Given the long-term use of carbon fibre reinforced polymers (CFRP) in harsh environments, this study investigates the isolated and combined effects of temperature and moisture variations on mode I fatigue delamination propagation. Several levels of temperature and relative humidity were applied as preconditioning and as in-service during fatigue testing to evaluate their effects on the Paris curve. In addition, statistical analyses, including analysis of variance (ANOVA), semi-empirical interpolation modelling, and fractographic assessments, were conducted to provide a comprehensive understanding of the failure mechanisms. The results indicate that the moisture absorbed during hygrothermal preconditioning and the in-service temperature applied during fatigue test individually affect the Paris curve slope. These factors interact synergistically, significantly altering the fatigue crack growth rate. An empirical model capturing this interaction showed good agreement with experimental data, enabling reliable prediction of environmental degradation trends. Fractographic evidence supported the observed changes in fracture patterns, linking changes in fibre bridging formation, surface roughness, and energy dissipation to the observed shifts in fatigue behaviour.

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.

Fibre bridging in laminated composites has a significant effect on Mode I delamination behaviour, resulting in improved opening resistance and altered fatigue crack growth rates. This study investigates the sensitivity of a recently developed superposition model to capture monotonic and cyclic bridging contributions to fatigue delamination. Double cantilever beam (DCB) specimens were tested under quasi-static and cyclic loading to derive R-curves and bridging stress profiles. A set of eleven different parameter combinations were used to generate zero-bridging Paris curves, followed by ANOVA based sensitivity analysis. The results show that the fracture toughness parameters G0 (initiation) and Gs (saturation) have the strongest influence on the Paris parameter coefficients, while the maximum end-opening δ* plays a secondary role. Heat map-based analysis shows that larger differences between G0 and Gs lead to stronger bridging effects and more conservative fatigue curves. This framework provides valuable insight into data-driven calibration of bridging models and supports application-specific fatigue design strategies.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Fiber metal laminates (FMLs) have mainly been used in aerospace applications with synthetic fibers. To improve their environmental credentials and address issues regarding the end-of-life of these materials, a shift to FMLs based on natural fibers can be a promising course of action. However, regarding them as conventional FMLs overlook some of the unique benefits of natural fibers. Therefore, this study pioneers the examination of FLAx-REinforced aluminum (FLARE) for its combined impact resistance and vibration damping. 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 (by 80% compared to the flax composite), approximated via an inverse mixture rule. Low-velocity impact tests highlight the role of aluminum layers in energy absorption and the composite strength as a critical factor in impact resistance. FLARE exhibits 25% less specific energy absorption compared to its glass fiber counterpart. A quasi-static analytical model suggests the potential of FLARE for practical applications. With its balance of properties and considering its potential advantages at end-of-life, allowing recycling of aluminum, and its expected lower carbon footprint, FLARE renders potential beyond the aerospace sector, e.g., in other forms of transportation.

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.

In this study, the hybridization of flax and silk fibre reinforced composites, at ply level, were studied and compared to their monolithic counterparts. Hybrid FRP laminates were produced via the filmstacking compression moulding method using the highly ductile thermoplastic high-density polyethylene grafted with maleic-anhydride as the matrix. The fibre volume fraction ratio between the two fibres and the lay-up configuration were studied, resulting in hybrid composites with different degrees of distribution of the flax and silk plies within the laminate. The results showed that more balanced properties in terms of stiffness, strength, strain to failure, and impact energy absorption can be achieved by varying those two parameters, thus increasing the designing freedom tailored to specific engineering applications. Additionally, the findings revealed that by optimizing those parameters, multiple fractures of the flax plies or “fragmentation” can be achieved as a toughening mechanism, leading to a pseudo-ductile hybrid composite with a gradual or delayed failure development. The fibre fragmentation mechanism, apart from the increased ductility, can be potentially used as a failure detection criterion for structural health monitoring.