Scarf Joints Research Articles

Adhesive strength improvement by providing steps in joints and differentiating initial and final debonding stresses

Step adhesive joints have a special characteristic quite different from other joints. When initial delamination occurs in other joints such as butt, lap and scarf joints, final failure always occurs. In these cases, the external stress causing initial delamination σcInitial is equal to the final failure stress as σcFinal = σcInitial. However, in step joints, the final failure stress σcFinal can be greater than the initial delamination stress σcInitial < σcFinal. To clarify the adhesive improvement mechanism, first, this paper discusses the ISSF (Intensity of Singular Stress Fields) for fully bonded step joint by varying the number of steps NS. Second, the singular stress field causing 2nd debonding is discussed by analyzing partially delaminated step joint. The results show that 2nd debonding requires larger external load than the initial debonding as σcInitial < σcFinal. This is because under the same external load the singular stress causing the 2nd deboning is smaller than the one causing the initial debonding. When NS≥6 and suitable overlap length, the final bond strength σcFinal can be more than 3.6 ∼ 4.4 times larger than the initial delamination stress σcInitial≪σcFinal resulting in much larger bond strength.

Failure investigation of woven scarf-repaired laminates reinforced with nanoparticles: Experimental and numerical investigations

This study explores the efficacy of multi-walled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs) as nano-reinforcement for the epoxy adhesive layer in scarf joints and 3D scarf-repaired laminates subjected to tensile loading. Experimental characterization of pure epoxy and nanocomposite mechanical properties was validated using the Mori–Tanaka method (incorporating agglomeration effects). A 3D finite element model considering material non-linearity and strain-based damage was employed to investigate joint failure behaviour, validated for 0.5 vol% MWCNT and GNP concentrations. A subsequent parametric study explored the influence of scarf angle, nanoparticle concentration, and patch configuration on 3D repaired specimens. The results suggest a 2.86° scarf angle with a [(+45/−45)]4s patch configuration reinforced with 1 vol% MWCNTs and 0.5 vol% GNPs exhibits the highest tensile strength improvement.

Uncertainty Analysis of Tensile Strength of Scarf Adhesive Joints Using Numerical Method

This study uses three-dimensional finite element (FE) analysis to investigate stress distribution and fatigue strength in adhesive scarf joints, which are complex due to their oblique geometry and varying material properties. ANSYS was used to model stress components within the adhesive and at the adhesive-adherent interface. The FE models, created with eight-node solid elements, utilized fine meshing to ensure accurate stress representation. A mesh size of 0.025 mm was optimal. The models, assuming linearly elastic materials, were validated by comparing simulated deformations with experimental results. Findings enhance understanding of local stress states, improving adhesive joint design reliability. Key Words: Adhesive scarf joints, finite element analysis, stress distribution, fatigue strength, ANSYS,

An energy-based 2D model for predicting mechanical behavior of adhesively bonded CFRP laminates

This paper presents a two-dimensional energy-based model for the mechanical behavior of adhesively bonded carbon fiber-reinforced polymer (CFRP) joints subjected to tensile loading. The proposed energy model is better suited than available discrete models to capture failure modes in adhesively bonded CFRP joints of different configurations having complex geometries. The governing equations are first developed for a three-stepped lap joint configuration by employing the minimum potential energy theorem and then solved using the finite element method (FEM). The disbond behavior of the joint configuration is modeled using the bilinear cohesive law. Subsequently, the developed model is extended to capture the mechanical response of adhesive joints for simple lap and scarf joint configurations. The proposed energy model is verified against 3D-FEA and experimental studies. Thereafter, a comparative analysis is undertaken for the three configurations of adhesive joints studied here. This is followed by a parametric study for the three-stepped lap joint to understand the influence of geometric and material properties of the adhesive and adherends on the stiffness and load-carrying capacity of the adhesively bonded CFRP joint. Finally, a design criterion is proposed for the three-stepped lap joint configuration, aimed at improving its load-carrying capacity/strength by reducing the peak stress levels at joint corners/overlap edges.

