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Variable-angle tow (VAT) manufacturing methods significantly increase the design space for elastic tailoring of composite structures by smoothly changing fiber angle and ply thickness across a component. Rapid tow shearing (RTS) is a VAT manufacturing technique that uses in-plane shearing (rather than in-plane bending) to steer tows of dry or pre-impregnated fibers. RTS offers a number of benefits over conventional bending-driven steering processes, including tessellation of adjacent tow courses; no overlaps or gaps between tows; and no fiber wrinkling or bridging. Further to this, RTS offers an additional design variable: fiber orientation to tow thickness coupling due to the volumetric relation between tow shearing and the tow’s thickness and width. Previous computational work has shown that through a judicious choice of curvilinear fiber trajectories along a cylinder’s length and across its circumference, the imperfection sensitivity of cylindrical shells under axial compression can be reduced and load-carrying capacity increased. The present work aims to verify these predictions by manufacturing and testing two cylinders: an RTS cylinder and a straight-fiber, quasi-isotropic cylinder as a benchmark. The tow-steered manufacturing process, imperfection measurements, instrumentation, and buckling tests of both cylinders are discussed herein. The experimental tests results are compared against high-fidelity geometrically nonlinear finite element models that include measured geometric and loading imperfections before and during the tests. Finally, a discussion is provided on the outstanding challenges in designing and manufacturing RTS cylinders for primary aerostructures.

Pressure vessels are widely used in a broad range of applications in many industries, including possible use for hydrogen storage. Additionally, pressure vessels comprise non-circular-cross-section fuselages such as in blended-wing–body aircraft. Failure performance, weight, and packing efficiency are important factors that should be considered when designing pressure vessels. Often, lightweight pressure vessels maximize their volume for a given available space volume and withstand substantial internal pressure. Bend-free super ellipsoids of revolution, enabled by stiffness tailoring via variable-angle tow placement of composite plies, present an appealing solution that makes them possible candidates for future composite pressure vessels. To date, the bend-free design methodology for super ellipsoids of revolution has only been developed under uniform internal pressure loading. However, in reality, pressure vessels can also be subjected to thermal loads during service. Therefore, this study develops an initial bend-free design methodology for super ellipsoids of revolution that are subjected to both uniform internal pressure and thermal loads.

Beam-like compliant elements have found wide-ranging application in many fields of engineering and science often where 3D large deflections can be of concern such as soft robotics, DNA mechanics and helicopter/wind turbine rotor blades. The homotopy analysis method (HAM) is used for the first time to obtain a novel analytical solution in converged series form for the arbitrary large deflection of geometrically exact beams subject to both conservative and follower loading scenarios. The homotopy analysis method, which offers desirable characteristics such as being free from small or large parameters, coupled with auxiliary parameters controlling convergence, is applied directly to the intrinsic governing equations of a geometrically exact beam theory. The system of first-order differential governing equations of geometrically exact beams with intrinsic formulation is free from rotation and displacement variables, and offers a low degree of nonlinearity (quadratic at most) and compact mathematical form, making it suitable for analytical solutions. Due to the relatively poor convergence of the original HAM algorithm, the iterative HAM technique is employed which is known to accelerate convergence and to improve the computational efficiency of the homotopy analysis method. The obtained homotopy series offers a number of novel features in the context of the analytical solutions for the large deflection of beams, including (a) the direct calculation of internal forces and moments which is significant for engineering design purposes, (b) being able to capture 3D deflections, (c) considering transverse shear effects which can be important for thicker beams or when the Young’s modulus to shear modulus ratio is significant (such as composite materials) and (d) considering conservative and follower tip and distributed loads, in a unified framework. In order to investigate the efficacy, applicability and accuracy of the proposed method, a number of numerical examples are considered in which a cantilever beam subject to tip or distributed loads undergoes large deflection. Large deflection results for both conservative and follower loads are compared against those of less comprehensive analytical solutions as well as against numerical methods including finite element and Chebyshev collocation methods where good agreement is observed. These results demonstrate the applicability and effectiveness of HAM for the large deflection analysis of geometrically exact beams.

