Behavior Of Single-lap Joints Research Articles

Effect of bonding area geometry on the behavior of composite single lap joints (SLJ) and estimation of adhesive properties using finite element method

ABSTRACT In this study, the mechanical behavior of Single Lap Joints (SLJ) subjected to tensile loading was investigated both experimentally and numerically by considering different SLJ sizes including adherend thickness (T:0.88, 1.76, 3.52 mm), joint width (W:10, 20, 30 mm), and overlap length (L:10, 20 mm). A polyurethane adhesive and carbon fiber composite adherends were used for the experimental activity. The experimental campaign was carried out to assess the effects of the SLJ geometry on the mechanical behavior of SLJ. Further, SLJ tests were used to estimate the fracture toughness in mode I and II by using Finite Element methods (FEM) coupled with optimization analysis. The results showed that all three parameters strongly change the load capacity of the joints. According to the Experiments, for every sample configuration, the higher the adherend thickness the higher the adhesive shear and the lower the substrate normal stresses. Moreover, the width showed negligible effect on adhesive shear and substrate normal stresses. Numerically, the effect of geometric parameters has been analyzed once at relative 25% of ultimate load and once at a fixed load for each sample. At 25% of ultimate load, it was observed that the increase in the joint width has nearly no significant effect on adhesive shear and peel stresses. However, at a fixed common load increasing L, W, and T resulted in a decrease in adhesive shear and peel stresses. A good agreement was found between the experimental and numerical results.

Response of Adhesively Bonded Single Lap Joint Doped with Graphene in Adhesive to Shear Loads

The adhesively bonded joint creates a strong bond between the adherend and the adhesive and are replacing the riveted joints in numerous fields such as aerospace, automobile and civil sectors. In this study, the consequence of compositions of graphene nanoparticle doped in the epoxy adhesive on the shear behaviour of single lap joints of adhesively bonded glass and carbon fibre as adherends were investigated. Glass and carbon fibre laminates were produced using Rotex EP-306 resin and Rotex EH-758 hardener. Adhesively bonded single lap joints were then made by using a two-part epoxy structural adhesive. Graphene was added in weight percentages of 0.1%, 0.5%, 1% and 2% into the resin and dispersed by using the method of ultrasonication. A shear test was carried to determine the bonding strength. Results indicated that joints with 0.5% wt. of graphene showed better bond strength compared to other configurations for both glass and carbon adherend. It was noted that with further increase in percentage of graphene, the shear strength decreased. Furthermore, the joints made with carbon as adherend showed better strength properties in comparison with joints made of glass adherend.

Experimental Study on the Effect of Bonding Area Dimensions on the Mechanical Behavior of Composite Single-Lap Joint with Epoxy and Polyurethane Adhesives

The effects of joint geometry parameters, such as adherend thickness (1.76, 3.52 mm), joint width (10, 20, 30 mm), and overlap length (10, 20 mm), on the behavior of single-lap joints (SLJs) under tensile loading are investigated in this study. Peak force, joint stiffness, shear stress, and normal stress are the investigated properties. SLJs are manufactured with carbon fiber composite adherends and two different types of adhesives, polyurethane and epoxy, which present a flexible and rigid mechanical response. The results showed that increasing all 3 geometric parameters (L, W, T) leads to a significant increase in the load capacity of polyurethane joints (on average, 88.4, 101.5, and 16.9%, respectively). For epoxy joints, these increases were 47.7, 100, and 46%, respectively. According to these results, W is the parameter with the most influence on the load capacity of the joints. However, it was observed that an increase in joint width has no significant effect on adhesive shear and a substrate’s normal stresses. Epoxy SLJs behave approximately elastically until failure, while polyurethane SLJ load-displacement curves include an initial linear elastic part followed by a more ductile behavior before the failure. Joint stiffness is affected by all the parameters for both adhesive types, except for overlap length, which led to a negligible effect on epoxy joints. Moreover, the damage surfaces for both types of joints are analyzed and the internal stresses (shear and peel) are assessed by using the analytical model of Bigwood and Crocombe.

