Composite Single-lap Joints Research Articles

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.

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.

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.

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.

A novel approach for damage assessment in adhesively bonded composite joints using backface strain technique

In this study, the backface strain (BFS) is measured by both digital image correlation (DIC) and fiber optic sensors (FOS) to detect the crack initiation and propagation in adhesively bonded composite single-lap joints (SLJ). BFS measures the resultant strain deriving from the positive strain, due to tensile load, and negative strain related to the bending moment. A point, called zero-strain point (ZSP), can be detected on the substrate surface of SLJ due to the concurrent effect of these positive and negative strains. The experimental activity shows that the value of the ZSP changes when the crack starts to propagate. Thus, this point can be used to monitor the service conditions of adhesive joints. The effect of joint dimensions on the position of the ZSP is investigated when the joint is subjected to quasi-static loading. In addition, the applicability of the method is investigated under a cyclic loading condition. The work shows that the ZSP can be used as an index to monitor joint healthiness. Furthermore, FOSs can be used for an in-situ monitoring of the joint.

Investigating fabric interlayer effects on tensile loading limits of adhesively bonded single-lap composite joints

This paper investigates enhancing the effectiveness of glass fiber/epoxy composite single-lap bonded joints by using fabric interlayers between the adherends. It was aimed to evaluate the changes in the joint tensile strength depending on the parameters such as different interlayer fabric types (S2-glass fabric and Kevlar fabric), numbers of interlayers (0, 1, 2, and 3), clamping pressure (0, 4, and 6 MPa), and temperature (20, 55, and 80°C) applied to the joint region throughout the curing period. Significant enhancements in peak tensile forces were observed by varying these parameters. The most substantial increase in tensile properties was achieved for the joint with two-layer S2 fabric fabricated at 4 MPa pressure and 55°C curing temperature, denoted as “2L-S2-P4-T55.” Compared to a non-layered joint, those with 1, 2, and 3 S2 fabric interlayers exhibited 35.5%, 39.6%, and 45.2% increases in tensile peak force, respectively. Increasing bonding pressure from 0 MPa to 4 MPa resulted in a 5.2% tensile failure load increase for double S2 fabric interlayered joints cured at 20°C, but a 6.7% decrease at 6 MPa. Using one-layer Kevlar fabric instead of one-layer S2 fabric caused a 15.2% drop in tensile peak force, still 14.9% higher than the non-layered joint.

Shear properties and failure mechanisms of adhesive bonded composite joint: Effect of glass particle reinforcement

AbstractHigh‐performance composite structures and effective joining processes are important for shipbuilding, and aerospace sectors to perform better in challenging situations. The importance of this study is to analyze the effect of glass powder‐modified adhesive and joining techniques such as co‐curing (CC), co‐bonding (CB), and secondary bonding (SB) on flexural, shear, and free vibrational characteristics such as natural frequency and damping factor for glass fiber‐reinforced polymer (GFRP) composites. Results revealed that, compared to SB and CB composite joints, 1 wt.% glass powder‐modified CC composite single lap joints (SLJ) enhanced the shear properties by 103.04% and 111.74%, respectively, due to rougher surface formation in the adhesive layer. As a result, it enhanced the fiber's adhesion to the matrix. On the other hand, CC bonded joints with 1 wt.% of glass powder exhibited the highest flexural strength (85.99 Mpa). The experimental free vibrational analysis showed that 1 wt.% glass powder CC composite had a high natural frequency, whereas higher wt.% glass powder increased the damping properties due to the matrix and fiber interaction. The current research reveals that CC technique with 1 wt.% of glass powder provided a significant improvement to the adhesively bonded composite's mechanical and free vibrational characteristics.Highlights Four distinct wt.% of glass powder were used to prepare the glass powder‐modified adhesive by sonication. SLJ glass fiber‐reinforced polymer composite joints were fabricated through CC, CB, and SB techniques Maximum shear and flexural strength were found at CC SLJ 1 wt.% glass powder. CC SLJ 1 wt.% glass powder enhanced better vibrational behavior. Failure surface analysis was conducted using FE‐SEM images.

