This work proposes a methodology for the characterisation of complex pore features in unidirectional composite prepregs, and provides insights into the interaction between fibre architecture and pores. The method showcased allows to compare spatial distributions at a three-dimensional level, highlighting in the tape analysed a significant correspondence between regions of elevated tortuosity and increased pore fractions. Regions associated with highly tortuous meandering fibres exhibit a pronounced association with porosity located both in the bulk and at the tape surface, suggesting a strong interaction between non-collective fibre displacement and the probability of pore location. Furthermore, our study quantifies the length scale of feature propagation, shedding light on the spatial extent of microstructural pore occurrence within the composite. These findings have significant implications from a characterisation perspective to aid modelling approaches and manufacturing processes for high-performance composite prepregs tapes.@en

This work studies the effect of compaction of tapes that have been heated using a laser during automated fibre placement. The deconsolidation has been shown to have a significant effect on the surface roughness, degree of effective intimate contact and void content. This work investigates whether after compaction the as-received (i.e., before heating) properties are obtained, or whether the deconsolidationcompaction cycle has an influence on the final tape quality. First, a lab test set-up is designed and manufactured to mimic the real manufacturing conditions. Next, the set-up is used to study the influence of the placement speed and pressure on the final quality. The results show that the effects of deconsolidation are mostly reversed, but the final tape still has slightly worse qualities. This effect will have to be taken into account for accurate modelling of the laser-assisted automated fibre placement process.@en

Double-curved composite structures that are manufactured via automated fiber placement, such as pressure vessels, can take advantage of tow steering to reduce weight. This design freedom comes with the cost of adding internal normal stresses to the tow, possibly leading to wrinkles or pull up. The present work investigates tow pull up both experimentally and analytically and details a correlation between tow pull up and the minimum critical steering radius, where the material tackiness and the modelled plate’s length are found to be the most influential parameters. The experimental determination of the material tackiness is the next step to improve the predictive capabilities of the proposed model.@en

In-situ Automated Fiber Placement (AFP) manufacturing of thermoplastic prepreg tapes has the potential to provide a fast and cost-effective manufacturing solution for large composite structures. However, it is prone to several defects, especially gaps and overlaps. One of the primary causes for this is the tape width deformation during placement. Current literature is not enough to understand this mechanism completely and the conventionally considered tape width deformation mechanism i.e., transverse squeeze flow has been suggested to be incorrect for AFP. Therefore, the research objective was to experimentally investigate the width deformation mechanism and the influence of processing parameters for thermoplastic prepreg tapes using humm3® as the heating device. The results show that the tape width deformation takes place both in the heating and consolidation phase of the process and involves spreading of the fiber-resin mixture. Additionally, the conformable roller has an influence on the post-processed tape cross-section profile. The surface roughness and the influence of the processing parameters indicate the role of temperature distribution as the influencing factor. Therefore, these results can be used to accurately model the tape width deformation to mitigate the problem of gaps and overlaps.@en

Manufacturing variations in the automated fiber placement (AFP) process are one of the causes of gaps and overlaps. These manufacturing variations can be due to robot inaccuracy, tow lateral movement on the roller, tow width variation or tow compaction. An experimental setup was built to measure and investigate these various sources of manufacturing variations and their relative contributions to gap and overlap defects. This setup consisted of a commercial AFP head instrumented with additional sensors. Among all the measured sources of variations, lateral movement of the tow on the compaction roller was the biggest contributor to gaps and overlaps. The distributions of these sources of variations were fit with probability density functions. Random samples from these fits were used to simulate adjacent tows and predict the occurrence of gap and overlap defects. The distribution of predicted gaps correlated closely with the distribution of experimentally measured gaps. Thus, this approach of using statistical information about the sources of manufacturing variations to predict the frequency and magnitude of defects in a layup was validated.@en

This work aims to improve the flexural behaviour of unidirectional fibre-reinforced laminates by means of coupling an optimization procedure for quasi-isotropic configurations with the design space opened by dispersed-ply orientations. The design approach consists of finding suitable alternatives to traditional laminates (with fibre orientations limited to 0°, ±45^∘, and 90°), while maintaining their stiffness characteristics. This strategy isolates the interlaminar response as the objective function that is optimized to improve their flexural behaviour. To this end, a modified Ant Colony Optimization was implemented and geared towards optimizing the interlaminar stress profile, allowing plies at every possible 5° orientation, with the ultimate goal of delaying delamination. To validate the approach, a traditional reference laminate and derived fully dispersed designs were experimentally tested. The correlated responses show that it was not possible to improve flexural resistance. However, the typical flexural brittleness of laminates can be modified into a pseudo-ductile behaviour.

