Bonded Scarf Joints Research Articles

Modeling of adhesively bonded single scarf CFRP joint behavior using energy-based approach

The mechanical behavior of an adhesively bonded single scarf joined Carbon fiber reinforced polymer (CFRP) laminates under tensile loading is studied. An energy-based analytical model is developed to capture the static response of adhesively bonded single scarf joint specimens. This is a high-fidelity and highly accurate alternative to 3D finite element models commonly employed in the literature. The derived governing differential equations (GDEs) are solved using the finite difference scheme. The numerical results for the global response of the adhesively bonded joints provided by the analytical model are compared and successfully validated with the experimental and FE results. For this purpose, the experimental whole-field technique of 2D digital image correlation (DIC) is used to capture the strain and displacement field over the specimen surface. A qualitative agreement of the field distributions is observed since the analytical model is a reduced-order model. Parametric studies are also undertaken to demonstrate the influence of design parameters, such as scarf angle, adhesive thickness, and adhesive modulus. These results provide quick solutions for the design space of adhesively bonded scarf joints. Therefore, a reliable model for the adhesively bonded single scarf joints is established, acting as a cost-effective and time-saving alternative solution.

Implementation of a probabilistic machine learning strategy for failure predictions of adhesively bonded joints using cohesive zone modeling

Adhesive bonding as an assembly procedure in aviation products is very efficient from both weight and recurring cost points of view. However, even with strict inspections, process control, and quality assurance protocols, it is difficult to evaluate and predict joint strength and performance. To manage this risk, regulators require determination of the maximum allowed disbond size as part of bonded joint design. This study proposes a methodology for determination of the bond-line strength characteristics based on a comprehensive test data from 88 Mixed Mode Bending (MMB) specimens. The Cohesive Zone Model (CZM) was employed to simulate disbond propagation, and nearly 2000 finite element simulations were conducted with a wide range of failure parameters. Combined experimental and numerical data was analyzed using the Gaussian Process Regression (GPR) machine learning algorithm to obtain the optimized CZM failure parameters. Statistical analysis was conducted to determine safety factors and B-Basis cohesive zone failure parameters, ensuring sufficient reliability of the failure predictions. The proposed failure parameters were validated by means of several benchmark problems, including single bonded lap joints and a bonded scarf joint, showing very good agreement with the test data.

Development of aviation industry-oriented methodology for failure predictions of brittle bonded joints using probabilistic machine learning

The bonding assembly concept of cured aerospace composite parts is considered an efficient approach from almost every perspective. It simplifies the design and provides great opportunities for weight and cost reductions. However, even with strict quality assurance, such assembly may lead to undetectable low bond-line strengths, usually referred to as “kissing bonds”. As part of a certification effort of bonded composites, the maximum allowed disbond size has to be determined. In this study, an approach for determination of the residual strength of bonded joints is proposed. Virtual crack closure failure parameters of the Loctite EA 9394 paste adhesive are determined based on a comprehensive test campaign. Predictions from more than 5,000 finite element simulations were analyzed using a machine learning strategy. The accuracy of the optimal failure parameters was assessed, and the failure parameters were statistically adjusted to account for predictions variability. The validity of the proposed approach and the corresponding failure parameters were examined using two classic bonded joints design concepts, bonded scarf joint and bonded T-joint.

Research on thermal buckling behaviour of the adhesively bonded scarf joints for glass connections

Adhesively bonded technique is preferred more than bolted joining technique for glass connections. Glass panels supported by metal frame are generally used to glazing applications. Obtaining the critical buckling temperature is highly important and also thermal buckling behaviour of the glass joints should be considered. Stability of the joint is influenced by many design parameters such as types of adhesive/adherent materials. This paper presents a finite element method (FEM) of adhesively bonded scarf joint under thermal loading. Finite element models for the glass joint having isotropic (Aluminum and Steel) or anisotropic (Boron-epoxy, Kevlar-epoxy and E glass-epoxy) adherents were established by ANSYS® commercial program. FEM analysis was based on the usage of special surface to surface contact elements. The effects of adherent properties on the critical buckling temperature were investigated. Adhesively bonded scarf joint mode shapes were presented for the first five modes. Among the anisotropic adherent materials, the highest critical buckling temperature was determined for the boron-epoxy/glass joint. Additionally, the results showed that the adherent materials should be chosen carefully for adhesively bonded glass joints. Copyright © 2017 VBRI Press.

