Double Cantilever Beam Joints Research Articles

Theoretical analysis of mode I fracture of adhesively bonded bi-material DCB joints

There has been a disputation about how to achieve a pure mode I fracture and thus measure its toughness for an adhesively bonded bi-material double cantilever beam (DCB) specimen. This paper therefore develops a theoretical methodology to calculate the normal and shear stress distributions in the adhesive layer and to perform data reduction and mode partitioning for generic bi-material DCB specimens. The theoretical model is validated using experimental data. The adhesive layer in a generic bi-material DCB joint is loaded both in normal and shear based on the predicted stress distributions in the adhesive layer, resulting in a mixed mode fracture behavior. The prediction results substantiate that a strain-based design principle eliminates the shear stresses and thus leads to pure mode I fracture in bi-material DCB specimens. This paper justifies that the established data reduction method is applicable for measuring mode I fracture toughness when a bi-material DCB is designed according to the strain-based design criterion.

Measuring traction–separation relationships of bonded Fe-SMA double cantilever beam joints

Adhesively bonded strengthening systems using self-prestressing iron-based shape memory alloys (Fe-SMA) have demonstrated significant potential. However, the premature nonlinear behavior of Fe-SMA during debonding failure presents challenges in ensuring the integrity of the adhesive joint. A recent study indicated that this behavior may lead to premature joint failure, and a similar conclusion can be drawn concerning plasticity in thin adherends. To characterize the failure process of Fe-SMA bonded joints, a method capable of measuring traction–separation relationships of thin adherends bonded with tough adhesives is essential. An algorithmic method, supported by an analytical nonlinear beam-on-foundation model and a minimization scheme, is shown to offer a solution. To benchmark the method against the J-integral approach, experiments are conducted using two types of adherends: thick steel with thickness-limiting plasticity and Fe-SMA with material behavior that inevitably leads to nonlinear deformation. Comparison of methods shows both measurement method are valid and allow for proper predictive modeling using the finite element method. The proposed method can be readily employed to characterize various thin adherends with elasto-plastic material behavior.

Mode I fracture of thick adhesively bonded GFRP composite joints for wind turbine rotor blades

This article investigates the effects of voids, joint geometry, and test conditions on the quasi-static Mode I fracture performance of thick adhesive Double Cantilever Beam (DCB) joints such as those prevailing in wind industry and shipbuilding. The specimens were made by glass fiber reinforced epoxy adherend and SikaPower®-830 epoxy adhesive in the cm thickness range. Side-grooved shape guided the crack propagation direction and assisted stable propagation, while lower cross-head displacement rates reduced the occurrence of unstable crack propagation and prevented crack deflection. Porosities, which are inevitable due to the high viscosity of the adhesive, led to unstable propagation and promoted crack path deviations. They could also decrease the apparent fracture energy release rate (SERR) since the crack surface is reduced. In conclusion, grooved DCB joints with low void content tested at low displacement rates showed stable crack propagation without significant crack path deviation. This method enables comparison with future adhesive formulations and the refinement of full-scale blade models.

Evaluation of XD 10 Polyamide Electrospun Nanofibers to Improve Mode I Fracture Toughness for Epoxy Adhesive Film Bonded Joints

The demand for ever-lighter structures raises the interest in bonding as a joining method, especially for materials that are difficult to join with traditional welding and bolting techniques. Structural adhesives, however, are susceptible to defects, but can be toughened in several ways: by changing their chemical composition or by adding fillers, even of nanometric size. Nanomaterials have a high surface area and limited structural defects, which can enhance the mechanical properties of adhesives depending on their nature, quantity, size, and interfacial adhesion. This work analyzes the Mode I fracture toughness of joints bonded with METLBOND® 1515-4M epoxy film and XantuLayr electrospun XD 10 polyamide nanofibers. Two joint configurations were studied, which differed according to the position of the nanomat within the adhesive layer: one had the nanofibers at the substrate/adhesive interfaces, and the other had the nanofibers in the center of the adhesive layer. Double cantilever beam joints were manufactured to evaluate the Mode I fracture toughness of the bonding with and without nano-reinforcement. The nanofibers applied at the substrate/adhesive interface improved the Mode-I fracture toughness by 32%, reaching the value of 0.55 N/mm. SEM images confirm the positive contribution of the nanofibers, which appear stretched and pulled out from the matrix. No fracture toughness variation was detected in the joints with the nanofibers placed in the middle of the adhesive layer.

