Single Lap Joints Research Articles

Synergetic effect of hybrid surface treatment on the mechanical properties of adhesively bonded AA2024-T3 joints

ABSTRACT Improving the surface properties of AA2024-T3 aluminium alloys is crucial for achieving durable and sustainable joints. For this purpose, AA2024-T3 alloys were first sandblasted at 4, 5 and 6 bar pressures using aluminium oxide and glass sphere abrasives. Then, the sandblasted samples were subjected to the chemical surface treatment resulting in a hybrid surface process. The surface properties of sandblasted and hybrid surface-treated AA2024-T3 alloys were characterised by 3D image analysis, contact angle and roughness measurement. Additionally, single-lap joints were fabricated using adhesives with low and high viscosity. The effect of changes in the viscosity of adhesives and the surface properties of the AA2024-T3 on the mechanical properties of the joints has been analysed in detail. It was found that both sandblasting and hybrid surface treatments applied to AA2024-T3 alloys caused significant improvement in the mechanical properties of the joints. AA2024-T3 alloys, which were chemically surface treated after being sandblasted with glass spheres at 6 bar pressure, were joined using low and high viscosity adhesives, and the failure load of these joints increased by approximately 110% and 68%, respectively, compared to untreated samples. In the case of aluminium oxide usage, the increase rates were determined to be 93% and 43%.

Effects of Stainless Steel Woven Wire Mesh Reinforcement on the Load-Bearing Capacity of Adhesively Bonded Aluminium Alloy Single-Lap Joints

Effects of Stainless Steel Woven Wire Mesh Reinforcement on the Load-Bearing Capacity of Adhesively Bonded Aluminium Alloy Single-Lap Joints

Nanoparticle integration in adhesive and hybrid single lap joints: effect on strength and fatigue life under environmental aging

Examining the mechanical performance of CFRP and aluminum samples subjected to environmental aging is crucial. Additionally, it is essential to develop methods to enhance their mechanical properties. This research investigates the impact of fullerene and single-walled carbon nanotubes (SWCNT) on the fatigue life and static strength of bonded and bonded/bolted joints. The study focuses on composite-to-composite (CTC) and composite-to-aluminum (CTA) substrates under three-point bending, both before and after hygrothermal aging. The samples were divided into four categories: (1) neat specimens, (2) specimens with added fullerene, (3) specimens with added SWCNT, and (4) specimens with a combination of 50% SWCNT and 50% fullerene. The experimental results indicated that the optimal nanoparticle ratio for bonded joints differs from that for bonded/bolted joints. Adding nanoparticles to the adhesive increased the fatigue life of SLJs, particularly in samples containing mixed particles and SWCNT. In some cases, nanoparticles amplified the effect of hygrothermal conditions, enhancing fatigue life further. The integration of nanoparticles into the adhesive and the use of bonded/bolted joints significantly improved joint strength, with the combination of both techniques yielding the best results. These modified joints offer a promising alternative to traditional joints in terms of strength and fatigue life. The study enhances understanding of the aging of adhesive and hybrid joints, especially dissimilar joints (composite to metal), and provides insights into their behavior under various environmental aging conditions. These methods show potential for optimizing joints and composite structures, improving durability, and reducing the likelihood of operational failures.

Enhanced shear and vibration behaviour of co-cured CFRP joints with innovative lamination techniques

ABSTRACT The adhesive bonding technique is extensively employed in large-scale component manufacturing to efficiently join the fibre-reinforced composite laminates. The following adhesive bonding techniques, such as secondary bonding, co-bonding and co-curing techniques, were modified by novel interleaved and covered lamination techniques. The shear and vibration behaviour of carbon fibre reinforced polymeric (CFRP) composite single lap joint were investigated per ASTM D5868 standards with innovative lamination techniques. The findings demonstrated that the Co-Curing with the Covered Lamination Technique (CL-CCT) showed an excellent shear strength of 88% higher than the secondary bonding technique. The Co-Curing with Interleaved Lamination Technique (IL-CCT) demonstrated a significant enhancement in shear strength, surpassing the secondary bonding technique by 74%. Furthermore, the Co-Bonding with the Covered Lamination Technique (CL-CBT) exhibited a 44% increase in natural frequency. The Co-Curing with Covered Lamination Technique (CL-CCT) also outperformed the conventional secondary bonding technique, demonstrating a remarkable 32% improvement in natural frequency. Eventually, the adhesive and cohesive failure modes are observed in the overlap zone of CFRP composite single-lap joints.

