Enhanced Mode I Fracture Toughness in Composite Joints: Synergistic Effects of CNT-Reinforced Epoxy Adhesives

Enhanced Mode I Fracture Toughness in Composite Joints: Synergistic Effects of CNT-Reinforced Epoxy Adhesives | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Enhanced Mode I Fracture Toughness in Composite Joints: Synergistic Effects of CNT-Reinforced Epoxy Adhesives Sabrina Khammassi, Fouad Erchiqui, Mostapha Tarfaoui This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5773014/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract High-performance adhesives are essential for creating durable, long-lasting bonds in various applications. This study explores the novel use of carbon nanotubes (CNTs) to enhance the fracture toughness of polymer adhesives, explicitly focusing on their effect on the Mode I fracture toughness of glass fibre-reinforced composite bonded joints. Unlike traditional research, this work investigates CNT-reinforced diglycidyl ether of bisphenol A (DGEBA) epoxy adhesives applied to double cantilever beam (DCB) joints. CNTs were incorporated at varying mass fractions (1, 2 and 5 wt.%), and the findings demonstrate that 1 wt.% CNTs yielded the most significant improvement, doubling fracture toughness compared to unreinforced adhesive (NE) due to optimal dispersion and strong matrix interaction. Numerical simulations supported the experimental results, confirming the effectiveness of CNTs in modifying cohesive and interfacial failure mechanisms. This study highlights a promising and less-explored avenue for developing advanced composite adhesives with superior mechanical performance. Bonded Joint Adhesion Carbon Nanotubes Composite Mode I Fracture Cohesive interface modelling Figures Figure 1 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 1. Introduction In modern engineering, composite materials have become indispensable across various industries, including aerospace, automotive, marine, and sports applications [1]. Their lightweight nature, combined with exceptional mechanical properties, makes them ideal for high-performance applications [2], [3] [4]. However, assembling composite structures often necessitates efficient joining techniques. While traditional mechanical fasteners remain widely used, adhesive bonding has emerged as a preferred alternative, offering advantages such as uniform stress distribution, corrosion resistance, ease of application, and cost-effectiveness [1], [5] [6][7][8][9]. Epoxy-based adhesives are favoured among adhesive systems for their superior bonding strength and durability. However, their inherent brittleness and limited toughness present significant challenges, especially in critical structural applications requiring high fracture resistance [10]. To address these limitations, researchers have explored various strategies to enhance the toughness of epoxy adhesives. The incorporation of nanomaterials as reinforcements has shown significant promise [11][12], [13], [14], [15], [16]. Organic and inorganic nanomaterials, such as short fibres, micro- and nanoparticles, and carbon-based nanofillers, have been extensively studied [17], [18], [19], [20], [21]. Carbon nanotubes (CNTs) [22] and graphene nanoplatelets (GNPs) [22] are particularly noteworthy due to their exceptional mechanical, thermal, and electrical properties. Toughening mechanisms associated with these nanofillers include crack-pinning [23], crack bifurcation [23], particle bridging [24], and pull-out effects [25]. Numerous studies have demonstrated the potential of CNTs to improve the performance of epoxy adhesives, even at low concentrations [26]. For example, Yu et al. [27] reported significant increases in bonding strength using the Boeing wedge test, while Sydlik et al. [28] observed lap shear strength improvements of 36% and 27% for functionalised and unfunctionalised CNT-reinforced epoxy adhesives, respectively. Similarly, Panta et al. [29] found a 53% increase in lap shear strength with 1 wt.% CNT incorporation, and Ruchi et al. [17] documented a 48.2% enhancement in shear strength at 1 wt.% CNT loading. These enhancements are attributed to the unique atomic structure, high aspect ratio, and interfacial solid interactions of CNTs with the epoxy matrix. However, most of these studies have focused on enhancing bulk mechanical properties, such as lap shear strength, while leaving the fracture behaviour of adhesively bonded composite joints, particularly under Mode I loading, insufficiently explored. Additionally, while CNT-reinforced diglycidyl ether of bisphenol A (DGEBA) epoxy adhesives have been studied, there has been limited investigation into the effects of CNT dispersion, matrix ductility, and the distinction between interfacial and cohesive failure mechanisms in bonded composite joints. The novelty of this study lies in its comprehensive investigation of the effects of CNT mass fraction on the Mode I fracture toughness of composite bonded joints. Using DGEBA epoxy adhesive, this work employs double cantilever beam (DCB) testing to analyse interlaminar fracture toughness in glass fibre-reinforced composite joints. Unlike prior research focusing primarily on bulk adhesive properties or lap shear strength, this study delves into crack propagation mechanisms. It examines the transition from brittle to ductile failure modes as a function of CNT concentration. Integrating experimental findings with advanced numerical simulations provides deeper insights into CNT-reinforced composite joints' stress distribution and failure behaviour. This research advances the application of CNT-reinforced adhesives [14], [19], [20], [28], [30], [31], [32], [33] by offering a unique approach to optimising the performance of composite bonded joints under critical loading conditions. Bridging the gap between experimental data and numerical modelling lays the groundwork for future innovations in adhesive bonding technologies, targeting high-performance structural applications in demanding environments. 2. Experimental investigation 2.1. Materials This study used a diglycidyl ether of bisphenol A (DGEBA) epoxy adhesive, supplied by MOMENTIVE, as the matrix material. The curing agent, Jeffamine D-230, was also procured from MOMENTIVE. Multiwalled carbon nanotubes (CNTs) were sourced from Nanocyl SA. For the substrates, glass fibre-reinforced composite (VETRONITE) plates with dimensions of 250 mm × 20 mm × 4 mm were obtained from Von Roll Group. Acetone was used as a cleaning agent to thoroughly prepare the composite substrate surfaces and ensure optimal adhesion by removing contaminants such as grease and dust. 2.2. Preparation of CNT-Reinforced Epoxy Adhesive A high-shear planetary mixer was employed to achieve a homogeneous dispersion of CNTs within the epoxy matrix, as shown in Fig. 1 . The CNTs were mixed with the DGEBA epoxy resin at a rotational speed of 1400 rpm for 60 seconds. This high-shear process minimises the risk of CNT agglomeration by generating sufficient shear forces between the nanofillers [34]. After dispersing the CNTs, the curing agent was added to the epoxy resin in a recommended volume ratio 100:32 (epoxy: curing agent). The mixture was stirred gently to avoid introducing air bubbles, ensuring uniformity across the adhesive samples. 2.3. Sample Preparation Glass fibre-reinforced composite substrates were precisely cut to the required dimensions of 250 mm × 20 mm × 4 mm, Fig. 2 . The surfaces of the composite substrates were cleaned with acetone to ensure proper bonding. The CNT-reinforced epoxy adhesive was then carefully applied to the bonding area of the composite substrates. Figure 2 shows the characteristic steps of the composite adherent surface treatment where: Degreasing with IPA. Polishing with P120 sandpaper. Use of IPA for degreasing. Finally, composite materials' bonding joints are carefully prepared, as presented in Fig. 3 . The main steps of this preparation are as follows: Positioning PTFE tape (0.25µm thick gasket). Inserting the board into the mold. Applying adhesive on the surface with tape (unreinforced and reinforced). Double-adhesive. Buttoning the opposite side. Pressing at least 20kN pressure for 16 hours. Completing the polymerisation reaction in an oven at 120°C for 1 hour. After assembling the samples, they were cured at room temperature under slight pressure for 24 hours, followed by a post-curing cycle at 80°C for 2 hours, per the manufacturer's recommendations. A steel plaque was bonded to each specimen's ends to enhance the cantilever arms' flexural rigidity. The final dimensions and setup of the Double Cantilever Beam (DCB) specimens are shown in Fig. 4 . 