Single-lap Adhesive Joints Research Articles

Investigation of Residual Strength in CFRP Double-Adhesive Single Lap Bonded Joints after Low-velocity Impact

ABSTRACT Single lap adhesive joints are prone to debonding and delamination during impact, affecting the residual tensile strength and mechanical properties. To improve the stress distribution and withstand larger loads, this paper proposes to use Araldite 2015 ductile adhesive at both ends of the joint and AV 138 brittle adhesive in the middle. Through finite element analysis and experiments, the residual strength and damage of the joint under different impact conditions are investigated, and a three-parameter Gram-Charlier model is established. The results show that the model can fit the data points accurately with a coefficient of determination of error above 0.99. As impact energy increases, the residual strength of the joint decreases, especially the AV 138 joint is more sensitive. The residual strength on one side of the impact joint is approximately 20% higher compared to the middle of the impact joint. For the same energy, the residual strength of the secondary impact was about 13% higher than that of the single impact. The main failure mode is cohesive failure, which gradually changes to fiber tear. The numerical simulation matches the experimental results, which verifies the reliability of the simulation and analysis methods.

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.

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.

Effect of microstructural roughness on the performance and fracture mechanism of multi-type single lap joints

Surface roughness of adherends is a crucial factor in determining the performance and failure mechanism of adhesive joints. However, the research dedicated to the examination of fracture mechanism for adhesive joints influenced by surface roughness at microscale is limited. This work conducts systematic experimental and numerical investigations into the effect of microstructural roughness on the performance and fracture mechanism of multi-type adhesive single lap joints (SLJ). The adherend materials are aluminium alloy (Al) and polyphthalamide (PPA), ground into three roughness grades for the fabrication of SLJs using an epoxy adhesive. The mechanical properties of the adhesive, adherends and SLJs derived from experimental studies are utilized to calibrate the microparameters in Discrete Element Method (DEM) models for a numerical analysis. The newly developed DEM models demonstrate efficacy in predicting the performance and capturing the failure modes of multi-type SLJs realistically with distinct microroughness profiles. Finally, the influencing mechanisms of microstructural roughness on the performance of multi-type SLJs are investigated, including the microscale interfacial bonds and mechanical interlockings. The effects of microstructural roughness on the microscale failure mechanisms of multi-type SLJs are also explored and discussed, including the crack initiation, coalescence, and propagation within the adhesive and interface.

Effects of laminate stacking sequence on the strength properties of aluminum alloy–carbon fiber-reinforced plastic dissimilar single-lap adhesive joints

Here, two single-lap adhesive joints (SLJs) of dissimilar materials were subjected to static and cyclic loading tests. A2017 aluminum alloy was used as an adherend for one, and carbon fiber reinforced plastic (CFRP) was used as the adherend for the other. Four types of orthogonal laminated CFRPs with different laminate stacking sequences were used as adherends to investigate the effect of adherend stiffness on the strength properties of the joints. Furthermore, the results of the finite element analysis of the dissimilar SLJs revealed that when the tensile load was applied to them, the out-of-plane deformation asymmetry increased with increasing difference in stiffness between both adherends. This asymmetry affected the peel and shear stress distributions. Furthermore, the experiments revealed that the static tensile strength of the SLJs increased with increasing stiffness of the CFRP adherend. Additionally, fracture simulation using cohesive-zone modeling (CZM) revealed that the SLJs with higher CFRP stiffness exhibited higher strength, qualitatively agreeing with the experimental results. CZM analysis and adhesion strain measurements during the tests indicated that failure occurred at the A2017 adherend–adhesive interface. In contrast, no differences were observed between the fatigue strengths of the different types of adherends in the short-life region, with a number of cycles to failure (Nf) being ≤ 2 × 105. However, in the long-life region, beyond Nf = 2 × 105, the SLJ bearing the unidirectional CFRP adherend exhibited lower fatigue strength than the others. The anodizing process on A2017 was found to improve fatigue strength by a factor of two or more.