An experimental-cohesive zone model approach to predict fatigue life of adhesive joints with varying modes of loading and joint configurations for automotive applications

ABSTRACT Predictive fatigue life models of adhesive joints are necessary to enable the assessment of automotive-bonded structures while reducing costly experimental testing. However, contemporary models have typically been calibrated for specific joint configurations and modes of loading, limiting their applicability to large-scale structures. Additionally, available models are based on simulation of cumulative fatigue cycling, making them computationally prohibitive. In the current study, cross-tension (CT) (load angles of 0°, 45°, and 90°) and single-lap shear joint (SLJ) configurations were bonded using an epoxy adhesive (BetaGuard CI6125R; PPG, France) used in automotive production (one part) and tested under fatigue cyclic loading. A total of nine joint configurations, having symmetrical (same material and thickness) and asymmetrical (dissimilar material or unequal thickness) joints, were tested. Fatigue tests at load levels between 25% and 75% of the static peak load were performed until joint failure or to runout (two million load cycles). The static tests of the joints were simulated to failure using finite element (FE) models with the cohesive zone method (CZM). The maximum strain energy release rates ( G max ) were calculated within the adhesive bond line at static loads corresponding to the peak loads of the fatigue tests. The G max values, computed from single cycle, specimen-specific FE simulations, were correlated with the measured fatigue life ( N f ) of the adhesive joints with varying modes of loading and joint configurations. The fatigue life prediction model, based on G max − N f correlation and following a crack propagation approach, predicted the cycles to failure for 85% of the fatigue tests, and 81% of the independent validation tests. The proposed fatigue life prediction approach provides computational efficiency and large-scale compatibility.

The effects of environments and adhesive layer thickness on the failure modes of composite material bonded joints

In this study, a structural adhesive was used to bond unidirectional prepreg and fiber fabric in a single lap joint. The mechanical properties of the structural adhesive were investigated under room temperature dry state (RTD) and elevated temperature wet state (ETW, 71 ℃/85% RH), and different adhesive layer thicknesses (0.5 mm, 1.0 mm, 1.5 mm, and 2.0 mm). The fracture surfaces of the bonded joints were examined using scanning electron microscopy (SEM), and finite element simulations were conducted to observe the failure modes and failure paths. Additionally, the specimens were immersed in water and hydraulic oil, and their tensile shear strength was tested to evaluate their liquid sensitivity. The experimental results indicated that with increasing adhesive layer thickness, the strength of the specimens decreased by 21% in the RTD and by 52% in the ETW. The strength differences between different environments were minimal for adhesive layer thicknesses of 1 mm and 1.5 mm. The shear strength of the specimens decreased after immersion in water and hydraulic oil, with reductions of 43.78% and 39.21%, compared to the room temperature dry respectively. SEM observations of the bonded joint sections revealed that the primary failure modes were adherend failure and adhesive layer failure. Finite element simulations indicated that fiber tearing and crack initiation occurred in stress concentration areas during loading, leading to structural failure.