Effects of interface angle, added powder and applied deformation on the transport current and structure of scarf joints of single- and multi-core unreacted MgB2 wires

The effects of the interface angle and applied deformation on the transport current and structure of scarf architecture joints between single- and multi-core MgB2 wires are studied in this paper. The transport currents of the joints were measured at 4.2 K, external magnetic fields of 2–8 T, and joint’s resistances between 27 and 42 K. Resistive transitions and critical currents of the prepared joints were compared with the transition and critical current of the unjointed MgB2 wires. The interface structure of joined in situ wires was analysed using optical microscopy. Increasing the interface area between the joined wires and optimizing deformation by pressing allows to enlarge the superconducting current path. In-field transport currents of up to 73% of wire’s I c were measured for joined single-core MgB2/Fe wires. In the case of six-filament wires, the maximal transport current up to 53% of wire’s I c was reached. The results show that the presented joint technique could potentially be used for superconducting MgB2 coils in a persistent mode.

Progress in adhesive-bonded composite joints: A comprehensive review

Among the myriad joining techniques, the adhesive bonding technique is widely used to join complex large-scale composite structures because of its numerous advantages compared to traditional joining techniques. This article profusely analysed the various techniques for ameliorating the performance of composite joints, such as bonding methods (secondary bonding, co-bonding, co-curing, and multi-material bonding), surface modification techniques (plasma, laser surface treatment, surface grinding, etc.), additional reinforcement techniques (Z pin, wire mesh, nanofiller, etc), and different joint geometries (stepped joints, half-stepped joints, balanced joints, and scarf joints). Also, the effect of various adhesives and fabrication techniques on the static and dynamic performance of CFRP and GFRP-based joints was studied in detail. Moreover, this review addresses the finite element modelling and optimisation techniques on adhesively bonded joints. It has been observed that the bonding methods, surface modification to enhance the roughness of the adherend, addition of nanofillers, and variations in joint geometry greatly influence the shear strength, fracture toughness, fatigue, and vibration behaviour of FRP composite joints.

Optimisation of bonded and hybrid step lap joints of thick carbon fibre/epoxy panels used in aircraft structures

The use of composite materials is widely spread across the Aerospace sector. Scarf and step lap joints are amongst the most commonly used methods for flush joint repairs; they restore original stiffness and nearly original strength. Prior investigations conducted by the authors have shown that bonded and hybrid step lap joints have superior static strength capabilities than bolted joints. Hybrid step lap joints have shown the highest fatigue resistance than its bolted and bonded counterparts. This investigation aims at optimising the previous step lap joint design containing five steps with a 90 mm long overlap length in order to improve joint efficiency. Four new step lap joint configurations are considered based on a parametric study conducted using Abaqus; two of which contain seven steps and a 130 mm long overlap (bonded and hybrid) whilst the other two contain six steps and a 105 mm long overlap (bonded and hybrid) with a tapered outer step region. Experimental tests right through to numerical modelling using three-dimensional (3D) finite element (FE) models have been investigated. Adhesive non-linear material properties, fastener contacts and friction forces were all included in the 3D FE models. The Multicontinuum Theory (MCT) is used to simulate the progressive failure process and determine the stress states in the four configurations. Overall the FE models are able to accurately predict the bonded and hybrid joint strengths and highlights the importance of not only optimising the number of steps and their lengths but also the individual step heights. This is a parameter which has often been overlooked by fellow researchers. Experimental results showed that all four step lap joint configurations have significant improvement under static and fatigue load cases; the greatest improvement is seen in the two bonded step lap joint configurations. Overall the hybrid joint configuration consistently outperformed the bonded joint configurations. The presence of fasteners supress crack propagation, minimise peak peeling stresses at the ends of the overlap and increase the durability of the joint. This enables early crack detection which can assist composite joint repair certification.