Cutouts are widely used to accommodate windows, openings for access purposes, or fasteners in the primary structural parts of airplanes. The presence of cutouts in composite panels results in stres...

Data availability: Data will be made available on request. Copyright © 2023 The Author(s). The present work studies the thermal degradation of laser-heated poly-ether-ether-ketone (PEEK) as the heating duration increases. Its damage morphology, chemical composition, crystallinity content, and mechanical properties are examined with optical microscopy, attenuated total reflection-Fourier transform infrared spectroscopy, differential scanning calorimetry, Raman spectroscopy, and continuous stiffness measurement nanoindentation. The applicability of those methods in detecting the thermal degradation of laser-heated PEEK and assessing the induced thermal damage is highlighted. Results show that short-time laser heating acts as an annealing process that improves the crystallinity and hardness on the affected surface of PEEK by up to 5.1% and 10.8% respectively. With a further increase in the heating duration, surface carbonisation occurs and a char layer is formed. Surface carbonisation is associated with the thermal limits of PEEK in laser heating decreasing by up to 50% its hardness and by 45% its indentation modulus. Finally, the char layer is found to act as a shielding mechanism that protects the bulk PEEK from the applied thermal load, resulting in mostly superficial thermally induced damage. This publication was made possible by the sponsorship and support of TWI. The work was enabled through, and undertaken at, the National Structural Integrity Research Centre (NSIRC), a postgraduate engineering facility for industry-led research into structural integrity established and managed by TWI through a network of both national and international Universities. Dimitrios Gaitanelis and Dr Angeliki Chanteli would like to thank Young European Research Universities Network (YERUN) for being awarded the YERUN Research Mobility Award 2021 to proceed to this collaboration. Dr Angeliki Chanteli and Professor Paul M. Weaver would like to thank Science Foundation Ireland (SFI) for funding Spatially and Temporally VARIable COMPosite Structures (VARICOMP) Grant No. (15/RP/2773) under its Research Professor programme.

New analytical solutions for the static deflection of anisotropic composite beams resting on variable stiffness elastic foundations are obtained by the means of the Homotopy Analysis Method (HAM). The method provides a closed-form series solution for the problem described by a non-homogeneous system of coupled ordinary differential equations with constant coefficients and one variable coefficient reflecting variable stiffness elastic foundation. Analytical solutions are obtained based on two different algorithms, namely conventional HAM and iterative HAM (iHAM). To investigate the computational efficiency and convergence of HAM solutions, the preliminary studies are performed for a composite beam without elastic foundation under the action of transverse uniformly distributed loads considering three different types of stacking sequence which provide different levels and types of anisotropy. It is shown that applying the iterative approach results in better convergence of the solution compared with conventional HAM for the same level of accuracy. Then, analytical solutions are developed for composite beams on elastic foundations. New analytical results based on HAM are presented for the static deflection of composite beams resting on variable stiffness elastic foundations. Results are compared to those reported in the literature and those obtained by the Chebyshev Collocation Method in order to verify the validity and accuracy of the method. Numerical experiments reveal the accuracy and efficiency of the Homotopy Analysis Method in static beam problems.

peer-reviewed Increasing applications of laminated composite structures necessitate the development of equivalent single layer (ESL) models that can achieve similar accuracy but are more computationally efficient than 3D or layer-wise models. Most ESL displacement-based models do not guarantee interfacial continuity of shear stresses within laminates. A possible remedy is the enforcement of interlaminar equilibrium in variational formulations, for example, in the framework of the Hellinger-Reissner variational principle, leading to a mixed force/displacement model. In this paper, the governing equations for bending and stretching of laminated beams, comprising only seven stress resultants and two displacement functionals, are obtained using global fifth-order and a local linear zigzag kinematics. As a strong-form solution technique, the differential quadrature method (DQM) is an efficient tool which can provide excellent convergence with relatively few number of grid points. However, in dealing with high-order differential equations, the conventional DQM can incur considerable errors due to the nature of numerical differentiation. Therefore, a mixed inverse differential quadrature method (iDQM) is proposed herein to solve the governing fourth-order differential equations for bending and stretching of laminated beams. This approach involves approximating the first derivatives of functional unknowns, thereby reducing the order of differentiation being performed. Using a non-uniform Chebychev-Gauss-Lobatto grid point profile, numerical results show that the accuracy of stress predictions is improved by using iDQM compared to DQM. In addition, the Cauchy’s equilibrium condition is satisfied more accurately by iDQM, especially in the vicinity of boundaries