Investigations of low-velocity impact behaviour of single-lap joints having dissimilar hybrid composite adherends through cohesive zone model approach

The low-velocity impact behavior of single-lap joints was investigated in this study. In single-lap joints, woven carbon fiber reinforced epoxy (CFRE) and plain woven glass fiber reinforced epoxy (GFRE) laminates as well as a hybrid composite laminate (HCGFRE), which is formed by stacking carbon and glass composite laminates, are used as adherends. Araldite 2011 and Araldite 138 M/HV998 epoxy adhesives were used with different mechanical properties for joining similar and dissimilar composite adherends. Low-velocity impact tests were carried out with 40 J impact energy. Reaction force-time and impact energy-time graphs were obtained from the experimental study as impact response. In numerical analyses, the damage approach of composite adherend was modeled using composite stress-based failure model (Mat 59) and adhesives were modeled utilizing a mixed-mode cohesive zone model (Mat 138) that existed in LS-DYNA explicit finite element software. It was seen that the numerical results indicated a similar tendency with the experimental results due to the modeling of the adhesives using the proper cohesive zone model, the application of correct boundary conditions, and the selection of the correct contact type.

On the influence of overlap topology on the tensile strength of composite bonded joints: Single overlap versus overlap stacking

The goal of this study is to explore new topologies for adhesively bonded composite overlap joints in order to improve their strength under tensile loading. Multiple stacked overlaps, also referred as finger joints, are compared with single overlap topologies. The quasi-static tensile behaviour of single lap joints with two overlap lengths 12.7 mm and 25.4 mm are compared to finger joints with 1 and 2 stacked overlaps through thickness with constant 12.7 mm overlap length. Two composite adherend stacking sequences are tested for each topology [0/90]4s and [90/0]4s.A non-linear FE-analysis is performed to analyse the shear and peel stresses along the adhesive bond line. A difference in peak shear and peel stress, at the tip of the bonded region could be observed: (i) for 1 finger, the peak peel stress is higher than in the single lap joint configurations because the beneficial effect of avoiding eccentricity in the finger joint is outperformed by the detrimental effect of reducing to half the adherend stiffness at the overlap; (ii) for 2 fingers, the stress field changes significantly leads to a 23% decrease in peak shear and 33% in peak peel stress, compared to the single lap joint topologies.In addition, experimental lap shear tests are performed and monitored using acoustic emission technique, to follow the damage events. Different trends at damage initation and at maximum load are believed to result from how the damage propagates inside the joint. A topology with 2 fingers and layup [90/0]4s, which fails entirely inside the adherend, provides the lowest peak shear and peel stress and the highest load at damage initiation. It is however outperformed in maximum load by a single lap joint topology with layup [0/90]4s, with mostly cohesive failure. It is further found that, unlike in single overlap topologies, the most dominant stress component for damage initiation inside the finger joints is the in-plane tensile stress, at the butt joint resin pockets, rather than peel stresses at the overlap region. Lastly, if weight efficiency is the main requirement, a finger joint design can effectively replace a single overlap joint design. However, for absolute maximum joint strength, the single overlap joint is a better choice than the finger joint.

Influence of hygrothermal aging on the quasi-static and impact behavior of single lap joints using CFRP and aluminum substrates

The use of adhesive to bond composites in vehicles continues to increase. Due to hygrothermal conditions, both adhesive and composite properties can be altered, making the mechanical behavior of a joint unpredictable. The novelty of this work is the assessment of the behavior of joints manufactured using composite and aluminum substrates tested under quasi-static and impact conditions in the following states: unaged, hygrothermally aged, and dried after hygrothermal aging. A significant reduction of failure load was registered for composite joints (quasi-static and impact) after hygrothermal aging. Joints dried after aging achieved a recovery in the failure load of quasi-static tests.