Influence of Failure-Load Prediction in Composite Single-Lap Joints with Brittle and Ductile Adhesives Using Different Progressive-Damage Techniques.

The usage of adhesively bonded joints, such as single-lap and double-lap joints, is increasing rapidly in aerospace composite structures as a popular alternative to bolts and rivets. Compared to the conventional joining methods such as fastening and riveting, adhesive-bonding technology better prevents damage to composite structures due to the smooth configuration and the mitigation of stress concentration around holes. In this work, the built-in progressive-damage-modeling techniques in Abaqus, including the cohesive zone model (CZM) and the virtual crack closure technique (VCCT), are used to predict the strength and progressive failure of composite single-lap joints subjected to tensile loading. Modeling of an adhesive layer by using a zero/non-zero-thickness cohesive element, cohesive surface, and VCCT is investigated, as is the effect of brittle and ductile adhesives. Two-dimensional finite-element models with different damage-modeling strategies are performed in this study. The failure-load predictions are compared with the experimental results obtained from the literature. For the ductile adhesive, the predicted failure loads using a zero/non-zero-thickness cohesive elements and a cohesive surface are all shown to be in good agreement with the experiments. However, the VCCT technique predicts higher failure loads. For a brittle adhesive, on the other hand, the predictions by zero/non-zero-thickness cohesive elements and cohesive surfaces reveal notable deviations compared to the experimental results. In contrast to the ductile adhesive, the VCCT technique is revealed to be accurate in predicting the brittle adhesive.

Enhancing shear strength in 3D printed single lap composite joints: A multi‐faceted exploration of GNP integration, print orientation, utilizing artificial neural networks, and dynamic analysis

AbstractThe state‐of‐the‐art manufacturing process known as additive manufacturing (AM) employs components that may be processed by AM, including ceramics, polymeric materials, metallic substances, titanium, metallic substances, and composites, to produce parts with intricate designs and exact properties. Fused deposition modeling (FDM) is a rapidly growing 3D printing technique. However, most FDM systems only support polylactic acid (PLA) or acrylonitrile butadiene styrene (ABS) as a printing medium. The impact of print orientations and graphene nanoparticles upon the tensile and shear properties of PLA single‐lap joint samples created by FDM has been investigated in this work. According to experimentation, the 0° orientation has the highest load‐bearing capacity and shear strength compared to 45° and 90°. Also, addition of GNP to epoxy adhesive improved greatly, with 0.25 to 1.00 weight percentages of 20.94, 12.34, 38.98, and 31.11%, respectively. FESEM has been used to analyze the failure criteria. The free vibrational analysis confirmed that the 3DS6 sample had the highest natural frequency (598.7 Hz) compared to all other samples. The artificial neural network (ANN) approach accurately predicted the failure load. The overall =0.8471 achieved is below the permissible margin of error, indicating that both the outcomes are reliable and in satisfactory agreement.

Enhancing shear and flexural strength of single lap composite joints with a graphene nanoparticle‐reinforced adhesive through a co‐curing technique

