Scarf Joints Research Articles

Damage Parameters of Adhesive Joints with General Triaxiality Part I: Finite Element Analysis

In part 1 of this paper, the triaxiality function of several adhesive joints was investigated using the finite element method. The main aim of this investigation was to decide on the most suitable joint type for fatigue experimental tests, which can be used to control triaxiality in adhesive layers. These fatigue experimental tests are presented in this paper (part 2). In part 1 of this paper, three types of adhesive joints were considered. These were: butt joint, cleavage joint and scarf joint. The geometry of the butt joint cannot be manipulated in order to control triaxiality, and therefore triaxiality is controlled through the tension/torsion loading ratios. It was found that the type of load had an effect on the map of triaxiality in the adhesive layer. For the cleavage joint, it was found that triaxiality can be manipulated by changing the adhesive bondline angle. However, the relationship between triaxiality and bondline angle was found to be complex. For the scarf joint, the triaxiality can also be manipulated in a similar way to that used for the cleavage joint by changing the bondline angle. The scarf joints has many advantages over cleavage joints including: (a) simpler relationship between triaxiality and bondline angle and (b) easier manufacturing process related to the geometry of the joint. It was shown that the scarf joint is the best practical choice to control triaxiality in fatigue experimental tests.

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.

Determination of loading capacity depending on bevel angle of the wooden bonded scarf joint

The paper is focused on comparison of experimental and simple theoretical method of determination of loading capacity depending on bevel angle of wooden bonded scarf joint. The Mohr's circle principle, thus shear stress dependence on normal stress, is used for loading capacity formula derivation. It has been established that for random bevel angle under approximately 70 degrees the future loading capacity can be calculated from knowledge of ultimate force for bevel angle 0 and 90 degrees.

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.

Study of Composite Interface Fracture and Crack Growth Monitoring Using Carbon Nanotubes

Interface fracture of woven fabric composite layers was studied using Mode II fracture testing. Both carbon fiber and E-glass fiber composites were used with a vinyl ester resin. First, the single-step cured (i.e., co-cured) composite interface strength was compared to that of the two-step cured interface as used in the scarf joint technique. The results showed that the two-step cured interface was as strong as the co-cured interface. Carbon nanotubes were then applied to the composite interface using two-step curing, and then followed by Mode II fracture testing. The results indicated a significant improvement of the interface fracture toughness due to the dispersed carbon nanotube layer for both carbon fiber and E-glass fiber composites. The carbon nanotube layer was then evaluated as a means to monitor crack growth along the interface. Because carbon nanotubes have very high electrical conductivity, the electrical resistance was measured through the interface as a crack grew, thus disrupting the carbon nanotube network and increasing the resistance. The results showed a linear relationship between crack length and interface resistance for the carbon fiber composites, and allowed initial detection of failure in the E-glass fiber composites. This study demonstrated that the application of carbon nanotubes along a critical composite interface not only improves fracture properties but can also be used to detect and monitor interfacial damage.

Scarf Joints of Composite Materials: Testing and Analysis

The objective of this study is to develop a reliable computational model in order to investigate joint strengths of scarf joint configurations constructed from carbon-fiber and glass-fiber woven fabric laminates with different material combinations like glass/glass, glass/carbon, carbon/glass, and carbon/carbon under various loading conditions such as axial, bending moment and shear loading. Both experimental and computational studies are conducted. For the experimental study, specimens made of hybrid scarf joints using carbon-fiber and glass-fiber woven fabrics are tested under compressive loadings to determine their joint failure strengths. Computational models are then developed using the discrete resin layer model along with fracture mechanics and virtual crack closure techniques. The numerical models are validated against the experimental data. The validate models are used to predict the joint strengths under different loading conditions such as axial, shear, and bending moment loadings.

Explosive scarf welding of aluminum to copper plates and their interface properties

AbstractExplosive welding was used to produce scarf joint between aluminum and copper plates. This process is known as explosive scarf welding (ESW). In a scarf joint, the final bond interface is oblique. In this study, chamfered end of aluminum and copper plates were joined explosively and named scarf joint, employing changes in chamfered angle at different stand‐off distance and explosive loading. The geometry of scarf joint enables consideration of both flyer and base plate thickness and explosive loading and the effects on mechanical properties of interface such as bond shear strength and micro‐hardness can be investigated.Mathematical models developed on the interface properties of scarf joint to make relationship between the bond shear strength and explosive loading ratio. To check the adequacy of developed models, mechanical properties of interface, such as bond shear strength was predicted and compared with actual values in explosive cladding process. The results show reasonable agreement with theoretical predictions.Consequently, mathematical model which is based on scarf joints, can predict bond shear strength of cladding metals under desired explosive loading and flyer plate thickness.

