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.

Adhesive bonded composite joints with an embedded insert consisting of an interfacial hybrid thermoset–thermoplastic bondline could activate an extrinsic toughening mechanism that quadruples the mode I fracture toughness. However, the mechanisms of extrinsic toughening (anchoring, debonding, stretching, detachment), their associated energy dissipation, and the role of bondline parameters (wavelength, porosity, ductility) have not been detailed thus far. Here, we developed double cantilever beam (DCB) finite element models consisting of two rigid composite adherends and an elastoplastic bondline. We prescribed a spatially arranged interfacial/cohesive pattern to simulate the extrinsic toughening and evaluate the increase in fracture toughness. DCB tests were performed to validate the load–displacement curves, fracture toughness, and extrinsic toughening mechanisms obtained from the finite element models. The elastic–plastic energy dissipation during the crack-bridging process was also evaluated using the models. Despite the two-dimensional nature, the modeling results are in reasonable agreement with the experiments, providing an option for further developing a new heterogeneous bondline concept.

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.

Bio-based epoxy materials face major challenges in 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, we manufactured polymer overlapping curls consisting of coiling fibers with sacrificial bonds and hidden lengths through 3D printing. These curls were embedded in a bio-based epoxy aiming to improve its toughness. The bio-based epoxy adhesive layer integrated by such 3D-printed coiling fibers was tested under mode I opening load using Double Cantilever Beam tests. The results show an extrinsic bridging triggered by the embedded curls that promote progressive failure and improve the mode I fracture toughness by 285%. The proposed 3D-printed coiling fibers can improve the performance of biobased epoxies and retard crack growth, opening new horizons for their use in structural applications and the use of these bio-inspired overlapping curls to control crack growth in adhesively bonded joints.

Interfaces play a critical role in modern structures, where integrating multiple materials and components is essential to achieve specific functions. Enhancing the mechanical performance of these interfaces, particularly their resistance to delamination, is essential to enable extremely lightweight designs and improve energy efficiency. Improving toughness (or increasing energy dissipation during delamination) has traditionally involved modifying materials to navigate the well-known strength-toughness trade-off. However, a more effective strategy involves promoting non-local or extrinsic energy dissipation. This approach encompasses complex degradation phenomena that extend beyond the crack tip, such as long-range bridging, crack fragmentation, and ligament formation. This work explores this innovative strategy within the arena of laminated structures, with a particular focus on fiber-reinforced polymers. This review highlights the substantial potential for improvement by presenting various strategies, from basic principles to proof-of-concept applications. This approach represents a significant design direction for integrating materials and structures, especially relevant in the emerging era of additive manufacturing. However, it also comes with new challenges in predictive modeling of such mechanisms at the structural scale, and here the latest development in this direction is highlighted. Through this perspective, greater durability and performance in advanced structural applications can be achieved.

Aiming to aid the sustainable transition to fossil fuel-free epoxy materials and enhance the toughness of bio-based epoxies, here we integrate an overlapping curl microstructure consisting of coiling fiber with sacrificial bonds and hidden lengths into a bio-based epoxy matrix. Inspired by natural material, where exceptional properties are achieved at low environmental cost, the microstructure mimics the molecular structures of spider silk, known for its exceptional fracture resistance. The 3D-printed overlapping curl shows a saw-tooth mechanical response with continuous load-carrying ability thanks to the break of sacrificial bonds and unfolding of the hidden lengths. By embedding the overlapping curl into the compact-tension configuration of the bio-based epoxy, an extrinsic toughening mechanism is triggered as the hidden length unfolds. Experimental results show that a single-sided overlapping curl structure is able to improve the toughness of bio-based epoxy by 19%.

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.

This work aims to improve the damage tolerance of secondary adhesively bonded joints under quasistatic mode I loading conditions by architecting the carbon fibre-reinforced polymer substrates’ stacking sequences [1]. Double Cantilever Beam tests show that architecting the stacking sequence of the laminates composite substrates in combination with the adhesive layer’s fracture toughness affects the crack onset and triggers different crack paths throughout the joints’ thickness. In specimens bonded with a low-toughness bi-component adhesive, the tailored design, including a co-cured toughening layer, could increase the effective fracture toughness of the composite bonded joints up to 200%. From this study, it was possible to recognise the complexity and benefits of moving from the traditional cohesive failure to outbreaking multiple crack path propagation.

