Prediction of mechanical behaviour of adhesively bonded CFRP scarf jointed specimen under tensile loading using localised DIC and CZM

The present study focuses on the mechanical behaviour of both single and double tapered scarf adhesively bonded joint of Carbon fibre reinforced polymer (CFRP) laminate as adherend subjected to tensile loading. The layup sequence of the CFRP adherend having unidirectional (UD) [0⁰]₁₆ and quasi [+45/−45/0/90]_2S are studied. The adhesive used here is Araldite 2015 supplied by Huntsman which is a two part epoxy system of intermediate toughness grade. Here, 2D digital image correlation (DIC) technique is used for capturing the whole field longitudinal, peel and shear strain distribution over the adhesive bond line of the CFRP specimen. Further, a localised DIC measurement is also carried out using microscopic tube lens for precisely capturing strain field over concentrated zones where damage initiation occurs. The evolution of whole field strain distribution with increasing load is captured to predict the mechanical behaviour and failure mechanism of a tapered scarf joint specimen. In addition, 2-D finite element analysis (FEA) of scarf joint model is carried out for validating the DIC results. In the finite element model cohesive zone elements are used for the modelling of both adhesive layer and inter/intra laminar interface of the composite laminate. Initially, to verify the proposed numerical model, joint's initial stiffness, failure load and corresponding displacement obtained from FEA are compared against the experimental load – displacement results. Later, qualitative and quantitative comparison of longitudinal, peel and shear strain values obtained over the adhesive layer by DIC and FEA is carried out to confirm the accuracy of the DIC results. A decent correlation is found to exist between the DIC predictions and numerical results thereby confirming the accuracy of the DIC technique. Analytical solutions are also derived for the same problem based on mechanics of material and further it is compared with both FEA and DIC predictions for completeness.     

A novel approach to analyse adhesive layer strain field in a stepped lap repaired carbon fiber reinforced polymer panel using digital image correlation

Abstract

The present work focuses on the critical strain analysis of thin adhesive layer present in single-sided stepped lap-repaired carbon fibre-reinforced polymer panels subjected to tensile loading. Digital image correlation technique is used for acquiring both the global and local whole field strain, to obtain the longitudinal, peel, and shear strain distribution over the adhesive layer. The evolution of strain field with increasing load is captured to predict its mechanical behaviour. Magnified optics is used to capture the localized strain field at critical zones. Step corners are identified as critical zones of damage. Debonding is observed as the primary source of damage in the adhesive layer. Overall, the load displacement behaviour and damage mechanism are captured from the experiment. A numerical study based on finite-element analysis is carried out for validating the experimental results. In the numerical study, the adhesive layer is modelled using zero thickness contact element with cohesive behaviour to mimic disbonding. The cohesive zone properties for mode-I and mode-II loading are experimentally obtained from DCBt to ENF test respectively. Microscopic load vs. displacement curve obtained from an experiment is found to be in good correlation with FE estimates.     

Numerical analysis of the stress distribution in single-lap shear tests under elastic assumption—Application to the optimisation of the mechanical behaviour

The single lap joint is the most used test in order to analyse the behaviour of an adhesive in an assembly as on one hand, the manufacturing of such specimens is quite easy, and on the other hand they require only a classic tensile testing machine. However, such specimens are associated with complex loading of the adhesive, i.e. non-uniform shear stress along the overlap length, quite large peel stress at the two ends of the overlap and significant edge effects associated with geometrical and material parameters. In addition, the stress concentrations can contribute to fracture initiation in the adhesive joints and thus can lead to an incorrect analysis of the adhesive behaviour. Therefore, understanding the stress distribution in an adhesive joint can lead to improvements in adhesively bonded assemblies. The first part of this paper presents the influence of edge effects on the stress concentrations in single lap joints under elastic assumption of the material and using a pressure-dependent elastic limit of the adhesive. In the second part, some usual geometries, proposed in the literature about stress limitation, are compared with respect to the maximum load transmitted by single lap joint. The last part presents some geometries, which significantly limit the influence of edge effects and are more appropriate for analysing the behaviour of the adhesive.     