AbstractAdhesive‐bonded joints in glass fiber‐reinforced polymer (GFRP) composite structures give numerous benefits, such as greater flexibility, good specific strength, improved fatigue resistance, and the ability to reduce stress concentration. Consequently, adhesive‐bonded joints are considered more influential if compared to fasteners. This research investigates the effects of incorporating graphene nanoparticles (GNPs) into the adhesive layer on several aspects of adhesively bonded co‐cure (CC) single‐lap GFRP composite joints. These aspects include shear strength, flexural strength, failure mechanisms, and fractography studies. The study examined six distinct weight percentages of GNP, specifically 0.25, 0.50, 0.75, 1.0, 1.25, and 1.5 wt.%, which are considered to enhance the interfacial strength of adhesive joints. Based on the research, 0.75 wt.% of GNP single‐lap joint (SLJ) shear strength improved by 69.40% when compared to the plain matrix SLJ. This suggests excellent load transmission between adherends. In addition, the flexural strength increased by 46.57% in the specimens containing 1.25 wt.% GNP when compared to the plain matrix used to bond in SLJ. After testing, fractured specimen microscopic images were used to examine failure mechanisms. According to fractography studies, adding GNPs to the adhesive causes surfaces to become rougher, which improves the adhesion between the adhesive and the adherends.Highlights Six distinct wt.% of GNPs were used to prepare the matrix by sonication. Single‐lap GFRP composite joints were fabricated through a CC technique. SLJ's maximum shear strength was found at 0.75 wt% of GNP. SLJ's maximum flexural strength was found at 1.25 wt% of GNP. Failure analysis was conducted using microscopes and FESEM images.

Enhancing the strength of adhesively bonded single-lap composite joints using fiber fabrics

The present study focused on enhancing the strength of adhesively bonded single-lap composite joints (ABSLCJ) using different fiber fabrics such as unidirectional and bidirectional glass-fiber, and bidirectional carbon-fiber, under tensile, shear, flexural, and fixed-free vibration. The impregnation of the carbon-fiber fabric in adhesive increases the strength of the joint. In comparison to only adhesive, the adhesive joint carbon-fiber fabric improved the tensile, flexural and stiffness by 23.93 %, 31 %, and 19.6 %.

Thermal and mechanical damage to carbon fibre reinforced composites with metallic fasteners under lightning strike

In this study, lightning strike (LS) damage to carbon fibre reinforced composites (CFRCs) with metallic fasteners was investigated by simulated LS tests and finite element (FE) model. First, the LS damage mode and mechanism of a single-lap composite joint and a CFRC laminate with a metallic rivet were studied. Resin pyrolysis was found on the surface but severe delamination was evident inside the composites due to a gas explosion caused by the sudden burn-off of polymer matrices and thermal expansion inside the laminate. Then, the effect of rivet spacing on the LS damage to CFRCs was examined. The results showed that the damaged area was significantly reduced because the surrounding rivets effectively suppressed impact-induced delamination and consumed more electrical energy. The results also showed that large-area delamination across the rivets was achieved due to high potential at the surrounding rivets when the spacing was short but dynamic deflection was confined by the surrounding rivets. In contrast, delamination was only concentrated at the central rivet when the spacing was large, which can be ascribed to fewer currents flowing into the surrounding rivets, and the confinement of the adjacent rivets was less significant.

Nonlinear CZM-based predictive analysis of damage in single-lap composite joints: An exploration of the FGM concept in Fiber–Matrix coupling

This study introduces an innovative numerical approach, validated through experiments, to incorporate functional graded materials (FGMs) volume fractions into mixing laws for fiber–matrix unidirectional composites. Nonlinear techniques address adhesive ductility, with the cohesive zone modelling (CZM) model integrated into finite element analysis using the ABAQUS code. The CZM model encompasses shear and detachment modes, characterized by adhesive stress increase in a trapezoidal shape. Proposed strategies introduce layer-specific fiber presence; concepts like C-1 exhibit denser fibers toward the middle, while C-2 and C-3 position them nearer to the plate edge. Stiffness from attached plates significantly governs overall responses, particularly under mixed loading conditions. The results have shown that the proposed grading concept C-2 provides an over-resistance capacity compared to other concepts. Additionally, using a special mesh in the composite grading, the results indicate that the adhesive formulation used in this analysis and the approach followed are primarily based on experimental results, demonstrating good agreement with the numerical model employed.

