Laura Ferrero

SMA Actuated Mechanism for an Adaptive Wing

A preliminary study of an adaptive unmanned aerial vehicle (UAV) wing actuated by shape memory alloy (SMA) devices is presented. The wing consists of a sandwich box substructure, flexible ribs, and a flexible laminated skin. The adaptation capability to the changing flight conditions is obtained via airfoil shape adjustments. Torsion SMA tubes are employed for wing camber control, while levers powered by SMA wires are employed for local shape control. A new architecture is proposed: the downward or upward actuation torque is provided by counterrotating concentric tubes connected through a clutch and a positioning piezoelectric motor to the flexible ribs. These actuator tubes are heated one at a time while the other is made free by the clutch in order to obtain any wanted shape without waiting for cooling. The capability of the wing to bear the aerodynamic loads, the power required by the actuators, and their force and torque are assessed by finite-element simulations. An improved version of a recently dev...

Optimization of sandwich panels to blast pulse loading

In this article, the optimization of the face sheet layers is carried out finding a spatially variable distribution of the stiffness properties that minimize the interlaminar stresses. The optimal orientation of the reinforcement fibers is the solution to the Euler—Lagrange equations representing the stationary conditions for the energy contributions under in-plane variation of plate stiffness coefficients. A refined multilayered plate model with a high-order, piecewise variation of displacements across the thickness is employed, that fulfill both the interfacial stress and displacement contact conditions, with the purpose to accurately and efficiently simulate the interlaminar stresses at the interfaces with the core.

Impact and Blast Pulse: Improving Energy Absorption of Fibre-Reinforced Composites Through Optimized Tailoring

This paper tries to conjugate an improvement of stiffness and delamination damage resistance. A number of published results allow us to guess the existence of fibre orientations that are a good compromise for an optimal absorption of the incoming energy and for maintaining of a high stiffness. Optimal absorption is herein intended as a way not involving weak properties, such as interlaminar strength. We seek for an optimal orientation of reinforcement fibres through definition of stationary conditions for bending and shear energy contributions under in-plane variation of plate stiffness coefficients. Our goal is to tune the energy absorption as desired. Two kinds of optimized layers are studied, that are compatible with current production technologies: type 1 reduces bending without substantially increasing the transverse shear stresses, type 2 reduces transverse shear stresses without substantially increasing deflections. Incorporation into the laminates of couples of these layers with opposite features and the same mean properties of those they substitute allows an energy transfer from an unwanted to a wanted mode, as shown by the numerical applications. In this way, the deflections and the stresses inducing delamination damage of laminates subjected to impact and blast pulse loads were reduced, while damping should not substantially change since the variation of the orientation of fibres lies in a range where mild variations of it are induced.Copyright

Optimization of Multi-Core Sandwich Composites Undergoing Impact Loads

In this paper, multiple cores sandwich composites undergoing impact loads are optimized in order to improve their resistance to the impact-induced delamination. This peculiar type of composites is characterized by one internal face splitting the core in two parts. Owing to their architecture with an intermediate and two external faces, their additional tailoring capability offers potential advantages in terms of energy absorption capability and damage tolerance behavior over conventional sandwich composites. Obviously, an accurate assessment of the interfacial stress fields, of their damage accumulation mechanisms and of their post-failure behavior are fundamental to fully exploit their potential advantages. Despite it is evident that structural models able to accurately describe the local behavior are needed to accomplish this task, the analysis is commonly still carried out using simplified sandwich models which postulate the overall variation of displacements and stresses across the thickness, because more detailed models could make the computational effort prohibitively large. No attempt is here made to review the ample literature about the sandwich composite models, since a plenty of comprehensive bibliographical review papers and monographs are available in the specialized literature. Likewise, no attempt is made for reviewing the methods used to model the damage. It is just remarked that the models to date available range from detailed models which discretize the real structure of the core, to FEM models by brick elements, to discrete-layer models and to sublaminate models. In these paper, two different models are used, to achieve a compromise between accuracy and limitation of costs. The time history of the contact force is computed by a C° eight-node plate element based on a 3D zig-zag model, in order to achieve the best accuracy using a plate model with the customary five functional d.o.f. This model is also used in the optimization process, since it is mathematically easily treatable and accurately describes the strain energy. In addition, it enables a comparison with the classical plate models, since they can be particularized from it. The counterpart plate element of this zig-zag model, which is obtained from a standard C° plate element through a strain energy updating (which successfully described the impact induced damage as shown by the comparison with the damage detected by c-scanning in a previous paper), is used for computing the contact force time history, to reach a good compromise between accuracy and computational costs. A mixed brick element with the three displacements and the three interlaminar stresses as nodal d.o.f. is used to compute the damage at each time step. The onset of damage is predicted in terms of matrix and fibers failure, cracks, delamination, rippling, wrinkling and face damping using different stress-based criteria. In this paper the effects of the accumulated damage are accounted for through the ply-discount theory, i.e. using reduced elastic properties for the layers and the cores that failed, although it is known that some cases exist for which this material degradation model could be unable to describe the real loss of load carrying capacity. The optimization technique recently proposed by the authors is used in this paper for optimizing the energy absorption properties of multi-core sandwiches undergoing impact loads. The effect of this technique is to act as an energy absorption tuning, since it minimizes or maximizes the amount of energy absorbed by specific modes through a suited in-plane variation of the plate stiffness properties (e.g., bending, in-plane and out-of-plane shears and membrane energies). The appropriate in-plane variable distributions of stiffness properties, making certain strain energy contributions of interest extremal, are found solving the Euler-Lagrange equations resulting from assumption of the laminate stiffness properties as the master field and setting to zero the first variation of wanted and unwanted strain energy contributions (e.g., bending, in-plane and out-of-plane shears and membrane energies). Our purpose is to minimize the energy absorbed through unwanted modes (i.e., involving interlaminar strengths) and maximize that absorbed through desired modes (i.e., involving membrane strengths). The final result is a ply with variable stiffness coefficient over its plane which is able to consistently reduce the through-the-thickness interlaminar stress concentrations, with beneficial effects on the delamination strength. All the solutions proposed can be obtained either varying the orientation of the reinforcement fibers, the fiber volume rate or the constituent materials by currently available manufacturing processes. The coefficients of the involved stiffness terms are computed enforcing conditions which range from the thermodynamic constraints, to imposition of the mean stiffness, to the choice of a convex or a concave shape (in order to minimize or maximize the energy contributions of interest). Two solutions of technical interest will be proposed, which both are based on a parabolic distribution of stiffness coefficients. The former reduces the bending of a lamina with moderately increasing the shear stresses, the second one reduces these stresses with a low increment in the bending contribution. The effects of the incorporation of these layers (with the same mean properties of the layers they replace) is shown hereafter.Copyright