Loading capacity determination of the wooden scarf joint

The paper describes the complete derivation of the theoretical bonded scarf joint loading capacity and the construction of the Mohr’s circle for linear state of stress. Then the method of the experimental determination of the bonded joint loading capacity is explained in detail. A part of this paper deals also with the bonded joint real loading capacity determination.

Study of composite joint strength with carbon nanotube reinforcement

In order to strengthen the interface of a composite scarf joint, this study investigated the benefits of using locally applied carbon nanotubes to reinforce a carbon fiber composite scarf joint. The effect of carbon nanotubes on enhancing the fracture toughness and interface strength was investigated by performing Mode I and Mode II fracture tests with and without carbon nanotubes applied locally at the joint interface. Furthermore, the effects of seawater absorption and different carbon nanotube concentration values on Mode II fracture were investigated. Finally, a partial application of carbon nanotubes only near the crack tip area was considered. During the study, the image correlation technique was used to examine the fracture mechanisms altered by the introduction of carbon nanotubes. The experimental study showed that an optimal amount of carbon nanotubes could increase the fracture toughness of the composite joint interface significantly, especially for Mode II, including a physical change in the fracture mechanism.

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.

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.

Performance of Dowel-Welded Wood Furniture Linear Joints

Comparison of three types of furniture joints such as scarf joints, step butt joints and dovetail joints held together either by one or two welded dowels, glued dowels and steel nails showed that the dowels always gave better shear strength and greater stiffness than the steel nails. The results of welded dowels and glued dowel joints were found to be comparable. The application of the welded joint technique to joints where the number of dowels is limited by the narrow space in which they can be applied, such as in furniture, can give results comparable to those obtained by gluing the same dowels.

Frequency Domain Structural Synthesis Applied to Quasi-Static Crack Growth Modeling

Quasi-static crack growth in a composite beam was modeled using the structural synthesis technique along with a finite element model. The considered crack was an interface crack in the shear mode (i.e. mode II), which occurs frequently in the scarf joint of composite structures. The analysis model was a composite beam with an edge crack at the midplane of the beam subjected to a three-point bending load. In the finite element model, beam finite elements with translational degrees of freedom only were used to model the crack conveniently. Then, frequency domain structural synthesis (substructure coupling) was applied to reduce the computational time associated with a repeated finite element calculation with crack growth. The quasi-static interface crack growth in a composite beam was predicted using the developed computational technique, and its result was compared to experimental data. The computational and experimental results agree well. In addition, the substructure-based synthesis technique showed the significantly improved computational efficiency when compared to the conventional full analysis.

Frequency Domain Structural Synthesis Applied to Quasi-Static Crack Growth Modeling

Quasi-static crack growth in a composite beam was modeled using the structural synthesis technique along with a finite element model. The considered crack was an interface crack in the shear mode (i.e. mode II), which occurs frequently in the scarf joint of composite structures. The analysis model was a composite beam with an edge crack at the midplane of the beam subjected to a three-point bending load. In the finite element model, beam finite elements with translational degrees of freedom only were used to model the crack conveniently. Then, frequency domain structural synthesis (substructure coupling) was applied to reduce the computational time associated with a repeated finite element calculation with crack growth. The quasi-static interface crack growth in a composite beam was predicted using the developed computational technique, and its result was compared to experimental data. The computational and experimental results agree well. In addition, the substructure-based synthesis technique showed the significantly improved computational efficiency when compared to the conventional full analysis.

Three-dimensional asymptotic stress field in the vicinity of an adhesively bonded scarf joint interface

A recently developed three-dimensional eigenfunction expansion approach for prediction of the singular stress field in the neighborhood of the interfacial front of an adhesively bonded scarf joint is presented. The plate is subjected to extension/bending (mode I) and in-plane shear/twisting (mode II) far field loading. Each material is assumed to be isotropic and elastic, but with different material properties. Numerical results include the dependence of the lowest eigenvalue (or stress singularity) on the wedge aperture angle of the plate material. Variation of the same with respect to the shear moduli ratio of the component plate and adhesive layer materials is also an important part of the present investigation. Hitherto unobserved interesting and physically meaningful conclusions are also presented.

