This study focuses on interlayer hybridisation of flax with silk fibres and the resulting damage mechanisms controlled by the hybrid composite configuration, can lead to an improved balance between stiffness, strength and toughness/ductility. The results demonstrate that a sandwich design configuration with flax layers at the outside of the laminate exhibit the highest increase in (pseudo-)ductility compared to monolithic flax fibre composites. X-ray computed tomography (XCT) revealed that fragmentation and debonding of the flax fibre layers can be achieved by optimising the hybrid laminate configuration and the volume fraction ratio between the two fibres, explaining the increased toughness of the hybrid composites.

Fiber metal laminates (FMLs) or metal-composite hybrid materials synergize the advantages of metals and composites, in particular, they combine the impact resistance of metals and the excellent fatigue and corrosion resistance of fiber-reinforced polymers. FMLs have been mainly used in aerospace applications with synthetic fibers as in GLARE. However, with the rising concerns about climate change, and the issues of recycling glass fiber composites, a new generation of FMLs with a reduced carbon footprint could be a promising course of action. This can be achieved by using bio-based fiber-reinforced composite layers, particularly flax instead of glass fiber composites, rendering FLAx REinforced Aluminum (FLARE), a partially biobased FML with lower embodied energy, in which aluminum layers can be recycled by incineration with energy recuperation of the flax composite. Contrary to conventional FMLs, FLARE can entail some unique benefits of natural fibers such as vibrational damping, thanks to the intricate flax fiber microstructure. Flax fibres demonstrate promising specific mechanical properties compared to glass fibres, particularly regarding tensile stiffness and bending stiffness and strength. This means that flax fibres can outperform glass fibres in stiffness-based designs, particularly in bending mode. This includes applications in the transportation and construction sectors as well as secondary structures for civil aircraft.

This study pioneers the examination of FLARE, focusing specifically on its key distinguishing features, namely its vibration damping and impact resistance capabilities which were not previously scrutinized. Dynamic mechanical analysis and vibration beam tests demonstrate that the metallic layer predominantly influences the damping behavior of FLARE. The loss factor notably decreases with aluminum addition approximated via an inverse mixture rule.

The low-velocity impact resistance of FLARE was compared with that of E-GLARE, with a focus on assessing the influence of MVF and fiber type. Impact tests highlight the role of aluminum layers in toughening and energy absorption and the composite strength as a critical factor in impact resistance. FLARE exhibits improved specific energy absorption compared to monolithic flax fiber composites, though 25% reduced energy absorption compared to E-GLARE counterpart. A quasi-static analytical model provides initial impact response estimations, validated by experimental data.

The study underscores the potential of FLARE to enhance the use of bio-based materials in structural applications, offering good mechanical properties thanks to FML concept, and improving the moisture sensitivity of bio-composites with metal acting as a protective layer. Combining flax fiber composites with metal results in a material with specific stiffness comparable to E-GLARE and superior to GFRP. Thus, for applications relying on stiffness-based designs, FLARE emerges as a more environmentally friendly alternative to both E-GLARE and GFRP, addressing recycling challenges effectively.

Finally, this study presents a first overview of the properties of FLARE and verifies the validity of the predictive tools developed for conventional FMLs which help in the design phase to optimize the structure according to specific requirements.

