Aeronautical Panel Research Articles

Eco-friendly composites with specific functional properties

The development of sustainable hydrophobic composite coatings is of high interest for aircraft applications. Currently, the use of natural derived functionalized microparticles as filler to obtain hydrophobic epoxy-based coatings was not deeply investigated. In this scenario, a novel hydrophobic epoxy-based composite including waste hemp microparticles functionalized with silica layer, 3-aminopropyltriethoxysilane, polypropylene-graft-maleic anhydride and silanes (hexadecyltrimethoxysilane and 1H,1H,2H,2H- Perfluorocotyltriethoxysilane) is presented. The resulting coating was casted on typical aeronautical panel, based on carbon-fiber-reinforced polymers, to achieve an improved hydrophobicity and anti-icing property induced by functionalized hemp microparticles.The wettability and anti-icing property were investigated. Compared to unfilled epoxy resin, the obtained composite coating achieved a greater water contact angle of 30° and doubled increase in icing time. Despite the low content (2 wt.%) of hemp particles, DSC analysis displayed a relevant increase in Tg value, confirming an efficient interaction between the epoxy matrix and the functionalized hemp filler. AFM analysis proved how the presence of hemp filler leads to an increase in roughness due to the hierarchical structure formed by the long chains of silane molecules. The combination of silane activity and rough morphology allows the development of hemp composite coatings with enhanced hydrophobicity, anti-icing behavior and thermal stability for aircraft applications.

Optimal design of stochastic aeroviscoelastic systems in subsonic regime using the doublet lattice

In a concerted effort to mitigate the global environmental impact, various governments and international organizations have put forth strategies aimed at enhancing the efficiency of products and solutions. The aeronautical and aerospace industries have set forth an ambitious target of eliminating their CO2 emissions by the year 2050. To realize this objective, manufacturers are adopting innovative approaches, including the incorporation of new blade engines, utilization of Sustainable Aviation Fuel (SAF), integration of lighter materials, and the adoption of novel geometric wing shapes designed for enhanced efficiency. However, the implementation of structural and aerodynamic modifications, such as those required for aeroelastic systems, can lead to instability effects, notably the flutter phenomenon. Addressing this challenge, one viable strategy involves the application of vibration control techniques. In this context, the use of passive control with viscoelastic materials emerges as an intriguing option due to its cost-effectiveness and ease of application. Additionally, considering the inherent variabilities in structural and aerodynamic parameters within these systems, there is a need for an efficient stochastic modeling methodology to cater to realistic applications of industrial significance. The present study endeavors to model a stochastic aeroviscoelastic system, specifically focusing on a plate-like wing in a subsonic regime. This is achieved through the integration of stochastic finite element modeling and the Doublet Lattice Method (DLM). The study aims to quantify the impact of increased mass and stiffness, particularly related to the addition of layers and the inclusion of damping from the viscoelastic material. The findings of the research demonstrate a noteworthy 31% increase in flutter speed with a corresponding 38% increase in mass. Although the ratio of speed gain to mass gain is below one in this instance, with a partial treatment approach, it can potentially reach 2. This underscores the advantageous nature of treating aeronautical panels with layers of viscoelastic material. Importantly, the engineer possesses the capability to strategically select deterministic parameters to align with specific objectives between the two defined functions.

Influence of Delamination Size and Depth on the Compression Fatigue Behaviour of a Stiffened Aerospace Composite Panel.

Delamination in reinforced panels is one of the primary challenges facing the safety and reliability of aerospace structures. This article presents a sensitivity analysis of the fatigue behaviour during the compression of a composite aeronautical stiffened panel experiencing delamination. The main objective is to assess the impact of delamination size and depth on the lifecycle and structural integrity of the panel. Different dimensions and positions of delamination are considered to cover a comprehensive range of damage scenarios. The key feature of this sensitivity analysis is the adoption of a numerical procedure that is mesh- and load-step-independent, ensuring reliable results and providing valuable insight into the criticality of delamination and its impact on the fatigue behaviour during the compression of reinforced aeronautical panels. Sensitivity analyses are essential for enhancing the design process of aerospace structures, thereby contributing to the increased safety and reliability of structural components. In this regard, the use of robust and effective numerical procedures is of crucial significance. This may be seen as the real added value of this paper.

