Double-lap Joints Research Articles

Compatibility and temperature effect on hybrid painted glass-aluminum bonded joints

The present work is based on an experimental campaign in which the mechanical properties of a set of adhesive painted aluminum-cloudy glass joint are investigated; the obtained results can be appropriately used for practical problems, since this particular junction is suitable in civil engineering applications.For the conducted experimental campaign, a commercial epoxy structural adhesive was selected. Since the performance of the adhesive depends on temperature, tensile test on dog bone specimens and double-lap joints were conducted at three different temperatures (representing laboratory conditions, work conditions and maximum service temperature).The work aims to verify the applicability and the compatibility of the tested junction on a building component, such as window frames, by reproducing the effective work conditions of both glass and aluminum.The obtained results show that the analyzed junction can be suitable used for practical applications up to work temperature, since a good performance in term of maximum load capacity and maximum displacement is maintained if compared with laboratory condition. Instead, a drastic reduction of mechanical properties is observed at maximum service temperature.

Constitutive relationships of different interlayer materials for laminated glass

The paper presents the results of an experimental investigation on the viscoelastic response of laminated glass joints with different interlayers. The problem is explored with double lap joints under shear loadings at temperatures between 0 °C and 60 °C. The viscoelastic mechanical parameters of a polymeric interlayer play a fundamental role in determining the response of laminated glass and thus, their knowledge is required for a fail-safe design of structural and non-structural members to avoid unexpected catastrophic collapses. Tests were performed on specimens made of three glass plies to study a resilient film produced from plasticized PVB (commercially known as DG41). As a comparison, two commonly used interlayer materials for laminated glass were also tested: the polyvinyl butyral (PVB), and the ionoplast polymer SG. Firstly, short-term tests were performed at different temperatures, secondly long-term properties of both materials at 20 °C were predicted from experimental curves using the time–temperature superposition principle (TTSP), which allows for a reference curve (master curve) to be drawn. A method to improve the drawing of the master curve and provide values of the relaxation modulus and inverse creep compliance modulus of PVB and DG41 at different temperatures to be used for design purposes, is proposed.

Revealing the competitive fatigue failure behaviour of CFRP-aluminum two-bolt, double-lap joints

The fatigue failure behaviour of CFRP-metal fastened joints in highly-loaded structures differs from that of CFRP-CFRP or metal-metal fastened joints because of the significant dissimilarity between fatigue damage mechanisms of CFRP and metal materials. In this paper, the competitive fatigue failure behaviour of a CFRP-aluminum two-bolt, double-lap joint was revealed via experiments and a novel numerical prediction method. In the proposed method, fatigue failures of the CFRP and aluminum joined plates were predicted by a novel progressive fatigue damage model and the nominal stress method, respectively. The predicted competitive fatigue life and failure mode of the joint were consistent with the experimental results, which validated the efficiency of the proposed method. It was found that, the collapse failure of the joint is caused by the failure of the CFRP laminate at high loading levels, whereas caused by the failure of the aluminum plate at low loading levels. Furthermore, influences of the thickness ratio of the joined plates on the competing fatigue failure of the joint were determined. Based on the critical thickness ratios corresponding to the transition of failure mode, the fatigue failure mode diagram of the joint was established, which is helpful for the design of the CFRP-aluminum bolted joint.

Effect of functionalized nano-SiO2 addition on bond behavior of adhesively bonded CFRP-steel double-lap joint