Studies have shown that temperature and moisture play a critical role in altering material properties, with both factors contributing to the overall degradation of structural components. This research aims to provide a deeper insight into the complex interplay between environmental factors and fatigue delamination behaviour in composite materials. To this end, the effect of hygrothermal aging and in-service temperature conditions on mode I fatigue delamination was investigated considering the relationship between fracture surface patterns and fatigue propagation behaviour. To investigate the effects of in-service temperature, Mode I delamination tests followed standardised protocols with pristine (unaged) samples tested at -40, 22, and 80ºC. Hygrothermal ageing (HA) was performed in a climatic chamber at 90ºC and 90% relative humidity for 60 days. After reaching an equilibrium moisture content of 1.2%, the samples were tested at in-service temperature levels of -40, 22, and 80ºC. The in-service humidity was set at 50% in all cases to ensure control of the absorbed moisture under the previous HA conditions. The fatigue curves were remarkbly affected by hygrothermal ageing, as a consequence of the change in the material properties. The individual effect of in-service temperature leads to a temperature-dependent variation in the slope of the da/dN versus dU/dN curves. On the other hand, hygrothermal ageing consistently reduced the slope of all curves, compared to pristine samples (Fig. 1a). The results confirm that temperature inversely affects the slope of the fatigue propagation curve, while hygrothermal ageing promotes a tougher fracture behaviour. The fractography investigation reveals that the surface roughness (Fig. 1b) follows the same trend as the dU/dN slope: higher slopes (lower temperature) correspond to less damage (lower tortuosity), while lower slopes (higher temperature) indicate more damage represented by higher surface roughness.

This study focuses on interlayer hybridisation of flax with silk fibres and the resulting damage mechanisms controlled by the hybrid composite configuration, can lead to an improved balance between stiffness, strength and toughness/ductility. The results demonstrate that a sandwich design configuration with flax layers at the outside of the laminate exhibit the highest increase in (pseudo-)ductility compared to monolithic flax fibre composites. X-ray computed tomography (XCT) revealed that fragmentation and debonding of the flax fibre layers can be achieved by optimising the hybrid laminate configuration and the volume fraction ratio between the two fibres, explaining the increased toughness of the hybrid composites.

Trees exhibit adaptability in response to external loads, which allows them to form an inosculated connection (self-growing connection) with a neighboring tree. Such connections have the mechanical potential to build living tree structures. Although qualitative studies have studied this phenomenon, quantitative analysis of its growth features remains limited. Self-growing connections fused by weeping figs (Ficus benjamina L.) are utilized to study growth features. X-ray scanning and optical microscopy techniques are employed to investigate parameters including density, geometry, fiber structures, and material compositions. Key findings demonstrate that the fused region of a connection has a larger volume and a higher density on the intersected surface. Microscopic analysis identifies that the enlarged wood in the fused area is tension wood characterized by G-layers. The key component that connects trees is referred to as merged fibers, and the pattern of their distribution is found to be mainly in the outer layer of the larger cross-angle of a connection. At the cellular level, crystals within cells are identified in the fused region, implying possible mechanical stresses the interface has experienced. The findings in self-growing connections can serve as inspiration for structural design in living structures, biomimicry, bioinspired structures, and advancements in bioeconomics.

Delamination fatigue propagation is known to cause a progressive degradation of stiffness and strength in composite laminates. Since delamination tends to follow a preferential plane, fracture resistance is conveniently analysed in terms of dominant loading modes at the crack tip: mode I (opening) and mode II (shearing). To this end, coupon tests can be performed to determine the growth rates under these particular stress states. Paris parameters from such tests are then often used in numerical implementations adopting mesoscale modelling, like in the case of cohesive element traction-separation laws. The majority of coupon tests available in the literature focus on interfaces where the fibres of the upper and lower plies have the same orientation, typically aligned with the direction of delamination growth. However, most practical applications involve multidirectional laminates, where delamination tends to develop at interfaces where the upper and lower plies have mismatching angles. Studying angled interfaces may lead to different results since some fracture phenomena, like fibre bridging and crack migration, are highly dependent on fibre orientations of plies adjacent to the delaminated interface [1].
The present work experimentally explored the various effects of fibre orientation on fatigue delamination growth in the different fracture modes. IM7/8552 carbon fibre epoxy prepreg (Hexcel), a material system commonly adopted in aerospace field, was tested under mode I Double Cantilever Beam (DCB), mode II End-Loaded Split (ELS), and Mixed-Mode Bending (MMB) tests. For all cases a combination of different interfacial fibre orientations were tested and the crack growth rate curves were compared in relation to the observed fracture behaviour.