Enhancing mechanical properties of GFRP–aluminum joints through Z pinning: a low velocity shear impact study

Studying the mechanical responses of GFRP and aluminum samples under varying loads is crucial for developing methods to enhance their mechanical properties. This research investigates the impact of z-pinning on single-lap adhesive joints through low-velocity shear impact tests. The study involves testing two types of joints: single-lap adhesive joints and single-lap hybrid pin-adhesive joints with six reinforcing pins, subjected to drop weight tests at load levels of 22.5, 27.5, and 32.4 J. The findings demonstrate that z-pinning can increase the joints’ load-bearing capacity by up to 18.87% and energy absorption by 20.58%. This enhancement is attributed to the additional composite substrate layers in the hybrid joint, which help absorb impact loads. In contrast, in the single-lap adhesive joint, only the layer adjacent to the adhesive bears the impact load. Observations from the tests indicate that in the single-lap adhesive joint, the first composite layer next to the adhesive undergoes complete delamination, whereas in the hybrid joint, all laminates fracture at the steel pins. In summary, this study underscores the significant potential of z-pinning to improve single-lap adhesive joints, providing an innovative approach to enhance their load-bearing capacity and energy absorption.

Effect of adhesive thickness and loading speed on bonding strength in single-lap adhesive joints with aluminum 5754-H111

This study provides shear test results of single-lap adhesive joints with aluminum 5754-H111. To investigate the effect of adhesive thickness on joint strength in single-lap adhesive joints, three different thicknesses (0.2, 1, and 2 mm) were applied. Increasing the adhesive thickness from 0.2 mm to 1 mm and from 0.2 mm to 2 mm resulted in a reduction in shear load of roughly 36% and 44%, respectively. It has been observed that the joint strength decreases with increasing adhesive thickness in single-lap adhesive joints with aluminum 5754-H111. In addition, single-lap adhesive joints were tested at various loading rates (1, 10, and 100 mm/min), and the influence of loading speed on adhesive strength was studied. The shear load increased with the loading speed in the test results of single-lap adhesive joints.

Substrate Thickness Optimization in Multi-Material Single Lap Adhesive Joints

Abstract Adhesive bonding of dissimilar materials introduces stress concentrations due to stiffness mismatch between the substrates, thereby exacerbating the peel and shear stresses leading to premature failures in single lap configurations. This work demonstrates that the stress distribution can be improved by decreasing the thickness of the stiffer substrate, and presents a structured approach to find the optimum thickness to improve overall joint performance. First, the critical stress components and critical locations in the single lap joint were identified for each mode of failure. Then, a minimax-type optimization framework was developed using severity-weighted parameters for each critical stress component. Optimal thickness obtained from the proposed framework agreed with FEA-based parametric studies within 10% variation. Overall, this approach can generate design charts and aid in efficient designs for multi-material joining.

Influence of adding nanomaterials on shear properties of epoxy resin at different temperatures

Adhesive joints play a vital role in different industries owing to their advantages and ease of application compared to other joining methods. This research focuses on enhancing the mechanical properties of epoxy adhesives by incorporating graphene nanoplatelets (G) and iron-oxide nanofillers (Fe<sub>3</sub>O<sub>4</sub>). Single-lap adhesive joints, including both G and Fe<sub>3</sub>O<sub>4</sub> nanoparticles, are fabricated at 2%, 3%, and 4% weight percentages and tested under tensile load at ambient, 45°C, and 88°C. The results reveal that adding G and Fe<sub>3</sub>O<sub>4</sub> nanofillers enhances shear strength at elevated and room temperatures without altering the epoxy glass transition temperature (Tg). Furthermore, G nanofiller performs better in improving shear strength than Fe3O4. The optimal weight percentage is identified as 3 wt% for G and Fe<sub>3</sub>O<sub>4</sub>, as higher percentages lead to decreased shear strength due to agglomerations. This study provides insight into tailoring epoxy adhesives for improved mechanical performance under varying temperature conditions.