Towards reducing the capital cost of manufacturing Laminated Veneer Lumbers: Investigating finger jointing solutions

The capital cost of setting up a Laminated Veneer Lumber (LVL) plant which produces continuous LVL billet products, through a continuous veneer assembly and hot-pressing processes, is significant. However, the utilisation of batch-type presses, similar to those employed in the plywood industry, could significantly reduce this initial cost and may provide new opportunities for small to medium scale operations. This process would produce shorter billet lengths which would need to be joined together to produce lengths viable for structural products. Scarf joints have been used commercially to join some veneer-based engineered wood products but have limitations, while finger joints are a common method for jointing sawn timber products and offer some key advantages but is not a common method to join veneer-based products. Consequently, this paper focusses on investigating the influence of key manufacturing parameters on the performance of finger jointed LVL. The effect of the joint orientation (horizontal or vertical), the finger length, the gluing pressure and the adhesive type on the joint strength and stiffness were investigated. The finger jointed LVL were tested in edge bending, flat bending and tension, and the results were compared to reference unjointed LVL. The bending performance of the finger jointed LVL was also compared to scarfed jointed LVL. In total 304 tests were performed. The results indicated that the average strength values of finger jointed LVL can reach up to 99% of the average strength of unjointed LVL and compares to scarf jointed LVL on flat bending. Horizontal joints, being more practical to produce for deep beams, performed similarly to vertical joints. The 25 mm joints were found to have no mechanical advantages over the 20 mm investigated finger joints. A gluing pressure lower than the Eurocode's recommended level for solid timber achieved sufficient bonding for the products to be utilised. The gluing pressure was also found not to influence the performance of the joint, for the range of pressures investigated. Both polyurethane and resorcinol-formaldehyde adhesives produced high performing products, with the latter displaying superior adhesive bond durability. The paper concludes that finger jointing LVL represents a viable solution to manufacture usable LVL lengths from short LVL billets, but have lower edge bending efficiency than scarf jointed LVL.

Effect of joint configurations on joint strength of different thickness AA6061-T6 friction stir welded blank

This paper presents a comparative study of the mechanical properties and microstructures of three friction stir welded joint configurations, namely square butt (SB), scarf (SC), and double butt lap (DBL) of the aluminium alloy 6061-T6. The experiment was conducted on two aluminium plates of different thicknesses, 2.0 mm on the advancing side and 1.5 mm on the retreating side, at constant welding parameters, such as welding speed, spindle rotation, tool tilt angle, and tool profile. The study compared the joints at different rolling angles, i.e. 0/0, 90/90, 0/90, and 90/0°. Tensile tests were conducted on the joints, and the ultimate tensile strength (UTS) was found to be 6–11% higher in the DBL joint configuration compared to the SC and SB joint configurations. The Vickers hardness values of the SC and DBL joints were also found to be higher than those of the SB joint configuration. The formation of the ‘S’ line was observed to influence the tensile test, hardness, and grain size, which in turn are determined by the shape of the faying surface.

Step and scarf joint elastic and failure behavior comparison under tensile and flexural loading

Step and scarf joints are the two most commonly used joint types in composite laminate bonding, offering distinct characteristics and behavior in response to specific loading conditions. In order to gain a comprehensive understanding of the mechanical behavior step and scarf joints, a comparative study of adhesive bonded step and scarf joint carbon fiber reinforced composite laminate under tensile and flexural loading condition was performed. The test specimens were fabricated with carbon fiber reinforced composite laminate. The step and scarf joints were precisely manufactured using a precise milling machine, and the joints were bonded with an epoxy adhesive with controlled bondline thickness. To assess the behavior of the bonded joint structure under various loading scenarios, both tensile and four-point bending flexural tests were performed. Strain measurement during these tests was carried out using digital image correlation (DIC) technology. The step joint showed multiple sequential bondline crack propagations while the scarf joint experienced a single catastrophic failure. The test result showed that the step joint retained 10% and the scarf joint retained 32% of tensile strength compared to the no-joint test specimen. Also, the step joint retained 13% and the scarf joint retained 44% of flexural strength compared to the no-joint test specimen. As a result, the scarf joint had significantly higher ultimate tensile strength, tensile modulus, and tensile strain at failure. The scarf joint also showed higher ultimate flexural strength and flexural modulus compared to the step joint.