The mechanical properties of adhesives significantly influence the bond behavior of steel structures strengthened by carbon fiber-reinforced polymer (CFRP) composites. In order to improve the toughness of the adhesive and the ductile behavior of adhesively bonded CRFP-steel joint, functionalized nano-sized silica (hereafter refer as nano-SiO2) toughening agent were added into the adhesive. Firstly, the effects of functionalized nano-SiO2 toughening agent on the mechanical properties of the adhesives after curing at room temperature were tested and analyzed. Then, a total of 19 adhesively bonded CFRP-steel double-lap joints were fabricated and tested under shear loading to investigate the effect of functionalized nano-SiO2 on the bonding behaviors, including the failure modes, bond–slip relationships, bond strength and so on. The test results indicate that tensile stress–strain relationship of the adhesive changes from linear to nonlinear with the increase of functionalized nano-SiO2 toughening agent, the elongation at break, strain energy and shear strength of functionalized nano-SiO2-toughened adhesive (ZR14-weight ratio of 1% functionalized nano-SiO2) was improved by 202.88%, 292.10% and 133.12% respectively in comparison to the adhesive without functionalized nano- SiO2 toughening agent. Moreover, the tensile strength, elongation at break, and strain energy of functionalized nano-SiO2-toughened adhesive ZR14 are much higher than those of commercial adhesives (e.g. Sika30, 330) without Nano-SiO2. In addition, the failure mode of CFRP-steel joints changed from interface failure to CFRP plate delamination, when the weight ratio of functionalized nano-SiO2 to adhesive increasing from 0 to 1%. The bond-slip curves could be simplified into a bilinear model (triangle-shape) for CFRP-steel joints without and with only weight ratio of 0.5% functionalized nano-SiO2, however, the bond-slip curves change to simplified trilinear model (trapezoidal shape) for the joints with 1% functionalized nano-SiO2 and the toughness of bonding interface is significantly improved. In all, the CFRP-steel lap joints bonded by adhesive with appropriate amount functionalized nano-SiO2 (i.e., wt. of 1%) shows superior interfacial performance.

Mixed-mode evaluation of ductile adhesive joints by the single-leg bending test

In the design of adhesive structures, it is extremely important to accurately predict their strength and fracture properties (critical strain energy release rate in tension, GIC, and shear, GIIC). In most cases, the loads occur in mixed-mode (tension plus shear). Thus, it is of great importance the perception of fracture in these conditions, namely of the strain energy release rates in tension, GI, and shear, GII, relative to different crack propagation criteria or fracture envelopes. This comparison allows to determine the most suitable energetic propagation criterion to be used in cohesive zone models (CZM). The main objective of this work is to verify, by CZM, which is the power law parameter (α) that best suits the energetic crack propagation criterion for CZM modelling, using single-lap joints (SLJ) and double-lap joints (DLJ) with aluminium adherends and bonded with a ductile adhesive. With this purpose, numerical simulations of the SLJ and DLJ are carried out, and the maximum load (Pm) is compared with experiments. For the tested materials and geometries, the energetic criterion resulting from the experimental work provided matching numerical results and, thus, the fracture envelope was validated.

Effects of elevated temperatures on mechanical behavior of epoxy adhesives and CFRP-steel hybrid joints

High environmental temperatures can significantly degrade the mechanical behavior of CFRP-steel composites, due to the reduced bond performance of epoxy adhesives. Here, we systematically investigate the effects of elevated temperatures on the mechanical behavior of epoxy adhesives and CFRP-steel hybrid joints. First, the glass transition behavior and mechanical properties of four distinct epoxy adhesives subjected to elevated temperatures were studied via dynamic mechanical analysis (DMA) and tensile tests. The failure process of CFRP-steel double-lap joints was subsequently investigated under different elevated temperatures. Digital image correlation (DIC) technique was utilized to capture the full-field strains on the specimens during the testing. The results indicate that adhesive Araldite 2014 possesses the highest glass transition temperature, followed by adhesive J133. However, adhesive J133 possesses the highest tensile strength and toughness under all testing temperatures and forms the most robust bonding between CFRP and steel at these temperatures. The bond strength is reduced by over 50% at 70 °C compared with that at room temperature, and the degradation of bond strength is correlated with that of the tensile strength of the adhesives. We also find that it is more appropriate to use Tg,o as an indicator to determine the working temperature limit of epoxy adhesives.

