Scarf Angle Research Articles

The effect of angle on the strain of scarf lap joints subjected to tensile loads

In this paper, the mechanical behavior of the scarf lap joints (SLJs) bonded with adhesive (Vinylester Atlac 580) under a tensile load was analyzed. The effects of scarf angle at the interface strain distributions of SLJs were examined. The stress analyses were performed via three dimensional Finite Element Method (3D-FEM). The 3D-FEM code employed was Ansys (12.0). Experimental results were compared with the 3D-FEM results and were found quite reasonable. The results indicated that the maximum values of the normalized εx strain values were determined at θ=60° in all joints.

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.

A computational analysis of the scarf angle on a composites repair

This study examined the relationship between the scarf angle and stress distribution, and estimated the strength recovery via a finite element analysis. The following conclusions were drawn from this study. Resin will fracture due to a tensile load with a high scarf angle, which is similar to the patch repair method. An applied stress can be loaded to a repaired laminate if the scarf angle is <TEX>$5^{\circ}$</TEX>. The Von-Mises stress increases with decreasing scarf angle, with the exception of a scarf angle of <TEX>$30^{\circ}$</TEX>, where the scarf angle can indicate the rates of shear and normal stresses. Strength recovery can be better if the scarf angle is decreased to a lower angle. However, scarf machining requires more time, a high skill level and considerable expense. Therefore, a scarf angle of <TEX>$5^{\circ}$</TEX> is the most effective for a repair. These results may provide a guide for engineers wishing to formulate a standard for repair. The scarf angle needs to be carefully managed for a more efficient composite repair.

Strength and Fracture Characteristics of SUS304/AL-Alloy Scarf Adhesive Joint with Various Adhesive Thicknesses

In this study, strength and fracture toughness of epoxy adhesively bonded scarf joints of dissimilar adherends, namely SUS304 stainless steel and YH75 aluminium alloy are examined on several scarf angles and various bond thicknesses under uniaxial tensile loading. Scarf angles, θ = 45°, 60° and 75° are employed. The bond thickness, t between dissimilar metals is controlled to be ranged between 0.1 mm to 1.2 mm. Finite element (FE) analysis is also executed to investigate the stress distributions in the scarf joints by ANSYS 11 code. From analytical solutions, stress singularity exists most pronouncedly at the steel/adhesive interface corner of joints having 45° to 75° scarf angle. This is not only in agreement with the FE analyses results but also confirmed by fracture surfaces observation wherein the fracture has always been initiated at this point. The strength of scarf joints increases as the bond thickness decreases. Interface corner toughness, Hc approach can be applied when predicting the failure stress of scarf joints. Besides, for scarf joints with an interfacial crack, the fracture toughness, Jc values are independent of bond thickness and less sensitive to adherends. Moreover, Jc increases as mode mixity increases.

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.

Stress analysis and strength evaluation of scarf adhesive joints subjected to static tensile loadings

The stress distributions in scarf adhesive joints under static tensile loadings are analyzed using three-dimensional finite-element calculations. The effects of adhesive Young’s modulus, adhesive thickness and scarf angle in the adherend on the interface stress distributions are examined. As the results, it is found that the maximum value of the maximum principal stress occurs at the edge of the interfaces. The differences in the interface stress distributions between the 2-D and the 3-D FEM results are demonstrated. It is also observed from the 3-D FEM results that the maximum value of the maximum principal stress is the smallest when the scarf angle is around 60 degree, while it is around 52 degree in the 2-D FEM when the singular stress at the edges vanishes. In addition, the joint strength is estimated using the interface stress distribution obtained from the FEM calculations. For verification of the FEM calculations, experiments were carried out to measure the strengths and the strains in the joints under static tensile loadings using strain gauges. Fairly good agreements were observed between the 3-D FEM and the measured results for strains. Therefore, for the joint strength, the results remain conservative.

