Single Lap Joints Research Articles

Construction of irregular and high-density microtexture on ultra-thin 304 austenitic stainless steel strip via laser surface treatment to improve steel/epoxy resin adhesive bonding properties

Ultra-thin steel strip can be used in composite materials to create high-performance and low-deadweight components for the aerospace industry. The present study used a laser to create surface microtextures on 304 austenitic ultra-thin stainless steel (SS) strips to improve their adhesion to epoxy resin. Modification was conducted at various laser power levels, resulting in ring-like textures of various qualities. The strips were bonded to epoxy resin at single-lap joints, which were then strength-tested. The bonding mechanisms were investigated via analyses of microstructure, surface chemical composition and interfacial failure modes. The laser power level affected the density and distinctiveness of the ring-like structures. The optimal level of 40 W produced a bond with a shear strength of 14.43 MPa. Bonding is enhanced by the interlocking effect of the texture, improved wettability between the steel and epoxy resin, and –C–O=C polar bonds. Lower and higher power levels provided weaker textures. In summary, irregular laser surface treatment can optimize the shear strength of SS–epoxy-resin joints, allowing the fabrication of stronger componentry.

The effect of tightening torque on the strength of composite-metal hybrid joints

In the manufacturing and assembly process of aircraft composite-metal hybrid structures, tightening torque is one of the important factors that control assembly quality and affect connection strength. This paper experimentally studies the mechanical properties of composite-metal single-bolt single-lap joints under different tightening torques, reveals the impact mechanism of different tightening torques on the manufacturing and assembly of composite-metal hybrid structures, and also establishes a finite element analysis model of hybrid connection structure considering the influence of drilling and assembly processes. The results show that the ultimate load of the composite-metal connection structure increased with the increase of tightening torque. When the tightening torques were 3 N·m, 6 N·m, and 9 N·m, the ultimate failure loads of the composite-metal hybrid joints were 12.9 kN, 13.8 kN, and 14.6 kN respectively.

Tailoring adherend surfaces for enhanced bonding in CF/PEKK composites: Comparative analysis of atmospheric plasma activation and conventional treatments

Here, we propose the utilization of atmospheric plasma activation (APA), which outperforms peel-ply (PP) treatment and mechanical abrasion (MA) in achieving high-performance adhesively bonded carbon fiber/polyetherketoneketone (CF/PEKK) composites. This study covers several key aspects, including the chemical and morphological characterization of treated surfaces and mechanical performance assessments of single lap-joints (SLJs) under tensile and flexural loading conditions. In addition, in-situ acoustic emission (AE) monitoring is employed during tensile tests to determine dominant damage types and failure modes in the SLJs. Surface analysis shows that MA increases roughness, PP treatment decreases wettability, while APA enhances wettability by modifying the surface chemistry. Tensile and flexural tests reveal that APA-treated joints surpassed non-treated (NT) ones, with up to 5- and 7-times higher load-carrying performance, respectively, while fracture analysis suggests a shift from adhesive to cohesive failure. AE results show that increased AE events related to cohesive failure align with improved interface interactions.

Hybrid Joining of Dissimilar Thin Metallic Sheets—Mechanical Joining and Adhesive Bonding

This paper deals with joining dissimilar materials using thermal drilling technology as well as the combination of thermal drilling and adhesive bonding. The base materials for the experimental work were deep-drawn low-carbon steel DC04, HSLA steel TL 1550-220 + Z, and structural aluminum alloy EN AW-6082 T6 (AlSi1MgMn). The geometry of the formed joints was tested metallographically as the load-bearing shear capacity under the tensile shear test of single-lapped joints and the resistance of the joints against corrosion-induced disbonding in a climate chamber. The energy dissipated by the joints up to fracture was calculated from the load–displacement curves. The hybrid joints were compared with the bonded joints with the same overlap area in terms of the load-bearing capacity and energy dissipated at joint failure. The hybrid joints formed by thermal drilling and adhesive bonding with a rubber-based adhesive confirmed the synergistic effect—the adhesive provides the high load-bearing capacity of the joint, and the bushing formed by thermal drilling increases the dissipated energy of the joint at failure. The exposure of the joints in the climatic chamber did not cause a relevant reduction in the characteristics of the joints.