Experimental Investigation of the Effect of Curing Pressure on Strength of Single Lap Adhesive Joints

It is a common method to apply a certain pressure to the bonded surfaces while bonding. In this way, it is ensured that the adhesive covers all surfaces and the air in the adhesive is evacuated. However, the effects of pressure changes on the curing process and mechanical properties, especially in adhesives that harden by chemical reaction, are an issue that should be handled with numerical data at a scientific level. When the literature is examined, there are limited studies on the effects of curing pressure on mechanical properties of adhesive joints. 
 In this study, the effect of curing pressure on bond strength in adhesive joints was investigated experimentally. Single lap adhesive joints were manufactured using steel plate and DP460 epoxy. At this stage, bonding joints with different curing pressure conditions were obtained by placing weights on the adhesive area. The samples were subjected to tensile tests. In consequence of the experiments, a reducing in the joint endurance was observed after a certain pressure value. Thus, it has been understood that the pressure applied to the adhesive area affects the curing process, and must be taken within certain limits.

Characterization of Bending Strength in Similar and Dissimilar Carbon-Fiber-Reinforced Polymer/Aluminum Single-Lap Adhesive Joints

In recent years, the adhesive bonding method has gained increased attention, especially in the automotive industry, for constructing efficient body structures from dissimilar and lightweight materials such as aluminum and polymeric composites. Adhesively bonded automotive structures endure complicated loading conditions, including tensile and bending loading, during their service lives. To the best of the authors’ knowledge, there is no published work on the assessment of bending strength in single-lap adhesive joints (SLJs) when considering dissimilar adherends under three-point bending. In this study, three-point bend experiments were carried on the bending strength and the failure mechanisms of dissimilar SLJs made of carbon-fiber-reinforced polymer (CFRP) and aluminum substrates bonded with Araldite 2015 adhesive. Additional experiments were conducted individually on similar SLJs, including aluminum/aluminum and CFRP/CFRP, to investigate and compare the effects of adherend material type on the bending strength and failure behavior of SLJs. The results indicate that a CFRP/CFRP single-lap adhesive joint exhibits significantly higher joint strength in comparison to an aluminum/aluminum single-lap adhesive joint under three-point bending. The strength of dissimilar CFRP/aluminum single-lap joints usually falls between that of an aluminum/aluminum and that of a CFRP/CFRP single-lap adhesive joint. When the CFRP adherend is situated at the bottom of the joint in three-point bending, it imparts significantly greater joint strength and deformation compared to situations where the aluminum adherend is placed at the bottom.

Investigation on shear and fatigue performance of CFRP/aluminum alloy single-lap adhesive joint

The size parameter is quite significant for the performance of adhesive joint. In order to investigate the effect of size parameters on the mechanical property and failure behavior of CFRP/aluminum alloy single-lap adhesive joint, T300 CFRP laminate and AA5182 aluminum alloy sheet were bonded by Sika Power 497 structural adhesive under different size parameters. The quasi-static shear test and fatigue test were implemented, and the failure behavior and failure mechanism were analyzed. Results showed that the increase of aluminum alloy sheet thickness or bonding area length could effectively improve the bonding strength of adhesive joint, when the strength of the aluminum alloy sheet or the adhesive layer was insufficient. The shear failure modes of adhesive joint mainly included adhesive failure, base material failure and mixed failure. The fatigue failure modes of adhesive joint were mainly adhesive cohesion failure and adhesive adhesion failure. At low stress levels, the aluminum alloy sheet might fracture. In addition, this work can provide a scientific guidance for the engineering application of adhesive bonding technology.