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Anisotropic materials formed by living organisms possess remarkable mechanical properties due to their intricate microstructure and directional freedom. In contrast, human-made materials face challenges in achieving similar levels of directionality due to material and manufacturability constraints. To overcome these limitations, an approach using 3D printing of self-assembling thermotropic liquid crystal polymers (LCPs) is presented. Their high stiffness and strength is granted by nematic domains aligning during the extrusion process. Here, a remarkably wide range of Young's modulus from 3 to 40 GPa is obtained during by utilizing directionality of the nematic flow during the printing process. By determining a relationship between stiffness, nozzle diameter, and line width, a design space where shaping and mechanical performance can be combined is identified. The ability to print LCPs with on-the-fly width changes to accommodate arbitrary spatially varying directions is demonstrated. This unlocks the possibility to manufacture exquisite patterns inspired by fluid dynamics with steep curvature variations. Utilizing the synergy between this path-planning method and LCPs, functional objects with stiffness and curvature gradients can be 3D-printed, offering potential applications in lightweight sustainable structures embedding crack-mitigation strategies. This method also opens avenues for studying and replicating intricate patterns observed in nature, such as wood or turbulent flow using 3D printing.

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Over the past 25 years, interest in thermoplastic composites in aircraft has steadily increased. Combining winding and laser-assisted tape placement is a promising method to manufacture thermoplastic structures using in-situ consolidation, as shown recently by manufacturing a variable stiffness, unitized, integrated-stiffener thermoplastic wingbox at the University of Limerick. The corner regions are a critical point of the structure and require in-depth characterization studies, for example by unfolding L-shaped samples in a 4-point bend test. In this work, samples with radii varying from 2 to 10 mm were manufactured and tested. Two manufacturing parameters were varied: the rotational speed and acceleration of the tool. Test data show that decreasing the radius increases the corner strength, but an optimum radius exists to withstand a maximum unfolding force/moment. In addition, the slowest deposition rate with least acceleration of the head used during manufacturing lead to the highest corner strength for the same radius.

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Finding new ways to evaluate the variability of microstructures, and its effect on macroscopic properties such as permeability and mechanical performance [1,2] is of increasing interest in the composite field. The variability of microstructural features at a three-dimensional level is not fully understood and its effect on macroscale properties is not well established, and mostly analyzed at a phenomenological level [3]. We introduced in recent work a method based on X-ray Computed Tomography for the threedimensional reconstruction of the fibrous microstructure of unidirectional tapes at a single fibre resolution [4]. A schematic of the workflow is represented in Figure 1. Three descriptors are introduced in the work to describe increasing level of complexity in the microstructural organization, from a single fiber path level with differential tortuosity, to group behavior with collective motion, to fibre network connectivity with length of contact. These descriptors and their interdependence highlight local effects like edge-core segregation in microstructural characteristics. However, in order to achieve a more complete definition of the unidirectional tape domain, understanding of matrix-based features and its interrelation with fiber architecture descriptors is needed. In this work, we expand the methodology of Gomarasca et al. [4], to account for matrix-based phenomena such as tape boundary variability, and void formation and morphology. This will be showcased on a unidirectional composite tape including both fiber-based and matrix-based analysis. These methods enable advanced characterization and modelling of microstructural formation and evolution during composite manufacturing. @en

This paper presents the collaborative model-based design of a business jet family. In family design, a trade-off is made between aircraft performance, reducing fuel burn, and commonality, reducing manufacturing costs. The family is designed using Model-Based Systems Engineering (MBSE) methods developed in the AGILE 4.0 project. The EC-funded AGILE 4.0 project extends the scope of the preliminary aircraft design process to also include systems engineering phases and new design domains like manufacturing, maintenance, and certification. Stakeholders, needs, requirements, and architecture models of the business jet family are presented. Then, the collaborative Multidisciplinary Design Analysis and Optimization (MDAO) capabilities are used to integrate various aircraft design disciplines, including overall aircraft design, onboard systems design, wing structural sizing, tailplane sizing, mission analysis, and cost estimation. Decisions regarding the degree of commonality are implemented by optionally fixing the design of a shared component when sizing an aircraft.@en

A retrofit analysis on a 90 passengers regional jet aircraft is performed through a multidisciplinary collaborative aircraft design and optimization highlighting the impact on costs and performance. Two different activities are accounted for selecting the best aircraft retrofit solution: a re-engining operation that allows to substitute a conventional power-plant platform with advanced geared turbofan and an on-board-systems architecture modernization, considering different levels of electrification. Besides the variables that are directly dependent from these activities, also scenario variables are considered during the optimization such as the fuel price, the fleet size and the years of utilization of the upgraded systems. The optimization is led by impacts of the retrofitting process on emissions, capital costs and saving costs, computed at industrial level. Overall aircraft design competences (aerodynamics, masses, performance, noise, and emissions) have been computed increasing the level of fidelity and reliability. The whole process is implemented in the framework of the AGILE 4.0 research project in a collaborative remote multidisciplinary approach. Results show that the engine retrofitting can be a profitable solution for both manufacturers and airliners. Conversely, the on-board-system electrification seems to be not convenient in a retrofitting process due to the high capital costs. Depending on the operative scenario, involved stakeholders can properly orient their decision on a retrofitting strategy.@en