2.4. Characterization The Mode I interlaminar fracture toughness (G Ic ) of the bonded composite joints was determined using the Double Cantilever Beam (DCB) method following ASTM D5528 standards. Each DCB specimen was prepared with two edges coated using white correction fluid and marked with black reference lines to enable optical tracking of crack propagation. A high-speed camera system was employed to monitor and record the crack propagation during the tests. The mechanical tests were conducted on an Instron Universal Testing Machine with a constant crosshead displacement rate of 1 mm/min, Fig. 5 . Three to five replicate tests were performed for each adhesive formulation to ensure reproducibility of the results. 2.5. Data Analysis The Mode I fracture energy, G Ic was calculated using the following formula: $$\:{G}_{I}=\frac{3P\delta\:}{2ba}$$ Where: P: Load applied to induce a displacement δ: Measured displacement, b: Specimen width, a: Crack length. This methodology provided an accurate evaluation of the fracture toughness of the composite bonded joints, enabling a comparison of the performance of the CNT-reinforced adhesives under Mode I loading conditions. 3. Results and discussion 3.1. Reproducibility Before proceeding with the analysis of the experimental results, it is essential to highlight the reproducibility of the tests, as demonstrated by the load-displacement curves obtained for the three specimens (#1-ne, #2-ne, and #3-ne), Fig. 6 . These curves represent a typical example of bonded joint specimens fabricated with neat epoxy as the adhesive. Reproducibility is a critical factor in experimental studies, as it ensures the reliability of the data and supports the validity of the conclusions regarding the mechanical behavior of the adhesive joints. The curves exhibit consistent trends across all specimens, characterized by distinct load peaks followed by abrupt drops and stepped progressions corresponding to successive stages of crack propagation. While minor variations are observed, particularly in the maximum loads sustained (e.g., a slightly higher peak for specimen #2-ne compared to #1-ne and #3-ne), these differences are within an acceptable range for bonded joint testing. Such variations can arise from inherent microscopic differences in the neat epoxy adhesive layer or subtle inconsistencies in the preparation or alignment of the bonded joints. This level of consistency reflects a well-controlled experimental setup, ensuring that the observed responses are primarily due to the mechanical properties of the neat epoxy adhesive and not influenced by external experimental factors. The reproducibility of the results validates the methodology and provides a robust basis for further analysis of the fracture behavior and mechanical performance of bonded joints under various loading conditions 3.2. Experimental results The DCB tests were conducted to evaluate the load-displacement (L-D) curves (Fig. 7a) and the R-curves (crack length increment) for both unreinforced (reference) and CNT-reinforced adhesive bonded joints. The L-D curves exhibit a moderate nonlinear zone near the peak force, indicating significant crack extension as the external force decreases abruptly. This behaviour suggests that the adhesive joint undergoes an initial brittle fracture before transitioning to a more stable propagation phase as the displacement increases. Once the critical fracture energy release rate (G IC ) is reached, the specimen continues to experience crack propagation, and the process is repeated until the bonded surfaces are separated. In the case of the reference specimen, significant and unstable load fluctuations were observed, indicative of a brittle failure mode with insufficient crack bridging. The long crack jumps observed in the load-displacement curve confirm the fragility of the adhesive joint. The maximum sustained load for the reference joint was approximately 97 N, which reflects the low fracture resistance of the unreinforced epoxy adhesive. In contrast, the CNT/DGEBA adhesive bonded joints showed a markedly different behaviour, requiring a substantially higher load for crack initiation and propagation. For instance, 1 wt.% CNT/DGEBA adhesive exhibited a maximum load of 237 N, representing a 144% increase compared to the reference adhesive. This significant increase is attributed to the reinforcement provided by CNTs, which enhance the adhesive's tensile strength by improving interfacial interactions between the adhesive and the composite substrate. However, the 2 wt.% CNT/DGEBA adhesive exhibited a decrease in maximum load to 128 N, which is 31% greater than the reference adhesive. This reduction in performance can be attributed to the onset of CNT agglomeration at higher loading levels, which impairs the dispersion and load transfer efficiency within the adhesive matrix. Interestingly, the L-D curves for CNT-reinforced adhesives became smoother as the CNT mass fraction increased, and the crack jumps were more gradual and stable. This trend reflects the transition from a brittle failure mode to a more ductile one. CNTs enhance crack bridging, bifurcation, and energy dissipation during fracture propagation, improving tensile strength at 1 wt.% CNT is due to the high aspect ratio of CNTs, which facilitates better interaction with the epoxy matrix and prevents premature crack initiation. However, the 2 wt.% CNT addition led to a reduction in the adhesive's strength compared to the 1 wt.% CNT/DGEBA adhesive. This decrease can be attributed to CNT agglomeration, which restricts the load transfer efficiency and introduces localised stress concentrations that promote crack initiation and accelerate crack propagation. These findings suggest an optimal CNT concentration (1 wt.%) maximises the adhesive's fracture toughness and tensile strength. Figure 7b presents the R-curves and the average critical strain energy release rate ( GIC ) as a function of crack growth (from 43 mm to 60 mm), providing further insights into the fracture mechanics of the bonded joints. The first point on the R-curve represents the fracture initiation, while subsequent points illustrate the progression of crack propagation. Incorporating CNT s into the DGEBA adhesive significantly improved the propagation energy (as indicated by the R-curve), particularly for the 1 wt.% CNT/DGEBA formulation, which demonstrated enhanced fracture toughness. This result suggests that CNTs are critical in reducing crack propagation rates by acting as a barrier to crack growth and promoting energy dissipation during fracture. Figure 8 illustrates the crack propagation stage of the Mode I fracture toughness test on the composite samples, showing that adding CNTs effectively mitigates crack propagation through improved interfacial adhesion and energy absorption. The average G IC values for both the initiation and propagation of the bonded joints are presented in Fig. 9 and Table 1 . The 1 wt.% CNT/DGEBA adhesive demonstrated the most significant improvement in fracture toughness, with GIC initiation increasing from 155 kJ/m ² (for the reference adhesive) to 354 kJ/m² and GIC propagation increasing from 330 kJ/m² to 768 kJ/m². These enhancements indicate that the presence of CNTs significantly increased the fracture energy absorption capacity of the adhesive. On the other hand, for 2 wt.% and 4 wt.% CNT concentrations, a moderate decrease in both initiation and propagation G IC values was observed, suggesting that excessive CNT loading leads to the agglomeration of CNTs within the adhesive matrix, as mentioned in Fig. 10. This agglomeration results in morphological heterogeneity, which creates regions of high-stress concentration and reduces the adhesive's overall performance. As the CNT concentration increases beyond the optimal value, these agglomerates interfere with the load transfer and crack resistance mechanisms, ultimately diminishing the fracture toughness of the adhesive. This study confirms that the 1 wt.% CNT/DGEBA adhesive produces the best Mode I fracture toughness performance, nearly doubling the GIC values of the reference DGEBA adhesive, underscoring CNT reinforcement's efficacy in enhancing adhesive bonding in composite materials. Table 1 Fracture energy progress of the unreinforced and reinforced joints Composite Bonded Joint 0 wt.