Obtaining strain-rate dependent traction-separation law parameters of epoxy adhesive joints and predicting fracture for dissimilar bonding adherends

This study investigated the mode I fracture behavior of double cantilever beam (DCB) epoxy adhesive joints with similar adherends on the both sides (i.e., aluminum-aluminum or copper-copper) at different strain rates; i.e., quasi-static (∼10 −3 s −1 ), low (∼7 s −1 ) and intermediate (∼14 s −1 ) rates. The fracture energy of the DCB joint in Al-adhesive-Al specimens decreased (i.e., by ∼62%, p = 0.0013) with an increase in the applied strain rate from quasi-static to low values, while it remained almost unchanged with further increase of stain rate to intermediate range (p > 0.05). For Cu-adhesive-Cu cases, however, the fracture energy was found to be almost insensitive to the applied strain rate over the range tested (p > 0.05). A cohesive zone model (CZM) was built and strain-rate dependent parameters of traction-separation (TS) law were obtained. These parameters were then used to predict the fracture of epoxy adhesive joints bonded with dissimilar substrates (i.e., copper-aluminum). Predicted fracture loads of the Cu-adhesive-Al cases obtained from the stain-rate dependent model were in reasonable agreement with measured forces (i.e., from ∼0 to 24%) at a given strain rate. Therefore, once TS law parameters for Al-adhesive and Cu-adhesive interfaces were determined, the CZM could predict the fracture of the joint bonded with Al and Cu on its sides. • Fracture tests were performed on 5 cm long, 250 μm thick epoxy adhesive joints at different strain rates. • Fracture energy of the Al–Al joints decreased significantly by increasing strain rate from quasi-static to low values. • By increase in the strain rate, J ci of Cu–Cu joints did not change significantly. • TS law parameters of Al–Al and Cu–Cu joints were determined by a CZM. • Fracture of Cu–Al joints were predicted using TS law parameters in a CZM at various strain rates.

Monitoring the crack length of adhesively bonded carbon-fiber-reinforced polymer by using aligned graphene

In this study, the lengths of cracks in composite joints bonded with epoxy were monitored by evaluating the electrical resistance. Samples of double cantilever beam joints bonded with epoxy-containing aligned graphene were subjected to Mode I fracture tests. During sample fabrication, 0.15 wt% graphene was included in the epoxy and then aligned under an electrical field. The presence of aligned graphene resulted in the formation of an electrically conductive medium between composite adherents. During the fracture tests, the variation in resistance with the crack length was recorded; the resistance of the adhesively bonded joint increased with the crack length. The experimental results for the relationship between the resistance and crack length agree well with theoretical predictions. The functionalized epoxy exhibited homogeneous electrical conductivity, which indicated that the aligned graphene was uniformly dispersed in the epoxy. Thus, epoxy functionalized with aligned graphene can be used to detect the lengths of cracks within composite joints.

Influence of cyclic aging on adhesive mode mixity in dissimilar composite/metal double cantilever beam joints

The adhesive layer in the adhesive joints can experience different modes of loading. Although the fracture energy of adhesive is generally considered to be a material parameter, it is found to be a function of the joint configuration too. Thus, to accurately simulate the behaviour of bonded joints, it is recommended to obtain the fracture energy of the joints using the same substrate(s) as in real applications. In some applications, it is necessary to join dissimilar substrates using adhesives. However, for pure mode fracture tests it is essential to reach the desired loading mode even in a dissimilar joint. Not only the joint configuration but also the environmental conditions need to be considered in fracture tests. In this condition, due to the aging, the stiffness of substrates and adhesive layer might change, and as a result, the adhesive may experience a mixed-mode loading condition. The current study aims to investigate the variation of the mode mixity for dissimilar double cantilever beam adhesive joints with composite/metal substrates subjected to cyclic aging. At different stages of the aging cycles, the mode mixity was calculated during the test using displacement fields obtained by digital image correlation and based on the Williams series expansion. In addition, the variation of flexural stiffness of polymer matrix composite substrates after cyclic aging was investigated using a three-point bending test Finally, based on the variation of composite substrate flexural stiffness and using the finite element method, the variation of the mode-mixity ratio was calculated numerically and compared to the experimental results. The obtained results show that during the cyclic aging the moisture diffusion decreases flexural stiffness of polymer matrix composite substrates significantly, but the variation of substrate flexural stiffness deviates the mode mixity in the aged double cantilever beam specimens.