The present paper deals with the buckling and post-buckling analysis of a multilayered composite reinforced panel. The panel, design for aeronautical applications, results in a complex stacking sequence, and the development of a refined model able to describe its geometrical nonlinear behavior is mandatory to avoid the usage of highly computational effort-required 3D finite elements. The proposed approach is a finite element analysis based on the Carrera Unified Formulation (CUF). Thanks to CUF, a 1D model of the composite panel can be formulated and complicated stress fields within the structure can be evaluated, so the nonlinear behavior is fully described. A refined Equivalent Single Layer (ESL) technique is employed, making use of Lagrange polynomials for the description of the stacking sequence. The results clearly demonstrate the reliability of this approach, comparing the linearized buckling and nonlinear post-buckling solutions with those from Nastran (1D, 2D and 3D) and experiments. @en

A computationally-efficient strength optimization method tailoring novel composite laminates using lamination parameters is developed. The method adopts a global p-norm approach to aggregate local failure indices into a global failure index, based on the Tsai-Wu failure criterion. For design purposes, the novel composite laminates are characterized via lamination parameters that can subsequently be transformed into locally variable fiber orientations in an existing three-step optimization method. An elliptical formulation of the conservative failure envelope is applied to represent the Tsai-Wu criterion in terms of lamination parameters. A lamination-parameter-based two-level approximation for the global failure index is derived, which guarantees the anticipated conservativeness and convexity in a gradient-based optimization framework. Numerical results show that the computational efficiency of the proposed strength optimization method improves remarkably with a proper value of p, compared to the existing local-based min-max method. The method is also shown to be robust and generate converged optimum designs even in the presence of stress concentrations and singularities.

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For pick-and-place processes to become widely implemented in industry a consistent and acceptable product quality needs to be achieved. In the state of the art it is assumed that reinforcements will be in perfect condition at the start of forming or draping. In reality the handling process can already result in undesired deformations. The current work will look at fiber angle deviations that occur during this process due to in-plane shear. It is shown that bounds can be set for these fiber angle deviations based on experimental work. Periodic representative volume element homogenization is used to obtain homogenized material properties for a bi-axial non-crimp fabric with a specific construction. With these material properties the in-plane shear strain, and thus the fiber angle deviations, can be predicted. The presented methodology and results obtained using it can be a basis in the design process for automated handling of reinforcements and for in-situ quality control of the pick-and-place process.

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Carbon fibre-reinforced polymer composites (CFRPs) outperform most structural engineering materials in specific stiffness and/or specific strength, especially in their unidirectional configuration. Unidirectional composites can be found as individual structural elements in cables or pin-loaded straps; they are however most commonly found in the form of tapes, representing a semi-finished product for subsequent processing to laminates by tape laying, winding or press moulding. The outstanding properties of such composites are affected by its microstructure. It influences the structural performance and fatigue life when architected into thin ply composites [1]. The microstructure is also affected by processing conditions, respectively recursively affects processability as observed in the deconsolidation [2] or intimate contact formation [3] during laser assisted tape laying. This work presents a novel approach to identify microstructural features. This is achieved by Voronoi tessellation-based evaluation of the fibre volume content on cross-sectional micrographs, with consideration of the matrix boundary. The method [4] is shown to be robust and is suitable to be automated and has the potential to be expanded into 3d imaging techniques [5]. It further has the potential to discriminate specific microstructural features and to relate them to processing behaviour. The method is experimentally validated on tape samples with characteristic processing history. @en

For pick-and-place processes to become widely implemented in industry a consistent and acceptable product quality needs to be achieved. One important quality criterion is the fiber angle deviations in the reinforcement. Handling a reinforcement will subject it to forces due to e.g. gravity and accelerations. These forces can result in in-plane shear and subsequently in fiber angle deviations. The current work looks at predicting and preventing in-plane shear induced fiber angle deviations by studying the positioning of pick-up points. In the state of the art the positioning of individual pick-up points is typically either not discussed or is based on the mould where the fabric is to be draped on – not on the effect of the handling on the fabric. The relationship between the positioning of the pick-up points and the behavior of the fabric should however also be considered. Finite element simulations validated through experimental work will be used to study the influence of pick-up point location on the in-plane shear strain for a bi-axial Non Crimp Fabric [NCF]. In [1] tolerances have been set for the fiber angle deviations, additionally the relationship between in-plane shear strain and fiber angle deviations has been demonstrated for the specific NCF. These results are used in the present work to evaluate the results from the simulations. The current work will demonstrate that it is possible to control the in-plane shear induced fiber angle deviations by varying the position and number of the pick-up points. Additionally, It will show whether simulations for the positioning of pick-up points on large reinforcements can be simplified by looking at one instance of a repeating pattern. The paper will provide a framework for the determination of positioning of pick-up points while predicting and preventing in-plane shear induced fiber angle deviations. @en