Hybrid Hemp Particles as Functional Fillers for the Manufacturing of Hydrophobic and Anti-icing Epoxy Composite Coatings.

The development of hydrophobic composite coatings is of great interest for several applications in the aerospace industry. Functionalized microparticles can be obtained from waste fabrics and employed as fillers to prepare sustainable hydrophobic epoxy-based coatings. Following a waste-to-wealth approach, a novel hydrophobic epoxy-based composite including hemp microparticles (HMPs) functionalized with waterglass solution, 3-aminopropyl triethoxysilane, polypropylene-graft-maleic anhydride, and either hexadecyltrimethoxysilane or 1H,1H,2H,2H-perfluorooctyltriethoxysilane is presented. The resulting epoxy coatings based on hydrophobic HMPs were cast on aeronautical carbon fiber-reinforced panels to improve their anti-icing performance. Wettability and anti-icing behavior of the prepared composites were investigated at 25 °C and -30 °C (complete icing time), respectively. Samples cast with the composite coating can achieve up to 30 °C higher water contact angle and doubled icing time than aeronautical panels treated with unfilled epoxy resin. A low content (2 wt %) of tailored HMPs causes an increase of ∼26% in the glass transition temperature of the coatings compared to pristine resin, confirming the good interaction between the hemp filler and epoxy matrix at the interphase. Finally, atomic force microscopy reveals that the HMPs can induce the formation of a hierarchical structure on the surface of casted panels. This rough morphology, combined with the silane activity, allows the preparation of aeronautical substrates with enhanced hydrophobicity, anti-icing capability, and thermal stability.

A data driven methodology for upscaling remaining useful life predictions: From single- to multi-stiffened composite panels

In this paper we execute a complex test campaign to develop a novel methodology for the Remaining Useful Life (RUL) estimation of complex multi-stiffened composite aeronautical panels utilizing Machine Learning models trained with Structural Health Monitoring (SHM) data from hierarchically simpler elements, i.e., single-stiffened panels. Distributed Fiber Optical sensors (DFOS) are employed to monitor the panels’ behavior undergoing variable amplitude compression-compression fatigue after multiple impacts. A data processing methodology is first applied to the DFOS data, to both alleviate the effect of the variable loading conditions on the monitored strain and ease the computational burden. In this upscaling endeavor, an advanced strain-based Health Indicator (HI) based on Genetic algorithms, created and validated on the single-stiffened panel data, is utilized as the prognostic feature for the RUL estimations of the multi-stiffened panels. The HI displays favorable characteristics in terms of monotonicity and prognosability which are highly desirable for more accurate RUL estimations. For the prognostic task, standard machine learning models are trained using the historical degradation data of the single-stiffened panels and a similarity analysis is performed to enhance the accuracy when predicting the RUL of the multi-stiffened panels. Despite the increased structural complexity of the multi-stiffened panels, we demonstrate that the RUL is able to be predicted with reasonable accuracy. The present work paves the road for upscaling and applying prognostic methodologies to more complex structures beyond simple coupons or generic elements.

Delamination Effect on the Buckling Behaviour of Carbon–Epoxy Composite Typical Aeronautical Panels

The instability of structures due to compression is one of the most critical issues related to aircraft components. Especially in composite materials, which have poor out-of-plane mechanical properties, the buckling load must be assessed to ensure that the structures are within the safe limits compared to the operating loads. In the presence of delamination, the compression instability of structures becomes catastrophic, as the propagation of delamination can dramatically reduce the stiffness of the structure almost instantaneously. During the operational life of composite aircraft components, one of the most common events that can occur is low-velocity impact with foreign objects, which is one of the primary reasons for delamination. In this paper, a sensitivity analysis is presented on a typical aerospace reinforced panel with a circular delamination, representative of an impact damage. Different configurations have been analysed, varying the radius and position along the thickness of the delamination. Furthermore, some geometric parameters of the panel have been modified to evaluate how the buckling load and the propagation of interlaminar damage evolve.