Experimental and analytical validation of mode II fracture toughness tests for a type of double-lap joint

Mode II fracture toughness tests for a type of double-lap joint referred to as the doubly end notched tension (DENT) tests were validated by experimental and numerical approaches. Crack growth behavior of two cracks was simultaneously observed during fracture toughness tests and analyzed by the digital image correlation (DIC) method. As one of the two cracks propagated first and the other followed, finite element (FE) analyses were carried out to clarify the mechanisms. The results of FE analyses showed that mode II energy release rate at the extended crack tip increased at the beginning of crack growth, and subsequently decreased under constant displacement condition. An opposite trend was found at the other crack tip. One cause of this phenomenon was due to local bending of the specimen. The effects of the geometry of specimens were also discussed. The FE analyses also revealed that the ratio of middle layer thickness to the outer layers thickness significantly affects the crack growth behavior in the DENT specimens.

Mechanical characterization of laser-welded double-lap joints in ultra-high and low strength steels for sandwich panel applications

This study is to investigate the microstructure and the mechanical properties (hardness, strength, fatigue resistance) of laser welded double-lap joints from two different steels, ultra-high strength abrasion resistant steel (AR400) and low strength structural (S355 MC) steels. The laser welding was conducted at two different energy density 170 and 958 J/mm2 and constant power 3 kW. The morphology and the microstructure of the cross-section lap joints have examined by optical microscopy. The shear strength and the fatigue resistance of the double-lap joints were evaluated by uniaxial tensile tests and fully reversed tension–compression strain-controlled fatigue tests, respectively. Whereas the hardness of the welded lap joints of UHS steel has been decreased due to the tempered martensite formation. The microstructure of the laser welded lap joints of LS steel shows martensite formation in the fusion zone. Hence, the hardness has been significantly increased to 300 Hv. The shear strengths of the welded lap joints have significantly influenced by the laser power density. Significant improvement in the strength could be observed in the strength of the lap joints of UHS steel at energy density 958 J/mm2. Fatigue cracking along steels sheets boundaries that intersect the weld zone in the welded lap joints and the crack propagation towards the weld zone observed to be the fatigue-induced microstructural features in the lap joints welded by laser.

Stress analysis of double-lap bi-material joints bonded with thick adhesive

Mechanics of double-lap Steel-to-CFRP adhesively-bonded joints loaded in tension are investigated experimentally using Digital Image Correlation (DIC) and Acoustic Emission (AE), analytically using a one-dimensional closed-form solution and numerically with Finite Element analysis. The double-lap bi-material joints are fabricated of a steel core adhesively bonded to two CFRP skins with adhesive thickness of ~ 8 mm, using an Epoxy-based and MMA-based adhesives. In order to capture the in-plane deformation of the joint, full field strain/displacement maps are obtained using DIC. This data is used to validate the shear-lag model predictions of the adhesive shear stress/strain distribution as well as the linear-elastic Finite Element Model (FEM) results. In addition, they are used to capture the susceptible damage locations and their effect on the displacement contour maps, strain distribution and load transfer between the joint's different constituents. A correlation between the DIC displacement and the AE signals is obtained for damage detection in both joints. Moreover, a good agreement amongst the analytical, FE and DIC strain/stress distributions along the bond-line is observed. This study introduces the analytical shear-lag model as an alternative to predict the stress state in thick-adhesive double-lap joints, with an acceptable level of accuracy and robustness.