%CNT 1wt.%CNT 2 wt.%CNT 5 wt.%CNT G Ii (kJ/m 2 ) Average 155 354 336 302 Progress 1 128% 117% 94% G Ic (kJ/m 2 ) Average 330 768 613 376 Progress 1 133% 85% 14% 3.3. Fracture surface morphology The fracture surfaces of the specimens obtained from the Mode I tests, both with and without CNT nanofillers, were analysed to investigate the toughening mechanisms further, as shown in Fig. 11. The morphology of the fracture surfaces revealed significant differences between the unreinforced and CNT-reinforced adhesive joints, providing valuable insights into the adhesive interfaces' fracture behaviour and failure mechanisms. The fracture surface exhibited a relatively smooth, uniform morphology for the unreinforced adhesive-bonded joint, characteristic of brittle failure. The absence of significant surface features suggests that the fracture propagation was rapid, with minimal energy dissipation and limited crack deflection. This type of failure is typical of materials with low ductility, where cracks propagate quickly without significant resistance or diversion. The smooth fracture surface confirms the inherent fragility of the unmodified epoxy adhesive. In contrast, the fracture surfaces of the CNT-reinforced adhesive bonded joints were significantly rougher and exhibited more irregularities (Figs. 11b, 11c, and 11d). Carbon nanotubes (CNTs) fundamentally altered the failure mode, promoting a ductile fracture mechanism. The increased surface roughness, coupled with the formation of wrinkles and ripples on the fracture surface of the 1 wt.% CNT/DGEBA adhesive indicates a more resilient bonding interface. This suggests that the CNTs were crucial in improving the adhesive's toughness, likely through mechanisms such as crack deflection, bridging, and energy dissipation. These mechanisms increase the fracture resistance by obstructing crack propagation and increasing the energy required for failure. At 2 wt.% CNT/DGEBA, the fracture surface showed a reduction in wrinkles compared to the 1 wt.% CNT/DGEBA joint, suggesting a decline in adhesive performance and the beginning of failure due to CNT agglomeration. The less pronounced surface roughness at this higher concentration points to the initiation of interfacial degradation, likely due to the formation of CNT clusters within the adhesive matrix, which disrupts the uniform distribution of nanofillers. This agglomeration diminishes the adhesive's ability to effectively transfer stress, leading to early crack initiation and reduced fracture toughness. At 5 wt.% CNT/DGEBA, the fracture surface exhibited predominantly interfacial failure, with significant porosity at the adhesive interface, as detailed in the SEM images (Fig. 12 ). This observation suggests a severe degradation of the adhesive properties due to increased viscosity and micro defects expansion in the adhesive layer. The excessive concentration of CNTs likely impaired the dispersal of the nanofillers within the adhesive, leading to morphological heterogeneity in the matrix. This non-uniform filler distribution can cause localised stress concentrations, promoting premature failure at the adhesive interface and reducing the bonded joint's mechanical integrity. In summary, incorporating CNTs improved the adhesive's fracture surface morphology and overall toughness, with the optimal performance observed at 1 wt.% CNTs. At higher CNT concentrations, however, the agglomeration of CNTs undermined the adhesive's performance by introducing morphological defects and stress concentrations, decreasing bond durability. These results highlight the critical importance of precise nanofiller dispersion in achieving the desired balance between enhanced toughness and maintaining adhesive integrity. 3.4. Numerical analysis Cohesive Zone Models (CZMs) were employed in ABAQUS software to model the debonding behaviour of the adhesive joint with high precision. As shown in Fig. 13 , the numerical model consisted of eight-node linear brick elements for the substrate and adhesive layers (C3D8R), with reduced integration and hourglass control, ensuring accurate simulation of the mechanical response. The adhesive bond was modelled using three-dimensional cohesive elements (COH3D8) to represent the interface between the adhesive and composite adherents. This approach captures the traction-separation law that governs the failure behaviour of both self-adhesive and CNT-reinforced adhesives. The traction-separation law models the relationship between the normal and tangential stresses and the relative displacements at the interface, which is essential for accurately predicting debonding and failure in adhesive joints. The parameters of the cohesive elements were defined based on the material properties of the adhesives, which are provided in Table 2 and Table 3 for the unreinforced and CNT-reinforced adhesives, respectively. The numerical displacement and experimental load-displacement (L-D) curves for both unreinforced and CNT-reinforced composite bonded joints are presented in Fig. 14 . The simulation results showed excellent agreement with the experimental data by comparing the initial slope and the peak load values of the numerical load-displacement curves with those obtained experimentally. The close match between the numerical and experimental results confirms the accuracy of the numerical model. It further validates the cohesive zone approach in capturing the failure mechanisms in adhesive joints. Furthermore, since the experimental Mode I fracture toughness (G IC ) values were directly integrated into the simulation, the good correlation between the numerical and experimental results underscores the effectiveness of the interface toughening technique involving CNT-reinforced adhesives. The results highlight the enhanced adhesive interface toughness, which is crucial for improving the structural integrity of composite bonded joints. The simulation results reveal detailed insights into how the adhesive layer interacts with the overall structure under tensile loading, especially regarding stress concentration and singularity at the adhesive interface. The numerical analysis, therefore, not only corroborates the experimental findings but also provides a comprehensive understanding of the stress distribution and failure behaviour in reinforced adhesive joints under Mode I loading. Table 2 Composite parameters outcome of DIGIMAT simulation \(\:{E}_{11}={E}_{22}\) (MPa) E 33 (MPa) \(\:{v}_{12}={v}_{21}\) \(\:{v}_{13}={v}_{23}\) \(\:{v}_{31}={v}_{32}\) G 12 (MPa) \(\:{G}_{13}={G}_{23}\) (MPa) \(\:\rho\:\) (kg.m 3) 16022 11920 0.46 0.22 0.16 10328 3752 2080 Table 3 CNTs/DGEBA properties \(\:E\) (MPa) \(\:v\) G (MPa) \(\:\rho\:\:\) (kg.m 3) 1wt.%CNT/ DGEBA 17478 0.26 6920 1509 2wt.%CNT/ DGEBA 12772 0.26 5037 1472 5wt.%CNT/ DGEBA 7264 0.28 2836 1607 4. Conclusion This study provides valuable insights into the role of carbon nanotubes (CNTs) in enhancing the Mode I fracture toughness of DGEBA adhesive bonded composite joints. The results demonstrate that incorporating CNTs significantly improves the fracture resistance at the adhesive-adherent interface, with the most notable enhancement observed at a 1 wt.% CNT concentration. At this level, CNTs effectively reinforce the adhesive, leading to a marked increase in toughness without introducing detrimental effects such as agglomeration or excessive porosity. However, when the CNT concentration exceeds this optimal threshold, poor dispersion and the formation of CNT clusters cause localised stress concentrations and porosity, ultimately deleting the adhesive's performance. This study not only underscores the critical role of CNT dispersion but also provides a comprehensive understanding of the interplay between CNT concentration, adhesive properties, and fracture behaviour. The findings contribute to the growing knowledge of advanced adhesive systems, offering a pathway for optimising CNT-reinforced adhesives for high-performance applications in industries such as aerospace and automotive. By highlighting the nuanced relationship between CNT dispersion and adhesive toughness, this research lays the groundwork for future studies to refine nanomaterial reinforcement strategies for structural adhesives. The innovative coupling of experimental data with numerical simulations adds a novel dimension to understanding bonded composite joints' stress distribution and failure mechanisms. Declarations Author Contribution "S.K. wrote the manuscript and prepared the figures and tables and M.T. and F.E reviewed the manuscript. All authors approved the final version of the manuscript." References D. G. dos Santos, R. J. C. Carbas, E. A. S. Marques, and L. F. 