Hybrid Single Lap Joints between 3D Printed Titanium Lattices and CFRP Composites: Experimental and Numerical Insights

In the contemporary emphasis on weight reduction, the utilization of advanced materials like Carbon Fiber Reinforced Polymers (CFRPs) and cutting-edge technologies such as 3D printing of metal is increasingly crucial. This study delves into the junction of CFRP and titanium, aiming to conduct Single Lap shear tests on specimens featuring a co-lamination of long fiber composite onto a metal lattice structure. Different specimens with different dimensions of the Simple Cubic (SC) unit cell were subjected to testing. A microscope investigation facilitated an exploration of junction failure and epoxy resin infiltration into the lattice substrate. Employing an efficient 2D Finite Element Model, the homogenization process yielded theoretical models underestimating the Young Modulus by approximately 10% compared to real specimens. Despite the challenges in bonding titanium and CFRP, the novel junction exhibited a shear stress of 17.25 MPa, which is nearly equivalent to those of a co-lamination between sandblasted steel and CFRP, that is 17.15 MPa.

Evaluation of Welded Lap Joints Using Ultrasonic Guided Waves.

Welded lap joints play a vital role in a wide range of engineering structures such as pipelines, storage tanks, pressure vessels, and ship hulls. This study aims to investigate the propagation of ultrasonic guided waves in steel welded lap joints for the baseline-free inspection of joint defects using the mode conversion of Lamb waves. The finite element method was used to simulate a single lap joint with common defects such as corrosion and disbonding. To identify the propagating wave modes, a wavenumber-frequency analysis was conducted using the 2D fast Fourier transform. The power loss of the transmitted modes was also determined to identify damage in the lap joints. The results indicate that the A0 incident in pristine conditions experienced significant transmission losses of about 9.5 dB compared to an attenuation of 2.8 dB for the S0 incident. The presence of corrosion was found to reduce these transmission losses by more than 28%. In contrast, introducing disbonding in the lap joint increased the transmission loss of the S0 incident, while a negligible loss was observed for the A0 incident. The mode-converted S0 (MC-S) and mode-converted A0 (MC-A0) incidents were found to exhibit a unique sensitivity to the presence of corrosion and disbonding. The results indicate that MC-S0 and MC-A0 as well as Lamb mode incidents interact differently in terms of corrosion and disbonding, providing a means to identify damage without relying on baseline signals.

Surface modification strategies of 304 austenitic stainless steel plate with different micromorphology and composition for improved mechanical interlocking and chemical bonding in metal–epoxy joints

304 austenitic stainless-steel (SS) strip-based carbon fiber metal laminates (CFMLs) have raised significant concerns, in terms of their mechanical performance. On top of that, the hybrid laminates’ interlamination performance is vital in regulating their failure patterns. Along these lines, in this work, SS strips with regulatable micromorphology and composition were obtained by using physical and chemical surface modification strategies, including polishing, acid etching, anodization, and thermal treatment. The shear strength of the treated-SS/epoxy single lap joints (two treated-SS strips bonded by the epoxy resin via hot curing process) was systematically explored, and the origins of the enhancement mechanism were attributed to the resulting surface microstructure, as well as the possible chemical reactions between the treated surface and the curing agent. The etching-anodizing-thermally treated-SS (EAT-SS)/epoxy joint exhibited the maximum shear strength (16.04 MPa) and work of fracture (5.85 kJ·m−2) due to the substantial cohesive failure pattern of the epoxy. The encouraging enhancement was mainly attributed to the synergistic effect of the micro-annular pit and the nanoporous array structure of the EAT-SS surface. These effects induced a significant mechanical interlock and a plausible chemical reaction at the interface, which led to improved bonding strength between the epoxy and treated-SS surface. Overall, in this work, it was demonstrated that the combination of the anodization and thermal treatment can effectively improve the interlamination bonding properties between the SS strip and the epoxy resin due to the combination effect of mechanical interlocking and chemical bonding.

Delamination prevention of composite joints with asymmetric carbon-fiber reinforced plastic adherends

The use of adhesive bonding in aeronautical applications has gained popularity due to its numerous advantages over traditional mechanical fastening methods. However, the issue of delamination still poses a significant challenge in fully embracing this technology in the industry. This study proposes a unique approach to optimize carbon fiber reinforced polymer (CFRP) single lap joints (SLJ) by introducing a curved joint design with varying adhesive thickness. The curvature is achieved through the intentional warpage caused by residual stresses resulting from an asymmetrical composite layup. While these residual stresses can potentially lead to cracks and distortions, this research aims to leverage their presence to enhance joint performance and mitigate delamination risks. A comprehensive analysis, involving both numerical simulations and experimental testing was conducted to compare the performance of the curved joint design with traditional configurations. The findings indicate that the curved joint design effectively prevents delamination while maintaining comparable joint strength. This demonstrates the potential benefits of employing curved joints for aeronautical components with curved geometries, affirming the feasibility and advantages of this approach. However, further optimization of the curved joint configuration is warranted to enhance its performance and expand its applicability to a broader range of applications.

