison with the results of the finite element (FE) simulation. The results obtained from the FE analysis were verified by experimental results. The FE strategy is demonstrated to be an effective means for predicting the capacity of such joints, where conducting a pure shear test is either impossible or difficult to accomplish. Contrary to the findings based on the elastic finite element analysis, the plasticity analysis revealed that the overlap length affects the ultimate strength of the joint.     

Delamination propagation analyses of spar wingskin joints made with curved laminated FRP composite panels

Initiation and propagation of inter-laminar delamination in adhesive bonded spar wingskin joint (SWJ) made with laminated fibre-reinforced plastic (FRP) composite curved panels have been studied employing three-dimensional finite element analyses. In-plane and out-of-plane normal and shear stress distributions are seen to be highly three-dimensional in nature. Tsai-Wu coupled stress failure criteria have been employed to identify critical locations of onset of delamination-induced damage. This occurs underneath the toe-end of the spar overlap and at the inter-laminar surface between the first and second plies of the curved FRP wingskin panel. Significant edge effects on the joint strength have been observed due to the curvature geometry of the composite wingskin panels. Non-linear finite element analyses have been carried out for study of delamination propagation using contact and multi point constraint (MPC) elements. The use of contact elements prevents inter-penetration of delaminated surfaces. Whereas, sequential release of MPC elements facilitates computation of opening, sliding and cross-sliding modes of delamination-induced strain energy release rates (SERR) by using virtual crack closure technique. Variation in delamination lengths significantly effects the variation of peel and inter-laminar shear stresses and different modes of SERRs. Variations on the two delamination fronts are seen to be quite different indicating dis-similar propagation rates. The Mode I SERR (G_I) predominantly governs the delamination propagation in the SWJ.     

Yield behavior of rubber-modified epoxy adhesives under multiaxial stress conditions

In rubber-modified epoxy resins, a damage zone is generated in the vicinity of the crack tip due to the cavitation of rubber particles, which improves fracture toughness dramatically. Hence, in evaluating the stress distribution in adhesive joints with rubber-modified adhesives, the void formation and growth should be taken into account. In most studies, however, the adhesive layer is still considered as a continuum material governed by the von Mises yield criterion. For many ductile materials, Gurson's model is used for the stress analysis, in which the void formation and growth is taken into account. In a previous study, using adhesively bonded scarf and torsional butt joints, the effect of stress triaxiality on the yield stress in the adhesive layer was investigated. In this study, these experimentally-obtained yield stresses were compared with those obtained by a finite element method, where Gurson's constitutive equations were applied to the adhesive layer. As a result, the calculated yield stresses agreed well with the experimentally-obtained yield stresses. This indicates that Gurson's model is a useful tool for estimating stress distributions in adhesive joints with rubbermodified adhesives.     

Fatigue delamination growth under cyclic compression in glass/epoxy composite beam/plates

Results are reported on the fatigue growth of internal delaminations in glass/epoxy composite beam/plates subjected to constant amplitude cyclic compression. Because of compressive loading, these structures undergo repeated buckling/unloading of the delaminated layer with a resulting reduction of the interlayer resistance. A noteworthy feature of the problem is that the state of stress near the delamination tip is of mixed mode (I and II). The present combined experimental/analytical investigation for the glass/epoxy composites complements our earlier studies on delamination growth under cyclic compression in unidirectional graphite/epoxy specimens. Several configurations are studied with the delamination located at different depths (through the thickness) and with different applied maximum compressive displacements. The experimental data are correlated with the predictions from a combined delamination buckling/postbuckling and fracture mechanics model. A mode-dependent fatigue delamination growth law is used together with an initial postbuckling solution for the deformation pattern of the delaminated layer and the substrate, which does not impose any restrictive assumptions on the delamination thickness and plate length. The experimental data seem to be adequately correlated with the theory and the fatigue delamination growth is found again to be strongly affected by the relative location of the delamination through the plate thickness. Finally, a comparison of the cyclic growth rate in glass/epoxy specimens with the corresponding one in graphite/epoxy specimens of the same geometry and applied loading shows that the delamination would grow much faster in the graphite/epoxy specimens.     

