Composite bonded joints are susceptible to sudden and brittle failure, especially under mode I opening loading conditions. To improve crack resistance under mode I, conventional solutions use through-thickness reinforcements, such as rivets, which is effective in delaying the crack growth but damage the performance of the composite. This study explores an alternative through-thickness reinforcement consisting of 3D-printed overlapping curl (OC) fibers with sacrificial bonds and hidden lengths (SBHL) to enhance fracture resistance under mode I of composite bonded joints. The OC fibers are embedded through the thickness of a structural epoxy, oriented perpendicular to the crack propagation direction, enabling effective activation of hidden length unfolding under mode I opening conditions. This through-thickness reinforcement fully exploits the SBHL toughening mechanism without introducing detrimental effects to the CFRP substrate, in contrast to conventional techniques such as Z-pinning or riveting. Double cantilever beam experiments demonstrate that the OC topology is essential for effective toughening, as it anchors the OCs within the adhesive and prevents pull-out, allowing hidden length unfolding that leads to extended crack bridging distance and increases the mode I energy release rate by up to 119 %. Furthermore, numerical simulations are conducted to systematically explore the influence of OC mechanical parameters, providing design guidelines for improving the fracture performance of CFRP adhesively bonded joints.

Conventional petroleum-based adhesives present environmental challenges due to their toxicity, limited recyclability, and reliance on finite resources. Because ancient technology may serve as an inspiration for future solutions, the BiDebA (BioBased Debondable Adhesives) Interreg NWE project, systematically evaluated three archaeo-inspired bio-based adhesives –birch tar, rosin-beeswax blends and hide glue. The aim is to generate performance data relevant for future sustainable adhesive development. Natural additives used historically and in contemporary bio-based formulation research, including hematite and biochar, were incorporated to assess their influence on mechanical and thermal behaviour. A multi-analytical approach combining single lap-shear testing, thermal and rheological characterisation, and structural analysis was performed. The adhesives exhibited average lap shear strengths between 1.5 and 3.5 MPa, consistent with other light-duty bio-based systems. Birch tar showed the broadest substrate compatibility, including effective bonding to plywood and carbon-fibre composites, while rosin-beeswax blends and hide glue demonstrated material-specific strengths and limitations. Among the fillers tested, hematite provided the most consistent enhancements, improving mechanical strength and thermal stability while lowering debonding temperatures, particularly in birch tar and rosin-based systems. This identifies hematite as a dual-function additive capable of tuning performance while maintaining reversibility, supporting the development of repairable and recyclable bio-based adhesive systems. These findings highlight the value of archaeological adhesive technologies as informative material models rather than direct formulations. The dataset produced here also provides a scientific foundation for tailoring the mechanical and thermal behaviour of natural adhesives and supports ongoing efforts toward sustainable innovation in adhesive technology.

Tow-Based Discontinuous Composites (TBDCs) are a new class of composite materials that combine high strength and stiffness with in-plane isotropy making them of interest in high-end structural applications. Despite their potential, efficient connection methods are currently lacking and the adhesive bonding behaviour of TBDC structures remains unexplored. This work, therefore, seeks to address this gap by analysing the quasi-static performance of TBDC adhesive joints under mode I loading condition. Double Cantilever Beam (DCB) tests were performed using two adhesives with contrasting toughness levels: a moderate (∼600 J/m 2) and a high toughness adhesive (> 2400 J/m2). When a moderate-toughness adhesive was used, a combination of cohesive failure and composite damage was observed, with only a small scatter in the experimental results. In contrast, the use of the high-toughness adhesive led to a shift in damage mechanisms towards the composite micro-architecture, resulting in fracture toughness values in the region of 800 J/m2, with a larger experimental scatter. Acoustic Emission analysis identified matrix cracking and fibre/matrix debonding as the dominant damage mechanisms. These findings were validated by the post-mortem fractography analysis via Scanning Electron Microscopy. This work therefore provides the first detailed analysis of the damage mechanism in adhesively bonded TBDCs, which have potential in aerospace and automotive applications.