Effect of infill ratio on fracture toughness of Fused Filament Fabrication (FFF) polymeric adhesive joints

ABSTRACT In order to optimize the stiffness/weight ratio and to reduce the manufacturing time, Additive Manufacturing (AM) components are built up by a solid skin and a partially filled core. The filled/total volume ratio is called infill ratio and it is one of the printing parameters to be selected. Since the infill ratio affects the global as well as the local adherend stiffness, i.e. the stiffness close to the joint interface, this paper aims at exploring the effect of this parameter on the Mode I fracture toughness of adhesive joints of AM materials, for a given infill pattern. Therefore, PLA (PolyLactic Acid) AM adherends have been manufactured with an FFF (Fused Filament Fabrication) printer at infill ratios ranging between 20% and 100% and bonded with a structural epoxy adhesive to form a DCB (Double Cantilever Beam) joint for Mode I fracture toughness testing. Finite Element (FE) simulation of a stationary crack in the adhesive layer has been done to evaluate the strain energy release rate along the crack front at different infill ratios in order to highlight local changes in its distribution that can correlate the variation of fracture toughness with infill ratio recorded in the experiments.

Towards pure mode I loading in dissimilar adhesively bonded double cantilever beams

Nowadays, dissimilar adhesive joints are used widely in different industrial sectors. For this type of joints, the investigation of the fracture behaviour under pure mode I loading is a vital issue. In order to study mode I fracture parameters of adhesive joints, scientists have proposed standard double cantilever beam (DCB) specimens. In dissimilar DCB adhesive joints, because of material asymmetry, obtaining pure mode I is challenging. To obtain pure mode I loading in dissimilar DCB joints, scientists have proposed two criteria based on (1) the flexural stiffness and (2) the distribution of the longitudinal strain along the bondline. However, the achievement of pure mode I in dissimilar DCB joints needs more research. In this paper, the mode-mixity in dissimilar DCB joints designed based on the two mentioned criteria is investigated numerically and experimentally. Metal-composite DCBs were fabricated and tested. During the tensile tests, the horizontal and vertical displacement fields of the adhesive layer were calculated using digital image correlation (DIC) and the finite element method (FEM). Based on the obtained displacement fields and using Williams’ series expansion, the mode-mixities were investigated. The results show that in order to achieve pure mode I loading in steel-composite joints, the criterion based on longitudinal strain is more suitable and in aluminium-composite joints the mode-mixities given by two criteria are almost the same.

Influence of infill percentage on the mode I fracture toughness of adhesively bonded Double Cantilever Beam joints with Additively Manufactured PLA adherends

Adhesive bonding can disclose the possibility to additively manufacture (AM) smaller, highly optimized objects to be bonded in post-production into/onto larger components. However, the infill shape and percentage can influence the fracture toughness of a bonded joint as it emerges from the literature Since the choice of the infill yields also a significant impact on manufacturing time, it is important to assess clearly its influence on bonded joint strength in order to balance strength and manufacturing issues. Therefore, in this work the Mode I fracture toughness of PLA bonded joints obtained by FFF will be assessed as a function of infill percentage, from solid (100% infill) to 20% infill. Test will be done using a Double Cantilever Beam (DCB) joint bonded with a PolyUrethane (PU), room temperature-curing adhesive.

The Effect of Adherend Thickness and Width on Fracture Behavior in Adhesively bonded Double Cantilever Beam Joints

In recent years, bonding joints are the most effective way to successfully and safely combine different materials in sectors such as automotive, aircraft and aerospace industries. In particular, the combination of composite materials, such as rivets, welds, non-conformity of traditional methods have been made use of bonding joints. The fracture behavior of the adhesive is important in adhesively bonded. In the present study, the fracture behavior of the adhesive was investigated experimentally under Mode-I loading of Double Cantilever Beam (DCB) joints obtained by using materials of different width and thickness. AA2024-T3 aluminum is used as the adherend and two component Araldite 2015 tough adhesive is used as adhesive. The fracture progression during the experiment was measured with a high-speed camera and the displacement was measured by a video extensometer. As a result; when the fracture energies of the experimentally obtained adhesive are examined, the fracture energy of the joint changes when the width and thickness of the adherend changes. In addition, the fracture energies of the joint obtained with the Corrected Beam Theory (CBT) and the Standard Test Method (SBT) are compared, and the fracture energy obtained with CBT is more accurate considering the elastic rotation in the adherend.