Numerical evaluation of three-dimensional scarf repairs in carbon-epoxy structures

The widespread employment of carbon-epoxy laminates in high responsibility and severely loaded applications introduces an issue regarding their handling after damage. Repair of these structures should be evaluated, instead of their disposal, for cost saving and ecological purposes. Under this perspective, the availability of efficient repair methods is essential to restore the strength of the structure. The development and validation of accurate predictive tools for the repairs behaviour are also extremely important, allowing the reduction of costs and time associated to extensive test programmes. Comparing with strap repairs, scarf repairs have the advantages of a higher efficiency and the absence of aerodynamic disturbance. This work reports on a numerical study of the tensile behaviour of three-dimensional scarf repairs in carbon-epoxy structures, using a ductile adhesive (Araldite ® 2015). The finite elements analysis was performed in ABAQUS ® and Cohesive Zone Modelling was used for the simulation of damage onset and growth in the adhesive layer. Trapezoidal cohesive laws in each pure mode were used to account for the ductility of the specific adhesive mentioned. A parametric study was performed on the repair width and scarf angle. The use of over-laminating plies covering the repaired region at the outer or both repair surfaces was also tested as an attempt to increase the repairs efficiency. The obtained results allowed the proposal of design principles for repairing composite structures.

ＣＦＲＰスカーフ継手の引張強度における衝撃損傷の影響

An experimental investigation on the mechanical properties of CFRP scarf joints with impact damage was carried out to understand the relationship between scarf angle and impact characteristics. This research shows the effect of the scarf angle on various characteristics, tensile strength, impact damage, and tensile after impact (TAI) strength. These characteristics were investigated experimentally about four scarf angles of 2o, 3o, 4.5o, and 6o. For the non-damaged specimen tensile test, it was found that the tensile strength decreases as the scarf angles increase. It was thought that the reason of decreasing of tensile strength was decrease of bonded area of scarf joint due to increase of scarf angle. Tensile fracture mode of non-damaged specimens were changed from net tension failure mode to adhesive failure mode as the scarf angles increase. As a result of the cross-sectional observation after impact test, impact damage area was increased by the increasing of scarf angle. At the higher scarf angle, debonding of adhesive layer was increased, but damage of laminate was decreased. It was thought that the damage decreasing of laminate was affected by the energy absorption due to significant debonding in the adhesive layer. For the damaged specimen tensile test, residual tensile strength decreased as the scarf angles increase. It was thought that the residual tensile strength of the scarf joints was affected by the increase of debonding in the adhesive layer at higher scarf angle.

Strength Prediction of Adhesively-Bonded Scarf Repairs in Composite Structures under Bending

This work reports on the experimental and numerical study of the bending behaviour of two-dimensional adhesively-bonded scarf repairs of carbon-epoxy laminates, bonded with the ductile adhesive Araldite 2015®. Scarf angles varying from 2 to 45º were tested. The experimental work performed was used to validate a numerical Finite Element analysis using ABAQUS® and a methodology developed by the authors to predict the strength of bonded assemblies. This methodology consists on replacing the adhesive layer by cohesive elements, including mixed-mode criteria to deal with the mixed-mode behaviour usually observed in structures. Trapezoidal laws in pure modes I and II were used to account for the ductility of the adhesive used. The cohesive laws in pure modes I and II were determined with Double Cantilever Beam and End-Notched Flexure tests, respectively, using an inverse method. Since in the experiments interlaminar and transverse intralaminar failures of the carbon-epoxy components also occurred in some regions, cohesive laws to simulate these failure modes were also obtained experimentally with a similar procedure. A good correlation with the experiments was found on the elastic stiffness, maximum load and failure mode of the repairs, showing that this methodology simulates accurately the mechanical behaviour of bonded assemblies.

