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.

Carbon fiber reinforced thermoplastic composites (TPCs) attracted significant attentions from the aerospace, transportation, and defense industries, due to their high specific stiffness and specific strength, outstanding thermal stability and good damage resistance, etc. As the demand of TPCs significantly increased for aerospace applications, the development of advanced joining technologies for TPC components becomes critical to ensure the structural integrity of aviation structures. This paper provides a comprehensive review of the historical development and recent advancements in welding technologies for TPCs, including ultrasonic welding, induction welding, resistance welding, and laser welding. Special emphasis is placed on ultrasonic welding due to its growing prominence in the field. The characteristics of various types of welding technologies for TPCs have been systematically discussed. Simultaneously, the strengths of the TPC joints manufactured by different welding technologies have been summarized and compared. The future development trend and research focuses for the welding technologies of TPC components are also proposed.

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.

Despite many favorable properties of sandwich panels, moisture penetration into the core of these panels has been known to cause catastrophic failures. To address these issues, developing an alternative panel with equivalent mechanical behavior can be a viable solution. Exploring the mechanical behavior equivalency between stiffened composite plates and existing sandwich panels is advantageous due to their potential for similar applications. This study employed explicit finite element modeling using LS-DYNA package to simulate the behavior of these panels. Also, experimental investigations were conducted on stiffened composite plates to examine the effect of stiffener arrangements and impact location on their static and dynamic behaviors. The experiments highlighted the significance of stiffener arrangements in influencing the static and impact behavior of the plates. Additionally, as a case study, an optimization procedure for designing an optimal stiffened plate under ice impact was studied, utilizing the Taguchi method and analysis of variance, to identify the optimal design point. The results indicated that the stiffened plate exhibited a maximum deflection similar to that of a sandwich panel under low-velocity impact, while having a 19.3% lower von Mises stress. This means that the equivalent stiffened plate demonstrated comparable deflection while providing enhanced strength during dynamic loading. Furthermore, the analysis of the parametric study showed that the thickness of stiffeners had the most pronounced influence on the behavior of stiffened plates subjected to hail impact.

Multidirectional (MD) composite laminates are extensively employed in structural applications owing to their superior mechanical characteristics. Nevertheless, the evaluation of the fracture toughness of composite laminates primarily relies on tests using unidirectional (UD) specimens. This study evaluates the reliability of characterizing mode II delamination behaviour in MD laminates by using UD specimens. The quantification of delamination area through Digital Image Correlation (DIC) analysis is integrated with a physical Energy Release Rate (ERR) method to ascertain the fracture resistance, which is compared with the ERR derived via a modified J-integral method and the standardized compliance methods. Fractographic analysis reveals similar fracture mechanisms in specimens with identical interfaces. The physical ERR increases notably due to large-scale fibre bridging induced by fibre nesting at 0^∘//0^∘ interfaces. Conversely, in 0^∘//90^∘ interfaces, large-area matrix cracking enhances the intrinsic fracture resistance, excluding the extrinsic toughening provided by fibre bridging.

Three-phase composites, especially those composed of high performance thermoplastics, have not been properly investigated with respect to their interlaminar fracture toughness. Therefore, this study investigates effect on the interlaminar fracture toughness by adding carbon nanotube buckypaper (BP), tested under cyclic loading in mode I and II. BP weakened the interlaminar fracture toughness in mode I, creating an easy path for crack growth and reducing the strain energy release (SERR) values in the Paris curves. Conversely, under mode II BPs presented no significant influence to the interlaminar fracture toughness and fatigue life; however, a slight improvement was observed due to the bridging effect. The energy balance principle model for opening delamination showed that BP composites require less energy per unit of area to crack growth, resulting in a smoother fracture surface with fewer failure mechanisms. In contrast, BP slightly increased the energy per unit of area for crack growth, leading to a rougher fracture surface with a higher prevalence of failure mechanisms under mode II. This work underscores the importance of examining the individual effects of mode I and II loadings on BP laminates since these interleaves affect the interlaminar toughness and fatigue life differently.

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.