Effect of Hygrothermal Cycle Aging on the Mechanical Behavior of Single-lap Adhesive Bonded Joints

The aging behavior of single lap joints (SLJ) in hygrothermal cycles was investigated and compared by using a shearing strength test, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG/DTG), energy dispersive spectrometry (EDS) and scanning electronic microscopy (SEM). The temperature/relative humidity was set at 80 ℃/95% and–40 ℃/30% for 20 cycles, 40 cycles, and 60 cycles (one cycle was 12 hours), respectively. The experimental results show that hygrothermal aging significantly decreases the failure strength of adhesive joints. However, the failure displacement increases as the number of aging cycles increases. In addition, hygrothermal aging changes the failure mode of the adhesive joints from a cohesive fracture in un-aged adhesive layers to an interfacial failure of aged adhesive joints.

Numerical analysis of the strength and interfacial properties of adhesive joints with graded adherends

The strength and interfacial behavior of single lap joints with graded adherends subjected to uniaxial tensile loading are investigated in the present paper. A bilinear cohesive zone model coupled with the finite element method is adopted to describe the damage and failure process of the adhesive layer. The peak loading, the rotation angle between the overlap of the joint and the horizontal direction, as well as the failure energy are investigated comprehensively. It is interesting to find that adopting different variation law in the graded adherends may result in varying strength of adhesive joints. By means of choosing proper material and geometry parameters of adhesive joints, the peak loading, the rotation angle and the failure energy of joints can be greatly improved. What is more, the strength of the SLJ is found to depend much more on the property of the soft part near the adhesive layer. The results should be helpful to guide the design of novel structures of adhesive joints in present and potential applications.

Strain rate dependence of adhesive joints for the automotive industry at low and high temperatures

In this study the impact and quasi-static mechanical behaviour of single lap joints (SLJ) using a new crash resistant epoxy adhesive has been characterized as a function of temperature. Single lap adhesive joints were tested using a drop weight impact machine (impact tests) and using a universal test machine. Induction heating and nitrogen gas cooling was used in order to achieve a homogeneous distribution of temperature along the overlap of + 80 °C and −20 °C, respectively. Adherends made of mild steel, similar to the steel used in automobile construction, were chosen in order to study the yielding effect on the strength of the SLJ. Results showed that at room temperature (RT) and low temperature (LT), failure was dictated by the adherends due to the high strength of the adhesive. At high temperature (HT), a decrease was found in the maximum load and energy absorbed by the joint due to the reduced strength of the adhesive at this temperature. The results were successfully modelled using the commercially available finite element software Abaqus®. Good correlation was found between experimental and numerical results, which allows the reduction of experimental testing.

Digital Image Correlation Analysis Of The Effects Of The Overlap Length, Adhesive Thickness And Adherends Yield Strength Over Similar And Dissimilar Joints Of High Strength Steel And Aluminum Alloys

This work encompasses the study of the mechanical behavior of single lap joints (SLJ) with similar and dissimilar materials, joined by epoxy adhesives. The objective was to analyze the effect of the variables overlap length (OL), adhesive thickness (AT) and adherends yield strength (YS) over the SLJ failure load (FL). The stress state of the joints was analyzed through an optical technique. Tensile tests were performed with samples containing three levels of the factors OL, AT and YS. This last variable analysis was performed using aluminum (AA5083 H111) and high strength steel (DP600) adherends, combined in three different levels – similar AA, similar HSS and dissimilar. The digital image correlation (DIC) technique was used in order to analyze and quantify the incidence of deformation on the adherends and adhesive during the tensile tests, in order to verify how the tensile load was distributed through the SLJ of this work. All the variables had significance over the SLJ. The OL increased the SLJ FL, but the effect reduced as the length is increased. The adherends YS also increased the SLJ FL, but not as much as the OL. Plastic deformation of the adherends leaded to the onset of high peeling stress, which reduced considerably the joints FL. The AT had a negative effect over the SLJ FL. This relationship varied when the adherends faced remarkable plastic strain.