Effect of bonding area geometry on the behavior of composite single lap joints (SLJ) and estimation of adhesive properties using finite element method

ABSTRACT In this study, the mechanical behavior of Single Lap Joints (SLJ) subjected to tensile loading was investigated both experimentally and numerically by considering different SLJ sizes including adherend thickness (T:0.88, 1.76, 3.52 mm), joint width (W:10, 20, 30 mm), and overlap length (L:10, 20 mm). A polyurethane adhesive and carbon fiber composite adherends were used for the experimental activity. The experimental campaign was carried out to assess the effects of the SLJ geometry on the mechanical behavior of SLJ. Further, SLJ tests were used to estimate the fracture toughness in mode I and II by using Finite Element methods (FEM) coupled with optimization analysis. The results showed that all three parameters strongly change the load capacity of the joints. According to the Experiments, for every sample configuration, the higher the adherend thickness the higher the adhesive shear and the lower the substrate normal stresses. Moreover, the width showed negligible effect on adhesive shear and substrate normal stresses. Numerically, the effect of geometric parameters has been analyzed once at relative 25% of ultimate load and once at a fixed load for each sample. At 25% of ultimate load, it was observed that the increase in the joint width has nearly no significant effect on adhesive shear and peel stresses. However, at a fixed common load increasing L, W, and T resulted in a decrease in adhesive shear and peel stresses. A good agreement was found between the experimental and numerical results.

Experimental Study on the Effect of Bonding Area Dimensions on the Mechanical Behavior of Composite Single-Lap Joint with Epoxy and Polyurethane Adhesives

The effects of joint geometry parameters, such as adherend thickness (1.76, 3.52 mm), joint width (10, 20, 30 mm), and overlap length (10, 20 mm), on the behavior of single-lap joints (SLJs) under tensile loading are investigated in this study. Peak force, joint stiffness, shear stress, and normal stress are the investigated properties. SLJs are manufactured with carbon fiber composite adherends and two different types of adhesives, polyurethane and epoxy, which present a flexible and rigid mechanical response. The results showed that increasing all 3 geometric parameters (L, W, T) leads to a significant increase in the load capacity of polyurethane joints (on average, 88.4, 101.5, and 16.9%, respectively). For epoxy joints, these increases were 47.7, 100, and 46%, respectively. According to these results, W is the parameter with the most influence on the load capacity of the joints. However, it was observed that an increase in joint width has no significant effect on adhesive shear and a substrate’s normal stresses. Epoxy SLJs behave approximately elastically until failure, while polyurethane SLJ load-displacement curves include an initial linear elastic part followed by a more ductile behavior before the failure. Joint stiffness is affected by all the parameters for both adhesive types, except for overlap length, which led to a negligible effect on epoxy joints. Moreover, the damage surfaces for both types of joints are analyzed and the internal stresses (shear and peel) are assessed by using the analytical model of Bigwood and Crocombe.

Numerical approach to disbonds in bonded composite Single Lap Joints: Comparison between Carrera Unified Formulation and classical Finite Element modeling

The scope of this study is the monitoring of the adhesion integrity of composite Single Lap Joints (SLJs) through the assessment of the MUL2 CODE, software developed by the MUL2 Research Group - Department of Mechanical and Aerospace Engineering of Politecnico di Torino. The MUL2 CODE is implemented through the Carrera Unified Formulation (CUF) for 2D structures based on Hierarchical Legendre Expansion (HLE) polynomials. An efficient method for the Structural Health Monitoring (SHM) of bonded composite joints is experimentally verified and simulated by commercial FE software and by the CUF approach, in order to reduce the computational cost of analyses: using transient excitations via toneburst signals, the structural health of damaged SLJ is analytically, experimentally and numerically evaluated. The interaction mechanism between the waves traveling through the investigated specimens is numerically simulated with a simple Finite Elements (FE) model and it is solved via MUL2 CODE and commercial software Ansys Workbench (with 2D and 3D models), respectively. Experimental campaigns data are compared with analytical, CUF and Ansys results demonstrating the consistence of the MUL2 formulation that is computationally simpler, but very efficient for the joint analysis. The presented and discussed CUF application is able to quantify with a high accuracy the debonding extension in the damaged SLJ, simply tuning the excitation frequency of the SHM technique.

