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.

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.

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.

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.

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.

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.

Parts manufactured with Laser Powder Bed Fusion (LPBF) are drawing interest in the adhesive joints research because of their high surface roughness, which is usually associated with good adhesion. This work aims to assess the adhesion strength of the inherent surface morphology of LPBF manufactured titanium.

Double Cantilever Beam (DCB) tests were carried out to determine the mode I fracture toughness of joints comprising as-printed titanium (Ti6Al4V) adherends, namely titanium-titanium secondary bonded and titanium-Carbon Fibre Reinforced Polymer (CFRP) co-bonded joints. The effect of high-temperature oxidation on the fracture toughness was also evaluated by testing a batch of joints in which the titanium underwent a post-printing thermal treatment. The as-printed specimens were compared to the same type of joints but with sandblasted titanium adherends to evaluate the effect of this surface pre-treatment on the value of fracture toughness.

The results indicate that non-oxidised titanium joints with untreated adherends had an average of 11% higher fracture toughness than their sandblasted counterparts. On the other hand, sandblasting proved beneficial for oxidised joints, increasing the fracture toughness by 64% on average over the untreated samples.

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.

In marine structures adhesive joint structures are often exposed to cyclic conditioning where the ambient humidity changes cyclically during their service. Though some comprehensive studies on the aging of adhesives exist, these researches mainly focus on monotonic aging conditions where the adhesive joints are exposed to a wet condition continuously for a long time. However, the few investigations performed on the cyclic moisture absorption of adhesive materials show that the parameters obtained in monotonic aging conditions are not suitable for estimation of the aging behaviour of adhesive exposed to alternating humidity. An important question that can arise is whether or not this frequency affects the mechanical behaviour of adhesive joints under cyclic aging condition. In this investigation bulk dogbone and square samples were manufactured, subjected to cyclic aging conditions with four different aging frequencies and tested. The results show that the moisture diffusion constant of adhesives exposed to higher aging frequencies increase more than those exposed to lower aging frequency conditions. In addition, the moisture content only affects the degradation of strength and stiffness of the tested adhesives in different aging frequencies.

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.

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%.

Here we investigate the adhesive properties of Selective Laser Melted (SLM) titanium surfaces in metal-composite cobonded joints without any prior surface treatment, to explore the inherent surface roughness of SLM parts to potentially create strong adhesive bonds. Double Cantilever Beam (DCB) tests were carried out to assess the joints mode I fracture toughness involving untreated SLM titanium and woven Carbon Fibre Reinforced Polymer (CFRP) adherends, co-bonded with an epoxy film adhesive. The same type of DCB joints, but now with sandblasted SLM titanium adherends, were tested to compare the adhesion strength of the untreated SLM surface versus the sandblasted surface. The findings reveal that joints with untreated SLM titanium adherends exhibit similar toughness to those with sandblasted SLM titanium adherends, indicating that the surface morphology of as-printed SLM titanium is suitable for manufacturing robust adhesive joints. The elimination of the surface treatment in the manufacturing of adhesive joints with SLM adherends could increase the interest of many industrial fields towards the use of adhesive bonding over other joining techniques.

For strengthening or rehabilitation of existing structures, patches of fiber-reinforced polymers (FRP) materials are being adhesively bonded to the existing metallic structures. So far, Carbon FRP (CFRP) are the main composite materials industrially implemented for metal structural reinforcement, due to their reliability and high mechanical performances. However, multiple researchers have highlighted the negative environmental impact of synthetic composite materials, and interest is shifting towards the use and development of bio-based composite materials. This paper presents an FE numerical investigation of the use of Hybrid Carbon/Flax FRP as an alternative solution for structural reinforcement of steel plates under flexural loading. Four different configurations of patches are studied to be bonded to steel rectangular plates, and three point bending tests are numerically modelled to simulate the flexural behaviour of the assemblies. Compared to the unreinforced steel plate baseline, reinforcements C5, F5, CFFFC, and FFFCC exhibited improvements of 149%, 120%, 137%, and 145% in bending stiffness, respectively.

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.

When a composite laminate is subjected to humidity, moisture diffusion occurs depending on the number and thickness of the lamina. Water diffusion changes the mechanical response of laminates and usually causes a significant reduction of the mechanical properties of the composite specimens in ocean structures. One of the most important mechanical properties of laminates is flexural stiffness which should be considered in the design procedure. Despite the extensive research on single cycle aging of composites, cyclic aging of these materials is less explored. The aim of the current research is to investigate the variation of mechanical properties of carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP) composites as a substrate in adhesive joints with the same initial flexural stiffness values subjected to cyclic wet/dry aging conditions for long-term structural applications. The matrix used in the CFRP and GFRP composites are based on epoxy and vinyl ester, respectively. Both unaged and cyclically aged samples were characterized by tensile and three-point bending tests. In order to simulate the moisture absorption condition of composites in adhesive joints, one side of the composite laminates was sealed with aluminum foils and three sides were exposed to humidity. The interaction between the composite thickness and the number of aging cycles was also investigated. The experimental results show that in cyclic aging condition, the reduction of flexural stiffness in CFRP is more than GFRP laminates and GFRP laminates is more suitable for ocean applications.

Adhesive joints are frequently exposed to cyclic ageing conditions during their service life, which can have a substantial impact on the mechanical properties of both the adhesive and the substrates. The safe life philosophy, commonly employed in the design of bonded joints, underscores the importance of obtaining an accurate estimate of the adhesive's durability. Therefore, it is essential to enhance the predictive capabilities of the adhesive's mechanical behavior under cyclic ageing conditions. This research aims to expand the use of quasi-static cohesive zone modelling (CZM) for damage and fracture analysis of dissimilar adhesive joints subjected to cyclic ageing environments. The first step involved measuring the mechanical properties of the adhesive through tensile tests on unaged and cyclically aged dogbone specimens, considering their moisture content and ageing cycles. Based on the results, a degradable CZM was developed. To validate the numerical model, dissimilar double cantilever beam specimens (DCBs) of glass fibre reinforced polymer (GFRP) and aluminium were manufactured and tested before and after ageing. The load-displacement curves of the bi-materials bonded joints were successfully predicted using the developed model where the properties of the material are defined as a function of the moisture uptake and ageing cycles at each material element. The obtained results showed that after 4 ageing cycles, the maximum load of DCB specimens decrease considerably.

This chapter discusses the mixed-mode loading of adhesive joints. The importance of mixed-mode loading is first introduced and then test methods commonly used to measure the mixed-mode fracture resistance of adhesive joints are presented and briefly discussed. The approaches to determine the fracture resistance are briefly reviewed and then the partitioning of mixed-mode fracture energies is discussed. The limitations of the local singular field and global approaches to mixed-mode partitioning are discussed and the use and application of a semianalytical cohesive zone analysis partitioning scheme is evaluated. The limitations of the global partitioning approach are further discussed in the context of developing a scheme to design and analyze adhesive joints with dissimilar adherends (a bi-material interface). A longitudinal strain criterion is proposed in addition to the matching of flexural rigidities and the approach is validated numerically. Finally, the practical issues of crack stability, failure path selection, and the use of mixed-mode failure envelopes is considered.