Improving energy absorption and dissipation of composites through optimized tailoring

Fibre-reinforced and sandwich composites with laminated faces are the best candidate materials in many engineering fields by the viewpoint of the impact resistance, containment of explosions, protection against projection of fragments, survivability and noise and vibration suppression. Besides, they offer the possibility to be tailored to meet design requirements. A great amount of the incoming energy is absorbed through local failures. The most important energy dissipation mechanisms are the hysteretic damping in the matrix and in the fibers and the frictional damping at the fiber-matrix interface. The dissipation of the incoming energy also partly takes place as a not well understood dissipation at the cracks and delamination sites. As self-evident, the local damage accumulation mechanism on the one hand is helpful from the standpoint of energy absorption, on the other hand it can have detrimental effects. To date sophisticated computational models are available, by which the potential advantages of composites can be fully exploited. A large amount of research work has been oriented to improve the impact resistance, the dissipation of vibrations and to oppose the propagation of delamination. These goals can be obtained with incorporation of viscoelastic layers. Unfortunately this makes quite compliant the laminates and reduce their strength. Studies have been recently published that seeks to comply stiffness and energy dissipation. The existence of fiber orientations that are a good compromise between optimal stiffness and optimal absorption of the incoming energy can be supposed by the results of a number of published studies. In this paper, a variable spatial distribution of plate stiffnesses, as it can be obtained varying the orientation of the reinforcement fibres along the plate and their constituent materials, is defined by an optimization process, so to obtain a wanted specific structural behaviour. The key feature is an optimized strain energy transfer from different deformation modes, such as bending, in-plane and out-of-plane shears. Suited plate stiffness distributions which identically fulfil the thermodynamic and material constraints are found that make stationary the energy contributions and transfer energy between the modes as desired. An application to low velocity impacts and to blast pulse loads is presented. The use of the optimized layers with the same mean properties of the layers they substitute were shown to reduce deflection and the stresses that induce delamination. A new discrete layer element is developed in this study, to accurately account for the local effects. Characteristic feature, it is based on a C° in-plane approximation and a general representation across the thickness which can either represent the kinematics of conventional plate models or the piecewise variation of layerwise models.Copyright

Impact Analysis of Laminated and Sandwich Composites Using a Plate Finite Element With Strain Energy Updating

This paper deals with finite element simulation of low velocity-low energy impacts on sandwich composites with laminated faces. A refined zig-zag model with a high-order piecewise representation of in-plane and transverse displacement components is used as a structural model in order to accurately simulate the effects of the transverse normal stress and strain. The goal is to develop a tool for improving the accuracy of conventional plate models, so as to enable the impact analysis of sandwich composites. A strain energy updating process is used for this purpose. As is customary, the Hertzian law and the Newmark implicit time integration scheme are used. The contact radius is computed within each load step by an iterative algorithm, which forces the impacted top surface to conform, in the least-squares sense, to the shape of the impactor. Then, the failure analysis is performed and the material properties of the failed areas reduced. Nonlinear strains of von Karman type are used because the transverse displacement can be quite large even when the plate deflection is small. Comparison with numerical and experimental results published in literature show the present model to be able to accurately predict the impact force and the damage it induces

Impact Analysis of Sandwich Composites

A finite element simulation of impacts on sandwich composites with laminated faces is presented; it is based on a refined multilayered plate model with a high-order zig-zag representation of displacements, which is incorporated through a strain energy updating process. This allows the implementation into existing commercial finite elements codes, preserving their program structure. As customary, the Hertzian law and the Newmark implicit time integration scheme are used for solving the contact problem. The contact radius and the force are computed within each time step by an iterative algorithm which forces the impacted top surface to conform, in the least-squares sense, to the shape of the impactor. Nonlinear strains of von Karman type are used. As appearing by the comparison with experimental results, the present model is able to accurately predict the impact force, the core damage and the damage of face sheets in sandwich composites with foam and or honeycomb core. Moreover, this paper also assesses the accuracy and the range of application of stress based criteria in predicting the onset and evolution of delamination in service. These criteria are widespread by virtue of their low run time and storage costs, although no exhaustive proofs are known weather they are accurate enough for a reasonably wide range of applications. Since where highly iterative solutions are involved (e.g., impact and geometric, or material nonlinear problems) they are the only currently affordable failure models, it appears of primary importance to fill this gap. Aimed to contribute to the knowledge advancement in this field, a comparison is presented with more sophisticate fracture mechanics and progressive delamination models.Copyright