As a sustainable and eco-friendly material, flax fibres offer a viable alternative to glass fibres in composite applications due to their good specific mechanical properties. However, addressing their moisture sensitivity is crucial to expanding their use in various applications. This study investigates the impact of enzymatic treatment on improving the moisture resistance of flax fibres. FlaxTapeTM 200 was treated with two types of polygalacturonase enzymes to selectively remove pectin. The moisture resistance of the treated fibres and their composites was compared with that of untreated samples. The results revealed a significant reduction in moisture uptake at high relative humidity conditions and a decrease in percentage water uptake in both longitudinal and transverse composites after enzymatic treatment. FTIR spectra and contact angle measurement results supported the observed improvement in the moisture resistance of flax fibres. This study highlights the effectiveness of enzymatic treatment in enhancing the moisture durability of flax fibres which further broadens their potential for structural and lightweight composite materials.

This study presents a new method with improved accuracy for measuring the tensile properties of elementary flax fibres using an automated single-fibre tester with Digital Image Correlation (DIC) for strain tracking, validated with glass fibres of known properties. Modulus values were obtained for glass fibres (83±5.17 GPa, 12mmGL; 79.4±1.33 GPa, MidFibre) and elementary flax fibres (77.0±15.3 GPa, 12mmGL; 74.8±20.2 GPa, MidFibre) using speckle patterns on both end tabs and on optical flags attached to the fibres. This study highlights the advantages of automated single-fibre testing and optical extensometry for reliable and efficient measurement of tensile properties of single fibres.

The accurate prediction of fatigue life in fibre-reinforced polymer (FRP) composites remains a central challenge in structural engineering, due to the extensive duration and cost of conventional fatigue characterisation. To address this, physics-based approaches offer an appealing alternative by reducing reliance on repeated mechanical testing. One such approach [1], [2], originally developed for metallic systems, estimates fatigue life by comparing the cumulative energy dissipated under cyclic loading to the total energy dissipated in a monotonic test. While promising, the application of this method without prior fatigue data necessitates assumptions regarding the evolution of energy dissipation during cyclic loading. Consequently, these assumptions may limit the accuracy and generalisability of the approach, and in practice, calibration with at least limited fatigue test data is often required to enable reliable application.

Therefore, this study proposes a novel methodology to estimate fatigue energy dissipation in FRP composites using only monotonic test data. The approach introduces the total work ratio (RW,tot), defined as the ratio between the cumulative dissipated work and the cumulative applied work over the fatigue life. Provided the applied work can be determined, based on material stiffness and loading parameters, RW,tot enables estimation of fatigue energy dissipation. Because the method is grounded in monotonic experiments, it inherently captures material-specific dissipative mechanisms.

The methodology is validated through experimental testing on a [0/90/0] glass FRP laminate and two flax fibre-reinforced biocomposite laminates: [0/90/0]S and [(+45/−45)2]S. Fatigue results indicate a linear dependence of RW,tot on the applied stress level that interestingly align with monotonic results. For the [0/90/0]S flax composite, this linear relationship intersects the origin, allowing direct estimation of RW,tot in fatigue solely from monotonic data under matched strain rates. In contrast, the [(+45/−45)2]S laminate does not exhibit origin-crossing linearity, potentially due to time-dependent mechanisms such as viscoelastic creep.

While further investigation is required to generalise the method across diverse laminate architectures, the findings highlight a simple, experimentally grounded, and physically interpretable approach for estimating energy dissipation in fatigue of FRP composites, potentially enabling more efficient fatigue life prediction.