A Numerical Study on the Influence of Nanosilica‐Reinforced Epoxy Resin on the Delamination Behavior of Composite Laminates

AbstractThe use of nanomodified epoxy resins can potentially increase composites application to aeronautical structural components, thanks to the potential enhancement, in terms of physical and mechanical properties, when compared to the neat epoxy matrix. In this work, the effects of silica nanoparticles (NPs) on the fracture toughness and, consequently, the crack growth resistance of fiber‐reinforced polymers (FRPs) have been numerically investigated. The skin‐stringer debonding initiation and growth have been studied by a tailored innovative numerical procedure considering an aeronautical panel reinforced with a single T‐shape stringer, made of carbon fibers/epoxy resin material, and subjected to compressive load. An analytical model has been used to evaluate the Mode I fracture toughness value of the nanomodified resin, and the Virtual Crack Closure Technique methodology has been employed to assess the delamination growth in the frame of a Finite Elements (FE) analysis performed in the Ansys FE environment. Numerical results presenting the comparison between charged and neat configurations have been assessed to provide a first understanding of the influence of nanoparticles on the static delamination growth in geometrically complex composite structures.

Vibro-Acoustic Modelling of Aeronautical Panels Reinforced by Unconventional Stiffeners

The purpose of this work is to characterise the vibro-acoustic behaviour of rectangular flat panels reinforced by “unconventional” stiffeners. Such panels are being increasingly employed in the aircraft industry in the case of composite fuselage, so that the assessment of the most efficient and accurate numerical techniques and modelling procedures to correctly predict their dynamic and acoustic behaviour is required. To this end, an analytical method, available from literature, has been initially employed to investigate on the main attributes of sound transmission loss properties of stiffened panels driven by an acoustic diffuse field excitation. Based on existing commercial codes, different numerical techniques have been implemented and deeply examined to assess their potentiality and restrictions. Among them, the Hybrid method has been eventually identified as the best compromise in terms of accuracy and computational effort. The drawbacks of deterministic and even Hybrid numerical approaches for medium–high frequency vibro-acoustic analysis when dealing with large structures, make use of the pure SEA approach compulsory. In particular, a refined tuning of a specific feature made available within the employed SEA analysis environment when dealing with reinforced shells is implemented as a potential solution to overcome the complexity in correctly modelling the examined unconventionally stiffened panels.

Simulation strategy to compensate spring-in deformations in aeronautical panel made by liquid resin infusion

Spring-in in composite manufacturing is a relevant non desired effect that complicates the industrialization process of new composite parts. Geometrical deviations of the part with regard to the nominal dimensions  hinders posterior assembly operations or even can cause unconformities (part rejection). Although some correction operations may be introduced in the assembly method to absorb these deviations, a preferred approach is to compensate the tool geometry to produce the part within tolerances. This contribution presents a methodology that, in an iterative process, links a MEF simulation workflow for the prediction of process induced distortions, a routine to compute deviations and updating tool cavity surface, till the deviations are within a target tolerance. The spring-in calculation engine is implemented in Abaqus Standard with specific subroutines to model resin curing kinetics, heat generation, cure dependent mechanical constitutive model with consideration of thermal expansion and cure shrinkage, and specific strategies to simulate the tool/part interactions. The spring-in engine is incorporated in an external Python manager in charge of post-processing results, updating the mesh and launching the executions. This methodology aims at proposing a modified tool surface geometry that could be used to build a compensated tool at first, reducing industrialization time and cost.