The optimal design of the postbuckling response of variable angle tow composite structures is an important consideration for future lightweight, high-performing structures. Based on this premise, a new optimisation tool is presented for shell-type structures. The starting point is an isogeometric framework which uses NURBS interpolation functions to provide a smooth description of the deformed shapes, thereby reducing the number of degrees of freedom with respect to standard finite elements. The stiffness variation is obtained by exploiting the same NURBS interpolation to describe lamination parameters, employed as intermediate optimisation variables. This choice allows the design space to be thoroughly explored with relatively few design variables in a smooth optimisation space. Therefore, the optimisation strategy is divided into two stages. Firstly, the optimal distribution of lamination parameters is determined using a gradient based algorithm. Afterwards, an actual distribution of fibre orientation is retrieved. The viability of the tool is tested firstly onto a cylindrical panel under compressive loading. Then, the postbuckling optimisation of a composite wingbox is given. For both structures, the optimised postbuckling response is compared with those of the corresponding quasi-isotropic baselines showing significant improvements.

A parametrically driven structural model based on Mindlin–Reissner plate theory is developed to capture the threedimensional deflections of a compliance-based morphing trailing edge device with severe structural discontinuities. The model is used to study the Fish Bone Active Camber (FishBAC) device, which is represented as a discontinuous plate structure that captures the step changes in stiffness created by the concept’s geometrical configuration. Courant’s penalty method is implemented in the form of artificial penalty springs to account for stiffness discontinuities. A numerical validation is performed using finite element analysis, followed by experimental validation under actuation loads. This analytical model represents a robust, efficient, mesh-independent, and parameter-driven solution tomodeling discontinuous plate structures. These traits make it useful for ongoing fluid–structure interaction analysisand optimization of the FishBAC concept and also for application to other complex composite structures.

Variable Angle Tow (VAT) composite panels, whereby fibre angle distributions vary continuously in-plane, have more scope for tuning structural properties than traditional straight fibre laminated composite materials. Currently, the geometrically non-linear response of such structures is typically modelled using path-following methods implemented in commercial finite element analysis. However the high computational cost given from both the incremental-iterative procedure and fine mesh required for accurate modelling of the variable trajectory of fibre direction, can be excessively time consuming. Driven by these shortcomings, we use a fast multi-modal Koiter asymptotic method for investigating the nonlinear buckling behaviour of VAT cylindrical panels in compression. In doing so, this paper provides new insight into the multi-modal description and nonlinear buckling mode interactions which are responsible for the highly nonlinear behaviour of cylindrical panels in compression with a particular emphasis on the nonlinear components in the mathematical asymptotic description. As such, through an extensive parametric virtual testing programme we construct, for each panel under investigation, an asymptotic solution projecting the equilibrium equations in the subspace of its first 30 buckling modes. Then, at several points of the resulting equilibrium path, the percentage contribution of each of these modes to the solution is measured. On the basis of the percentage obtained, those modes which give the largest contribution to the solution are identified. The main novelty of this work is that these results are used as a priori information for re-projecting the equilibrium equations into a new modal subspace to which only the buckling modes with the largest participation belong. Moreover, for the first time, we observe that in a multi-modal Koiter-inspired description the modal shapes giving the largest contribution to the asymptotic solution have the same degree of symmetry. In this way, for a generic panel under consideration, we show that it is possible to obtain a more computationally-efficient asymptotic description than current approaches. Finally, the new computed equilibrium paths are compared to benchmark results using the commercial finite element software ABAQUS and the computational advantages given by using a priori information in the asymptotic expansion are commented upon.