Most MDO problems currently do not include manufacturing as an optimization domain. Within the H2020 project AGILE 4.0 the intent is to bring manufacturing into the MDO domain using MBSE techniques developed within the project. To demonstrate how manufacturing can be brought into the MDO domain application cases are set up that resemble MDO problems from industry. In this paper, the MDO techniques will be used for the design of a Flap for a regional jet. The manufacturing aspect is represented by including the manufacturing cost of the flap in a Design Of Experiments (DOE). In this DOE different flap kinematic mechanisms and different flap sizes and paths are explored. The DOE is set up using the MDO toolset developed within AGILE 4.0. It allows for an automatic definition of the DOE workflow. The DOE results show that the choice of flap configuration has a significant effect on the Flap manufacturing cost, the flap wright and the landing performance of the aircraft. Next steps will be to investigate more flap configurations, improved the manufacturing cost model used and to set up a true flap optimization.

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Representative stiffened panels are optimized such that multiple buckling modes and failure (using open hole allowables) occur within a range of 10% of the lowest buckling load. This implies the panels cannot be loaded up to the buckling load without risking failure, hence vibrational correlation testing was used to estimate the buckling loads and modes. At the same time, a finite element model was created using the Carrera Unified Formulation. This model was validated using the tests and a good correlation between both was observed. Three panels were manufactured and each panel was put in place for testing twice. Each time a panel was put in place, the test was repeated three times. This allowed us to get a ballpark estimate for the variation due to replicas of the panel, the test set-up and repeating the tests. @en

Despite their importance in benchmarking numerical simulations, buckling tests still feature compromises between component-level and high-fidelity large-scale tests. For example, compression-induced buckling tests cannot capture the through-thickness or span-wise stress gradients in wing skins. Consequently, the results obtained often require careful interpretation and conservative considerations before applying to a structure. Alternatively, a system-level large-scale test can be used, yet at considerably increased time and expense. There has been little progression towards capturing system-level behaviour in a simplified test. Herein, for buckling behaviour assessment, a three-point bending test is used, which is quick, simple to implement, and cost-effective compared to existing conventional methods. The proposed method relies on subjecting a panel with auxiliary stiffeners to bending to introduce compression-induced buckling in the skin. The three-point bend test is used, because it provides readily controllable loading and boundary conditions. The location of the neutral plane can be tailored via design of the stiffeners, thus allowing for control of the through-thickness stress gradient induced in the skin. This method is applicable to buckling of stiffened structures subject to bending (e.g., aircraft wingboxes). Numerical models are used to explore the limits of the proposed method and comparing it against traditional coupon and full-scale structural level tests. The test method is experimentally demonstrated for capturing the buckling behaviour of a thermoplastic composite panel made via automated fibre placement. The proposed approach is shown to reliably capture the buckling behaviour of a large-scale test using a simpler and more time and cost-efficient setup than conventional methods.

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This work presents a new aeroelastic optimisation framework for the preliminary design of variable stiffness composite wing structures. The framework is constructed by sequentially and iteratively solving two sub-problems: aeroelastic tailoring and lay-up retrieval, using gradient-based algorithms with full-analytical sensitivities provided. During aeroelastic tailoring, the wing mass is minimised by optimising the lamination parameters and thickness of wing laminates together with wing jig twist distribution. The load cases cover not only static loads, but also the critical gust loads that are identified across the entire flight envelop at every iteration of optimisation. Further, a cruise shape constraint is included in addition to other aerostructural constraints, so that the optimal aircraft performance can be ensured. During lay-up retrieval, the manufacturable stacking sequence is retrieved according to the optimal lamination parameters with the consideration of minimal steering radius constraint. Moreover, to fix the possible constraint violations caused by lay-up retrieval, a correction strategy is incorporated to tighten the violated constraints for repeating aeroelastic tailoring. Finally, several case studies on the design of NASA common research model wing are carried out and investigated. The results indicate that the critical gust loads and cruise shape constraint have a large influence on the design of tow-steered composite wing structures, which therefore demonstrate the usefulness and benefits of the proposed optimisation framework.

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