This study investigates the interface between hemp fibre and thermoplastic polymer matrices such as polypropylene (PP) and poly-acrylate (Elium®). The analysis was conducted using both traditional dynamic contact angle measurements with various probe liquids and a more novel approach involving inverse gas chromatography (IGC) with multiple gaseous probes. The goal was to predict fibre–matrix compatibility based on surface chemical characterisation. Both methods showed good correlation, with the dispersive surface energy of the natural fibres ranging between 33 and 39 mJ/m², and a significant basic component contributing to the overall polar surface energy. IGC proved more effective at distinguishing between the predominantly apolar PP and the more polar poly(methyl methacrylate) (PMMA) based Elium, with the predicted thermodynamic work of adhesion notably higher for hemp–Elium interfaces compared to hemp–PP. Micromechanical testing, combined with microscopic imaging, further supported the observation of improved intermolecular adhesion between the natural fibres and the Elium matrix.

This study investigates the effects of hygrothermal conditions on the fatigue performance of flax FRP composites. Cross-ply laminates were tested in tension-tension fatigue in five different hygrothermal conditions. Humidity was initially expected to enhance fatigue life at 30% RH and reduce it at 90% RH relative to the reference 50% RH, based on the modulus variations observed in quasi-static tests. However, experimental results indicated the opposite trend, with a remarkable ~10-fold increase in fatigue life under high-humidity conditions. Temperature effects were also found to have a significant impact but only at high temperature and high stresses displayed by a change of the S-N curve slope.

Fibre bridging is an important phenomenon influencing the mode I delamination growth behaviour in composite materials. Accurate modelling of this phenomenon is required in order to be able to account for its effects in damage tolerance evaluation of composite structures. Therefore, this study introduces a novel physical model to isolate and quantify the contribution of fibre bridging to Mode I fatigue delamination. The model distinguishes between monotonic and cyclic components of fibre bridging stress, capturing their individual effects on the strain energy release rate (SERR) in the Paris curve. The monotonic component, based on the Sørensen model, accounts for pre-cracking effects, while the cyclic component is derived by integrating a bridging stress function over the end-opening displacement, with both components modelled by empirical exponential relationships. The model has been validated against established methods such as the Yao model and specific extrapolation techniques, demonstrating improved accuracy in fitting the Paris curve, particularly in accounting for the monotonic influence in the shift of the SERR and the cyclic contribution to the curve slope. Importantly, the model requires only one quasi-static and one fatigue test, reducing the experimental workload. In conclusion, this method provides a more accurate characterisation of fibre bridging effects, making it a robust tool for fatigue delamination analysis.

Fatigue behaviour of fibre-reinforced polymers (FRPs) in laboratory is typically evaluated under continuous loading. However, real-life loading scenarios of structures, e.g. bridges or wind turbine blades, often involve complex histories. These include fatigue loading interruptions, creep, combined creep-fatigue, or peak loads. While such variations may be negligible for elastic carbon and glass fibres, the viscoelastic nature of flax fibres makes them sensitive to complex loading patterns, potentially affecting the fatigue performance. Moreover, some flax preforms are made of twisted yarns, adding one more level of complexity to the hierarchical microstructure of flax FRP laminates. However, the effects of auxiliary loading sequences and the microstructure at the yarn/fibre levels, on the fatigue behaviour of flax FRPs remain largely unexplored. Therefore, this paper pioneers investigation of these effects, giving insights on how to exploit microstructural re-arrangements, preloading, and load interruptions to tailor fatigue response of flax FRPs in comparison to glass FRPs. The findings reveal that the yarn un-twisting significantly influences fatigue behaviour, leading to a doubling of strain accumulation, and dynamic stiffness increment, compared to flax FRPs with straight fibres.
Additionally, the pre-creeping and fatigue interruptions were found to substantially impact fatigue life, particularly in laminates with yarn twist, leading to a 1.7-fold increase due to interruptions and a threefold increase following pre-creeping. The latter also yielding a near-elimination of strain accumulation. Therefore, pre-creeping is proposed as an effective strategy to reduce in-service strain accumulation and extend fatigue life in predominantly UD flax FRPs with twisted yarns.