Enhancement of composite scarf joint interface strength through carbon nanotube reinforcement

It was investigated whether there was a potentially significant improvement to scarf joint bonding that was achieved through the dispersion of carbon nanotubes (CNTs) along the interface of the composite joint. The study examined various factors that might affect CNT-reinforced joint interface strength. Each composite joint consisted of a vinyl-ester matrix base (DERAKANE 510-A) interlaced with a carbon fiber weave (TORAY T700CF). During the curing process, the research explored several variables concerning the CNT application. The testing included single-walled CNTs (SWCNT), and conventional and bamboo-structure multi-walled CNTs (MWCNT) with varying length, purity, and concentration levels along the surface area of the joint interface. This wide array of data demonstrated the effect of CNTs introduction at the joint interface, and provided the ideal type, size, purity level, and concentration level for composite scarf joint bond reinforcement using CNTs. Furthermore, a computational model was developed to predict the strength of the scarf joints. The predicted model agreed well with the experimental data.

A Finite Element Analysis of Scarf Joint for Controlling the Triaxiality Function in Adhesive Bonding

Triaxiality function (Rv) has been known as one of the important factors that responsible for damage initiation in adhesive bonding. Damage evolution law for low cycle fatigue (LCF) is function of Rv, von Mises equivalent stress (Seqv) and number of cycles (N). From previous research, it was found that the Rv values of two cases: bulk adhesives and single lap joint (SLJ), were close to unity. Those values are uncontrollable. Meanwhile, the damage equation for general solution contains Rv as an independent variable. There is need to choose another joint type that can characterise Rv as an independent variable. This paper presents the choice of scarf joint as specimen that can simulate variation of Rv. Several types of adhesive joints have been modelled and analysed using ANSYS as finite element analysis (FEA) tool. In ANSYS, Rv values were calculated directly from direct output results: von Misses equivalent stress and Hydrostatic stress. From FEA, it was shown that Rv changed as a function of adhesive bondline angle of the scarf joint. The values of Rv are constant along adhesive line except at the free edges. This choice is better than Cleavage joint where the values of Rv are not constant along adhesive line due to the presence of bending moment.

Experimental and numerical analysis of a halved and tabled traditional timber scarf joint

Traditionally constructed heavy timber trusses, found in timber framed buildings and bridges, employ various traditional joints, among them the lower chord scarf joint. This paper examines the behavior of a halved and tabled scarf joint, which was studied as an isolated structural component using experimental tests and finite element analyses. Experimental tests identified two different limit states for these particular scarf joints: shear failure parallel to grain and tension failure perpendicular to grain. The possibility of failure due to tension perpendicular to grain results from variations in grain angle and means that the limit state of shear failure parallel to grain, typically assumed in analysis and design, is unconservative. For the purposes of design and rehabilitation, the authors propose that the scarf joint be treated as a member subject to combined bending and axial tension forces. The results of the finite element analysis, performed using solid continuum elements in ABAQUS, are in good agreement with the experimental test results. In addition to finite element models, the authors use analytical spring models to demonstrate that when developing a two-dimensional model of a truss with lower chord scarf joints, serviceability limit states be checked with a model that reduces the lower chord section properties in the vicinity of the scarf joints.

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.

Experimental and numerical analysis of a stop-splayed traditional timber scarf joint with key

Traditionally constructed heavy timber trusses employ scarf joints to transfer tensile forces between lower chord members. Scarf joints significantly impact the global stiffness of heavy timber trusses, yet little is known about their structural behavior. This paper describes a study recently completed on the commonly used stop-splayed scarf joint with a key. Experimental tests were performed on scarf joints replicated from a covered wooden bridge in Pennsylvania and compared to the numerical results of full, three-dimensional finite element models created using ABAQUS. The authors determined that the key has the most influence on scarf joint behavior as its orientation causes it to be loaded in compression perpendicular to grain. Experimental tests also revealed the importance of clamping bolts, without which a limit state of “key rolling” was observed. With two clamping bolts in place, the limit state was shear failure parallel to grain, which provides a more stable failure and a higher ultimate strength. The authors recommend to engineers that the scarf joint not be analyzed as a pure tension member; rather, it should be considered a member subject to combined tension and bending forces.