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Khammassi","email":"data:image/png;base64,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","orcid":"","institution":"ENSTA Bretagne, IRDL - UMR CNRS 6027","correspondingAuthor":true,"prefix":"","firstName":"Sabrina","middleName":"","lastName":"Khammassi","suffix":""},{"id":427146660,"identity":"d9a0ab8a-d93b-42c6-b9f6-4bd0f2b0ec26","order_by":1,"name":"Fouad Erchiqui","email":"","orcid":"","institution":"Université du Québec en Abitibi-Témiscamingue","correspondingAuthor":false,"prefix":"","firstName":"Fouad","middleName":"","lastName":"Erchiqui","suffix":""},{"id":427146661,"identity":"9c61faba-5596-4e3a-9470-eb785a7caa90","order_by":2,"name":"Mostapha Tarfaoui","email":"","orcid":"","institution":"ENSTA Bretagne, IRDL - UMR CNRS 6027","correspondingAuthor":false,"prefix":"","firstName":"Mostapha","middleName":"","lastName":"Tarfaoui","suffix":""}],"badges":[],"createdAt":"2025-01-06 10:40:48","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5773014/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5773014/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":78419580,"identity":"1cb90776-1c99-49d5-8afa-2c47e7080de1","added_by":"auto","created_at":"2025-03-13 05:28:56","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":171627,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic Workflow for Fabricating CNT-Reinforced DGEBA Nanocomposite Adhesive and Composite Bonded Specimens.\u003c/p\u003e","description":"","filename":"FIGURE1.png","url":"https://assets-eu.researchsquare.com/files/rs-5773014/v1/f2550379a433d3a8b5b8039a.png"},{"id":78420163,"identity":"ff58f541-9597-4bb5-9f4d-514d20fd2488","added_by":"auto","created_at":"2025-03-13 05:36:56","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":347270,"visible":true,"origin":"","legend":"\u003cp\u003eManufacture of composite/composite bonded joints.\u003c/p\u003e","description":"","filename":"FIGURE3.png","url":"https://assets-eu.researchsquare.com/files/rs-5773014/v1/81c75699a655241e6fa265d6.png"},{"id":78422663,"identity":"f96dbf2c-2454-463b-bbc7-9a1efac89ee0","added_by":"auto","created_at":"2025-03-13 06:00:56","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":77817,"visible":true,"origin":"","legend":"\u003cp\u003eThe geometry of DCB specimens (dimensions in mm).\u003c/p\u003e","description":"","filename":"FIGURE4.png","url":"https://assets-eu.researchsquare.com/files/rs-5773014/v1/30a975777e6dbc622822addd.png"},{"id":78422664,"identity":"d4fe61df-3e50-4094-9764-71e756a5edad","added_by":"auto","created_at":"2025-03-13 06:00:56","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":462366,"visible":true,"origin":"","legend":"\u003cp\u003eMode I test with DCB specimen using INSTRON machine.\u003c/p\u003e","description":"","filename":"FIGURE5.png","url":"https://assets-eu.researchsquare.com/files/rs-5773014/v1/05d3dbfd9ee4ba4d507c185d.png"},{"id":78419586,"identity":"ce74e611-0847-435b-ac8d-267e85107677","added_by":"auto","created_at":"2025-03-13 05:28:56","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":39520,"visible":true,"origin":"","legend":"\u003cp\u003eReproducibility of Load-Displacement Curves for Bonded Joints with Neat Epoxy Adhesive\u003c/p\u003e","description":"","filename":"FIGURE6.png","url":"https://assets-eu.researchsquare.com/files/rs-5773014/v1/40ee694475b1e86b46b0d849.png"},{"id":78422807,"identity":"414ce961-fd94-423e-bac6-dc4f57ee78c5","added_by":"auto","created_at":"2025-03-13 06:01:53","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":130271,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Comparison of DCB Load-Displacement Curves and (b) Corresponding R-Curves for Reinforced and Unreinforced Adhesive Bonded Composite Specimens.\u003c/p\u003e","description":"","filename":"FIGURE7.png","url":"https://assets-eu.researchsquare.com/files/rs-5773014/v1/3163e11b1afd3a78763deb25.png"},{"id":78419603,"identity":"43c3fdcb-4dcf-4d41-8cac-df0fd7e1c1ad","added_by":"auto","created_at":"2025-03-13 05:28:57","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":106699,"visible":true,"origin":"","legend":"\u003cp\u003eIntegration of CNT-Reinforced Adhesive Matrix into Composite Bonded Joints and Mode I Fracture Toughness Testing Using the DCB Method.\u003c/p\u003e","description":"","filename":"FIGURE8.png","url":"https://assets-eu.researchsquare.com/files/rs-5773014/v1/49062f4d84edbad66a57d2a4.png"},{"id":78419590,"identity":"d48b14b3-9309-4d4c-b9bc-6b66f0ec9a90","added_by":"auto","created_at":"2025-03-13 05:28:57","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":399562,"visible":true,"origin":"","legend":"\u003cp\u003eVisualization of Carbon Nanotube Dispersion States in DGEBA Adhesive Matrix for Various CNT Concentrations (1 wt.%, 2 wt.%, and 5 wt.%) Simulated Using DIGIMA\u003c/p\u003e","description":"","filename":"FIGURE10.png","url":"https://assets-eu.researchsquare.com/files/rs-5773014/v1/65f9526a4235f2cbaff25227.png"},{"id":78422809,"identity":"5d645a6c-8ef4-46e3-ae6a-4ffbd26b63ef","added_by":"auto","created_at":"2025-03-13 06:01:54","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":1559561,"visible":true,"origin":"","legend":"\u003cp\u003eCharacterization of fracture surfaces in DCB specimens bonded with DGEBA adhesive reinforced with different CNT mass fractions.\u003c/p\u003e","description":"","filename":"FIGURE11.png","url":"https://assets-eu.researchsquare.com/files/rs-5773014/v1/73f6690eb6689dab4fe262a6.png"},{"id":78419595,"identity":"d71396f1-0f6a-4d20-bc3a-98e986092fc6","added_by":"auto","created_at":"2025-03-13 05:28:57","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":83278,"visible":true,"origin":"","legend":"\u003cp\u003eMicroscopic analysis of agglomeration phenomena in the adhesives interface reinforced with 5wt.% CNTs post-degradation.\u003c/p\u003e","description":"","filename":"FIGURE12.png","url":"https://assets-eu.researchsquare.com/files/rs-5773014/v1/0a96a3d5c60ed1142414af06.png"},{"id":78422800,"identity":"824f5af0-def8-4180-b905-3c07497ff6ee","added_by":"auto","created_at":"2025-03-13 06:01:48","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":47812,"visible":true,"origin":"","legend":"\u003cp\u003eFinite element modelling of DCB test: Visualization of cohesive zone elements and traction-separation behaviour\u003c/p\u003e","description":"","filename":"FIGURE13.png","url":"https://assets-eu.researchsquare.com/files/rs-5773014/v1/650ccc6862c09034420683d0.png"},{"id":78420196,"identity":"72ae1ad4-abd3-4b67-9083-4c609d6c4919","added_by":"auto","created_at":"2025-03-13 05:36:58","extension":"png","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":258685,"visible":true,"origin":"","legend":"\u003cp\u003eCorrelation between numerical and experimental P-d RESPONSES FOR Mode I DCB tests, highlighting the effect of varying CNT mass fractions on adhesive performance.\u003c/p\u003e","description":"","filename":"FIGURE14.png","url":"https://assets-eu.researchsquare.com/files/rs-5773014/v1/3d87631b3bc3f605eaa03d19.png"},{"id":80474094,"identity":"d111d3cb-ec44-4448-afe4-c91bef2b6a2f","added_by":"auto","created_at":"2025-04-13 09:46:38","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4293371,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5773014/v1/68b06cc6-fe8f-419f-afd9-297493c11b49.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Enhanced Mode I Fracture Toughness in Composite Joints: Synergistic Effects of CNT-Reinforced Epoxy Adhesives","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eIn modern engineering, composite materials have become indispensable across various industries, including aerospace, automotive, marine, and sports applications [1]. Their lightweight nature, combined with exceptional mechanical properties, makes them ideal for high-performance applications [2], [3] [4]. However, assembling composite structures often necessitates efficient joining techniques. While traditional mechanical fasteners remain widely used, adhesive bonding has emerged as a preferred alternative, offering advantages such as uniform stress distribution, corrosion resistance, ease of application, and cost-effectiveness [1], [5] [6][7][8][9]. Epoxy-based adhesives are favoured among adhesive systems for their superior bonding strength and durability. However, their inherent brittleness and limited toughness present significant challenges, especially in critical structural applications requiring high fracture resistance [10].