Effect of outdoor environmental exposure on the mechanical behavior of steel/GFRP single-lap joints

ABSTRACT Adhesive joints GFRP/Steel bonded with epoxy adhesive were exposed to the external environment for one year, a tropical environment with temperature variations, humidity, radiation, and salt spray, due to its proximity to the sea. The shear strength of the joint was measured over the exposure time, as well as their chemical and physical changes. Tg of the adhesive was evaluated through DMA, and changes in the composite matrix were studied using FTIR. A significant reduction in joint strength was observed within the first 30 days, but this reduction quickly stabilized. High corrosion of the metal adherend was evident, which was also observed in the bonded region, significantly impacting the adhesive strength of the joints. Changes in the color of the composite were also identified due to exposure to solar radiation, which also affected the chemical bonds, as indicated by FTIR. The Tg of the adhesive decreased in the first 90 days and then gradually increased. These changes may have been related to the humidity, initially decreasing the Tg, and the exposure to higher ambient temperatures, which causes post cure effect. Therefore, the proposed external exposure conditions contain factors capable of causing significant changes in the joint and its components.

Mechanical response of reduced graphene oxide (rGO) reinforced epoxy adhesive joints

The present work aims at investigating the effect of adhesive modification on the lap shear strength of the adhesively bonded joints. The epoxy-based adhesive is reinforced with reduced graphene oxide (rGO) at different weight fractions. Single lap joints (SLJ) have been prepared by considering the commercially available aerospace-grade aluminum alloy AA8011 as the adherend material and Araldite 2014 A/B as the adhesive. Experiments showed that the addition of 0.5 wt.% graphene to the epoxy adhesive resulted an approximately 27% increase in the lap shear strength of the SLJ. Further increase in the reinforcement loading decreases the lap shear strength due to the agglomeration of graphene particles. A detailed analysis of the effect of reinforcement loading and overlap length on the shear strength has been presented and failure mechanisms are discussed.

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.

Numerical modeling on load bearing and bypass behavior of bolted single lap joints of laminates

AbstractBolted joints are widely used in aircraft structures. Proper design and accurate strength assessment of these joints are of great importance in aircraft design. In this paper, to investigate the mechanical behavior of single lap shear joints of carbon fiber reinforce plastic, four types of fastener of numerical model methods are developed. Based on the same principle of lap joint, different double bolt, and single‐lap joint structures with three distinct width‐diameter ratios (w/D) are designed and manufactured. Each specimen is loaded under tension until complete failure. The experimental results are used not only to verify the fastener models, but also to investigate the failure transformation induced from the composite load bypass and the bolt‐hole load bearing. It is found that the increase of w/D, the failure model could transfer from the net cross section damage caused by load bypass to the bolt hole failure caused by load bearing. By comparison, the numerical results show that the Cbush+RBE3 model has high adaptability and good fidelity in elastic stage without increasing the model scale in terms of the level of details. To improve the accuracy in nonlinear stage, hole geometry details need to be considered. Additionally, single bolt is created to reveal the nonlinear deformation affected by laminate thickness. Three/four/five row of bolt models are used to show the bolt load distribution characteristics. Finally, the influences of the element size and the bolt modeling methods on the bolt load distribution are analyzed, respectively.

Mechanical behavior investigation of fused deposition modeling joints by using different bonding geometry with variable adhesive thickness

PurposeAdhesively bonded joints are used in many fields, especially in the automotive, marine, aviation, defense and outdoor industries. Adhesive bonding offers advantages over traditional mechanical methods, including the ability to join diverse materials, even load distribution and efficient thermal-electrical insulation. This study aims to investigate the mechanical properties of adhesively bonded joints, focusing on adherends produced with auxetic and flat surfaces adhered with varying adhesive thicknesses.Design/methodology/approachThe research uses three-dimensional (3D)-printed materials, polyethylene terephthalate glycol and polylactic acid, and two adhesive types with ductile and brittle properties for single lap joints, analyzing their mechanical performance through tensile testing. The adhesion region of one of these adherends was formed with a flat surface and the other with an auxetic surface. Adhesively bonded joints were produced with 0.2, 0.3 and 0.4 mm bonding thickness.FindingsResults reveal that auxetic adherends exhibit higher strength compared to flat surfaces. Interestingly, the strength of ductile adhesives in auxetic bonded joints increases with adhesive thickness, while brittle adhesive strength decreases with thicker auxetic bonds. Moreover, the auxetic structure displays reduced elongation under comparable force.Originality/valueThe findings emphasize the intricate interplay between adhesive type, bonded surface configuration of adherend and bonding thickness, crucial for understanding the mechanical behavior of adhesively bonded joints in the context of 3D-printed materials.