Finite Element Analysis of Low-Speed Oblique Impact Behavior of Adhesively Bonded Composite Single-Lap Joints

Finite Element Analysis of Low-Speed Oblique Impact Behavior of Adhesively Bonded Composite Single-Lap Joints

Effects of adherend notching on the bonding performance of composite single-lap joints

The use of adhesively bonded joint is a mainstream and effective approach for solving connection problems of composite materials. Single-lap joint (SLJ) is one of the most frequently used joint types in the industry due to its simple geometry and easy applicability. The main factors in designing SLJ are the notch geometrical configurations (depths and lengths), the bond overlap lengths, and the types of adhesives. However, the influence of the notch geometrical configurations (depths and lengths) on joint bonding performances under different adhesive types (brittle and ductile adhesives) is still not fully understood. In this study, both the un-notched and notched 2D finite element models of SLJs have been developed and verified by comparison with literature-reported data and self-performed experiments, respectively. Subsequently, this validated 2D modeling approach is used to evaluate the influence of notch geometrical configurations and overlap lengths on the SLJs bonding performances in terms of failure loads, peel stresses and shear stresses distributions under two different brittle and ductile adhesives. In addition, a three-point bending resistance numerical analysis of SLJs has been carried out to investigate their flexural properties. The research conclusions will serve as a guide for the SLJs to choose proper adhesive types and notching configurations to increase joint strength and decrease stress concentrations.

An experimental investigation on press joining mechanism and bearing behavior of single-lap composite riveted joints

This paper reports an experimental study on the forming mechanism and bearing behavior of single-lap composite riveted joints with different hole diameters (4.82 mm and 4.9 mm) and squeeze forces (15 KN, 18 KN, 20 KN, and 23 KN). Pressure riveting and tensile tests are conducted to composite riveted joints to obtain the joint interference, joint stiffness and bearing strength needed to quantitatively evaluate their deformation features and mechanical properties. Furthermore, microscopic studies on damaged areas using applying ultra-depth three-dimensional microscope to monitor damage evolution are conducted to comprehend the joint shear failure mechanism. Results show that the hole diameter and riveting force have a significant effect on the performance of the joint. The forming damage occurs mainly in the hole wall near the driven head and is dominated by fiber crushing and interfacial shear cracks. The radial expansion of the rivet shank is not uniform under the riveting force and the whole is drum shaped. But the hole expansion of the joint at 4.82 mm hole diameter is more uniform than 4.9 mm, which would lead to better joint stiffness and bearing strength. Moreover, high interference fit would cause the shear fracture of the rivet, while lower interference joints are dominated by compression-pull mixed failure. In general, the size of interference fit is an unneglectable factor in the composite riveted joints that should be thoroughly investigated.

Full-field through-the-thickness strain distribution study of hybrid multi-bolted/bonded single-lap composite joints using digital image correlation

This study proposes a digital image correlation (DIC) technique for through-the-thickness deformations of multi-bolted single-lap (SL) shear hybrid bolted/bonded joints made of carbon/epoxy laminates. Through-the-thickness peel strains are obtained throughout the overlap region during the entire tensile test. It is observed that real-time monitoring of the strain in the overlap region informs on the location of the failure before the load reaches 50% of its ultimate value. The peel strain-load curves for both the overlap ends superimpose perfectly up to about 40% − 50% of the applied tensile load and diverge considerably beyond this level. Similar behavior occurs for shear strain and is confirmed for quasi-isotropic (QI) and cross-ply (CP) laminate stacking sequences. It is also found that peel and shear strains, as well as joint rotation in CP, are higher than those of QI. These results can be useful for the design of this type of joint.