\u003c/p\u003e \u003cp\u003eTo address these limitations, researchers have explored various strategies to enhance the toughness of epoxy adhesives. The incorporation of nanomaterials as reinforcements has shown significant promise [11][12], [13], [14], [15], [16]. Organic and inorganic nanomaterials, such as short fibres, micro- and nanoparticles, and carbon-based nanofillers, have been extensively studied [17], [18], [19], [20], [21]. Carbon nanotubes (CNTs) [22] and graphene nanoplatelets (GNPs) [22] are particularly noteworthy due to their exceptional mechanical, thermal, and electrical properties. Toughening mechanisms associated with these nanofillers include crack-pinning [23], crack bifurcation [23], particle bridging [24], and pull-out effects [25].\u003c/p\u003e \u003cp\u003eNumerous studies have demonstrated the potential of CNTs to improve the performance of epoxy adhesives, even at low concentrations [26]. For example, Yu et al. [27] reported significant increases in bonding strength using the Boeing wedge test, while Sydlik et al. [28] observed lap shear strength improvements of 36% and 27% for functionalised and unfunctionalised CNT-reinforced epoxy adhesives, respectively. Similarly, Panta et al. [29] found a 53% increase in lap shear strength with 1 wt.% CNT incorporation, and Ruchi et al. [17] documented a 48.2% enhancement in shear strength at 1 wt.% CNT loading. These enhancements are attributed to the unique atomic structure, high aspect ratio, and interfacial solid interactions of CNTs with the epoxy matrix.\u003c/p\u003e \u003cp\u003eHowever, most of these studies have focused on enhancing bulk mechanical properties, such as lap shear strength, while leaving the fracture behaviour of adhesively bonded composite joints, particularly under Mode I loading, insufficiently explored. Additionally, while CNT-reinforced diglycidyl ether of bisphenol A (DGEBA) epoxy adhesives have been studied, there has been limited investigation into the effects of CNT dispersion, matrix ductility, and the distinction between interfacial and cohesive failure mechanisms in bonded composite joints.\u003c/p\u003e \u003cp\u003eThe novelty of this study lies in its comprehensive investigation of the effects of CNT mass fraction on the Mode I fracture toughness of composite bonded joints. Using DGEBA epoxy adhesive, this work employs double cantilever beam (DCB) testing to analyse interlaminar fracture toughness in glass fibre-reinforced composite joints. Unlike prior research focusing primarily on bulk adhesive properties or lap shear strength, this study delves into crack propagation mechanisms. It examines the transition from brittle to ductile failure modes as a function of CNT concentration. Integrating experimental findings with advanced numerical simulations provides deeper insights into CNT-reinforced composite joints' stress distribution and failure behaviour.\u003c/p\u003e \u003cp\u003eThis research advances the application of CNT-reinforced adhesives [14], [19], [20], [28], [30], [31], [32], [33] by offering a unique approach to optimising the performance of composite bonded joints under critical loading conditions. Bridging the gap between experimental data and numerical modelling lays the groundwork for future innovations in adhesive bonding technologies, targeting high-performance structural applications in demanding environments.\u003c/p\u003e"},{"header":"2. Experimental investigation","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Materials\u003c/h2\u003e \u003cp\u003eThis study used a diglycidyl ether of bisphenol A (DGEBA) epoxy adhesive, supplied by MOMENTIVE, as the matrix material. The curing agent, Jeffamine D-230, was also procured from MOMENTIVE. Multiwalled carbon nanotubes (CNTs) were sourced from Nanocyl SA. For the substrates, glass fibre-reinforced composite (VETRONITE) plates with dimensions of 250 mm \u0026times; 20 mm \u0026times; 4 mm were obtained from Von Roll Group. Acetone was used as a cleaning agent to thoroughly prepare the composite substrate surfaces and ensure optimal adhesion by removing contaminants such as grease and dust.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Preparation of CNT-Reinforced Epoxy Adhesive\u003c/h2\u003e \u003cp\u003eA high-shear planetary mixer was employed to achieve a homogeneous dispersion of CNTs within the epoxy matrix, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The CNTs were mixed with the DGEBA epoxy resin at a rotational speed of 1400 rpm for 60 seconds. This high-shear process minimises the risk of CNT agglomeration by generating sufficient shear forces between the nanofillers [34]. After dispersing the CNTs, the curing agent was added to the epoxy resin in a recommended volume ratio 100:32 (epoxy: curing agent). The mixture was stirred gently to avoid introducing air bubbles, ensuring uniformity across the adhesive samples.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Sample Preparation\u003c/h2\u003e \u003cp\u003eGlass fibre-reinforced composite substrates were precisely cut to the required dimensions of 250 mm \u0026times; 20 mm \u0026times; 4 mm, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The surfaces of the composite substrates were cleaned with acetone to ensure proper bonding. The CNT-reinforced epoxy adhesive was then carefully applied to the bonding area of the composite substrates.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the characteristic steps of the composite adherent surface treatment where:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eDegreasing with IPA.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003ePolishing with P120 sandpaper.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eUse of IPA for degreasing.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e \u003cp\u003eFinally, composite materials' bonding joints are carefully prepared, as presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The main steps of this preparation are as follows:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003ePositioning PTFE tape (0.25\u0026micro;m thick gasket).\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eInserting the board into the mold.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eApplying adhesive on the surface with tape (unreinforced and reinforced).\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eDouble-adhesive.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eButtoning the opposite side.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003ePressing at least 20kN pressure for 16 hours.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eCompleting the polymerisation reaction in an oven at 120\u0026deg;C for 1 hour.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAfter assembling the samples, they were cured at room temperature under slight pressure for 24 hours, followed by a post-curing cycle at 80\u0026deg;C for 2 hours, per the manufacturer's recommendations. A steel plaque was bonded to each specimen's ends to enhance the cantilever arms' flexural rigidity. The final dimensions and setup of the Double Cantilever Beam (DCB) specimens are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4. Characterization\u003c/h2\u003e \u003cp\u003eThe Mode I interlaminar fracture toughness (G\u003csub\u003eIc\u003c/sub\u003e) of the bonded composite joints was determined using the Double Cantilever Beam (DCB) method following ASTM D5528 standards. Each DCB specimen was prepared with two edges coated using white correction fluid and marked with black reference lines to enable optical tracking of crack propagation. A high-speed camera system was employed to monitor and record the crack propagation during the tests. The mechanical tests were conducted on an Instron Universal Testing Machine with a constant crosshead displacement rate of 1 mm/min, Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. Three to five replicate tests were performed for each adhesive formulation to ensure reproducibility of the results.