Evaluation of hot-pressing scarf joints between polyamide 6,6 (PA66) composite and AA5754-H111 under tensile and flexural loading

As a direct bonding method, hot pressing connection offers versatility for manufacturing high quality multi-material structures. This experimental study focuses on the hot-press bonding of short-glass-fiber reinforced PA66 and Al5754 alloy. Laser surface structuring with grid pattern is performed on the metallic adherend to enhance the bonding strength and the scarfing ratio of 1:4 is analyzed for the dissimilar joint. Contact angle measurements and X-ray photoelectron spectroscopy (XPS) are used to evaluate the surface modification. Mechanical performance of the joints is studied in terms of the tensile-shear and flexural properties. Morphology of the fracture surfaces is scrutinized through a scanning electron microscopy (SEM). According to the results, it is found that the laser ablation treatment significantly affects the surface of the metallic adherend and subsequent joint strength. The scarf adhesiveless joint shows a mixed fracture with a negligible elastic deformation under tensile-shear loading. Furthermore, the joint performs better under the bending load from the metallic adherend.

Investigation of GFRP/AA7075-T6 Scarf Joint as a Robot Arm for High Natural Frequency and Damping Ratio

Investigation of GFRP/AA7075-T6 Scarf Joint as a Robot Arm for High Natural Frequency and Damping Ratio

Bonding Additively Manufactured PLA Materials: Effects of Joint Scarf Angle and Substrate Raster Orientation

The aim of this study is to investigate the effect of the scarf angle of the bonding region and the raster orientation of 3D printed substrates for the adhesive scarf joints made of additively manufactured Polylactic acid (PLA) adherends. In the first step, single PLA specimens were 3D printed using the fused filament fabrication (FFF) process in three different raster orientations of 0º, 45º, and 90º. Tensile and compression tests of them showed that the raster orientation of 90º gives the weakest results. Hence, in the second step, for the scarf joints, the PLA adherends were 3D printed in two different raster orientations of 0º and 45º. The joints were built with five different scarf angles of 15º, 30º, 45º, 60º, and 90º. The tensile and compression tests of all the specimens were conducted to determine the failure loads for different scarf angles and raster orientations. It is found that the endured load before rupture varies measurably as a function of the raster orientation of printed substrates and scarf angle of the joint, but differently for tensile and compression loadings. Eventually, the scarf angle for PLA scarf joints is determined which gives similar failure loads under both tension and compression loadings.

Fast realization of high-strength, repeatable, damp-heat resistant bamboo-strip scarf bonding joints via hot-melt adhesives

Bamboo, which has excellent mechanical strength, is an ideal raw material for manufacturing high-performance bio-composites. However, due to a lack of fast, high-strength lengthening technology for bamboo units, the large-scale, mechanized, continuous production of bamboo woven materials has not been fully developed. In this work, bamboo strips (BS) with 0.5 mm thickness were used as a basic unit to prepare lengthened bamboo strips with the scarf jointing method. The effects of scarf jointed angles and hot-melt adhesive type on the scarf bonding performance were systematically investigated. The tensile strength of BS scarf joints appears to increase as the scarf joint angle decrease. Scarf joints prepared with low-cost thermoplastic polyurethane (TPU) film outperform those prepared with reactive polyurethane (PUR) or ethylene vinyl acetate copolymer (EVA) hot-melt adhesives in terms of maximum tensile strength (197.9 MPa), secondary repeated bonding performance, and damp-heat aging resistance, where tensile strength of 136 MPa was retained after a 120 h cycle aging test. Specimens bonded with PUR adhesive are resistant to damp-heat aging, but show unsatisfactory secondary repeated bonding performance. The tensile strength and damp-heat aging resistance of EVA bonded specimens are unacceptable due to an unreliable bonding interface. We developed a facile solution for high-strength BS lengthening with repeated bonding and damp-heat resistant characteristics, which may provide technical support for the large-scale, mechanized manufacturing of bamboo-based composites.