Interface crack model using finite fracture mechanics applied to the double pull-push shear test

An analytical procedure predicting a debond (interface crack) onset and growth in an adhesive joint between two beams or plates is developed and applied to a specific configuration often used in reinforcement tests in civil engineering. The procedure is based on Timoshenko beam theory and Linear Elastic-(Perfectly) Brittle Interface Model (LEBIM) combined with the Coupled Criterion of the Finite Fracture Mechanics (CC-FFM) for mixed-mode fracture. First, a sixth order differential equation in the shear stresses along the adhesive layer is deduced and solved, leading to closed form expressions for both shear and normal stresses in the adhesive. Then, the critical value of the applied load necessary to produce debonding is predicted by coupling a stress and an energy condition based on: (i) the stress distribution produced in the interface before the debond onset and (ii) the energy released during the debonding process along the interface. Although the developed procedure can be applied to several types of joints with different geometries, materials and loads (e.g., double lap joint tests including adherents made of steel or composites), herein it is applied to the double pull-push shear test where the debond onset and growth between a Carbon Fibre Reinforced Polymer (CFRP) laminate and a concrete block occurs. For such a case, the debond is produced under predominant fracture mode II; nevertheless, it is shown that relevant normal (peeling) stresses associated to mode I may appear as well. A comparison of the present solution with a previous one by the shear-lag model is provided as well.

Enhanced comprehensive performance of bonding interface between CFRP and steel by a novel film adhesive

A novel film adhesive was proposed to bond carbon fiber reinforced polymer (CFRP) laminas to steel substrates, and experimental validations for improved comprehensive bond performances are presented. This film adhesive was shown to possess excellent high-temperature resistance via dynamic mechanical analysis (DMA). A series of CFRP/steel double-lap joints (DLJs) with different bond lengths were experimentally studied, focusing on the failure modes, effective bond length, bond-slip relationship, and bond strength. The results show that the film-adhesive bonded joints fail in the mode of CFRP delamination, which indicates a stronger interfacial bond between the adhesive layer and adherends than the intra-laminar strength of CFRPs. The bonding interface has an effective bond length of approximately 65 mm, beyond which no increase of ultimate load can be achieved with the increase of bond length. The bond-slip relationship of the film-adhesive bonding interface exhibits a trapezoidal (ductile) shape, which is significantly different from the triangular (brittle) shapes for most paste-adhesive interfaces. Superior strength, ductility, and high-temperature resistance of the bonding interfaces in CFRP/steel composites are achieved due to the use of this film adhesive. The proposed study offers an alternative solution to an enhanced comprehensive property of the bonding interface in CFRP/steel composites.

An experimental and numerical investigation of adhesive bond strength in Al-GFRP single lap and double butt lap joints due to applied longitudinal loads

ABSTRACTFailure and fracture response of two shear resistive joints namely, single lap joint (SLJ) and double butt lap joint (DBLJ) of aluminium and GFRP adherents bonded with a ductile adhesive, has been investigated experimentally and numerically under static tensile loading. Numerical simulations were carried out by the CZM approach through 2D and 3D FE models. The joints progressive damages and effects of various cohesive parameters were analysed. In spite of some previous studies showing lower strengths in SLJs compared to DBLJs with aluminium adherents, here, the results indicated that in an AL-GFRP bonded SLJ, tensile strength may averagely be 33% higher than that for DBLJ. Failure load and stiffness were overestimated by the FEM with the CZM approach. A compound failure mechanism was observed in both type of joints with the most dominant failure mode of light fibre tear in mat layer of composite adherents and yielding of aluminium adherent.

Comparative Study of Ultimate Bond Strength Models for FRP-to-Steel Bonded Joints

This paper presents a comparative study of current ultimate bond strength models for fiber reinforced polymer (FRP)-to-steel bonded joints. A database that includes the test results of 106 single and 93 double lap joints was assembled from the open literature for this comparison. Comparative results show that Xia and Teng’s and Hart-Smith’s models have the highest accuracy for predicting the ultimate bond strengths of single and double lap joints, respectively. Current ultimate bond strength models for single or double lap joints have some applicability to both single and double lap joints. Xia and Teng’s model has the best performance for predicting the ultimate bond strengths of both single and double lap joints. However, the predictions by Xia and Teng’s model have a large deviation for double lap joints and for joints whose adhesives have trilinear bond–slip behavior. Based on comparative results, an improved ultimate bond strength model for both single and double lap joints is proposed. The proposed model further considers the effects of the joint type, the stiffness ratio of the double lap joint and the bond–slip behavior of the adhesive. The comparison between model predictions and test results indicates that the improved model is more accurate and applicable than current models for both single and double lap joints.