Strength of scarf-bonded CFRP repairs containing disc-shaped zones of weak bonding considering hot-wet conditioning

A commonly used design for the repair of carbon fibre reinforced polymers (CFRP) is the bonded scarf joint. As contaminations on the adherend surface or bondline flaws can reduce the bonding strength drastically, the effect of such strength reducing phenomena has to be known. This is also the case for bonded joints that had to sustain hot-wet conditions. Therefore, the aim of this study was to investigate the influence of weak bonds and hot-wet conditioning on the mechanical properties of scarf bonded CFRP joints. Circular zones of weak bonding have been artificially produced by application of release agent on the adherend surface. Different sizes and locations were studied. Additionally, two scarf ratios and two lay-ups have been investigated, to account for different stress states in the bondline. The investigation showed that the effect of a weak bond on bond strength is strongly dependent on the location, size and lay-up of the adherend, while hot-wet conditioning reduces the bond strength in all cases.

Prediction of failure behavior of adhesively bonded CFRP scarf joints using a cohesive zone model

In this article, the tensile behavior of adhesively bonded scarf joints (SAJs) of CFRP laminates with ductile adhesive is investigated by experiments and finite element (FE) simulations. Experimental tests considering joints with six different scarf angles are performed, and three-dimensional (3D) FE models are established in ABAQUS to study the failure mechanism. A user-defined cohesive zone model (UCZM) is proposed to predict the failure of ductile adhesive. The load-displacement response and failure modes of SAJs obtained by FE simulations are compared with the experimental results to validate the accuracy and applicability of UCZM. Moreover, the widely used triangular cohesive zone model (TCZM) is also employed to predict the failure. The comparison between experimental and numerical results exhibits that UCZM is able to predict the joint strength and failure displacement more precisely although TCZM also achieves certain accuracy. Furthermore, the influence of adhesive properties and composite stacking sequence on the tensile performance of joints is discussed using UCZM. Finally, the stress distribution within the adhesive layer and its variation along with different scarf angles are analyzed.

An improved progressive failure modelling and damage tolerant design methodology for composite scarf joints with bondline flaws

There is a lack of understanding on the damage tolerant design of bonded scarf joints, specifically when an existing bondline flaw affects the performance. This paper presents an improved progressive damage modelling methodology for assessing the damage tolerant performance of composite scarf joints containing artificial flaws in the bondline. The developed methodology is validated using an experimental study investigating the influences of design parameters on the strength of scarf joints under quasi-static tensile loading. The presented work gives excellent predictions of joint strength, and significant insight into the initiation, interaction and progression of damage. A new damage tolerant design approach is subsequently proposed for the application of bonded composite scarf joints, particularly in primary aerospace structures. The proposed design approach assesses the state of the stress at the failure-critical regions of adhesive and adherend to inform on damage-tolerant safe design loads, thus improving the future application of bonded composite scarf joints.

Tensile strength of composite bonded scarf joint in various thermal environmental conditions

This study analyzed variations of tensile strength and the failure mode of composite bonded scarf joint in various thermal environments. A total of 70 scarf bonded joints were manufactured using secondary bonding and co-bonding methods, in which a number of 10 scarf joints were additionally attached to an external ply over the joint area to examine its effect. Six different environmental temperatures ranged from 25°C to 150°C were chosen and tested. A pure laminate having the same stacking sequence as the parent laminate was prepared and tested at the six temperatures for references. Experimental results revealed that tensile strengths of the secondary bonded (SB) and co-bonded (CB) joints at 75°C reduced by 9.3% and 10.5% in comparison with strengths of the corresponding joints at 25°C, respectively. Particularly, at 125°C, which is higher than the resin glass transition temperature, significant reductions of 57.1% and 53.6% in strength were found for SB and CB joints, respectively. In terms of the manufacturing methods, the strengths of the SB joints were higher than those of the CB at 25°C, 50°C, and 75°C by 6.5%, 7.6%, and 8.0%, respectively. However, at high temperatures (100°C, 125°C, and 150°C) these differences were negligible. Strengths of SB joints attached external ply increased by 5.9% and 37.5% compared to those of SB joints without the external ply at 25°C and 100°C, respectively.