Fatigue resistance of an aluminium one-component polyurethane adhesive joint for the automotive industry: Effect of surface roughness and adhesive thickness

Fatigue is a crucial type of loading for many structural components that contain adhesive bonding systems such as the automotive industry. One of the main advantages of adhesives is their weight-reduction aptitudes. Furthermore, adhesively bonded joints allow a good damping to the fatigue's solicitation with fewer sources of stress concentration. In addition, a suitable surface treatment allows a great joint interface adhesion under cyclic loading. Hence, the mechanical preparation of the bonded surface influences directly the bonded joint performance. Therefore, a careful consideration should be given to the choice of the bonded adhesive thickness. The aim of this paper is to investigate the influence of surface roughness of an aluminium alloy and the adhesive thickness of a one-component polyurethane adhesive, to be used in several parts for buses structure, on the fatigue behaviour of single lap joints. To achieve this purpose, four different surface roughnesses were prepared on aluminium specimens, with an arithmetic average height from Ra ≈ 0.6 µm to Ra ≈ 1.5 µm. Bonded specimens with four adhesive thicknesses from 0.3 mm to 2 mm were manufactured and tested in dynamic tests. The results confirm that there is an optimum combination of surface roughness and adhesive thickness providing the maximum fatigue life. Moreover, good agreement was found between this investigation and a previous work using the same parameters under static loading.

Effect of Reduced Graphene Oxide Reinforcement on Creep Behavior of Adhesively Bonded Joints

The creep behaviour of adhesively bonded joints is one of the major concerns in modern industry due to their application at relatively high temperature. In this research, the effect of added RGO on the creep behaviour of the Araldite 2011 adhesive is investigated. Uni-axial creep tests have been carried out on neat Araldite 2011 and RGO added adhesive at different stress and temperature levels. It has been shown that adhesive with RGO particles demonstrate higher creep resistance. Also, general time hardening models are developed for both materials and their constitutive coefficients have been determined using creep test data. Based on these models, the creep behavior of single lap joints has been simulated using finite element solution method. The results are compared with experimental data and good agreement has been observed that reflects the robustness of the model. It has been shown that joints with added RGO adhesive have significantly higher creep strength.DOI: http://dx.doi.org/10.5755/j01.mech.23.5.19360

Dynamic behaviour of composite adhesive joints for the automotive industry

The automotive industry has significantly increased the use of adhesive joints in vehicle construction, which can be explained in part by the widespread adoption of composite materials and structures. The combined use of composites and bonding allows the manufacture of structures with high mechanical strength and reduced weight. However, to ensure vehicle safety, these adhesive joints must be able to sustain large impact loads, transmitting the load to the structure without damaging the joint. This work aims to study the impact behaviour of composite adhesive joints bonded with a ductile epoxy adhesive, comparing different overlap lengths. For this purpose, a characterization of the behaviour of single lap joints was performed under quasi-static and impact conditions. Dynamic tests were also performed using vibration analysis to assess the damping capabilities of the studied joints. Numerical models were developed with cohesive elements in ABAQUS® software, including both quasi-static and dynamic models. It was demonstrated that joints manufactured with this type of adhesives and substrates can exhibit excellent impact strength and damping capabilities. It was also shown that the behaviour of these joints can be successfully modelled using static and dynamic finite element analysis.

Damage tolerance analysis of adhesively bonded composite single lap joints containing a debond flaw

ABSTRACTAdhesive bonding technology is being increasingly used in the assembly and repair processes of composite structures. The existence of debond flaws weakens the performance of adhesively bonded structures. This article presents the results from an investigation into the effects of debond flaws on the mechanical performance of adhesively bonded single lap joints. The experimental results show that both the load-carrying capability and the failure mode of the single lap joints vary with the location of the debond flaws. Three-dimensional progressive damage finite-element models were developed in ABAQUS to simulate the tensile behaviour of single lap joints. The simulation results agree with the experimental data. The flaws located at 1/4 lap length result in a more pronounced reduction in the load-carrying capability than those located at the edge and the middle portion of the bond region. Compared with the other two locations, the residual strength of the single lap joint with a flaw at 1/2 lap length possesses a higher value. Moreover, the effects of flaws on strength reduction are more prominent for damage propagation than damage initiation.