Improvement of strength in single-lap adhesive joints of AlSi10Mg alloys fabricated by laser powder bed fusion

This study deals with the mechanical strength of metallic single-lap joints bonded with adhesive and fabricated by Additive Manufacturing (AM). To this aim, AlSi10Mg alloy has been used to manufacture the specimens using Laser Powder Bed Fusion (LPBF) process. Based on a series of tensile tests on AMed joints, the mechanical strength of the joints was determined. In this paper, a procedure is recommended to improve the strength of AMed single-lap joints. Particularly, a notch is introduced in the adherends, which plays a crucial role in the mechanical strength and failure load of the joints. Therefore, a series of single-lap joints with notch have been printed and examined. Based on the conducted tests, failure load and fracture behavior of the joints with and without notches have been compared. Here, we used the digital image correlation technique to determine strain fields on the surface of the joints. In addition, a series of finite element models has been developed to investigate the stress distributions in the adhesive and simulate the load-carrying behavior of the joints. The obtained results confirm the effective role of notches in adhesive joints for strength improvement of AMed single-lap joints. The outcome of this study can be utilized for fabrication of AMed metallic joints with a higher mechanical strength.

FE-based machine learning model for predictive damage assessment in bonded composite joints via acoustic emission

This study systematically evaluated the mechanical performance of bonded composite single-lap joints with varying overlap lengths under tensile load. By integrating Acoustic Emission (AE) signals with Finite Element (FE) modeling, this study established a predictive machine learning model to enhance the understanding and prediction of failure mechanisms. Firstly, AE signal features were extracted and utilized as inputs for a hierarchical clustering model to classify AE signals into matrix cracking, fiber breakage, and adhesive debonding. AE signals classified as adhesive debonding were further analyzed by accumulating counts, amplitude, and energy. A cohesive zone-based Finite Element (FE) model was developed to simulate progressive debonding damage in the adhesive layer. After calibration with experimental results, various physical quantities such as stress, the damage initiation indicator, and the stiffness degradation indicator were extracted for correlation analysis. The experiments and simulations unravel the missing relationship between the AE and FE data. A strong correlation was observed between the cumulative count and cumulative amplitude with the damage initiation indicator, and between cumulative energy with the damage propagation indicator. Finally, based on the disclosed relationship between AE and FE data, a Support Vector Regression (SVR) model was established to predict the damage indicator. The developed model accurately predicted testing datasets within low absolute error ranges, underscoring its high predictive accuracy. Comprehensive stress analysis proposed a quantitative damage initiation indicator of 0.32 for structural fractures and a damage indicator of 0.12 for allowable stress levels. Consequently, combining AE techniques with FE modeling provided an effective tool for evaluating damage evolution in bonded composite joints.

Strain measurement of CFRP-Steel bonded joints using distributed optical fibre sensors

ABSTRACT Externally bonded carbon fibre reinforced polymer (CFRP) is widely employed for strengthening steel structures. The CFRP-steel bond behaviour is typically conducted using the single-lap joint (SLJ) and double-lap joint (DLJ) shear tests. This study utilizes advanced distributed optical fibre sensors (DOFS) to measure strains in CFRP-steel SLJs and DLJs, with a specific focus on examining the bending effect. An experimental investigation was conducted to verify the appropriate adhesive and fibre coating materials, achieving synergistic deformation between optical fibre sensors and adherends. The strain results reveal that PI-coated fibres bonded with epoxy resin enhance deformation compatibility. The CFRP strain distributions within the joints are accurately measured by DOFS, enabling the calculation of bending strain to assess joint bending effects. The observed strain aligns well with the finite element (FE) predictions. A more significant bending effect is observed in SLJs compared to DLJs, predominantly concentrated at the loaded end. It can be inferred that the SLJs are subjected to mixed-mode I/II loading in shear tests, and the DLJs appears to be mainly influenced by mode II loading. Additionally, the verified FE model demonstrates that the shear strain across the thickness of the adhesive layer exhibits significant variation at the bond ends.