Effect of infrared laser surface treatment on the morphology and adhesive properties of scarfed CFRP surfaces

The bonded scarf joint is a common design for the repair of carbon fibre reinforced polymer (CFRP). Surface treatment is an essential factor for the performance of such joints. Infrared (IR) laser surface treatment is of particular interest, as it has the potential for a fast and automatable process. The effect of the IR laser surface treatment on the morphology was investigated by scanning electron microscopy in combination with ion beam milling. Change of surface chemical composition was assessed by X-ray photoelectron spectroscopy. Additionally, IR laser treated CFRP scarf joints were tested under tensile load. Surface analysis revealed a stripping of carbon fibres in combination with a roughening. On the other hand, a decrease of oxygen and nitrogen on the surface was detected. The increase of the bonding strength was attributed to the increase of the effective surface and mechanical interlocking due to the roughening of the CFRP surface.

An experimental investigation of the fatigue damage behaviour of adhesively bonded joints under the combined effect of variable amplitude stress and temperature variation

Under working conditions, adhesively bonded structures are exposed to variable cyclic loading and temperature variation. However, there is still limited number of works dealing with their relationship. The present study investigates the combined effect of variable amplitude stress and variation of temperature on the fatigue damage behaviour of adhesively bonded scarf joints. Initially, experiments at constant amplitude stress, which showed reduction of fatigue strength with increasing temperature, were carried out to establish base SN curves for calculations of cumulative damage. Then, tests under variable amplitude fatigue (VAF) were performed with constant temperature and with two, four and six temperature changes. Temperatures for VAF experiments were −10, 23 and 50 °C. VAF at constant temperature demonstrated: (i) damage retardation at −10 and 23 °C and damage acceleration at 50 °C and (ii) no clear effect on the fracture mode. Conversely, VAF with temperature variation showed: (i) high damage acceleration with an increase proportional to the number of temperature changes and (ii) a trend of more adhesive failure. For lifetime predictions made with a linear damage rule, good correlation was obtained at constant temperature. However, under variable temperature conditions caution must be taken since predictions overestimated fatigue lifetime.

RETRACTED ARTICLE: Stress distribution in adhesively bonded scarf joint under thermo-mechanical loads

(2019). RETRACTED ARTICLE: Stress distribution in adhesively bonded scarf joint under thermo-mechanical loads. The Journal of Adhesion: Vol. 95, No. 2, pp. 103-125.

Health monitoring of scarfed CFRP joints under cyclic loading via electrical resistance measurements using carbon nanotube modified adhesive films

This study deals with a structural health monitoring approach for adhesively bonded carbon fiber reinforced polymer joints. A modification of an epoxy based adhesive film with single wall carbon nanotubes allows for electrical resistance measurements through the joint. Cyclic fatigue tests of adhesively bonded scarf joints with simultaneous electrical resistance measurements are conducted to investigate the damage detection and localization of repaired composite parts during operation. The measured electrical resistance changes are compared to results from digital image correlation. Crack initiation and growth can be detected by an increase of electrical resistance. Furthermore, it is possible with parallel oriented ink-jet printed circuits to localize the damages occurred.

Recent progress on damage mechanisms in polymeric adhesively bonded high-performance composite joints under fatigue