Effect of nanoalumina in epoxy adhesive on lap shear strength and fracture toughness of aluminium joints

ABSTRACT Lap shear strength and opening mode fracture toughness of epoxy/alumina nanocomposite adhesives in bonded aluminium alloy joints were determined using a single lap joint, and double cantilever beam and contoured double cantilever beam joints, respectively. Epoxy/alumina nanocomposite adhesives were prepared with 0.5, 1.0, 1.5 and 2.0 wt.% of alumina nanospheres (diameter 23–47 nm) and alumina nanorods (diameter in the range of 10 nm and length less than 50 nm). A significant improvement in the lap shear strength and fracture toughness of nanocomposite adhesives was observed over that of neat epoxy adhesive. Maximum lap shear strength of joints bonded with nanocomposite adhesives was observed at 1.5 wt.% of nanospheres and at 1.0 wt.% of nanorods. Maximum fracture toughness of both types of joint was observed for adhesive having 1.5 wt.% of nanospheres and for 1.0 wt.% of nanorods. However, the fracture toughness of joints having 1.5 wt.% of nanospheres was higher in comparison to the fracture toughness of joints having 1.0 wt.% of nanorods. Lap shear strength and fracture toughness decreased on further increment in the wt.% of nanoalumina i.e. at 2.0 wt.% of nanospheres and, at 1.5 and 2.0 wt.% of nanorods.

Influence of laser ablation-induced surface topology on the mechanical behaviour of aluminium bonded joints

The state of the surface plays a crucial role in defining the mechanical properties of bonded joints, in particular, the chemical state of the adherend surface and its structural morphology have been proved to be the main elements of the bonding process. The structural morphology of the surface strictly depends from its topological features, like roughness, presence and distribution of grooves, homogeneity, etc. In a previous work, the effect provided by pulsed Yb-laser ablation on the mode I energy release rate of aluminium double cantilever beam joints was evaluated, in order to identify a relation between the combination of laser parameters and the morphological characteristics of the ablated surfaces which can support in identifying the optimal process configuration. The experimental tests showed that the fracture toughness of the double cantilever beam joints grew up as the surface roughness increased until a threshold value, after which the grooves resulted too narrow to allow the adhesive to completely fill them, inducing a higher amount of entrapped air into the grooves and therefore reducing the level of failure propagation energy. In this work, the problem dealing with the influence that the presence of air bubbles has over the mechanical behaviour of bonded joints was considered and the study went deeper into the investigation on the relation between laser-induced surface topology and mechanical response of joints. Different surface textures involving different directions and conditions of treatment were realized and their effect on the mechanical properties of aluminium bonded specimens was evaluated through experimental tests. The characterization of the treated surfaces was carried out by observing them with a 3D optical profiler and with SEM analysis and by measuring some geometrical features of the patterns created as a result of laser ablation.

Development and fracture toughness characterization of a nylon nanomat epoxy adhesive reinforcement

The potential of an electrospun nylon nanofibrous mat as adhesive carrier and reinforcing web in adhesive bonding has been proven by the authors in a previous work. In that work, a pre-preg nanomat was developed using a low-viscosity epoxy resin for composite hand lay-up, in order to favour wetting of the nanofibres and to minimize air entrapment. However, the resin for hand lay-up exhibited a poor bonding performance when compared to the one typical of epoxy adhesives. The present work is therefore aimed at developing a laboratory route to add an electrospun polymeric nanomat to a two-part epoxy adhesive joint. Three different adhesives with increasing viscosity have been preliminarily evaluated regarding the entrapment of air after curing. The most promising one has been used to manufacture a small-size, Al-alloy double cantilever beam joint and compare the performance with and without the nanomat. Three different precracking procedures have also been developed and evaluated, namely fatigue precracking ...

The fracture behaviour of nanostructure added adhesives under ambient temperature and thermal cyclic conditions

In this study, the fracture behavior of nanocomposite adhesives produced by adding nanostructure in to the adhesives were investigated using Double Cantilever Beam (DCB) test under ambient temperature and thermal cycle conditions. Adhesively bonded DCB joints were produced using DP460 toughened adhesive type and DP125 flexible adhesive type as the adhesives; AA2024-T3 aluminum alloy was used as the adherend, and 1 wt.% Graphene-COOH, Carbon Nanotube-COOH and Fullerene C60 were used as the added nanostructures. As a result, when the experimental fracture energy was examined, the nanocomposite adhesives obtained by adding nanostructure were found to have increased the fracture energy of the joint. In the joints bonded with the flexible DP125 adhesive that were subjected to thermal cycles, the addition of Graphene-COOH into the adhesive increases the fracture energy of the joint by 55%, the addition of Fullerene increases the fracture energy of the joint by 135%. Also, it was observed that there is a significant difference between the displacements that were obtained directly from the test machines stroke and measured via video extensometer giving the crack opening between the top and bottom adherends during the DCB test. This situation significantly effects the correct calculation of the fracture energy of adhesive.