An analytical solution is presented for the 3D static response of variable stiffness non-uniform composite beams. Based on Euler-Bernoulli theory, a set of governing differential equations are obtained, in which four degrees of freedom are fully coupled. For the variable stiffness beam, the governing field equations have variable coefficients reflecting the stiffness variation along the beam. Using the direct integration technique, the general analytical solution is derived in the integral form and the closed-form expressions of the obtained solutions are presented employing a series expansion approximation. The series expansion representation enables the proposed approach to be applicable for variable stiffness composite beams with arbitrary span-wise variation of properties. As an alternative solution, the Chebyshev collocation method is applied to the proposed formulation to verify the results obtained from the analytical solution. A number of variable stiffness composite beams made by fibre steering with various boundary conditions and stacking sequences are considered as the test cases. The static response are presented based on the analytical solution and Chebyshev collocation method and excellent agreement is observed for all test cases. The proposed model presents a reliable and efficient approach for capturing the complicated behaviour of variable stiffness non-uniform composite beams.

The effect of stochastic variation in material and geometric properties on structural performance is important for robust design. Knowledge of such effects can be acquired by applying variation pat...

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.

Cut-outs are inevitable in many structural components such as in aircrafts to accommodate windows or openings for access purposes or fasteners. Engineers usually view cut-outs, especially in primary structures, with disfavour as they result in stress/strain concentration and consequently reduced load carrying capability. Local reinforcements usually increase cost and weight to the overall design which is not favourable in aerospace applications. In case of composite panels, emerging advanced manufacturing methods such as 3D printing of automated fiber placement made it possible to continuously steer fibers/tows around a cut-out to potentially alleviate stress/strain concentration problem. Another advantage of tow steering in this case is maintaining the continuity of fiber/tow paths without any fiber cut which precludes ply-level, 3D stress/strain concentration which could otherwise lead to delaminationinduced damage. In this study, potential capability of tow steering around an elliptical cut-out (manhole) in reducing stress/strain concentration in a composite wingbox is investigated Buckling response under compression loading together with stress and strain concentrations under both tensile and compression loads are examined. Under tensile loading, the maximum stress and strain concentration factors around the cut-out in the straight fiber design are shown to be approximately 29% and 32% larger than its counterpart with steered tows around the cut-out. For the compression loading condition, the direct strain of the panel with straight fiber orientations was found to be three times that of steered fiber trajectories in the vicinity of the cut-out.

Pressure vessels are designed to store liquids and gases and have various applications spanning from chemical plants to automotive and aerospace industries. Currently, lightweight composite pressure vessels are desirable, especially in transportation industry applications because of their subsequent benefits in fuel consumption, cost and environmental issues. Using composite materials for pressure vessels along with advanced manufacturing technologies such as automated fiber placement provides excellent scope to tailor stiffness through the structural surface using fiber steering to achieve desirable structural performance. Recently, variable angle tow (VAT) technology has been used to suppress bending in super ellipsoids of revolution composite pressure vessels, resulting in minimizing the inefficient bending stresses and deformations and increasing their load-carrying capacity. It is worth noting that such geometries can provide excellent packing efficiency. These advantages make the bend-free super ellipsoids of revolution composite pressure vessels potential candidates for the next generation of pressure vessels. Therefore, their failure performance as the most important design factor should be studied carefully due to safety reasons. In this study, the maximum allowable internal pressure for VAT bend-free ellipsoidal pressure vessels, using the first-ply failure based on both Tsai-Wu and three-dimensional invariant-based failure criteria is determined. Subsequently, VAT bend-free pressure vessels’ failure performance is compared against that obtained for conventional constant stiffness composite vessels. Among structures considered, the VAT bend-free composite vessel has the best failure performance. Moreover, the predicted failure load using the three-dimensional invariant-based failure criterion for the VAT bend-free design is 34% lower than the failure load predicted by the Tsai- Wu. Finally, the effect of various material properties on the difference in predicted failure load using these criteria is assessed. Results provide physical insight useful for designers in materials selection.