Use of polymeric emeraldine salt for conductive adhesive applications

The wide range of electrical, electrochemical, and optical properties associated with Polyaniline (PANI) and its composites has made them attractive for many industrial applications. In this study, Emeraldine Salt (ES), which is a doped conducting form of PANI, was chemically prepared in situ using the oxidizing agent ammonium persulphate in the presence of aqueous HCl solution. In order to gain insight into the efficiency of electrical conduction in relation to the chemical and viscoelastic behaviors of ES in homogeneous powder and as filler in composite adhesive forms, the interrelationship between their electrical resistivity and morphology was studied. The pressure-dependent electrical conduction behavior of ES powder shows, among other factors, the dependence of electrical resistivity on the intrinsic chemical and viscoelastic properties of powders. In order to obtain electrically conductive composite adhesive forms, a nonconducting nitrocellulose solution based adhesive was filled with as-synthesized ES in the amount 30%, 40%, and 50% by volume, and the effects of filler concentration on the composite's electrical resistivity were investigated. The results of our investigation revealed a typical percolation threshold behavior with a critical concentration of approximately 30% by volume. Finally, single lap joints were made using aluminum and zinc (plated on copper) as well as silver substrates bonded using the ES filled nitrocellulose adhesive developed, and the corresponding electrical and mechanical properties of these bonded interconnections were investigated. A complex redox-reaction mechanism catalyzed by ES filler is thought to be occurring at the boundary layer between the adhesive and the substrate for conversion from semiconductor to insulator of the joints in the cases of aluminum and zinc (plated) substrates.     

Finite element analysis of torsional free vibration of adhesively bonded single-lap joints

Adhesively bonding is a high-speed fastening technique which is suitable for joining advanced lightweight sheet materials that are dissimilar, coated and hard to weld. In this paper, the free torsional vibration characteristics of adhesively bonded single-lap joints are investigated in detail using finite element method. The effectiveness of finite element analysis technique used in the study is validated by experimental tests. The focus of the analysis is to reveal the influence on the torsional natural frequencies and mode shapes of these joints caused by variations in the material properties of adhesives. It is shown that the torsional natural frequencies and the torsional natural frequency ratios of the adhesively bonded single-lap joints increases significantly as the Young′s modulus of the adhesives increase, but only slight changes are encountered for variations of Poisson's ratio. The mode shapes analysis show that the adhesive stiffness has a significant effect on the torsional mode shapes. When the adhesive is relatively soft, the torsional mode shapes at the lap joint are slightly distorted. But when the adhesive is relatively very stiff, the torsional mode shapes at the lap joint are fairly smooth and there is a relatively higher local stiffening effect. The consequence of this is that higher stresses will be developed in the stiffer adhesive than in the softer adhesive.     

Strength model of adhesive bonded composite pipe joints under tension

A theoretical model is developed to predict the strain of the pipe, coupling, and adhesive under tensile loading of an adhesive bonded joint. The model is found to be within 10 percent of the experimental pipe and coupling strain. Based on the model, several failure modes and their locations are defined and related to the measured data. In this investigation, delamination is the dominating mode of failure. The delamination stress for each test sample is within 7 percent of the average theoretical delamination stress. In addition, the effect of the coupling length, coupling Young's modulus, adhesive shear modulus, and adhesive thickness on the delamination failure are investigated. The model shows that decreasing the modulus of the coupling improves the delamination failure load; however, the coupling strain at the middle of the joint is increased by this variation. Increasing the shear modulus of the adhesive provides the most significant improvement of the joint delamination failure load. Two geometric factors, the joint length and the adhesive thickness also affect the joint failure load. The joint delamination failure load can only be significantly improved by increasing the bonding length up to a certain limit. Increasing the adhesive thickness increases the delamination failure load, however, a large gap between the pipe and coupling may contribute to misalignment during installation which may result in imposed moments under tensile loading. This study can supply the manufacturers with the appropriate design parameters to improve the joint performance significantly under tensile loading.