To achieve the integrity of the honeycomb structure without interlocking or bonding, a 3D braided honeycomb structure was designed and developed using natural fiber (jute) reinforced epoxy resin. To optimize the in-plane compressive performance of 3D braided composite honeycombs, the effects of resin, unit cell opening angle, and joint wall length on the in-plane compressive strength, deformability, stiffness and energy absorption were investigated. Furthermore, a theoretical model was established between braiding parameters, geometric parameters of honeycomb cell structure, and in-plane compressive modulus. It is found that if 3D braided composite honeycombs are designed for high load-bearing capacity, it is necessary to reduce the resin strain to failure, increase free wall columns, align the angle between the free wall with the main loading direction, or load along the braiding direction. If the design objective is to maximize the energy absorption, the number of cell rows must be maximized. This study establishes the relationship between braiding parameters, honeycomb geometric parameters, and the in-plane compression performance of the 3D braided composite honeycomb, providing a reference for its structural design and performance optimization.

Characterisation of the effect of lay-up on the delamination growth revealed a complex set of damage characteristics. Tortuous propagation resulted in higher fibre bridge densification in the off-axis laminates, requiring an increased number of experimental tests to validate the characterisation of fibre bridge saturation effects. The main objective of this research is to apply a modified Sørensen model [1] to measure the fibre bridge (FB) stress curve. In addition, the research aims to estimate the full saturation curve. These objectives will contribute to a more efficient and accurate characterisation of fatigue delamination behaviour in composite materials. The procedure was carried out using a model proposed in the literature [2] for unidirectional composites, to estimate positions of the sull-saturated and fibre bridge Paris curves. The proposed model, based on physical considerations of FB stress formation, was applied to a quasi-static curve and a fatigue curve for each lay-up: 0//0, 0//45 and 0//90. Based on the resulting R-curve, the FB stress curve is derived for each case. This approach accurately models the fatigue behaviour and bridging effects in composites by providing the zero bridging curve (Fig. 1). Following a similar procedure, but now adding the full saturation stress integrated along the end opening delamination, provides the additional strain energy in the steady state region (Fig. 1). This approach allows a more efficient and cost-effective characterisation of fatigue delamination behaviour with a reduced number of experiments, and evaluates the feasibility of applying the proposed model to a single quasi-static and fatigue curve. In addition, the proposed method addresses a comprehensive understanding of fatigue behaviour considering the effect of fibre orientation, thereby contributing to a more comprehensive understanding of fatigue behaviour.

The aim of this study was to determine the effect of moisture cycling (environmental relative humidity cycles) on the durability of flax-epoxy composites and investigate the influence of cycling duration and temperature on the stiffening and strengthening of flax fibres. Four moisture cycling protocols for flax fibres were employed in this research which includes 4D21 (4days per cycle at 21ºC), 4D60 (4days per cycle at 60ºC), 3H27 (3hours per cycle at 27ºC) and 3H60 (3hours per cycle at 60ºC). To measure the impact of high-low humidity cycling at different cycling durations and temperature, tensile testing of impregnated fibre bundle test (IFBT) samples was done. Results of the back-calculated properties revealed that the applied cycling protocols enhanced both the tensile strength and modulus of the fibres. Better improvement of tensile properties was observed in fibres cycled at longer duration. The fibres undergoing 4 days of cycling at 21ºC (4D21 fibres) showed the highest improvements in tensile strength (18%), as well as tensile moduli E1 (19%) and E2 (18%) after 10 cycles. Interestingly, all fibres showed increased stiffness (E1) in the range of 8-20% after 10 cycles and 4-8% after 20 cycles. This fibreimprovement in mechanical strength and stiffness of the fibres can possibly be attributed to a phenomenon similar to a hornification effect in wood or possibly by fibre repair due to pectin migration, which produces the strengthening and stiffening effect.