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.

Understanding the relationship between the sensors’ outputs and the damage evolution within the joints is becoming increasingly crucial to improving structural health monitoring systems and collecting data to improve the joint’s design. Therefore, a study of the acoustic emission method associated with visual fracture evaluation was proposed to give insights into the toughening of composite bonded joints and better understand the relationship between the acoustic emission features and the damage mechanism involved. Thus, two different layups were proposed for the substrates: [0]8 and [0/902/0]S. In addition, a toughened epoxy adhesive with an embedded carrier (AF163-2k) was used to bond the substrates. Five specimens of each stacking sequence were tested under quasi-static mode I loading conditions. A travelling microscope and a regular digital camera were used on the lateral sides of the specimens to track the crack propagation paths. One piezoelectric sensor linked to the AMSY-6 Vallen system was used to assess the acoustic emission features produced within the joints during the tests. Unsupervised machine learning algorithms based on artificial neural networks and the Morlet continuous wavelet transformation were used to pattern recognition of the acoustic emission data. Self-organising maps, together with k-means algorithms, were used for data clustering. Following that, the acoustic emission features of each cluster were associated with the insights obtained from the crack propagation images. Finally, it was observed that the different layups triggered simultaneous toughening mechanisms. The combination of the acoustic emission and the visual evaluation was crucial for a deeper understanding of the underlying phenomena.

Aiming to increase damage tolerance of adhesively bonded joints, this work explores the influence of CFRP layup of the adherends on the crack onset and crack propagation of composite bonded joints under mode I loading. Quasi-static Double Cantilever Beam tests were performed using four different CFRP layups bonded with two adhesives. Parallel to the experimental program, finite element analyses were performed to aid in understanding and identifying the various damage mechanisms in each specimen type. The results show that the CFRP layup and adhesive fracture toughness significantly influence the joint fracture phenomena at crack onset and further crack propagation. An enhancement of the joint's mode I fracture toughness values at crack onset was observed in the specimens where a crack competition between the propagation within the bondline and the composite's layers was triggered. During crack propagation, the fracture toughness of the joint increases at crack deflections between the different plies of the CFRP layup until reaching the 0° ply, where sudden delamination occurs. It has been shown that CFRP layup tailoring is a promising toughening method that, when carefully designed, has the potential to increase the maximum effective fracture toughness up to 100% when compared to pure cohesive failure.

Carbon fiber-reinforced polymers (CFRPs) have widely attracted the aerospace and automotive industries due to high stiffness and lightweight. Secondary adhesive bonding of CFRPs is a promising research field to fully explore their potential. However, multiple challenges have limited the further application of adhesively-bonded composite joints since it is difficult to inspect the premature debonding, which leads to catastrophic failure once initiated. Thus, it is crucial to introduce crack arrest features, to slow down (or even stop) the crack growth and achieve progressive failure. Various methods have been reported to introduce crack arrest features, including z-pins and corrugated substrates. Our previous work directly utilized the adhesive layer to bridge the separating CFRP parts, through the extrinsic bridging of adhesive ligaments. The bridging adhesive ligaments are triggered by the patterning of distinct surface treatments. These extrinsic bridging ligaments largely enhance the energy release rate (ERR) and successfully arrest the crack propagation. However, a large portion of the required energy for the further crack propagation is stored elastically in the stretching ligaments, which would cause catastrophic fast joint debonding after the failure of ligaments. In this work, the adhesive layer was architected in order to improve its plasticity. By promoting the plastic energy dissipation, the bridging, stretching, and failure of generated adhesive ligaments could result in tougher and safer joints. CFRP substrates were alternatively patterned by two distinct surface treatments to achieve different interfacial strength and toughness values. Then, double-cantilever beams (DCB) were manufactured by bonding treated substrates with the architected adhesive material, such as integrating 3D-printed nylon wires or newly synthesized adhesive material. Results showed that the proposed joint toughening strategy could improve ERR compared to conventional uniform treatments and increasd adhesive plasticity could also stabilize the crack propagation, leading to a safer joint.