Stress and Failure Analysis of Repaired Sandwich Composite Beams using a Cohesive Damage Model

The behavior of a repaired sandwich beam subjected to four-point bending is investigated numerically using the ABAQUS® software and specially developed interface elements including a cohesive mixed-mode damage model based on the indirect use of fracture mechanics. The two major repair configurations for sandwich structures, namely overlap and scarf repair, are studied. The interface elements, placed at the middle of the adhesive, parent laminate/adhesive and adhesive/patch interfaces, allow to obtain stress distributions at these locations as well as to simulate damage onset and growth. The influence of several geometrical parameters, such as overlap length and patch thickness for overlap repairs, and scarf angle for scarf repairs, is evaluated in terms of stress analysis and strength predictions. Conclusions were drawn about design guidelines of the sandwich composite repair.

Interface microstructure and mechanical properties of laser welding copper–steel dissimilar joint

Relatively the high reflectivity of copper to CO 2 laser led to the difficulty in joining copper to steel using laser welding. In this paper, a new method was proposed to complete the copper–steel laser butt welding. The scarf joint geometry was used, i.e., the sides of the copper and steel were in obtuse and acute angles, respectively. During the welding process, the laser beam was fixed on the steel side and the dilution ratio of copper to steel was controlled by properly selecting the deviation of the laser beam. The offset of laser beam depended on the scarf angle between the copper and steel, the thickness of plate and the processing parameters used in the laser welding. The microstructure near the interface between Cu plate and the intermixing zone was investigated. Experimental results showed that for the welded joint with high dilution ratio of copper, there was a transition zone with numerous filler particles near the interface. However, if the dilution ratio of copper is low, the transition zone is only generated near the upper side of the interface. At the lower side of the interface, the turbulent bursting behavior in the welding pool led to the penetration of liquid metal into Cu. The welded joint with lower dilution ratio of copper in the fusion zone exhibited higher tensile strength. On the bases of the microstructural evaluation at the interface of the welded joint, a physical model was proposed to describe the formation mechanism of the dissimilar joint with low dilution ratio of copper.

Optimum shapes of scarf repairs

Adhesively bonded scarf repairs are the preferred method for repairing composite structures, limited mainly by the amount of material removal associated with scarfing. In addition to the high strength restoration, scarf repairs also enable recovery of the original external surface as required by aerodynamic and/or external mould line considerations. However, scarf repairs almost inevitably result in the removal of undamaged material to make way for the scarf insert. This can be a particularly significant issue for thick structures, because the scarf length can vary between 20 and 100 times the thicknesses of the parent structure. In this investigation, an optimisation method has been developed for determining the optimum repair shapes for a given biaxial loading condition. The optimum scarf shape is determined by numerically solving the resulting non-linear differential equation governing the scarf angle. The optimum and near-optimum shapes are presented and discussed with respect to computational modelling using the finite element method.

Comparison between experimental and numerical study of the adhesively bonded scarf joint and double scarf joint: Influence of internal singularity created by geometry of the double scarf joint on the damage evolution

The objective is to investigate the influence of singularity created by the internal geometry of the double scarf joint, on the damage evolution to the adhesively bonded joint. Several finely instrumented tests are carried out to characterize the damage process unlike the ultimate failure of the adhesively bonded joint. The approach is to compare the micromechanical behavior of adhesively bonded scarf joint and double scarf joint, with the same scarf angle value. Experimental results are obtained by strain gauge measurements to distinguish three damage thresholds, which characterize the damage evolution of the adhesively bonded joint. Numerical results use a damage criterion in order to predict progressive damage evolution. The numerical results show the viability of the failure criterion and the complementarity between experimental and numerical works allows refining our final conclusions.

Stress Analysis and Strength Evaluation of Scarf Adhesive Joints with Dissimilar Adherends Subjected to Static Tensile Loadings

The interface stress distributions in scarf adhesive joints with dissimilar adherends under static tensile loadings are analyzed using two-dimensional and three-dimensional finite element calculations. The effects of the adherends and adhesive Young's modulus, the scarf angle and the adhesive thickness on the interface stress distributions are examined. In addition, the joint strength is predicted using the interface stress distributions based on the maximum principal stress theory. It is found that when the scarf angle is around 60°, the singular stress at the edges of the interfaces is minimum in the 3-Dimensional FEM calculations. Furthermore, it is noticed that the strength of the joints with dissimilar adherends is smaller than that of the joints with similar adherends. For verification of the FEM calculations, the strains in the adherends and the joint strengths were measured in the experiments. The measured strains are in fairly good agreement with those obtained from FEM calculations. Also, the measured joint strength is fairly consistent with the calculated results.