Design Concepts for Peel-Dominant Adhesive Joints in Aeronautic Applications

The adhesive bonding technique is employed from the aeronautical/aerospace industry to current house products. To comply with the requirements of distinct applications, different joint configurations are available to the designer. While single-lap joints (SLJ) are the most common in application and research, double-lap joints, scarf joints and T-joints find specific applications. T-joints are seldom studied in the literature, but these are used, for instance, in aircraft to bond the stiffener beams to the skin, or in the cars between the B-pillar and the rocker. Due to the high stress concentrations, T-joints often fail under average stresses much lower than the adhesive strengths, giving rise to the necessity for proper design and strength improvement methodologies. This work initially aims to validate the cohesive zone modelling (CZM) technique with experiments, and then use it to numerically evaluate and optimize the performance of T-joints subjected to peel loads. CZM is nowadays regarded as the most powerful strength prediction tool for adhesive joints, and can be a valuable tool to improve T-joints. Different features are addressed for a complete analysis: adhesive type, geometrical parameters, dual-adhesive technique for strength improvement, and composite joints. The evaluated geometrical parameters are the base adherend thickness (a), T-part thickness (t), overlap or bonding length (l) and curvature radius (r). As a result of this work, the model was successfully validated, and clear design guidelines were provided to define the ideal geometric and material (adhesive) conditions for best performance.

On the static failure behaviour of co-cured composite joints with enhanced interlaminar fracture energies via multiscale toughening

This paper is focused on the multiscale toughened co-cured composite interfaces of carbon/epoxy laminates using vacuum infusion and out-of-autoclave curing—with an emphasis on the role of enhanced interlaminar fracture energies on the static failure of laminate joints. To toughen co-cured interlaminar regions, three toughening routes are used: (i) epoxy matrix toughening with 10 wt% core–shell rubber (CSR) nanoparticles, (ii) 10 g/m2 polyphenylene sulfide (PPS) non-woven micro-fibre veil toughening, and (iii) multiscale toughening by combining epoxy matrix toughening with 10 wt% CSR nanoparticles and 10 g/m2 PPS veils. Double cantilever beam and end-notch flexure tests are conducted with the toughened interfaces for mode-I and mode-II fracture energies. Moreover, three co-cured joint configurations (viz. double-notch single lap, stepped scarf and skin-stiffener joints with the three toughening routes) are tested to characterise the role of enhanced interlaminar fracture energies on the static failure of laminate assemblies. The results show that the three toughening routes have significantly altered the R-curve behaviour and interlaminar fracture energies and that the degree of enhancement in mode-I and mode-II fracture energies is sensitive to the type of toughening route. Based on the material systems tested, the mode-I fracture initiation energy is enhanced by ∼150% with multiscale toughening, ∼65% with particle toughening, and ∼60% with veil toughening; whereas, the mode-II fracture initiation energy is enhanced by ∼85% with veil toughening and ∼35% with multiscale toughening—but reduced by ∼20% with particle toughening. The joint tests show that the toughening routes affect the static failure of the three joint configurations and that the strength and damage evolution of the co-cured joints depend on the interlaminar fracture energies. The strength of the co-cured single lap joints is increased by ∼50% with multiscale toughening and ∼40% with veil toughening; whereas, the strength of the co-cured skin-stiffener joints is enhanced by ∼35% with multiscale toughening and ∼45% with veil toughening. The failure mechanisms observed on the fracture surfaces indicate that the strength/failure behaviour of the co-cured joints depends on the dominant interlaminar fracture mode and the corresponding fracture energies. Overall, it is shown that the multiscale interlaminar toughening routes that are used to address the issue of delamination can also be effectively explored to enhance the static failure of co-cured laminate assemblies by toughening the co-cured interfaces/bondlines.