The design of a multi-part aerospace structural component, such as a wingbox, is a challenging process because of the complexity arising from assembly and integration, and their associated limitations and considerations. In this study, a design process of a stiffeners-integrated variable stiffness three-bay wingbox is presented. The wingbox has been designed for a prescribed buckling and post-buckling performance (a prescribed real testing scenario) and made from thermoplastic composite material system (Carbon-PEEK) with the total length of three meters. The stiffeners and spars are integrated into the top and bottom panels of the wingbox resulting a single-piece blended structure with no fasteners or joints. The bottom skin also has an elliptical cut-out for access purposes. The composite tows are steered around this cutout for strain concentration reduction purposes. The fiber/tow steering in the top skin bays (compression side) has also been considered for improved compression-induced buckling load carrying capacity. The proposed design has been virtually verified via high fidelity finite element analysis.

Bamboo poles, and other one-dimensional thin-walled structures are usually loaded under compression, which may also be subject to bending arising from eccentric loading. Many of these structures contain diaphragms or circumferential stiffeners to prevent cross-sectional distortions and so enhance overall load-carrying response. Such hierarchical structures can compartmentalize buckling to local regions in addition to withstanding global buckling phenomena. Predicting the buckling mode shapes of such structures for a range of geometric parameters is challenging due to the interaction of these global and local modes. Abaqus finite element software is used to model thousands of circular hollow tubes with random geometric parameters such that the ratios of radius to periodic length range from 1/3-1/7, the ratio of wall thickness to radius varies from 1/4-1/10. The material used in this study is a type of bamboo, where the Young’s and shear moduli are point-wise orthotropic and gradually increase in magnitude in the radial direction. Under eccentric loads with varying eccentricity, the structures can buckle into a global mode or local modes within an internode, i.e. periodic unit. Moreover, the local modes may contain only one wave or multiple waves in the circumferential direction. As expected, numerical results show that the global mode is more likely to occur in small and thick tubes, whereas the local modes are observed in larger tubes with a smaller number of circumferential waves present in thicker walls. Also, greater eccentricity pushes the local mode domains towards smaller tubes. An efficient classification method is developed herein to identify the domains of each mode shape in terms of radius, wall thickness and eccentricity. Based on linear discriminant analysis, explicit boundary surfaces for the three domains are defined for the obtained data, which can help designers in predicting the mode shapes of tubular structures under axial bending.

With the introduction of the variable stiffness concept, the design space for highperformance lightweight composite structures has expanded significantly. A larger design space, in particular, allows designers to find more effective solutions with higher overall stiffness and fundamental frequency when considering prestressed dynamically excited aerospace components. In this context, an efficient and versatile Ritz method for the transient analysis of prestressed variable stiffness laminated doubly-curved shell structures is presented. The considered theoretical framework is the first-order shear deformation theory without further assumptions on the shallowness or on the thinness of the structure. A rational Bézier surface representation is adopted for the description of the shell allowing general orthogonal surfaces to be represented. General stacking sequences are considered and the unknown displacement field is approximated by Legendre orthogonal polynomials. Stiffened variable angle tow shell structures are modelled as an assembly of shell-like domains and penalty techniques are used to enforce the displacement continuity of the assembled multidomain structure and the kinematical boundary conditions. For the transient analysis of prestressed variable stiffness structures, classical Rayleigh damping is considered and solutions are obtained through the Newmark integration scheme. The proposed approach is validated by comparison with literature and finite elements results and original solutions are presented for prestressed free and forced vibrations of VS stiffened shell structure, proving the ability of the present method in dealing with the analysis of complex aerospace structures.

Planar cellular lattice structures subject to axial compression may undergo elastic bending or buckling of the unit cells. If sufficient compression is applied, the columns of adjacent cells make contact. This changes the topology of the lattice by establishing new load paths. This topology change induces a corresponding shift in the effective stiffness characteristics of the lattice — in particular, the shear modulus undergoes a step-change. The ability to embed adaptive stiffness characteristics through a topology change allows structural reconfiguration to meet changing load/operational requirements efficiently. The concept, of topological reconfiguration, can be exploited across a range of length scales, from (meta-)materials to components. Here we focus on macroscopic behaviour presenting results obtained from finite element analysis that shows excellent correlation with the observed response of 3D-printed PLA lattices. Through a parametric study, we explore the role of key geometric and stiffness parameters and identify desirable regions of the design space. The non-linear responses demonstrated by this topology morphing lattice structure may offer designers a route to develop bespoke elastic systems.