Analytical higher-order-theories for noise reduction analysis of viscoelastic composite multilayered shells

The noise transmission of aeronautical panels is an important phase of the design process of an airplane. In this work an analytical Navier-type solution, based on higher-order layer-wise shell models, is proposed for the analysis of the sound insulation of laminated panels. The considered multilayered structures are laminated with cross-ply composite layers embedded with interlaminar viscoelastic sheets. The use of the soft interlayers permits to have a passive insulation effect in the study of the sound transmission. In order to take into account the frequency depedent properties of a realistic viscoelastic layer, the damping behavior is modeled through a fractional derivative Zener model. The Rayleigh integral method is used to extrapolate the acoustic indicators for the sound transmission analysis. Some results are presented to validate the efficiency of the present approach, comparing the present solutions with others taken from the literature.

Fatigue damage diagnosis and prognosis of an aeronautical structure based on surrogate modelling and particle filter

A key issue affecting the performances of every human-conceived engineering system is its degradation, fatigue crack growth being one of the major structural deterioration phenomena. Fatigue crack growth is usually modelled as a stochastic process: uncertainty sources lie both in the item and in the physical degradation process variability. Fatigue crack growth deserves close attention, especially considering that condition-based maintenance methodologies are recently experiencing a major drive to increase their technology readiness level, requiring validated diagnostic and prognostic methodologies which should be capable of operating online and in real-time. In this regard, particle filters provide a consistent Bayesian framework, where the posterior distribution of the system degradation state is recursively approximated based on a time-growing stream of observations measuring the system response, enabling, in general, increasingly informed lifetime estimates. However, the real-time operation capability of such methods is hindered by their requirements in terms of computational power, which is mainly due to the complexity of the structural models they rely upon. Within this work, a comprehensive particle filter framework, able to deal with fatigue crack growth uncertainty sources while simultaneously addressing the computational burden issue, is proposed. The algorithm structure enables to simultaneously perform the diagnosis and prognosis of fatigue crack growth, while the adoption of the augmented state formulation allows to address scenarios where the degradation process of fatigue crack growth fails to meet the degradation model ruling the particle filter. Artificial neural networks–based surrogate modelling is adopted at different stages and embedded within the particle filter algorithm, relieving the computational burden associated with the evaluation of the trajectory likelihoods as well as enabling a fast estimation of the remaining useful life. Both simulated and experimental data sets regarding fatigue crack growth in an aluminium aeronautical panel are used for the algorithm testing, additionally proving the validity and effectiveness thereof by means of common prognostic performance metrics.

Optimum Design of Damage Resistant Reinforced Composite Panels

In this work, an optimization procedure able to determine the optimum design of a stiffened aeronautical panel subjected to low velocity impacts is presented. As design variables, the number of plies and the stacking sequence of the panel have been considered. The optimization is based on a genetic algorithm, while the onset of the impact induced damage is predicted by means of an approach based on the critical impact force threshold. A multi-objective optimization has been carried out to determine, among the configurations able to withstand the low velocity impacts, the ones characterized by the maximum buckling load and the minimum weight.

Particle filtering‐based adaptive training of neural networks for real‐time structural damage diagnosis and prognosis

Real-time failure diagnosis and prognosis play key roles in prognostics and health management of critical engineering assets. Physics-based (or empirical–phenomenological) and data-driven methods have been proposed in the literature to solve damage diagnosis and prognosis challenges. Among data-driven methods, neural networks have often successfully proven their regression capabilities for the complex, nonlinear problems typical of this field research field. Indeed, the need to resort to machine learning techniques (e.g., neural networks) becomes stronger when physics-based (or empirical–phenomenological) models are lacking for describing the degradation behavior of the systems under consideration. In this work, we propose to exploit the flexibility offered by neural networks in order to adaptively, and in real time, learn from the monitored structure and derive models for diagnosis and prognosis of structural components subject to damage processes. To this aim, neural networks are embedded within a particle filtering scheme, and the training of the network is performed sequentially and in real time, as observations become available during the damage evolution. The structural health monitoring tool is capable of sequentially updating itself using the observations gathered during the system's operation, avoiding the typical limitations associated to off-line training schemes, which are, in general, not capable of capturing possible changes in the dynamical degradation behavior, when not properly included in the training patterns. For validation and comparison purposes, the method is demonstrated on simulated fatigue crack growths and real crack evolution dynamics observed in a metallic aeronautical panel loaded with a changing-amplitude fatigue load, during an accelerated laboratory test.