Hydrogen is a promising candidate for achieving aviation sustainability, but storage aboard aircraft presents significant challenges. All-composite, double-walled, vacuum-insulated cryo-compressed storage vessels offer a potential solution by achieving high volumetric and gravimetric efficiencies. Load transfer connections between the tank's shells and the surrounding structure introduce concentrated loads in the composite shells. This work develops analytical models to characterize the stress state in composite shells under discrete in-plane loading, showing how stress concentrations decay and how laminate selection influences the decay rate. Discrepancies between the analytical and numerical models are noted, with suggestions for improving both. Additionally, the current model’s limitations due to the number of roots obtained from the governing equations are addressed by proposing additional boundary conditions. This research supports the structural and thermal analysis of composite hydrogen storage vessels, aiding the adoption of hydrogen as a sustainable aviation fuel.

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.

This study investigates the Mode-I fracture toughness of laminates with varying interface angles. A method for identifying crack tip location using grayscale characteristic parameters in DIC is proposed. The findings demonstrate that both initial and steady-state fracture toughness exhibit a bilinear relationship with interface angle. A cohesive constitutive model incorporating the interface angle was developed and integrated into a double cantilever beam finite element model, predicting delamination propagation behavior that was highly consistent with experimental results. Numerical analysis suggests that zigzag cracks may improve fracture toughness before steady-state toughness is achieved, with peak toughness correlating to the length of the zigzag cracks.

Interlaminar crack propagation in layered materials, such as composites, is still a not fully understood phenomenon in fracture mechanics. Experimental observations reveal a broad spectrum of crack propagation velocities during the interlaminar fracture of layered materials under varying loading conditions and rates. While there have been numerous studies examining interlaminar crack propagation under quasi-static, fatigue, and more recently, high-speed loading, there has been a notable lack of endeavors to present and interpret crack growth data within a unified framework. To this direction, we present a new interpretation of interlaminar crack propagation data acquired from a broad test campaign under various mode-I loading conditions (quasi-static, fatigue and high-speed). We connect the crack tip velocity to the rate of change of the SERR to propose a new equation/model that shows potential in presenting and explaining the variety of experimental crack growth data. This correlation arises from a simple mathematical analysis of the rate of change of the SERR using the chain differentiation rule. Subsequently, it is further substantiated by experimental evidence, in an effort to accommodate results from various experiments under a unified description. A robust correlation of the test data is established through a range of slopes which are proven -experimentally- to be characteristic constants of the mode-I interlaminar fracture of a specific material system.

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.

Hydrogen is being investigated as aviation fuel, with the objective to achieve an energy transition for the aviation sector. Effective storage solutions are crucial to mitigate the aerodynamic penalty caused by its low volumetric energy density. The focus of this study is the integration of a cryo-compressed vacuum-insulated storage vessel into the primary structure of aircraft, aiming to enhance structural efficiency. This is achieved by implementing analytical methods to analyse the thermo-mechanical loading of the inner and outer walls of the fuel tank. It is envisioned that the inner wall rather than the outer wall is more suitable to sustain additional loads. However, it is unclear how the cryogenic environment affects the stress state of the composite material.

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.

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).

Crashworthiness of the Flying-V Aircraft Concept with VerticalDrop Test SimulationsM. Desiderio, M.J. Schuurman, R.C. Alderliesten and S.G.P. Castro∗.Department of Aerospace Structures and Materials, Faculty of Aerospace Engineering, Delft University of Technology,Kluyverweg Street No. 1, 2629HS, Delft, The Netherlands.The following presents a preliminary assessment on the crash characteristics of the Flying-Vaircraft, an unconventional configuration consisting of a V-shaped flying wing with an oval cabincross section, currently being actively researched at TU Delft. Successively, the preliminaryassessment is carried out by means of design of experiments, where four crash structureconcepts are defined and evaluated. Virtual drop tests of the Flying-V typical fuselage sectionare performed while measuring the energy absorption of the fuselage, and the dynamic responseindex (DRI) and selected locations. The finite element modeling scheme is validated using theFokker F-28 Fellowship typical section, for which physical drop test data is available. Whilea crashworthy typical section for the Flying-V could not be designed, it has been found thata conventional crash concept with a total of four oblique floor struts is able to absorb 72%of the total kinetic energy, with a DRI reaching 18.2 units. A sensitivity analysis shows thatthe bending stiffness of the frames has a critical role in the crashworthiness of the Flying-V,due to the increase in rigidity following pressurization loads of the oval fuselage section andthat, additionally, the structural simplifications applied in the context of the research likelyrendered the results overly-conservative. A 16% frame thickness reduction resulted in a DRIof 16.2 units, just above the 16 units typically required by regulators. Recommendations forfuture work include a structural sizing optimization where requirements from crashworthinessand airworthiness can be evaluated simultaneously as design constraints, enabling design forcrashworthiness at the preliminary design.