The determination of dynamic strength of single lap joints using the split Hopkinson pressure bar

The current investigation focuses on the determination of the strength of adhesive-bonded single lap joints under impact with the use of a split Hopkinson pressure bar (Kolsky bar). For this, experiments were conducted at different loading rates, for identical metallic adherends bonded by a two-part epoxy adhesive. Four different types of specimens were adopted, all with a given adhesive thickness. The length of overlap and the width of the adherends were varied resulting in four different areas of overlap. It was found that the average strength, as calculated from the readings obtained from a Kolsky bar, increases with decrease of overlap area. An elastodynamic model for the shear strain of the adhesive-bonded single lap joint was developed to investigate this drastic effect of overlap area on the average strength of the joint. The mathematical model was found to be dependent on both the material properties of the adherend and adhesive, as well as the structural properties of the joint, viz. the width and the thickness of the adhesive layer. A combined experimental-numerical technique was used to predict the strain distribution over the length of the bond in the adhesive. It was found that the edges of the adhesive were subjected to maximum strain, while a large part of the adhesive was found to exhibit zero shear strain. The effect of the lap length and the width was studied individually. The cumulative effect of averaging the strain over the entire overlap area, was decreased shear strain for an increased overlap area. The Kolsky bar was identified to give conservative values of the shear strength of an adhesive bonded lap joint under high rates of loading.     

Stress redistributions in adhesively bonded double-lap joints, with elastic–perfectly plastic adhesive behavior, subjected to axial lap-shear cyclic loading

A shear-lag model is developed in order to evaluate stress redistributions in double-lap joints under axial (tensile) lap-shear cyclic loading. The adherend materials exhibit linear elastic behavior, whereas the material of the adhesive layer satisfies the elastic–perfectly plastic shear stress–strain constitutive relation. The reference state (from which the stresses are redistributed) is based on the standard elastic–perfectly plastic shear-lag analysis for double-lap joints. The main conclusion of the current analysis is that, during unloading, shear stresses of opposite sign may develop in the plastic zones of the adhesive layer, at the ends of the overlap, without reversing the direction of the applied load. A simple model for evaluating the variation of the maximum peel stress in the adhesive layer, based on the variation of the peak shear stress, demonstrates that the sign of peel stresses may alternate, as well. Under cyclic (fatigue) loading, the range of the peak stresses in the adhesive layer is the basic parameter for the evaluation of the variation of the energy release rate and the associated crack growth rate in the overlap. In this framework, the current simplified analysis may provide a reference model for comparisons with experimental data or with results which are based on more complex numerical models. The current model can be readily extended to cover the cases of development of plastic zones in the adhesive layer with shear stresses and plastic strains of opposite sign (during unloading or during load direction change).     

Single and dual adhesive bond strength analysis of single lap joint between dissimilar adherends

The emerging trends for joining of aircraft structural parts made up of different materials are essential for structural optimization. Adhesively bonded joints are widely used in the aircraft structural constructions for joining of the similar and dissimilar materials. The bond strength mainly depends on the type of adhesive and its properties. Dual adhesive bonded single lap joint concept is preferred where there is large difference in properties of the two dissimilar adherends and demanding environmental conditions. In this work, Araldite-2015 ductile and AV138 brittle adhesives have been used separately between the dissimilar adherends such as, CFRP and aluminium adherends. In the dual adhesive case, the ductile adhesive Araldite-2015 has been used at the ends of the overlap because of high shear and peel strength, whereas in the middle of the bonded region the brittle adhesive AV138 has been used at different dimensions. The bond strength and corresponding failure patterns have been evaluated. The Digital Image Correlation (DIC) method has been used to monitor the relative displacements between the dissimilar adherends. Finite element analysis (FEA) has been carried-out using ABAQUS software. The variation of peel and shear stresses along the single and dual adhesive bond length have been captured. Comparison of experimental and numerical studies have been carried-out and the results of numerical values are closely matching with the experimental values. From the studies it is found that, the use of dual adhesive helps in increasing the bond strength.     

Effects of Different Curvature Patches on the Strength of Double-Strap Adhesive Joints

In general, the damage in adhesively bonded joints initiates from and propagates through the ends of the overlap area due to high stress concentration in that area. The reduction of these stress concentrations results in an increase in the strength of the joints. For this reason, the rounding of the overlap region before bonding and then applying compression during the bonding process will exert compressive residual stresses on the adhesive layer in the overlap end regions. It is known that the residual stresses formed in this process increase the failure strength of the joint and hence delay the initiation of the damage.

In this study, the effects of overlap length (L = 50,75, and 100 mm), patch thickness (h = 1.6, 3.2, and 5 mm) and patch materials (AA2024 aluminum alloy, AISI 304 steel, AISI 1040 steel) on bond strength were experimentally investigated for adhesively bonded double-strap joint (DSJ) and curvature double-strap joint (CDSJ) subjected totensile loading. The experimental study showed that the overlap length, patch thickness and patch materials have considerable influence on the failure strength and displacement capacity of the joints.     