Interfacial Stress Analysis of Adhesively Bonded Lap Joint

The use of adhesively bonded joints in place of traditional joining techniques such as bolted or rivet joints is becoming greatly popular in recent years. Interfacial stress in the adhesive is critical to the strength of adhesively bonded joints. It is necessary to predict the interfacial stresses accurately to ensure the safety of joints. In this work, an analytical model is explicitly presented to evaluate the stresses in a double lap joint. The equilibrium equations in the adhesive overlap region are derived on the basis of elasticity theory. The governing equations are presented in terms of shear and peel stresses in the adhesive. Analytical solutions are derived for the shear and peel stresses, which are considered to be the main reason for the failure of the double lap joint. To verify the analytical solutions, the finite element method is conducted using the commercial package ANSYS. Results from the analytical solution agree well with finite element results and numerical investigations available in the literature. The effect of the adhesive thickness, shear modulus, adherend Young’s modulus and bonding length on the shear and peel stresses in the adhesive of the double lap joint are studied. Numerical results demonstrate that both the maximum shear and peel stress occur at both ends of the bonding region. The maximum values of the shear and peel stresses increase as the adhesive thickness decreases and as the adhesive shear modulus increases provided that the adhesive thickness is sufficiently small. The simplicity and capability to obtain analytical expressions of the shear and peel stresses for double lap adhesive bonded joints makes the proposed analytical model applicable for the stress analysis and preliminary structural design.

A critical assessment of design tools for stress analysis of adhesively bonded double lap joints

Despite the proliferation of high fidelity finite-element (FE) models, lower fidelity models remain commonly used in adhesively bonded joint design. These design models can save both computational and user time due to their simplicity and ease of use. This study presents a detailed assessment of local stress fields predicted by five design models: A4EI, HyperSizer, Joint Element Designer, Carrera Unified Formulation, and a Continuum Solid Shell FE model. All models were compared with a high fidelity, dense mesh FE model. Six double lap joint cases with different combinations of features like different adhereds, a core, and tapers were compared.

General formulation of macro-elements for the simulation of multi-layered bonded structures

ABSTRACTAdhesively bonded joints are often addressed through Finite Element (FE). However, analyses based on FE models are computationally expensive, especially when the number of adherends increases. Simplified approaches are suitable for intensive parametric studies. Firstly, a resolution approach for a 1D-beam simplified model of bonded joint stress analysis under linear elastic material is presented. This approach, named the macro-element (ME) technique, is presented and solved through two different methodologies. Secondly, a new methodology for the formulation of ME stiffness matrices is presented. This methodology offers the ability to easily take into account for the modification of simplifying hypotheses while providing the shape of solutions, which reduced then the computational time. It is illustrated with the 1D-beam ME resolution and compared with the previous ones. Perfect agreement is shown. Thirdly, a 1D-beam multi-layered ME formulation involving various local equilibrium equations and constitutive equations is described. It is able to address the stress analysis of multi-layered structures. It is illustrated on a double lap joint (DLJ) with the presented method.

A modified analytical model for stress analysis of adhesively bonded stepped-lap joints under tensile load

A modified analytical model with closed-form solution is proposed in this study to investigate interfacial shear and normal stresses in adhesively bonded double-lap joints with stepped outer adherend, subjected to tensile loading. By extending some researchers' prior related works, explicit expressions for adhesive interfacial shear and normal stresses are derived, with considerations of shear deformation in the adherends and plasticity in the adhesive layer. Continuity conditions are introduced to fit the variation of the outer adherend thicknesses. To obtain unknown constants of general solutions for adhesive normal stress, some approximate boundary conditions are reconstructed and several simplifications are imposed. The adhesive stress distributions along the bondline are obtained to investigate the effects of joint's material properties and geometric parameters. Adhesive with elastic-perfect plastic behavior is also considered in the analyses. The results obtained from the present model show an excellent agreement with corresponding finite element results. The present method is applicable to implement parametric analysis and optimal design for adhesively bonded double-lap joint.