Inspired by the sacrificial bonds and hidden lengths mechanism found on the molecular structure of spider silk, this study investigates a bio-inspired overlapping curl (OC) 2D-mesh fabricated via 3D-printing and integrated into the adhesive layer as a reinforcement architecture. Mode I double cantilever beam tests were conducted to investigate the effects of mesh orientation (0° and 45°) and adhesion patterning on crack propagation and fracture toughening. The results show that the 2D-OC mesh itself effectively activates extensive filament bridging, leading to an apparent energy release rate (ERR) enhancement up to 47 %. In contrast, adhesion patterning partially suppresses bridging. X-ray computed tomography and fracture surface analyses further reveal that reinforcement orientation governs crack front evolution and bridging activation. In particular, the 45°-oriented mesh induces a zig-zag crack front that promotes crack deflection and bifurcation, thereby activating more bridging filaments than the 0°-oriented configuration. Based on these observations, a coupled toughening framework is established, linking the apparent ERR enhancement to the combined effects of adhesion patterning width, reinforcement orientation, bridging activation, and filament material behavior. The findings provide mechanistic insight into crack–reinforcement interactions and offer design guidelines for architected bio-inspired reinforcements in adhesively bonded composite structures.

The strength of adhesive joints is influenced by the surface of the adherends, which is often treated before bonding to prevent interfacial (adhesive) failure. Laser Powder Bed Fusion (LPBF) offers promising potential for bonding without time-consuming surface treatments, since LPBF parts have an inherently rough surface, which is usually associated with good adhesion strength. Here we study the effect of the printing parameters on the mode I fracture toughness of co-bonded joints between untreated LPBF Ti6Al4V and Carbon Fiber Reinforced Polymer (CFRP) substrates. A factorial Design of Experiment (DoE) was set varying the laser scan speed and the build angle of the Ti6Al4V substrates, which were co-bonded with a CFRP woven laminate to form Double Cantilever Beam (DCB) joints. The results showed that increasing the scan speed from 500 mm/s to 2000 mm/s led to higher titanium surface roughness (+125% on average). On the other hand, the mode I fracture toughness was mainly affected by the build angle: the joints with vertically printed (90° with respect to the build platform) titanium adherends exhibited, on average, a 200% increase in toughness compared to the samples with titanium printed at an angle. This behavior was due to the higher number of partially melted particles on the surface of the vertical joints. A particle counting method was introduced to quantify the partially fused particles and their correlation with the mode I fracture toughness was demonstrated. Moreover, to the authors’ knowledge, for the first time an original approach was proposed to assess their interlocking contribution to joint toughness.

Adhesive bonding has emerged as an attractive solution for the joining of lightweight structures, yet accurate stress analysis remains computationally demanding when relying on Finite Elements (FE). This paper introduces a novel plate Macro-Element (ME) formulation that extends previous beam-type approaches to enable three-dimensional stress analysis of bonded joints. High-order polynomial expansions are employed to describe the displacement field of the adherends, while the adhesive is modeled as an elastic foundation. Governing equations are derived using a variational principle and integrated within a standard FE framework. Through the derivation of a special stiffness matrix, a ME can simulate an entire overlap with just one element. The proposed methodology is validated against FE results for a single-lap bonded joint with a thin adhesive layer. The influence of different higher-order displacement assumptions and constitutive models is investigated. The results show that their inclusion in the formulation improves the solution accuracy.

Bio-based epoxy adhesives face significant challenges due to their relatively poor mechanical properties compared to their petroleum-based competitors, including low fracture toughness and abrupt failure. By mimicking the molecular structure of spider silk, which is one of the toughest materials in nature, 3D-printed polymer overlapping curls consisting of coiling fibers with sacrificial bonds and hidden lengths, were impregnated into a bio-based epoxy adhesive to improve its mode I fracture toughness. Such bio-inspired structures were designed specifically to toughen and improve the crack resistance of adhesive joints. These overlapping curls were embedded in the bio-based epoxy bondline with various adhesion patterning strategies, aiming to architect the fracture scenario and increase mode I energy dissipation. Double cantilever beam test results show that an extrinsic bridging is triggered by the embedded curls that promote progressive failure and delay crack growth, which improved the mean energy release rate by 133% and enhanced the mean peak energy release rate up to 313%. The proposed 3D-printed coiling fibers successfully improved the mechanical performance of the bio-based epoxy and retarded the crack growth within the bondline, opening new horizons for their use as carriers of bondlines in structural applications to control crack growth in adhesively bonded joints.

