Giuliano Allegri

Design and Modeling of Selective Reinforcements for Integral Aircraft Structures

A numerical simulation is presented in this paper on the performance of crack retarders bonded to integral metallic structures. The work is described in two main parts. First, a novel modeling approach employing the finite element method has been developed for simulating the various failure mechanisms of a bonded structure and for predicting fatigue crack growth life. Crack growth in the substrate and the substrate/strap interface disbond failure aremodeledintheframeworkoflinearelasticfracturemechanics.Acomputercodeinterfacingwiththecommercial package MSC NASTRAN has been developed and validated by experimental tests. Second, the effectiveness of differentstrapconfigurationsoncrackgrowthretardationhasbeenmodeled;theseincludedifferentstrapmaterials, strap dimensions, and their locations on the substrate. The research has included two substrate materials and four strap materials, and at this stage the specimens were cured at room temperature. Strap stiffness and adhesive toughness are found to be the most influential parameters in designing crack retarders. A design tool has been developedbasedonthe numericalsimulationto achieveoptimalcrackretarderdesigninterms ofprescribedfatigue life target and minimum structural weight added by the bonded reinforcement.

An integrated conceptual design study using span morphing technology

A comprehensive conceptual design study is performed to assess the potential benefits of span morphing technology and to determine its feasibility when incorporated on medium altitude long endurance unmanned air vehicles. A representative medium altitude long endurance unmanned air vehicle based on the BAE Systems Herti unmanned air vehicle was selected. Stability and control benefits are investigated by operating the morphing span asymmetrically to replace conventional ailerons. The Tornado vortex lattice method was incorporated for aerodynamic predictions. The sensitivity of rolling moment generated by span morphing for different flight parameters (instantaneous vehicular weight and angle of attack) is studied. The variation of roll rate (steady and transient response) with span morphing (for constant rolling moment) for different rolling strategies (extension and retraction) is investigated. It turns out that the optimum rolling strategy is to extend one side of the wing by 22% while retract the other by 22%. Operational performance benefits are investigated by operating the morphing span symmetrically to reduce drag, increase endurance and reduce take-off and landing distances. Twenty-two per cent symmetric span morphing reduces the total drag by 13%, enhances the endurance capability by 6.5% and reduces the take-off field length and landing distance by 28% and 10%, respectively.

Conceptual Modeling of an Adaptive Torsion Wing Structure

This paper presents the conceptual analysis of a novel Active Aeroelastic Structure (AAS) device, which allows tailored twist deformations of wing structures to be achieved. The Adaptive Torsion Wing (ATW) concept is a thin-walled closed section two-spar wing-box whose torsional stiffness can be adjusted by changing the area enclosed between the front and rear spar webs. This is done by translating the spar webs in the chord-wise direction inward and towards each other using internal actuators. As the webs move closer to each other, the torsional stiffness of the structure reduces, while its bending stiffness in the span-wise direction is unaffected. The reduction in torsional stiffness allows external aerodynamic loads to induce twist on the structure and to maintain its deformed shape. These twist deformations can be controlled by changing the relative position of the webs as a function of the flight conditions to obtain an optimal or targeted level of performance. A Quasistatic Aeroelastic Suite has been developed in MATLAB TM to model the ATW concept and to study its behavior with respect to different web shifting strategies. Finally, the variation of structural figures of merit such as torsion constant, tip twist, shear centre position, and minimum actuation energy are evaluated and discussed.

Performance and control optimisations using the adaptive torsion wing

This paper presents the Adaptive Torsion Wing (ATW) concept and performs two multidisciplinary design optimisation (MDO) studies by employing this novel concept across the wing of a representative UAV. The ATW concept varies the torsional stiffness of a two-spar wingbox by changing the enclosed area through the relative chordwise positions of the front and rear spar webs. The first study investigates the use of the ATW concept to improve the aerodynamic efficiency (lift-to-drag ratio) of the UAV. In contrast, the second study investigates the use of the concept to replace conventional ailerons and provide roll control. In both studies, the semi-span of the wing is split into five equal partitions and the concept is employed in each of them. The partitions are connected through thick ribs that allow the spar webs of each partition to translate independently of the webs of adjacent partitions and maintain a continuous load path across the wing span. An MDO suite consisting of a Genetic Algorithm (GA) optimiser coupled with a high-end low-fidelity aero-structural model was developed and employed in this paper.

Dynamic modelling and actuation of the adaptive torsion wing

This article presents the dynamical modelling of a novel active aeroelastic structure. The adaptive torsion wing concept is a thin-wall, two-spar wingbox whose torsional stiffness can be adjusted by translating the spar webs in the chordwise direction inward and towards each other using internal actuators. The reduction in torsional stiffness allows external aerodynamic loads to induce twist on the structure and maintain its deformed shape. Here, the adaptive torsion wing system is considered as integrated within the wing of a representative unmanned aerial vehicle to replace conventional ailerons and provide roll control. The adaptive torsion wing is modelled as a two-dimensional equivalent aerofoil using bending and torsion shape functions to express the equations of motion in terms of the twist angle and plunge displacement at the wingtip. The full equations of motion for the adaptive torsion wing equivalent aerofoil were derived using Lagrangian mechanics. The aerodynamic lift and moment acting on the aerofoil were modelled using Theodorsen’s unsteady aerodynamic theory. A low-dimensional, state-space representation of an empirical Theodorsen’s transfer function was adopted to allow time-domain analyses. Four actuation strategies were investigated. Figures of merit, including plunge displacement, twist angle, actuation forces and actuation powers, were quantified and discussed for each of the scenarios. This study allows the conceptual design and sizing of the internal actuators that are required to drive the webs.