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.

Repair of Aerospace Composite Structures Using Liquid Thermoplastic Resin

In this study, two types of carbon-fiber-reinforced plastic (CFRP) composite scarf geometries were created using two scarf angles, i.e., 1.43° and 5.71°. The scarf joints were adhesively bonded using a novel liquid thermoplastic resin at two different temperatures. The performance of the repaired laminates was compared with pristine samples in terms of residual flexural strength using four-point bending tests. The repair quality of the laminates was examined by optical micrographs, and the failure modes after flexural tests were analyzed using a scanning electron microscope. The thermal stability of the resin was evaluated by thermogravimetric analysis (TGA), whereas the stiffness of the pristine samples was determined using dynamic mechanical analysis (DMA). The results showed that the laminates were not fully repaired under ambient conditions, and the highest recovery strength at room temperature was only 57% of the total strength exhibited by pristine laminates. Increasing the bonding temperature to an optimal repair temperature of 210 °C resulted in a significant improvement in the recovery strength. The best results were achieved for laminates with a higher scarf angle (5.71°). The highest residual flexural strength was recorded as 97% that of the pristine sample repaired at 210 °C with a scarf angle of 5.71°. The SEM micrographs showed that all the repaired samples exhibited delamination as the dominant failure mode, whereas the pristine samples exhibited dominant fiber fracture and fiber pullout failure modes. The residual strength recovered using liquid thermoplastic resin was found to be much higher than that reported for conventional epoxy adhesives.

Analysis of the tensile and bending strengths of the joints of “Gigantochloa apus” bamboo composite laminated boards with epoxy resin matrix

Abstract The need for wood in the ship building industry continues to grow every year. An alternative raw material is needed to replace wood at a more affordable price, namely, bamboo laminated boards. However, bamboo has a weak connection between its segments, with a maximum length between components of less than 40 cm. To reduce these weaknesses, the connection between bamboo segments with laminated boards is carried out as follows: scarf joint, butt joint, finger joint, desk joint, and tongue and groove joint. The study aims to determine the connection’s effect on each connection variation’s strength. Tensile tests and bending tests were carried out on the test specimens. The average results obtained were quite varied for the tensile test, which were in the range of 81.36–118.62 MPa, while the results of buckling test were in the range of 395.28–475.89 MPa. This study revealed that the connection of the specimen with seven layers had a value of 118.62 MPa in the tensile strength test and 475.89 MPa in the buckling strength test, while 3 layers finger joint samples with the lowest buckling tensile strength value had a value of 81.36 MPa tensile strength and 395.28 MPa bending strength.

Superconducting joints between MgB2/Ni and MgB2/Nb composite wires, their transport currents and micro-structure

Superconducting joints between single-core MgB2/Ni and MgB2/Nb wires made by an internal Mg diffusion (IMD) process have been manufactured by using scarf joints architecture. Joint’s transport current were measured at 4.2 K and external magnetic fields of 2–8 T and compared with the currents of used MgB2 wires. Structure of selected joints were analysed by an optical microscope. It was found that addition of boron powder between the joined wires allows the creation of superconducting current paths. Observed Mg–Ni interaction inside the joint area reduces the creation of well-connecting MgB2 phase, and consequently, degrades the critical current of joint by 6.7% of wire’s Ic. Joints between MgB2/Nb wires have no inter-metallic interactions, and consequently, large critical currents up to 60% of wire’s Ic can obtained. Presented results show that wires with inert metallic sheath and added boron powder could be used for the fabrication of applicable superconducting joints and MgB2 coils working in persistent mode.