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5. Data Analysis\u003c/h2\u003e \u003cp\u003eThe Mode I fracture energy, G\u003csub\u003eIc\u003c/sub\u003e was calculated using the following formula:\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:{G}_{I}=\\frac{3P\\delta\\:}{2ba}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eP: Load applied to induce a displacement\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eδ: Measured displacement,\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eb: Specimen width,\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003ea: Crack length.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eThis methodology provided an accurate evaluation of the fracture toughness of the composite bonded joints, enabling a comparison of the performance of the CNT-reinforced adhesives under Mode I loading conditions.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Reproducibility\u003c/h2\u003e \u003cp\u003eBefore proceeding with the analysis of the experimental results, it is essential to highlight the reproducibility of the tests, as demonstrated by the load-displacement curves obtained for the three specimens (#1-ne, #2-ne, and #3-ne), Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. These curves represent a typical example of bonded joint specimens fabricated with neat epoxy as the adhesive. Reproducibility is a critical factor in experimental studies, as it ensures the reliability of the data and supports the validity of the conclusions regarding the mechanical behavior of the adhesive joints.\u003c/p\u003e \u003cp\u003eThe curves exhibit consistent trends across all specimens, characterized by distinct load peaks followed by abrupt drops and stepped progressions corresponding to successive stages of crack propagation. While minor variations are observed, particularly in the maximum loads sustained (e.g., a slightly higher peak for specimen #2-ne compared to #1-ne and #3-ne), these differences are within an acceptable range for bonded joint testing. Such variations can arise from inherent microscopic differences in the neat epoxy adhesive layer or subtle inconsistencies in the preparation or alignment of the bonded joints.\u003c/p\u003e \u003cp\u003eThis level of consistency reflects a well-controlled experimental setup, ensuring that the observed responses are primarily due to the mechanical properties of the neat epoxy adhesive and not influenced by external experimental factors. The reproducibility of the results validates the methodology and provides a robust basis for further analysis of the fracture behavior and mechanical performance of bonded joints under various loading conditions\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Experimental results\u003c/h2\u003e \u003cp\u003eThe DCB tests were conducted to evaluate the load-displacement (L-D) curves (Fig.\u0026nbsp;7a) and the R-curves (crack length increment) for both unreinforced (reference) and CNT-reinforced adhesive bonded joints. The L-D curves exhibit a moderate nonlinear zone near the peak force, indicating significant crack extension as the external force decreases abruptly. This behaviour suggests that the adhesive joint undergoes an initial brittle fracture before transitioning to a more stable propagation phase as the displacement increases. Once the critical fracture energy release rate (G\u003csub\u003eIC\u003c/sub\u003e) is reached, the specimen continues to experience crack propagation, and the process is repeated until the bonded surfaces are separated.\u003c/p\u003e \u003cp\u003eIn the case of the reference specimen, significant and unstable load fluctuations were observed, indicative of a brittle failure mode with insufficient crack bridging. The long crack jumps observed in the load-displacement curve confirm the fragility of the adhesive joint. The maximum sustained load for the reference joint was approximately 97 N, which reflects the low fracture resistance of the unreinforced epoxy adhesive.\u003c/p\u003e \u003cp\u003eIn contrast, the CNT/DGEBA adhesive bonded joints showed a markedly different behaviour, requiring a substantially higher load for crack initiation and propagation. For instance, 1 wt.% CNT/DGEBA adhesive exhibited a maximum load of 237 N, representing a 144% increase compared to the reference adhesive. This significant increase is attributed to the reinforcement provided by CNTs, which enhance the adhesive's tensile strength by improving interfacial interactions between the adhesive and the composite substrate. However, the 2 wt.% CNT/DGEBA adhesive exhibited a decrease in maximum load to 128 N, which is 31% greater than the reference adhesive. This reduction in performance can be attributed to the onset of CNT agglomeration at higher loading levels, which impairs the dispersion and load transfer efficiency within the adhesive matrix.\u003c/p\u003e \u003cp\u003eInterestingly, the L-D curves for CNT-reinforced adhesives became smoother as the CNT mass fraction increased, and the crack jumps were more gradual and stable. This trend reflects the transition from a brittle failure mode to a more ductile one. CNTs enhance crack bridging, bifurcation, and energy dissipation during fracture propagation, improving tensile strength at 1 wt.% CNT is due to the high aspect ratio of CNTs, which facilitates better interaction with the epoxy matrix and prevents premature crack initiation.\u003c/p\u003e \u003cp\u003eHowever, the 2 wt.% CNT addition led to a reduction in the adhesive's strength compared to the 1 wt.% CNT/DGEBA adhesive. This decrease can be attributed to CNT agglomeration, which restricts the load transfer efficiency and introduces localised stress concentrations that promote crack initiation and accelerate crack propagation. These findings suggest an optimal CNT concentration (1 wt.%) maximises the adhesive's fracture toughness and tensile strength.\u003c/p\u003e \u003cp\u003eFigure 7b presents the R-curves and the average critical strain energy release rate \u003cb\u003e(\u003c/b\u003eGIC\u003cb\u003e)\u003c/b\u003e as a function of crack growth (from 43 mm to 60 mm), providing further insights into the fracture mechanics of the bonded joints. The first point on the R-curve represents the fracture initiation, while subsequent points illustrate the progression of crack propagation. Incorporating CNT\u003cb\u003es\u003c/b\u003e into the DGEBA adhesive significantly improved the propagation energy (as indicated by the R-curve), particularly for the 1 wt.% CNT/DGEBA formulation, which demonstrated enhanced fracture toughness. This result suggests that CNTs are critical in reducing crack propagation rates by acting as a barrier to crack growth and promoting energy dissipation during fracture.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e8\u003c/span\u003e illustrates the crack propagation stage of the Mode I fracture toughness test on the composite samples, showing that adding CNTs effectively mitigates crack propagation through improved interfacial adhesion and energy absorption.\u003c/p\u003e \u003cp\u003eThe average G\u003csub\u003eIC\u003c/sub\u003e values for both the initiation and propagation of the bonded joints are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e9\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The 1 wt.% CNT/DGEBA adhesive demonstrated the most significant improvement in fracture toughness, with GIC initiation increasing from 155 kJ/m\u003cb\u003e\u0026sup2;\u003c/b\u003e (for the reference adhesive) to 354 kJ/m\u0026sup2; and GIC propagation increasing from 330 kJ/m\u0026sup2; to 768 kJ/m\u0026sup2;. These enhancements indicate that the presence of CNTs significantly increased the fracture energy absorption capacity of the adhesive.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eOn the other hand, for 2 wt.% and 4 wt.% CNT concentrations, a moderate decrease in both initiation and propagation G\u003csub\u003eIC\u003c/sub\u003e values was observed, suggesting that excessive CNT loading leads to the agglomeration of CNTs within the adhesive matrix, as mentioned in Fig.\u0026nbsp;10. This agglomeration results in morphological heterogeneity, which creates regions of high-stress concentration and reduces the adhesive's overall performance. As the CNT concentration increases beyond the optimal value, these agglomerates interfere with the load transfer and crack resistance mechanisms, ultimately diminishing the fracture toughness of the adhesive.