Accurate full field stress responses are often necessary to predict the structural performance of composite structures. The Unified Formulation (UF) is a promising approach to realise efficient and accurate 3D stress predictions of beam-like structures by using recently proposed hierarchical Serendipity Lagrange Elements (SLE) for capturing the 2D response of the beam cross-section while the 1D behaviour along the beam axis is captured using the Finite Element Method (FEM). Despite the computational merits of SLE elements, the performance of UF-SLE-FEM model is strongly influenced by FEM mesh discretisation along the beam's longitudinal axis. With respect to multi-layered beam structures, a high density FEM mesh may lead to loss of efficiency of the UF-SLE-FEM model due to a significant increase in the number of degrees of freedom required to obtain convergence. This study proposes high-order refined formulations of the 1D structure based on strong-form, Differential Quadrature Method (DQM) and weak form, FEM for 3D stress analysis of composite structures. The proposed high-order strong-form DQM and weak-form FEM models, which are benchmarked against exact solutions, lead to significant computational advantages over UF-SLE-FEM models of similar accuracies. However, the symmetry and positive definiteness of the stiffness matrices of FEM models offer a numerical advantage over the strong-form DQM model with non-symmetric, non-positive definite stiffness matrices.

Porous graded materials found in nature can be regarded as variable stiffness optimised load carrier elements that exhibit beneficial properties for real-life engineering designs. In order to investigate the nonlinear behaviour of variable stiffness bioinspired materials, the large deflection of functionally graded beams made from porous materials is considered in this work. Our purpose is to present an efficient and accurate methodology capable of capturing spatially large deflections of these structures with different types of loading conditions and porosity distributions. A geometrically exact beam model with fully intrinsic formulation is employed for the first time to study the large deflection behaviour of functionally graded beams under conservative and non-conservative (follower) loading scenarios. An orthogonal Chebyshev collocation method is used for the discretisation of the fully intrinsic formulation. Two types of porosity distributions, namely cross-sectional and span-wise, are considered and the effect of porosity distribution has been investigated for various benchmark classical test cases. For a given level of accuracy, it is shown that the span-wise functionally graded beam is computationally more demanding compared to the cross-sectional functionally graded beam. In addition to classical problems, two examples demonstrating 3D deflections of highly flexible structures made from porous material subject to combined loads are investigated. It is shown that the current paradigm, while being computationally efficient, can effectively capture the large deflections of functionally graded beams with excellent accuracy.

Accurate modeling of composite structures is important for their safe application under different loading conditions. To provide accurate predictions of three-dimensional (3D) stress fields in an efficient computational framework, in this study, a modeling approach that builds upon the recently developed hierarchical Serendipity Lagrange finite elements (FEs) is employed. The approach provides layerwise (LW) and equivalent single-layer (ESL) models for analyzing constant- and variable-stiffness laminated beam structures. To enhance the capability of the ESL model, two zig-zag (ZZ) functions, namely,Murakami’s ZZ function (MZZF) and the refined ZZ theory function (RZT), are implemented. For constant-stiffness laminated and sandwich beams, the RZT ZZ function predicts the structural response more accurately than the MZZF. However, for variable-stiffness laminated structures, the applicability of RZT remains unknown and its accuracy is therefore tested within the present formulation. Results obtained are validated against 3D closed-form and 3D FE solutions available from the literature. For similar levels of accuracy, significant gains in computational efficiency are achieved over 3D FE and LW models by using the ESL approach with RZT ZZ functions.

