In this second international permeability benchmark, the in-plane permeability values of a carbon fabric were studied by twelve research groups worldwide. One participant also investigated the ...deformation of the tested carbon fabric. The aim of this work was to obtain comparable results in order to make a step toward standardization of permeability measurements. Unidirectional injections were thus conducted to determine the unsaturated in-plane permeability tensor of the fabric. Procedures used by participants were specified in the guidelines defined for this benchmark. Participants were asked to use the same values for parameters such as fiber volume fraction, injection pressure and fluid viscosity to minimize sources of scatter. The comparison of the results from each participant was encouraging. The scatter between data obtained while respecting the guidelines was below 25%. However, a higher dispersion was observed when some parameters differed from the recommendations of this exercise.
Morphing systems able to efficiently adjust their characteristics to resolve the conflicting demands of changing operating conditions offer great potential for enhanced performance and functionality. ...The main practical challenge, however, consists in combining the desired compliance to accomplish radical reversible geometry modifications at reduced actuation effort with the requirement of high stiffness imposed by operational functions. A potential decoupling strategy entails combining the conformal shape adaptation benefits of distributed compliance with purely elastic stiffness variability provided by embedded bi-stable laminates. This selective compliance can allow for on-demand stiffness adaptation by switching between the stable states of the internal elements. The current paper considers the optimal positioning of the bi-stable components within the structure while assessing the energy required for morphing under aerodynamic loading. Compared to a time-invariant system, activating specific deformation modes permits decreasing the amount of actuation energy, and hence the amount of actuation material to be carried. A concurrent design and optimisation framework is implemented to develop selective configurations targeting different flight conditions. First, an aerodynamically favourable high-lift mode achieves large geometric changes due to reduced actuation demands. This is only possible by virtue of the internally tailored compliance, arising from the stable state switch of the embedded bi-stable components. A second, stiff configuration, targets operation under increased aerodynamic loading. The dynamic adequacy of the design is proved via high fidelity fluid-structure interaction simulations.
Airborne wind energy (AWE) vehicles maximize energy production by constantly operating at extreme wing loading, permitted by high flight speeds. Additionally, the wide range of wind speeds and the ...presence of flow inhomogeneities and gusts create a complex and demanding flight environment for AWE systems. Adaptation to different flow conditions is normally achieved by conventional wing control surfaces and, in case of ground generator-based systems, by varying the reel-out speed. These control degrees of freedom enable to remain within the operational envelope, but cause significant penalties in terms of energy output. A significantly greater adaptability is offered by shape-morphing wings, which have the potential to achieve optimal performance at different flight conditions by tailoring their airfoil shape and lift distribution at different levels along the wingspan. Hence, the application of compliant structures for AWE wings is very promising. Furthermore, active gust load alleviation can be achieved through morphing, which leads to a lower weight and an expanded flight envelope, thus increasing the power production of the AWE system. This work presents a procedure to concurrently optimize the aerodynamic shape, compliant structure, and composite layup of a morphing wing for AWE applications. The morphing concept is based on distributed compliance ribs, actuated by electromechanical linear actuators, guiding the deformation of the flexible-yet load-carrying-composite skin. The goal of the aerostructural optimization is formulated as a high-level requirement, namely to maximize the average annual power production per wing area of an AWE system by tailoring the shape of the wing, and to extend the flight envelope of the wing by actively alleviating gust loads. The results of the concurrent multidisciplinary optimization show a 50.7% increase of extracted power with respect to a sequentially optimized design, highlighting the benefits of morphing and the potential of the proposed approach.
Fiber-reinforcements’ permeability and compressibility are crucial parameters in Liquid Composite Molding Processes, as they represent the main textile properties that influence the behavior of the ...injected resin. Before designing LCM molds and optimizing manufacturing processes, permeability and compaction characterizations are performed; however, the results obtained often show considerable deviations. One reason for this behavior is the variable compaction histories of fiber-beds, which can be altered during storage or handling. This work presents a novel method to normalize compaction histories through cyclic compaction, thus increasing the reproducibility of compaction and through-thickness permeability responses. It also shows how fiber-bed’s viscoelasticity can be reduced with minor influence on permeability. It is greatly beneficial in developing processing routes for vacuum assisted processes, and for future compaction or permeability benchmarks, in defining optimal characterization methods. It provides also solid basis for the comparison and validation of compaction and through-thickness permeability models with reduced measurement errors.
In this international permeability benchmark exercise, in-plane permeability data for two reinforcement fabrics, obtained using a total of 16 different experimental procedures, were compared. ...Although, for each procedure, the results appear consistent, different procedures result in a scatter of up to one order of magnitude in principal permeability values for each fabric at any given fibre volume fraction. The ratio of the principal permeability values varies by factors of up to 2. While experimental uncertainties and variability of the specimens affect the scatter in results for any single series of experiments, it is suspected that the main source of scatter in results from different procedures is related to human factors. Aiming at standardisation of measurement methods and interchangeability of results, “good practice” guidelines will be formulated in order to eliminate sources of scatter.