Buckling Behaviour of Carbon–Epoxy Adhesively-Bonded Scarf Repairs

The present work is dedicated to the experimental and numerical study of the buckling behaviour under pure compression of carbon–epoxy adhesively-bonded scarf repairs, with scarf angles varying from 2 to 45°. The experimental results were used to validate a numerical methodology using the Finite Element Method and a mixed-mode cohesive damage model implemented in the ABAQUS® software. The adhesive layer was simulated using cohesive elements with trapezoidal traction–separation laws in pure modes I and II to account for the ductility of the adhesive used. The cohesive laws in pure modes I and II were determined with Double Cantilever Beam and End-Notched Flexure tests, respectively, using an inverse method. Since in the experiments interlaminar and transverse intralaminar failures also occurred, cohesive laws to simulate these failure modes were also obtained experimentally following a similar procedure. Good correlations were found between the numerical predictions and experimental results for the elastic stiffness, maximum load and the corresponding displacement, plateau displacement and failure mode of the repairs.

101 エポキシ系異材継手の強度に及ぼす接合角度と接着層厚さの影響(OS7-(1) 材料損傷の機構・実験評価,オーガナイズドセッション)

101 エポキシ系異材継手の強度に及ぼす接合角度と接着層厚さの影響(OS7-(1) 材料損傷の機構・実験評価,オーガナイズドセッション)

Modelling the tensile fracture behaviour of CFRP scarf repairs

An experimental and numerical study concerning the tensile behaviour of adhesively-bonded carbon–epoxy scarf repairs is presented, using scarf angles ranging from 2° to 45°. A mixed-mode cohesive damage model adequate for ductile adhesives was used to simulate the adhesive layer. The cohesive laws of the adhesive layer, composite interlaminar and composite intralaminar (in the transverse and fibre directions) in pure modes I and II, necessary to simulate numerically the experimental failure paths, were previously characterized using an inverse method. Validation of this methodology was accomplished in terms of repair initial stiffness, maximum load and the corresponding displacement, as well as the failure mode. A good agreement between the numerical predictions and the experiments showed that the proposed methodology can be successfully applied to joints or repairs bonded with ductile adhesives.

Experimental and Numerical Analysis on Adhesively Bonded Scarf Joints: Effects of the Substrate's Material and Adhesively Bonded Joint Geometry on the Damage Evolution

This work characterizes the damage evolution of adhesively bonded scarf joints according to the substrate materials (AU4G-type aluminum alloy and XC18-type steel), the adhesive thickness (ej) and the scarf angle (α). Experimental results were obtained by strain gauge measurements. This method allows distinguishing three damage thresholds, which characterize the damage evolution of the adhesively bonded scarf joint. Contrary to most numerical studies, which determine the strength of the bonded joints according to the ultimate failure load, in this study a failure criterion implemented in ABAQUS was used to predict progressive damage evolution of the adhesively bonded scarf joints. The obtained results show the viability of the numerical model to predict the damage evolution of the bonded joint as a function of the studied key parameters. Nevertheless, for high adhesive thickness or a very low scarf angle the comparison also shows the limits of the numerical model.

Optimum shapes for minimising bond stress in scarf repairs

Bonded scarf repairs are used in composite structures when high strength recovery is needed or when there is a requirement for a flush surface to satisfy aerodynamic or stealth requirements. However, scarf repairs are complex to design and require the removal of significant parent structure, particularly for thick skins. In this investigation, analytical and numerical approaches have been developed to investigate whether an optimum repair shape for a known biaxial load can be determined. The results clearly demonstrate that the strength of a repaired panel can be improved by optimising both the initial damage cut-out shape and the scarf angle distribution.