This article responds to increasing public and academic discourses on social innovation, which often rest on the assumption that social innovation can drive societal change and empower actors to deal with societal challenges and a retreating welfare state. In order to scrutinise this assumption, this article proposes a set of concepts to study the dynamics of transformative social innovation and underlying processes of multi-actor (dis)empowerment.First, the concept of transformative social innovation is unpacked by proposing four foundational concepts to help distinguish between different pertinent ‘shades’ of change and innovation: 1) social innovation, (2) system innovation, (3) game-changers, and (4) narratives of change. These concepts, invoking insights from transitions studies and social innovations literature, are used to construct a conceptual account of how transformative social innovation emerges as a co-evolutionary interaction between diverse shades of change andinnovation. Second, the paper critically discusses the dialectic nature of multi-actor (dis)empowerment that underlies such processes of change and innovation. The paper then demonstrates how the conceptualisations areapplied to three empirical case-studies of transformative social innovation: Impact Hub, Time Banks and Credit Unions. In the conclusion we synthesise how the concepts and the empirical examples help to understand contemporary shifts in societal power relations and the changing role of the welfare state.

Variable angle tow (VAT) composites have demonstrated better performance in buckling and post-buckling over straight fiber composites based on the mechanics of load redistribution from critical regions to supported edges. In this work, the dynamic instability behavior of a curved VAT composite panel subjected to periodic axial compression load is investigated. The governing energy functional of a curved symmetric VAT panel under external loading is derived using Donnell's shallow shell theory. Later, the discretized equations of motion are derived using the Rayleigh–Ritz method combined with the generalized differential integral quadrature method (GDIQM). Initially, the pre-buckling problem is solved by applying a uniform compression load to compute the stress resultant distribution which is used to evaluate the buckling load of the curved VAT panel. Subsequently, the dynamic/parametric instability region of a curved VAT panel subjected to periodic axial compression load is determined using Bolotin's first-order approximation. Then, the dynamic instability performance is evaluated for a curved VAT panel with linear fiber angle distribution and compared with straight fiber laminates. Finally, the influence of fiber angle orientation, the radius of curvature, aspect ratio and plate boundary conditions on the dynamic instability of VAT panel is presented.

peer-reviewed The stiffness-tailoring capability of Variable Angle Tow (VAT) laminates gives enhanced freedom to design thin-walled structures. One key advantage of tow steering is the ability to redistribute stresses improving buckling performance, leading to reduction in material weight and costs. The aim of this work is to optimise the initial postbuckling behaviour of a recently proposed VAT composite wingbox. The optimisation process is based on a fibre path parameterisation. It involves seeking the stacking sequence that minimises the displacements occurring in the postbuckling regime. This problem is solved by coupling the multi-modal Koiter asymptotic approach implemented with a solid-shell Finite Element environment through stochastic optimisation strategies. Results obtained regarding different optimisation scenarios show a much improved performance for the buckling and postbuckling response of the wingbox with respect to the initial VAT design. Additionally, manufacturing constraints are readily included in the optimisation program. The possibility of performing an efficient and robust optimisation process of a complex structure with a multi-modal Koiter asymptotic approach is demonstrated, showing its viability as a design tool for buckling dominated structures. A parametric study regarding the influence of steering radii shows that overcoming the current manufacturing constraint on minimum radius is worthy of investigation.

Thermoplastic composites offer the potential for reducing the overall manufacturing costs of aircraft structures by allowing continuous production methods to be applied without the ancillary need f...

Fibre reinforced composite materials are finding increasing application in aerospace structures due to their superior specific properties. Aerospace structures make widespread use of stiffening elements, such as stringers, for example in wingboxes and fuselage structures. Sizing of stiffeners to fulfil strength, stiffness and manufacturing considerations is a significant challenge. Herein, a novel manufacturing approach using winding in combination with laser-assisted tape placement is used to manufacture an omega-shaped stiffener made from carbon fibre thermoplastic material . This paper discusses the integrated approach taken by considering material choice, manufacturing constraints and structural design on the performance of a closed-section omega stiffener. The sizing is based on the buckling response of the wingbox, with manufacturing constraints taken into consideration. In the collapsible mould, a low-melt aluminium alloy is used as a spacer, which can be removed post-process by exposing the mould to the alloy melt temperature, which is below the glass transition temperature of the thermoplastic composite material. Manufacturing tests show that repeatable stiffeners of the appropriate dimensions are manufactured. Characterisation tests show that both the bond strength, measured using an interlaminar shear strength test, and the corner strength, assessed using a four-point bend test, are satisfactory.