On the influence of joining processes on the vibration of structures

The joining of new and dissimilar materials in the transportation sector, to meet the demands for lighter structures, requires the use of alternative joining processes. However, it is not clear how different types of structural joints compare in terms of their contribution to the vibration and damping of structures. The present work aims to provide a comprehensive experimental and numerical analysis on the contribution of three structural joints – butt friction stir welding, single lap adhesive joint and hole hemmed joint – to the vibration and damping of two joined aluminium sheets. For this purpose, experimental analysis is performed to study the free-free vibration of the three structural joints. In addition, finite element models are developed to better understand the behaviour of these joints and discuss the challenges of the numerical modelling procedure. The first four natural frequencies, experimentally obtained for each structure, suggest that the adhesive joint has significantly higher natural frequencies, due to the thickness increase at overlap, while the hole hemmed joint presents the most significant contribution to damping, owing to sliding at the overlap region. The numerical models show a very good agreement with the experimental results in terms of the natural frequencies and mode shapes, with simple modelling providing accurate results. In conclusion, the main findings are that the adhesive joints allow for a stiffer structure, with the natural frequencies increasing with the overlap dimension, while hole hemming enhances damping. The butt-butt friction stir welding has a small effect on the structural behaviour, showing a similar dynamic stiffness and damping when compared to a solid sheet of the same dimension.

An experimental investigation of static and fatigue behavior of various adhesive single lap joints under bending loads subjected to hygrothermal and thermal conditions

ABSTRACT The objective of this study is to investigate the impact of hygrothermal and thermal aging on adhesive bonding joints of similar and dissimilar specimens. The research focuses on the single lap joint (SLJ) configuration using Aluminum/Aluminum and CFRP/CFRP for similar bonding and Aluminum/CFRP for dissimilar bonding. Understanding the behavior of such joints over time and eventual failure can enhance their durability, reliability, and safety in structures. The study submerged SLJ specimens in tap water at a temperature of 53°C for 20 days to assess hygrothermal aging and kept another group of joints in an oven set at the same temperature for thermal aging. The research evaluated the strength of samples that underwent no aging, hygrothermal and thermal aging, tested through three-point bending tests. Various load levels were tested utilizing a recently designed fixture, including 70%, 60%, and 50% of the static load level. Results showed the highest joint strength in Aluminum/Aluminum SLJs and reduced strength following thermal and hygrothermal aging. The failure patterns and number of cycles leading to failure also varied with load percent and aging. Ultimately, the study aimed to evaluate and contrast the strength of samples that underwent different aging processes.

Fatigue behavior of single-lap adhesive joints with similar and dissimilar adherends under cyclic loading: A combined experimental and simulation study

This study aims to research the fatigue behavior and reveal the failure mechanism of the adhesive joints with similar and dissimilar adherends, which made of the aluminum alloys (Al) and carbon fiber reinforced plastics (CFRP). Firstly, the electronic universal testing machine and fatigue testing machine were used to obtained the failure strength and fatigue life of the joints. Secondly, the joints were modeled by cyclic cohesive zone model (CCZM) to obtained the damage evolution process of the joint. Finally, the fatigue failure mechanism of the joint was analyzed based on the experimental failure mode and microscopic morphology. The results showed that in the fatigue process, the damage in CFRP-Al joints extends from the CFRP end to the Al end and eventually fractures near the Al end, while the damage in CFRP-CFRP and Al-Al joints are symmetrically distributed along the central axis of the lap area. In addition, the failure characteristics of the joints are mainly affected by the peel stress, and compared to differences in substrate stiffness (the cause of peel stress in joints with dissimilar adherends), the differences in the strength of the substrate-adhesive connection (the cause of peel stress in joints with similar adherends) plays a greater role in the generation of peel stress.