A prominent critical problem with the use of adhesively bonded composite scarf joint in aircraft structures is their susceptibility to unpredictable failure. Any efficient and robust design approach for adhesively bonded scarf joints under fatigue loading should account for the damage mechanisms in the joint and this cannot be achieved without understanding damage development with initiation and propagation. Research into damage development using methodical approach were conducted on the stiffness of polymer adhesively bonded composite scarf joint since it is the prime physical parameter that can be measured most accurately. The research produced significant new knowledge of damage development under fatigue as strain hardening and a methodology to characterize damage initiation and propagation in adhesively bonded joints. Under uninterrupted fatigue, stiffness degradation and plasticity effects led to strain hardening of the joint adhesive until failure. But there was in-homogenous stiffness degradation in regions within the joint due to defects and anomalies so that the general stiffness degradation of the scarf joint could be either a stiffness increase or decrease to failure.The changes in the rate of stiffness over the fatigue life of a scarf joint reveal three characteristic regions. Region I & II marked the damage initiation life of scarf joints and this was (66±9)% of the fatigue life for specimens tested, and Region III marked the onset of damage propagation to failure. Mean strain effect was evident in some scarf joint under fatigue cycle. It is shown that the damage initiation & propagation life under variable and constant fatigue loading for scarf joints is independent of the adhesive thickness, the fatigue load application frequency, and the severity of the applied load. Damage propagation in adhesively bonded scarf joint was found to be a combination (1) strain hardening of the adhesive layer leading to brittle cracking of the adhesive layer (2) cracking of the adhesive layer at 45° or 90° to the axial loading, leading to break in stress distribution in the adhesive joint as it matures with strain hardening (3) leading to gradual quasi-static failure of the joint particularly since there was no evidence of direct residual fatigue pattern on the fracture surface either in this research work or from any existing literature for similar loading situation. In all, the paper addressed one of the most critical and important steps that will drive correct quality implementation of design for fatigue in polymer adhesively bonded joints towards addressing the current airworthiness limitations on use of these joints in the aerospace industry and further to expand the scope of application of adhesively bonded joints.

On the interfacial strength of bonded scarf joints

The interfacial strength of bi-material bonded scarf joints is evaluated by an integrated experimental and numerical investigation. The effects of the scarf angle, interface dimension and the stress singularity on the interfacial strength were examined using a series of aluminum/PMMA and aluminum/polycarbonate bi-material specimens under static bending moments. The experimental results showed that the maximum principal tensile stress at failure did not vary significantly with interface dimension. The measured maximum principal tensile stress of scarf joints at failure decreases as the scarf angle increases. We correlated fracture initiation with critical values of the stress intensities of the singular fields near the three-dimensional interface corner. The order of the singularity at three-dimensional interface corners and edges was obtained numerically using common finite element methods. Finally, an empirical equation based on interface fracture mechanics was determined by curve fitting of the experimental results to predict the interfacial strength of bonded scarf joints.

Analysis of the Influence of Geometric Parameters on the Stress Distributions in Adhesively Bonded Scarf Joints Using 2D Models Under Elastic Assumption

The scarf joint is a usual experimental assembly employed to analyze the mechanical behavior of an adhesive. In fact, using a unique type of bonded assembly with a classic tensile testing machine, various tensile-shear loadings of the adhesive can be applied by changing the value of the scarf angle. In this paper, accurate numerical analyses of the stress distributions within the adhesive in scarf joints under elastic assumption using 2D models are developed. Numerical results underline the influence of the adhesive thickness and mainly the influence of the scarf angle on the edge effects, and confirm the presence of an optimal scarf angle associated with very low stress concentrations. Moreover, the use of a suited elastic limit for the adhesive, defined from the two stress invariants, hydrostatic stress and von Mises equivalent stress, allows the more stressed parts of the adhesive with respect to the scarf angle to be defined. These results also underline the possible influences of the edge effects on the experimental results, i.e., possible crack initiations close to the free edges of the adhesive for some scarf angles. Finally, it is shown that a little modification of the free edges of the adhesive (a so-called “cleaning”) can strongly reduce the influences of the edge effects and thus can improve the experimental results for a wide range of scarf angles.

Effects of bondline flaws on the damage tolerance of composite scarf joints

Scarf repairs to aircraft structures need to sustain design ultimate load in the presence of flaws due to manufacturing and impact by foreign objects, in order to demonstrate compliance with airworthiness regulations. This paper presents an investigation into the effect of disbonds on the load-carrying capacity of adhesively bonded scarf joints. Experiments were conducted on scarf joints containing disbonds of varying lengths. The results showed that the load-carrying capacity of scarf joints decreases with the size of the bondline flaw at a faster rate than the reduction in the effective bond area. Fractographic analysis showed that the fracture occurred in the composite matrix adjacent to the adhesive-composite interface, at a distance equal to a small fraction of ply thickness. Computational analyses using the virtual crack closure technique (VCCT) and the cohesive zone model (CZM) confirmed these experimental observations: model predictions using composite material properties were in better correlation with experimental results than those using adhesive properties. Furthermore, CZM is capable of predicting the effects of flaws of all sized being considered, while the VCCT model is only applicable to joints containing flaws greater than a certain size.