Fatigue crack propagation rate of CFRP/aluminum adhesively bonded DCB joints with acrylic and epoxy adhesives

Recently, structural acrylic adhesives have received increased interest because they can be cured at room temperature. However, there are few studies investigating the strength behavior of CFRP/metal joints bonded with acrylic adhesive, especially under cyclic loading. In this study, the fatigue crack growth rate was measured using adhesively bonded CFRP/aluminum double cantilever beam (DCB) joints made with an acrylic adhesive. To compare fatigue crack growth behavior with heat-cured type adhesive, fatigue testing was also conducted for DCB joints with an epoxy adhesive. For CFRP/aluminum asymmetrical joints, the ratio of the thickness of the lower adherend to the upper adherend is an important factor determining the mode ratio and stress distribution at the crack tip which are affected by the residual stress generated in the curing process. The effect of the thickness ratio for the DCB joints on the fatigue crack growth rate was investigated for DCB joints with both acrylic and epoxy adhesives.

Influence of laser treatment parameters on the mode I strain energy release rate of aluminum double cantilever beam joints

Surface texturing produced by laser ablation is an efficient and effective technology for treating substrates to improve adhesive bond strength. In the literature, its effect has been extensively studied for different adherends materials, adhesives and laser sources. Laser ablation produces both morphological and chemical modifications of the surfaces, promoting mechanical interlocking and chemical bonding between the adhesive and the substrates. In this work, the effect of pulsed Yb-fiber laser ablation over the quasi-static mode I fracture energy of Double Cantilever Beam (DCB) aluminum bonded joints has been assessed for different combinations of processing parameters, with the aim of optimizing the treatment for industrial purposes. The mechanical tests show that the treatment becomes effective when a laser energy density threshold is overcome. On the other hand, a further increase in the energy density leads to a slight reduction of the joints fracture energy. This is related to the viscosity of the adhesive and to the high roughness produced by high energy treatments, resulting in the presence of air bubbles in the adhesive layer. In order to understand this phenomenon, the treated surfaces are characterized from the morphological point of view using a 3D optical profiler and SEM analysis.

A general analytical model based on elastic foundation beam theory for adhesively bonded DCB joints either with flexible or rigid adhesives

A new general analytical model based on elastic foundation beam theory is proposed for adhesively bonded double cantilever beam (DCB) specimens. The new model accounts for any stress state (from plane stress to plane strain) in the adhesive and therefore, the model is able to predict the mechanical response of a DCB specimens whatever the adhesive type (from rigid to flexible), the stress state, and the width/thickness relationship of the specimen are. The model is validated with DCB tests for flexible Silkron-H100 and stiff Araldite-2021 adhesives. A comparison with other models from the literature demonstrates that more accurate predictions in terms of stress distribution in the middle plane of the adhesive layer, adherend deflections and load predictions are obtained. The model is especially suitable when flexible adhesives and/or thick adhesives are used. In addition, a new method to accurately obtain Poisson’s ratio for flexible adhesives is proposed.

An improved model for adhesively bonded DCB joints

An improved analytical model is proposed for characterizing the fracture behavior of an adhesively bonded double cantilever beam joint under Mode I loading. Novel interfacial normal stress distribution function is used with a key parameter c that is determined using continuum mixture theory. In addition to the mechanical and sectional properties of the adherends, crack length, and overlap area, the model also incorporates the adhesive thickness and material properties as well as the crack tip rotation. Model prediction of the fracture toughness of the joint is entered into finite element analysis to simulate crack propagation under peel loading. The effect of various parameters on the joint fracture properties is discussed. Results show that the proposed model provides better correlation with published experimental data.

Modeling Mode I Fracture of Bitumen Films

AbstractThe fracture behavior of thin films of bitumen in double cantilever beam (DCB) specimens was investigated over a wide range of temperature and loading rate conditions using finite-element analysis. The model includes a phenomenological model for the mechanical behavior of bitumen, implemented into a special-purpose finite-element user material subroutine, combined with a cohesive zone model (CZM) for simulating the fracture process. The finite-element model is validated against experimental results from laboratory tests of DCB specimens by comparing measured and predicted load–line deflection histories and fracture energy release rates. Computer simulation results agreed well with experimental data of DCB joints containing bitumen films in terms of peak stress, fracture toughness, and stress-strain history response. The predicted “normalized toughness,” G/2h, was found to increase in a power-law manner with effective temperature-compensated strain rate in the ductile region as previously observed ...