Effect of Surface Treatment on Tensile Strength of Steel Single Lap Joints Bonded with Double-Sided Acrylic Foam Tapes for Naval Applications

This study investigated single lap joints in steel used for naval carpentry. The surface was mechanically treated, and then a double-sided acrylic foam tape was applied with varying surface preparation conditions. Specifically, three different conditions were examined. Tensile tests revealed that changing the type of surface preparation significantly affects the mechanical strength of the joints. The best mechanical properties were achieved when a primer was used. Our results demonstrate that this method can be effectively employed in naval applications as an alternative to welding for non-structural applications, such as the installation of brackets for mounting electrical devices (e.g., sockets).

Investigation of the degradation over steel/GFRP single lap joint: UV radiation and immersion at different temperatures

Adhesive joints with composite components are used in various fields and under diverse environmental conditions. Exposure to UV radiation, temperature fluctuations, and immersion in different mediums can influence their performance. This study focuses on the degradation of steel/GFRP (glass fiber reinforced polymer) adhesive joints by immersing them in distilled and salt water at three different temperatures (40 °C, room temperature, and 4 °C), with and without prior exposure to UV radiation. Water absorption over time was evaluated under different degradation conditions, studying the absorption of the joints, composite, and adhesive, both individually and in combination. Mechanical shear tests on the joints and three-point bending tests on the composites were conducted, along with an assessment of failure modes influenced by water absorption and UV degradation. The results indicate that immersion temperature affects water absorption, post-curing, and the stiffness of the matrix and polymers, while UV radiation promotes post-curing and facilitates water absorption. The combined degradation conditions exhibit different effects on the materials compared to individual degradation, highlighting the complexity of service environmental conditions.

Effects of temperature and loading rate on the shear performance of GFRP-steel single-lap joints

This research examines how the shear properties of glass fiber-reinforced polymer (GFRP) and steel single-lap joints react under various temperatures (−25, 0, 25, 50, and 100℃) and loading speeds (0.625, 1.25, 2.5, and 5.0 m/s). The GFRP sheets, aligned unidirectionally, were produced using vacuum-assisted resin transfer molding and were then adhesively attached to steel plates with epoxy resin. The assemblies were subjected to a high-speed servo-hydraulic testing apparatus for evaluation. The results indicate that the joint's performance is relatively unaffected by changes in the loading speed, but the overall toughness tends to decrease at higher speeds. However, the adhesive bond strength increases when temperatures are between −25 and 50℃ but diminishes as temperatures rise from 50 to 100℃. Most samples exhibited debonding at the steel-adhesive interface under varied speeds and colder temperatures, whereas debonding at the GFRP-adhesive interface was more prevalent at elevated temperatures. These insights are vital for both theoretical analyses and practical implementations of GFRP-steel single-lap joints in environments subject to extreme loading conditions and temperatures.

Effects of surface micro-texturing laser-etching on adhesive property and failure behaviors of basalt fiber composite single-lap-joint

Laser-etching technology can perform micro-nano processing on the sub-surface of composite, thereby enhancing its mechanical interlocking to improve the adhesive performance. However, it can easily cause surface fiber damage and destroy the integrity of composite. Hence, the influences of surface micro-texturing and laser-etching effect on the adhesive property and failure behaviors of basalt fiber reinforced polymer (BFRP) composite single-lap-joint are investigated. Here, different surface micro-texturing of BFRP with different laser-etching power and line space are processed by laser-etching. The surface morphology and wetting property of experimental samples are characterized, and single lap tests are conducted to evaluate the adhesive properties. Results show that the adhesive property firstly increases and then decreases with the laser power increase, where the samples etched with 10 W and line space of 0.1 mm perform the best. It is found that the epoxy on the surface is removed by laser-etching to improve the contact area, but the fiber damage is formed to affect the adhesive property. When the laser-etching reaches a certain depth in spite of fiber damage, it can achieve the best bonding performance with the balance of performance and structural integrity. Finally, the multi-scale simulations are conducted to effectively demonstrate the laser-etched interface bonding behavior and damage evolution process of BFRP single lap joints.