Comparative of conventional and alternative Digital Image Correlation techniques for 3D modal characterisation

The integration of Fringe Projection and 2D-DIC is a low-cost optical technique which employs a single camera for full-field 3D displacement measurements using RGB colour encoding to filter coexisting fringe and speckle patterns out from a single image. In this work, this technique was explored together with conventional 3D-DIC for challenging modal analysis on a large aeronautical panel under random excitation. Modal identification was performed using the Least-Squares Complex Exponential approach. Multiple degree-of-freedom methods have been avoided in previous works or just employed in one direction due to high memory requirements to process 3D full-field data. In this work, the 3D modal characterisation was addressed with especial mention to mode shapes. Hardware limitations makes FP + DIC less sensitive than 3D-DIC and hence more suitable for lower frequencies. Nevertheless, the obtained modal parameters showed high agreement and hence FP + DIC demonstrated to be a cost-effective alternative for 3D modal analyses.

Sound Transmission Analysis of Viscoelastic Composite Multilayered Shells Structures

In the development of aircraft comfort, one of the main issues is the sound transmission analysis to estimate the insulation capability of aeronautical panels. In this work, a higher-order shell finite element is proposed for the passive noise insulation analysis of composite laminated structures embedding viscoelastic layers. Starting from the Principle of Virtual Displacements, the present Finite Elements are obtained by making use of higher-order Layer-Wise theories, employing the Mixed Interpolated Tensorial Components (MITC) method to avoid the shear locking effect and taking into account the frequency dependence of the viscoelastic material through the use of a fractional derivative model. The Rayleigh integral method is considered for the evaluation of the acoustic insulation of the panels. Numerical studies are carried out to demonstrate that the present shell finite element is an efficient and accurate tool for the sound transmission analysis. Different lamination sequences, different boundary conditions and various radius to thickness ratios are taken into account.

FP+DIC for low-cost 3D full-field experimental modal analysis in industrial components

Abstract High Speed 3D Digital Image Correlation has reached notorious popularity in dynamic characterisations because of the full-field non-invasive performance. It is being especially relevant for experimental modal analysis due to the importance of the modal parameters to define the dynamic behaviour. However, the economic cost of a stereoscopic system of two high speed cameras is a main concern regarding traditional instrumentations. In the search for low-cost alternatives, different methodologies have arisen that employ a single camera for 3D measurements. One approach consists in obtaining a stereo-vision in a single image using, for instance, a mirror system. These approaches involve important field-of-view restrictions. An interesting approach employs the full resolution by combining Fringe Projection and 2D-DIC for out-of-plane and in-plane displacements, respectively. In this study, this technique is explored to perform experimental modal analysis in a large non-flat aeronautical panel. The validation of the results has been performed using accelerometer measurements during an impact hammer test. The results of the study stand this technique as an attractive low-cost technique for 3D full-field modal characterisations of complex components.

