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.

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.

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.

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.

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.

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. 

3-D braided composites are a promising material for manufacturing tubular structures. However, a thorough understanding of their damage mechanisms under torsion is required to maximize their potential applications. The present work constructed a multiscale equivalent model, integrating mesoscopic and homogeneous structures to reveal torsional behavior of 3-D braided carbon fiber/epoxy resin composite tubes. The cumulative failure process, spatial stress distribution and interface damage were calculated to illustrate stress transfer and damage initiation and propagation. It is found that stress varies on the surface and internally within the representative unit cell (RUC). The yarns experience both axial tension parallel to the direction of torsion and axial compression perpendicular to the direction of torsion. The stress difference between them leads to damage initiation and propagation. Interfacial cracking as main damage mode hinders the stress transfer between resin and fiber bundles. The results show that the braided yarn path, axial stress dispersion in two directions and localization of damage effectively impede the torsional damage propagation.

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.

This study aims to improve laboratory aging procedures for bituminous materials to better replicate field conditions. Two binders and mixtures were subjected to various levels of humidity, temperatures, pressures, film thicknesses, and aging durations. By comparing these lab-aged samples to field-aged samples, the study aims to simulate real-world aging more accurately. Fourier-transform infrared (FTIR) spectroscopy and frequency sweep tests were employed to analyse these samples. Multivariate techniques—Principal Component Analysis (PCA), Multiple Linear Regression (MLR), and Support Vector Regression (SVR)—were used to explore chemical and rheological relationships, evaluate the interchangeability of aging factors, and quantify the equivalency between laboratory and field aging. The findings revealed that increased temperature, pressure, and duration lead to more oxidative products. The PCA distinguished between two binders and aging trends, highlighting the importance of both FTIR and rheological measurements. The SVR model demonstrated strong predictive performance for rheological properties, identifying critical FTIR region, 710–912 -1cm. By MLR model, optimal aging conditions to simulate nine years of field aging for porous asphalt and stone mastic asphalt were back-calculated. The Euclidean distance found laboratory conditions that closely match field-aged samples. SVR models provided predictions of simulated field aging time for various laboratory aging conditions.

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.

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.

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).

The development of bio-based fibre-reinforced polymer composites (FPRs) has accelerated in recent years aiming at replacing synthetic FRPs in primary structures. Comparative case studies on biobased fibres (such as flax and hemp) indicate their potential to replace synthetic glass and carbon fibres in certain FRP structures as more sustainable and environmentally friendly alternatives. In those studies, the effects of in-situ hygrothermal conditions on the physical and mechanical performance of the natural fibre composites were generally overlooked, however, due to hygroscopicity of lignocellulosic natural fibres, flax fibre composites mechanical response is sensitive to its environmental temperature and relative humidity. To quantify and understand the effects of in-situ hygrothermal conditions on flax FRP composite, a quasi-static tensile testing campaign was conducted at various temperatures and relative humidities. Glass and flax FRP laminates were tested in cross-ply and angle-ply configurations. It was observed that the strength and stiffness of cross-ply and angle-ply FFRP laminates were significantly affected by relative humidity contrary to GFRP counterparts. All laminates exhibited strength and/or stiffness variation with temperature that can be attributed to the sensitivity of the common epoxy matrix. However, a distinct difference was observed between the flax and glass cross-ply laminates with the modulus of the flax FRP being highly affected by temperature and relative humidity while the glass FRP modulus was unaffected.

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.

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.

To comply with the trend in the development of engineering materials towards lightweight, high strength, eco-friendly, sustainable, and multi-functional, a three-dimensional braided integrated composite honeycomb is designed. The effects of geometrical parameters particularly joint wall lengths on the in-plane mechanical behavior of the honeycombs were investigated. The results show that the in-plane mechanical properties are related to the number of cell walls, and the angle between the cell wall and the loading direction. Increasing the number of cell rows to double and triple at similar areal density lead to an improvement of the maximum load up to 2.5, and 3.8 times, respectively. Similarly, the total absorbed strain energy increased up to 2.6 and 5.9 times, respectively. The displacement at the maximum load is increased by 1.6 and 2.7 times as a result of increasing the cell row number. The total absorbed strain energy increased to 1.7 and 1.3 times, respectively. The failure angle of the 3D braided composite honeycomb is about 4°–7°. This investigation presents the geometrical factors of a 3D braided composite honeycomb can be further designed and optimized, but it also provides a reference for the development and design of a new composite honeycomb.

Fibre metal laminates (FML) were originally developed as a hybrid material, to create synergy between the impact resistance of metals and excellent fatigue and corrosion resistance of fibre reinforced polymers, and to overcome the shortcomings of monolithic materials. Yet, the scope of the FML concept is predominantly limited to GLAss REinforced laminates (GLARE) for aerospace structures [1]. However, with the rising concerns about climate change, and the issues of recycling glass fibre composites, a new generation of FMLs with a reduced carbon footprint should be envisaged. This can be achieved by using bio-based fibre reinforced composite layers, particularly flax fibre instead of glass fibre composite, rendering FMLs with lower embodied energy, in which aluminium layers can be easily recycled by incineration with energy recuperation of the flax composite. 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, and in applications in which the loading mode is predominantly in bending. This includes applications in the transportation and construction sectors as well as secondary structures for civil aircraft, such as automotive panels, flooring, and bridge decks. Additionally, flax fibre composites demonstrate high damping capabilities due to the unique hierarchical structure of these fibres. This makes them particularly suitable for applications where vibrational and acoustic damping is of interest which includes many of the above given examples. However, they also have disadvantages such as high moisture absorption that can restrict their use [2]. The FML concept would overcome these limitations and thus allow the introduction of these materials in primary structures.

In this study, the combination of flax fibre reinforced epoxy with thin aluminium layers is realised as a partially biobased alternative to current FMLs, aiming to obtain primarily good vibration damping properties and improved impact resistance. The impact behaviour of the flax fibre reinforced aluminium (FLARE) will be evaluated by low velocity impact and quasi-static indentation tests to identify the role of each material constituent. The results will be compared with a predictive model based on the work of F. Morinière et al. [3]. For the damping properties, to cover a wide range of frequencies and to compare methods, the vibration absorption capacities will be measured by dynamic mechanical analysis and vibration beam tests. The results will be compared to the model predictions from the metal volume fraction method. Finally, this study will give a first overview of the properties of FLARE and will verify the validity of the predictive tools developed for conventional FMLs, which help in the design phase to optimise the structure according to specific requirements

In this study, the tensile and bending properties of silk fibre composites using three (0, 90) woven fabrics with different architectures are investigated. The tensile results show that the silk composites can achieve high strain to failure (more than 20%) and toughness (up to 13 MJ/m3), which can be further manipulated based on the architecture of the fabrics, thus providing more tailored properties and design freedom in applications. XCT and SEM characterization are used to investigate and explain the outstanding toughness of these composites. In tension, a high density of microcracking was observed away from the failed location, which could explain the intrinsic high ductility and energy absorption of silk fibre composites by means of damage spreading throughout its volume in contrast to inherently brittle materials. In bending, significantly lower properties were observed with the more striking being the strain at failure reaching only 30% of the tensile value, thus limiting the potential of silk fibres in bending-dominated loading configurations. XCT revealed that the lower performance is due to failure on the compressive side of the composites, where a clear characteristic kink-band was observed in all composites subjected to bending, while there was no visible damage on the side under tension. This behaviour is also linked to the soft HDPE polymer matrix used, which provides little resistance to fibre buckling.