Corrugated laminates have highly anisotropic stiffness properties. Therefore, they are interesting candidates for flexible skins. The geometrically non-linear stiffness response of corrugated sheets ...is crucial for applications with large deformations. The present paper analyzes the governing mechanisms that drive the non-linear tensile behavior and suggests a model to analyze corrugated structures. The mechanical response mainly depends on the geometry such as amplitude and thickness of the corrugation and the material properties. The proposed model calculates the mechanical response based on a simplified model consisting of rods and discrete torsional springs. It is verified by comparison with non-linear structural analysis with FEM and experimentally validated by using samples manufactured by 3D printing. The paper also presents a parametric study investigating the influence of the geometry on the non-linear behavior and we identify the limits of linear approximation of the structural response of corrugated laminates.
This paper presents a study of the mechanical response of hexachiral honeycombs with transversally curved ligaments under planar uniaxial tensile, compressive and shear loads. The impact of the ...chiral cell design parameters on the resulting macroscopic behaviour is assessed utilising finite elements calculations. It is shown that the presence of ligament curvature permits to attain mechanical responses which are not achievable through conventional chiral honeycomb designs. In addition, the resulting responses exhibit, for all considered load cases, significant tunability through the investigated geometrical design parameters. Two chiral lattices with identical geometries, only differing in their ligament curvature, were manufactured and experimentally tested to validate the finite elements predictions. A connection and assembly strategy is presented and utilised, offering a fast and robust approach to build larger finite lattice structures through 3D printed single basic cells. The hexachiral lattices were tested in tension, compression and in-plane shear, showing good agreement with the numerical predictions.
Although good progress was made by two international benchmark exercises on in-plane permeability, existing methods have not yet been standardized. This paper presents the results of a third ...benchmark exercise using in-plane permeability measurement, based on systems applying the radial unsaturated injection method. 19 participants using 20 systems characterized a non-crimp and a woven fabric at three different fiber volume contents, using a commercially available silicone oil as impregnating fluid. They followed a detailed characterization procedure and also completed a questionnaire on their set-up and analysis methods. Excluding outliers (2 of 20), the average coefficient of variation (cv) between the participant’s results was 32% and 44% (non-crimp and woven fabric), while the average cv for individual participants was 8% and 12%, respectively. This indicates statistically significant variations between the measurement systems. Cavity deformation was identified as a major influence, besides fluid pressure/viscosity measurement, textile variations, and data analysis.
This paper presents the design, optimization, realization and testing of a novel wing morphing concept, based on distributed compliance structures, and actuated by piezoelectric elements. The ...adaptive wing features ribs with a selectively compliant inner structure, numerically optimized to achieve aerodynamically efficient shape changes while simultaneously withstanding aeroelastic loads. The static and dynamic aeroelastic behavior of the wing, and the effect of activating the actuators, is assessed by means of coupled 3D aerodynamic and structural simulations. To demonstrate the capabilities of the proposed morphing concept and optimization procedure, the wings of a model airplane are designed and manufactured according to the presented approach. The goal is to replace conventional ailerons, thus to achieve controllability in roll purely by morphing. The mechanical properties of the manufactured components are characterized experimentally, and used to create a refined and correlated finite element model. The overall stiffness, strength, and actuation capabilities are experimentally tested and successfully compared with the numerical prediction. To counteract the nonlinear hysteretic behavior of the piezoelectric actuators, a closed-loop controller is implemented, and its capability of accurately achieving the desired shape adaptation is evaluated experimentally. Using the correlated finite element model, the aeroelastic behavior of the manufactured wing is simulated, showing that the morphing concept can provide sufficient roll authority to allow controllability of the flight. The additional degrees of freedom offered by morphing can be also used to vary the plane lift coefficient, similarly to conventional flaps. The efficiency improvements offered by this technique are evaluated numerically, and compared to the performance of a rigid wing.
Corrugated laminates with large corrugation amplitude have highly anisotropic stiffness properties. Here, the bending response of rectangular corrugated laminates to homogeneous transverse pressure ...is investigated. The center-point deflection of thin laminates with high corrugation amplitude is controlled by the higher bending-stiffness value alone. Simple formulas for calculating the center-point deflection are developed from beam formulas by replacing beam bending stiffness with plate bending stiffness and the line load with pressure. A finite-element model is used to verify the results and to illustrate the peculiar load-carrying action of the highly anisotropic plate. Further an error analysis is presented to indicate for which stiffness and aspect ratios the beam-equation modeling approach is valid and can be used for preliminary design.
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