Design of a noise reduction passive control system based on viscoelastic multilayered plate using PDSO

Abstract The sound transmission and insulation of aeronautical panels represents one of the major problem of the aircraft comfort. In the present work, a plate finite element with advanced higher-order kinematic field is proposed for the analysis of noise reduction passive control system in laminated structures. A novel Decline Population Swarm Optimization ( P D SO ) procedure is introduced and used to select the optimal parameters for the sound control. The present sound insulation passive control is based on Finite Elements obtained by applying the Principle of Virtual Displacements, taking into account the frequency dependence of the viscoelastic material, making use of higher-order Layer-Wise theories, and employing the Mixed Interpolated Tensorial Components (MITC) method to contrast the shear locking effect. The P D SO procedure is based on a decline demographic model and is characterized by high global search capabilities with reduced computational costs. The acoustic insulation of the panels is evaluated by computing its sound transmission factor using Rayleigh integral method. Several numerical investigations are carried out to validate and demonstrate the accuracy and efficiency of the acoustic optimization procedure for the design of viscoelastic plates.

Sound proofing and thermal properties of an innovative viscoelastic treatment for the turboprop aircraft fuselage

Low-resilience polyurethane foams including several additive constituents were synthesized to improve their vibro-acoustic performances, as well as the thermal insulation. viscoelastic polymer additive can attenuate vibrations and absorb sound energy. the vibro-acoustic properties of two innovative viscoelastic treatments fabricated with polyurethane foams are discussed in this paper using a typical aeronautical panel test setup. Since an aircraft insulation arrangement must provide both noise and thermal insulation for the specified operating conditions and expected thermal comfort of passengers, the thermal conductivity of the samples has been examined assuming a testing range between 20 °C (room temperature) and − 40 °C (cruise altitude). the results highlighted an optimal behavior of the novel viscoelastic foams in terms of both acoustic and thermal performance, offering a very interesting self-embedded solution with a good weight to performance ratio, compared to standard blanket composed by extra viscoelastic treatments.

Flutter suppression of plates using passive constrained viscoelastic layers

Flutter in aeronautical panels is a self-excited aeroelastic phenomenon which occurs during supersonic flights due to dynamic instability of inertia, elastic and aerodynamic forces of the system. In the flutter condition, when the critical aerodynamic pressure is reached, the vibration amplitudes of the panel become dynamically unstable and increase exponentially with time, significantly affecting the fatigue life of the existing aeronautical components. Thus, in this paper, the interest is to investigate the possibility reducing the effects of the supersonic aeroelastic instability of rectangular plates by applying passive constrained viscoelastic layers. The rationale for such study is the fact that as the addition of viscoelastic materials provides decreased vibration amplitudes it becomes important to quantify the suppression of plate flutter coalescence modes that can be obtained. Moreover, despite the fact that much research on the suppression of panel flutter has been carried out by using passive, semi-active and active control techniques, few works have been proposed to deal with the problem of predicting the flutter boundary of aeroviscoelastic systems, since they must conveniently account for the frequency- and temperature-dependent behavior of the viscoelastic material. After the presentation of the theoretical foundations of the methodology, the description of a numerical study on the flutter analysis of a three-layer sandwich plate is addressed.

Flutter of stiffened composite panels considering the stiffener’s base as a structural element

Flutter in aeronautical panels is a type of self-excited oscillation which can occur during supersonic flights. At the flutter point the vibrations of the panel become unstable and increase significantly in time. This manuscript presents a semi-analytical model taking into account the stiffener’s base effects, in order to predict the aeroelastic response of laminated composite stiffened panels under supersonic flow. Krumhaar’s modified supersonic piston theory, which considers the radius effect, is adopted to model the aerodynamic loading. The proposed model has been validated against results available in the literature for various configurations. A parametric study considering different panels and stiffener configurations is also presented. The numerical results indicate that the stiffener base significantly affects the panel aeroelastic behavior. Preliminary studies also indicate that redistributing the laminate plies from the stiffener’s flange to its base significantly increases the torsion stiffness of the panel locally, opening new design possibilities that may lead to higher critical flutter speeds and therefore to better designs. The results also indicate that designs with plies distributed on the base may lead to a better flutter performance when the airflow is transverse to the longitudinal stiffener direction.