Epoxy adhesives are key enablers of lightweight, high-performance structures, providing efficient bonding of dissimilar materials for demanding engineering applications. Meanwhile, sustainable bio-based formulations with performances comparable to petroleum-derived systems are emerging as environmentally friendly alternatives. By incorporating carbon nanotubes (CNTs), these adhesives become multifunctional materials capable of simultaneously carrying loads and monitoring their structural state. Herein, a commercial Araldite epoxy and a bio-based resin, together with their composites containing 1, 3, and 5 wt% CNTs, were processed using a three-roll mill and systematically characterized. The bioresin exhibited higher tensile strength but lower strain-at-break than Araldite, while CNT incorporation caused only a slight reduction in the mechanical performance of both matrices. Although lap shear strength decreased after functionalization, all formulations remained suitable for semi-structural bonding applications. Thermoresistive sensitivity was maximized near the electrical percolation threshold, reaching ST ≈ −0.29 and −0.23% °C⁻¹ between 30°C and 100°C for Araldite and the bioresin, respectively. Load sensing was successfully demonstrated under shear loading in single-lap joints up to 50 N, with piezoresistive sensitivities approaching 300 Ω/N for Araldite containing 3 wt% CNTs and 25 Ω/N for the bioresin with 5 wt% CNTs. These findings demonstrate that both petroleum-based and bio-based epoxy adhesives can be transformed into sustainable multifunctional bonding materials with integrated thermo- and piezoresistive sensing capabilities for next-generation structural applications.

Tailoring the stacking sequence of composites bonded joints improves fracture toughness and damage tolerance of the joint by encouraging extrinsic toughening mechanisms, such as crack deflection and crack branching. Previous works show that in composite substrates with tailored laminates, each crack deflection into a new ply can increase the joint's toughness. Still, once a 0° layer is reached, toughness drops abruptly due to sudden delamination. To overcome this limitation, this work explores embedding a co-cured film-adhesive layer to prevent delamination in 0° plies. It examines how the substrate's bending stiffness influences the effectiveness of this toughening strategy. Quasi-static double cantilever beam tests on four different carbon fibre reinforced laminates, with and without the co-cured layer, revealed two regimes: (i) compliant substrates lead to high peel stresses, triggered crack deflection into ±45° plies, enabling bridging and rising R-curves—up to 200% toughness increase; (ii) stiffer substrates suppressed near-tip rotation, and promoted cleavage-like crack growth with minimal toughening.

Further development of thermoplastic composites for advanced structural applications, such as in aerospace, requires tough interfaces at bimaterials junctions such as composite-metal interfaces. Mode I failure being the most critical failure mode of interfaces, surface roughening or patterning techniques are commonly used to improve the mode I interface toughness. Patterning typically involves creating grooves on the surface via laser ablation or 3D printing. However, crack propagation may follow two distinct paths: along the groove pattern (interfacial failure) or through the polymer within the grooves (cohesive failure). Cohesive failure is often the toughest mechanism. However, design criteria linking groove geometry to joint materials are currently lacking. This study investigates the influence of groove dimensions, joint dimensions, and material and interface properties on the resulting failure mechanism using a cohesive zone model. First, a small-scale yielding (SSY) model is developed. The results indicate that the characteristic fracture length of the material filling the grooves plays a critical role in determining the failure mechanism. Specifically, cohesive failure is promoted when the groove depth is at least ten times greater than the characteristic length, and when the groove aspect ratio (depth-to-width) exceeds 10. Additionally, filling the grooves with a more compliant material, such as a polymer, helps to prevent interfacial failure. Finally, a double-cantilever model is developed, indicating that the loading configuration significantly influences the failure mechanisms taking place. For the DCB configuration, crack propagation along the interface is promoted, compared to the SSY case, owing to the bending of the adherends.