Laminated pultruded composite plates are gaining interest for use in wind turbine blades due to their excellent structural performance with affordable cost. However, there is limited understanding of their fracture properties. The present work explores the interlaminar fracture behaviour of pultruded composite plates, bonded through resin infusion, to form thick CFRP structures. Mode-I, −II, and mixed-mode (I/II) tests were performed to obtain fracture properties at different mixed-mode ratios. Mode I crack propagation exhibits stick–slip behaviour, resulting in brittle failure in a few steps, while mode II provides more stable crack propagation along with cohesive failure. The mixed-mode fracture patterns follow the trend of the mode-mix ratios, in which higher mode-mix ratios (more mode II) induce more stable crack propagation. Benzeggagh-Kenane and power law criteria were compared regarding their prediction of crack initiation toughness given a mode mix ratio, and a linear relation between the mixed mode I/II fracture toughness components could exist at interfaces of laminated pultruded plates. Meanwhile, applicability of testing standards and the effect of manufacturing-induced defects on fracture properties are thoroughly discussed. The results show that existing standards provide sufficient support for characterising fracture properties of bonded pultruded plates; and that manufacturing-induced defects can be detrimental to crack propagation and cause more brittle behaviour in mode I dominant cases, while beneficial effect of defects by toughening the interface was exhibited in mode II dominant cases.

This paper investigates the fatigue-induced delamination growth in carbon fibre-reinforced polymer (CFRP), considering different fibre orientation combinations. The study explores the application of Artificial Neural Networks (ANN) in the simulation of fatigue delamination behaviour to reduce the number of experimental tests required for fatigue evaluation and eventual certification. The research aims to evaluate the effectiveness of ANN at different stages of data processing, including raw data simulation and final curve estimation. The results show that applying ANN at the raw data stage provides flexibility in modelling, with error < 10%. In addition, when ANN is applied directly to the final Paris curve, it minimises errors and increases reliability, allowing for a more cost-effective fatigue evaluation process. The study highlights the importance of the data processing stages in determining the accuracy of fatigue delamination predictions with AI modelling, thus informing strategies for efficient fatigue evaluation of CFRP components in structural applications. 

Similar to wood, adhesives may exhibit duration of load effects. When loaded for longer periods of time, damage processes in the material may develop, eventually leading to failure. From wood research it is known that load level, temperature and relative humidity have an important influence on this behaviour. In general, higher stress levels, temperatures, and moisture content will lead to shorter times to failure and these effects may be more pronounced in loading directions such as shear or tension perpendicular to the grain. It is shown that the reaction kinetics based approach for damage accumulation effects in polyurethane based adhesives can be described using the same non-linear damage accumulation expression as used for wood. The relationship between the time to failure and load-level as influenced by for instance temperature is determined for lap joints, immersed in hot water with temperature of 60oC and 90oC, and at load levels varying between 30 and 90% of the mean short term shear strength.

It is shown that a non-linear damage accumulation expression as used for wood, can also be used for damage accumulation effects in melamine-urea-formaldehyde adhesives. The relationship between the time to failure and load-level as influenced by temperature is determined for beech lap joints loaded in tensile shear. The specimens have been immersed in hot water with temperatures of 60oC and 90oC respectively, and at load levels varying between 30 and 90% of the mean short term shear strength.

Driven by the energy transition and the reduction of carbon emissions, pultruded CFRP plates have emerged as an affordable high performance material for designing wind turbines with larger rotors. To enable the creation of thicker laminates, pre-cured plates are bonded together using an epoxy resin. The present study focuses on characterizing the fracture behaviours of these laminated bonded plates under different modes, and to generate fracture criteria for numerical simulations establishing design allowables and service life. Double cantilever beam tests, end-loaded split tests, and mixed-mode bending tests were conducted under quasi static loading for such plates. Mode I crack propagation showed brittle failure while mode II fracture tests on the other hand, presented a more stable crack growth. Finally, power law and Benzeggagh-Kenane law criteria were established to numerically describe the fracture behaviours. Overall, the results show that the available standards for fracture toughness testing are also suitable for these laminated-pultruded composite plate structures, enabling the material characterization needed for design.