A failure-envelope-based method for the probabilistic failure prediction of composite multi-bolt double-lap joints

A prediction method is proposed for the probabilistic failure of composite multi-bolt double-lap joints based on a single-parameter spring-based method, a failure envelope method and Monte Carlo simulation. In this method, the effects of the randomness of the laminate bearing and open-hole strength, laminate's properties, geometric parameters, clearances, tightening torques and bolt bearing chord stiffness are considered. And the application of failure envelope method leads to an advantage of a very small amount of calculation. To validate the proposed method, composite two-bolt and three-bolt double-lap joints were designed and tested to determine the stochastic failure load. Following the testing of all the necessary parameters, satisfactory agreement between the numerical and experimental stochastic failure loads of the multi-bolt joints was found. Furthermore, the sensitivities of the probabilistic failure load of multi-bolt joints were investigated. It follows that the randomness of the laminate open-hole tensile and bearing strengths have the most significant influences on the failure load, while the influences of the randomness of the other parameters are slight, which affect bolt load distribution directly, and indirectly affect the failure load through influencing the bolt load distribution.

Numerical stress analysis in adhesive joints under thermo-mechanical load using model with special boundary conditions

A numerical study of the adhesivejoint made of similar and dissimilar adherends subjected to thermo-mechanical loading is presented. A comprehensive numerical model was used for this purpose with the novel displacement coupling conditions which are able to correctly represent monoclinic materials (off-axis layers of composite laminates). The geometrical nonlinearity as well as nonlinear material model are also taken into account. Three different types of single-lap and double-lap adhesive joints are considered in this study: a) metal-metal; b) composite-composite; c) composite-metal. In case of composite laminates, four lay-ups are evaluated: uni-directional ([08]T and [908]T) and quasi-isotropic laminates ([0/45/90/-45]S and [90/45/0/-45]S). This paper focuses on the parameters which have the majoreffect on the peel and shear stress distribution within adhesive layer at the overlap ends. The comparison of behaviour of single-and double-lap joints in relation to these parameters is made. The master curves for maximum stress (peel and shear) at the ends of the overlap with respect to the bending stiffness and axial modulus of the adherends are constructed by analysing stress distributions in the middle of the adhesive.The main conclusions of this paper are: the maximum peel stress value for SLJ is reduced with increase of the adherend bending stiffness and for DLJ,similar behaviour was observed at the end next to the inner plate corner, while, at the end next to the outer plate corner peel stress is reduced with increase of adherend axial modulus.

Numerical and experimental investigation of the effect of the cold expansion process on the fatigue behavior of hybrid (bonded-bolted) double shear lap aluminum joints

In this study, the effect of cold expansion process on the fatigue behavior of hybrid (bonded-bolted) double shear lap joints has been investigated, numerically and experimentally. Also the effects of cold expansion process and adhesive bonding process on the fatigue behavior of simple bolted double shear lap joint are studied, separately. For this purpose, six batches of specimens were made from 7075-T6 aluminum alloy sheet in Simple and Hybrid groups. Simple group consists of simple, 1.5% cold expanded and 4.7% cold expanded batches. Hybrid group consists of hybrid, 1.5% cold expanded hybrid and 4.7% cold expanded hybrid batches. All of the specimens after assembling were put under cyclic loading. The S-N curves of the experimental results show that the cold expansion process increases the fatigue lives of Hybrid and Simple group specimens. This increase is more appreciable by increasing the cold expansion percent. The adhesive bonding of Simple group specimens increases the fatigue life of these specimens. For all specimens the fatigue life enhancement is more appreciable at the lower loads. Also, the rate of crack growth in Hybrid group is slower than that of Simple group.