Experimental behaviour and fatigue life prediction of bonded joints subjected to variable amplitude fatigue

The variable amplitude fatigue behaviour of adhesive joints has not been extensively considered in literature and considerable knowledge gaps regarding its underlying mechanisms still exist, hindering accurate life prediction. This study investigates the effect of loading sequence and number of load transitions on single lap joints (SLJs) bonded with a toughened epoxy adhesive. Life prediction is also conducted using established cumulative rules: Palmgren–Miner (PM), Broutman and Sahu (BS) and, Hashin and Rotem (HR). To overcome their limitations, a modified Cortan–Dolan approach is proposed. For this, SLJs were subjected to single transition (low-to-high and high-to-low load transitions at different stages of the fatigue life and with different magnitudes) and multi-block loading spectra with different numbers of transitions.Results showed an extension of the fatigue life for high-to-low transitions. This was attributed to a combination of load interaction effects, as evidenced by an elastoplastic numerical simulation, and non-linear damage accumulation. Conversely, low-to-high transitions exhibited a damaging effect. For single transition spectra, BS and HR models were seen to improve prediction over PM by indirectly considering these effects. The proposed approach further improved this prediction. Multi-block results showed no significant influence of the number of load transitions and were satisfactorily predicted using PM.

Influence of reinforcement on vibration control in adhesively bonded single lap joints: a numerical and experimental validation

In this growing world, it is imperative to employ innovative techniques that effectively manage the structural response of materials without inducing adverse effects on the original structure. This can be achieved mainly by changing the material and geometrical features of the adhesive. In this work, an attempt has been made to control the eigenvalues of adhesively bonded single-lap joints (SLJs) by reinforcing them with polymer patches. Numerical techniques were utilized to adhere the polymer patches to the single-lap bonded joints using ABAQUS software. Subsequently, the eigenvalue responses of SLJs, both with and without patches, were experimentally well agreed with the numerical predictions. The validated numerical model was then used to investigate its structural response by modifying the parameters such as patch shape, patch position, and adhesive geometry. Additionally, it has been observed that a square-shaped polymer patch at the overlapping edge is more effective in reducing the eigenvalues compared to patches of different shapes and positions. The eigenvalue response follows a declining trend as the adhesive thickness increases, forming thicker bonds.

Curing and Failure Monitoring of Composite Material Single-Lap Joint Based on CNT/PSF Sensor

Abstract Composite bonded structures are widely used in the field of aerospace. The safety of composite structures can be ensured by the health monitoring and early warning of their whole life. In this paper, a composite CNT/PSF film sensor based on asymmetric porous PSF film was designed and prepared. Through integrated molding with epoxy film, in-situ monitoring of gel point (142.1°C) and curing time (36min) of epoxy film is realized. After optimization of curing process, tensile strength of epoxy film casting body is increased by 86.3%. The single lap shear strength of the composite is increased by 45.7%. The CNT/PSF film sensor with composite material has high response sensitivity (GF≈1.4) and linearity (0.998) to cyclic strain (0-3000με). In the shear experiment of composite single lap joint, the adhesive layer presents a typical cohesive failure mode of first compression, then tensile and finally failure. The resistance changes of CNT/PSF film sensor first decreases from 0 to -1‰, and then rises to 2.7‰. Finally, the fracture response is good, and the in-situ failure behavior monitoring of the joint can be fully realized.

Bond-slip models on interfacial behavior between Fe-SMA and steel in hygrothermal environments