Reliability analysis of adhesive bonded scarf joints

A probabilistic model for the reliability analysis of adhesive bonded scarfed lap joints subjected to static loading is developed. It is representative for the main laminate in a wind turbine blade subjected to flapwise bending. The structural analysis is based on a three dimensional (3D) finite element analysis (FEA). For the reliability analysis a design equation is considered which is related to a deterministic code-based design equation where reliability is secured by partial safety factors together with characteristic values for the material properties and loads. The failure criteria are formulated using a von Mises, a modified von Mises and a maximum stress failure criterion. The reliability level is estimated for the scarfed lap joint and this is compared with the target reliability level implicitly used in the wind turbine standard IEC 61400-1. A convergence study is performed to validate the FEA model, and a sensitivity analysis on the influence of various geometrical parameters and material properties on the maximum stress is conducted. Because the yield behavior of many polymeric structural adhesives is dependent on both deviatoric and hydrostatic stress components, different ratios of the compressive to tensile adhesive yield stresses in the failure criterion are considered. It is shown that the chosen failure criterion, the scarf angle and the load are significant for the assessment of the probability of failure.

Fatigue life evaluation of adhesively bonded scarf joints

The fatigue life of adhesively bonded scarf joints with various scarf angles was analyzed experimentally in this study. Experimental results indicate that the fatigue strength increases significantly with the scarf angle of the studied specimen. The main failure modes for the specimens with small and large scarf angles are adhesive and cohesive, respectively. Additionally, the finite element method was employed to obtain the stress states within the studied scarf joint specimens, and the fatigue data of specimens with 0°/30° scarf angles and 45°/60° scarf angles were successfully correlated using the interfacial peeling stress parameter and the von Mises stress within the adhesive, respectively.

Strength prediction of epoxy adhesively bonded scarf joints of dissimilar adherends

In this study, strength of epoxy adhesively bonded scarf joints of dissimilar adherends, namely SUS304 stainless steel and YH75 aluminum alloy is examined on several scarf angles and various bond thicknesses under uniaxial tensile loading. Scarf angle, θ=45°, 60° and 75° are employed. The bond thickness, t between the dissimilar adherends is controlled to be ranged between 0.1 and 1.2 mm. Finite element (FE) analysis is also executed to investigate the stress distributions in the adhesive layer of scarf joints by ANSYS 11 code. As a result, the apparent Young's modulus of adhesive layer in scarf joints is found to be 1.5–5 times higher than those of bulk epoxy adhesive, which has been obtained from tensile tests. For scarf joint strength prediction, the existing failure criteria (i.e. maximum principal stress and Mises equivalent stress) cannot satisfactorily estimate the present experimental results. Though the measured stress multiaxiality of scarf joints proportionally increases as the scarf angle increases, the experimental results do not agree with the theoretical values. From analytical solutions, stress singularity exists most pronouncedly at the steel/adhesive interface corner of joint having 45–75° scarf angle. The failure surface observations confirm that the failure has always initiated at this apex. This is also in agreement with stress- y distribution obtained within FE analysis. Finally, the strength of scarf joints bonded with brittle adhesive can be best predicted by interface corner toughness, Hc parameter.

Analysis of the Effect of Piezoelectric Patches on the Behavior of Adhesively Bonded Scarf Joints — An Analytical Study

In order to reduce the maximum peel and shear stress concentrations in the adhesive layer, a smart adhesively bonded scarf joint system was developed by surface bonding of piezoelectric patches onto a typical scarf joint. The forces and bending moments at the edges of the developed smart joint system can be adaptively controlled by adjusting the applied electric field on the piezoelectric patches, thus reducing the peel and shear stresses concentration in the adhesive layer. In order to verify the effect of surface bonding of piezoelectric patches in smart scarf adhesive joints, an analytical model was developed to evaluate the shear stress distribution and to predict the peel stress. It was established that the piezoelectric patched joint could reduce the stress concentrations at the scarf joint edges. The influence of the electric field and the effects of the scarf angle and the adherend Young's modulus on the peel and shear stresses were investigated. It was found that the effect of scarf angle is more significant at higher angles to raise the stresses. The effect of the electric field on the shear stress is more significant than on the peel stress.