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.

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 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.

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.

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.

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 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.

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.

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.

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.

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.

Lining techniques for the treatment of structurally damaged canvas paintings have been in use since at least the seventeenth century, with on-going invention, development, and refinement. These systems can be categorised based on their adhesive component – natural or synthetic – or by their application procedure, such as water-based, hot-melt, cold-lining, nap-bond lining, or mist-lining. The choice of lining system is often influenced by geographical practice and individual expertise rather than purely material-technical considerations, as comprehensive data for benchmarking different systems is limited in both literature and practice. This paper aims to address this gap in knowledge by comparing various lining adhesives and their application techniques. The adhesives under examination include glue-paste, wax-resin, BEVA® 371, Plextol® B500, and Dispersion K360:Plextol® D512. Mock-linings were designed to reduce variables and ensure standardisation. Previously reported recipes and descriptions of studio application techniques were used where possible to create the mock-linings. These were subsequently subjected to stress/strain through exposure to cyclic fluctuations in relative humidity (RH) and temperature (T) to simulate mechanical-physical ageing. Both unaged and aged samples underwent lap-shear and T-peel tests according to ASTM standards, as reported in earlier studies. The data presented here can assist conservators and scientists in establishing requirements and making informed decisions tailored to the specific needs of each painting. Results indicate that each lining technique has its own limitations, and the suitability of a given technique will depend on the type of treatment necessary for stabilising individual paintings.

Biobased fibre-reinforced polymer (FRP) composites, consisting of natural lignocellulosic fibres such as flax or hemp, are great alternatives to synthetic fibres to mitigate the environmental impact of high-performance composites in engineering structures. Natural fibres such as flax have damping and specific mechanical properties suitable to potentially replace glass fibres in FRP composites in engineering structures. However, structural design with flax FRPs can be challenging for engineers due to their rather peculiar mechanical responses thanks to the complex multi-scale microstructure of the flax fibres. In particular, flax FRP composites have shown large ratcheting (accumulation of plastic deformation) and stiffness increase when subjected to tensile fatigue loading. Therefore, this paper proposes a novel yet simple 'pre-straining' method as a promising strategy for improving the fatigue response of flax FRP, to potentially replace synthetic glass FRP in various engineering structures. To this end, cross-ply flax, and glass FRP composite laminates were manufactured and subsequently tensile-tensile fatigue experiments were performed. It was observed that pre-straining of flax FRP composite coupons can improve their mechanical performance by increasing stiffness and reducing ratcheting during fatigue which is attributed to further alignment of the fibres within the twisted yarns, as well as possible microfibril alignment. The pre-straining of glass fibre reinforced composites samples did not lead to any remarkable reduction in ratcheting nor increase in stiffness.

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.

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.

Synthetic fibre-reinforced polymer composites (FRPs) have long been favored in structural engineering for their exceptional mechanical properties. However, their environmental impact due to energy intensive manufacturing, and disposal has prompted exploration into sustainable biobased alternatives such as flax FRP composites (FFRPs). While using flax fibres as composite reinforcement has lightness and damping benefits from a structural point of view, it also introduces design challenges as the complex microstructure of flax fibres and flax FRPs induces a more viscoelastic fatigue response. Therefore, prestraining and pre-creeping are proposed in this study as simple methods to improve fatigue performance by taking advantage of alignment mechanisms intrinsic to flax fibre and yarn microstructure. An experimental campaign was conducted on [0/90/0]S flax FRP laminates (hence, predominantly UD) to compare the tension-tension fatigue performance of reference specimens to pre-strained and pre-creeped specimens. It was observed that pre-straining and especially pre-creeping are effective at improving FFRPs fatigue performance with significant increase in fatigue life, increase in dynamic modulus, and decrease in accumulation of deformation (ratcheting).

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. 

Gelatine adhesives, also known as animal glues, are collagen-based water-soluble biopolymers derived from vertebrate connective tissues. One of the various fields in which gelatine adhesives are widely used is the conservation of cultural heritage such as decorated furniture and panel paintings. It is observed that, with time, the failure in these objects often occurs along the adhesive bondlines. Given the moisture and temperature sensitivity of these adhesives, obtaining knowledge of their long-term behaviour, when exposed to climate variations, is pivotal. Here, the influence of hygrothermal ageing (exposure to a combination of elevated temperature and relative humidity (RH) cycling) on the microstructure and macroscopic properties of four different types of gelatine adhesives is investigated. These adhesives were selected from different animal origins namely bovine, rabbit, and fish with different Bloom strengths. It was observed that ageing cycles interfere with the most critical structural feature of protein chains namely triple helices. A clear decay in triple helix content at the micro-scale, determined by Differential Scanning Calorimetry (DSC) and X-Ray Diffraction (XRD) techniques, was observed which had implications on the macroscopic properties of these adhesives such as reduction of strain to failure and toughness (strain energy density to failure). The rate of decay in properties was revealed to be the highest in the adhesives with the lowest triple helix content. This study provides a scientific view of microstructure-property relationships in gelatinous adhesives as a function of environmental ageing, and stipulates the underlying mechanism of the degradation of mechanical properties as the loss of structural triple helices, regardless of the animal origin.

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.