Mechanical response of carbon fiber reinforced epoxy composite parts joined with varying bonding techniques for aerospace applications

As a result of the widespread use of composite materials in primary structures of aerospace platforms, composite joining became more crucial. This study addresses the effect of joining methods on the strength of composite joints experimentally, numerically and analytically. Single lap joint shear strengths of carbon fiber reinforced epoxy composite parts joined by mechanical fastening with a pop and solid rivet, secondary bonding with a paste adhesive, co-curing and co-bonding techniques were compared. In addition, the effect of adhesive thicknesses (0.2, 0.4, 0.6, 0.76 mm) on the single lap shear strength was investigated. Carbon fiber reinforced composite (CFRP) samples were produced according to the ASTM 5868 standard. After the production of samples with varying joining methods, single lap shear tests were implemented. Moreover, the interface damage in the composites was examined by use of a scanning electron microscope (SEM) for the purpose of studying the damage mechanism. Fracture mechanisms corresponding with bonding methods were also assessed by examining the fracture surface of the composite samples. Furthermore, results were analyzed by Hypermesh, ABAQUS and ESAComp. For instance, the co-bonded sample with an adhesive film exhibits an experimental shear strength of 24.03 MPa which deviates only 3 % from the numerical expectation.

A Novel Technique for Substrate Toughening in Wood Single Lap Joints Using a Zero-Thickness Bio-Adhesive.

In contemporary engineering practices, the utilization of sustainable materials and eco-friendly techniques has gained significant importance. Wooden joints, particularly those created with polyurethan-based bio-adhesives, have garnered significant attention owing to their intrinsic environmental advantages and desirable mechanical properties. In comparison to conventional joining methods, adhesive joints offer distinct benefits such as an enhanced load distribution, reduced stress concentration, and improved aesthetic appeal. In this study, reference and toughened single-lap joint samples were investigated experimentally and numerically under quasi-static loading conditions. The proposed research methodology involves the infusion of a bio-adhesive into the wooden substrate, reinforcing the matrix of its surfaces. This innovative approach was developed to explore a synergetic effect of both wood and bio-adhesive. The experimentally validated results showcase a significant enhancement in joint strength, demonstrating an 85% increase when compared to joints with regular pine substrates. Moreover, the increased delamination thickness observed in toughened joints was found to increase the energy absorption of the joint.

S$$^4$$: simple quasi-1D model for structural health monitoring of single lap joint software

S$$^4$$: simple quasi-1D model for structural health monitoring of single lap joint software

Effect of Different Step-Lap Joints on the Natural Frequencies of Different Adhesively Bonded Metallic Materials: A Numerical Study

Due to their many applications' benefits, adhesively bonded joints are widely utilized in nearly every industry, including space, marine, automotive, and aeronautics. Since unpredicted loadings may cause resonance in the structures, an accurate prediction of the bonded joints' dynamic characteristics is crucial. Therefore, in this study, modal analysis was performed on the two-, three-, four- and double-step adhesively bonded lap joints of Aluminum (Al), Copper (Cu), and Mild steel (Ms) materials with Epoxy Araldite adhesive. Ansys commercial program was utilized to analyze it numerically. The results showed that modeling the bonding region of single lap joints as two-, three-, and four-step adhesively bonded lap joints has no significant effect on the natural frequencies. This modeling has a minor incremental effect on the natural frequencies. However, Double-step lap joints were found to cause a considerable reduction in natural frequencies compared to not only single lap joints but also two-, three-, and four-step adhesively bonded lap joints. Double-step bonding caused a decrease of 8.82%, 8.57%, and 8.73% for Al-Al, Cu-Cu, and Ms-Ms. In general, in all models, the best increase or decrease in terms of natural frequencies was found to be Cu-Cu adhesively lap joints.