Damage-tolerant composite structures by Z-pinning

Abstract This chapter presents a focused update and the addition of new information to published reviews of the manufacture and performance of Z-pinned composite structures, concentrating on the mechanisms of enhancement of delamination damage resistance. Special emphasis is placed on the effect of manufacturing routes in terms of generation of the mesostructure of the composite. Effects of manufacturing defects, such as Z-pin misalignment, on the apparent toughness of the composite are explored in experimental and modeling terms, detailing the use of single Z-pin coupons to generate data and to validate models. The strong relationship between the mesostructure of the Z-pinned composite and its performance in both in-plane and out-of-plane properties is highlighted in relation to the differences in dominant failure mechanisms between control and Z-pinned structural elements. The chapter concludes with an update on recent advances in modeling of the effects of load mixity on the apparent toughness of Z-pinned composites.

Wave Propagation in Periodically Supported Nanoribbons: A Nonlocal Elasticity Approach

We develop an analytical formulation describing propagating flexural waves in periodically simply supported nanoribbons by means of Eringen’s nonlocal elasticity. The nonlocal length scale is identified via atomistic finite element (FE) models of graphene nanoribbons with Floquet’s boundary conditions. The analytical model is calibrated through the atomistic finite element approach. This is done by matching the nondimensional frequencies predicted by the analytical nonlocal model and those obtained by the atomistic FE simulations. We show that a nanoribbon with periodically supported boundary conditions does exhibit artificial pass-stop band characteristics. Moreover, the nonlocal elasticity solution proposed in this paper captures the dispersive behavior of nanoribbons when an increasing number of flexural modes are considered. [DOI: 10.1115/1.4023953]

Buffeting mitigation using carbon nanotube composites: a feasibility study

The article describes a feasibility study to assess the use of nanotubes-based composites to mitigate tail buffeting. The buffeting of a representative business jet rudder is considered as case study. The baseline rudder configuration consists in a sandwich structure with honeycomb core and carbon/epoxy IM7/8552 skins. The damping characteristics of the baseline rudder configuration are compared to those achieved employing constrained layer Al/3M467 skin patches, and those obtained by dispersing multi-walled carbon nanotubes in the baseline carbon/epoxy material. The loads applied to the rudder during flight are obtained by airworthiness standards. Static and dynamic finite element analyses of the rudder under flight loads are carried out to evaluate the structural response at two different temperatures, −40 ℃ and +30 ℃. IM7/8552/MWNT with 1.5 wt% nanofiller is shown to have the best overall performance for the case study considered here, with the potential of outperforming conventional constrained layer patches for buffeting mitigation.

Numerical Assessment of Using Sherman-Morrison, Neumann Expansion Techniques for Stochastic Analysis of Mistuned Bladed Disc System

The paper presents an assessment about using two classical reduced-order techniques, the Sherman-Morrison-Woodbury (SMW) formula and the Neumann expansion method, to enhance the computational efficiency of the stochastic analysis in mistuned bladed disc systems. The frequency responses of the blades are evaluated for different mistuning patterns via stiffness perturbations. A standard matrix factorization method is used as baseline to benchmark the results obtained from the SMW formula and Neumann expansion methods. The modified SMW algorithm can effectively update the inversion of an uncertainty matrix without the need of separated inversions, however with a limited increase of the computational efficiency. Neumann expansion techniques are shown to significantly decrease the required CPU time, while maintaining a low relative error. The convergence of the Neumann expansion however is not guaranteed when the excitation frequency approaches resonance when the mistuned system has either a low damping or high mistuning level. A scalar-modified Neumann expansion is therefore introduced to improve convergence in the neighbourhood of the resonance frequency.Copyright

Enhanced 2-D modeling technique for single-sided patch repairs

This paper reviews the current modeling techniques in the literature and proposes a new one to model bonded patch repaired metallic plates by including all known effects on fatigue crack growth rate. The modeling technique makes use of 2-D plate finite elements and takes into account adhesive progressive failure and the secondary bending effect due to the unsymmetrical configuration of single-sided repair patches. First, comparisons between this 2-D model and a 3-D model were carried out to validate the 2-D bending analysis and the calculated crack-tip stress intensity factors. This modeling technique was then implemented into a computer code interfacing with the commercial finite elements package MSC NASTRAN. Calculated crack growth lives were validated by published experimental tests for three different aluminum plates. Second, the computer code was used to conduct a parametric study on the patch geometries for two different substrate thicknesses showing that for a thinner substrate fatigue crack growth life can be improved by a smaller patch. Finally, a design tool has been developed to determine the most effective patch dimensions for a prescribed fatigue life target.