\u003c/p\u003e \u003cp\u003eThis study confirms that the 1 wt.% CNT/DGEBA adhesive produces the best Mode I fracture toughness performance, nearly doubling the GIC values of the reference DGEBA adhesive, underscoring CNT reinforcement's efficacy in enhancing adhesive bonding in composite materials.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eFracture energy progress of the unreinforced and reinforced joints\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c6\" namest=\"c3\"\u003e \u003cp\u003eComposite Bonded Joint\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e0 wt.%CNT\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e1wt.%CNT\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003e2 wt.%CNT\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cb\u003e5 wt.%CNT\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eG\u003csub\u003eIi\u003c/sub\u003e\u003c/p\u003e \u003cp\u003e(kJ/m\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eAverage\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e155\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e354\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e336\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e302\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eProgress\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e128%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e117%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e94%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eG\u003csub\u003eIc\u003c/sub\u003e\u003c/p\u003e \u003cp\u003e(kJ/m\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eAverage\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e330\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e768\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e613\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e376\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eProgress\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e133%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e85%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e14%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.3. Fracture surface morphology\u003c/h2\u003e \u003cp\u003eThe fracture surfaces of the specimens obtained from the Mode I tests, both with and without CNT nanofillers, were analysed to investigate the toughening mechanisms further, as shown in Fig.\u0026nbsp;11. The morphology of the fracture surfaces revealed significant differences between the unreinforced and CNT-reinforced adhesive joints, providing valuable insights into the adhesive interfaces' fracture behaviour and failure mechanisms.\u003c/p\u003e \u003cp\u003eThe fracture surface exhibited a relatively smooth, uniform morphology for the unreinforced adhesive-bonded joint, characteristic of brittle failure. The absence of significant surface features suggests that the fracture propagation was rapid, with minimal energy dissipation and limited crack deflection. This type of failure is typical of materials with low ductility, where cracks propagate quickly without significant resistance or diversion. The smooth fracture surface confirms the inherent fragility of the unmodified epoxy adhesive.\u003c/p\u003e \u003cp\u003eIn contrast, the fracture surfaces of the CNT-reinforced adhesive bonded joints were significantly rougher and exhibited more irregularities (Figs.\u0026nbsp;11b, 11c, and 11d). Carbon nanotubes (CNTs) fundamentally altered the failure mode, promoting a ductile fracture mechanism. The increased surface roughness, coupled with the formation of wrinkles and ripples on the fracture surface of the 1 wt.% CNT/DGEBA adhesive indicates a more resilient bonding interface. This suggests that the CNTs were crucial in improving the adhesive's toughness, likely through mechanisms such as crack deflection, bridging, and energy dissipation. These mechanisms increase the fracture resistance by obstructing crack propagation and increasing the energy required for failure.\u003c/p\u003e \u003cp\u003eAt 2 wt.% CNT/DGEBA, the fracture surface showed a reduction in wrinkles compared to the 1 wt.% CNT/DGEBA joint, suggesting a decline in adhesive performance and the beginning of failure due to CNT agglomeration. The less pronounced surface roughness at this higher concentration points to the initiation of interfacial degradation, likely due to the formation of CNT clusters within the adhesive matrix, which disrupts the uniform distribution of nanofillers. This agglomeration diminishes the adhesive's ability to effectively transfer stress, leading to early crack initiation and reduced fracture toughness.\u003c/p\u003e \u003cp\u003eAt 5 wt.% CNT/DGEBA, the fracture surface exhibited predominantly interfacial failure, with significant porosity at the adhesive interface, as detailed in the SEM images (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e12\u003c/span\u003e). This observation suggests a severe degradation of the adhesive properties due to increased viscosity and micro defects expansion in the adhesive layer. The excessive concentration of CNTs likely impaired the dispersal of the nanofillers within the adhesive, leading to morphological heterogeneity in the matrix. This non-uniform filler distribution can cause localised stress concentrations, promoting premature failure at the adhesive interface and reducing the bonded joint's mechanical integrity.\u003c/p\u003e \u003cp\u003eIn summary, incorporating CNTs improved the adhesive's fracture surface morphology and overall toughness, with the optimal performance observed at 1 wt.% CNTs. At higher CNT concentrations, however, the agglomeration of CNTs undermined the adhesive's performance by introducing morphological defects and stress concentrations, decreasing bond durability. These results highlight the critical importance of precise nanofiller dispersion in achieving the desired balance between enhanced toughness and maintaining adhesive integrity.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.4. Numerical analysis\u003c/h2\u003e \u003cp\u003eCohesive Zone Models (CZMs) were employed in ABAQUS software to model the debonding behaviour of the adhesive joint with high precision. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e13\u003c/span\u003e, the numerical model consisted of eight-node linear brick elements for the substrate and adhesive layers (C3D8R), with reduced integration and hourglass control, ensuring accurate simulation of the mechanical response. The adhesive bond was modelled using three-dimensional cohesive elements (COH3D8) to represent the interface between the adhesive and composite adherents. This approach captures the traction-separation law that governs the failure behaviour of both self-adhesive and CNT-reinforced adhesives. The traction-separation law models the relationship between the normal and tangential stresses and the relative displacements at the interface, which is essential for accurately predicting debonding and failure in adhesive joints. The parameters of the cohesive elements were defined based on the material properties of the adhesives, which are provided in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e for the unreinforced and CNT-reinforced adhesives, respectively.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe numerical displacement and experimental load-displacement (L-D) curves for both unreinforced and CNT-reinforced composite bonded joints are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e14\u003c/span\u003e. The simulation results showed excellent agreement with the experimental data by comparing the initial slope and the peak load values of the numerical load-displacement curves with those obtained experimentally. The close match between the numerical and experimental results confirms the accuracy of the numerical model. It further validates the cohesive zone approach in capturing the failure mechanisms in adhesive joints.