A new testing methodology for the assessment of fatigue properties of structural adhesives

The monotonic and fatigue shear behaviour of an epoxy adhesive joint has been studied using a short overlap — thick adherend configuration. A specific in situ bonding procedure has been developed in order to accurately control the initial stress state of the joint before testing. The strength of the joint has been found to be strongly dependent on the strain rate and the joint thickness. This dependency was associated with a change in damage mechanisms from cohesive failure (for small joint thickness or elevated speed) to adhesive failure (for large joint thickness and low speed). This was attributed to changes in peel stresses and/or joint morphology with thickness. Fatigue tests were carried out at imposed strain amplitude. Fatigue logs giving the changes in the shape of the transverse loaddisplacement loops were at first considered. The latter have been found to be useful to differentiate between the different failure modes (i.e. cohesive and adhesive). In a second step, all the results were summarised in a fatigue map giving the endurance properties and the failure mode of the joint as a function of the joint thickness and the strain level. This method should be generally useful whenever an accurate determination of the contribution of adhesive and interface behaviour is required for the assessment of joint durability.     

Load transfer in adhesive double-sided patch joints

In this work, the application of adhesively bonded joints to connect two structural elements with a double-sided patch is studied. On the basis of the shear lag model, a simple closed-form solution was obtained. The analytical solutions can be used to predict the shear stress in the adhesive and the load transfer between the structural elements and the external patches. The load and shear stress distributions in the adhesively bonded region are presented. For verification of the analytical model, finite element analyses were employed to calculate the load transfer and shear stress for the double-sided patch joint under static tensile loadings. Good agreement was found between the theoretical predictions and numerical results. To obtain a better understanding of the joints, the effects of adhesive thickness, adhesive shear modulus and patch Young's modulus on the load transfer and shear stress distributions were investigated. The results show that the maximum shear stress occurs at the edge of the adhesive. The maximum value of the shear stress increases as the adhesive shear modulus and patch Young's modulus increase and as the adhesive thickness decreases. A more gradual load transfer can be achieved by increasing the adhesive thickness and decreasing the adhesive shear modulus. The simple analytical solution presented in this paper has the advantages of avoiding the numerical difficulties and giving explicit relationship between the stress state and joint parameters. Moreover, from the designer's point of view a closed-form and easy-to-use solution is preferred.     

Experimental and numerical investigations of adhesively bonded CFRP single-lap joints subjected to tensile loads

In this study, both experimental tests and numerical simulation are implemented to investigate the tensile performance of adhesively bonded CFRP single-lap joints (SLJs). The study considers 7 different overlap lengths, 5 adherend widths and 3 stacking sequences of the joints. Three-dimensional (3D) finite element (FE) models are established to simulate the tensile behavior of SLJs. The failure loads and failure modes of SLJs are investigated systematically by means of FE models and they are in good agreement with those of experiments, proving the accuracy of finite element method (FEM). It is found that increasing the adherend width can improve the load-carrying capacity of the joint better than increasing the overlap length does. Moreover, choosing 0° ply as the first ply is also beneficial for upgrading joint's strength. With respect to failure modes, cohesive failure in adhesive and delamination in adherend take dominant, while matrix cracking and fiber fracture only play a small part. With overlap length increasing or adherend width decreasing, cohesive failure takes up a smaller and smaller proportion of whole failure area, but the opposite is true for delamination. SLJs bonded with [0/45/-45/90]_3S adherends are prone to cohesive failure, and [90/-45/45/0]_3S adherends are easy to appear delamination. Both shear and peel stress along the bondline indicate symmetrical and non-uniform distributions with great stress gradient near the overlap ends. As the load increases, the high stress zone shifts from the end to the middle of the bondline, corresponding to the damage initiation and propagation in the adhesive layer.     