Fatigue fracture criteria in single-lap adhesive joints with cumulative dissipated energy

This study focuses on predicting the fatigue life of structurally bonded joints. Herein, fatigue tests were conducted using single-lap adhesive joints subjected to varying stress ratios (R = − 1.0 and − 0.5) and cyclic frequencies (f = 2, 5, 10, and 20 Hz) with a digital image correlation to measure shear strain in adhesives. The relation between tensile–shear stress and the number of cycles to failure Nf showed a dependence on R and f, which suggested that reduced mean stress and increased f positively impacted fatigue life of single-lap adhesive joints. Cumulative dissipated energy Wd was developed by considering adhesive strain rate owing to cyclic loading, and a fracture criterion Wd–Nf was established to approximate the relation with the number of cycles regardless of the R and f conditions. The fracture criterion established under R = 0.2 was expected to include the relationships between Wd and Nf for other conditions within ranging from 1/3 to 3 times. In a range of 104 to 107 cycles, the prediction accuracy of the fatigue fracture criteria based on the Wd exceeded that of the conventional fatigue curves by approximately three times.

Adhesive Single-Lap Joint Evaluation Using Ultrasound Guided Waves

This work deals with the evaluation of adhesive single-lap joints using ultrasound guided waves; in particular, it is intended to characterize the signal propagation when defects are present in the adhesive joint by simulation and experimental approaches. The propagation of guided waves in the joint is developed from matrix formulations. The behavior of the guided wave modes that exist in the bonded region is characterized. It is found that its amplitudes can be estimated from the properties of the incoming wave that propagates in the non-bonded region. It is verified that the excitation of these modes is related to the degree to which the shapes of both modes match each other. A 3D simulation of two aluminum-bonded plates using 500 kHz ultrasonic transducers in a pitch-and-catch configuration was implemented using the Matlab k-Wave toolbox. Scattering effects, due to some defects located in the bond line of the joints, were simulated. The experimental setup with some artificial defects produced in the aluminum joints was used in order to compare it with the simulation. Qualitative agreement was observed between the two approaches. The observed deviation can be due to the different characteristics of the experimental and simulated defects.

The Influence of MSR-B Mg Alloy Surface Preparation on Bonding Properties.

Nowadays, industrial adhesives are replacing conventional bonding methods in many industries, including the automotive, aviation, and power industries, among others. The continuous development of joining technology has promoted adhesive bonding as one of the basic methods of joining metal materials. This article presents the influence of surface preparation of magnesium alloys on the strength properties of a single-lap adhesive joint using a one-component epoxy adhesive. The samples were subjected to shear strength tests and metallographic observations. The lowest properties of the adhesive joint were obtained on samples degreased with isopropyl alcohol. The lack of surface treatment before joining led to destruction by adhesive and mixed mechanisms. Higher properties were obtained for samples ground with sandpaper. The depressions created as a result of grinding increased the contact area of the adhesive with the magnesium alloys. The highest properties were noticed for samples after sandblasting. This proved that the development of the surface layer and the formation of larger grooves increased both the shear strength and the resistance of the adhesive bonding to fracture toughness. It was found that the method of surface preparation had a significant influence on the resulting failure mechanism, and the adhesive bonding of the casting of magnesium alloy QE22 can be used successfully.

Joining technology of additively manufactured components: effects on the bonding strength for the adhesive application through inner channels

The maximum size of additively manufactured (AM) components is restricted due to the confined building space of the manufacturing machines. Component separation and subsequent joining can be an effective way of manufacturing larger components using AM processes. For joining of AM components, adhesive bonding provides great potential for not constraining the adherend’s geometry, as long as the adhesive can still be applied to the adhesive surfaces of the adherends. This work investigates the effectiveness and applicability of additively manufactured inner channels to improve the adhesive application. A circular adhesive single lap joint between a laser-based powder bed fusion (PBF-LB) component made of AlSi10Mg and a cold drawn aluminum round bar was considered. The PBF-LB components were designed with varying geometric complexity to implement different adhesive application concepts. Subsequently, the bonded joints were subjected to static tensile tests. The fracture strength of joints where the adhesive was applied by injection into AM inner channels exceeds the fracture strength of joints where the adhesive was injected into geometries manufacturable by subtractive machining, and also exceeds the fracture strength of joints where the adhesive was pre-applied.