Adhesive bonding of Fiber-Reinforced Polymer (FRP) patches is increasingly used to strengthen steel structures. Considering that carbon FRP (CFRP) and epoxy adhesives are the primary materials in industrial applications, this study explores the feasibility of hybridizing CFRPs with Flax FRPs (FFRPs) and the effects of using different adhesives on the mechanical performance of reinforced steel plates under flexural loading. Four configurations of composite layups were manufactured, namely: F5 (flax), C5 (carbon), CF3C and F3C2 (carbon/flax hybrids). These patches were bonded to steel plates using three adhesives: a rigid and brittle epoxy (adhesive A), a medium flexibility-ductility acrylate (adhesive B), and highly flexible ductile silane-modified polymer (adhesive C), representing a wide range of adhesive properties. Quasi-static three-point bending tests were conducted to evaluate mechanical performance of these novel hybrid composite-adhesive-steel structures compared to unreinforced steel plates, and Digital Image Correlation (DIC) was performed to monitor displacements and strains on the outer surface of the composite patch. Furthermore, finite element models were built and validated by experimental data, which were then used to predict development of stresses in each part of the assembly. Overall, it has been demonstrated that composite patch bonding can significantly enhance the load-bearing capacity of the reinforced steel substrate. Adhesive A provided the highest flexural strengthening but displayed brittle failure behavior, followed by adhesive B which showed second best flexural strengthening performance while keeping a ductile behavior at failure. Adhesive C showed poor flexural strengthening enhancement due to the poor stress transfer capabilities of the adhesive. Hybrid F3C2 and CF3C configurations achieved flexural performance comparable to pure carbon (C5), highlighting the potential of hybrid designs for structural applications.

Adhesive bonding of fiber-reinforced polymer (FRP) patches is increasingly used to strengthen steel structures. While carbon FRP (CFRP) and epoxy adhesives are the primary materials in industrial applications, this study explores hybrid Carbon/Flax FRP as an alternative for reinforcing steel plates under flexural loading. Four composite layups were tested: F5 (flax), C5 (carbon), CFC, and FC (carbon/flax hybrids). These patches were bonded to steel plates using three adhesives: a flexible and ductile silane-modified polymer (SMP-FD), a medium flexibility-ductility acrylate (ACR-MFD), and a rigid and brittle epoxy (EP-RB), representing a wide range of adhesive properties. Three-point bending tests were conducted to evaluate mechanical performance compared to unreinforced steel plates. Results demonstrated that composite patch bonding significantly enhances load-bearing capacity. The EP-RB adhesive provided the highest reinforcement, followed by ACR-MFD and SMP-FD. Hybrid FC and CFC configurations achieved reinforcement comparable to or greater than pure carbon (C5), highlighting the potential of hybrid designs for structural applications.

In this study, a 3D-printed biomimetic overlapping curl structure inspired by spider silk molecular structure, containing sacrificial bonds and hidden lengths, is studied as a toughening mechanism for a bio-based epoxy. Experimental results of the fracture phenomena of the overlapping curl-reinforced bio-based epoxy identify three toughening mechanisms triggered by the overlapping curl: (1) crack re-initiation, (2) overlapping curl bridging, and (3) epoxy ligament. First, the integrated overlapping curl creates a void within the epoxy matrix. As the crack tip reaches the end of this void, the crack re-initiates. Then, as the hidden length of overlapping curl unfolds, it leads to a bridging effect in resisting crack growth. In addition, for the smallest hidden length, an epoxy ligament is formed due to crack branching, significantly improving the energy release rate. The epoxy fracture energy release rate increased by 13 %. The overall modest improvement is attributed to the large plastic dissipation energy of the epoxy and the relatively low overlapping curl load-capacity. However, when expanding the design space numerically, it was shown that as the failure load of the overlapping curl increases, the bridging effect increases progressively. The introduction of the bio-inspired overlapping curl structure into bio-based epoxy proves the concept of a toughening strategy for developing high-performance sustainable composite materials.