Effect of the mean stress on the fatigue behaviour of single lap joints

ABSTRACTSteel is the most important construction material for the mass production of engineered structures, especially in the transport industry. On the other hand, adhesive joints are typically used to join load-bearing components. Therefore, this work intends to investigate the stress ratio effects on the fatigue behaviour of adhesively bonded steel lap joints. S–N diagrams of fatigue tests, under constant amplitude loading, were obtained for stress ratios ranging between 0.05 and 0.7. It was observed that the fatigue life of the adhesive joints has very little dependence on the stress amplitude, indicating that only the maximum stress is important. The combination of a linear equation with a quadratic equation seems to be the best formulation to fit the experimental results. Finally, the Palmgren–Miner’s Law is accurate enough to predict the fatigue design for sequential block loadings.

Editorial for thematic series: 1st Luso-Brazilian conference on adhesion and adhesives

© A m This special issue of Applied Adhesion Science (AAS) contains selected papers presented at the 1 Luso-Brazilian Conference on Adhesion and Adhesives (CLBA 2012) held in Rio de Janeiro, Brazil during November 8–9, 2012. CLBA 2012 was chaired by both of us and was co-organized by the Brazilian Association of Adhesion and Adhesives (ABAA) and the Portuguese Association of Adhesion and Adhesives (APAA). The goal of the conference was to join the Portuguese and Brazilian communities to discuss issues related to adhesive bonding research lately developed in both countries. Approximately 60 papers were presented by researchers from many universities and research centers. In order to disseminate the work presented in CLBA 2012, selected papers were prepared which resulted in the present Special Issue. In the first paper, Queiroz et al. study the creep behavior of single-lap joints (SLJ). They developed a pneumatic creep equipment capable of testing ten specimens simultaneously. Results show that the average tensile strength is not enough to guarantee project safety, for long term applications. An initial model for the creep behavior of bonded joints was also proposed. In the next paper, Sales et al. investigated the effect of periodontal disease on the bond strength of fiberglass posts. Push-out tests were performed in specimens prepared with different adhesive systems and resin cements. It was possible to conclude that periodontal disease did not affected the bond strength mean values of fiberglass posts cementation. In the third paper, Silva Neto et al. present a numerical and experimental investigation on the adhesion between the steel tube and cement paste in petroleum wells. Push-out tests were performed with three different interface conditions in order to validate the numerical model. They observed the influence of the different conditions used in the experiments and the sensibility of the model variables to these changes. Antunes et al. worked on the characterization of FeCr and FeCoCr alloy coatings of carbon steels for marine environment applications. The adhesive strength of alloy coatings using thermal spray process was studied and related to chemical composition. Five combinations of wires and intermediate bonds were used. The coatings were characterized by morphological aspects, corrosion and adhesion tests. They concluded that the FeCoCr deposition, with an epoxy sealing, is suitable to be used as an efficient coating of carbon steel in aggressive marine environments. The last paper by Souza and Reis is related to the thermal behavior of DGEBA (Diglycidyl Ether of Bisphenol A) adhesives. Three different epoxy adhesives used in the offshore industry are characterized by Differential Scanning Calorimetry (DSC), Dynamic Mechanical Analysis (DMA) and Thermogravimetric Analysis (TGA). The

The behaviour of single lap joints under bending loading

Four-point bend tests were performed on single lap joints with hard steel adherends and a structural epoxy adhesive. The effect of the overlap, the adherend thickness and the adhesive thickness was studied. It was found that the length of the overlap has no significant effect on the strength of the joints. This is because the load transfer is occurring in a very localised area around the edges of the overlap, being the failure governed by peel mechanisms. The thickness of the adherends strongly affects the strength of the joints. The thicker the adherend, the stronger is the joint. The experimental results are compared with a finite element model and reinforce the fact that the failure takes place due to local strains at the ends of the overlap in tension. An analytical model is also given to predict in a simple but effective way the joint strength and its dependence on the adherend thickness.