Observations of T‐peel behavior of flexible polymers using digital image correlation

This work investigates the T‐peel behavior of a flexible polymer using digital image correlation (DIC). In particular, the method enabled the direct observation of the full‐field strain of the T‐peel system in fabrics coated with thermoplastic polyurethane elastomers. The strain field is divided into three parts defined as the high‐strain zone, low‐strain zone, and far‐field zone. The correlation between the strain variation and the load‐displacement curve can be used to identify the full‐field beginning of the peeling process, which is defined as the “peel initiation.” Furthermore, a model is developed to predict the strain variation as a function of the load. To validate this model, the calculated strain in the root region is compared with the experimental results measured by DIC and is found to be in excellent agreement. Finally, to address the inhomogeneity of the strain distribution, an engineering index based on the relative displacement of the upper and lower peeling arms is proposed to evaluate the deformation inhomogeneity. It is found that the index increases exponentially near the welded zone. POLYM. ENG. SCI., 55:196–204, 2015. © 2014 Society of Plastics Engineers     

The Differential Displacements and the Failure in Solvent-Welded Lap Joints

A recent analytical model for adhesive joints proposed by Yue and Cherry for analysing and predicting the strength of solvent-welded lap joints is examined. The experimental verification of an important assumed basis of the applicability of this model to solvent-welded joints is considered. The differential strain in the composite adhesive layer of the solvent-welded joint was shown to be approximately equal to the differential strain in its final adhesive layer. The differential strain and hence the stress concentration was largest near the edge of the overlap. Fractography suggested that failure of the joint initiated at the edge of the overlap.     

The Differential Displacements and the Failure in Solvent-Welded Lap Joints

A recent analytical model for adhesive joints proposed by Yue and Cherry for analysing and predicting the strength of solvent-welded lap joints is examined. The experimental verification of an important assumed basis of the applicability of this model to solvent-welded joints is considered. The differential strain in the composite adhesive layer of the solvent-welded joint was shown to be approximately equal to the differential strain in its final adhesive layer. The differential strain and hence the stress concentration was largest near the edge of the overlap. Fractography suggested that failure of the joint initiated at the edge of the overlap.     

Study of tensile failure mechanisms in scarf repaired CFRP laminates

Bonded scarf repairs are used in composite structures when high strength recovery is desired or when there is a requirement for a smooth surface to satisfy aerodynamic requirements. Experimental and finite element study is carried out to understand the damage propagation and ultimate strength of scarf patch repaired CFRP laminates under uni-axial tensile load here. The ultimate strength and damage propagation in scarf repaired laminates has been investigated with respect to change in scarf angle and patch diameter. It is revealed from the results that damage is initiated in adhesive film at the bonded interface of 0° plies of parent laminate and patch in and then propagates in circumferential direction around scarf patch. It is found that some damage occurs in the parent laminate before the damage initiation in the adhesive film. After the adhesive film failure, the damage in parent laminate quickly propagates transversely to the free edge sides of the laminate and failure occurs. The results of this investigation provide further insight into the damage mechanisms in scarf repairs of composite structures under tensile load. This study may be helpful in improving the design and analysis techniques for scarf patch repair of composite structures.     

Progressive failure and experimental study of adhesively bonded composite single-lap joints subjected to axial tensile loads

Experimental tests and finite element method (FEM) simulation were implemented to investigate T700/TDE86 composite laminate single-lap joints with different adhesive overlap areas and adherend laminate thickness. Three-dimensional finite element models of the joints having various overlap experimental parameters have been established. The damage initiation and progressive evolution of the laminates were predicted based on Hashin criterion and continuum damage mechanics. The delamination of the laminates and the failure of the adhesive were simulated by cohesive zone model. The simulation results agree well with the experimental results, proving the applicability of FEM. Damage contours and stress distribution analysis of the joints show that the failure modes of single-lap joints are related to various adhesive areas and adherend thickness. The minimum strength of the lap with defective adhesive layer was obtained, but the influence of the adhesive with defect zone on lap strength was not decisive. Moreover, the adhesive with spew-fillets can enhance the lap strength of joint. The shear and normal stress concentrations are severe at the ends of single-lap joints, and are the initiation of the failure. Analysis of the stress distribution of SL-2-0.2-P/D/S joints indicates that the maximum normal and shear stresses of the adhesive layer emerge on the overlap ends along the adhesive length. However, for the SL-2-0.2-D joint, the maximum normal stress emerges at the adjacent middle position of the defect zone along the adhesive width; for the SL-2-0.2-S joint, the maximum normal stress and shear stress emerge on both edges along the adhesive width.     