Driven by sustainability goals outlined in the European Green Deal, most of the industrial sectors (i.e. automotive, aerospace and civil infrastructures) require reliable, lightweight, and durable materials. Accurate crack detection significantly extends the operational life of bonded structural components, reducing maintenance, waste, and environmental impact. This study presents acoustic emission (AE) techniques for accurately monitoring crack length in adhesively bonded joints, primarily targeting Titanium-Carbon Fiber Reinforced Polymer (Ti-CFRP) bi-material specimens, with Titanium-Titanium (Ti-Ti) joints included as a benchmark. Titanium Ti6Al4V substrates fabricated via Laser Powder Bed Fusion (LPBF) were prepared with various surface conditions: as-printed and sandblasted. The mode I fracture toughness was evaluated via Double Cantilever Beam tests, which were supported by continuous AE monitoring with high-resolution equipment capturing around 200,000 waveforms. Principal Component Analysis and machine learning techniques, including Self-Organising Maps and K-means clustering, classified AE signals into clusters associated with damage or background noise. A linear localisation algorithm tracked crack initiation and growth phases. Results validated the accuracy of AE signals to localise crack propagation under the bi-material quasi-static mode I load condition. The study highlights AE's potential for precise and sustainable structural health monitoring, informing future numerical modelling to predict joint durability.

Spider silk is known for its excellent strength and fracture resistance properties due to its molecular design structure, characterized by sacrificial bonds and hidden lengths. These structures have inspired reinforcements of synthetic polymer materials to enhance toughness. In this study, we mimic these natural toughening mechanisms by designing and manufacturing 3D-printed polymeric structures incorporating overlapping curls consisting of coiling fiber with sacrificial bonds and hidden lengths. Utilizing the liquid rope coiling effect, we manufactured overlapping curls using three polymers: polylactic acid (PLA), liquid crystal polymer (LCP), and polyamide 6 (PA6). Uniaxial tensile tests were performed to characterize the mechanical properties of overlapping curl as a function of geometries, post-treatments, and material constitutive parameters. Our results show that single-sided overlapping curls can fully unfold while double-sided curls are prone to premature failure. Heat-pressure post-treatment was found to significantly increase the load-capacity of the sacrificial bonds by up to [Formula presented] due to increased contact area. However, the defects introduced in the fibre after the break of the sacrificial bonds, make the structure more susceptible to premature failure, limit the complete unfolding of the hidden length, and lead to a decrease up to [Formula presented] of the toughness. To guarantee the complete unfolding of the hidden lengths and improve the toughness, we demonstrate that selecting a polymer material with either high fracture strength (e.g., LCP, [Formula presented]) or high fracture strain (e.g., PA6, >2) is crucial, and increase toughness up to [Formula presented] and [Formula presented], respectively.

This study uses the acoustic emission structural health monitoring method to identify fracture mechanisms in composite bonded joints when varying the substrate stacking sequence. Quasi-static mode I loading tests were performed on secondary adhesively bonded multidirectional composite substrates (0, 90, 45, −45, 60 and −60° fibre orientations). An unsupervised artificial neural network combined with the visual fracture evaluation of the specimens and the Morlet continuous wavelet transform was used to cluster and give the acoustic emission signals a physical meaning. Different fracture mechanisms could be identified within the adhesive layer (i.e., cohesive failure) and in the composite substrates, including non-visible damage mechanisms (matrix micro-cracking, fibre/matrix debonding, fibre pull-out and fibre breakage). Using the Morlet continuous wavelet transform, it was possible to recognise that the highest peak frequency does not always represent the most relevant signature of the fracture mechanism. Moreover, multiple peak frequencies can be associated with multiple fracture mechanisms, such as the fibre pull-out that occurs in the combination of matrix cracking and fibre breakage. Furthermore, no differences were observed in mode I loading conditions between the acoustic emission signatures from the cohesive failure in the adhesive layer and the matrix cracking within the composite substrate. The findings of this study present a great opportunity to gain more insight into the fracture behaviour of polymer materials and fibre-reinforced polymer materials and to improve the quality of adhesively bonded joints.