Evaluation of mixed-mode stress intensity factors for anisotropic elastic solids with interface singularity

Abstract When a structure is composed of many different parts and each part is made of different materials, it is very possible that many interface corners appear in several local fields of the structure. Due to the mismatch of elastic properties, stress singularity may occur at the interface corners. The singular orders of stresses and their associated stress intensity factors (SIFs) are two important parameters for understanding failure initiation. Since the singular orders of general interface corners may be real or complex, distinct or repeated, several different definitions of SIFs have been proposed for different types of interfaces. In order to build a direct connection among all general interface corners (including cracks and interface cracks), a unified definition of SIFs was proposed in our previous studies. To calculate the uni-defined SIF accurately and efficiently, a special boundary element using the fundamental solution satisfying interface continuity conditions is established. The singular integrals involved in the associated boundary integral equations are solved by the setting of proper rigid body motions and the approximation through interpolation of nearby non-singular solutions. To avoid the unstable and inaccurate region near the corners, in this study we employ the path-independent H-integral, and derive the relations between H-integral and SIFs. Moreover, the auxiliary solutions of displacements and tractions required in the H-integral are provided for different integral paths. Several numerical examples, such as interface cracks, interface corners, single lap joints and delamination, are presented to demonstrate the methodology employed in this paper.

Experimental and numerical investigation on failure behavior of hybrid bonded/bolted GFRP single-lap joints under static shear loading

The mechanical performance of the GFRP (Glass Fiber Reinforced Polymer) hybrid bonded/bolted single-lap joints under static shear loading is investigated. Static loading tests considering the hybrid joints with one bolt, four bolts, and nine bolts were first conducted to study the failure mechanism of hybrid joints, where failure process, rigidity, failure load, and failure modes were focused. Finite element (FE) analysis involving the material degradation and failure laws was also carried out for exploring the internal mechanism of joints’ failure. Based on the verified FE model, the effects of plate thickness and bolt edge distance on the failure capacity and strength of the hybrid joints were further investigated by performing a numerical parametric analysis. Results show that, the failure process of the hybrid joints can be divided into two stages which correspond to the failures of adhesive layer and GFRP plates, respectively. The joint rigidity of joints increases by 98.9 %∼133.1 % comparing the second loading stage to the first stage, and it is found that the rigidity in the first stage is hardly affected by the number of fasten bolts, while it decreases with more bolts in the second stage due to the severe incompleteness of GFRP plates. The failure patterns of the hybrid joints mainly include the slippage between connected plates, the compressive failure and shear failure of GFRP material, and the inclination of bolts. A proper arrangement of bolts can lead to shear failure of GFRP which makes full use of the material property. The final failure loads of four-bolt and nine-bolt joints are respectively 14.5 % and 22.9 % higher than the single-bolt specimen. As the plate thickness getting greater, the shear bearing capacity of hybrid joint increases almost linearly, while it would increase first and then descend with increasing bolt edge distance. In general, the achievement of this work can provide references for the design and further study of the GFRP hybrid joints.

Modelling the static and fatigue behavior of hybrid spot welded-adhesively bonded single lap joints

Hybrid joints formed by combining two or more joining methods, including spot welding and adhesive bonding, are relatively new joints designed especially for use in the automotive industry. The aim of this work is to study numerically the static and fatigue behavior of ultrasonically spot-welded, adhesively bonded and hybrid spot welded – adhesive bonded joints. The Virtual Crack Closure Technique was used to model the experimental static and fatigue tests for single lap joints reported in the literature. By introducing a short fictitious initial crack, in static loading the simulations matched the experimental results, characterized by a higher stiffness and static strength of the bonded and hybrid joints compared to the just spot-welded ones, and by an approximately similar static strength of the bonded and the hybrid joints. By the same method, a good match between the numerical and the experimental fatigue strengths was found, with the hybrid joints providing higher strength compared to the just spot-welded joints at 0.1 load ratio. According to the numerical analyses, S–N curves of bonded joints are expected to be statistically similar to those of the hybrid joints. This indicates that the adhesive plays the major role in the fatigue behaviour of the hybrid joint. Finally, the fatigue strength of hybrid joints was simulated at 0.3 load ratio. It was found a reasonable agreement between the experimental and numerical observations at 0.3 load ratio. The VCCT proved to be capable of simulating the static and the fatigue behaviour of the hybrid joints, thus proving to be a tool useful for the interpretation of the results and for the design of other hybrid joint configurations.