搭接长度对钛合金-芳纶纤维复合材料单搭接接头胶接性能的影响

采用钛合金与芳纶纤维复合材料制备不同搭接长度的单搭接接头。利用数字图像相关技术（DIC）、万能试验机等表征方法，对接头拉伸应变与极限载荷进行表征，研究了搭接长度对异质材料单搭接接头胶接性能与破坏模式的变化规律。结果表明，随着搭接长度的增加，单搭接接头极限载荷提升，胶接强度降低，高搭接长度接头出现渐进损伤；偏心弯矩引起的接头偏移减少，搭接部位纵向应变区域面积占比降低；芳纶纤维复合材料层间破坏模式增多，钛合金?胶层界面破坏模式减少，剥离复合材料层数增加。     

Interlaminar shear behaviors of 2D needled C/SiC composites under compressive and tensile loading

The interlaminar shear strength of 2D needled C/SiC composites was measured using the double-notch shear test method. Interlaminar shear tests were performed under compressive and tensile loading. Shear stress–strain response and shear strain field evolution were studied using the digital image correlation (DIC) technique. The results show that the interlaminar shear strength of the specimen using the compressive loading method is 15% higher than that of the tensile loading method. Severe shear strain concentration was observed near the upper notch of the tensile loading specimen. Acoustic emission (AE) was utilized to monitor the damage during the tests. Typical damage mechanisms were categorized according to AE signal characteristics. The statistical results show that more matrix cracks were produced in the tensile loading specimen and no separate fiber/matrix debonding signal was detected in both specimens.     

Analytical solutions for nonlinear analysis of composite single-lap adhesive joints

This paper presents analytical nonlinear solutions for composite single-lap adhesive joints. The ply layups of each composite adherend can be arbitrary, but in the overlap region the ply layups of the upper and lower adherends are assumed to be symmetrical about the adhesive layer. In the present formulation, equilibrium equations of the overlap are derived on the basis of geometrical nonlinear analysis. The governing equations are presented in terms of adherend displacements by taking into account large deflections of the overlap adherends and adhesive shear and peel stresses simultaneously. Closed-form nonlinear solutions for adherend displacements, an edge moment factor and adhesive stresses are formulated and then simplified for practical applications. To verify the present analytical solutions for nonlinear analysis of composite single-lap joints, the geometrically nonlinear 2D finite element analysis is conducted using commercial package MSC/NASTRAN. The numerical results of the edge moment factor, deflections and adhesive stresses predicted by the present solutions correlate well with those of the geometrically nonlinear finite element analysis. This indicates that the present analytical solutions capture key features of geometrical nonlinearity of composite single-lap adhesive joints.     

The Use of Peel Ply as a Method to Create Reproduceable But Contaminated Surfaces for Structural Adhesive Bonding of Carbon Fiber Reinforced Plastics

In the fabrication of fiber-reinforced plastics materials peel plies are commonly used as an additional layer on top of the laminates to sponge up the surplus resin and to create an activated surface for adhesive bonding or coating by peel ply removal. In theory, the peel ply removal results in a new and uncontaminated fracture surface that is activated by polymer chain scission. The peel ply method is often presented as being a good surface treatment for structural bonding.

In this study carbon fiber-reinforced plastics (Hexcel® 8552/ IM7) were produced by the use of five different peel plies and a release foil made of polytetrafluorethylene (PTFE). The peel plies themselves and the surfaces on the CFRP created by peeling were examined by scanning electron microscopy (SEM), x-ray photo electron spectroscopy (XPS), energy-dispersive x-ray spectroscopy (EDX), infrared (IR) spectroscopy, atomic force microscopy (AFM), and contact angle measurements to characterize the surfaces produced. Furthermore, the bond strength of lap shear and floating roller peel samples was determined with and without additional plasma treatment. For bonding, a room temperature-curing two-component-epoxy adhesive (Hysol® 9395) was used to prove the applicability of different peel plies for structural adhesive bonding under repair conditions.     

Low-speed bending impact behavior of adhesively bonded single-lap joints

This study addresses the low-speed impact behavior of adhesively bonded single-lap joints. An explicit dynamic finite element analysis was conducted in order to determine the damage initiation and propagation in the adhesive layers of adhesive single-lap joints under a bending impact load. A cohesive zone model was implemented to predict probable failure initiation and propagation along adhesive–adherend interfaces whereas an elasto-plastic material model was used for the adhesive zone between upper and lower adhesive interfaces as well as the adherends. The effect of the plastic deformation ability of adherend material on the damage mechanism of the adhesive layer was also studied for two aluminum materials Al 2024-T3 and Al 5754-0 having different strength and plastic deformation ability. The effects of impact energy (3 and 11 J) and the overlap length (25 and 40 mm) were also investigated. The predicted contact force-time, contact force-central displacement variations, the damage initiation and propagation mechanism were verified with experimental ones. The SEM and macroscope photographs of the adhesive fracture surfaces were similar to those of the explicit dynamic finite element analysis.