Temperature can significantly affect fatigue delamination growth (FDG) behavior in composites, while fiber bridging has been frequently reported during FDG. The focus of this study was therefore on investigating temperature effects on FDG behavior with fiber bridging. Mode I fatigue delamination experiments were conducted on a thermoset composite laminates M30SC/DT120 at different temperatures. The Paris relation and fatigue resistance curve (i.e. fatigue R-curve) were used to interpret bridging effects on FDG behavior and to explore temperature effects on fiber bridging development. A modified Paris relation was employed to determine the effects of temperature on the intrinsic FDG behavior at the crack front excluding fiber bridging. The Paris interpretations clearly demonstrate that fiber bridging can significantly retard FDG behavior at different temperatures. Temperature can have different effects on fiber bridging development and the intrinsic FDG behavior. Particularly, elevated temperature can promote more bridging fibers, whereas decreased temperature has negligible influence on fiber bridging. When looking at the intrinsic delamination resistance, mode I FDG can accelerate at elevated temperature but decrease at freezing temperature. Fractographic examinations indicate that fiber/matrix interface debonding is the dominant damage mechanism in mode I FDG at different temperatures. Elevated temperature can lead to the weakening of interface adhesion, contributing to faster intrinsic mode I FDG behavior and more fiber bridging development. And a semi-empirical fatigue model based on normalization was finally proposed to determine mode I intrinsic FDG behavior at different temperatures for engineering applications.

Carbon emissions from commercial aircraft are expected to reach more than twice as much as the current levels by 2050. Unconventional aircraft, such as the Flying-V, are projected to achieve more than 20% fuel savings when compared to conventional configurations. However, these unconventional aircraft configurations pose a unique set of design challenges, being one of them the crashworthiness of wing-fuselage structures, which have an oval-shaped cross section that leads to a significant reduction in space underneath the cabin floor. Evaluating the feasibility of a design early in the design phase is vital to avoid cost overruns and minimize the need for drastic design changes. For assessing crashworthiness early in the design phase, the development of low-fidelity models is an attractive as well as a viable option because these models offer both low computational cost and the capability to conduct parametric studies on the crash structure. To develop and implement such low-fidelity models, we propose to explore the analytical modeling of various energy-absorbing mechanisms, namely axial crushing, plastic bending, and joint failure. In the present study, we present the modelling of plastic bending for beam-like structural members. We also present an envisaged method applying user-defined elements to simulate plastic bending in structural members for cases where the location of plastic hinges cannot be predetermined.

Fatigue delamination growth (FDG) is an important failure in composite structures during their long-term operations. Hygrothermal aging can have significant effects on interlaminar resistance. It is therefore really necessary to explore FDG behavior in composite laminates with hygrothermal aging. Dynamic mechanical thermal analysis (DMTA), mode I FDG experiments and fractographic examinations were conducted to fully investigate hygrothermal aging effects and the corresponding mechanisms on FDG behavior. The DMTA results indicated that environmental aging can induce obvious T_g decrease. Mode I experimental fatigue data interpreted via different Paris-type correlations demonstrated that: Bridging has obvious retardation effects on FDG behavior via the Paris interpretations; The modified Paris relation can well characterize the intrinsic FDG behavior around the crack front; The use of the two-parameter Paris-type relation can appropriately account for R-ratio effects, contributing to a master resistance curve in determining mode I FDG behavior. According to these interpretations, it can be concluded that hygrothermal aging can have adverse effects on mode I FDG behavior. SEM examinations demonstrated that moisture absorption can cause fibre/matrix debonding and resin matrix pores/voids in the composite. However, no obvious difference in damage mechanisms was identified in mode I fatigue delamination for composite with/without environmental conditioning. Both fibre/matrix debonding and matrix brittle fracture were identified on fatigue fracture surfaces. Accordingly, it was concluded that fibre/matrix interface and matrix degradation induced by water absorption were the main reasons for a faster mode I fatigue crack growth in environmental aged composite.

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.