Strengthening the existing steel structures to enhance performance can prolong their service life and reduce carbon emissions. Previous studies have employed iron-based shape memory alloy (Fe-SMA) to adhesively and actively retrofit long-span steel bridges in practice, achieving satisfactory repair efficiency. Nonetheless, concerns persist regarding the long-term performance of Fe-SMA-bonded parent structures, principally stemming from the performance degradation of the adhesives in hygrothermal environments. In this study, the damage evolution and debonding mechanisms of Fe-SMA/steel single-lap joints (SLJs) in hygrothermal environments are investigated through 54 lap-shear tests. The evolutionary process of Fe-SMA strain and shear stress within the overlapping zone is assessed, varying with aging time and adhesive type under shear load. Furthermore, the bond-slip models of the Fe-SMA/steel SLJs are put forward, which change with the aforementioned influencing factors in hygrothermal environments. The lap-shear tests demonstrate that the ultimate shear stress of SLJs is transmitted approximately up to 50 mm away from the initial end, with shear stress developing swiftly towards the free end during this phase. In hygrothermal environments, the effective overlapping length of the SLJs is highly contingent on the material properties of adhesives, improving with the prolonged aging time. This study proposes bond-slip models that describe the interfacial performance between Fe-SMA and steel, accounting for the detrimental influence of temperature, humidity, plastic deformation of components and strain relaxation. The experimental outcomes can guide the analysis of the debonding characteristics of Fe-SMA-bonded structures, and the theoretical study can provide reliable predictions for the service performance of reinforcement systems in hygrothermal environments.

Bending of fusion-bonded thermoplastic single lap joints

The assembly of thermoplastic composites is one of the most important issues for their reliable use in the aerospace and automotive industries. Assembly through co-curing is a process that enables simultaneous bonding and curing of adherends and is a preferable option for these industries. This paper presents a study of fusion-bonded, co-cured single lap joints with thermoplastic adherends that were subjected to four-point bending. Processing of the joint created a 45° notch at one edge of the overlap. The effect of this notch on the strength of the joint was assessed in tension and compression by flipping the joint. Compared to samples without such notches, experiments indicated that the tensile stresses in the vicinity of the notch decreased the maximum load and applied displacements at failure initiation by about 30 % and 33 %, respectively. In order to better understand this behavior, the joints were also analyzed using finite element simulations. The initial failure was predicted using the Maximum Stress Theory. Both experimental and numerical results were consistent up to the initial failure. The analysis did not account for the subsequently observed interlaminar delamination in the adherends. This type of failure would most likely be approached using measured interlaminar traction-separation relations in a cohesive zone modelling approach.

Evaluation of Fatigue Damage Monitoring of Single-Lap Composite Adhesive Joint Using Conductivity.

The widely used adhesive joining technique suffers from the drawback of being unable to be dismantled to examine for degradation. To counteract this weakness, several structural health monitoring (SHM) methods have been proposed to reveal the joint integrity status. Among these, doping the adhesive with carbon nanotubes to make the joint conductive and monitoring its electrical resistance change is a promising candidate as it is of relatively low cost and easy to implement. In this work, resistance change to monitor fatigue debonding of composite single-lap adhesive joints has been attempted. The debonded area, recorded with a liquid penetrant technique, related linearly to the fatigue life expended. However, it correlates with the resistance change in two different trends. Scanning electron microscopy on the fracture surface reveals that the two trends are associated with distinct failure micromechanisms. Implications of these observations on the practical use of the resistance change for SHM are discussed.

Failure analysis of CFRP/Al single lap adhesive joint with enhancing porous metal foam insert

Cohesive failure is one of the main causes of adhesive bonding failure. The enhanced method and mechanism for adhesive signify a critical research issue for better adhesive joint design. This study proposes using a porous metal foam insert adhesive to enhance the carbon fiber reinforced polymer/aluminum single lap joint. In particular, stress distribution and failure locations of joints with and without insert were compared by simulation modeling. The insert of the Cu-foam, Fe-foam, and Ni-foam, compared to no insert, increased the shear strength of the bonded joint by 32.23 %, 36.66 %, and 66.82 %, respectively. Compared to no insert, the crack propagation of the joint with a metal foam insert was changed from the cross-section of the adhesive layer to the plane of the foam insert. The highest strength of the adhesive joint with the Ni-foam insert was further analyzed to determine fatigue behavior compared with pure adhesive joint under different stress levels. Compared to no insert, the fatigue life increased highly, and the crack path was prolonged.