\u003c/p\u003e \u003cp\u003eFurthermore, since the experimental Mode I fracture toughness (G\u003csub\u003eIC\u003c/sub\u003e) values were directly integrated into the simulation, the good correlation between the numerical and experimental results underscores the effectiveness of the interface toughening technique involving CNT-reinforced adhesives. The results highlight the enhanced adhesive interface toughness, which is crucial for improving the structural integrity of composite bonded joints. The simulation results reveal detailed insights into how the adhesive layer interacts with the overall structure under tensile loading, especially regarding stress concentration and singularity at the adhesive interface. The numerical analysis, therefore, not only corroborates the experimental findings but also provides a comprehensive understanding of the stress distribution and failure behaviour in reinforced adhesive joints under Mode I loading.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eComposite parameters outcome of DIGIMAT simulation\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{E}_{11}={E}_{22}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003cp\u003e(MPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eE\u003csub\u003e33\u003c/sub\u003e\u003c/p\u003e \u003cp\u003e(MPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{v}_{12}={v}_{21}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{v}_{13}={v}_{23}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{v}_{31}={v}_{32}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eG\u003csub\u003e12\u003c/sub\u003e\u003c/p\u003e \u003cp\u003e(MPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{G}_{13}={G}_{23}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003cp\u003e(MPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\rho\\:\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003cp\u003e(kg.m\u003csup\u003e3)\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e16022\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e11920\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.46\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e10328\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e3752\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e2080\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCNTs/DGEBA properties\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:E\\)\u003c/span\u003e\u003c/span\u003e (MPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:v\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eG (MPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\rho\\:\\:\\)\u003c/span\u003e\u003c/span\u003e(kg.m\u003csup\u003e3)\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1wt.%CNT/ DGEBA\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e17478\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e6920\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1509\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2wt.%CNT/ DGEBA\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e12772\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e5037\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1472\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5wt.%CNT/ DGEBA\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e7264\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2836\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1607\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e "},{"header":"4. Conclusion","content":"\u003cp\u003eThis study provides valuable insights into the role of carbon nanotubes (CNTs) in enhancing the Mode I fracture toughness of DGEBA adhesive bonded composite joints. The results demonstrate that incorporating CNTs significantly improves the fracture resistance at the adhesive-adherent interface, with the most notable enhancement observed at a 1 wt.% CNT concentration. At this level, CNTs effectively reinforce the adhesive, leading to a marked increase in toughness without introducing detrimental effects such as agglomeration or excessive porosity. However, when the CNT concentration exceeds this optimal threshold, poor dispersion and the formation of CNT clusters cause localised stress concentrations and porosity, ultimately deleting the adhesive's performance.\u003c/p\u003e \u003cp\u003eThis study not only underscores the critical role of CNT dispersion but also provides a comprehensive understanding of the interplay between CNT concentration, adhesive properties, and fracture behaviour. The findings contribute to the growing knowledge of advanced adhesive systems, offering a pathway for optimising CNT-reinforced adhesives for high-performance applications in industries such as aerospace and automotive. By highlighting the nuanced relationship between CNT dispersion and adhesive toughness, this research lays the groundwork for future studies to refine nanomaterial reinforcement strategies for structural adhesives. The innovative coupling of experimental data with numerical simulations adds a novel dimension to understanding bonded composite joints' stress distribution and failure mechanisms.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003e\"S.K. wrote the manuscript and prepared the figures and tables and M.T. and F.E reviewed the manuscript. All authors approved the final version of the manuscript.\"\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eD. G. dos Santos, R. J. C. Carbas, E. A. S. Marques, and L. F. M. da Silva, \u0026quot;Reinforcement of CFRP joints with fibre metal laminates and additional adhesive layers,\u0026quot; \u003cem\u003eCompos B Eng\u003c/em\u003e, vol. 165, pp. 386\u0026ndash;396, 2019.\u003c/li\u003e\n\u003cli\u003eJ. Njuguna, K. Pielichowski, and J. R. 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Lafdi, \u0026quot;Study of mechanical performance of polymer nanocomposites reinforced with exfoliated graphite of different mesh sizes using micro-indentation,\u0026quot; \u003cem\u003eJ Compos Mater\u003c/em\u003e, p. 0021998321993211, 2021.\u003c/li\u003e\n\u003cli\u003eS. Khammassi and M. Tarfaoui, \u0026quot;Influence of exfoliated graphite filler size on the electrical, thermal, and mechanical polymer properties,\u0026quot; \u003cem\u003eJ Compos Mater\u003c/em\u003e, 2020, doi: 10.1177/0021998320918639.\u003c/li\u003e\n\u003cli\u003eD. Quan, J. L. Urd\u0026aacute;niz, and A. Ivanković, \u0026quot;Enhancing mode-I and mode-II fracture toughness of epoxy and carbon fibre reinforced epoxy composites using multiwalled carbon nanotubes,\u0026quot; \u003cem\u003eMater Des\u003c/em\u003e, vol. 143, pp. 81\u0026ndash;92, 2018, doi: https://doi.org/10.1016/j.matdes.2018.01.051.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Bonded Joint, Adhesion, Carbon Nanotubes, Composite, Mode I, Fracture, Cohesive interface modelling","lastPublishedDoi":"10.21203/rs.3.rs-5773014/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5773014/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eHigh-performance adhesives are essential for creating durable, long-lasting bonds in various applications. This study explores the novel use of carbon nanotubes (CNTs) to enhance the fracture toughness of polymer adhesives, explicitly focusing on their effect on the Mode I fracture toughness of glass fibre-reinforced composite bonded joints. Unlike traditional research, this work investigates CNT-reinforced diglycidyl ether of bisphenol A (DGEBA) epoxy adhesives applied to double cantilever beam (DCB) joints. CNTs were incorporated at varying mass fractions (1, 2 and 5 wt.%), and the findings demonstrate that 1 wt.% CNTs yielded the most significant improvement, doubling fracture toughness compared to unreinforced adhesive (NE) due to optimal dispersion and strong matrix interaction. Numerical simulations supported the experimental results, confirming the effectiveness of CNTs in modifying cohesive and interfacial failure mechanisms. This study highlights a promising and less-explored avenue for developing advanced composite adhesives with superior mechanical performance.\u003c/p\u003e","manuscriptTitle":"Enhanced Mode I Fracture Toughness in Composite Joints: Synergistic Effects of CNT-Reinforced Epoxy Adhesives","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-03-13 05:28:51","doi":"10.21203/rs.3.rs-5773014/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"490560af-c321-46d0-b8e3-16e98f68a484","owner":[],"postedDate":"March 13th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-04-13T09:38:23+00:00","versionOfRecord":[],"versionCreatedAt":"2025-03-13 05:28:51","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5773014","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5773014","identity":"rs-5773014","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}