Cold spray is well-known to be an attractive method for aerospace repair based on its solid-state deposition nature, demonstrating significant benefits compared to its conventional thermal spray counterparts. However, usage of cold spray for repair within an aviation context is currently limited to geometric restoration for minor damage, including nicks, dents, scratches, and minor corrosion, and will remain limited until the durability and damage tolerance of such repairs can be reliably demonstrated.

Current limitations in this regard surround the manufacturing-dependence of the produced depositions combined with lack of process standardization. In service, the quality of thermal spray coatings are typically evaluated using metrics such as adhesion strength, porosity, and hardness, and these parameters are therefore also often suggested for quality assurance of cold spray deposits. However, it is not clear if these metrics are also suitable and sufficient for assuring the quality of structural repairs.

The present work concerns assessment of the quality of cold spray repairs subjected to high cycle fatigue loading. Fatigue test results for Al6061 cold spray blend-out repair coupons on a like substrate manufactured using different process parameters and surface preparation methods are presented, with an emphasis on variability of, and interactions between, damage modes in fatigue. A comparison between quality metrics used for thermal spray coatings and cold spray fatigue performance are presented.

Acknowledgement: The work presented was supported by the Dutch Research Council Open Technology Programme ‘CSAR’ (grant no: 20434).

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.

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.

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.

Fiber-metal laminates are a well-known and established material concept featuring an enhanced crack propagation resistance when compared to their metal and fiber reinforced plastic (FRP) constituents. In this paper, this approach is transferred to purely carbon fiber reinforced plastic (CFRP) based laminates made from layers having polyetherimide (PEI) and epoxy matrices in an alternating laminate architecture. The laminates are manufactured via hot pressing. Double-cantilever beam (DCB) tests are performed on standard samples for both the hybrid laminates in different configurations as well for the both constituent materials, i.e. carbon fiber reinforced PEI (CFR-PEI) and carbon fiber reinforced epoxy. As the formation of an interphase is already reported in literature for this matrix combination, microstructural investigations have also been carried out in addition to fractography on crack surfaces. It is shown that the hybrid materials outperform both constituents regarding the crack resistance when crack initiation starts in the tougher CFR-PEI layer and the laminate layup is 0/90°. In the other configurations investigated, there is no significant effect. The energy dissipating mechanisms are crack jumping and the formation of several parallel cracks. Consequently, crack resistance in such hybrids might be controlled in future by adjusting the crack resistance of the constituents as well as the laminate architecture.

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.

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.

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.

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.

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.

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.

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.

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.

To design crash structures for disruptive aircraft designs, it is required to have fast and accurate methods that can predict crashworthiness of aircraft structures early in the design phase. Axial crushing is one of the key energy absorbing mechanisms during a crash event. In this study, various analytical models proposed for calculation of mean crushing force for thin-walled tubular structures are compared with a database of numerical and experimental values to ascertain their accuracy. Improvements to some of the models have also been proposed. Finally a generalized model based on the studied and improved analytical models for prediction of mean crushing force for closed section thin-walled tubular structures is introduced. The generalized model demonstrates high accuracy when compared against experimental/numerical dataset as evidenced by a high coefficient of determination (R²) value of 0.97 and can therefore be used to estimate the mean crushing force for closed-section thin-walled metallic tubular structures with various cross-sectional shapes and crushing modes early in the design phase.

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.

Vacuum-insulated, all-composite hydrogen storage vessels are promising for achieving viable gravimetric and volumetric efficiencies in aviation applications. However, stress concentrations arise due to the connections between the composite shells and to the surrounding structure. This study develops analytical models which capture the stress state of composite cylinders subjected to pressure loading and discrete in-plane edge loads. The models allow for easy adjustments in tank geometry, lamination, and load introduction parameters, including the number and width of connection points. This aids in the preliminary structural and thermal analysis of composite hydrogen storage vessels, pushing the implementation of hydrogen as a sustainable fuel for aviation.

In this study, a novel experimental approach was devised to investigate shear dominant and combined opening-shear planar delamination behaviours in composite laminates subjected to quasi-static out-of-plane loading. The patterns of planar delamination growth were depicted through different inspection techniques, including digital image correlation (DIC), C-scan, and microscopic observation. The artificially embedded delamination propagated in the direction parallel to the fibre orientation of the layer above the mid interface, but migrated to an upper interface in the direction transverse to the directing ply. A continuous stiffening process was recognized with increasing delamination area. Furthermore, a numerical analysis based on virtual crack closure technique (VCCT) indicated that the local mode II was dominant for delamination growth, while the local mode III triggered delamination migration.