Use of artificial neural networks for modelling rate dependent behaviour of adhesive materials

This research work highlights the use of artificial neural networks (ANN) for modelling the rate-dependent response of adhesive materials with the purpose of expanding the established method for modelling the response of adhesively bonded structures, and in particular single lap joints. The motivation for this work comes after a viscoplastic model developed in a previous research work failed to predict the response of single lap joints bonded with a rate dependent adhesive material. The viscoplastic model, however, was successful in replicating both bulk and shear properties of the used adhesive system. Predictions made using the rate-dependent von Mises material model proved to be successful in predicting the behaviour of single lap joints, but it could not model the shear data using the tensile data due to hydrostatic stress sensitivity in the adhesive itself. Accurate predictions of the rate-dependent behaviour using artificial neural networks are possible with the availability of stress and strain data sets from experiments. This is where the neural network constitutive model directly acquires the information on the material behaviour from experimental data sets. Material data defining both the tensile and shear response of the adhesive system was extracted from previous research work. An artificial neural network constitutive model was developed and then used to replicate experimental data and also to generate further data at other strain rates. The available model could be slightly modified and then used to investigate various geometrical parameters, such as overlap length, plate thickness and adhesive thickness on joint strength.

Creep analysis of fibre reinforced adhesives in single lap joints—Experimental study

In this paper, the effect of reinforcing the adhesive on the creep behaviour of single lap joints was studied experimentally. The reinforcement was in the form of fibre and three types of fibres namely aramid, carbon and glass were used. The test was performed at a temperature above the glass transition temperature of the adhesive. The effect of fibre orientation was also investigated. The failure time and initial strain for all the specimens were evaluated and compared to the un-reinforced adhesive joint. According to the results, adding fibres in the bondline considerably affects both the initial strain and the failure time and these effects are dependent on the fibre type and orientations. The fracture surfaces of the specimens were also studied to investigate the failure mechanisms of the reinforced adhesive in creep. The fibre breakage was observed along with adhesive and cohesive failures.

The prediction of crack growth in bonded joints under cyclic-fatigue loading II. Analytical and finite element studies

In Part I (Int. J. Adhesion Adhesives (2003) in press) the performance of adhesively bonded joints under monotonic and cyclic-fatigue loading was investigated. The joints consisted of an epoxy-film adhesive which was employed to bond aluminium-alloy substrates. The effects of undertaking cyclic-fatigue tests in (a) a ‘dry’ environment of 55% relative humidity at 23°C, and (b) a ‘wet’ environment of immersion in distilled water at 28°C were studied. The basic fracture-mechanics data for these different joints in the two environments were measured, as well as the behaviour of single-lap joints. In the present paper, Part II, a method for predicting the lifetime of adhesively bonded joints and components has been investigated. This prediction method consists of three steps. Firstly, the fracture-mechanics data obtained under cyclic loading in the environment of interest have been modelled, resulting in an expression which relates the rate of crack growth per cycle, d a/d N, to the maximum applied strain-energy release-rate, G max, in a fatigue cycle. Secondly, this relationship is then combined with an analytical or a computational description of the variation of G max with the crack length, a, and the maximum applied load per unit width, T max, per cycle in the joint, or component. Thirdly, these data are combined and the resulting equation is integrated to give a prediction for the cyclic-fatigue lifetime of the bonded joint or component. The theoretical predictions from the above method, using different approaches to describe the variation of G max with the crack length, a, and applied load, T max, in the single-lap joint, have been compared and contrasted with each other, and compared with the cyclic-fatigue behaviour of the lap joints as ascertained from direct experimental measurements.